Dose Modulation Technique in CT

Transcription

1 Content Dose Modulation Technique in CT Short Overview - Overview - Dose Saving Features - Take Home Point - Conclusions Senior Application specialist Siemens Limited, Taiwan For internal use only / Copyright Siemens AG All rights reserved. Page 2 Scanner Generations - Overview Page 3 Page 4 Scanner Generations Brain CT Development of CT MDCT Spiral MSCT EBCT Whole? FPCT CT body CT? Low Matrix Electron Continuous Pulse Flat Voltage X-ray Plan formating gun X-ray & CTSlip multi design Ringtarget SS design detector Transaxial Scaninfusion Contrast enhancement? FPCT Helical Volume Heart Dynamical Topo & Scan Scan Fast Scan Scan Power dual Power Hand-push air-drip head injector injector infusion injector Multi-phase Dynamic Contrast enhancement scan CTA / Multi-organ single / multi-organ Page 5? MTCT Page 6? Multi-Tube design? MTCT Volume ScanPower injectormulti-phase CCA /? Single heart heart beat beat 1

2 Radiation Risk and Effective Dose Risk Vs Benefit Radiation Risk Schematic (!) Graph: Assumption for radiation protection (ICRP): Linear extrapolation without threshold Data from radiation victims (Hiroshima, Nagasaki) imaginable correlations for low dose Typ. range of CT Eff. Dose (msv) For children: Higher Risk! Page 7 Page 8 Cataract in eye of interventionist after repeated use of over table x-ray tube Example of chronic skin injury due to cumulative skin dose of ~2, mgy (2 Gy) from coronary angiography and x2 angioplasties 21 months after first procedure, base of ulcer exposes spinous process Page 9 Page 1 News 1 On Friday, 8 Oct. 29, the FDA sent out a notice concerning "Safety Investigation of CT Brain Perfusion Scans: Initial Notification." According to the FDA notification, 26 patients were exposed over a time period of 18 months with inappropriately high levels of radiation dose during CT Perfusion (CTP) examinations. As a consequence of the exposure, some patients suffered from erythema and partial hair loss. Take from Managing Patient Dose in Multi-Detector Computed Tomography (MDCT) Madan Rehani C 3 etc. Page 11 Page 12 2

3 News 2 CHICAGO (Reuters) - Radiation from CT scans done in 27 will cause 29, cancers and kill nearly 15, Americans, researchers said on Monday CHICAGO Mon Dec 14, 29 4:3pm EST About 7 million CT scans were done on Americans in 27, up from 3 million in 198. Amy Berrington de Gonzalez of the National Cancer Institute and colleagues developed a computer model to estimate the impact of so many scans. They estimated the scans done in 27 will cause 29, cancers. A third of the projected cancers will occur in people who were ages 35 to 54 when they got their CT, two-thirds will occur in women and 15 percent will arise from scans done in children or teens. The researchers estimated there will be an extra 2, excess breast cancers just from CT scans done in 27. Page 13 Page 14 Distribution of S or E US for the categories of exposure early 1981s 26 occupationa / occupationa / industrial consumer 2% industrial % % consumer 2% medical 15% Background 83% medical 48% Background 5% Data from NCRP Report 16 Page 15 Page 16 Over view Frequency of radiological examination Proportions of collective effective dose CT 6% rest 53% CT 47% rest 94% The figures shown refer to Germany for the year 23 [BfS, 23] Page 17 Page 18 3

4 Page 19 Page 2 Reference Dose Levels Various countries or organisations have defined Reference Levels for CT dose e.g. American College of Radiology (ACR): e.g. Germany (Federal Office of Radiation Protection): Diagnostic Reference Values for CT-Examination for adults Examination Brain Cranium (Face) / Paranasal Sinuses Thorax Abdomen Pelvis Upper Abdomen Lumbar Spine CTDIw (mgy) DLP(mGy cm) Page 21 Page 22 Define diagnostic image quality by adding noise Advantage and Risk of Diagnostic with X-rays 16mAs 13mAs 6mAs significant information about pathology Image Quality Radiation Risk basically radiation induced cancer* A careful consideration of advantage against risk is demanded for every individual patient! *) "stochastic damage". The "deterministic damage" caused by very high doses (e.g. radiation burns) are not taken into account here. 4mAs 56mAs Page 23 28mAs 2mAs Page 24 4

