Comparison of true unenhanced and virtual unenhanced (VUE) attenuation values in abdominopelvic single-source rapid kilovoltage-switching spectral CT

Objective: To assess the agreement between the true non-contrast (TNC) attenuation values of intra-abdominal structures and attenuation values obtained on virtual-unenhanced (VUE) images based on rapid kVp-switching dual-energy CT. The effects of contrast phase and patient characteristics (e.g., BMI, hematocrit, hemoglobin content) on VUE values were also investigated. Methods: Ninety four patients who underwent triphasic abdominal CT (liver mass protocol, n = 47; pancreas mass protocol, n = 47) between August 2014 and May 2015 were retrospectively reviewed. Unenhanced series was performed using conventional single-energy mode at 120 kVp. Late arterial and venous phase post-contrast series were obtained utilizing rapid kVp-switching dual-energy CT technique. VUE images were processed off of arterial (VUE-art) and venous (VUE-ven) phase series. Attenuation values of liver, pancreas, kidneys, adrenal glands, muscle, subcutaneous fat, aorta, IVC, and main portal vein were recorded on TNC and VUE sets of images. Attenuation values were compared using univariate linear regression and Student two-tailed paired t test. Results: There was excellent correlation between TNC, VUE-art, and VUE-ven attenuation values across all organs (p < 0.0001). Paired Student t test, however, showed significant difference between TNC and VUE-art attenuation of kidneys, right adrenal gland, paraspinal muscle, and aorta. There was also significant difference between TNC and VUE-ven attenuation of left kidney. Percentage of cases which had > 10 HU difference between VUE and TNC for an individual was calculated which ranged between 13% (right kidney) and 42% (right adrenal gland). Conclusion: Although the correlation between VUE and TNC attenuation values was excellent and mean difference between TNC and VUE attenuation values was negligible (ranging between -5.94 HU for paraspinal muscles to 6.2 HU in aorta), intra-patient analysis showed a considerable number of cases which had > 10 HU difference between VUE and TNC. VUE-ven generally offered a better approximation of TNC values. Further optimization of post-processing algorithms might be necessary before complete replacement of TNC with VUE images.