MO‐D‐134‐05: Size Specific Dose Estimation in Abdominal CT: Impact of Longitudinal Variations in Patient Size

Purpose: Size specific dose estimates (SSDE) represent the mean dose to a patient of a specific size. However, patient size can vary significantly within the scan range, affecting the choice of conversion factor and, with tube current modulation, CTDIvol. The purpose of this study was to evaluate the impact of patient size variations in the abdomen on SSDE. Methods: Images from 100 consecutive abdominal CT exams (top of liver to top of crest) were evaluated. A Matlab program was used to automatically remove the patient table and calculate water equivalent diameter (Dw). SSDE was calculated in two ways and compared: 1) The mean CTDIvol over the scan range was used with the conversion factor corresponding to Dw at the middle of the scan range. 2) The mean mAs(z) and the CTDIvol/mAs were used to calculate the CTDIvol(z); Dw(z) used to determine the conversion factor(z); SSDE(z) and mean SSDE over the scan range calculated, where z=longitudinal position. Results: A range of body sizes was included, with BMI ranging from 14.9 to 41.5. However, the standard deviation of Dw within each patients' scan range was on average only 1.3cm (range: 0.4–2.9cm), indicating a high degree of uniformity in patient size over the abdomen. The difference between the two groups of SSDE values ranged from −0.3 to 1.9mGy, with root mean square difference of 0.3 mGy. A paired t‐test showed no significant difference between these two groups of SSDE values (p<0.001). Conclusion: For abdominal CT, there was no significant difference between the mean SSDE values calculated using 1) the mean CTDIvol over the scan range and Dw at the middle of the scan range and 2) the CTDIvol and Dw at each position in the scan range. In body regions with greater longitudinal variation in attenuation, this may not be the case. This work was supported in part by NIH grant R01 EB071095 from the National Institute of Biomedical Imaging and Bioengineering.