An improved method for automated calculation of the water-equivalent diameter for estimating size-specific dose in CT.

Purpose: The aim of this study is to propose an algorithm for the automated calculation of water-equivalent diameter (D ) and size-specific dose estimation (SSDE) from clinical computed tomography (CT) images containing one or more substantial body part.

Methods: All CT datasets were retrospectively acquired by the Toshiba Aquilion 128 CT scanner. The proposed algorithm consisted of a contouring stage for the D calculation, carried out by taking the six largest objects in the cross-sectional image of the patient's body, followed by the removal of the CT table depending on the center position (y-axis) of each object. Validation of the proposed algorithm used images of patients who had undergone chest examination with both arms raised up, one arm placed down and both arms placed down, images of the pelvic region consisting of one substantial object, and images of the lower extremities consisting of two separated areas.

Results: The proposed algorithm gave the same results for D and SSDE as the previous algorithm when images consisted of one substantial body part. However, when images consisted of more than one substantial body part, the new algorithm was able to detect all parts of the patient within the image. The D values from the proposed algorithm were 9.5%, 15.4%, and 39.6% greater than the previous algorithm for the chest region with one arm placed down, both arms placed down, and images with two legs, respectively. The SSDE values from the proposed algorithm were 8.2%, 11.2%, and 20.6% lower than the previous algorithm for the same images, respectively.

Conclusions: We have presented an improved algorithm for automated calculation of D and SSDE. The proposed algorithm is more general and gives accurate results for both D and SSDE whether the CT images contain one or more than one substantial body part.