5 Advantage and Risk of Diagnostic with X-rays Body Part Spatial Resoluction Low contrast Noise significant information about pathology kvp ma Sec Algorithm AEC E&T basically radiation induced cancer* How to give a definition about Good Image Quality? higher Image Contrast & less noise or less image contrast & acceptable noise *) "stochastic damage". The "deterministic damage" caused by very high doses (e.g. radiation burns) are not taken into account here. Page 25 Page 26 Advantage and Risk of Diagnostic with X-rays Diagnostic Information significant information about pathology Radiation Risk basically radiation induced cancer* Image Quality A careful consideration of advantage against risk is demanded for every individual patient! *) "stochastic damage". The "deterministic damage" caused by very high doses (e.g. radiation burns) are not taken into account here. Page 27 Page 28 Define diagnostic image quality by adding noise Advantage and Risk of Diagnostic with X-rays Lesion Location Size characteristic significant information about pathology Radiation Risk basically radiation induced cancer* 6mAs 13mAs 16mAs Image Quality A careful consideration of advantage against risk is demanded for every individual patient! *) "stochastic damage". The "deterministic damage" caused by very high doses (e.g. radiation burns) are not taken into account here. Page 29 4mAs 56mAs Page 3 2mAs 28mAs 5

6 An Innovation Leader in Low Dose Computed Tomography - Dose Saving Features Up to 68% Up to 3% Up to 7% Up to 5% Up to 5% CARE Dose 4D Ultra Fast Ceramic (UFC) Hand CARE Pediatric 8 kv Protocols Siemens 1999 Exclusive 22 DSCT Siemens 25 Exclusive 1-3 msv Adaptive ECG- Pulsing/Sequence 27 X-ray low Up to 25% < 1 msv No penalty Up to 5% Up to 4% Up to 6% Adaptive Dose Shield Flash Spiral Selective Photon Shield 4D Noise Reduction X-CARE IRIS Page Page Siemens Exclusive Siemens Siemens Siemens 28 Exclusive 28 Exclusive 28 Exclusive 29 CT Dose Modulation Technique CARE Dose 4D Automatic Exposure Control RadioGraphics 28; 28: , Chang Hyun Lee, MD etc Page 34 The human body is not a homogeneous cylinder X-ray attenuation varies along the spiral path of a CT scan X-ray tube Accordingly, the noise in projection data varies (for constant tube output) Detector x body axis Optimal diagnostic image quality in every slice at lowest dose levels Adaptation of tube current to attenuation Automatic Exposure Control Page 35 attenuation "On each CT SCANNER, AUTOMATIC EXPOSURE CONTROL (AEC) shall be provided as a MODE(S) OF OPERATION alternative to the manual selection of CT CONDITIONS OF OPERATION." Page 36 6

7 mas-adaptation along the patients z-axis and to patient size Define diagnostic image quality by adding noise Constant image noise Example: Thorax Protocol 16mAs 13mAs 6mAs 75kg-Patient: Neck Thorax Abdomen Shoulder Obese Patient (Example): Neck Thorax Abdomen Shoulder 4mAs 56mAs Page 38 28mAs 2mAs Results of Clinical Image Quality Assessment mas-adaptation along the patients z-axis and to patient size Image Noise should not be constant resp. mas should not be proportional to object attenuation Constant image noise Clinical reasonable mas Example: Thorax Protocol Noise in pediatric patients would be too high mas for obese patients would deliver excessive dose levels and exceed the power limits of current scanners mas should be adapted by an empirical function, according to diagnostic information requirements CARE Dose 4D: ma is set proportional to (A/A ref) b Default Settings for b: Slim patients / low attenuation: b=.5 Obese patients / high attenuation b=.33 75kg-Patient: Neck Thorax Abdomen Shoulder Obese Patient (Example): For internal Neck use Thorax only / Copyright Abdomen Siemens Shoulder AG 21. All rights reserved. How does CARE Dose 4D work? Care Dose 1. Evaluation of Topogram for attenuation in and AP direction 2. Calculation of appropriate axial tube current profiles in and AP direction 3. Axial tube current variation during scan 4. Angular online tube current modulation during scan body axis x late ral attenuation Page 41 Page 42 7

