Estimation of size-specific dose estimate (SSDE) of CT scans using an effective diameter electron density

An assessment of the effective diameter of a patient's body using electron densities of tissues inside the scan area (D_eff^ρe) was proposed to overcome challenges associated with the estimation of water-equivalent diameter (D_w), which is used for size-specific dose estimate (SSDE). The aims of this study were to (1) investigate the D_eff^ρe method in two different forms using a wide range of patient sizes and scanning protocols, and (2) compare between four methods used to estimate the patient size for SSDE.

Under IRB approval, a total of 350 patients of varying sizes have been collected retrospectively from the Hospital. The D_w values were assessed over six different CT body protocols: (1) chest with contrast media, (2) chest High-Resolution Computed Tomography (HRCT) without contrast media, (3) abdomen-pelvis with contrast media, (4) abdomen-pelvis without contrast media, (5) chest-abdomen-pelvis with contrast media, and (6) pelvis without contrast media. A MATLAB-based code was developed in-house to assess the size of each patient using the conventional effective diameter method (D_eff), D_eff^ρe by correcting either both the lateral (LAT) and anterior-posterior (AP) dimensions (D_eff,LAT+AP^ρe) or LAT only (D_eff,LAT^ρe), and D_w at the mid-CT slice of the patient images.

The results of D_eff,LAT+AP^ρe and D_eff,LAT^ρe provided a better estimation for the chest protocols with the averages of absolute percentage difference (PD) values in the range of 3 - 7 % for all patient sizes as compared to the D_w method, whereas the averages of PD values for the D_eff method were 9 - 15 %. However, D_eff gave a better estimation for D_w values for the other body protocols, with differences of 2 - 4 %, which were lower than those obtained with the D_eff,LAT+AP^ρe and D_eff,LAT^ρe methods. For the chest protocols, statistically significant differences were found between D_eff and the other methods, but there were no significant differences between all the methods for the other scanning protocols. The results show that the correction of both dimensions, LAT and AP, did not improve the accuracy of the D_eff^ρe method, and, for most protocols, D_eff,LAT+AP^ρe gave larger range differences compared to those based on correction of the LAT dimension only.

If the D_w cannot be assessed, the D_eff,LAT^ρe method may only be considered for the chest protocols as an alternative approach. The D_eff method may also be used for all regions taking into account the application of a correction factor for the chest protocols to avoid a significant under or overestimation of the patient dose.