SU‐E‐I‐37: Lesion Circularity as a Potential Indicator for Detectability in Low Dose Abdomen CT

Abstract

Purpose: Low‐contrast lesion detection is critical for abdomen CT. However, there is no standardized, objective measure of the detectability of low‐contrast objects. The purpose of this study is to investigate a new measure, the ability of a circular lesion to maintain its shape or circularity, as a potential indicator of lesion detectability. Methods: An anthropomorphic phantom with embedded spherical liver lesions was custom‐designed to assess the effectiveness of iterative reconstruction for improving the low contrast lesion detection. This phantom consists of 36 spherical lesions with 3 different sizes (5mm, 10mm, and 15mm). Each size has three different contrast levels. All spherical lesions were embedded non‐uniformly to minimize the memory bias for a concurrent human observer study. Phantom was scanned with a MDCT (Siemens Definition Flash) at multiple dose levels and images were reconstructed with both filtered back‐projection (FBP) and Iterative Reconstruction (SAFIRE). An adaptive Wiener filter was applied during the lesion detection. The circularity was calculated based on the ratio of the area to (perimeter)2 of each detected lesion. Results: The circularity was normalized to 1 when the detected lesion has a perfectly circular shape and the boundary is connected. At 50mAs, the circularity for a 10mm spherical lesion with 18HU contrast was 0.618 with FBP vs. 0.650 with SAFIRE; While at the same mAs, for a 15mm spherical lesion with 12HU contrast, the circularity was 0.666 with FBP vs. 0.763 for SAFIRE . At 200mAs, the circularity for a 10mm spherical lesion was 0.811with FBP vs. 0.870 with SAFIRE ; for a 15mm lesion, the circularity was 0.818 with FBP vs. 0.863 with SAFIRE. Conclusion: Circularity decreased with decreasing lesion size, contrast differential and dose. Iterative reconstruction techniques improved upon lesion circularity when compared to FBP at the same dose level, at similar lesion size and density differences. Dr. Herts and Dr. Baker are currently funded by Siemens Healthcare for research in low dose abdominal CT.