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

Abstract

Objectives: To compare the effectiveness of two different patient size metrics based on water equivalent diameter (D_w ), the mid-scan water equivalent diameter D_{w_c} , and the mean (average) water equivalent diameter in the imaged region, D_{w_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). D_{w_c} and D_{w_ave} were calculated from the archived CT images. Size-specific dose estimates (SSDE) were obtained from volume CT dose index (CTDI_vol ), using the conversion factors for a patient diameter of D_{w_c} .

Results: CTDI_vol and SSDE correlate better with D_{w_ave} than with D_{w_c} . R-squared values of linear fits to CTDI_vol of CAP examinations were 0.81-0.89 for D_{w_c} and 0.93-0.94 for D_{w_ave} (SSDE: 0.69-080 for D_{w_c} , 0.87-0.92 for D_{w_ave} ). Percentage differences between D_{w_c} and D_{w_ave} were -4 ± 4% for chest and +5 ± 4% for CAP examinations (in % of D_{w_ave} ). However, small D_w variations translated as larger variations in CTDI_vol for these ATCM systems (e.g., a 24% increase in D_w doubled CTDI_vol ). The dependence of CTDI_vol on D_{w_ave} was similar for chest and CAP examinations performed with similar ATCM parameters, while use of D_{w_c} resulted in a clear separation of the same data according to examination type. Maximum D_w variation in the imaged region was 5.6 ± 1.6 cm for chest and 6.5 ± 1.4 cm for CAP examinations.

Conclusions: D_{w_ave} is a better metric than D_{w_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 D_w for SSDE calculations, automatic calculation of D_{w_ave} should be considered.

Keywords: computed tomography; examination doses; patient size metrics; water equivalent diameter.