Patient Doses for CT Examinations in Denmark

Transcription

1 Patient Doses for CT Examinations in Denmark Revision of Guidelines for Diagnostic Reference Levels by Mikkel Øberg A Thesis Submitted to the Department of Electrical Engineering at the Technical University of Denmark as partial fulfillment of the requirement for the Degree of Master of Science in Engineering (Medicine and Technology) July 2011 In collaboration with

2 Patient Doses for CT Examinations in Denmark Revision of Guidelines for Diagnostic Reference Levels Project Counselors Risø National Laboratory for Sustainable Energy Radiation Research Division Senior Scientist Claus Erik Andersen Risø National Laboratory for Sustainable Energy Radiation Research Division Senior Scientist Lars René Lindvold National Institute of Radiation Protection Medical Use of Ionizing Radiation Chief Advisor Hanne Neergaard Waltenburg Primary Institute Contact National Institute of Radiation Protection Medical Use of Ionizing Radiation Radiographer Britta Højgaard Project Degree Master of Science in Engineering (Medicine and Technology) Project Duration From February 1 to July 29, 2011 Project Scope 35 ECTS Points July 2011 Mikkel Øberg [s032364]

3 Abstract The dangers and risks associated with X-rays are well established. The use of X-rays in medical diagnostics is governed by orders and guideline, and these are in Denmark published by the National Institute of Radiation Protection. The current guideline was published in 2001 and concerns three main areas: mammography, conventional X-ray and computed tomography (CT). This guideline is used by the departments and clinics carrying out examinations on the approximately 125 CT-scanners in Denmark that are used for diagnostics. The current guideline does not take the increased opportunities for differentiated patient therapy into account, and the main objective of the thesis was to form the argumentative basis for the revision of the part of the current 2001 guideline that specifically relates to CT examinations. The divide was confirmed by analyzing new measurements obtained 2010/2011, by revealing a significant variance between doses within most categories. The extent and course for this variance was determined within the limits of the data. Additional relevant studies were analyzed, and the argumentative basis for the individual revisions was developed The guideline was revised into a pilot that was sent to a select number of departments, leading to a number of additional revisions that further strengthened the link between the guideline and current practice. This project has developed a revised guideline that meets the challenge of the divide between new possibilities arising from the technological development and the supervision provided by the guideline. v P a g e

4 Preface The thesis is titled Patient Doses for CT Examinations in Denmark: Revision of Guidelines for Diagnostic Reference Levels. It was submitted as partial fulfillment of the requirement for the Degree of Master of Science in Engineering (Medicine and Technology). The project was conducted in collaboration between the author, the National Institute of Radiation Protection (SIS) and Risø, the National Laboratory for Sustainable Energy at the Technical University of Denmark DTU (Risø). It was initiated February 1, 2011, concluded July 31, 2011, and corresponded to 35 ECTS points. Three project counselors have been attached to the project, each imparting their expertise in different areas. Senior Scientist Claus Erik Andersen, Risø, has been counselor on statistics, Chief Advisor Hanne Neergaard Waltenburg, SIS, has been counselor on public administration, while Senior Scientist Lars René Lindvold, Risø, has taken care of several administrative issues. In addition, Radiographer Britta Højgaard, SIS, has been the primary institute contact in relation to the development of the revised guideline. It has been the ambition of the author to conduct a master project that fully utilized the duality inherent in the combination of medicine and technology. The project was developed to be as close as possible to a genuine development process, as this is a prime objective of the degree studied by the author. The combination of developing within the public administration while contributing with statistical and analytical knowledge has been an interesting challenge. vi P a g e

5 Acknowledgements The author would first of all like to thank the project counselors, Senior Professors Claus E. Andersen and Lars R. Lindvold from Risø, and Chief Advisor Hanne Waltenburg from SIS, for their aid in planning, organizing and executing the project. The readings and discussions have been an aid in clarifying the project. The cooperation with SIS was continuously fruitful and effective, in no small part due to the tremendous aid offered by Radiographer Britta Højgaard. Without her commitment, sparring and constant disposal, the project would not have gone as far as it did. The project has taken its toll on other commitments, and the author would like to thank Head of Department of Clinical Physiology and Nuclear Medicine and PET, Rigshospitalet, Copenhagen University Hospital Liselotte Højgaard and Computer Scientist Flemming Andersen, for giving him free rein to focus his efforts on the project. For proofreading and enlightening discussions, the author would like to thank his friends Peter Schmidt and Nikolaj Kjelgaard Vedel-Smith. Rune Juhl provided feedback and suggestions regarding the statistics, and for this, the author is grateful. Last, but certainly not least, the patience and tireless support of my wife Ann will not be forgotten. vii P a g e

6 Table of Contents Abstract... v Preface... vi Acknowledgements...vii Table of Contents... ix List of Figures... xiii List of Tables... xv Abbreviations... xvii Chapter 1 Project Description Background Objectives Delimitation Method Chapter 2 Ionizing Radiation and Medical Imaging Introduction Types of Ionizing Radiation Interactions between Photons and Absorber X-ray Imaging The X-ray Tube Conventional X-ray Imaging CT-Imaging System Parameters The Necessity of Dosimetry and DRLs Risks Associated with Radiation Regulations Regarding Radiation The Mathematics of Dosimetry Chapter 3 Measurements 2010/ Introduction Collection of Data Sorting of Data Analysis of the Data Basic Distribution of the Data Sets Main Subjects for Analysis Outliers and Evaluation Linear Model Spread of Height, Weight and BMI ix P a g e

7 3.4.6 Anatomical Regions of 2001 Guideline Chapter 4 Pilot for the Revised Guideline Introduction Optimization of Image Quality in Relation to Patient Dose Input from Resource Persons Revisions in the Pilot Commissioning Changes in Protocols ALARA-argumentation Definition of Examinations Other Protocols Types of Dose Measurements Multiple Scans or Series Number of Patients Standard Patient Deviant Mean Multiple CT-scanners Number of Spreadsheets Information to be Submitted Considerations that were Omitted Pilot Spreadsheet Chapter 5 Revision of Pilot into the Revised Guideline Introduction Measurements from Pilot Fifth Annual Danish Quality Meeting in Radiology Revisions in the Revised Guideline Clarification of Parameters Additions Removals Proposed, but Rejected Revised Guideline Spreadsheet Chapter 6 Conclusion Summary Future Work Bibliography Appendix A General Appendixes... A-93 A-1 Guideline Regarding Diagnostic Reference Levels for X-ray Examinations... A-94 Appendix B Collection of Data 2010/ B-101 x P a g e

8 B-1 sent on December 7, 2010 to All Departments and Clinics...B-102 B-2 Spreadsheet Provided by SIS for Submitting Data [excerpt]...b-103 B-3 Sorted Data [excerpt]...b-105 Appendix C Optimization of Image Quality in Relation to Patient Dose...C-107 C-1 sent on November 22, 2010 to Department Managements... C-108 C-2 Questionnaire... C-109 C-3 Responses... C-111 Appendix D Input from Resource Persons... D-125 D-1 sent on January 19, 2011 to Resource Persons... D-126 D-2 Letter attached to sent on January 19, D-127 D-3 Questionnaire... D-129 D-4 Responses Document... D-133 Appendix E Pilot... E-139 E-1 sent on May 6, 2011 to Select Resource Persons... E-140 E-2 Pilot for Revised Guideline... E-141 E-3 Pilot Spreadsheet Developed for Submission of Data [excerpt]... E-145 Appendix F Revised Guideline... F-149 F-1 Revised Guideline... F-150 Appendix G Additional Figures... G-155 Appendix H Complete R Code... H-159 H-1 data_load.r... H-160 H-2 output.r... H-161 H-3 plots.r... H-164 xi P a g e

9 List of Figures Figure 2-1 The emission of a photon by transition of an electron to vacant position (7) Figure 2-2 The Compton effect and scattering of the photon (7) Figure 2-3 The creation of the electron pair in pair production and subsequent annihilation of the positron (6) Figure 2-4 The three mechanisms for energy loss by photons in absorber, note logarithmic x- axis (edited (6)) Figure 2-5 X-ray tube Figure 2-6 The emission of a photon as a result of bremsstrahlung (7) Figure 2-7 A conventional X-ray imaging system (10) Figure 2-8 Simple overview of a third generation CT-imaging system (12) Figure 2-9 The concept of simple back projection (16) Figure 2-10 The concept of filtered back projection (16) Figure 2-11 The effect of pitch on irradiated area, with a overlap for pitch < 1 (23) Figure 2-12 Relationship between dose and effect for deterministic and stochastic radiation damage (35) Figure 2-13 Standard two-part Plexiglas phantom, with a large body-phantom and a smaller head-phantom (54) Figure 2-14 Coordinate system used for CT imaging (55) Figure 3-1 Examination vs. DLP: Basic distribution of all data obtained 2010/2011, sorted by median [Jitter, Box] Figure 3-2 Examination vs. log(dlp): Basic distribution of all data, sorted by median [Jitter, Box] Figure 3-3 DLP for cerebrum, abdomen, thorax abdomen and thorax [Histogram] Figure 3-4 Examination vs. log(dlp): Distribution based on overweight patients, sorted by median [Jitter, Box] Figure 3-5 Hospital vs. Estimated log(dlp): Estimates and ±2 SD of the Hospital categories [Point, Error Bar] Figure 3-6 Hospital vs. Estimated log(dlp): Estimates and ±2 SD of the Cerebrum Hospitals [Point, Error Bar] Figure 3-7 DLP vs. BMI: Measurements containing BMI, colored by BMI ranges [Jitter] Figure 3-8 DLP vs. BMI: Measurements containing BMI, colored by BMI ranges [Box] xiii P a g e

10 List of Tables Table 2-1 The relevant vendors and their AEC (24) Table 2-2 Tissue weighing factors, with denoting undefined in Pub. 60 (26) (40) Table 3-1 The International Classification of adult BMI ranges defined by the WHO (56) Table 3-2 Sorting and reduction of the initial categories submitted to SIS Table 3-3 Number of data sets in each category, and the percentage which included Scanner, Model, kv and BMI Table 3-4 Distribution of gender, vendor and BMI Table 3-5 Median, third quartile and mean for the histograms in Figure Table 3-6 ANOVA Type 3 of the reduced linear model Table 3-7 Comparison of reduced model and the omission of Hospital Table 3-8 Mean and SD of height, weight and BMI of measurements containing BMI Table 3-9 Third quartile and maximum DLP for 2001 guideline anatomical regions Table 3-10 Preliminary DRLs defined by the third quartile of normal range BMI measurements Table 4-1 Quantification of responses regarding optimization (appx. C-3) Table 4-2 Quantification of input persons responses regarding the revision of the guideline (appx. D-4) Table 4-3 Revisions in the pilot from the 2001 guideline Table 4-4 The International Classification of adult underweight, overweight and obesity according to BMI (56) Table 4-5 Mean height, weight and BMI, excerpt from Table Table 4-6 Percentage of BMI patients constituting standard patients (57) Table 4-7 Overweight in Denmark, the United States and comparable countries Table 4-8 Overview of information to be submitted in the pilot (appx. E-2) Table 4-9 Comparison of spreadsheets used for submission of measurements Table 5-1 The usage of parameter-fields in the pilot spreadsheet Table 5-2 Restricted typing of data in the revised guideline xv P a g e

11 Abbreviations Risø ICRP NCRP SIS CT DRL SSCT MSCT SNR FBP AEC IR IFBP IAEA UNSCEAR FDA Gy Sv W T ALARA CTDI TLD CTDI w CTDI vol DLP BMI WHO AIC R 2 ANOVA DF SD RIS PACS OECD Risø, the National Laboratory for Sustainable Energy at the Technical University of Denmark DTU International Commission on Radiological Protection National Council on Radiation Protection and Measurements Statens Institut for Strålebeskyttelse The National Institute of Radiation protection Computed Tomography Diagnostic Reference Level Single-Slice CT Multi-Slice CT Signal-to-Noise-Ratio Filtered Back Projection Automatic Exposure Control Iterative Reconstruction Inverse Filtered Back Projection International Atomic Energy Agency United Nations Scientific Committee on the Effects of Atomic Radiation Food and Drug Administration Gray - a unit for absorbed dose [J/kg] Sievert - a unit for effective dose [J/kg] Tissue Weighing Factor As Low As Reasonably Achievable Computed Tomography Dose Index Termo Luminens Dosimetri Computed Tomography Dose Index, Weighted Computed Tomography Dose Index, Volume Dose Length Product Body Mass Index World Health Organization Akaike Information Criterion Multiple-R-squared Analysis Of Variance Degrees of Freedom Standard deviation - In a normally distributed population with a mean of µ, 68.3% and 95.5% of the data is within ±1 and ±2 SD of µ, respectively, Radiology Information System Picture Archiving and Communications System Organisation for Economic Co-operation and Development xvii P a g e

12 Chapter 1 Project Description 1.1 Background The dangers and risks associated with X-rays have been extensively researched during the last century, and it is apparent that government supervision of the usage of X-rays is necessary. In 1928, the forerunner of the independent advisory body International Commission on Radiological Protection (ICRP) was founded, with the National Council on Radiation Protection and Measurements (NCRP), a U.S. advisory body, being founded the year after (1). Since then, the ICRP have published guidelines relating to radiation protection, and the European countries use these as basis for their own national guidelines. In Denmark, these guidelines are published by the National Institute of Radiation Protection (in Danish: Statens Institut for Strålebeskyttelse, abbreviated SIS). SIS became a dedicated institute under the National Board of Health (in Danish: Sundhedsstyrelsen) in 1961 as the field of radiation protection became sufficiently extensive. SIS monitors the radiological departments and clinics 1, and bases their guidelines on a combination of ICRP publications, national measurements, related research and cross-nation comparison with Nordic countries. The current guideline regarding the usage of X-ray examinations is Guideline regarding diagnostic reference levels for X-ray examinations, which was published in This guideline concerns three main areas: mammography, conventional X-ray and computed tomography (CT). The field of measuring the dose of radiation is called dosimetry, and this is used by the departments and clinics carrying out examinations on the approximately 125 CT-scanners in Denmark that are used for diagnostics. For these scanners, patient doses of nine different anatomical regions are measured to validate that they are below the relevant diagnostic reference level (DRL) put forth by SIS in their guideline. DRLs are not thresholds, but rather representation of good clinical practice. A properly performed, regular CT examination should not exceed the corresponding DRL. The constant evolution and steady increase in the usage of X-ray imaging makes DRLs a dynamic issue, with an absolute need for periodic revision. The great technological development in the health care sector in general has increased the opportunities for differentiated patient therapy, and thus greater demands that CT examinations are more specific and detailed than previously. The current guideline does not take this development into account, and the data collected 2010/2011 are not expected to reflect the complexities of the examinations performed in practice. 1.2 Objectives The main objective of the thesis is to form the argumentative basis for the revision of the part of the current 2001 guideline that specifically relates to CT examinations. The work has to be continuously centered on the safety of the patient, and is divided into a number of minor objectives: 1 The terms department and clinic are used interchangeably unless specified otherwise. 1-1 P a g e

13 Confirm or reject the expectation by SIS that the collected data does not represent current practice by analyzing the new measurements obtained 2010/2011. If the expectation is confirmed, extract and analyze information from an ongoing project on the optimization of image quality in relation to patient dose. Based on the analysis, determine the areas requiring revision. Revise the 2001 guideline into a pilot in continuous interaction with SIS, by producing the argumentative basis and formulating the revisions. Revise the pilot into the proposal for the revised guideline, based on the author s technical grounding. Additionally, separate from the revision of the guideline, the new measurements obtained 2010/2011 will be analyzed in order to determine if there exist significant trends between different hospitals, scanners, patient-sizes and types of examination. 1.3 Delimitation The thesis concerns the revision of the part of the current guideline relating to CT examinations of adults. Therefore, it will cover neither mammography, conventional X-ray nor the field of pediatrics, except where this is necessary for the further understanding of CT. The data used will primarily encompass the measurements obtained 2010/2011 in conjunction with data from the ongoing optimization project. The priority language in the Danish government is Danish, and the legislations regarding the availability of official texts in a multitude of languages do not apply to specialized text not meant for the common populace. As such, there exist no official English translation of the existing orders and guidelines from SIS. The revised guideline developed in the thesis will not be translated into English, nor will any existing documents, as the effort required would not measure up to the yield. Therefore, anyone reading the thesis without the ability to read Danish will find much of the appendixes incomprehensible, however the analysis, argumentation and conclusions will still be understandable. 1.4 Method The data obtained 2010/2011 as measurements at the individual departments and clinics are analyzed (Chapter 3) together with analysis of qualitative and quantitative responses from the department management. These results, combined with experience and procedures from comparable countries, serve as a basis for a pilot of the revised guideline (Chapter 4). This pilot is sent to a select number of departments and clinics that have expressed an interest as resource persons. The measurements submitted by these are collected and analyzed in conjunction with the qualitative responses. The pilot is then revised based upon this new input into a revised guideline (Chapter 5), which forms the basis for future measurements in order to establish new DRLs after the thesis (Chapter 6). 1-2 P a g e

14 Chapter 2 Ionizing Radiation and Medical Imaging 2.1 Introduction This chapter presents the theory necessary to understand the basic principles of a CT-scanner and the practice of SIS, beginning with ionizing radiation, and continuing to conventional X-ray imaging and CT imaging. It concludes with the argumentation for the necessity of regulations. Subjects important for further understanding and argumentation will be explored in depth, while those not essential will be covered cursorily. 2.2 Types of Ionizing Radiation Radiation is the travel of energy through a medium or space: some we can detect with our senses, such as heat or visible light, while most others are undetectable to us, including X-rays. Radiation is classified as either ionizing or non-ionizing according to its ability to affect matter at an atomic level, specifically whether or not it has sufficient energy to break chemical bonds and separate electrons from atoms. This loss of electrons creates ions, altering their physical properties as a result of irradiating the parent material. Ionizing radiation is furthermore divided into two separate groups, directly- and indirectly ionizing, based on the nature of the ionizing particle. Charged particles (electrons, protons, alpha particles) are included in directly ionizing radiation, as they carry sufficient energy to ionize or excite atoms and molecules. Uncharged particles (neutrons, photons), however, initiate direct ionizing radiation, but are not in themselves directly ionizing (2)(3). Photons are the minute energy packets of electromagnetic radiation(4), and the thesis will focus on the subclass of electromagnetic radiation named X-rays, which are defined by extremely short wavelengths of 10-8 to meters(4) and having resulting energies in the range of 120 electron volts [ev] to 1.20 MeV as per Eq E p = hυ Eq. 2-1 where E p is the energy of the photon, h is Planck s constant 2 and υ is the frequency of the photon (5). X-rays carry the image information when acquiring a CT-image, in the form of absorption of photons. The basic principles of interaction between photons and matter are covered in the following Interactions between Photons and Absorber Photons have zero mass and are electrically neutral, and therefore do not lose energy via coulombic interactions with the atomic electrons of the absorber. They are far more penetrating than charged particles of similar energy, and will travel a considerable distance 2 Planck s constant (h) has a numerical value equal to J s. 2-5 P a g e

15 before undergoing a more catastrophic interaction, which will result in a partial or total transfer of the photon energy to electron energy. There are three possible mechanisms for this energy loss (6): Photoelectric effect Compton effect Pair production Each mechanism will be individually covered in the following paragraphs, with a conclusion of which are of relevance to CT imaging. Photoelectric Effect In the photoelectric absorption process, an incident photon undergoes an interaction with an absorber atom in which the photon is completely absorbed and disappears. In its place an energetic photoelectron is ejected from one of the bound shells of the atom, with energy equal to the difference between the energy of the incident photon and the binding energy of the photoelectron in its original shell(6). The atom hit by the photon, having lost an electron, is in an excited (ionized) state. It must now fill the gap in its orbital, as this is an unstable state of energy for the atom: it will decay. Electrons of higher orbitals fall into the vacant positions of the lower orbitals, always a transition to a lower energy level: as a result, a new photon is emitted, with a wavelength equal to the energy difference between the two orbitals, as illustrated on Figure 2-1. E H E p = E H - E L E L Figure 2-1 The emission of a photon by transition of an electron to vacant position (7) This phenomenon is exploited in chemical analysis and radiation detection. However, it is not these emitted photons that constitute an interest in X-ray imaging, as they do not carry image information. Compton Effect The Compton effect takes place between the incident photon and an electron in the absorber. The incident photon is deflected through an angle θ with respect to its original position, transferring a portion of its energy to the electron, ejecting it from the shell as a recoil electron(6). The atom is ionized and there is the possibility for emission of a photon by decay as described for the photoelectric effect. 2-6 P a g e

