SU-F-I-33: Estimating Radiation Dose in Abdominal Fat Quantitative CT.

PURPOSE: To compare size-specific dose estimate (SSDE) in abdominal fat quantitative CT with another dose estimate Dsize,L that also takes into account scan length.

METHODS: This study complied with the requirements of the Health Insurance Portability and Accountability Act. At our institution, abdominal fat CT is performed with scan length = 1 cm and CTDIvol = 4.66 mGy (referenced to body CTDI phantom). A previously developed CT simulation program was used to simulate single rotation axial scans of 6-55 cm diameter water cylinders, and dose integral of the longitudinal dose profile over the central 1 cm length was used to predict the dose at the center of one-cm scan range. SSDE and Dsize,L were assessed for 182 consecutive abdominal fat CT examinations with mean water-equivalent diameter (WED) of 27.8 cm ± 6.0 (range, 17.9 - 42.2 cm). Patient age ranged from 18 to 75 years, and weight ranged from 39 to 163 kg.

RESULTS: Mean SSDE was 6.37 mGy ± 1.33 (range, 3.67-8.95 mGy); mean Dsize,L was 2.99 mGy ± 0.85 (range, 1.48 - 4.88 mGy); and mean Dsize,L /SSDE ratio was 0.46 ± 0.04 (range, 0.40 - 0.55).

CONCLUSION: The conversion factors for size-specific dose estimate in AAPM Report No. 204 were generated using 15 - 30 cm scan lengths. One needs to be cautious in applying SSDE to small length CT scans. For abdominal fat CT, SSDE was 80-150% higher than the dose of 1 cm scan length.