RAD 451. Managing Patient Dose in Computed Tomography (CT)

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Byron Bell
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1 RAD 451 Managing Patient Dose in Computed Tomography (CT) Objectives: Identify basic terminology associated with CT dosimetry Determine How CT dose is measured Explain How CT dose can be reduced Demonstrate CT radiation safety practices as a technologist 1

Contents Measurement Terminology Why increased dose? Is the dose really high? How high? What can be done to manage patient dose? What can operator do?

2 Contents Measurement Terminology Why increased dose? Is the dose really high? How high? What can be done to manage patient dose? What can operator do? Action for physicians Action for manufacturers References 1. Lois Romans for Technologists: A Comprehensive Text, Lippincott Williams & Wilkins, ISBN: Euclid Seeram.: Physical Principles, Clinical Applications, and Quality Control, 3rd ed. Saunders, ISBN: Text book of radiographic positioning and related anatomy; by Kenneth L.Bontrager,8 th edition. 4 2

3 Patients are exposed to higher radiation levels from the use of CT compared to most imaging techniques How much dose is scanner delivering? Inter-scanner comparison of dose Estimate patient potential risk Weigh risk against benefit 3

4 Measurement Terminology The ionizing radiation used in CT is an x-ray with maximum energy from 120 to 40 kev and an average energy near 70 kev. The unit of x-ray exposure in air = the Roentgen (R). When the x-rays from a CT scanner strike a patient and interact with tissue, most of the energy is absorbed, and some of it passes through to the detector. The unit of absorbed dose = Radiation Absorbed Dose, or RAD. This unit describes the amount of energy absorbed per unit mass. The International System of Units (abbreviated SI ) is a newer system that is used internationally, The SI unit of absorbed dose is the gray (Gy). There are 100 rad in 1 Gy. A centigray (cgy) equals 1 rad. Measurement Terminology Dose Index (CTDI) special quantity expresses radiation dose in CT Measured with a dosimeter inserted into a phantom that represents a patient Dose is measured by scanning one slice factors are applied to convert the measured phantom CTDI to an actual patient scan a reasonable estimation of the actual dose to the patient 4

5 Measurement Terminology Multiple Scan Average Dose A dose delivered to patient when a series of scans are performed Why increased dose? Unlike radiography where over-exposure results in blackening of film, better image quality is obtained with higher exposures in CT There is a tendency to increase the volume covered in a particular examination Modern helical CT involves volume scanning with no inter-slice gap and with possibility of overlapping scans Repeat CT examinations 5

6 What is the dose from CT? How high? The effective dose in chest CT is in the order of 8 msv (around 400 times more than chest radiograph dose) and in some CT examinations like that of pelvic region, it may be around 20 msv The absorbed dose to tissues from CT can often approach or exceed the levels known to increase the probability of cancer as shown in epidemiological studies Effective doses in CT and radiography CT examination Effective dose (msv) Radiographic examination Effective dose (msv) Head 2 Skull 0.07 Chest 8 Chest PA 0.02 Abdomen Abdomen 1.0 Ba enema 7 Pelvis Pelvis 0.7 6

7 Organ doses in CT Breast dose in thorax CT may be as much as mgy, even though breasts are not the target of imaging procedure Eye lens dose in brain CT, thyroid in brain or in thorax CT and gonads in pelvic CT receive high doses Does spiral CT give more or less radiation dose? It depends upon the choice of factors Even though it is possible to perform a spiral CT with lower radiation dose than slice-by-slice CT, in practice the patient gets higher dose due to the factors chosen (scan volume, mas, pitch, slice width) 7

8 Does multi-slice CT impart more or less radiation dose? An increase by 10-30% may occur with multi-slice detector array mas Pitch Collimation Scan volume Slice thickness Slice gap Repeats shielding Alignment ( Positioning) patient care 8