16 The deflected photon will continue away from the atom as illustrated on Figure 2-2 below. It will continue to eject orbital electrons until it is either through the absorber or its energy is too low for the Compton effect to occur, after which it will be absorbed as per the photoelectric effect (5). Figure 2-2 The Compton effect and scattering of the photon (7) Pair Production A photon with energy in excess of MeV may interact by pair production, a process in which a photon, passing near the nucleus of the atom, is subjected to strong field effects from the nucleus. The photon disappears, and reappears as an electron pair as illustrated on Figure 2-3 below. The pair consists of a positive electron (positron) and a regular negative electron. The excess energy from the photon is shared by the two particles in the form of kinetic energy. Figure 2-3 The creation of the electron pair in pair production and subsequent annihilation of the positron (6) Note that the positron and electron created are not scattered orbital electrons, but are created by the conversion of the photon. The positron is not a stable particle, and after losing its kinetic energy by interacting with electrons in the absorber, it will undergo a nuclear reaction with any electron, annihilating both particles. This creates two photons with energy of 511 kev each, travelling in opposite directions. 2-7 P a g e

17 Interaction Probability Each of these mechanisms has an interaction probability dependent on the photon energy (hυ) and the atomic number of the absorber (Z), as presented in Figure 2-4 below. The individual interaction probabilities for photoelectric effect, Compton effect and pair production are τ, σ and κ, respectively. Figure 2-4 The three mechanisms for energy loss by photons in absorber, note logarithmic x-axis (edited (6)) The green lines in Figure 2-4 represent the combination of photon energy and atomic number which have an equal probability of the two adjacent mechanisms occurring. As is evident from Figure 2-4, for very low atomic numbers, the Compton effect is dominant for all energies, and as the atomic number of the absorber increases, the photoelectric effect begins dominating for low energies while pair production begins dominating for high energies. The most dominant interaction mechanism in absorption of X-rays in soft tissue is the Compton effect (6), apparent from the atomic numbers of the elements constituting the human body: oxygen (Z=8), carbon (Z=6), hydrogen (Z=1) and nitrogen (Z=7). Together these four elements constitute 96 percent by mass of the human body (8), and are all found below the red line in Figure 2-4. Though X-rays can have energies as low as 120 ev, the diagnostic imaging range is near 100 kev, marked by a red circle on Figure 2-4, where the Compton effect is dominant (9). The photoelectric effect is still relevant for X-ray imaging, as the photon loses energy when it is scattered by the Compton effect, increasing τ while lowering σ. Pair production is of no relevance, as a photon energy of at least MeV is required, which is above the diagnostic imaging range. 2.3 X-ray Imaging The production of an X-ray image is the result of the successful detection of the incident photons, which initially hit the patient and subsequently pass through the patient without being absorbed. Conventional X-ray imaging goes back to Wilhelm Roentgen creating the first 2-8 P a g e

18 X-ray image in history on December 22, The basic principle behind X-ray imaging has not changed significantly since then, as presented in the following subsections The X-ray Tube X-ray tubes used in modern scanners are still based on the same principle as the models used by Roentgen in the late 19th century, as shown on Figure 2-5 below. Figure 2-5 X-ray tube A glass envelope constitutes the exterior shell of the tube, with a vacuum inside. In this vacuum, a cathode emits a steady stream of electrons whose paths are controlled by use of a focusing cup. An anode is positioned directly opposite the cathode, with a metal target fastened to the anode. Normally copper or tungsten is used, either alone or in combination. A high voltage exists across the anode and cathode, usually in the magnitude of kv, and as a result, the electrons wander towards the anode. Because of the vacuum, the electrodes do not interact with anything before reaching the metal target, with which they collide. As explained in section 2.2, electrons are charged particles. They are directly ionizing radiation and will bring the atoms of the metal target to an excited state. This will result in the emission of X-rays as discussed in subsection These X-rays exit the tube through a window in the glass envelope, and are called characteristic X-rays, as their energy is characteristic for the type of metal target. There is, however, another type of X-rays being emitted from the X-ray tube in addition to the characteristic X-rays covered previously. When the electrons enter the metal target, they will deviate from their linear entry-path as a result of the negatively charged electron being affected by the much larger positive charge of the atomic nucleus. In this deviation the electron decelerates, losing kinetic energy, which is consequently emitted as a photon. 2-9 P a g e

19 The energy of the emitted photon is equal to the energy loss of the electron, as illustrated on Figure 2-6 below. Figure 2-6 The emission of a photon as a result of bremsstrahlung (7) This emission of X-rays is bremsstrahlung, and is distinguished from characteristic X-rays by the photons having a continuous energy-distribution. The amount of kinetic energy lost is not limited to discrete levels, as with characteristic X-rays. Together, these two types of X-rays constitute the beam emitted from an X-ray tube. The beam is composed of with a continuous energy distribution arising from bremsstrahlung combined with discrete spikes corresponding to the difference in orbital energy levels of the target metal s electrons; the characteristic X-rays. With the X-ray tube and origin of the photons described, it is possible to regard the X-ray imaging system as a whole. This is described in the following two subsections: the conventional X-ray imaging system is the subject of subsection 2.3.2, whereas its evolvement to the modern CT-scanner is covered in subsection Conventional X-ray Imaging A conventional X-ray imaging system is illustrated in Figure 2-7, and is composed of five distinct steps from the origin of the X-rays all the way through detection and data processing: the X-ray tube, filtration, projection, detection and acquisition, each presented in the following. Figure 2-7 A conventional X-ray imaging system (10) 2-10 P a g e

20 X-ray Tube The X-ray tube is the site for generation of the photons, and is covered in detail in subsection Filtration As previously covered, the X-rays exiting the tube have a continuous energy distribution, with a significant percentage of the photons having energies below the energy of the desirable characteristic X-rays. These low-energy photons are undesirable as they posses insufficient energy to avoid absorption in the patient. They therefore only add to the patient s dose and not the final image, as only the X-rays passing through the patient can be detected. Therefore, the X-rays pass through a low-energy filter that filters out these low-energy photons. The filter is usually in the form of a thin plate of metal (7). As seen on Figure 2-5, the X-rays exit the tube in a cone-formed beam, which is desired, as the photon-beam will be required to encompass the entirety of area of the patient being scanned. However, it is necessary to modify the beam intensity based on patient density, which may vary across the scanned region. In order to achieve this level of control, a wedge filter is inserted after the low-energy filter. The wedge filter consists of a block of material, constructed so that the thickness varies continuously in the shape of a wedge. Furthermore, as different examinations will require a variation in the field size of the beam at the patient s position, a physical collimation is required. It consists of an adjustable window as the final component between the tube and the patient. Projection Having filtered the beam, it exits the window in a cone-shape towards the patient, the angle of the beam adjusted to fit the area of interest and the intensity modulated by the wedge filter. The individual photons will reach the patient, pass through the skin, and continue on their individual linear paths. A number of the photons will lose a portion of their energy, relative to the density of the tissue passed through. Higher density tissue will have more molecules per distance compared to lower density tissue, and as such the chance of a photon interacting with tissue increases with its density. The result is that a portion of the X-ray beam will exit the patient and travel towards the detector. Detection Photons travelling through the patient will initiate indirect ionizing radiation as described in section 2.2. The new photons created through this effect is not representative of the density of the tissue, and their angle differing from the path of the incident photons. Additionally, photons may change direction as a result of Compton scattering, and the photons may bounce off walls or other objects before reaching the detector array. These photons reach the detector at an angle different from the incident X-rays, having passed through the patient in a non-linear matter. They carry image information from one region, but possibly being detected in another. This is not necessarily a problem in conventional X-ray imaging systems, as the level of the noise from these scattered photons is insignificant relative to the signal. This is true for small objects in the scan field, such as a patient s arm or leg. For larger objects like the chest, however, it may be necessary to employ a collimator in the form of a grid. It will filter out these scattered photons, thus improving the image quality. Regardless of whether or not a grid 2-11 P a g e

21 is employed, the photons will reach the detector. It can be either a fluorescent foil, a photographic plate (either with or without thin film transistors), a scintillation detector or a semiconductor detector (11). The types of detectors used exclusively in conventional X-ray imaging are of no relevance to the thesis, and the subject of detectors will be elaborated in the corresponding subsection regarding CT. Acquisition Having successfully registered the intensity of the X-rays in a 2-dimensional array, and having filtered the X-rays initially as to create uniform intensity prior to reaching the patient, the variance in intensity across the array must be a result of the travel through the patient. By adding all acquisitions in each element of the array, the total intensity in that particular part of the image is achieved. Then it is simply a matter of reconstructing an image, with highintensity elements representing areas with low density and low-intensity elements representing high-density areas. This technique is mostly unaltered in CT-imaging, with the significant changes described in the following subsection CT-Imaging System The technique used in CT-scanners share most of its characteristics with conventional X-ray imaging, and the prime differences are seen in projection, detection and acquisition as presented in Figure 2-8 below. Figure 2-8 Simple overview of a third generation CT-imaging system (12) Projection A CT-scanner uses a rotating, circular frame, called the gantry, in which the X-ray tube is mounted opposite an array of detectors. The patient is in the middle on a motorized table, and by moving the table, image slices can be obtained down through the patient. This structure is only representative for third-generation scanners, which were introduced in 1975 and constitutes the vast majority of CT-scanners in Denmark. Few fourth generation CT scanners exist in Denmark, and they differ by having detectors in the entire circumference of the gantry P a g e

22 Detection In a conventional X-ray imaging system, a 2-dimensional array of detectors are used for data acquisition, as the final image is to be constructed from a single projection. In single-slice CT (SSCT) systems, the detectors are placed in a 1-dimensional array along a portion of the rotating gantry s inner circumference. Modern scanners are multi-slice CT (MSCT) where multiple rows of detectors are placed longitudinally in rows of 4, 16 or 64, resulting in multiple slices being acquired per rotation. The detection of scattered photons is a significant contributor of noise in CT-imaging, therefore all these photons should be removed before reaching the detector, as they do not contribute patient information to the image, but only increases the noise 3. Therefore, a Söller collimator is used, consisting of a piece of metal opaque to X-rays. Corridors are drilled in a 2- dimensional array which will only allow passage of the photons travelling parallel to these, whereas photons reaching the collimator at an angle will be stopped, being absorbed either between holes or in the walls of a corridor. At the other side of the Söller collimator, the remaining X-rays reach an array of detector elements, and the intensity in each separate element of the array is measured while the frame rotates. The resulting 2-dimensional image is called a sinogram, with the separate detector elements along one axis, and the angle of the frame (a relation to time) along the other. This sinogram constitutes the basis for the later reconstruction of the image, a transition from the data space to the image space. There are three principal types of detectors used in CT-imaging systems: Scintillation detector Ionizing detector Solid-state detector The scintillation detector consists of a crystal which, when hit by the photons that traveled through the patient, is the site of Compton Effects and the consequent emission of photons within the crystal as described above. These photons reach a photocathode with a photomultiplier tube at the back of the crystal, and the photons are registered as light and converted to an electrical signal, proportional to the energy of the photons. This type of detectors were used in first and second generation CT-scanners, but are too large to be packed as close together as required in modern detector arrays (13). The ionizing detector, also known as the gas-ionizing detector, uses noble gases embedded in small aluminum-chambers under pressures of atm. The primary gas used is xenon, but some detectors using krypton exists. The two sides of each chamber consist of a metal anodecathode pair, the voltage across which is approximately 500 V. When a photon reaches the 3 The prevalent noise in CT-imaging is quantified as the signal-to-noise-ratio (SNR), which specifies the ratio of desired signal power to the noise power present in the total signal. The higher the SNR, the higher the relative proportion of signal to noise, with a ratio of 1 indicating an equal distribution of signal to noise P a g e

23 detector, the gas will ionize proportional to the energy of the incident photon. The ejected electron and positive ion will travel toward the anode and cathode, respectively; this will induce a current in the metal, proportional to the energy of the incident photon (13). These detectors are widely used in third-generation CT-scanners, although their limitations regarding spatial resolution have resulted in a decrease in their use in favor of solid-state detectors, which yield higher detective quantum efficiency 4 (14). Solid-state detectors, also known as semiconductor detectors, share their basic principle with scintillation detectors, in that they use the photoelectric ionization of the detector material. Instead of the creation of electron/ion-pairs seen in scintillation detector crystals, solid-state detectors use a p-n type semiconductor to create electron/hole-pairs. The p-type material has an abundance of holes (electron-deficient sites), while the n-type has an abundance of electrons. The two types are placed adjacent (creating a junction) under the influence of a voltage, constituting a semiconductor. When this material absorbs ionizing radiation, it gives rise to the generation of additional electron/hole-pairs. These will move towards the junction, creating a variation in the current. This variation is proportional to the energy of the absorbed photons, as is a measure of the energy deposited in the detector similar to the other types of detectors. Generally, a crystal of either silicon or germanium is used for the p-n type semiconductor; however, newer scanners use a ceramic semi-conductor instead (15). Acquisition It is necessary to distinguish between the older axial systems, and the newer helical systems. In axial systems, the frame is rotated 360 to obtain a single slice, after which the table is moved a fixed distance. The process is repeated until image acquisition is complete, resulting in an axial acquisition of data. These systems have the advantage of requiring less processing power as there is a pause between acquisitions of data when the table moves, which can be used for processing the newly acquired slice data. The newer scanners use a helical system, where the motorized table is moving at constant speed with the frame also rotating at constant speed, resulting in the X-ray tube and detectors moving in a helical path. Data is collected continuously, and as such require substantially more processing power compared to axial systems, as there is no pause in data acquisition. Data processing is significantly more complicated in CT-imaging systems compared to conventional X-ray imaging systems, as the individual images need to be reconstructed from the sinogram of the individual slices. Remember that each line in the sinogram represents the array of detectors in one specific discrete angle of the rotation. The sinogram contains data collected during discrete intervals of a rotation; therefore, the process for reconstructing the image is a matter of reversing the rotation. 4 Detective quantum efficiency a measure of how effectively an imaging system can produce images with a high SNR relative to an ideal detector P a g e

24 This concept is known as back projection and is illustrated in Figure 2-9 below. Figure 2-9 The concept of simple back projection (16) In back projection, the image is reconstructed by projecting the sinogram (which is in the data space) into an image (i.e. into the image space). Each line in the sinogram is projected back into a reconstruction image with respect to the angle at which it was collected. By having collected data at a sufficient number of discrete angles, it is possible to reconstruct the image. As evident from Figure 2-9, the image (which should show a sharply contoured dot) is blurry and has radiant artifacts 5. This can be avoided by using filtered back projection (FBP), which takes into account the sharp contours. It does so by creating darker-than-average wells around sharp contours, combined with a flattening of the spike as seen in Figure 2-10 below. Figure 2-10 The concept of filtered back projection (16) The process of filtered back projection is here simplified, as the type and degree of filtering has a significant impact on the image. However, it is not the subject of the thesis to give a comprehensive understanding of the image reconstruction process, instead focusing on the subjects important for proper understanding of dosimetry and the challenges it pose Parameters In order to properly calculate and compare doses, it is imperative to have a standardized nomenclature to ensure that all data is comparative (17). Without this, it will be difficult to 5 Radiant artifacts the blooming, like ripples in water, which is the result of sharp contours P a g e

25 reproduce measurements, and to develop consistent protocols. When performing a CT examination, a number of parameters are defined by the operator. The thesis will cover the parameters deemed important for correct, uniform dosimetry: tube current, tube voltage, rotation time, total scan length, slice thickness and pitch. Automatic exposure control (AEC) and iterative reconstruction will be briefly covered, as their impact on dose and image quality is more of a qualitative influence than a quantitative one. Tube current The tube current [ma] influences the number of photons exiting the X-ray tube, as it determines the number of electrons leaving the cathode. The tube current is directly proportional to radiation dose, and as such is a prime parameter in adjusting the dose. Instead of tube current is sometimes used the tube-current-time-product [mas], which is the tube current multiplied with the scan time. Tube Voltage The tube voltage [kv] determines the voltage across the anode and cathode of the X-ray tube, and therefore the acceleration of the electrodes across the interior vacuum. This determines the kinetic energy of the electrodes when they reach the anode, and therefore the number of interactions they can initiate before being absorbed. As a consequence, an increase in tube voltage will increase the dose, all other factors kept constant; however, the increase is not directly proportional as was the case with current. Voltage determines the energy of the electrons, and therefore the energy distribution of the incident X-rays. It is rarely adjusted from the customary value of 120 kv. Certain examinations use a different voltage, but seldom outside the range of 80 to 140 kv (17). Rotation Time The rotation time of the gantry [s] has decreased greatly over the last few decades, with modern scanners having a rotation time in the area of 0.4 seconds (18). The main consequence of the decreased rotation time is an increase in the noise and a reduction in absorbed dose. To avoid the noise, it is customary to increase the tube current accordingly (19). Total Scan Length It is apparent that the total scan length [cm] influence the absorbed dose, as an increase in scan length will expose a larger part of the patient to radiation. Therefore, it is imperative that scan length is to be limited to cover just the diagnostically relevant part of the patient; otherwise, an unnecessary increase in dose will be seen (20). This is relatively easy with SSCT; however, the situation is more complicated for MSCT. At the initiation of the scan, the X-ray tube will be activated the moment the first row of detectors reach the diagnostic area. The X- ray beam will irradiate the entire detector-array, but only the first row of detectors will be acquiring image data. The remaining detector rows will not acquire data, but the area will still be irradiated. This is called overscan, and a small degree of overscan is required for correct reconstruction. As the table moves, more rows of detectors are entering the diagnostic area, contributing to the image. At the reverse end of the patient, the same scenario occurs, and a noteworthy part of the dose is absorbed in the patient outside the diagnostic area (19) P a g e

26 Slice Thickness In SSCT, with only a single row of detectors, the slice thickness [cm] is determined by simple collimation. The maximum slice thickness is limited by the width of the individual detector element (typically 10 mm (19)), and by collimating the beam, this thickness can be decreased. In other words, the width of the beam is equal to slice thickness. In MSCT, the width of each individual detector element in the longitudinal direction determines the minimum slice thickness, and by merging multiple adjacent detector elements during detection, one can increase the slice thickness. This has a significant impact on image quality, as thin slices have better spatial resolution compared to thick slices, but lower SNR. To address the decrease in SNR, it is necessary to increase for instance the tube current, resulting in a significant increase in dose to the patient (21). As an example, changing the slice thickness from 10 mm to 1 mm will increase the noise by a factor of 3.2, other factors held constant (22). Pitch With the prevalence of helical MSCT, it is necessary to incorporate the incremental movement of the table, in relation to the irradiated area. This is defined as pitch, being the increment of the table per rotation, divided by the width of the beam. In Figure 2-11 below, a 4-slice MSCT is rotated twice around the patient, resulting in the acquisition of eight slices in pairs of two (indicated by color). The slices are in reality at an incline, as the patient is moving during exposure. Figure 2-11 The effect of pitch on irradiated area, with a overlap for pitch < 1 (23) With pitch of 1, the last slice of the first rotation will be directly adjacent to the first slice of the second rotation, i.e. a distance of zero between them. With increasing pitch, this distance will increase. With pitch of 2 it is equal to the beam width, as the table has moved twice the beam width during a single rotation. This result in less irradiation of the patient, but the lack of full 360 degrees image date for all slices lowers the SNR. With pitch lower than 1, the slices will be overlapping, resulting in an increased dose to the patient as some areas are exposed multiple times. The SNR, however, improves as a result of overlapping image data. Having presented the basic parameters relating to CT examinations, following is two techniques that have more of a qualitative effect on dose and image quality P a g e

27 Automatic Exposure Control Technological advances lead to the development of a technique where the tube current is modulated in real-time, in order to minimize the dose while retaining image quality. This technique, Automatic Exposure Control (AEC) varies the tube current during exposure. The variance is relative to patient thickness, optimized to achieve dose distribution defined by a desirable image quality. It is possible to achieve a significant reduction in dose based on which type of AEC is used: either the exposure varies within a single slice, i.e. in the image plane of the slice, or it is modulated in the longitudinal direction of the patient. It is also possible to combine these two types of AEC. A complete overview of all relevant vendors 6 and their AEC is presented in Table 2-1 below. Vendor AEC Slice-plane Longitudinal plane Combined GE Smart Scan Auto ma Smart ma Philips DOM Z-DOM Siemens Care Dose CareDose 4D Toshiba 3D Real E.C. Sure Exposure Table 2-1 The relevant vendors and their AEC (24) The difference in naming and definitions between the vendors indicates their different approaches to AEC, in large part a result of the absence of a standardized nomenclature in the field. Each vendor has their own method for quantification of image noise and image quality, making it difficult to compare the use and effect of AECs across vendors (25)(26). Iterative Reconstruction When reconstructing the image as described in subsection 2.3.3, the sinogram is initially filtered. The image is then back-projected from the data space into the image space. This represents a single reconstruction of the slice, and constitutes a single conversion of the acquired data from the data space into the image space. A large number of conversions in conventional engineering use iterative processing, which represents a combination of multiple conversions between the data space and image space, and/or a recurring conversion within the same space. This process was introduced into the field of CT-imaging in the late 2000s, and is now featured by all relevant vendors. When used in relation to image acquisition, the term iterative reconstruction (IR) is used to denote a process in which the sinogram is first converted into an initial image using FBP. This image is defined as the master reconstruction, and here it diverges into two possible methods of IR: the classic method where the iterations are between image space and data space, and the newer method confined to iterations within the image space. The vendors are split between the two, with some using a combination. Therefore, both will be briefly covered in the following. 6 Relevant vendors are defined as vendors having a Danish market share of above 5% (57) P a g e

