Size-specific dose estimates for adult patients at CT of the torso

Purpose: To determine relationships among patient size, scanner radiation output, and size-specific dose estimates (SSDEs) for adults who underwent computed tomography (CT) of the torso. Materials and Methods: Informed consent was waived for this institutional review board-approved study of existing data from 545 adult patients (322 men, 223 women) who underwent clinically indicated CT of the torso between April 1, 2007, and May 13, 2007. Automatic exposure control was used to adjust scanner output for each patient according to the measured CT attenuation. The volume CT dose index (CTDI_vol) was used with measurements of patient size (anterioposterior plus lateral dimensions) and the conversion factors from the American Association of Physicists in Medicine Report 204 to determine SSDE. Linear regression models were used to assess the dependence of CTDI_vol and SSDE on patient size. Results: Patient sizes ranged from 42 to 84 cm. In this range, CTDI_vol was significantly correlated with size (slope = 0.34 mGy/cm; 95% confidence interval [CI]: 0.31, 0.37 mGy/cm; R² = 0.48; P < .001), but SSDE was independent of size (slope = 0.02 mGy/cm; 95% CI: -0.02, 0.07 mGy/cm; R ² = 0.003; P = .3). These R² values indicated that patient size explained 48% of the observed variability in CTDI_volbut less than 1% of the observed variability in SSDE. The regression of CTDI _vol versus patient size demonstrated that, in the 42-84-cm range, CTDI_vol varied from 12 to 26 mGy. However, use of the evaluated automatic exposure control system to adjust scanner output for patient size resulted in SSDE values that were independent of size. Conclusion: For the evaluated automatic exposure control system, CTDI_vol (scanner output) increased linearly with patient size; however, patient dose (as indicated by SSDE) was independent of size.