Dose reference level based on size-specific dose estimate (SSDE) and feasibility of deriving effective body diameter using tube current and time product (mAs) for adult chest and abdomen computed tomography (CT) procedures

This study aimed to establish dose reference level (NDRL $_\mathrm$ ) based on size-specific dose estimate (SSDE) derived using effective diameter ( $D_\mathrm$ ) for adult chest and abdomen computed tomography (CT) procedures and to explore the feasibility of driving $D_\mathrm$ using the product of tube current and time (mAs). In this retrospective study, dose data, scan parameters and patient body dimensions at the mid-slice level from 14 CT units (out of 63 total) were extracted. Additionally, the mAs values of the axial slice at the same z-location where the diameter measurements were made (mAsz) were recorded. Pearson's correlation (r) analysis was used to determine the relationship of $D_\mathrm$ with patient BMI, weight, and mAsz. The NDRL $_\mathrm$ for the chest and abdomen were 9.72 mGy and 13.4 mGy, respectively. The BMI and body weight were less correlated (r = 0.24 and r = 0.33, respectively) with $D_\mathrm$ . The correlation between mAsz and $D_\mathrm$ was considerably strong (r = 0.78) and can be used to predict $D_\mathrm$ accurately. The absolute dose differences between SSDEs calculated using the AAPM–204 method and mAsz was less than 1.1 mGy (15%). Therefore, mAsz is an efficient parameter to derive $D_\mathrm$ . Further, the direct conversion factors to estimate SSDEs at different locations along the z-direction in the scan region from corresponding mAs and CTDI $_\mathrm$ were calculated. The NDRL $_\mathrm$ suggested in the present study can be used as a reference for size-dependent dose optimisation in Sri Lanka, and existing NDRL based on CTDI $_$ underestimate the average adult CT dose by 36.0% and 39.7% for chest and abdomen regions respectively. The results show that using mAsz to determine SSDE is a simple and practical approach with an accuracy of 95% and 85% for abdomen and chest scans, respectively. However, the obtained linear relationship between $D_\mathrm$ and mAs is highly dependent on the ATCM technique and the user-determined noise levels of the scanning protocol. Finally, the phantom study resulted in the strongest correlation (r = 0.99) between the D $^z_$ and mAsz, and the prediction of patient size would be more precise than $D_\mathrm$ method.