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Computed Tomography Dose Index (CTDI) Effective Dose Why is CT dose important?

CT Radiation Dose: Units and Measurement Methods Walter Huda PhD SUNY Upstate Medical University, Syracuse NY. Computed Tomography Dose Index (CTDI) Effective Dose Why is CT dose important?. Conventional radiography. x-ray skin dose 2 mGy/0.2 rad. Body CT scan. Head CT scan.

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Computed Tomography Dose Index (CTDI) Effective Dose Why is CT dose important?

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  1. CT Radiation Dose: Units and Measurement MethodsWalter Huda PhDSUNY Upstate Medical University, Syracuse NY

  2. Computed Tomography • Dose Index (CTDI) • Effective Dose • Why is CT dose • important?

  3. Conventional radiography x-ray skin dose 2 mGy/0.2 rad

  4. Body CT scan Head CT scan

  5. Cataracts Deterministic effect

  6. Carcinogenesis Stochastic effect

  7. In CT, CTDI is "radiation concentration" Effective dose is the "total radiation"

  8. Dose distributions in CT measured in phantoms (center & periphery)

  9. 16 cm and • 32 cm • diameter • Acrylic • phantoms • = 1.19 Z = 6.56 100 mm ionization chamber

  10. Computed Tomography Dose Index (CTDI) CTDI = 1/h  D(z) dz

  11. Integration limits for  D(z) dz  7T for CTDIFDA  50 mm for CTDI100 CTDI100 = (f X L) /T f is exposure to dose factor X is measured exposure L is ionization chamber length T is slice thickness

  12. CTDIw (weighted)CTDIw =(1/3 CTDI100)center + (2/3 CTDI100)periphery

  13. CTDI100 is Proportional to mAsApproximately independent of slice thickness

  14. CTDIw (mGy) 300 mAs/10 mm

  15. Helical scanning & dose

  16. Pitch of 1.0 has dose  axial scan Pitch of 0.75 has 33% more dose Pitch of 1.5 has 66% less dose

  17. Volume CTDIvol CTDIvol = CTDIw/Pitch Pitch defined as: Table distance traveled in 360 rotation  Total collimated width of x-ray beam

  18. Multi-slice CT ‘Overbeaming’ caused by wider collimator settings to avoid penumbral effects;

  19. Body CTWIw values (300 mAs/120 kV)

  20. Contiguous CT imaging

  21. Dose-length product (DLP)DLP = CTDIvol x Scan LengthDLP has units of mGy-cmGiven on console of some CTsCan be used to obtain effective dose E

  22. CTDI is independent of section thickness T & number of sections N Patient risk does depend on section thickness T & number of sections N

  23. Dose (risk) in CTis best measured by effective dose (E) E = i wi x Di

  24. Sensitive organs (wi = 0.12)Red bone marrowColonLungStomach

  25. Moderately sensitive organs (wi = 0.05)BladderBreastLiverEsophagusThyroid

  26. Organ doses can be measured in phantoms, or computed using MC

  27. Effective doses can be estimated using DLP conversion factors mSv/mGy-cm Nagel HD: Radiation Exposure in CT (2001) cocir@zvei.org

  28. Model patient as cylinder of waterDetermine energy impartedConvert energy imparted into effective dose Huda et al, Radiology 1997; 203:417-422

  29. Head CT scan ~1 mSv 120 kVp 340 mAs 18 x 7 mm

  30. Chest CT ~5 mSv 120 kVp 280 mAs 43 x 7 mm

  31. Abdominal CT scan ~4 mSv 120 kVp 280 mAs 32 x 7 mm

  32. Doses from CT are a major concern

  33. Frequency of CT examinations Germany 1994

  34. CT contribution to collective medical dose Germany 1994

  35. In US HospitalsCT ~10% of examsCT contributes 2/3rd of patient doseFred Mettler – 2000 study in New Mexico

  36. Individual and collective doses will continue to increase

  37. Dose is related to image quality

  38. High dose scan Low dose scan

  39. Doses should be As Low As Reasonably Achievable (ALARA) 120 kVp 5 mm 206 mAs

  40. 120 kVp 5 mm 43mAs C Small gas inclusion D ureter opacified c contrast A Metastasis in vertebral body B Subcutaneous gluteal metastatis

  41. Conclusions The Beatles Godfrey Hounsfield

  42. CTDI parametersquantify CT scanner dose characteristics,not patient dosesCTDIw & CTDIvol

  43. Stochastic risks are important in CTDeterministic riskshould not occurDoses should be keptALARA

  44. Effective doses in CT~1 mSv for head~5 mSv for bodyCT doses dominatemedical exposures

  45. Radiation Dose in CTMF McNitt-GrayRadiographics (2002) 22:1541-1553

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