28 In classic IR, the master reconstruction is converted reversely into the data space (a sinogram) using inverse FBP (IFBP), and an algorithm compares the new sinogram with the original, extracting all differences. These must be the result of noise or inherent errors in the FBP; the noise is sorted and a new image is reconstructed with the noise minimized. This image is once more converted by IFBP into a sinogram, the algorithm compares this new sinogram with the last, and the process repeats a number of times. Each new image is based upon a comparison between the current sinogram and the one prior, defining an iterative process, and will hereby improve the image by reducing noise to an acceptable level. In IR within the image space, the iterative comparison is between to images, which are not converted back into sinograms. The images are compared directly, and the same sinogram is used for the reconstruction of the corrected image under influence of an iterative algorithm that adapts and minimizes the noise. The process of IR as a basis for dose reduction is still not extensively researched; however, preliminary results are positive. Vendors claim a reduction in effective dose of up to 80% (27), but more importantly independent studies show realizable reductions in the range of 32-65% (28)(29). The International Atomic Energy Agency (IAEA) expects a reduction in the collected dose from medical diagnostic imaging of an estimated 50% in the next decade. This includes not only IR, but also a significant reduction in the number of low-dose scans that IAEA find are in unnecessary in some cases (27). IR is the prime focus of all the relevant vendors, in response to the lack of significant technological reductions in dose in the last five years (27). It is, however, still in the early stages of implementation. This is also the case with AEC. The vendors have not defined a standard nomenclature in this field, making it statistically uncertain to use it as a standard parameter for DRLs. Protocols All of the above parameters are defined in protocols, which are a basic set of parameters that are hereafter modified in order to accommodate the individual patient. Protocols serve as basic guidelines for a specific examination on a specific scanner. For each individual patient, a number of the parameters will be modified. The following patient will initially use the original, basic parameters, which will again be modified to suit the individual patient. Having presented the basic theoretical principles for CT-imaging systems, the rest of the chapter will focus on dosimetry and DRLs. Section 2.4 argues for the necessity of DRLs, while section 2.5 presents the mathematics used in dosimetry. 2.4 The Necessity of Dosimetry and DRLs X-ray examinations constitutes a risk to the patient s health, as any exposure to medical X-rays adds to the total burden of carcinogen 7 exposure, and as such is the subject of regulations (30). This is an unavoidable condition of the imaging process covered in subsection 2.3.2, where the 7 Carcinogen - any substance, radionuclide or radiation that is an agent directly involved in causing cancer (7) P a g e

29 decrease in intensity reaching the detectors is the result of a portion of the photons entering the patient having been absorbed. The first subsection will describe the risks associated with radiation, while the second will present the regulatory background for dosimetry Risks Associated with Radiation The majority of the damages and risks associated with X-rays are a result of the ejection of electrons, creating free radicals; ions and molecules with unpaired electrons in an open shell formation. These free radicals are highly chemically reactive and will seek to regain their electron from another cell, a phenomenon not covered earlier as it had no relevance to the physics of X-ray absorption. As the free radical regains its electron, it will change its structure. It therefore does not regain all its physical properties, which possibly results in a mutation (31). The free radical may damage cells in a multitude of ways before becoming stable, the most severe of these being lipid peroxidation 8. Furthermore, the donator of the missing electron in turn becomes a free radical a process called autocatalysis where the stabilization of one free radical leads to another being formed. This self-propagation can cause widespread damage before being terminated, with the damage lingering after the chain reaction is broken (30). As with all mutations within the cells, it can possibly lead to cancer as these free radicals can change bases in the DNA, making it unrecognizable to DNA polymerase 9, or break the sugarphosphate backbone and hereby cause chromosomal abnormalities 10 (31). It is not the subject of the thesis to go into more detail on the levels and types of radiation damage, as this is not within the subject of dosimetry; instead, arguments will be presented for the need to assess this risk. In 1945, near the end of the Second World War, two nuclear bombs were dropped on the Japanese cities of Hiroshima and Nagasaki, resulting in the acute death 11 of an estimated people (32). However, a large number of people who survived the blast had still been exposed to high levels of radiation; people within three kilometers of the hypocenters still received readily measurable levels of radiation (32). These people have been monitored since, to extract as much information as possible regarding the long-term effects of ionizing radiation. This forms the basis for the collected knowledge regarding these effects. 8 Lipid peroxidation the oxidative degration of cell membrane lipids, can result in puncture of the membrane. 9 DNA polymerase a compound of prime importance in the replication of DNA; it reads a single DNAstrand and uses it as a template to synthesize a new complementary DNA-string, resulting in a new double helix. 10 Chromosomal abnormalities the chromosomes contains the full human genome, and abnormalities including an excess of loss of one or more chromosome, loss of a piece of one, or the transfer of a piece of one to another and are significant genetic hindrances that might be the cause of disease (31). 11 Acute deaths within four months, an estimated 150, ,000 people had died P a g e

30 Radiation damage is traditionally separated into two main types, based on their different pathophysiological effects (33)(34) and is presented in Figure 2-12 below. Figure 2-12 Relationship between dose and effect for deterministic and stochastic radiation damage (35) Deterministic Effects Deterministic effects are clinical symptoms, which require doses higher than the threshold of that particular tissue to manifest. They are characterized by demonstrating a relatively fast onset, and their severity, symptoms and timing being dependent on dose. Skin burns is an example of a deterministic effect. Stochastic Effects Stochastic effects are late, randomly occurring effects. They arise independent of dose, and are by definition without a lower threshold value. They include a risk for manifestation of cancer or other genetic damage after a latency of years, decades or, for genetic damage, generations. The risk of manifestation of disease is relative to dose; the severity is not. Regarding cancer, it is either cancer or not; there is no variance in severity. The data gathered from the surviving victims from Hiroshima and Nagasaki, combined with the Chernobyl Catastrophe in 1986 (26), form the basis for the models related to stochastic effects. Regression and interpolation are used to assess the risk of lower doses where the high latency makes it difficult to attain sufficient clean data (33). However, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) concluded in their 2006 report regarding the effects of ionizing radiation that pathophysiological events were evident with doses in the range of CT examinations (36). Additional studies of lung cancer following exposure to radon in mines and homes, bone cancer in patients and workers exposed to radium, and liver cancer and leukemia in patients given Thorotrast during the 1930s and 1940s all concur with the risk estimates from the nuclear bomb survivor studies (37). As per Figure 2-12, the probability for an event decreases for very high doses, as patients will suffer acute death because of the deterministic damage and not have a sufficient lifespan for stochastic effects to occur. Some literature defines a third category: teratogenic effects (damage to the fetus) which have special symptoms. The thesis does not categorize teratogenic effects as a separate class, as dosimetry relating to pregnancy is not covered in the thesis P a g e

31 In regards to CT-imaging, stochastic effects are more relevant because doses are summed; with repeated scans, the probability of a patient developing a pathophysiological event increases. In contrast, deterministic effects are of lesser importance since a single regular CT examination does not reach the deterministic threshold dose for any tissue, and therefore no event can occur (38). However, if done incorrectly, even a regular CT examination can give rise to deterministic effects. In 2009, a number of United States patients having received a regular CT brain perfusion examination began losing hair where they had been irradiated, and in 2010, more than 300 patients showed this symptom of a severe overdose. As a direct consequence the Food and Drug Administration (FDA) is currently conducting an enquiry into the circumstances regarding the overdose of United States clinics (39). Regarding dose it is important to mention the different quantities used in dosimetry; one can either quantify the dose as simply the absorbed energy relative to mass, or go further and take into account the nature of the radiation and the tissue radiated. There are three common quantities for dose (33)(38): Absorbed Dose Absorbed dose is a measure of the energy deposited in the patient, per unit of mass, and is given in the unit of gray (Gy), equal to [J/kg]. Absorbed dose contains no information regarding the type of ionizing radiation, only the energy deposited, and is therefore a poor indicator of the probability of a pathophysiological event occurring. Equivalent Dose Equivalent dose take into account the type of radiation, by multiplying the absorbed dose with the radiation weighting factor specific to this type of radiation. As such, equivalent dose is a measure of the relative biological effect of a particular dose of a specific type of radiation. X- rays have a radiation weighting factor of 1, and as such the equivalent dose will for all examinations be equal to the absorbed dose. However, in order to denote that equivalent dose contains information regarding the type of radiation, the unit is different from absorbed dose; sievert (Sv), equal to [J/kg]. Effective Dose Effective dose is the energy absorbed, per unit of mass, and is calculated by multiplying the equivalent dose by the corresponding tissue weighting factor. The tissue weighing factor represents the radiological sensitivity, which varies for different tissue. This is a consequence of the type of damage caused by ionizing radiation, as well as tissue density. X-rays damage cells by causing mutations in the DNA-replication-chain, and during replication the DNA is exposed and most vulnerable. Therefore tissue containing cells with a high mitotic rate (gonads, bone marrow, colon) is more receptive of radiation damage. It has a higher stochastic probability for a pathophysiological event, most often cancer (34). Tissue consisting of cells with a low mitotic rate (bone, skin) has higher resistance to radiation damage, as the DNA is seldom exposed (31). Most CT examinations expose more than one type of tissue. If the tissue weighing factors for these tissues are unequal, it is necessary to evaluate the relative proportions of dose absorbed 2-22 P a g e

32 in the different tissue in the scanned region. These proportions have been estimated previously, using either body phantoms or mathematical modeling. The total equivalent dose is then proportioned relative to each type of tissue, and the equivalent dose in each is then multiplied by the tissue weighing factor specific to that tissue. These are then summed to get the effective dose for the whole scan. The tissue weighing factors were originally defined by the ICRP in ICRP Publication 27 in 1977, and first revised in ICRP Publication 60 in 1990 based upon limited empirical data, combined with theoretical assumptions and measurements (40). In 2007, ICRP published ICRP Publication 103, which effectively superseded the old publication, by providing revised tissue weighing factors incorporating the significant increase in empirical knowledge acquired in the intervening time (26). The tissue weighing factors (W T ) currently used are presented in Table 2-2 below. Tissue or organ ICRP Pub. 60 ICRP Pub. 103 W T W T W T W T Gonads Red bone marrow, colon, lung, stomach Bladder, liver, esophagus, thyroid Breast Bone surface, skin Brain, salivary glands Remainder of body Table 2-2 Tissue weighing factors, with denoting undefined in Pub. 60 (26) (40) In summary, if the total effective dose is 1 msv, the probability of a pathophysiological event corresponds to a dose of 1 msv = 25 msv given only to the liver (having a tissue weighing 0.04 factor of 0.04). Although ionizing radiation is a strong carcinogen, it is worth relating the proportions of CT examinations in regards to the radiation of the population as a whole. The backgroundradiation 12 in Denmark is approximately 3 msv/yr (41), and the added contribution to the mean dose of ionizing radiation per capita from medical diagnostics is 1 msv/yr (42), resulting in a collected dose per capita of 4 msv/yr. This is the nation mean, and the majority of the population of approximately 80% will receive 3 msv/yr. A minority of approximately 20% will undergo one or more radiological diagnostic scans. These will be significantly above 4 msv/yr (43), as a single whole-body CT-scan nets an effective dose in the area of 10 msv. Cancer is the most frequent cause of death in Denmark, with a probability of developing a lethal cancer of 25% (33). It is, however, difficult to assess the contribution from medical ionizing radiation for the individual, as the majority of the population will never undergo this procedure. The basic 12 Background-radiation the radiation dose not caused by medical or other human factors. Includes internal radiation from food (0.4 msv/yr), inhalation of radon from housing (2.0 msv/yr), cosmic radiation (0.3 msv/yr) and gamma radiation from radioactive substances in the ground ( msv/yr) (41) P a g e

33 contribution from ionizing radiation regarding the probability of developing a lethal cancer is defined as 5% per Sievert, or 0.005% per msv (34). Nonetheless, any CT examination is considered a carcinogen, increasing the probability for a pathophysilogical event relative to the dose, and as such should be optimized. In summary, the main pathological event of radiation damage is development of a cancer, as ionizing radiation is a strong carcinogen. Cancer is a stochastic effect, and as all regular CT examinations are well below the deterministic threshold, only stochastic effects are considered in regular dosimetry. The risk between dose and probability for stochastic effects is a continuous function, with no lower threshold. Therefore, it is not possible to conclude that the lower the dose, the better for the patient; lower dose results in lower quality of the reconstructed image, increasing the challenge of a correct diagnose. This balancing act between dose and diagnostic certainty is the prime reasoning behind government-regulation of dosimetry. Each patient is to be individually evaluated regarding expected diagnose and the dose required to achieve this. The government is required to establish guidelines and DRLs to act as a uniform backbone for the individual healthcare professional Regulations Regarding Radiation Many years have passed since the basic knowledge regarding the harmful potential of radiation as a carcinogen was unknown in the general population, as evident in the following excerpt from an article in TIME in 1956 (44): Patients are getting worried about X rays [sic]. Following the National Academy of Sciences' Report to the Public on the biological effects of radiation (TIME, June 25), more and more people have begun to nag doctors and dentists about possible harmful effects. Many have flatly refused to submit to X-ray examination or treatment. Just how safe or dangerous are X rays? Since its introduction to the market, the CT-scanner has seen a rapid increase in usage. The number of CT examinations being performed in the United States has increased by 10% annually in the period of , to account for 15% of all procedures in radiology in The same year CT examinations contributed with 50% of the collective population dose resulting from the diagnostic use of ionizing radiation (45), a figure that is equivalent with findings from comparative countries. Switzerland has most recently published theirs: an increase in the number of CT examinations by almost 70% from 1998 to 2003 and a collective dose contribution of an estimated 50% (46). The corresponding values in Denmark is an increase in the number of performed CT examinations from 4.2% in 1995 to 9.6% in 2003, and then a steady increase of approximately 10% annually to 16.3% in 2009 (43). The share of the collective dose contributed by CT increased from 37% in 1995 (43) to 65% in 2008 (47). Regarding the number of examinations performed, even though the relative increase is identical for the United States and Denmark, the number of examinations in relation to the size of the population is not. In 2007, the United States performed CT examinations per 1000 population, while Denmark performed 73.6 (48). Note that these numbers on the contribution to the collective population dose of CT is not directly comparative. There is no 2-24 P a g e

34 common consensus on which examinations to include, and there is an undefined difference in the frequency of dosimetry reporting. As a result of the inherent risk regarding ionizing radiation, ICRP have since its founding in 1928 advised on issues regarding radiation safety. In 1977, they issued ICRP Publication 26, defining the three fundamental principles regarding the usage of radiological examinations (20)(33)(49): The Justification Principle: No practice shall be adopted unless its introduction produces a positive net benefit. The Optimization Principle: All exposures shall be kept as low as reasonably achievable, economic and social factors being taken into account. The Limitation Principle: The exposures to individuals shall not exceed the limits recommended for the appropriate circumstances by the commission. The first principle, the justification principle, is not directly related to dosimetry. It dictates the assessment of the healthcare professional considering the examination, and its effect on dose is all-or-nothing, not gradual. The second principle, the optimization principle, is the reasoning behind dosimetry as described earlier; the balancing act between dose and diagnose, summed in the sentence as low as reasonably achievable (ALARA). This guiding principle underlines every assessment made for each individual patient, within the limits set forth by the ICRP in the third principle, the limitation principle. The first regulation regarding government supervision of radiation was put forth in 1930, as Law Regarding the Use of X-rays (in Danish: Lov om Brugen af Røntgenstråler). It outlined the basic principles and the responsibility for compliance with the law was given to the National Board of Health (in Danish: Sundhedsstyrelsen) (50). This responsibility is, as mentioned in section 1.1, now managed by SIS. They have subsequently published orders regarding conventional X-ray and CT imaging systems in medical use, the latest in 1998 (51). They also publish guidelines regarding dosimetry and DRLs, the latest encompassing all X-ray imaging systems in 2001 (52). Herein lays the eligibility for the thesis. The constant evolution and steady increase in the usage of X-ray imaging makes DRLs a dynamic issue, with the absolute need for periodic revision. The guideline from 2001 specified three main areas: mammography, conventional X- ray and CT. Mammography is of no importance to the thesis, as CT is not employed in this field. While a revision of the part of the guideline regarding conventional X-ray imaging was published in 2006 (53), the part concerning CT has not been revised since With the increased technological divide between the diagnostic possibilities of conventional X-ray and CT, the need is pressing for a revision of the part of the 2001 guideline specifically aimed at covering the considerations related to CT imaging. The necessary mathematics for proper dosimetry is presented in the following subsection, which concludes the chapter P a g e

35 2.5 The Mathematics of Dosimetry It is nearly impossible to measure the dose to individual organs directly during CT examinations, but it is possible to estimate the expected dose by use of phantoms as reference. In contrast to conventional X-ray imaging, in CT imaging the X-ray tube is rotated around the patient during exposure. Therefore, the highest dose will be given to the skin, and the lowest dose to the isocenter. It is necessary to know the relation between parameters, the dose for the individual scanner and its associated protocols. This value is defined the CT Dose Index (CTDI) and is calculated using dose-measurements on a Plexiglas phantom capable of both head (16 cm diameter) and body (32 cm diameter), as shown in Figure 2-13 below. Figure 2-13 Standard two-part Plexiglas phantom, with a large body-phantom and a smaller head-phantom (54) The dosimetry is conducted using either a 100x10 mm pencil ion chamber or TLD (Thermoluminescent Dosimeter). Five measurements are made, one in the centre (CTDI 100,c ) and four in the periphery (CTDI 100,p ), 10 mm below the surface. From the four periphery measurements the mean CTDI 100,p is calculated, and from this value, combined with CTDI 100,c the total CTDI is calculated. Centre and periphery are weighted differently, as per the uneven dose-distribution mentioned earlier and shown in Eq. 2-2 below (52). CTDI w = 1 3 CTDI 100,c CTDI 100,p Eq. 2-2 CTDI w has the unit of [mgy] and is an indicator of dose in relation to tube current and time, in a single rotation of the gantry. In other words, it represents the dose in the x- and y-directions, with x being horizontal and y vertical in Figure 2-14 below. Figure 2-14 Coordinate system used for CT imaging (55) 2-26 P a g e

36 In order to include the gaps and overlaps between the radiation dose profiles from consecutive rotations of the X-ray source, one must include information from the z direction. This information is included in CTDI vol, which is calculated differently for axial and helical scans. For an axial scan, Eq. 2-3 below is used. CTDI vol = N T I CTDI w Eq. 2-3 where N is the number of slices per rotation, T is the nominal thickness [cm] and I is the table increment per rotation of the gantry [cm] (55). For a helical scan, Eq. 2-4 is used instead. CTDI vol = 1 pitch CTDI w Eq. 2-4 where pitch is the pitch of the helical scan as presented in subsection (55). CTDI vol represents only the exposure of individual rotations of the gantry, not the combined exposure to the patient. It is necessary to sum the dose over the entire CT examination, i.e. the total scan length, and multiply this with the dose exposed at a single rotation and table increment. This is defined the Dose Length Product (DLP) and is calculated as per Eq where L is the total scan length [cm]. DLP = CTDI vol L Eq. 2-5 In summary, DLP is an indicator of the collected dose of the entire CT examination, calculated from the basic scan parameters (tube current, tube voltage and time), as well as the number and width of scans and pitch. This, along with CTDI vol, is given directly as output on the scanner interface in most modern scanners, and is therefore readily available. It is however necessary to validate these periodically, as to verify the correctness of the calibrations. For a given examination, the average DLP over a number of patients, currently 10 as per the 2001 guideline, is compared to the corresponding DRL. If the DLP is above the DRL, the clinic is to conduct an internal review to elicit the responsible factors, and if possible change practice. Having presented the necessary theory, the following chapter will detail the analysis of the measurements submitted to SIS 2010/2011. This analysis will evaluate the expectation from SIS that there is a disparity between the 2001 guideline and current practice P a g e

