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Dose reduction
1) CT dose index (CTDI) measures radiation output of CT scanners. Modern measures include CTDI100, CTDIvol, and dose length product (DLP). 2) Automatic exposure control (AEC) modulates tube current based on patient attenuation to maintain consistent image quality while reducing dose. 3) Dose reduction techniques in CT include AEC, bowtie filters, iterative reconstruction, prospective gating, and dynamic collimation.
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Dose reduction
- 1. Radiation dose modulationand dose reduction techniques
- 2. Introduction to presentation CTDose Index CTDI100 CTDIvol DLP Effective dose CT dose reduction techniques AEC Filter Reconstruction
- 3. CT Dose Index CTdose index (CTDI) is the standardized measure of the radiation output of a CT system, Currently used measures are the CTDI100 and CTDIw, modified to CTDIvol for modern helical scanners.
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- 6. Measurement of CTDI •Ionisation pencil chamber – Air-filled chamber – X-rays cause ionisations in chamber, causing current to flow, proportional to exposure – Instant readings, good accuracy, easy to use 100 mm
- 7. Measurement of CTDI •Single slice measurement (z-axis) Dose T Nominal beam width ( )dzzD T 1 CTDI - ∫ +∞ ∞ = ( )dzzD T 1 CTDI 50 50- 100 ∫ + = -50 +50 ∫ + − = 50 50 D(z)dz n.T 1 CTDI100 n = no. slices imaged simultaneously T = nominal imaged width Davge T 100Davge CTDI100 × =
- 8. CTDI100 • CTDI100 isa measure of radiation dose on a 100mm long pencil ionization chamber. • Centre • Periphery - 1 cm depth (mean of 4 positions) C P1 P2 P3 P4 320 mm ‘Body’ ‘Head’ 160 mm 140 mm
- 9. Measurement of MSAD(MultipleScan Average Dose) CTDI ≈ ‘Series’ dose Multi-scan ‘Series’ dose CTDI× × = I TN MSAD T T is the nominal scan width (mm) I is the distance between scans (mm) I N is the number of scans N × T is the total nominal scan width
- 10. CTDI100 • Typical CTDI100values (mGy) 40 Head Body 40 4040 40 20 2020 20 10 Periphery : centre ≅ 1:1 Periphery : centre ≅ 2:1
- 11. Derivatives of CTDI100 •CTDIW • Weighted average CTDI: represents the average dose for contiguous irradiation • CTDIw = 1/3 CTDI100,C + 2/3 CTDI100,P CTDIC CTDIP
- 12. Derivatives of CTDI100 •CTDI100 & CTDIw refer to a pitch of 1 • Average absorbed dose dependent on pitch Contiguous Extended Overlapping
- 13. CTDIvol • Volume CTDI:Considers contiguous scanning • CTDIvol = CTDIw Pitch Pitch = 1 CTDIvol = CTDIw Pitch = 2 CTDIvol = CTDIw 2 Pitch = 0.5 CTDIvol = 2 x CTDIw
- 14. Radiation risk: DLP •CTDIvol is a measure of absorbed dose, energy absorbed per unit mass • To measure radiation risk from stochastic effects the total energy absorbed must be considered • In CT this can be estimated using the Dose Length Product, DLP
- 15. DLP = CTDIvol. L (mGy.cm) DLP1 ≅ DLP2 where L = scan length Dose length product L1 T Pitch 1 8 rotations Pitch 2 8 rotations L2 T as CTDIvol2 = CTDIvol1 2
- 16. Estimates of effectivedose (E) • Can be obtained from DLP • Effective dose = DLP. CF (mSv) • • • • • • Conversion factors not scanner specific or location specific Region of body Conversion factor,EDLP (mSv mGy-1 cm-1 ) Head & neck 0.0031 Head 0.0021 Neck 0.0059 Chest 0.014 Abdomen& pelvis 0.015 Trunk 0.015
- 17. CT dose reductiontechniques AEC systems have a number of potential advantages, including better control of patient radiation dose, avoidance of photon starvation artifacts, reduced load on the x-ray tube, and the maintenance of image quality in spite of different attenuation values on CT scans
- 18. Types of AEC Patient-SizeAEC Z-axis AEC Rotational or Angular AEC Operation of AEC Systems on Different Multidetector CT Scanners Exposure 3D CARE Dose 4D
- 19. Patient-Size AECPatient sizeAEC: the tube current is adjusted based on the overall size of the patient to reduce the variation in image quality between small patients and large patients. For a given patient size, the appropriate milliamperage is selected and is used for the entire examination or scan series Hi-mA Lo-mA
