1. IAEA International Atomic Energy Agency Optimization of Protection in Computed Tomography (CT)-What can radiographers do? IAEA Regional Training Course on Radiation Protection of patients for Radiographers, Accra, Ghana, 11-15 July 2011

2. IAEA Introduction 18: Optimization of Protection in CT Scanner2

3. IAEA Multislice CT scanner

4. IAEA The CT image is mathematically reconstructed from the measured data.

5. IAEA Outline Introduction Factors affecting image quality Factors affecting dose Optimization 18: Optimization of Protection in CT Scanner5

6. IAEA 18: Optimization of Protection in CT Scanner6 CT scanning offers immense medical benefits (improved low contrast resolution) Patient doses can be high in CT CT trauma: 59 mGy (normal chest 0.33 mGy x ~ 150) Image quality in CT is often higher than necessary for diagnostic confidence Need for patient dose management Radiographer plays a big role in this endeavor …………………Introduction

7. IAEA …….Introduction 18: Optimization of Protection in CT Scanner7 CT scanner is a complex equipment User must understand relationship between scan protocols and patient dose and image quality Such understanding is not intuitive/training & experience needed

8. IAEA Scanning protocols can not simply be transferred between different manufacturers

9. IAEA Factors affecting image quality 18: Optimization of Protection in CT Scanner9 Spatial resolution Contrast discrimination Spatial uniformity Noise Pixel size Slice thickness mAs Tube voltage Reconstruction algorithm Sampling frequency Pitch (in multislice) Patient size

10. IAEA Spatial resolution 18: Optimization of Protection in CT Scanner10 The ability to resolve neighbouring structures Size of detectors Number of detectors X-ray focus Distance between source and detector Sampling frequency

11. IAEA Spatial Resolution

12. IAEA Contrast Discrimination The ability to detect differences between neighbouring structures of similar density (CT number)

13. IAEA Spatial Uniformity The faithful representation of shapes and contrast throughout the image Identical

14. IAEA Spatial Uniformity The faithful representation of shapes and contrast throughout the image Non-Uniform Contrast

15. IAEA Spatial Uniformity The faithful representation of shapes and contrast throughout the image Non-Uniform Shape

16. IAEA Noise What is noise? Anything that contributes to the degradation of CT images

17. IAEA Noise Decreases the quality of the image Makes diagnosis harder by Masking information Not presenting correct information

18. IAEA Other Factors - Image Quality & Patient Dose Pixel Size Slice Thickness mAs Algorithm Sampling Frequency Artefacts

19. IAEA Slice Thickness Thin slice -> smaller signal Smaller signal -> more noise More noise -> poor image Solution increase dose!!

20. IAEA mAs Current directly proportional to intensity of X- rays Low current, low intensity Low intensity, low signal Low quality image produced Solution increase dose!!

21. IAEA mAs 100 mAs50 mAs

22. IAEA Paper of Interest http://www.oucom.ohiou.edu/ou- microct/Downloads/Tradeoffs_in_CT_Image _Quality_and_Dose_9794-13379.pdf http://www.oucom.ohiou.edu/ou- microct/Downloads/Tradeoffs_in_CT_Image _Quality_and_Dose_9794-13379.pdf Good paper on CT image quality & dose tradeoffs

23. IAEA 18: Optimization of Protection in CT Scanner23 Monitoring of dose An issue to be considered Factors should be adjusted to produce necessary image quality without resulting to unnecessary dose

24. IAEA 18: Optimization of Protection in CT Scanner24 Monitoring of dose CTDI: special quantity to express radiation Dose in CT Useful since it is not easy to measure actual dose to internal organs Not a measure of patient dose

25. IAEA 18: Optimization of Protection in CT Scanner25 Dose Length Product DLP is a practical way for expressing total radiation dose deposited in body A measure of patient dose/risk DLP=CTDIvol x scan length

26. IAEA Factors affecting dose 18: Optimization of Protection in CT Scanner26 Tube current (mA) Scan (rotation) time (s) Tube voltage Beam (slice) width (mm) Helical pitch Number of slices/tube rotations

27. IAEA Effect of scan parameters on CTDIvol 18: Optimization of Protection in CT Scanner27 mAs and scan (rotation) time -At a given pitch, CTDIvol increases linearly with mA and time e.g. 2 x mAs= 2x CTDIvol - Given 100 mA 1s, one can double mAs by 200 mA x 1s or 100 mA x 2s)

28. IAEA Effect of scan parameters on CTDIvol 18: Optimization of Protection in CT Scanner28 CTDIvol increases with kV (approx. kV squared) - Variation of dose with kV if other parameters are constant kV relative CTDIvol 80 0.4 120 1.0 140 1.4 Beam width: CTDI increases if beam width wider than nominal imaged width (otherwise remains constant)