37 2-28 P a g e

38 Chapter 3 Measurements 2010/ Introduction Every CT-scanner in Denmark is monitored by SIS, with the individual departments and clinics collecting the dose for each examination and filing it in their dose-profile archives. They are obligated to ensure their doses are optimized. They must measure their doses at least once every two years, and certify that they are within the DRLs defined by SIS. If not the case, they must determine the reason and, if possible, address the issue. These data are not normally obtained by SIS; however, when it is decided to revise the DRLs all departments and clinics are asked to send in doses. They must be measured for ten standard patients for each of nine predefined anatomical regions. These data are pooled, and for each anatomical region, the third quartile is calculated and used as a basis for the revised DRL. The initial DRLs were defined primarily by pooling data from the Nordic countries, and evaluating these with basis in demographic statistics from the individual countries while including the relevant ICRP publications. The revision of the DRLs regarding CT has been planned since The existing data at SIS is not recent enough as to constitute a proper representation of the current state of CT examinations. On December 7, 2010 SIS sent a notice to all departments and clinics, asking them to send in measurements, using a specialized spreadsheet located online if possible. These data were obtained during the winter of 2010/2011. When work began on the thesis February 1, little more than half of the final amount of data had been received. The remaining followed in the coming months, with the final dataset including measurements from all Danish hospital departments and most clinics. This chapter presents the work on collecting the data and the subsequent sorting into relevant and manageable categories, which is a requirement for proper statistical analysis. Having sorted and uniformed the data, the chapter concludes with a statistical analysis of relevant phenomena and tendencies within the data. This will be used as basis for the revised guideline in the subsequent chapter, in accordance with the defined goal. 3.2 Collection of Data The measurements were submitted to SIS by all departments performing diagnostic radiology examinations, including private clinics. The departments were asked to submit data in the nine anatomical regions defined by the 2001 guideline: Cerebrum Face and sinuses Vertebral (trauma) Thorax HRCT Thorax Abdomen (ordinary) 3-29 P a g e

39 Liver and spleen Cymbals (ordinary) Cymbals [Bone] The sent to the departments (appx. B-1) advised using the spreadsheet on the SIS homepage for submitting the data (appx. B-2). For each examination, the following parameters were entered: Room, Generator (scanner), kv (tube current), Gender, Height, Weight and DLP. Room and generator were translated to scanner vendor and scanner model using SIS internal equipment database. These values were more interesting for statistical purposes, as they might possibly allow for analysis of scanner dependency. Gender, height and weight were normalized to *m/f+ (male/female), *cm+ and *kg+ for proper calculation of the patient s Body Mass Index (BMI). BMI is defined as the weight in kilograms divided by the square of the height in meters, and has the unit of [kg/m 2 ]. It is a widespread measure of underweight and obesity, and is a basic diagnostic indicator. There is no set definition of which classifications of BMI ranges are standard. The ones defined by the World Health Organization (WHO), however, constitute a good basis. The different ranges of BMI as defined by the WHO are presented in Table 3-1 below. Classification BMI [kg/m 2 ] Underweight < Normal range Overweight 25 Pre-obese Obese 30 Obese class I Obese class II Obese class III 40 Table 3-1 The International Classification of adult BMI ranges defined by the WHO (56) The thesis will use the classifications and ranges defined in Table 3-1 as its basis for analyzing BMI. Note that the DLP for the patients are not directly measured for each patient, instead they are based on the phantom measurements as described in section 2.5. They are therefore not actual measurements of absorbed dose, but rather representative calculations based on phantoms and, possibly, mathematical modelling. 3.3 Sorting of Data Having collected sufficient data to begin sorting, the data was inserted in to a single spreadsheet, constituting a database. An examination, as defined by SIS, is a single scan séance, and any scout- or contrast scans were pooled as a single measurement. The departments had submitted a wide range of different examinations, and each was given its own unique sheet in the spreadsheet. This resulted in 39 different categories P a g e

40 These 39 categories encompassed anatomical areas, pathophysiological indicators, highly specialized examinations, and examinations with very few patients. Therefore, it was necessary to sort a number of these categories, reducing them to increase the statistical basis. This reduction and sorting of the required categories is outlined in Table 3-2 below, with the argumentation for the reduction. The argumentation of same region was proposed by SIS, while too few patients were evaluated by SIS in collaboration with the author. Category Action Argumentation Abdomen Cymbals Pooled with Abdomen Same region Aorta Aneurism Pooled with Aorta Same region Cranium Pooled with Cerebrum Same region Head (Standard) Pooled with Cerebrum Same region Kidney Pooled with Urography Same region Lumbar Discs Pooled with Columnar Same region Myelography Pooled with Columnar Same region Neck Thorax Abdomen Pooled with Thorax Abdomen Same region Overview Over Abdomen Pooled with Abdomen Same region Overview Over Urography Pooled with Urography Same region Stones Examination Pooled with Urography Same region Thorax Abdomen Cymbals Pooled with Thorax Abdomen Same region Thorax Liver Pooled with Thorax Upper Abdomen Same region Upper Abdomen Pooled with Abdomen Same region Angiography (Body) Removed Too few patients (13) Aorta Removed Too few patients (4) Calcium Score Removed Too few patients (16) Cervical Removed Too few patients (8) Colonoscopy Removed Too few patients (19) Coronary Removed Too few patients (8) Coronary Arteries Removed Too few patients (1) Indicative biopsy Removed Too few patients (8) Neck Removed Too few patients (23) Shoulder Metering Removed Too few patients (7) Small Intestine Removed Too few patients (3) Pancreas Removed Too large range [616;4077] considering few patients (48) Table 3-2 Sorting and reduction of the initial categories submitted to SIS The reduction of categories outlined in Table 3-2 above resulted in 15 categories in the final database, as presented below. Abdomen Cerebrum Cymbals Cymbals [Bone] Facial Skeleton Heart HRCT Thorax Liver 3-31 P a g e

41 Sinuses Spine Thorax Thorax Abdomen Thorax Upper Abdomen Trauma Urography 3.4 Analysis of the Data The measurements are representative as a whole, as measurements have been submitted from all Danish hospitals, and most clinics. The analysis of the data is separated into three stages. The first subsection gives a short overview of the basic distribution of the data, and avoids going into detail with any specific subject. The second subsection presents the main subjects to be analyzed, the results of which will be presented in the subsequent subsections. All analysis is conducted in the statistical programming language R, a freely available language for statistical computing and graphics, available at The complete scripts can be found in Appendix H. A number of figures are placed in Appendix G, and they are referenced by their figure number. Note that the source for all figures of data is (57) unless otherwise noted Basic Distribution of the Data Sets The data was sorted into 15 categories, their characteristics presented in Table 3-3 below. Each patient, with all relating parameters, constitutes a single data number. Category Number of Percentage of data including: Data Sets Scanner Model kv BMI Abdomen Cerebrum Cymbals Cymbals [Bone] Facial Skeleton Heart HRCT Thorax Liver Sinuses Spine Thorax Thorax Abdomen Thorax Upper Abdomen Trauma Urography Summed Table 3-3 Number of data sets in each category, and the percentage which included Scanner, Model, kv and BMI 3-32 P a g e

42 All the data contained Hospital, Department, Gender and DLP. Scanner, Model, kv, Height, Weight and/or BMI were lacking from some data sets. The distribution of these parameters, for each category, is evident from Table 3-3. Height and weight is included in BMI, as no data contains only one or two of these three parameters. These omissions result in a weaker statistical grounding; most severe is the lack of BMI for 56.7% of the patients, as it is challenging to evaluate the DLP without a measure of patient size. The inclusion of tube voltage [kv] in only 61.2% of the data sets is of little consequence as 91.9% of the submitted tube voltages are 120 kv. The scanner vendor is submitted for 71.8% of the patients, which should provide for satisfactory statistical grounds in the later analysis of scanner dependency. The distribution of gender, scanner vendor and BMI is shown in Table 3-4 below. Parameter Percentage Gender Male 51.8 Female 48.2 Vendor Vendor # Vendor # Vendor # Vendor #4 7.5 BMI Underweight [<18.5] 4.4 Normal Range [ ] 49.0 Pre-Obese [ ] 34.9 Obese [ 30] 11.7 Table 3-4 Distribution of gender, vendor and BMI The distribution between male and female patients is close to a 50/50 split, while the spread of vendors clearly show a shift towards a single vendor, vendor #1. The normal BMI range constitutes 49% of the data, even though the measurements for submission were specifically required to be standard patients. The normal range is actually wider than the recommended BMI between 20 and 25 recommended in the spreadsheet used with the 2001 guideline. The large portion of overweight patients (BMI 25) of 46.6% would be expected to raise the overall doses measured, which will be analyzed in subsection The fact that the data is unbalanced is apparent from Table 3-3 and Table 3-4, as the proportion of data within each factor differs significantly. The data sets are observational data, and had it been a fully randomized study it would have been designed with an equal distribution of the different parameters. The bias seen in the distribution of BMI, scanner model and especially the number of data sets in each examination all add to an unbalancing of the analysis. This is an expected challenge when analyzing real-world data and it is necessary to keep this in mind when evaluating the results P a g e

43 The most important parameter is DLP, and its distribution is best shown via a box-and-whiskers plot as shown on Figure 3-1 below. The box encompasses half the datasets, beginning with the first quartile and ending with the third quartile, with a horizontal line showing the median. The width of the box is defined the interquartile range (IQR), being the difference between the first and third quartile. The lower whisker shows the lowest data still within 1.5 times the IQR of the first quartile, while the upper whisker shows the highest data still within 1.5 IQR of the third quartile. The measurements not included in this range (spanning 4 times the IQR) are defined as outliers. Figure 3-1 Examination vs. DLP: Basic distribution of all data obtained 2010/2011, sorted by median [Jitter, Box] Considering the large variance in DLP between examinations, the DLP is transformed using the common logarithmic function. Furthermore, this transformation will lower the influence from outliers. This will better illustrate the distribution of DLP within the different examinations, as shown of Figure 3-2 below. This transformed DLP will be used in the majority of the following plots, but note the y-axis of each for expectations. Figure 3-2 Examination vs. log(dlp): Basic distribution of all data, sorted by median [Jitter, Box] 3-34 P a g e

44 It is evident from Figure 3-2 that the majority of categories are not normally distributed, as there are a large number of outliers, all in the upper range. There are two major reasons for this; first, Figure 3-2 includes all the data, even that lacking BMI. BMI is expected positively proportional to dose, but when plotting all data it is impossible to determine the influence of BMI on DLP. Second, as evident from Table 3-2, a number of categories encompass multiple different. The variance between the different examinations pooled into a single category could give rise to an increased spread. Looking at the spread of the outliers for Abdomen, Cerebrum, Spine, Liver, Thorax, Thorax Abdomen and Urography it is also apparent that they are concentrated around a value slightly higher than the upper whisker. This suggests that there are instead two distributions, one centered about patients in the normal BMI range, and one with higher DLP for the overweight patients. This will be analyzed later in the chapter, as will the influence of only looking at the measurements containing BMI. Trauma has by far the highest DLPs, with both the highest mean, IQR, minimum and maximum value. This is to be expected given the diagnostic background for a trauma-examination. The patient is in a state of high distress, and even though the optimization principle is still valid, the justification principle becomes dominant as the patient is in a higher threatening stage compared to regular examinations. In order to evaluate the distribution of datasets containing BMI compared to those lacking BMI, a histogram for the four examinations with the most data is shown in Figure 3-3 below. The examinations are cerebrum, abdomen, thorax abdomen and thorax. Figure 3-3 DLP for cerebrum, abdomen, thorax abdomen and thorax [Histogram] From the histograms in Figure 3-3, it is apparent that the expectation by SIS concerning the non-uniformity of the data is confirmed. The histograms are largely different in structure and form, even when considering only the data containing BMI. Cerebrum has a drop around DLP=750, possibly indicating the presence of two different examinations with mean DLP on either side being pooled into a single category P a g e

45 In order to evaluate if the datasets lacking BMI has a different BMI, the median, third quartile and mean of the histograms in Figure 3-3 is shown in Table 3-5 below. Examination Median Third Quartile Mean Cerebrum (BMI) Cerebrum (No BMI) Abdomen (BMI) Abdomen (No BMI) Thorax Abdomen (BMI) Thorax Abdomen (No BMI) Thorax (BMI) Thorax (No BMI) Table 3-5 Median, third quartile and mean for the histograms in Figure 3-3 It is evident from Table 3-5 that datasets lacking BMI in all cases except Abdomen have a median 10% or more above the median for BMI datasets. The third quartile is relevant for establishing DRLs, and this is significantly higher for all examinations. The mean is higher as well for non-bmi data. Having presented a short overview on the basic distribution of the measurements, the remainder of the chapter will cover the analysis. These subjects are formulated by the author, in accordance with the considerations provided by SIS, to be used as argumentation for revisions of the guideline. Furthermore, the data will be analyzed for proportionality, tendencies and other trends as per the defined goal Main Subjects for Analysis The analysis consists of two types of subjects: those subjects arguing the revisions of the guideline, and those describing more specific tendencies and trends. There is no clear distinction between these two subjects, as the arguments for the revisions proposed by SIS and the author will draw upon different aspects of each individual subject. The reminder of this chapter will therefore describe a number of statistical tendencies, analysis and comparisons, each in their own separate subsection. Their possible usage as argumentation for the revisions will be covered in Chapter 4. Outliers and Evaluation: First, the spread of the DLP within the different categories is not sufficiently described by being normally distributed. An evaluation of the outliers and their relation to BMI would provide insight into the influence of the outliers. Linear Model: Second, a linear model will be constructed explaining the proportionality between individual factors and DLP. The correlation between scanner vendor and dose is also analyzed. With 71.8% of the measurements containing scanner vendor, there is a possible basis for evaluation. The aim of this model is to give an insight into the influence of different parameters on dose P a g e

46 Spread of Height, Weight and BMI: Third, having researched the correlation between BMI and dose, the spread of the patient parameters of height, weight and BMI is analyzed. This leads to a discussion of the standard patient and its representation, or lack thereof, in the measurements. Anatomical Regions of 2001 Guideline: Fourth, building upon the conclusions of the linear model and outliers, the measurements are compared to the nine anatomical areas defined in the 2001 guideline. A preliminary revision of the DRLs using the new measurements is presented. These preliminary revisions use the third quartile, and include an evaluation of the influence of using only standard patients Outliers and Evaluation The outliers of DLP as presented in Figure 3-1 are not all normally distributed. This is suspected to be the result of only 43.3% of the data sets containing BMI, and even those data sets containing the 46.6% of patients with a BMI above the normal range. In order to evaluate the validity of the claim, Figure 3-4 shows the distribution of all data, color-coded for not overweight [green], overweight [red] and those patients with no defined BMI [black]. Box-andwhiskers plots are overlaying each examination; the left contains the data with BMI while the right contains the data lacking BMI. Figure 3-4 Examination vs. log(dlp): Distribution based on overweight patients, sorted by median [Jitter, Box] Based on Figure 3-4, a number of observations become apparent. It shows that the overweight patients have higher DLP, but it is difficult to conclude as the large numbers of data sets lacking BMI dominate the plot. Another plot, with limited DLP range, will be produced in subsection to evaluate this. The assumption that the data lacking BMI corresponded to the large DLP is reinforced, as of all the data with DLP above 3,000, only three contains BMI: two overweight patients for thorax and a single overweight thorax abdomen examination. For the 31 BMI patients with a DLP between 2,000 and 3,000, 32% are trauma patients dominating the range. The remaining 68% contains 42% obese (BMI 30) and 26% non-obese (Figure 3-37 P a g e

47 Appx. G-1). The exclusion of these would allow for the lowering of the DLP range to 2,000, increasing the graphical spread of the data for easier analysis. The box-and-whisker plots are not readily different at this DLP range for most examinations. However, for liver and trauma, the two with the largest spread, the data without BMI has a larger spread and overall larger values. This further underlines the impact of data lacking BMI on the non-normal distribution of the DLP. These observations concerning the distribution will be evaluated in the linear model, which is the subject of the following subsection Linear Model To evaluate the proportionality between the individual factors and DLP and evaluate the outliers further, a linear model is constructed. The initial model contained all data, and defined a possible proportionality between all relevant factors and DLP. The relevant factors were hospital, scanner, gender, height, weight and BMI. Department was removed because apart from a few oncology departments, all were radiology departments. Scanner model was omitted as the 2,932 measurements containing this parameter encompassed 39 different models, 33.12% of which were a single model (57). Tube voltage was removed as 91.9% of these are 120 kv, and the direct influence of tube voltage on dose is already known. It is apparent from the basic distribution that by far the greatest influence on DLP comes from the type of examination, and therefore it will be included in the linear model. Height and Weight were removed, as they are already included in the calculation of BMI. It is therefore necessary to include either height and weight, or only BMI. BMI was chosen, as it is more widespread as a reference in the health care sector. The model was created using R s lm() for fitting linear models, set to ignore missing values, and does not consider interactions between factors. Creating the model, it therefore removed all measurements that did not include all the above factors. If it was later found that a given factor had no influence on dose, it could be removed and more data included as a result. Furthermore, as per a general suspicion by radiologists regarding dose, the model included the BMI squared as a factor. The model would then be able to evaluate if DLP was influenced by the second power of BMI or just BMI. DLP will be transformed by the natural logarithmic function, to minimize the influence of outliers and make the examinations comparable. Note also that neither BMI nor DLP is factorized, as opposed to all other factors. The raw model was therefore defined as follows: log(dlp)~factor(examination)+factor(hospital)+bmi+bmi 2 +factor(scanner)+factor(gender) The raw model requires trimming, as it currently assumes correlation between all given parameters. This is achieved by the function step() that trims away the parameters having no influence on dose by use of the Akaike Information Criterion (AIC). This value is a measure of 3-38 P a g e

48 the goodness of fit for compared models, i.e. how well the models fit the measurements, compared to each other. It does not contain information regarding the null-hypothesis, i.e. it does not tell how well the individual model fits the data, only which of two compared models is the best fit. The model with the lower AIC is the superior fit, and the step() function creates a series of models. Each model is one level simpler (one factor less) than the previous, and if the reduced model has a lower AIC than its more complex counterpart, it is selected and a new model is created. The result of this iteration is that the superfluous factors are removed, and only the factors influencing DLP are left. The resulting linear model includes Examination, Hospital, BMI, Scanner and Gender. The only factor removed in step() was BMI squared, and the influence on DLP from BMI is sufficiently covered without this value. The reduced model is therefore defined as follows: log(dlp)~factor(examination)+factor(hospital)+bmi+factor(scanner)+factor(gender) When comparing the AIC of the raw model and the reduced, it is seen that the reduced model is superior, as expected. The raw model has an AIC of , while the reduced model has an AIC of It is a very small difference, but note that AIC does not reflect the validity of the individual model, and any difference is therefore sufficient argument. To evaluate the validity of the reduced model, the multiple R-squared (R 2 ) was calculated. The multiple R- squared is equal to the proportion of the variance in DLP that is explained by the factors included in the model, and is as such a measure of model s explanation of the data. The reduced model had a multiple-r-squared value of 66.26%, in other words the included factors accounts for two-thirds of the variance seen in the data. Having validated the mode, an Analysis of Variance (ANOVA) Type 3 is conducted, but with an important comment. When the analysis was attempted, vendor #4 created an error output: there are aliased coefficients in the model. This implies that there are coefficients that are aliased between different factors. In this particular situation, the error arose as vendor #4 has a low proportion of a mere 7.5%, and is distributed on only five hospitals. Four of these have scanners exclusively from vendor #4, and when constructing the model, Hospital is included before Scanner. When the model tries to include scanner, it correctly concludes that any influence from vendor #4 is already included by the respective hospitals. Therefore, there is an alias between vendor #4 and the four hospitals, and therefore the analysis fails. As a result, vendor #4 was removed as a coefficient, and the resulting ANOVA analysis is presented in Table 3-6 below. The degrees of freedom (DF), F-value and the corresponding probability are shown. The ANOVA Type 3 will be influenced by the order in which the factors are added to the model, as opposed to Type 2, and it is therefore used. Factor DF F-Value Pr(>F) Examination < Hospital < BMI < Scanner < Gender < Table 3-6 ANOVA Type 3 of the reduced linear model 3-39 P a g e

49 ANOVA try to validate the null hypothesis, which states that there is no correlation between the factors and DLP. If the null hypothesis is rejected, there is a correlation between the factors and the measured value, e.g. BMI has an influence on DLP. The probability listed in Table 3-6 is the probability that the null hypothesis cannot be rejected, i.e. the probability that the given factor is not related to the DLP. The lower this is, the more certain is the influence of the factor on DLP. There is no defined cut-off for significance, but 0.05 (5%) is often used. It is, however, important that one always compare the different probabilities when deciding if a given factor is significant. The DF is a measure of the number of values that are free to vary within each factor. Hospital has 28 DF, as there are = 29 different hospitals, and one has to be defined as the standard to which all other (28) hospitals are compared. There are more hospitals in the collected data, but note that data not containing all factors have been removed as part of the model. The F-value is a measure of the relation between values of a specific factor, the variance between the data compared to the variance within the data. If the F-value is high, there is significantly greater variance when comparing the data between the different values of that factor, compared to comparing the data within a single value of that factor. In the example of the hospitals, the high F-value indicates that there is much greater variance when looking at data from different hospitals, than when looking at data from the same hospital. Given the values in Table 3-6, it is apparent that all the remaining factors have a significant impact on DLP, and while Examination and BMI was expected, Hospital and Scanner are more interesting. Gender is also significant, which is assumed to relate to men having larger BMI (Figure Appx. G-3). Scanner and Hospital is significant in relation to DLP, but the dominance of Vendor #1 by 55.9% makes an evaluation of the different scanners challenging. If more data with both BMI and scanner vendor was collected at a later date, it would be relevant to analyze the influence of Scanner alone, while minimizing the influence from BMI. The analysis of Scanner is further complicated by the individual hospitals having a bias towards different scanners. A majority of the departments has scanners from only a single vendor, and it is therefore challenging to split the influence from Scanner apart from Hospital P a g e

