1. What are the dose quantities in CT ? IAEA/RCA Kampala Kampala

2. 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 CT Principles Kampala

4. Reminder : CT terminology Axial scanning –1 rotation of beam followed by stepwise couch movement –Increment – distance moved by couch Helical/spiral scanning –Continuous table movement & beam rotation –Pitch – table movement relative to nominal slice width Kampala

5. CT Slice Kampala

6. CT Slice Kampala

7. CT Slice Kampala

8. Dose Profile Kampala

9. Dose profile for a series of slices Kampala

10. Dose Profiles in Phantoms / Patients Effects of scatter Broadened profile Peak kerma dependent on beam width Long scatter tails for wide beams Kampala

11. Which Dosemeter ? Kampala

12. Integrated Profile Kampala

13. Integrated Profile Kampala

14. CTDose Index Measure of dose for a single axial slice (mGy) –no account of Helical/MDCT gaps or overlaps CTDI = area under dose profile (mGy.mm) nominal slice width, T (mm) Kampala kerm a T CTDI (mGy) Z-axis (mm) CTDI = height of rectangle if area under profile was all contained within nominal slice width

15. CTDI 100 – What is it ? Analagous to Multiple Scan Average Dose. Originally defined for single slice scanners. Kampala

16. In air : CTDI 100,Air Sometimes normalise to 1 mAs: n CTDI 100,Air Just like in radiography! Kampala

17. In Perspex / PMMA Kampala

18. CT dosimetry in Perspex The computed tomography air kerma index is also measured inside PMMA head and body phantoms The quantity CTDI 100,c is measured at the centre of the standard CT dosimetry phantom and CTDI 100,p is the average of values measured at four positions around the periphery of the same phantom. Kampala

19. CTDI 100 CTDI can be measured in air or in phantoms CTDI in air can be used as a measure of tube output For dosimetry purposes CTDI is measured in perspex phantoms 16cm diameter ‘head’ 32cm diameter ‘body’ Kampala

20. Patient Size. CTDI is a measure of scanner output. CTDI does take patient size into account CTDI is NOT patient dose Important for paediatrics, small adults and possibly bariatrics. You can correct to patient size –To be mentioned later. Kampala

21. Phantoms for CT dosimetry Cylindrical PMMA phantoms with holes for pencil chamber 32 cm body phantom 16 cm head phantom Kampala

22. Centre Periphery – 1cm depth (mean of 4 positions) C P1P1 P2P2 P3P3 P4P4 32 cm ‘Head’ ‘Body’ 16 cm 140 mm CTDI Phantoms Kampala

23. 40 ‘Head’ ‘Body’ 40 20 10 Periphery : centre  1:1 Periphery : centre  2:1 Typical CTDI 100 Values CTDI 100,p values quoted in mGy Kampala

24. CTDI W Weighted average CTDI: represents the average dose for contiguous irradiation CTDI w = 1/3 CTDI centre + 2/3 CTDI periphery n CTDI w = CTDI w /mAs CTDI C CTDI P Kampala

25. CTDI vol Pitch = 1 CTDI vol = CTDI w Pitch = 2 CTDI vol = CTDI w 2 Pitch = 0.5 CTDI vol = 2 x CTDI w Kampala

26. CTDI vol CTDI W is the average dose in the scan plane Volume CTDI considers non-contiguous scanning The CTDI vol can be considered to be the average CTDI W in the scan volume CTDI vol = CTDI W Pitch Kampala

27. Dose Length Product CTDI vol is a measure of absorbed dose, energy absorbed per unit mass Radiation risk is based on total energy absorbed Estimate using dose length product (DLP) DLP = CTDI vol x scan length Units are mGy cm Kampala

28. Dose Quantities in CT Kampala

29. Dose Summary Sheet - Commercial CT Scanner Huda W, Mettler F A Radiology 2011;258:236-242 Kampala

30. CTDI on the Console The CTDI value displayed will depend on the phantom that it is based on. Body : 32 cm; Head 16 cm. CTDI for 16cm reference twice that of CTDI for 32cm reference. Paediatric protocol MAY reference to 16 cm phantom Implementation manufacturer dependent. Kampala

31. CTDI MEASUREMENT IN PRACTICE Kampala

32. Equipment for CT measurements CT ionization chamber and electrometer calibrated for CT beam qualities Chamber support stand CT head & body phantoms Kampala

33. Methodology for CT measurements in air Position chamber in stand so beyond couch Adjust couch position so chamber along axis of scanner and beam centred at central point of chamber Record dose measurements for single axial scan at selected parameters (no couch movement) Record all data Kampala

34. Methodology for CT measurements in phantoms Position phantom on couch (use head rest if appropriate) Position chamber in central position in phantom Adjust couch position so chamber centred as before Record dose measurements for single axial scan at selected parameters (no couch movement) Repeat for peripheral positions (north, south, east & west) Kampala