Comparison of true unenhanced and virtual unenhanced (VUE) attenuation values in abdominopelvic single source rapid kilovoltage switching spectral CT
ObjectiveTo 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.MethodsNinety 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.ResultsThere 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).ConclusionAlthough 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.
Abdominal Radiology