50 One possibility is to remove hospital as a factor from the reduced model, and compare the models using ANOVA. This comparison is shown in Table 3-7 below, including other statistical values of interest. Note that the comparison of models by ANOVA defines one of the models as the norm with which to compare the second model. In this analysis, the reduced model is the norm, and therefore lacks some values. Value Reduced Omit Hosp. ANOVA DF F-Value - 13 Pr(>F) - < Other Values AIC R % 60.19% Table 3-7 Comparison of reduced model and the omission of Hospital The increased AIC and lowered R 2 seen when omitting Hospital are clear indicators that this weakens the model. The impact is significant, as seen from the probability. Having evaluated the influence of Hospital it would be interesting to determine if it is a single deviating hospital, or if it is a general tendency. Therefore, the R-function summary is used to evaluate the individual coefficients of each factor, and the resulting estimates and standard deviations are illustrated in Figure 3-5 below. Figure 3-5 Hospital vs. Estimated log(dlp): Estimates and ±2 SD of the Hospital categories [Point, Error Bar] The error bars in Figure 3-5 indicates the extend of the double standard deviation to each side of the estimated value, and as such encompass 95% of the estimation. The color corresponds to the P-value, and it is evident that there is a significant variation between hospitals. It appears the influence from Hospital on DLP is not limited to a few hospitals, but rather is a general phenomenon. Note, however, that the data are unbalanced to a high degree, and therefore the variation from especially Examination is challenging to separate from the influence from Hospital P a g e

51 In order to reduce the influence of examination, a new linear model was constructed. This model used data only from the Cerebrum category, as its BMI-patients encompassed 25 hospitals. The estimates and standard deviations for this new model focusing on Cerebrum are illustrated in Figure 3-6 below. Figure 3-6 Hospital vs. Estimated log(dlp): Estimates and ±2 SD of the Cerebrum Hospitals [Point, Error Bar] The distribution evident from Figure 3-6 is similar to that from Figure 3-5, and therefore the conclusion is identical. The influence from Hospital on DLP is not the result of a few hospitals deviating significantly from the mean dose. It is a more general challenge Spread of Height, Weight and BMI From subsection it became apparent that the outliers with DLP > 2,000 primarily were the data lacking BMI. Restricting the DLP range, combined with only using measurements containing BMI, would be advantageous. The current DLP range going up to 10,000 is obscuring any further analysis of the distribution of BMI. Therefore Figure 3-7 and Figure 3-8 below remove the non-bmi data and show further details regarding BMI. The range of DLP for both figures has been limited to 2,000 to make the tendencies more apparent. Figure 3-7 is a jitter plot, while Figure 3-8 is the corresponding box plot (underweight patients removed) P a g e

52 Figure 3-7 DLP vs. BMI: Measurements containing BMI, colored by BMI ranges [Jitter] The 4.4% of patients who are underweight are challenging to conclude on. For the 13 examinations containing patients in this range, the mean DLP of these patients within each examination is lower than the mean for the patients with a BMI 18.5 for 11 (85%) of the examinations (Figure Appx. G-2). For the remaining three BMI ranges, Figure 3-7 illustrates the lack of these data in a number of examinations. There is a general tendency of the higher BMI ranges of pre-obese and obese to center on a higher DLP, compared to the normal range. This distribution is more apparent on Figure 3-8 below, where the underweight patients have been removed. Figure 3-8 DLP vs. BMI: Measurements containing BMI, colored by BMI ranges [Box] While Figure 3-7 illustrated the proportion of measurements containing BMI within each examination, Figure 3-8 more clearly shows the distribution of these. It is apparent that the mean DLP increases with BMI for most examinations. The exceptions are cerebrum, facial skeleton, heart, sinuses and trauma. It was suspected that there would be little or no proportionality between BMI and DLP for cerebrum, facial skeleton and sinuses. These 3-43 P a g e

53 anatomical regions are not as affected by weight compared to other regions a claim supported by these observations. Trauma has lost the majority of its data by limiting the DLP range, and therefore it is challenging to conclude on the remaining data. Additionally, only 7.3% of the trauma data included BMI and thus it would be irrelevant to conclude on the distribution of all trauma data based on the minority of data including BMI. Regarding the heart, the low number of data with BMI of 29 is insufficient to evaluate the relevant distribution. Based on the above, it is assumed that the BMI (and by extension height and weight) can be considered normally distributed. As such, it is valid to compute the mean and standard deviation [SD] of these data, as presented in Table 3-8 below. Anatomical Region Height [cm] Weight [kg] BMI Mean SD Mean SD Mean SD Abdomen Cerebrum Cymbals Cymbals [Bone] Facial Skeleton Heart HRCT Thorax Liver Sinuses Spine Thorax Thorax Abdomen Thorax Upper Abdomen Trauma Urography Mean Table 3-8 Mean and SD of height, weight and BMI of measurements containing BMI First, it is seen that the distribution of height, weight and BMI is comparable between examinations (excluding trauma), as the largest deviation from the mean height, weight and BMI is 1.53%, 6.42% and 5.10% respectively (57). Second, as BMI is calculated directly from height and weight, the SD of BMI will be based on the SD for these two parameters. The SD for height is low compared to the SD for weight. Further analysis of this subject will be presented in the argumentation of subsection regarding the standard patient Anatomical Regions of 2001 Guideline The 2001 guideline defines nine anatomical regions for which to submit doses, as presented in section 3.2. The DRLs for each of these regions were defined based on the third quartile of the submitted measurements, in comparison with other Nordic countries. It is therefore relevant to evaluate the third quartile of the measurements obtained 2010/2011, and the impact of 3-44 P a g e

54 limiting BMI to the normal BMI range. The third quartile for all three ranges, along with the DRLs from the 2001 guideline, is shown in Table 3-9 below. The three ranges are all the data sets [All], only those with BMI [BMI], and those in the normal BMI range [NR]. The Facial skeleton and sinuses are separated in the following for clarity and in compliance with SIS, but note that the 2001 guideline pooled these two regions into a single category. Anatomical Region Third Quartile [Q 3 ] Maximum DLP 2001 DRL All BMI NR All BMI NR Cerebrum Facial Skeleton Sinuses Spine Thorax HRCT Thorax Abdomen Liver Cymbals Cymbals [Bone] Table 3-9 Third quartile and maximum DLP for 2001 guideline anatomical regions Including all the data, as seen in Table 3-9, give third quartiles that are for the most part larger than the corresponding DRL. This was to be expected, based on the basic distribution of the measurements presented in subsection The presence of outliers is apparent when comparing the third quartile with the maximum DLP, where several regions have a large spread between the two. Therefore, it is more relevant to consider only the data containing BMI. By removing all data lacking BMI, the third quartile drops for most regions. By further limitation to the normal range, the third quartile is most comparable with the 2001 DRLs. Using these values, albeit lacking comparison with Nordic countries, Table 3-10 below lists the preliminary changes to DRLs. These preliminary DRLs are defined by the author together with SIS. Anatomical Region NR Q DRL Preliminary DRL Change Cerebrum % Facial Skeleton % Sinuses % Spine % Thorax % HRCT Thorax % Abdomen % Liver % Cymbals % Cymbals [Bone] % Table 3-10 Preliminary DRLs defined by the third quartile of normal range BMI measurements Prior to further detailing the new DRLs, note that facial skeleton and sinuses were collected as a single category in the 2001 guideline, and therefore has the same DRL. Based on the large 3-45 P a g e

55 difference in Q 3 of these two categories, it was decided to separate them in these preliminary DRLs. The preliminary DRLs have been lowered for a number of examinations, notably sinuses that has benefitted from having its own category. Two categories have had their DRL s increased, being spine and cymbals [bone]. This is a result of the emergence of 3D reconstructions in CT imaging, where a fully rendered 3D volumetric image is created based on the individual 2D slices. In order to achieve acceptable image quality in these reconstructions, the slice-thickness must be sufficiently low. Therefore, the dose is increased when comparing with earlier scans. These 3D reconstructions are primarily used in diagnosing fractures, and are therefore seeing widespread use. It is insufficient to only revise the DRLs for the revision of the 2001 guideline. The distribution of the data shows a significant spread, as each individual category encompass a wide variety of different examinations. The expectation from SIS is confirmed; the data collected does not represent current practice. It is therefore necessary to conduct a fundamental revision of the guideline, by looking at each aspect and determine which require revision. This will be the subject of the following chapter P a g e

56 3-47 P a g e

57 3-48 P a g e

58 Chapter 4 Pilot for the Revised Guideline 4.1 Introduction As described in section 1.2, the main goal of the thesis is to form the argumentative basis for a revision of the part of the current Guideline regarding diagnostic reference levels for X-ray examinations that specifically relates to CT examinations. This chapter presents these revisions, some proposed by SIS alone and some in collaboration with the author. This revision is a re-envisioning of the practice of dosimetry in CT-imaging, as it is not merely a revision of the DRLs based on current practices. One such analysis of the preliminary revised DRLs using the measurements obtained 2010/2011 is presented in subsection This is, however, not included in the pilot. The numerous changes, primarily the shift away from anatomical regions, make direct comparison between the DRLs and the new examinations infeasible. This chapter presents the work on the pilot for the revised guideline, presented in full in appx. E-2. Two sections present the other works constituting the background for the revision. First is the questionnaire circulated in November 2010 regarding optimization of image quality in relation to dose, inviting all departments and clinics to participate in a constructive dialogue regarding this issue. Second is the questionnaire circulated in January 2011 on input from resource persons. These are select involved individuals who have previously declared their interest in assisting with developing revised guidelines for CT-imaging. This is followed by a section dedicated to an overview of all the changes from the original 2001 guideline, which is presented for comparison in its entirety as appx. A-1. Each revision has a dedicated subsection where they will be presented, compared, and argued. The chapter concludes with a comparison of the spreadsheets used for submitting the data. 4.2 Optimization of Image Quality in Relation to Patient Dose Prior to the thesis, an (appx. C-1) was sent November 22, 2010, bearing the headline Study on optimization of X-ray examinations. The recipients were the management of departments and clinics performing X-ray examinations. It presented image quality optimization in relation to patient dose as a cornerstone in radiation protection. It described that SIS wished to collect knowledge regarding the current initiatives in this field in Denmark. The mention of CT directly was purposely avoided, in order to avoid biased answers. SIS was concerned that if asked directly on a specific subject, the departments would answer overaffirmatively to avoid negative evaluation. The respondents were promised anonymity if the information was to be released into the public domain and a questionnaire established (appx. C-2) for submitting the responses. The questions were formulated by SIS alone. Had the author taken part in the formulation of the questions, they would have included aspects that are more technical. It would have asked about protocols, and how the departments used AEC and IR. Whether they were used as early-planning methods, or rather added after optimization P a g e

59 In the questionnaire the departments were asked if they were currently taking steps in relation to optimization. To minimize bias towards CT in the answers, the questions were all formulated in general terms, to include all modalities utilizing X-ray radiation. Those that responded affirmatively were asked to provide a short overview in the form of a description of the individual projects. This included how the work was organized, and the results disseminated and implemented. To evaluate the departments use and design of protocols, they were asked whose protocols they used when commissioning new equipment. A total of 26 respondents chose to fill in the questionnaire, representing departments from all five regions. Therefore, it is reasonable to assume that the answers are representative for an assessment of the optimization initiatives currently ongoing at the departments. Of the 26 respondents, 25 are currently running projects on optimization, and a quantification of their responses is presented in Table 4-1 below. The responses on organization are omitted, as they are of no relevance to the revision of the guideline. The table is read thus: 28% of the respondents have specific ongoing initiatives to minimize the number of discarded images. Initiative % Initiative % Specific Initiatives Implementation of Results Minimize discarded images 28% Teaching or instruction 60% Mentions CT specifically 56% Change of protocols 44% Optimize/revise protocols 48% New internal guidelines 52% Dissemination Protocols used at Commissioning Not at present time 4% Vendor 40% Only locally 68% Regional 32% additionally regionally 24% Own 48% additionally at a congress 4% Other: vendor s modified to own 24% Table 4-1 Quantification of responses regarding optimization (appx. C-3) There are several conclusions that can be drawn from the responses supporting the argumentation for the revision. The majority of the arguments will be presented here and subsequently referred in the corresponding revision. Other conclusions are more general arguments for the necessity of a revision and as such will be presented in full in the following. Regarding the specific initiatives, the large percentage of 56% who specifically mentions CT is remarkable considering the general wording of the questions. This supports the argument that optimized dosage of CT examinations is a priority in the departments, and that a prime method is by optimizing and revising protocols (48%). When looking at dissemination however, it becomes apparent that this ongoing gain of knowledge on optimized protocols is not shared sufficiently between the departments. Of the respondents, 72% share only locally or not at all, leaving a mere 28% who share either regionally or at a congress. This is a major argument for an increased detail in the submissions to SIS, as a large majority of the departments each optimizes their protocols independently of one another. SIS has the capabilities to revise the guideline to create transparency on a national level P a g e

60 The implementation is primarily done through teaching. SIS has no influence, however, on how the individual departments use teaching and instruction of their staff, or on the wording of internal guidelines. A total of 44% specifically mention the changing of protocols as a prime method for implementing optimization results. This signifies the importance of protocols and the validity of obtaining their parameters as part of the revised guideline. Another argument for the shift to focus on protocols is the large dispersion in the origin of these, as seen with the protocols used during commissioning: there is a lack of common consensus on which protocols to use. 40% use the vendor s with little or no modification, while 48% use their own protocols without specifying on what they are based. The 32% using the regional protocols is in strong opposition to the mere 24% who share their optimization-results regionally. When third of the departments rely on protocols that are optimized by a minority of departments, the usability of such protocols can therefore be questioned. In summary, there is a significant spread in the method and practice of optimization. There are many ongoing projects with relation to CT, the results of which are implemented in protocols. This knowledge is unfortunately not sufficiently disseminated outside the department, and beneficial initiatives are not used as widely as desired, to the loss of the patient. 4.3 Input from Resource Persons Knowing that the revision of the guideline would be more effectively implemented if developed in cooperation with the departments, all diagnostic departments were asked for their participation as resource persons. These persons would be asked to contribute by answering a questionnaire prior to the revision, and further on possibly evaluate and measure the pilot when it was published. Prior to the initiation of the thesis, an was sent on January 19, 2011 (appx. D-1), asking the involved personnel to fill in a questionnaire (appx. D-3). This detailed their input on SIS proposals for examinations as presented in the attached letter (appx. D-2). SIS proposed a number of examinations relevant for patient dosimetry, together with a selection of scan parameters. These were evaluated by the resource persons, who were asked to consider if anything was missing. The responses were saved in a database, but the high number of questions (47) combined with the relatively large portion of polar questions made it unmanageable to analyze the data in this form. Therefore, the polar questions were converted into a fraction, with 0 and 1 equaling no and yes, respectively. These fractions were then presented, together with the qualitative responses, in a collected document as presented in appx. D-4. Presented below in Table 4-2 is an excerpt of questions related to the revision. As with the prior section regarding optimization, only the general observations will be presented here. The results underlining specific revisions will be presented in the corresponding subsection, including responses regarding height and weight and other primarily qualitative arguments P a g e

61 Question Responses Question Responses Parameters Indicators DLP 1.00 Cerebrum 1.00 CTDI vol 0.50 Facial skeleton 0.20 kv 0.80 Sinuses 0.86 mas 0.60 Trauma 0.83 Vendor 1.00 Thorax 0.67 Model 1.00 Heart 1.00 Slices per rotation 0.50 Lungs 0.67 Number of X-ray tubes 0.33 HRCT Lungs 1.00 Axial/spiral 0.40 Abdomen 0.71 AEC 0.80 Urography 1.00 Iterative reconstruction 0.50 Colon 0.60 Pitch 0.50 Misc Slice thickness 0.25 Urography multiple indic Scan length 0.75 Perfusion studies 0.60 Table 4-2 Quantification of input persons responses regarding the revision of the guideline (appx. D-4) For a number of parameters, the support is lower than expected, and the qualitative arguments for these are presented in the following. Explanatory Responses The low support of number of X-ray tubes and Axial/spiral is because the resource persons argued it is apparent from the model. It is, however, necessary for a smooth evaluation that this basic information is readily available. Furthermore, axial/spiral is tied to a specific protocol; as a spiral scanner is still capable of doing axial scans, it is not certain that a spiral scanner exclusively performs spiral examinations. There is a significant difference in the calculation of dose between axial and spiral, as shown in section 2.5. This makes it an important parameter. Iterative reconstruction is a concept that have not yet spread to all departments, therefore two out of four resource persons argued that it in no way can reduce dose on the performed examination (appx. D-4). This is incorrect, as argued in subsection 2.3.4, although the misconception is understandable. The vendors have no common nomenclature in this field, and they do not necessarily use the wording iterative reconstruction in their instructions and teachings. As such, the concept as worded here might not be known to the departments. That, however, does not alter the fact that it is an important parameter regarding dose. The backing for CTDI vol of 0.50 should be seen in conjunction with the support for scan length of 0.75, as the single argument against CTDI vol is that it is included in DLP (appx. D-4). This is correct in the sense that DLP is calculated from CTDI vol, but scan length is also included in DLP. It is a value of less interest when evaluating dose. As the prime relevance of the guideline in relation to the departments it the evaluation of dose, the submission of CTDI vol in favor of scan length is advantageous P a g e

62 Slice thickness is necessary for proper evaluation of CTDI vol and is therefore tied to the submission of this value, and the low support may be influenced by asking only diagnostic departments. According to SIS, the departments primarily use a small number of standard slice thicknesses, and it is therefore of lesser interest to them. This guideline, however, applies to all medical departments using CT-scanners. Other types of departments will have a larger spread in their slice thicknesses, and therefore it is a valuable parameter when evaluating dose. 4.4 Revisions in the Pilot Table 4-3 below lists all revisions in the pilot, sorted in the order they appear within the guideline. Each will be presented in its own dedicated subsection. Revision 2001 Guideline Pilot Commissioning - Timeframe of six months Changes in protocols - New measurements ALARA-argumentation - Included Definition of examinations Anatomical regions Pathophysiological indicators Other protocols - Measure three of these Types of dose DLP DLP and CTDI vol measurements Multiple scans or series Individual Combined Number of patients 10 At least 20 Standard patient Weight: 70±10 kg Weight 70±20 kg Deviant mean - Part of evaluation Multiple CT-scanners Distribute measurements Repeat measurements Number of spreadsheets One per clinic One per scanner Information to be submitted Appendix B-2 Appendix E-2, Table 2 Table 4-3 Revisions in the pilot from the 2001 guideline The pilot opens with an introduction, translated and presented here in English, as it is an essential requirement for understanding the input and responses presented in Chapter 5: You are now reading the pilot version of a revised guideline regarding collection of patient doses for CT examinations. Thank you for the assistance in trying out the method, before it is send out to the rest of the country. We would like to obtain measurements for each examination even if you do not have sufficient time to measure 20 patients. As you can see, height and weight is especially important, but we have excluded the trauma-protocol out of practical considerations, as well as cerebrum and sinuses as it in these cases have very little impact on dose. Please give us all comments; either by writing in the spreadsheet, by or by calling us. The final version will also contain information regarding DRLs and weighing factors. We would, among other things, like you to test for: Readability and usability Does the information largest diameter in the field of scan cause difficulties? Is the SKS-code an aid? Please add missing codes P a g e

63 Does the method make sense for you as a radiographer/x-ray nurse, physician, or physicist? Please provide input for the choice of indicators we think it is a bit messy with Latin and Danish, but have chosen the wording we find usable. (58) This introduction will naturally be removed from the revised guideline, but it serves as an introduction to the mindset behind the revised guideline: that the opinions and experiences of physicists and other healthcare-professionals who use these guidelines in their daily routines are valued, and that a significant role of SIS is that of a consultant. As this is a pilot, we ask for their input regarding the basic structure of the new categories of indicators. This includes their naming and the overall wording of the guideline. The feedback received, and the subsequent revision of the guideline, will be the subject of Chapter 5. This chapter will continue with an analysis of each revision of the pilot, to provide a thorough comprehension of the basis for the revisions. Keep in mind that the 2001 guideline is presented in full as appx. A-1, while the pilot constitutes appx. E Commissioning In the 2001 guideline, it is stated that the departments and clinics must measure their patient doses for the examinations performed every two years, with doses in relation to CT being defined as DLP. This is in addition to the acceptance tests performed as part of commissioning. It is not specifically stated that measurements of protocols and evaluation against DRLs should be performed in conjunction with commissioning. This omission could potentially result in the specific dosage of the particular scanner not being fully reviewed until two years after commissioning. The pilot specifies that they must perform these measurements within a timeframe of six months after commissioning. The argument is that the medical physics experts (in Danish: ansvarlige fysikere) carry the responsibility that the dosage is within the limits defined by SIS. Therefore, it is imperative that they determine the validity of the calibration 13 regarding their own protocols within an appropriate timeframe. It was suggested to set an even shorter timeframe, based on the recent revision from Sweden, who published their revised guideline on DRLs in medical diagnostic radiology in This guideline, the Radiation Safety Authority regulations on diagnostic standard doses and reference levels in medical diagnostic radiology, specifies that when changing equipment or method of examination, the expected influence on the diagnostic standard dose must be measured. (...) For unplanned changes however, it must happen within three months. (59) The Swedish guideline does not specify the timeframe for commission, but one could argue that it would be logical to make the timeframe for a scheduled change equal to or shorter than for an unplanned change. It was argued by SIS that a timeframe of three months would be too short, as it would result in submission of measurements based on unfinished protocols. These measurements would constitute an only 13 The scanners have built-in CTDI tables based on phantom measurements, requiring periodic validation P a g e

