Using body mass index to estimate individualised patient radiation dose in abdominal computed tomography

BACKGROUND: The size-specific dose estimate (SSDE) is a dose-related metrics that incorporates patient size into its calculation. It is usually derived from the volume computed tomography dose index (CTDI(vol)) by applying a conversion factor determined from manually measured anteroposterior and lateral skin-to-skin patient diameters at the midslice level on computed tomography (CT) localiser images, an awkward, time-consuming, and not highly reproducible technique. The objective of this study was to evaluate the potential for the use of body mass index (BMI) as a size-related metrics alternative to the midslice effective diameter (D(E)) to obtain a size-specific dose (SSDE) in abdominal CT. METHODS: In this retrospective study of patients who underwent abdominal CT for the investigation of inflammatory bowel disease, the D(E) was measured on the midslice level on CT-localiser images of each patient. This was correlated with patient BMI and the linear regression equation relating the quantities was calculated. The ratio between the internal and the external abdominal diameters (D(RATIO)) was also measured to assess correlation with radiation dose. Pearson correlation analysis and linear regression models were used. RESULTS: There was good correlation between D(E) and patient BMI (r = 0.88). An equation allowing calculation of D(E) from BMI was calculated by linear regression analysis as follows: D(E) = 0.76 (BMI) + 9.4. A weak correlation between radiation dose and D(RATIO) was demonstrated (r = 0.45). CONCLUSIONS: Patient BMI can be used to accurately estimate D(E), obviating the need to measure anteroposterior and lateral diameters in order to calculate a SSDE for abdominal CT.