- 20. Z-axis AEC: tubecurrent is modulated according to patient attenuation along the z- axis.The goal is to reduce the variation in image quality of images from the same series. attenuation
- 21. Patient-Size AECRotational AEC:The tube current is decreased and increased rapidly (modulated) during the course of each rotation to compensate for differences in attenuation between lateral (left-right) and A-P (anterior-posterior) projections . highattenuationlow attenuation
- 22. Combination of AECfunctions • Tube current is adjusted during scanning to compensate for attenuation differences – dose applied to patient only where needed, avoiding dose where it isn’t mA position
- 23. On line’ modulation–usesattenuation data from previous rotation– adapts tube current to patient attenuation ‘on the fly’
- 25. Scan projection radiographs(SPRs, known as scout, scanogram or topogram views) are the main way that AEC systems assess the attenuation of the patient in order to set the tube current
- 26. To obtain correctattenuation data from SPR always centre the patient carefully Patient is positioned in the isocenter – optimal dose and image quality Patient is positioned too high - increased mAsPatient is positioned too low - reduced mAs and increased noise
- 27. Toshiba:defining image quality requirements Specifys.d. level (or ‘image quality level)–patient mAcalculated to achieve this noise level at any scan parameter settings Set min & max mA
- 28. Benefits of CTscanner AEC Consistent image quality Potential for dose reduction through exposure optimisation Reduced tube loading Extended scan runs(OLP) Reduction in photon starvation artifact
- 30. 30 • Images alonglength of phantom (no AEC) • • • • • • • • Constant mA Testing the AEC
- 31. 31 Testing the AEC •Measure noise with AEC off and on • Monitor mA, CTDIvol 0 4 8 12 16 20 24 28 -150 -100 -50 0 50 100 150 Z-position (mm) Noise(%) automA off Noise Index 12 Increased mA Decreased mA Constant mA Same mA Increased mA Decreased mA
- 32. 32 Coronal view Sagittalview z-axis AEC off z-axis AEC on Noise increases Constant noise Testing the AEC – Viewing with MPR
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- 35. without angular mAmodulation with angular mA modulation
- 36. Bowtie Filter Longer bowtiepath lines up with shorter patient path
- 37. Reduce x-ray scatter(noise and artifact) Maintain uniform x-ray at detector Reduce surface dose by 50%
- 38. • Retrospectively gatedCCTA(dose modulation ) ~15 mSv100% 20% ~30% reduction
- 39. Image obtained inmiddiastole (75% of R-R interval) reconstructed using 1.5-mm slice thickness at high tube current shows low-noise Image obtained in midsystole (30% of R-R interval) reconstructed using 1.5-mm slice thickness shows higher-noise image obtained at low radiation dose
- 40. • Prospectively gatedCCTA ~ 1 mSv Prospective Gating of CTA(snapshot plus) 40mm 5mm overlap
- 41. Scan range Conventional technology withoutDose Shield SOMATOM Definition AS+ with Adaptive Dose Shield Scan range Dynamic Collimation • In helical scanning extra rotations are needed at end of imaged volume – Significant extra dose: wide beam widths and short scans • Dynamic collimation - collimator blades open and close asymetrically at start and end of scan
- 42. (a) conventional and(b) adaptive section collimation CT scanning protocols. For adaptive section collimation, shape of x-ray cone beam at beginning and end of spiral acquisition is controlled by two collimators made of absorbent material.
- 43. Image reconstruction(FBP) attenuation detectorposition 90°2 projections4projections8 projections16 projections attenuation detector position 0°
- 44. 0(3) 0(9) 0(7) 0(5) 66 6 6 10 14 12 12 12 12 5 7 5 7 3 9 7 5 3 5 9 7 816 12 12 +1-1 -2+2 Iterative reconstruction
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- 47. 120 kVp and300 mA FBP 120 kVp and 300 mA ASiR 100% 120 kVp and 300 mA ASiR 50%