64 intermediate representation of patient dose, and therefore be of no interest. This is supported by the responses regarding optimization in Table 4-1, where 60% of the respondents used protocols different from the vendors. These departments require time to optimize their protocols to the specifications of the new scanner Changes in Protocols The departments do not set each individual parameter for each examination performed. The parameters are instead defined in protocols, as presented in subsection These protocols constitute a base set of parameters, which are adjusted to accommodate the examination of the individual patient. Each patient, however, initially have the same basic parameters. These protocols, however, are not set in stone, and there are a number of situations that will require an update of certain protocols: software-updates can change the basic functions of the scanner, re-calibration can shift the expected dose and new practices can change the procedure of an examination. All these influence the parameters for all coming examinations, and therefore the dose-profile, CTDI vol and DLP. It will be necessary to obtain new measurements using these updated protocols, as they are not readily comparable to the measurements obtained using the old protocols. The 2001 guideline does not explicitly mention updating of protocols, but this is introduced in the pilot: For major changes to a protocol, the measurements must be performed again, as part of the dose optimization process in accordance with the ALARA principle.(58) By including the above statement, it is made clear to the medical physics experts that their measurements must at all times be representative of the dose. It is insufficient to rely on measurements obtained every two years if the procedure of certain examinations undergoes significant changes. These periodic revisions of protocols are sure to occur, as 44% of the departments use protocols directly as a method for implementing results from optimization projects (Table 4-1). In these situations, it is necessary to reassess the doses by performing new measurements ALARA-argumentation The principles behind the need for periodical measurements, as presented in subsection 2.4.1, are not mentioned in the 2001 guideline. It is assumed common knowledge for medical physics experts, but one of the prime differences between a guideline and an order is in the presentation. An order is the law consisting of articles and paragraphs, whereas the guideline is a linguistically fluent summation of the order. It contains expanded explanations and a presentation of recommended methods. To this end, it can be advantageous in the guideline to present the background and reasoning behind the order. This way, the guideline more effectively conveys its content as the reader understands its eligibility. To provide this reasoning, the following was added to the pilot: 4-55 P a g e

65 The measurement of patient doses is primarily an aid to optimize the patient doses in the individual department in relation to the ALARA-principle. In addition, it helps to maintain attention on the dose-level for CT examinations across departments in Denmark, and gives us the opportunity to compare with other countries.(58) Furthermore, because of the shift in the required data and categorization, the inclusion of this snippet would serve as a reminder for the medical physics experts as to why it is in their own interest to measure patient doses, and why SIS requests these doses at times Definition of Examinations The major revision of the pilot, and a main goal of the thesis, is to accommodate the increased range of examinations and diagnostic possibilities. This is primarily implemented by the shift from examinations being defined by anatomical region to being defined by pathophysiological indicators. These more correctly reflect the examinations being performed at the departments and clinics. The 2001 guideline is based on ICRP Publication 60 from 1990, and as such the methods and practices described was obtained during the mid 1990s, according to SIS. The concepts are, on average, 15 years old. The number of diagnostic possibilities (and by extension examinations and protocols) was severely limited when compared with today. Time was the limiting factor, with examination times of approximately 20 minutes for a simple chest scan. Compared with no more than a few minutes today, the range of possible examinations was relatively limited. All but a few scanners were axial, with few discrete slice thicknesses. The existing categories of DRLs defined by anatomical region as per the 2001 guideline were an appropriate categorization at the time, considering the technological possibilities. The nine categories in the 2001 guideline are: Cerebrum Face and sinuses Vertebral (trauma) Thorax HRCT Thorax Abdomen (ordinary) Liver and spleen Cymbals (ordinary) Cymbals [Bone] However, as evident from the measurements collected 2010/2011, the departments today perform a multitude of examinations not easily classified as one specific of the nine categories. Excluding SPECT-CT, PET-CT, and pediatrics, there were still 39 examinations. 24 of these were either pooled with others or excluded because of too few and/or scattered data. This left 15 categories, but a number of categories encompass a multitude of different examinations, as described prior. A variation is introduced in the doses within these categories that is not representative of the patient, but of the specifics of the examination. This complicates comparison between different departments and scanners for the same category, additionally 4-56 P a g e

66 not accommodating the difference in doses for different examinations within the same anatomical region. A shift to using pathophysiological indicators counters this challenge. Each examination, with its unique protocol, constitutes a single category for measuring. The basic structure of CT dosimetry will focus on the potential harmful effects of individual examinations and give a firmer statistical basis for defining appropriate DRLs. With onset in the responses from the resource persons, the only examinations without at least a two-third majority support were facial skeleton, colon and perfusion studies (Table 4-2). Facial skeleton was removed entirely, as only 20% of responses were positive. Colon and perfusion studies were both included, in spite of only 60% positive responses for both. Colography was not a CT examination when the measurements forming the basis for the 2001 guideline were obtained during the 1990s. The diagnosis of colon cancer was instead by use of X-ray fluoroscopy. In line with the technological development, the golden standard has changed to at first a colonoscopy, and if that does not provide sufficient diagnostic basis, the next step is either an examination of the colon using conventional X-ray or a colography using CT. According to SIS, colography is still a relatively specialized examination performed at a few select departments. Therefore, the general assessment of the need for the examination as a basic protocol defined by SIS is skewed with a negative bias. Regarding perfusion studies, all three non-positive responses argue that it is not yet performed regularly: 1) We think it is at too early a stage yet. 2) We are not currently performing perfusion studies in (...) and can therefore not make a qualified comment. 3) I have no experience with perfusion studies sorry. (appx. D-4) As the only argument against perfusion studies is that it is not widely performed yet, and it is therefore preferable to include it in the new categories. PET-CT/SPECT-CT is omitted from the new categories, based on the large variation in the examinations and the small amount of data from each. Furthermore, the departments are too divergently specialized at present time to be comparable. It was concluded that the statistical basis would be too weak, and standardization in the form of common practice, protocols and DRLs would be too challenging at this point in time. The new categories, based on indicators, for submission in the pilot are: CT-scanning of cerebrum: Obs. bleeding Perfusion study Cerebrum CT-scanning of sinuses, incl. cavum nasi: Pre-operation CT-scanning of trauma patients (head, thorax, abdomen): High-energy trauma CT-scanning of thorax: Obs. lung cancer CT-scanning of the heart: Coronary disease High Resolution CT-scanning of lungs: Obs. interstitial lung disease 4-57 P a g e

67 CT-scanning of abdomen: Acute abdomen CT-urography: Obs. malignity CT-urography: Obs. stones CT-scan of colon and rectum (CT-colography): Obs. cancer CT-scan of thorax and abdomen: Obs. malignity and tumor control The protocols used by the different departments and clinics are not fully standardized. Instead, there is a significant overlap in the protocols being used, but the exact parameters differentiate between departments. This is the result of vendor, model and practice, resulting in a multitude of different protocols for the same examination. By collecting all the relevant parameters constituting the different protocols, it would allow the departments to investigate the doses more thoroughly. This would not be limited locally within a single department, but if relevant also at a national level. This could possibly prove advantageous as only 28% of the departments share their optimization results beyond a local level (Table 4-1) Other Protocols The categories of the 2001 guideline were defined by anatomical region, and therefore not directly related to the protocols used by the departments. With the revision of the categories as described in the previous subsection, the opportunity arose to clarify that the measurements are to be performed on all examinations, i.e. using any protocol. This includes those protocols not included in the 12 pre-defined indicators. Using the 2001 guideline, the number of categories submitted by the departments was 39, more than four times the nine categories defined. The new definition of categories should minimize this issue, but as to not exclude any examination from dosimetry, the following was added in the pilot: Patient doses are to be measured for the specified examinations, to the extent that they are performed on the CT-scanner. If the list does not include the department s most frequently used protocols, three of these are also to be measured. Hereby measurements on all CT-scanners are guaranteed, including [PET-CT] and SPECT-CT.(58) This tighter relationship between the categories defined by SIS and the protocols used by the individual departments and clinics as the focus for relevant measurements is reinforced. The result is that the departments measure and submit what was intended: the examinations they carry out through their daily activities, instead of limiting themselves to the examinations in the categories defined by SIS Types of Dose Measurements The 2001 guideline defines relevant patient dose for CT as DLP. This is representative of the combined dose absorbed in the patient, as described in section 2.5. It has become a worldwide standard for CT-dosimetry, being representative of the total patient dose although it is only an index of the patient dose. The calculation of DLP is based upon phantom measurements, and 4-58 P a g e

68 not the true dose to a patient. It is, however, the best estimate currently widely available, and as such is a required output on all scanners. As DLP is dependent on total scan length, it will vary across multiple otherwise identical examinations of different patients, even if all other scan parameters are kept constant. In other words, the variation within the DLPs collected by a single department is consistent with a variation in patients. This is assuming that the patients are standard and as such do not require significant modifications of the protocol. It becomes more complicated when trying to compare doses between departments, as the variation in DLP is now disrupted with the variation within protocols for a specific examination. Protocols are not standardized, and therefore large variations exist as a result of practice, scanner model/-type, and optimization efforts. These variations can only be partially separated, and therefore it would be beneficial to obtain a measure having a lower dependency on patient size. CTDI vol is independent of total scan length, and is therefore a more valid indicator for the dosage of a specific protocol. This validity is relative to the standardization of the patient, as obese or otherwise non-standard patients will require modifications to the basic scan parameters. This will shift the CTDI vol, and create a correlation with patient size. The data collected by SIS according to the 2001 guideline is by requirement obtained from standard patients. Therefore, CTDI vol is a satisfactory measure for comparing the different departments optimization of their protocols. Variations within the CTDI vol of a specific protocol can arise from both non-standard patients and the usage of AEC and IR. AEC especially is used on standard patients, and as such impacts doses for regular examinations. In summary, the validity of CTDI vol as a measure in combination with DLP is that it allows for better evaluation of the doses between examinations within a single department. Additionally, it creates the possibility to better assess the doses between departments for identical examinations, with less variation due to patient size. The pilot describes it thus: It is the total DLP and CTDI vol for the entire examination, which is to be reported (...). By including both measurements, the information obtained more adequately represents the examination dose.(58) This together with the shift in the categorization of examinations will make it more valid, prospectively, to assess the dose-distribution. Not only within a single department, but also between departments, should the need arise Multiple Scans or Series Specific examinations may require a scout- or pre-scan prior to the diagnostic scan. These are low-dose scans performed in order to obtain an anatomical overview of the region to be scanned, to aid in planning the diagnostic scan. At other times, an intravenous contrast is given 4-59 P a g e

69 to make certain physiological features stand out during scanning, and multiple scans are performed. In both these situations, the scans will be performed within a short timeframe. This is defined as the same scan séance, constituting the time from the patient lies down on the table until she rises again. Considering these factors, the individuality of the scans is of no interest in relation to patient dose, as it is the collected dose for the entire scan séance that is relevant. Evident in the measurements obtained 2010/2011, a number of these multiple scan patients were typed with two or more DLP. As these multiple DLP are simply pooled into a single, combined dose, the pilot clarifies this issue by adding the following: It is the total DLP and CTDI vol for the entire examination, which is to be reported, including scout-scans and series with/without contrasting agent. (58) As a result, the typing, collecting and sorting of data will be more streamlined, while minimizing the risk of inconsistencies Number of Patients When asked for by SIS, the individual departments must measure 10 standard patients per the 2001 guideline. This number is based on a balance between having a sufficiently high number of patients required for a solid mean, while maintaining a workload of adequate proportions in the individual departments. There are considerations both for and against increasing the required number of patient doses. It was decided, however, that priority for a better statistical basis outweighed the increased workload for the department, considering the infrequency with which these data are collected. The Swedish revision from 2008 increases the number of patients to be measured to 20, to avoid a too great statistical uncertainty regarding the diagnostic standard dose. (59) The feedback from the resource persons further argued for an increase in the number of patients: More patients than 10 (e.g. 50) for each type of examination, as to increase the statistical basis. (appx. D-4) Additional input asked for even more patients, up to a whole year s examinations, provided the collection of data be automated. This is not currently the case, and it was deemed irrelevant at the present time. As a result, the number of patients required for each examination was increased from 10 in the 2001 guideline to at least 20 (58) in the pilot. This creates a stronger statistical basis while accommodating the increased spread for the standard patient, as presented in the following subsection P a g e

70 4.4.9 Standard Patient The 2001 guideline uses the definition of the standard patient as defined by ICRP. First defined in ICRP Publication 23 from 1975, and later expanded in ICRP Publication 89 from 2002, the standard patient, or reference man, is defined as follows in ICRP Publication 23: Reference man is defined as being between years of age, weighing 70 kg, is 170 cm in height, and lives in a climate with an average temperature of from 10 C to 20 C. He is a Caucasian and is a Western European or North American in habitat and custom. (60) This information was not changed in ICRP Publication 89, instead other aspects of the reference man, and more categories, were introduced (61). The definition cited above still stands as the standard used worldwide, including in the United States (62). There is, however, a complication in relation to this definition of the standard patient. The BMI of the standard patient is significantly higher than the mean of the normal BMI range. The table of BMI ranges defined by WHO from Chapter 3 is presented as Table 4-4 below for clarity. Classification BMI [kg/m 2 ] Underweight < Normal range Overweight 25 Pre-obese Obese 30 Obese class I Obese class II Obese class III 40 Table 4-4 The International Classification of adult underweight, overweight and obesity according to BMI (56) The standard patient, with a height of 170 cm and a weight of 70 kg, has a BMI of 24.22, which is in the upper end of the normal range of to The 2001 guideline requires the 10 patients to be of standard size, i.e. in accordance with the ICRP definition, and the spreadsheet (appx. B-2) suggests a 20 to 25 as being normal. From the measurements collected 2010/2011 was calculated the mean height, weight and BMI, which is recapped in Table 4-5 below. Height [cm] Weight [kg] BMI Source Mean SD Mean SD Mean SD 2010/2011 Data Table 4-5 Mean height, weight and BMI, excerpt from Table 3-8 The mean height is not significantly different from the standard patient, a difference of a minor 1.83 cm (1.08%) with a SD of The resulting BMI for two-thirds of the patients, with weight locked at 70 kg, would be from to 26.47, a range of P a g e

71 The mean weight, however, is not only shifted significantly more from the standard patient by 4.46 kg (6.37%). It also has a significantly wider spread, with a SD of 14.39, which is even more significant as SD for these types of aspect ratio data (height/weight) is expected to increase relative with mean. The contribution to the SD of BMI from weight is as a result more profound than that of height. Two-thirds of patients had a BMI between and 29.20, a range of 9.96 as a result of height, with height locked at 170 cm. In other words, the influence on BMI from height was 5.11, compared to the influence from weight what was 9.96, nearly doubled. The percentage of the measurements obtained 2010/2011 constituting a standard patient varies by definition. Dependent on which parameter is deemed most important in defining the standard patient, resulting percentages is shown in Table 4-6 below. As there is no standard spread for the height, the accepted range for standard height is based having a normal BMI between 18.5 and and a mean weight of 70 kg. This gives a resulting range of cm to cm. Percentage of BMI Patients Height Weight BMI Normal Range Standard patients 65.67% 52.18% 49.13% 43.83% Table 4-6 Percentage of BMI patients constituting standard patients (57) It is apparent that the weight is the factor, having the largest impact on defining the patient, compared to height. Therefore, it is imperative that the standard patient is redefined from the current practice. The 2001 guideline does not specifically mention the allowed width of the distribution, i.e. the difference from the mean still constituting a standard patient. Common practice, and the recent Swedish revision, does indicate that a range of ±10 kg is within reasonable limits (59), giving a range of 60 to 80 kg. Even though the majority of resource persons indicated the importance of height and weight, there were also those who noted that they would rather not measure these values. They had three main arguments: 1) Rather not [measure height and weight], as standard patients are rarities. 2) In the future, patient doses will be collected via RIS (Radiology Information System), and as such, the patient s height and weight will not be part of the data. 3) Collection of patient doses based on the familiar method with noting of height and weight I consider as being unfit, as these notations are always filled with many errors and misunderstandings. Furthermore, it is always a hassle to collect these data. (appx. D-4) The first argument is the main reason for the redefining of the standard patient, as far too few of the data measured were within the defined standard size. This is even more problematic considering these measurements were specifically collected for standard patients. If the departments and clinics are having this much difficulty finding a sufficient amount of standard patients to measure, it seems reasonable to reevaluate their definition. The second argument relating to collection via RIS or possibly PACS (Picture Archiving and Communications System) is irrelevant in regards to dose measurements submitted to SIS. The 4-62 P a g e

72 departments can collect the data by any method they choose, but as argued earlier the height and especially weight has a significant effect on dose, and as such is required for proper assessment. Therefore, these values are required for submission, regardless of the departments individual methods of collecting. The third argument is disproved for two reasons: following regulations, the departments are required to collect these data at two-year intervals as per Order 975, chapter 17, section 96, 2 nd paragraph. However, as these data require height and weight, there is naturally an increased workload in collecting these data compared to regular examinations. Regarding the amount of errors, it can refer to two types: either an error relating to measuring the values, or an error when entering the values. Regarding the former, it is not possible to determine the extent of the issue, as SIS has not participated in the measurement of height and weight. Of the latter error, out of the 2,585 data sets containing height, weight and possibly BMI, only five had identifiable errors related to the typing of values. This is not counting those with unit of mm instead of cm for height, which is more an issue of differences in practice. BMI is left undefined in the pilot, as the mean BMI of the submitted data is in the upper end of the normal range. This is primarily the result of increased weight. If BMI is to be incorporated directly as a normal range as part of the revised guideline, the range would have to include both normal and overweight (pre-obese) patients. It is difficult to argue that this range constitute the standard patient. These considerations regarding the definitions of BMI are not confined to radiology. In line with the worldwide increase in mean weight, the notion of the 70 kg standard patient is becoming outdated, and the ranges of BMI need to be redefined (63). The importance of weight warrants a dedicated paragraph, as its importance cannot be sufficiently stressed. The Organisation for Economic Co-operation and Development (OECD) monitors this growing obesity. It is an independent organization established 1961 in France to promote policies that will improve the economic and social well-being of people around the world(64) and now constituting 34 countries. In 2010, they published their latest findings in Health at a Glance, OECD Health Data 2010 Frequently Requested Data (48) and Obesity and the Economics of Prevention: Fit not Fat (65), as presented in Table 4-7 below. Country % Adult Population (65) % Obese Adults(48) Overweight Also Obese Male Female Denmark Sweden Norway Germany Switzerland United States OECD average EU average (66) Table 4-7 Overweight in Denmark, the United States and comparable countries The weight is rapidly increasing in Europe and the United States, and when the population as a whole is breaking the ranges defined for normal BMI, it is apparent that a redefining of BMI is required. One aspect is the increasing number of obese patients; another more important 4-63 P a g e

73 aspect is the significant percentage of the adult population who are overweight. Note that BMI in dosimetry is used to calculate correct dosage, and not to evaluate if the patient is medically obese. It stands that the range of the standard patient s weight should be redefined, as weight is the parameter with the highest uncertainty. As the mean weight is kg compared to 70 kg for the standard patient, it was proposed to change the definition from the existing 70±10 (60-80) kg to 75±15 (60-90) kg. This would incorporate both the shifted mean and the SD of This revision would be based solely on the measurements from 2010/2011, but would contain one major hindrance: the mean weight would shift. This would make it difficult to compare doses internationally. As this is one of the important methods for establishing national DRLs, this loss would have too great of an impact when considering the gain. Therefore, it was decided to keep the mean of the standard patient for time being, as an effective shift of the mean would require international consensus. The range, however, was changed from ±10 kg to ±20 kg, to include the same upper limit (90 kg) in weight. This is described thus in the pilot: For each type of examination, the measurements are to be performed on 20 patients of standard size, which is defined by being between 50 and 90 kg (...). The mean must converge on 70 kg. (58) Deviant Mean The 2001 guideline defines that the departments are required to measure ten patients of standard size, which is elsewhere defined as having a mean weight of 70 kg. This guideline, however, does not specify what is to be done if the measured mean deviates from this value. As the range of permitted standard patients has been widened, the mean can more easily deviate from 70 kg if the departments are not attentive. This would have a profound influence on their mean doses. As the demographic distribution is unknown, however, it is not possible to categorically prohibit that the mean deviates significantly from 70 kg. The pilot provides a practice regarding this challenge in the form of a single sentence following the defining of the optimal mean: The mean must converge on 70 kg. If the mean is significantly different, then this is to be included in the evaluation of the doses.(58) By this wording, it is evident that the department must strive towards a mean of 70 kg. They are still allowed a deviation, providing it is included in their evaluation of dose Multiple CT-scanners The number of CT-scanners in the departments has risen during the last decade, from 11.4 per million population in 2000 to 23.7 in 2009 (48). As a consequence of this, the same 4-64 P a g e

