Virtual Unenhanced Image in Abdominal Single Source Rapid Kilovoltage Switching CT Scan: Is It Comparable to True Unenhanced Image?

Abstract

Objective: To compare the attenuation values of intra-abdominal organs obtained from virtual unenhanced (VUE) images and true unenhanced (TUE) images using a single source dual-energy computed tomography (DECT) with rapid kV switching and evaluate the visibility of stones on VUE images. Radiation dose of both VUE and TUE were reported. Materials and Methods: A retrospective cross-sectional study included 95 patients that underwent abdominal CT scan using single source DECT with rapid kV switching technique and had both TUE phase and post-contrast portovenous phase between May and November 2019. Attenuation of the abdominal organs, size, and attenuation of calyceal stones and gallstones observed on VUE and TUE were recorded. Paired t-test was calculated for comparison. Dose length product (DLP) and effective dose were also recorded. Results: Statistically significant difference in attenuation values between VUE and TUE images were observed for liver, spleen, left adrenal gland, kidneys, aorta, inferior vena cava (IVC), main portal vein, and subcutaneous fat. No significant difference was observed for right adrenal gland (p=0.482) and paravertebral muscle (p=0.891). The mean attenuation values of the organs obtained from VUE images tended to be lower and per-case analysis showed 1.1% to 35% of cases with more than 10 HU difference. Calyceal stones and gallstones also showed smaller size and decreased attenuation on VUE images. VUE showed 75.8% and 97.4% for calyceal stones and radiopaque gallstones, respectively. By omitting TUE phase, the effective dose would be decreased by 53.2% and 53.8% for routine non-contrast and post-contrast upper abdominal and whole abdomen CT scan. Conclusion: There were significant differences in attenuation values of abdominal organs and less sensitivity for stones detection on VUE images. Although replacing TUE with VUE could reduce radiation, some disease may be missed or misinterpreted due to erroneous attenuation values. The use of VUE may be appropriate in follow-up or with prior imaging. Keywords: Dual energy CT, Virtual unenhanced, True unenhanced, Attenuation