Automatic calculation of patient size metrics in computed tomography: What level of computational accuracy do we need?

OBJECTIVES To compare the effectiveness of two different patient size metrics based on water equivalent diameter (Dw ), the mid-scan water equivalent diameter Dw_c , and the mean (average) water equivalent diameter in the imaged region, Dw_ave , for automatic detection of accidental changes in computed tomography (CT) acquisition protocols. METHODS Patient biometric data (height and weight) were available from a previous survey for 80 adult chest examinations, and 119 adult single-acquisition chest-abdomen-pelvis (CAP) examinations for two 16 slice scanners (GE LightSpeed and Toshiba Aquilion RXL) equipped with automatic tube current modulation (ATCM). Dw_c and Dw_ave were calculated from the archived CT images. Size-specific dose estimates (SSDE) were obtained from volume CT dose index (CTDIvol ), using the conversion factors for a patient diameter of Dw_c . RESULTS CTDIvol and SSDE correlate better with Dw_ave than with Dw_c . R-squared values of linear fits to CTDIvol of CAP examinations were 0.81-0.89 for Dw_c and 0.93-0.94 for Dw_ave (SSDE: 0.69-080 for Dw_c , 0.87-0.92 for Dw_ave ). Percentage differences between Dw_c and Dw_ave were -4 ± 4% for chest and +5 ± 4% for CAP examinations (in % of Dw_ave ). However, small Dw variations translated as larger variations in CTDIvol for these ATCM systems (e.g., a 24% increase in Dw doubled CTDIvol ). The dependence of CTDIvol on Dw_ave was similar for chest and CAP examinations performed with similar ATCM parameters, while use of Dw_c resulted in a clear separation of the same data according to examination type. Maximum Dw variation in the imaged region was 5.6 ± 1.6 cm for chest and 6.5 ± 1.4 cm for CAP examinations. CONCLUSIONS Dw_ave is a better metric than Dw_c for binning similar-sized patients in dose comparison studies, despite the additional computational effort required for its calculation Therefore, when implementing automatic determination of Dw for SSDE calculations, automatic calculation of Dw_ave should be considered.