Truncation Effects on Effective Diameter Measurement for Size-specific Dose Estimation in Computed Tomography (CT) Imaging

Abstract

The determination of size specific dose estimation (SSDE) has been proposed for accurate CT dose assessment. The SSDE introduction has prompted numerous manufacturers and researchers to develop various methodologies that have progressed from manual to automated methods in SSDE determination. Few studies reported on truncation effects with respect to specific size and SSDE determination and CT manufacturers are yet to incorporate truncation correction in their system [1,2].
 This study aimed to evaluate the effects of truncation artifacts on the measurement of effective diameter (Deff) and SSDE in CT imaging. A phantom study was performed using CTDI phantom of different diameters, 22 and 32 cm with Siemens SOMATOM Definition AS+ CT scanner. Phantom images of different truncation percentage (TP) of 5%, 10%, 15%, and 20% were simulated and acquired. The Deff and SSDE were determined based on the displayed CTDIvol, and the AAPM correction factor table was used to calculate the SSDE accurately [3]. The percentage error for each TP in comparison with non-truncated image (TP 0%) was computed.
 Results showed the percentage error (PE) for both Deff and SSDE increased as the TP increased. The PE was determined by comparing the Deff and SSDE of each TP with non-truncated (TP 0%) image. Table 1 shows the values of PE of truncated images of different TP (5%, 10%, 15%, and 20%) for both Deff and SSDE in phantom sizes of 22 and 32 cm. However, it can be observed that PE values for SSDE of 5% and 10% truncation were similar for both phantom sizes. This is because the correction factors for 5% and 10% were similar and had resulted in similar calculated SSDE. The highest recorded PE was observed in 20% truncation that determined the largest difference in Deff and SSDE with non-truncated image.
 This study demonstrates that there is a linear relationship between truncation percentage with measured Deff and SSDE computation. It can be concluded that the percentage error in Deff measurement and calculated SSDE will increase as TP increased. Therefore, the effects of truncation artefacts should be carefully considered in SSDE calculation for accurate estimation especially in development of automated calculation.