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.