A simple manual method to estimate water-equivalent diameter for calculating size-specific dose estimate in chest computed tomography

Abstract

Objectives: The American Association of Physicists in Medicine (AAPM) Task Groups (TG) 204 and 220 introduced a method to estimate patient dose by introducing the Size-Specific Dose Estimate (SSDE). They provided patient size-specific conversion factors that could be applied to volumetric CT Dose Index CTDIvol to estimate patient dose in terms of SSDE based on either effective diameter (Deff) or water equivalent diameter (Dw). Our study presented an alternative method to manually estimate SSDE for the everyday clinical routine chest CT that can be readily used and does not require sophisticated computer programming. Methods: For 16 adult patients undergoing chest CT, the method employed an average relative electron density (ρelung = 0.3) for the lung tissue and a ρetissue of 1.0 for the other tissues to scale the lateral thickness and compute the effective lateral thickness on the patient’s axial image. The proposed method estimated a “corrected” Deff (Deffcorr) to replace Dw and compared results with TG220 and a second method proposed by Huda et al, for the same set of CT studies. Results: The results showed comparable behavior for all methods. There is overall agreement especially between this study and TG220. Largest differences were +13.3% and+15.9% from TG220 and Huda values, respectively. Patient size correlation showed strong correlation with the TG220 and Huda et al methods. Conclusions: A simple, quick manual method to estimate CT patient radiation dose in terms of SSDE was proposed as an alternative where sophisticated computer programming is not available. It can be readily used during any clinical chest CT scanning. Advances in knowledge: The paper is novel as it presents simple, quick manual method to estimate CT patient radiation dose in chest imaging. The process can be used as alternative in cases no sophisticated computer programming is available.