74 examinations are being carried out on multiple scanners. In these cases, the 2001 guideline calls for a distribution of measurements among the scanners: If, in a single department, multiple X-ray apparatus are used for the same type of examination, the ten measurements are to be distributed on several of these. (52) This is not optimal, considering the difference between vendors. The measurements obtained 2010/2011 did not contain a sufficient spread of vendors, nor sufficient BMI, to evaluate the influence of a specific scanner vendor. This variance between vendors and models lead to the segregation of scanners and examinations. The full number of patients (at least 20) should be measured using each protocol on each individual scanner, and from the statistical basis, it would be possible to determine any correlation between scanner and examination. The pilot specifies this new approach as such: If the department uses multiple CT-scanners for the same type of examination, the measurements must be performed on each of these. (58) The data are not to be pooled for a specific examination, as this could potentially hide unoptimized scan protocols. If a specific examination is performed on two scanners at the department, and one scanner is significantly above the DRL while the other is similarly below, the pooled mean would even out. Therefore, the departments are to calculate the mean for each specific examination, performed on each individual scanner Number of Spreadsheets In accordance with the previous subsection regarding multiple scanners, the existing spreadsheet which is tied to a given department or clinic is no longer optimal. The new categorization by pathophysiological indicators ties examinations together with protocols. These protocols are not uniform between vendors or models, and therefore, each spreadsheet is tied to a particular scanner. The spreadsheet used with the measurements collected 2010/2011 is recommended in the e- mail sent to the departments: On our homepage is Template for collecting patient doses, 5 th edition March 2009, which advantageously can be used for the submission. (Appx. B-1) As less than 20% of the data was submitted using the template provided by SIS(57). Therefore, the new spreadsheet is mentioned in the pilot specifically, instead of being restricted to the e- mail: Attached to the is a template Excel-spreadsheet, in which the measurements are to be entered. A new document is created for each scanner. That way, the information regarding scanner type and protocol for the individual examinations only needs to be 4-65 P a g e

75 entered once. When a protocol is modified, the measurements are to be performed anew and entered in a separate sheet. The average dose for an examination/indication therefore only represents a single scanner protocol. (58) The transition to the new workflow is eased by including the argumentation for the change, providing the medical physics experts with the background information Information to be Submitted There are three arguments for the change in the information to be submitted to by the departments. First is the shift from categories being defined by anatomical regions to pathophysiological indicators. This changes the order and types of data, as parameters are of increased importance. Second is the introduction of CTDI vol as a required value. Third is the change from the whole department sharing a single spreadsheet to having a unique spreadsheet for each individual scanner. This changes the order and amount of information required P a g e

76 The composition of the new spreadsheet, and a comparison with the existing used for the measurements 2010/2011, is the subject of the separate section 4.5. The present subsection presents an overview of the information to be submitted in Table 4-8 below, with a short description of each. The table is read as such: the parameter Hospital is included in the pilot, it was also included in the 2001 guideline, and it is the name of the hospital. Pilot 2001 Guideline Description/Argument Hospital Included Name of hospital Department Included Name of department/clinic Room number Included - Vendor Included In 2001, Generator included both vendor and model Model Included In 2001, Generator included both vendor and model Number of X-ray tubes Basic parameter of scanner Max number of slices per Basic parameter of scanner rotation kv Included Basic parameter mas Basic parameter of protocol Axial/spiral Basic parameter of protocol Pitch Basic parameter of protocol Slice thickness Basic parameter of protocol Dose-modulation (AEC) Advanced parameter of protocol Number of phases, including Significant for evaluating high DLP scout(s) Iterative reconstruction Significant for evaluating high DLP Height Included Basic parameter of patient Weight Included Basic parameter of patient Gender Included Basic parameter of patient DLP Included Basic parameter of patient CTDI vol Significant in comparison between departments and scanners Largest diameter in scan field [cm] In conjunction with BMI, describes prime patient variance on dose Sheet name (indicator) Sheet name (region) Shift in definition of examination Table 4-8 Overview of information to be submitted in the pilot (appx. E-2) The majority of the required information is either unchanged or basic parameters related to the focus on individual scanners instead of departments. These are described in full in subsection 2.3.4, section 2.5 and previous in this chapter. The omission of BMI is in accordance with the absence of BMI from the pilot, instead focusing on height, weight and largest diameter in scan field. As mentioned in the opening of this section, height and weight were deemed irrelevant for three protocols: trauma, cerebrum and sinuses P a g e

77 The argument for obtaining the largest patient diameter in scan field for each patient is that it includes information on the x- and y-direction of the patient. This is already included in DLP, but its influence is difficult to separate from other parameters. The size of the patient in the z- direction is already taken into account, as DLP is calculated by multiplying the CTDI vol with the total scan length. In summary: the varying changes arising from the width and weight of the patient (the x- and y-directions) affect CTDI vol and thereby DLP, whereas the linear changes arising from the length of the patient (the z-direction) affect DLP only. Weight is a significant contributor to dose, and is of prime importance that its contribution can be separated from height. This challenge can be eased by measuring the largest patient diameter in scan field Considerations that were Omitted Two factors were omitted from the pilot: pediatrics and age. As defined in the delimitations, this revision relates only to the CT examinations of adults, and therefore pediatrics was omitted. By extension age was omitted, as it is only of significant importance in pediatrics P a g e

78 4.5 Pilot Spreadsheet For submission of data for the pilot, a new pilot spreadsheet was designed which incorporated the revisions described in the previous section. The pilot spreadsheet is presented in excerpt as appx. E-3, while the existing spreadsheet is excerpted in appx. B-2. A comparison of the spreadsheets is summarized in Table 4-9 below. Pilot Spreadsheet 2001 Guideline Spreadsheet Entered once for each Spreadsheet Hospital Hospital Department Department Room number Vendor Model Number of X-ray tubes Max number of slices per rotation Entered once for each Sheet (Examination/Protocol) kv mas Axial/spiral Pitch Slice thickness Dose-modulation (AEC) Number of phases, including scout(s) Iterative reconstruction Entered once for each Patient Height Room Weight Generator Gender kv DLP Gender CTDI vol Height Largest diameter in scan field [cm] Weight BMI DLP Locked or Calculated Sheet name (indicator) Sheet name (region) Diagnostic reference level (DLP) Average of measured doses Table 4-9 Comparison of spreadsheets used for submission of measurements Entered once for each Spreadsheet The first sheet of the existing spreadsheet is an introduction. This is absent from the pilot spreadsheet, as the spreadsheet is not for ordinary clinical use. The first sheet of the pilot is instead used for entering the information relating to department and scanner. This is only to be entered once for each spreadsheet, as they each represent an individual scanner P a g e

79 Entered once for each Sheet (Examination/Protocol) This first sheet is followed by a dedicated sheet for each examination, the types of which is described in subsection These sheets differ from those in the existing spreadsheet in that the information relating to the protocol is entered at the top, and as such is constant for all patients. As per the revised guideline, any permanent change in the protocol should be considered a different protocol and require new measurements. Entered once for each Patient The information relating to protocol is entered separately, and therefore less information is required for each patient. The examinations for which height and weight are not relevant have these cells grayed out, as to underline this. Locked or Calculated This is a pilot and not the revised guideline, and therefore DRLs are absent from the pilot as there are no data on which to base these DRLs. Additionally, the objective of the pilot study is not to evaluate the patient doses of the departments, but rather to evaluate the design, wording and data required. Having revised the 2001 guideline into a pilot, and designed the accompanying spreadsheet, it can be sent to the select resource persons who wished to participate in the evaluation. The following chapter will present the feedback, measurements and analysis related to the pilot. This will lead to a further revision of the pilot into the author s proposal of the revised guideline P a g e

80 Chapter 5 Revision of Pilot into the Revised Guideline 5.1 Introduction This chapter details the feedback received from the resource persons regarding the pilot, and the subsequent revision of the pilot into the revised guideline, presented in full in appx. F-1. The considerations and argumentation behind a number of the changes in the pilot has been reconsidered, since the pilot was sent to a limited number of resource persons on May 6, New input and feedback from the resource persons have also provided new insight. First is presented the analysis of the measurements obtained from the pilot, as submitted by the resource persons. Due to the low number of data, and their large spread, the statistical grounding is weaker. Therefore, the analysis is more basic compared to the measurements obtained 2010/2011. It is, however, possible to derive an understanding of the usage of the pilot and the new required parameters. This is followed by feedback and observations made when the pilot was presented at the Fifth Annual Danish Quality Meeting in Radiology. This feedback will be used, together with the feedback from the pilot, as argumentation in the revisions from the pilot and 2001 guideline into the revised guideline. These revisions will be presented in groups with dedicated subsections. 5.2 Measurements from Pilot Five departments agreed to measure using the new categories of the pilot, which gave a total of 151 measurements. These were performed on a total of 25 examinations, spread across nine of the 12 pathophysiological indicators. The amount of data within each examination is insufficient for an analysis of DLP and other parameters. They, however, do provide a valid overview of the departments understanding of the different parameters. Table 5-1 below show how the parameters of the 25 examinations we entered. They were either typed correctly, incorrectly, or left blank. The table is read as such: The parameter Dose-modulation (AEC) was entered as only either a yes or a no in 64% of the examinations, entered as something else in 20%, and left blank in 16%. Parameter Correct [%] Incorrect [%] Left Blank [%] kv mas Axial/spiral Pitch Slice thickness Dose-modulation (AEC) Number of phases, including scout(s) Iterative reconstruction Table 5-1 The usage of parameter-fields in the pilot spreadsheet 5-73 P a g e

81 The values presented in Table 5-1 provide an understanding that will serve as argumentation for several revisions in the revised guideline, together with the feedback presented in the following section. 5.3 Fifth Annual Danish Quality Meeting in Radiology The pilot and the underlying argumentation were presented at the Fifth Annual Danish Quality Meeting in Radiology (hereafter known as the meeting) on June 15, This proved to be a good opportunity for feedback from the departments. The meeting was a one-day conference with 33 participants, mostly quality coordinator radiographers. They represented 26 departments, clinics, and regions, spanning all five regions. The feedback and discussion concerning the pilot is summarized in the following in bold paragraphs. These are each followed by the responses given by Britta Højgaard, the representative from SIS. It will serve as part of the argumentation for the revision, by either confirming or disproving changes in the pilot. Selection of examinations: Is an eventual prior examination, performed on suspicion of the specific indicator, to be summed with the later, full examination? E.g. patients who are scanned with low-dose CT to confirm the suspicion prior to the full examination, should these be added together? SIS: It should not be included, because the radiographer performing the full examination would need to look back for any earlier examinations with the same indicator, and that would be unnecessarily troublesome. Furthermore, we sort doses as each continuous examination, consisting of a single scan séance with a single problem. Missing scan length: The scan length is missing from the required information of the patient. SIS: Scan length can be calculated by dividing DLP with CTDI vol. More patients: It would be preferable with an increased number of patients, if this could remove height and weight as required information. SIS: As the deviation of BMI is as large as it is, it would require a significantly larger number of patients to minimize this effect. The timeframe for obtaining these measurements would possibly extent across revisions of the protocols, which would require a renewed number of measurements. Standard patient: It would still be preferable to avoid height and weight; however, it is acknowledged that more patients will be available for measuring with the new range of 70±20 instead of 70±10. SIS: The importance of BMI has been argued earlier, and height and weight will still be required. We have, however, removed height and weight from trauma for practical reasons, and also from cerebrum and sinuses, and included Appendix 2 arguing the necessity of BMI, as promised in the pilot P a g e

82 Multiple scanners: The existing 2001 guideline where measurements are spread out across multiple scanners makes no sense, as it is inadvisable to mix different vendors in the same mean. SIS: We agree, as this allows for the vendor to be a factor in the evaluation of doses, and for better optimization. If measurements show that a specific vendor (or individual scanner) has a lower mean dose on certain examinations, it is beneficial to prioritize these examinations on that specific scanner. Largest diameter in scan field: There is not a consensus between the vendors on how to incorporate the diameter in scan field for calculation of necessary dose, as vendors use anterior-posterior (thickness), right-left (width) or a differently weighted sum of both. SIS: We were unaware of this large difference between the vendors, and we will reconsider its validity as a requirement. Commentary text: We would like to be able to enter commentary text, instead of only numbers, in relation to several parameters. This includes AEC, iterative reconstruction and number of phases. SIS: We will provide fields for commentary text for the requested parameters. Metal inside patients: What about information regarding metal inside the patient: pacemaker, bolts used for fractures, etc. Should this be submitted? SIS: We will consider it. Noise figure instead of mas: It was proposed to submit the noise figure (in Danish: støjtal) instead of mas. SIS: We will consider it. 5.4 Revisions in the Revised Guideline Following is a list of all major revisions in the revised guideline, sorted by type, with each type being presented afterwards in its own dedicated subsection. Clarification of parameters Additions Collected Dose Iterative reconstruction and AEC CTDI vol Metal inside the Patient Appendix 1 regarding CTDI and DLP Appendix 2 regarding the standard patient 5-75 P a g e

83 Removals Introduction ALARA-argumentation Number of X-ray tubes mas Additions to the spreadsheet Introduction sheet Commentary field Restricted typing of data The revision from the pilot into the revised guideline was primarily in clarifying and optimizing the methods for communicating the intents and revisions presented in the previous chapter. Therefore, a number of minor revisions in wording and layout will not be covered Clarification of Parameters Based on the departments measurements using the pilot, a number of the parameters required clarification. This is in line with the feedback from the meeting, where it was noted that the lack of commentary fields in the spreadsheet caused explanatory information for internal use to be entered in the fields reserved for numbers. Collected Dose As described earlier, the DRLs relate to the entire dose of the patient, in a single scan séance. Therefore, the dose submitted to SIS must be the collected dose, including any scout- or prescans. A number of departments submitted multiple doses for each individual patient, creating a possibility for misunderstandings and the calculation of incorrect means. To address the issue, a commentary field was added in the spreadsheet next to the number of scans, combined with a mouse-over-explanation in the spreadsheet. Iterative Reconstruction and AEC The field iterative reconstruction was consistently used incorrectly to enter information describing the regular reconstruction planes. This is contributed to the lack of widespread use of the term, as evident by the input from the resource persons. Further research show that all four relevant vendors correctly use the specific term iterative reconstruction in their technical documents. Their instructions and teaching material seldom use the term, however, instead incorporating it into their existing dose-modulations, such as AEC. These doubts about the terms is evident in the measurements from the pilot, where 64% entered AEC correctly, while a lower 24% correctly entered the use of IR. More importantly, 48% incorrectly entered IR. This omission of the term iterative reconstruction by the vendor is addressed by the addition of a mouse-over-explanation in the spreadsheet explaining the term, and a short description in 5-76 P a g e

84 the revised guideline. The related errors in the entering of AEC by the inclusion of text warrants the addition of a commentary field for additional text. CTDI vol CTDI vol was mostly entered correct, although two examinations had switched CTDI vol and DLP for all patients. This error was easily spotted because both values were submitted. Total scan length is an important parameter, as evident from the resource persons, with a positive support of Its relation between DLP and CTDI vol was, however, not sufficiently apparent from the pilot. The issue of the missing total scan length as a parameter was brought up at the meeting, where its calculation by dividing DLP with CTDI vol was presented and acknowledged Additions Three main additions were included, one definition and two appendixes. The definition regarded patients with metal inside their bodies. Appendix 1 was a revision of the corresponding technical appendix from the 2001 guideline, while Appendix 2 presented the arguments for the revised standard patient. Metal inside the Patient At the meeting, the question was raised to register if the patient had metal in the body. This was rejected for two reasons: first, the measurements submitted to SIS are standard patients. A small minority with metal inside their bodies would disrupt the mean significantly, and the doses would no longer be representative of a standard patient. Second, it would be challenging to compare patients, as the dose would vary greatly with the metal s type, form, amount and location. Therefore, it was concluded that patients with metal implants could not be considered representative standard patients. This was done by adding a sentence (in bold below) before mentioning the standard weight: Appendix 1 regarding CTDI and DLP The revised Appendix 1 describes the standardized mathematics for dosimetry as presented in section 2.5. The major shift is from the 2001 appendix that used the normalized CTDI w [ n CTDI w ] and from this calculated two different DLP; one for axial and one for spiral scans. The revised Appendix 1 incorporates the difference between axial and spiral scans in CTDI vol, from which DLP is calculated. Portions of the text have been reworded, now arguing for the necessity of CTDI vol and including the new requirement of at least 20 patients. Appendix 2 regarding the Standard Patient Mentioned in the pilot, Appendix 2 presents the necessary argumentation for the revised standard patient, with a mean of 70 kg and an accepted range of ±20 kg. It is a condensed summary of subsection 4.4.9, including the argument of comparison with other countries. It constitutes a more comprehensive argument for the inclusion of height and weight P a g e

85 5.4.3 Removals Two paragraphs and three parameters were removed from the pilot in the revision. Introduction The introduction presented the reasoning behind the shift to pathophysiological indicators, and asked for further input and reflections on a number of considerations. It was removed, as it was only relevant for the pilot. The guideline is not in itself to be viewed as a continuation of the one prior; the most recent guideline is independent of those before it. A number of other wordings were removed for the same reason. ALARA-Argumentation The ALARA-argumentation served as an expanded explanation of the purpose of the guideline, and dosimetry in general. Even though a guideline is the fluent summary of the order, the paragraph was deemed insignificant for the single page of text constituting the guideline. It contained no information that aided the departments in proper dosimetry, and could be read as a patronizingly educational statement. Therefore, the paragraph was removed from the revised guideline. Number of X-Ray Tubes Dual-Energy is the concept of having two different tube voltages in a single scan, and was initially obtained by switching the voltage within a single tube. During the 2000s, scanners with two X-ray tubes reached the market, defined dual source. These newer scanners perform dualenergy by having each individual X-ray tube at a different voltage. The usage of dual-energy CT seemingly has a positive effect on dose, especially for large patients (67). It is therefore an important parameter for evaluating patient dose. The number of X-ray tubes was removed as a parameter in the revised pilot. It is the usage of dual-energy that is of interest, and not the technological method in which this is achieved. This information is already included in the parameter kv (specify both in dual-energy). If SIS in the future wishes to know the number of X-ray tubes, this factor should be specified when the scanner is initially registered. Largest Diameter in Scan Field The largest diameter in scan field is not used uniformly in the calculation of dose, and this pose a problem in using it as a parameter in evaluating patient dose. This became apparent at the meeting, where it was presented that at least one vendor used a weighted sum of both the anterior-posterior- and right-left diameter. Therefore, the validity of largest diameter in scan field as a value to evaluate dose is weakened, and it was removed in the revision. It is possible it will be included in a future revision, when there is a common usage in the industry. mas Tube current as a parameter was essential in evaluating dose in the 2001 guideline, but the increased usage of AEC has led to the use of effective mas instead. Effective mas describes the mean mas per slice, but AEC results in a varying tube current. Therefore, the effective mas is not guaranteed to accurately describe the dose for different regions. At the meeting, it was argued that the value is not always computed directly by the scanner, as it is included in 5-78 P a g e

86 CTDI vol. This view is supported by the evaluation of mas in the measurements from the pilot, where 36% left the field blank, while 28% entered it incorrectly, leaving 36% who entered it correctly. The challenges regarding this parameter, and the question of its validity in the immediate evaluation of dose, lead to it being removed from the revised guideline Proposed, but Rejected During the debate at the meeting, a number of suggestions for revisions were raised and discussed, but ultimately rejected. The major of these are presented in the following. Suspicion on Specific Indication With the new categorization based in pathophysiological indicators, it was asked at the meeting if earlier examinations performed on suspicion of the same indicator should be added together. This was rejected with the argument that the evaluation of individual patient dose is for each individual scan séance. It would be a source of error, as SIS would not be able to validate whether or not the examinations had been summed. Noise Figure The noise figure is only relevant for homogenous tissue and is dependent on what is measured. This makes it challenging to compare between indicators, and the difference between vendors make any use difficult. 5.5 Revised Guideline Spreadsheet The spreadsheet for use with the revised guideline required a number of changes to accommodate the revisions. Introduction Sheet The first sheet from the existing spreadsheet used with the 2001 guideline, as seen in appendix B-2, was rewritten and included in the revised spreadsheet. This introduction describes the structure of the spreadsheet, including the procedure for creating new sheets to accommodate protocols used at the department not already included. This overview serves as a practical introduction to the use of the spreadsheet, to ease the workflow of the departments. Commentary Field It became apparent that the departments required a commentary field for entering nonessential information for their own internal evaluation and use. This was evident from the use of the data fields in the pilot spreadsheet, with many fields containing information of no immediate importance to SIS. It was further argued at the meeting, and was accommodated by including commentary fields next to the parameters frequently used incorrectly in the pilot as evident from Table 5-1. These were Slice thickness, Dose-modulation (AEC) and Iterative reconstruction P a g e

