CT Radiation Dose Units and Measurement Methods

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 Head CT scan Body CT scan Cataracts Deterministic effect Carcinogenesis Stochastic effect In CT, CTDI is "radiation concentration" Effective dose is the "total radiation" Dose distributions in CT measured in phantoms (center & periphery) 16 cm and 32 cm diameter Acrylic phantoms = 1.19 Z = 6.56 100 mm ionization chamber Computed Tomography Dose Index (CTDI) CTDI = 1/h  D(z) dz 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 CTDIw (weighted) CTDIw = (1/3 CTDI100)center + (2/3 CTDI100)periphery CTDI100 is Proportional to mAs Approximately independent of slice thickness CTDIw (mGy) kV 80 120 140 Head 14 40 55 Body 5.8 18 25 300 mAs/10 mm Helical scanning & dose Pitch of 1.0 has dose  axial scan Pitch of 0.75 has 33% more dose Pitch of 1.5 has 66% less dose Volume CTDIvol CTDIvol = CTDIw/Pitch Pitch defined as: Table distance traveled in 360 rotation  Total collimated width of x-ray beam Multi-slice CT ‘Overbeaming’ caused by wider collimator settings to avoid penumbral effects; Body CTWIw values (300 mAs/120 kV) Collimation 4 x 1.25 4 x 2.5 4x5 Beam width (mm) 5 10 20 CTDIw mGy 33 24 20 Contiguous CT imaging Dose-length product (DLP) DLP = CTDIvol x Scan Length DLP has units of mGy-cm Given on console of some CTs Can be used to obtain effective dose E CTDI is independent of section thickness T & number of sections N Patient risk does depend on section thickness T & number of sections N Dose (risk) in CT is best measured by effective dose (E) E = i wi x Di Sensitive organs (wi = 0.12) Red bone marrow Colon Lung Stomach Moderately sensitive organs (wi = 0.05) Bladder Breast Liver Esophagus Thyroid Organ doses can be measured in phantoms, or computed using MC Effective doses can be estimated using DLP conversion factors mSv/mGy-cm Nagel HD: Radiation Exposure in CT (2001) cocir@zvei.org Model patient as cylinder of water Determine energy imparted Convert energy imparted into effective dose Huda et al, Radiology 1997; 203:417-422 Head CT scan ~1 mSv 120 kVp 340 mAs 18 x 7 mm Chest CT ~5 mSv 120 kVp 280 mAs 43 x 7 mm Abdominal CT scan ~4 mSv 120 kVp 280 mAs 32 x 7 mm Doses from CT are a major concern Frequency of CT examinations Germany 1994 CT contribution to collective medical dose Germany 1994 In US Hospitals CT ~10% of exams CT contributes patient dose rd 2/3 of Fred Mettler – 2000 study in New Mexico Individual and collective doses will continue to increase Dose is related to image quality High dose scan Low dose scan Doses should be As Low As Reasonably Achievable (ALARA) 120 kVp 5 mm 206 mAs 120 kVp 5 mm 43mAs A Metastasis in vertebral body B Subcutaneous gluteal metastatis C Small gas inclusion D ureter opacified c contrast Conclusions Godfrey Hounsfield The Beatles CTDI parameters quantify CT scanner dose characteristics, not patient doses CTDIw & CTDIvol Stochastic risks are important in CT Deterministic risk should not occur Doses should be kept ALARA Effective doses in CT ~1 mSv for head ~5 mSv for body CT doses dominate medical exposures Radiation Dose in CT MF McNitt-Gray Radiographics (2002) 22:1541-1553