8 mean Measuring the attenuation in z-axis CARE Dose 4D: ma is set roportional to (A/Aref) b max CareDose ma = X/Y ma tube current in ma Page 43 Page 44 table position Dose Modulation Technical Aspects Why is a angular modulation needed? AP X-ray tube Generator Elliptical/ irregular objects Current modulation unit Detector Data acquisition system Without angular dose modulation: Detector The image noise is mainly determined by those angles with the highest X- ray attenuation! Page 45 Page 46 Using angle modulation Dose Modulation Phantom scan AP Shoulder phantom, 14cm x 4cm Scan with constant ma Scan with automatic ma adaption 199mAs 189mAs Detector s = 12.9HU s = 9.4HU Lower Image Noise due to better ma distribution Effective Dose Reduction: 49% measured / 5% calculated Page 47 Page 48 8

9 Improved image quality at lower dose Advantage of Online Modulation Scan without dose modulation Scan with dose modulation Saved Dose Example: Shoulder Scan 327mAs 171mAs Dose reduced by 51%, p.a. Page 49 Page 5 Optimal ma for AP and Views: On-line ma modulation CARE Dose 4D: Whole body scan Headline Optimal image for all organs (adult) tube current Attenuation tube current attenuation I_ / I 14mAs 55mAs table position in mm 1 Page 51 Page 52 11mAs 13mAs Courtesy of Erlangen University, Stone Germany Chen -5 Page 53 Care Dose 4D Spiral scan of Carotid Arteries table position 4 tube current 35 Att 3 ma max attenuation I_ / I tube current in m A table position For internal -15 use only / Copyright -2 Siemens -25 AG 21. All rights reserved. Stone table Chenposition attenuation I_ / I CARE Dose 4D First Results Page 54 Dose reduction in %, compared to SOMATOM Sensation16 standard Protocols Dose Reduction Neck Thorax Thorax / Abdomen Abdomen / Pelvis Average 38% Average 13% Average 37% Average 39% protocols (14 patient studies) Organ specific Reduction in Scan Range Head 51% Shoulder -13% Shoulder 26% Neck 66% Thorax 38% Thorax 64% Thorax 67% Shoulder 17% Abdomen 4% Abdomen 37% Abdomen 4% Pelvis 33% Pelvis 34% [Gress et al, RSNA 23] 9

10 CARE Dose 4D Automatic Exposure Control for Siemens CT-Scanners (since SW VB1n) How to work with CARE Dose 4D Features: mas setting automatically adapted to patient size full mas variation over the patient s long axis real-time modulation during tube rotation (current CARE Dose) Since SW version VB1/VB19, Siemens default scan protocols use CARE Dose 4D, resulting in ability to scan immediately without any adjusting of mas after performing a Topogram (*resulting in default image quality). Benefits: scan protocols for slim/obese, (adult/pediatric patients) Adjustment of image quality to individual preference is possible. optimal diagnostic image quality in every slice achieved at lowest dose levels Page 55 Page 56 CARE Dose 4D: Topogram Data CARE Dose 4D: Topogram A Topogram is needed to utilize CARE Dose 4D If the scan range exceeds the Topogram range: Outside the Topogram range the last measured/calculated ma-value of the Topogram range will be used (warning pops up). If more than one Topogram of the current examination exists: Information of all valid Topograms will be used to calculate ma values (if overlapping in the direction, e.g., the newest information will be used) Patient (centered) Isocenter positioning of patient is essential! X-ray tube Detector Page 57 Page 58 CARE Dose 4D: Topogram CARE Dose 4D Topogram Isocenter positioning of patient is essential! Isocenter positioning of patient is essential! Patient (not centered) X-ray tube Patient (not centered) X-ray tube Detector Distorted Topogram does influence the mas Calculation! Detector Distorted Topogram does influence the mas Calculation! Page 59 Page 6 1