87 Restricted Typing of Data In relation to the inclusion of commentary fields as described prior, a number of the otherwise simple parameters had an unexpectedly large number of incorrect or blank entries. Most notably is Axial/Spiral, which only 52% entered correctly, while 44% left it blank. To streamline the typing of data, the following fields should be locked for specific typing, using the build-in Data Validation in Microsoft Office Excel as shown in Table 5-2: Parameter Type Valid values Axial/Spiral Text/Dropdown Axial/Spiral Number of phases, including scout(s) Integer > 0 Dose-modulation (AEC) Text/Dropdown Yes/No Iterative reconstruction Text/Dropdown Yes/No Height Integer [150;200] Weight Integer [50;90] Gender Text/Dropdown M/K Table 5-2 Restricted typing of data in the revised guideline For Axial/Spiral, Dose-modulation (AEC), Iterative reconstruction and Gender, a dropdown menu was implemented in the fields. This would still allow direct typing of the value, but creating a useful reference of the valid values. For Number of phases, including scout(s), only 4% of the examinations left the field blank, but it was just as easy to validate all relevant fields at once. Height is restricted to an integer between 150 cm and 200 cm, encompassing what would be considered standard patients. Weight must be an integer between 50 and 90 kg, to avoid measurements of non-standard patients. These validations and restrictions, in combination with the commentary fields, would allow for greater usability within the departments. This concludes the work on revising the 2001 guideline into a revised guideline. It is the author s proposal, and the following, concluding chapter will present the future development of the guideline from this time forward P a g e

88 Chapter 6 Conclusion 6.1 Summary The main objective of the thesis was to form the argumentative basis for the revision of the part of the current 2001 guideline that specifically relates to CT examinations. It was not necessarily restricted to a revision of the DRLs, as there was an expectation by SIS that the technological development in the health care sector had resulted in a divide between the guideline and practice. This expectation was confirmed by analyzing new measurements obtained 2010/2011. This analysis revealed a significant variance between doses within most categories, and while a part of this was attributed to unbalanced data, it was sufficiently argued that the expectation from SIS was confirmed. Prior to continuing the revision of the guideline, the measurements obtained 2010/2011 were analyzed using ANOVA. A linear model provided insight into the influence on dose by different factors, and a variance in dose between hospitals was argued. This provided firm reasoning for the necessity for stronger data in the future, which would allow for a firmer statistical analysis on the variance seen in dosimetry. The expected influence on dose from BMI was researched and identified, providing SIS with important arguments against the departments for the necessity of collecting these. The areas of the guideline requiring revision were additionally based on analyzing an ongoing project on the optimization of image quality in relation to patient dose. This provided the revision with the required link with the practice at the departments and clinics. The author developed the argumentative basis for these revisions, additionally determining new areas in need of revision. The guideline was revised into a pilot that was sent to a select number of departments and the feedback and results were analyzed. This lead to a number of additional revisions that further strengthened the link between the guideline and current practice. The research, analysis and revisions resulted in the final proposal for the revised guideline. The revisions of this are all fully argued, and based on the author s technical grounding. This revised guideline meets the challenge of the divide between new possibilities arising from the technological development and the supervision provided by the guideline. It reflects the current examinations and practice of the departments and clinics, based on analysis of their own measurements and continuous feedback P a g e

89 6.2 Future Work The revised guideline as presented in the thesis is the author s final proposal. Finalization of the guideline is under the responsibility of SIS, with no influence from the author. In other words, the development of the accompanying spreadsheet for submitting data and possible changes to the revised guideline will be carried out by SIS alone. However, it will not take place immediately following the thesis. The publication of a guideline of this type during the holidays would be suboptimal, as a number of key personnel are on vacation. Following the final publication by SIS, new DRLs are to be based on measurements collected using the methods and categories of the revised guideline. These will be devised in comparison with other Nordic countries, in accordance with current practice. Based on the feedback from the departments, and the corresponding revisions in other Nordic countries, the timeframe for submitting data could possibly be lowered from every second year to annually. This significant change was postponed, as it required no argumentative basis from the author. The author has collaborated with project counselor Hanne N. Waltenburg and primary institute contact Britta Højgaard on an abstract titled Creating New Guidelines for CT Reference Dosimetry. This abstract includes data from the thesis, and will be presented by Britta Højgaard in the form of a poster at the upcoming conference for the Nordic Society for Radiation Protection in Iceland on August 22 to August 25, This will be an important forum for discussing a number of the revisions, including the new standard patient and the shift from anatomical regions to pathophysiological indicators P a g e

90 Bibliography 1. Kathren, R and Ziemer, P. Health Physics: A Backward Glace. s.l. : Pergamon Press, SIS. Strålebeskyttelse. Statens Institut for Strålebeskyttelse. [Online] [Cited: May 8, 2011.] 3. IAEA: Division of Human Health. Dosimetry and Medical Radiation Physics. [Online] juli 24, [Cited: april 14, 2011.] 4. Encyclopædia Britannica. Encyclopædia Britannica Online. "photon", "X-ray". [Online] [Cited: June 23, 2011.] 5. Young, H.D. Sears and Zemansky's university physics: with modern physics. 11th edition. San Fransisco : Pearson Education, imc., as Addison Wesley, Coderre, J Introduction to Ionizing Radiation, Fall MIT OpenCourseWare: Nuclear Science and Engineering. [Online] [Cited: June 24, 2011.] /index.htm. 7. Wikipedia. Wikipedia. [Online] [Cited: May 11, 2011.] 8. Chang, R. General Chemistry: The Essential Concepts. New York : McGraw-Hill Companies, Inc., Petterson, H. et al. The Encyclopedia of Medical Imaging. Oslo : The NICER Institute, Evolving role of cardiac CT in the diagnosis of coronary artery disease. Gupta, R. et.al. 87: , s.l. : Postgrad. Med. J., 2011, Vols. March 1, Jørgensen, P.M. Radiografisk Billeddannelse, Digital Røntgen. [Teaching material] København : s.n., FDA. Computed Tomography (CT). [Online] september 11, [Cited: april 14, 2011.] EmittingProducts/RadiationEmittingProductsandProcedures/MedicalImaging/MedicalX- Rays/ucm htm. 13. Jørgensen, B.H. CT-teknik: Indføring i CT-teknikkens grundprincipper. 7th Edition. Ballerup : Forlaget Utopia, Direct comparison of a xenon and a solid-state CT detector system: measurements under working conditions. Fuchs, T., Kachelriess, M., Kalender, W.A Sep; 19(9), Erlangen : IEEE Trans Med Imaging, Bushong, S.C. Radiologic Science for Technologists: Physics, Biology, and Protection. 9th Edition. s.l. : Mosby Elsevier, Smith, S.W. The Scientist and Engineer's Guide to Digital Signal Processing. San Diego : California Technical Publishing, Bib-87 P a g e

91 17. Standardized Nomenclature and Description of CT Scanning Techniques. Kalra, M. K. and Saini, S. 241: , s.l. : RSNA, December 2006, Radiology. 18. Philips. Philips Healthcare. [Online] [Cited: May 25, 2011.] Strategies for CT radiation dose optimization. Kalra, M. K., Maher, M. M., Toth, T. L., Hamberg, L. M., Blake, M. A., Shepard, J. A. and Saini, S. 203(3): , s.l. : RSNA, March 2004, Radiology. 20. ICRP. Managing Patient Dose in Computed Tomography. s.l. : ICRP Publication 87, Kalender, W.A. Computed tomography : fundamentals, system technology, image quality, applications. Chichester : Wiley, AAPM/RSNA Physics Tutorial for Residents: Topics in CT. McNitt-Gray, M.F. 22: , s.l. : RadioGraphics, November Cattin, P. Principles of Medical Imaging. [Presentation] Basel : University of Basel, Silkoset, R.D. Analyse av variasjon i representative doser ved undersøkelser med computertomografi (CT). Østerås : Statens stråleværn, ICRP. Managing Patient Dose in Multi-Detector Computed Tomography (MDCT). Orlando : ICRP Publication 102, ICRP. The 2007 Recommendations of the International Commission on Radiological Protection. Orlando : ICRP Publication 103, Øberg, M. CT dosimetry and image optimisation for medical physicists. [Presentation] Herlev : s.n., Iterative Reconstruction Technique for Reducing Body Radiation Dose at CT. Hara, A.K., Padven, R.G., Silva, A.C., Kujak, J.L., Lawder, H.J. Pavlicek, W. 193: , Scottsdale : American Journal of Roentgenology, A comparison of radiation doses between state-of-the-art multislice CT coronary angiography with iterative reconstruction, multislice CT coronary angiography with standard filtered back-projection and invasive diagnostic coronary angiography. Gosling, O. et al. 96: , Exeter : Heart 2010, Nowak, T.J. Pathophysiology : concepts and applications for health care professionals. 3rd Edition. New York : McGraw-Hill, Purves, W.K. et al. Life, the science of biology. 7th. Sunderland : Sinauer Associates, inc., Radiation Effects Research Foundation. Frequently Asked Questions. RERF. [Online] [Cited: May 12, 2011.] Dige-Petersen, H. Medicinsk Kompendium [Medical Compendium]. [book auth.] Ib Lorenzen et. al. 15th Edition. Copenhagen : Nyt Nordisk Forlag Arnold Busck, 1999, Vol. 2, Chapter 77, pp P a g e

92 34. SIS. Helbredsrisici ved ioniserende stråling. Statens Institut for Strålebeskyttelse. [Online] [Cited: May 15, 2011.] Korea Atomic Energy Research Institute. Nuclear Safety and Radiation Protection. KNTC- Nuclear Power Project. [Online] [Cited: May 13th, 2011.] UNSCEAR 2006 Report. Effects of Ionizing Radiation: Report to the General Assembly. s.l. : United Nations, Mobbs, S.F., Muirhead, C.R. and Harrison, J.D. Risks from Ionising Radiation. s.l. : Health Protection Agency, RadDaily.com. Quantifying CT Radiation Dose. RadDaily.com. [Online] Gage Media. [Cited: May 11, 2011.] Bogdanish, W. After Stroke Scans, Patients Face Serious Health Risks. The New York Times. 2010, July ICRP Recommendations of the International Commission on Radiological Protection. s.l. : ICRP Publication 60, Højgaard, B. Myndighedsarbejde. [Presentation] Herlev : SIS, SIS. Stråledoser. Statens Institut for Strålebeskyttelse. [Online] [Cited: May 25, 2011.] Pedersen, K.M. Patientbeskyttelse. [Presentation] Herlev : SIS, TIME Magazine. Medicine: X-Ray Danger. Monday, 1956, Oct Population and patient risks from CT scans. Sutton, D. Issue 2, 2008, C2I2, Vol. Volume VI. 46. Patient Doses in CT Examinations in Switzerland: Implementation of National Diagnostic Reference Levels. Treier, P., Aroua, A., Verdun, F.R., Samara, E., Stuessi, A. and Trueb, Ph. R. 2-4: , Lausanne : National Board of Health, 2010, Radiation Protection Dosimetry (2010), Vol Collective doses from medical exposures: an inter-comparison of the TOP 20 radiological examinations based on the EC guidelines RP 154. Aroua, A., Aubert, B., Back, C., Biernaux, M., Einarsson, G., Frank, A., Friberg, E.G., Griebel, J., Hart, D., Järvinen, H., Leitz, W., Muru, K., Nekolla, E., Olerud, H.M., Tenkanen-Rautakoski, P., Trueb, P., Valero, M., Waard, I., Waltenburg, H. and Zili. Helsinki : IRPA Congress, OECD. OECD Health Data Frequently Requested Data. [Spreadsheet] s.l. : OECD, ICRP. Recommendations of the ICRP. s.l. : ICRP Publication 27, Lov nr. 147 af 15/04/1930. Lov om Brug af Røntgenstråler. København : Indenrigs- og Sundhedsministeriet, Bib-89 P a g e

93 51. BEK nr. 975 af 16/12/1998. Bekendtgørelse om medicinske røntgenanlæg til undersøgelse af patienter. København : Indenrigs- og Sundhedsministeriet, SIS. Vejledning om referencedoser for røntgenundersøgelser. København : SIS, J.nr SIS. Vejledning om patientdoser og referencedoser for røntgenundersøgelser: Konventionelle røntgenundersøgelser. København : SIS, J.nr CIRS. CT & X-Ray Phantoms. CIRS: Tissue Simulation & Phantom Technology. [Online] Computerized Imaging Reference Systems, [Cited: May 30, 2011.] Radiation Dose in Computed Tomography of the Heart. Morin, R.L. PhD. et al. Circulation 2003;107: , s.l. : American Heart Association, Inc., WHO. BMI Classification. Global Database on Body Mass Index. [Online] WHO, June 13, [Cited: June 13, 2011.] Øberg, M. Measurements of CT-doses in Denmark 2010/2011. [Excel-spreadsheet] Herlev : SIS, Øberg, M., Højgaard, B., Waltenburg, H. Vejledning om referencedoser for røntgenundersøgelser - Pilotprojekt maj-juni Herlev : SES, Strålsäkerhetsmyndigheten. Strålsäkerhetsmyndighetens föreskrifter om diagnostiska standarddoser och referensnivåer inom medicinsk röntgendiagnostik ICRP. Report on the Task Group on Reference Man. s.l. : ICRP Publication 23, ICRP. Basic Anatomical and Physiological Data for Use in Radiological Protection Reference Values. s.l. : ICRP Publication 89, Obama, B., Waxman, H.A. Letter from Senator Barack Obama and Representative Henry A. Waxman to Stephen L. Johnson Administrator, U.S. Environmental Protection Agency. [Letter as PDF] Washington : Institute for Energy and Environmental Research, July Reexamining the Physical Examination for Obese Patients. Silk, A.W., McTigue, K.M. 2011;305(2): , s.l. : JAMA, OECD. About OECD. [Online] OECD. [Cited: June 14, 2011.] OECD. Obesity and the Economics of Prevention: Fit not Fat. s.l. : OECD, OECD. Health at a Glance: Europe s.l. : OECD, Dual-source dual-energy CT with additional tin filtration: Dose and image quality evaluation in phantoms and in vivo. Primak, A.N., Giraldo, J.C., Eusemann, C.D., Schmidt, B., Kantor, B., Fletcher, J.G., McCollough, C.H Nov;195(5), Rochester : American Journal of Roentgenology, P a g e

94 Appendix A General Appendixes This appendix contains the guideline from 2001, the partial revision of which constitutes the basis for the thesis. A-1: Guideline Regarding Diagnostic Reference Levels for X-ray Examinations A-93 P a g e

95 A-1 Guideline Regarding Diagnostic Reference Levels for X-ray Examinations A-94 P a g e

96 A-95 P a g e

97 A-96 P a g e

98 A-97 P a g e

99 A-98 P a g e

100 Appendix B Collection of Data 2010/2011 This appendix contains the material related to the collection of data, which began on December 7, 2010, and is covered extensively in Chapter 3. The spreadsheet for reporting data in B-2 is only an excerpt, as the data-sheets are identical save for the DRL and title, with the following data-sheets in the complete spreadsheet: Guideline (excerpted), CT Cerebrum (excerpted), CT Sinuses, CT Spine, CT Thorax, HRCT Thorax, CT Abdomen, CT Liver, CT Cymbals, CT Cymbals [Bone]. The sorted data presented in B-3 is only an excerpt, as the complete dataset consists of 5966 individual patients; however an excerpt demonstrates the basic formatting of the datastructure, and the different sheets included in the complete spreadsheet are: Cerebrum (excerpted), Sinuses, Spine, Thorax, HRCT Thorax, Abdomen, Liver, Cymbals, Cymbals [Bone], Thorax Abdomen, Thorax Upper Abdomen, Urography, Facial Skeleton, Heart, Trauma. B-1: sent on December 7, 2010 to All Departments and Clinics B-2: Spreadsheet Provided by SIS for Submitting Data [excerpt] B-3: Sorted Data [excerpt] B-101 P a g e

101 B-1 sent on December 7, 2010 to All Departments and Clinics B-102 P a g e

102 B-2 Spreadsheet Provided by SIS for Submitting Data [excerpt] B-103 P a g e

103 B-104 P a g e

104 B-3 Sorted Data [excerpt] B-105 P a g e

105 Appendix C Optimization of Image Quality in Relation to Patient Dose This appendix contains the material related to the questionnaire circulated November 22, 2010 regarding optimization of image quality in relation to patient dose, which is used as part of the basis for the pilot in Chapter 40. C-1: sent on November 22, 2010 to Department Management C-2: Questionnaire C-3: Responses C-107 P a g e

106 C-1 sent on November 22, 2010 to Department Managements C-108 P a g e

107 C-2 Questionnaire C-109 P a g e

108 C-110 P a g e

109 C-3 Responses C-111 P a g e

110 C-112 P a g e

111 C-113 P a g e

112 C-114 P a g e

113 C-115 P a g e

114 C-116 P a g e

115 C-117 P a g e

116 C-118 P a g e

117 C-119 P a g e

118 C-120 P a g e

119 C-121 P a g e

120 C-122 P a g e

121 Appendix D Input from Resource Persons This appendix contains the questionnaire sent to the resource persons on January 19, 2011 where every physicist, physician and other related healthcare personnel was invited to give their input regarding the revised guideline as covered in Chapter 40. The responses were saved in a spreadsheet, but the high number of questions (47) combined with the relatively large portion of polar questions made analyzing the data unmanageable. Therefore, the polar questions were converted into a fraction, where 0 and 1 equals no and yes, respectively. These fractions were then presented, together with the qualitative responses, in a collected document instead as presented in D-4. D-1: sent on January 19, 2011 to Resource Persons D-2: Letter attached to sent on January 19, 2011 D-3: Questionnaire D-4: Responses Document D-125 P a g e

122 D-1 sent on January 19, 2011 to Resource Persons D-126 P a g e

123 D-2 Letter attached to sent on January 19, 2011 D-127 P a g e

124 D-128 P a g e

125 D-3 Questionnaire D-129 P a g e

126 D-130 P a g e

127 D-131 P a g e

128 D-132 P a g e

129 D-4 Responses Document D-133 P a g e

130 D-134 P a g e

131 D-135 P a g e

132 D-136 P a g e

133 Appendix E Pilot This appendix contains the documents related to the pilot for the revised guideline, which was sent to a limited number of resource persons on May 6, 2011 and is covered extensively in Chapter 40. The pilot spreadsheet for reporting data in E-3 is only an excerpt, as the data-sheets are identical save for the title and whether height, weight and BMI is to be reported. The following data-sheets are in the complete spreadsheet: Hospital- and scanner information (excerpted), CT-scanning of cerebrum: Obs. bleeding (excerpted), Perfusion study Cerebrum, CT-scanning of sinuses, incl. cavum nasi: Pre-operation, CT-scanning of trauma patients (head, thorax, abdomen): High-energy trauma, CT-scanning of thorax: Obs. lung cancer (excerpted), CTscanning of the heart: Coronary disease, High Resolution CT-scanning of lungs: Obs. intestinal lung disease, CT-scanning of abdomen: Acute abdomen, CT-urography: Obs. malignity, CTurography: Obs. stones, CT-scan of colon and rectum (CT-colography): Obs. cancer, CT-scan of thorax and abdomen: Obs. malignity and tumor control. E-1: sent on May 6, 2011 to Select Resource Persons E-2: Pilot for Revised Guideline E-3: Pilot Spreadsheet Developed for Submission of Data [excerpt] E-139 P a g e

134 E-1 sent on May 6, 2011 to Select Resource Persons E-140 P a g e

135 E-2 Pilot for Revised Guideline E-141 P a g e

136 E-142 P a g e

137 E-143 P a g e

138 E-144 P a g e

139 E-3 Pilot Spreadsheet Developed for Submission of Data [excerpt] E-145 P a g e

140 E-146 P a g e

141 E-147 P a g e

142 Appendix F Revised Guideline This appendix contains the documents related to the revision of the pilot into the revised guideline, and is covered in Chapter 5. F-1: Revised Guideline F-149 P a g e

143 F-1 Revised Guideline F-150 P a g e

144 F-151 P a g e

145 F-152 P a g e

146 F-153 P a g e