29. IAEA Effect of scan parameters on CTDIvol 18: Optimization of Protection in CT Scanner29 For some image noise, dose can increase with decreasing kV kVCTDIvol per unit noise headbody 801.62.6 1101.2 1301.0 e.g. Siemens Emotion 6

30. IAEA Effect of scan parameters on CTDIvol 18: Optimization of Protection in CT Scanner30 Slice width : indirect effect of imaged slice width on dose In theory: halve image slice width means double mAs In practice: have increased contrast so compromise on increase in mAs Scan length: Number of slices (tube rotations) CTDIvol is approx. independent of scan length DLP is directly related to scan length

31. IAEA Pitch Pitch = table travel per rotation nominal slice width Single Slice Multi Slice

32. IAEA Pitch Single Slice CT Table travel = 5 mm per rotation Slice width = 5 mm Pitch = 5/5 =1 Pitch = table travel per rotation nominal slice width

33. IAEA Pitch Multi Slice CT Pitch x = table travel per rotation xray beam width

34. IAEA Pitch Multi Slice CT 20 mm 5 mm 15 mm Pitch x = 20 /15 = 1.33 Pitch d = 20 / 5 = 4

35. IAEA Pitch Pitch = 1, no gaps in helical path Pitch > 1, gaps Pitch < 1, overlap

36. IAEA Helical considerations 18: Optimization of Protection in CT Scanner36 For equivalent scan parameters: helical dose is approx.equal to sequential (axial dose) For constant mAs, CTDIvol is inversely proportional to pitch On single slice scanners, increasing pitch is used for dose reduction)

37. IAEA Multi-slice considerations 18: Optimization of Protection in CT Scanner37 Dose differences between single and multi-slice - Use of pitch for dose reduction - Extent of additional rotations in helical scanning -’over-beaming’-penumbra lies outside active detectors Multi-slice pitch: 200 mAs (200 mA, 1s rotation) Effective mAs or mAs per slice= True mAs/ pitch On MSCT, mAs is often adjusted to keep effective mAs constant Therefore CTDIvol will remain constant with pitch

38. IAEA Multislice: scan protocol 18: Optimization of Protection in CT Scanner38 Greater flexibility of MSCT allows user to - increase scan length - scan more phases in multiphase studies - increase mAs to keep noise down in thin slices

39. IAEA Effect of patient size 18: Optimization of Protection in CT Scanner39 Theory of x-ray CT: HVL in tissue is approx. 4 cm To maintain constant noise, double mAs for extra 4 cm of tissue Adjusting mAs for patient size: - small patients: require lower noise -large patients : higher noise accepted Some manufacturers recommend doubling mAs for - approx. 10 cm in abdomen scans -approx. 13 cm in lung scans Automatic tube current control (mA modulation) (20-50% dose reductions without compromising image quality) -manufacturer’s weight/age based pre-programmed protocol - mAs adjusted for lateral patient dimensions

40. IAEA CH McCallough: Dose optimization in CT: Implementation and clinical acceptance of size based charts (RSNA, 2002) 18: Optimization of Protection in CT Scanner40 Patient width* (cm)Relative mAs > 21-260.4 > 26-310.5 > 31-360.7 > 36-411.0 > 41-461.4 > 24-512.0 * Lateral width based on A-P scout at level of liver

41. IAEA Radiation Protection in Paediatric Radiology L06. Radiation protection in computed tomography 41 Example of successful story. IAEA TECDOC-1621(2009): Dose reduction in CT while maintaining Diagnostic Confidence: A feasibility /Demonstration study Six hospitals (Canada, Greece, India,Poland,Thailand,UK) developed a relationship between image noise and patient weight. Used the relationship to adapt the noise in the image to a pre-selected value. Dose reductions: 25-62% (abdomen CT ) + 12- 79% (Chest CT) Paediatric chest CT

42. IAEA 18: Optimization of protection in CT scanner42 Quality Control Tests In CT Quality control test description on: CT accuracy, uniformity, linearity and noise, Low and high contrast resolution Alignment, Couch travel accuracy Gantry tilt measurement Dosimetry

43. IAEA Summary Scanning parameters should be based on study indication, patient size and body region being scanned Manufacturer protocols should be the starting point. Any adjustments must be done in consultation of radiologist. Image quality in CT is often higher than necessary for diagnostic confidence Implementation of QC programme is important for patient dose management Training of physicians and CT staff can help in management of protocols and patient dose 18: Optimization of Protection in CT Scanner43

44. IAEA THANK YOU! 18: Optimization of Protection in CT Scanner44