9 Reduction of Dose You,the operator, must know dose You, the operator, how to keep it at minimum What can be done to reduce Multiple Scan Average Dose (MSAD) mas When Reducing the mas, the patient dose is decreased 9

10 Collimation Practical collimation = decreased patient dose Scanned volume When Reducing the scan volume the patient dose is decreased 10

Slice thickness When Reducing the slice thickness the patient dose is increased Slice gap When Reducing the slice gap the patient dose is increased Pitch Pitch = The

11 Slice thickness When Reducing the slice thickness the patient dose is increased Slice gap When Reducing the slice gap the patient dose is increased Pitch Pitch = The distance the patient couch travels during one 360 degree turn As pitch increases, the time spent in any one point in space is decreased Decreasing Pitch = Higher Radiation Dose 11

12 shielding Shield thyroid, breast, eye lens, gonads to reduce organ dose by 30 60% Other: Patient alignment(positioning) Patient preparation and care Avoid Repeats 12

13 Patient Alignment Correctly centering patient on CT gantry can reduce radiation dose by as much as 56% When the patient is in the incorrect position the patient must be moved and the CT scan repeated Others Patient preparation and care? Avoid of repeats? 13

14 What can be done to manage patient dose in CT? What can operators do? What can Physicians do? What can Manufacturers do? 14

15 What can be done to manage patient dose in CT? What can operators do? Limit the scanned volume Reduce mas values Adapting the scanning parameters to the patient cross section % reduction in dose documented, without any loss of image quality Separate factors for children. Can reduce dose by a factor of 5 or more Adequate selection of image reconstruction parameters What can be done to manage patient dose in CT? What can operators do? Use of spiral CT with pitch factor>1 and calculation of overlapping images instead of acquiring overlapping single scans Shielding of superficial organs such as thyroid, breast, eye lens and gonads particularly in children and young adults. This results in 30-60% dose reduction to the organ Patient preparation and care? Accurate postioning? 15

16 What can be done to manage patient dose in CT? What can physicians do? Justification: Ensure that patients are not irradiated unjustifiably Request for CT examination should be generated only by properly qualified medical or dental practitioners depending upon national educational and qualification system. The physician is responsible for weighing the benefits against risks Clinical guidelines advising which examinations are appropriate and acceptable should be available to clinicians and radiologists What can be done to manage patient dose in CT? What can physicians do? Consider whether the required information be obtained by MRI, ultrasonography Consider value of contrast medium enhancement prior to commencing examination CT scanning in pregnancy may not be contraindicated, particularly in emergency situations, although examinations of the abdomen or pelvis should be carefully justified 16

17 What can be done to manage patient dose in CT? What can physicians do? CT examination should not be repeated without clinical justification and should be limited to the area of interest Clinician has the responsibility to communicate to the radiologist about previous CT examination of the patient CT examination for research purpose that do not have clinical justification (immediate benefit to the person undergoing the examination) should be subject to critical evaluation by an ethics committee What can be done to manage patient dose in CT? What can physicians do? CT examination of chest in young girls and young females needs to be justified in view of high breast dose Once the examination has been justified, radiologist has the primary responsibility for ensuring that the examination is carried out with good technique 17

18 What can be done to manage patient dose in CT? What can manufacturers do? Introduce automatic exposure control Be conscious of high doses in CT Include safety features to avoid unnecessary dose Display of dose Convenience in using low dose protocols Draw attention of users to selecting separate protocols for paediatric patients Conclusion Justification of practice in CT is of particular importance and base line for Radiation Protection A series of clinical factors play a special part Adequate clinical information, including the records of previous imaging investigations, must be available In certain applications prior investigation of the patient by alternative imaging techniques might be required An additional training in radiation protection is for staff is an added value 36 18

Staff Doses & Practical Radiation Protection in DEXA

Patient Xray X Doses Staff Doses & Practical Radiation Protection in DEXA Una O ConnorO Dept. of Medical Physics & Bioengineering, St. James s s Hospital. Examination Types General XrayX Fluoroscopy /