11 The Quality Reference mas value defines the overall image quality displays the average eff. mas (noise) of the current protocol and that will be applied by CARE may be adapted for each protocol to Dose 4D for the current scan the user's individual preference of displays the average eff. eff. mas that range image quality will be applied by CARE Dose 4D for indicates CARE Dose 4D is the current scan range (will be switched on for the current updated to the real applied eff. eff. mas protocol after the scan - migth differ somewhat) somewhat) Switch for CARE Dose 4D Page 61 Page 62 Page 63 Page 64 Page 65 Page 66 11

12 Impact of system limits Impact of system limits tube current upper system Limit (max. tube current) angle modulated tube current tube current upper system Limit (max. tube current) angle modulated tube current mean tube current per rotation mean tube current per rotation scan range z scan range z Page 67 Page 68 Impact of system limits Why is this CARE Dose a 4D?? CD4D adjusts the tube current over the patient s long axis tube current upper system Limit (max. tube current) mean tube current per rotation angle modulated tube current The 1D approach, like our competitors on some scanners CD4D adjusts the tube current in x and y (tube angle): 3D The 3D approach based on Topogram evaluation Data needed to plan the scan: dose information, tube load During the scan: Real-time measurement of attenuation and ma modulation Real 4D modulation in space and time scan range z Page 69 Page 7 Dose Saving Features (Extract) - Automatic Exposure Control (CARE Dose 4D) => see e-learning! tube current Attenuation tube current table position in mm 1 - Image Noise Reduction (Adaptive Filter, Image filtration) attenuation I_ / I 1.) Reference-Level-based Dose-Adaptation to patient size 2.) Dose-Modulation along z-axis 3.) Angular Online-Dose-Modulation Note: Without CARE Dose 4D - the dose has manually adapted to patient size, which will frequently result in too high patient dose or reduced image quality. - the dose is constant over the whole scan range.excessive dose in thin regions and excessive image noise in thick regions will result! - in inhomogeneous regions (shoulder, pelvis) the dose will be too high, without improvement of image quality. - Adaptive Dose Shield - ECG-Pulsing -... Page 71 Page 72 12

13 Principle of Radiation Protection Time Distance Shielding Principle of Radiation Protection for Patient (ASAP) Time (exp time) : As soon as possible Distance (Coverage) : As short as possible Shielding (H/W improvement) : As shielding as possible Page 73 Page 74 Page 75 Page 76 Page 77 Page 78 13

14 Adaptive cardiac Sequence scan Page 79 Page 8 Flash scan 1 heart beat scan Page 81 Page 82 Cardiac Dose Approaches Overview Dose (in msv) SOMATOM Definition Flash Spiral CTA Bhatti, SCCT 27 abstract 19 Dewey, Rofo, Jun 27 Mori, EurJRad, Jul 27 Hausleiter, JAMA, Feb 29 Step-and-Shoot Stolzmann, Eur Radiol 27 McCollough, Radiology 27 Hausleiter, SCCT 27 abstract 17 Spiral CTA Scheffel, Heart 28 Stolzmann, Radiology 28 Step-and-Shoot Page 83 Rybicki, J Card Im, Mar 28 Earls, SCCT 27, abstract 16 Cole, SCCT 27 abstract 15 Sablayrolles, RSNA 27, abstract Page 84 2 Run and Shoot 14

