Feasibility of Low-Tube-Voltage Excretory Phase Images During CT Urography: Assessment Using a Dual-Energy CT Scanner

Abstract

The purpose of this study is to assess the feasibility of low-tube-voltage images during excretory phase CT urography. In this retrospective study, we examined 70 consecutive CT urograms (35 men and 35 women; mean age, 58.5 years) performed on a dual-energy CT scanner and compared excretory phase images obtained at 80 kVp and 340 mAs with blended images (0.3 × 140 kVp and 80 mAs; and 0.7 × 80 kVp and 340 mAs). Quantitative measurements of urinary system opacification (Hounsfield units), image noise (Hounsfield units), and effective dose (millisieverts) were compared using Student paired t test. Image noise was correlated with patient thickness. Two independent blinded readers qualitatively assessed opacification, image quality (both compared using Wilcoxon test), overall acceptability (compared using McNemar test), and detectability of urinary and extraurinary findings. The 80-kVp images yielded significantly higher opacification of renal pelvis (p < 0.0001), ureter (p < 0.0001), bladder (p < 0.0001), and aorta (p < 0.0001); higher image noise (p < 0.0001); and lower radiation dose (5.2 vs 11.9 mSv). Image noise increased along with increasing patient thickness (r = 0.86 for 80-kVp images). Qualitative opacification scores were better only in the bladder on 80-kVp images (p = 0.002). