For 545 adult patients who underwent CT examinations of the torso and who varied in size by a factor of two, the automatic exposure control system used increased CTDIvol from 12 to 26 mGy, but average estimated patient dose was 22 mGy ±?3, independent of size.
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 (CTDIvol) 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 CTDIvol and SSDE on patient size.
Patient sizes ranged from 42 to 84 cm. In this range,CTDIvol 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; R2 = 0.003; P = .3). These R2 values indicated that patient size explained 48% of the observed variability in CTDIvol but less than 1% of the observed variability in SSDE. The regression of CTDIvol versus patient size demonstrated that, in the 42–84-cm range, CTDIvol 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.
For the evaluated automatic exposure control system,CTDIvol (scanner output) increased linearly with patient size; however, patient dose (as indicated by SSDE) was independent of size.
© RSNA, 2012
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Article HistoryReceived November 7, 2011; revision requested December 19; revision received May 15, 2012; accepted June 8; final version accepted June 25.
Published online: Dec 2012
Published in print: Dec 2012