15 - Take Home Point How dose technologist can do more better? Page 85 Page 86 Dose Saving Features (Extract) Dual Energy - Field of View - Proper filtration of X-ray beam (avoid quanta energies that produce only dose and don't contribute to the image) - Shaped Filters Focal Spot Shaped Filter Patient Patient Positioning Important Hints Some general but Important Hints Dual Energy information is only available in a FOV of 26 cm. - Predefined Siemens Scan Protocols (in particular for children) => Proper setting of kv, eff. mas, collimation, etc. Page 87 Page 88 Lower Dose Technique For Conventional Radiography Image contrast of Film = Subject Contrast * Film Contrast (γ) * Dose Image contrast = Subject Contrast * Film Contrast (γ) * kvp * ma * sec CXR = Lung * (γ) * kvp * ma * sec ( ) For CT: Image contrast of CT = Subject Contrast * kvp *ma*sec Tube Voltage (kv) The tube voltage controls the used x-ray spectrum. Increasing the kv (at constant mas) will increase Pt dose x 1 4 decrease image noise 14 decrease image contrast 12 (especially iodine contrast) 1 improve image quality for big patients 8 Change CT No. quanta per mm 2 and mas in 1m distance kv 1 kv 12 kv 14 kv Lower Dose CT Technique for Lung Screening kev Page 89 Page 9 15

16 Dose correlation - Overview Tube current (ma) 1.) CTDIvol ~ mas 2.) CTDIvol kv 6-8 mgy/1mas 8 kv 1kV 12kV 14kV 3% 6% 1% 15% for typical Body Parameters* at 12kV (referred to Phantom 32 cm) mgy/1mas for typical Body Parameters* at 12kV (referred to Phantom 16 cm) *) Note: Special parameter settings (e.g. UHR) may lead to significantly higher values The tube current controls the no, of photon. Increasing the ma (at constant kvp) will increase Pt dose decrease image noise improve image quality for big patients Page 91 Page 92 Slice Sensitivity Profile (SSP) The user controls the SSP by choosing the effective slice width. Increasing the slice width will have the effect of reducing image noise, blurring structures in the image with strong z dependence, reducing contrast of small structures. Note: The effective slice width can only be wider than the collimation! Pitch In spiral CT, the pitch is defined as Table feed per rotation Total detector collimation Increasing the pitch has the following impact: The tube ma has to be increased to achieve the noise or dose (this will be done automatically by the SureView concept) The maximum achievable dose is reduced. The scan time is shortened. In scans without z-sharp, stronger windmill artifacts may appear. Page 93 Page 94 SureView a unique Feature of all SOMATOM Scanners SureView Effective mas Concept Due to the patented SOMATOM reconstruction algorithms, Siemens offers image quality independent of the pitch. At a given ma (or mas) value, the noise will vary with the pitch. To achieve the noise and dose with varying pitch, it makes sense to introduce a new ma-related quantity ma RotTime effective mas = PitchFactor Holding the eff. mas constant, the tube current increases with pitch. Image noise depends only on eff. mas. Patient dose depends only on eff. mas System mas limitation, effective ma = (ma * sec). Only eff. mas appear on UI (without CARE Dose 4D). Page 95 Page 96 16

17 Overview Scan & Recon Parameters Will affect Increasing Page 97 Scan Parameters Recon Params. Parameter Pitch Coll. Slice Rot Time Eff. Slice Kernel kv Eff. mas Noise (SureView) less less less (SureView) less more more (more) worse Sharpness less (z) less (z) Contrast less (z) more less (z) Artifacts worse (without z- sharp) worse worse (motion) less Obese Patients slightly worse improved improved improved improved improved -Conclusions Page 98 Conclusion => Patient Dose has to be weighted against the diagnostic use to reduce the radiation risk => CTDIvol and DLP are well established to describe the physical dose of a CT-examination => Dose saving features help to reduce dose, if applied => Careful adjustment of scan parameters to patient type and examination type has to be done, => Displayed dose values have to be observed Siemens Computed Tomography. Thank you for your attention! Page 99 Page 1 For internal use only / Copyright Siemens AG All rights reserved. 17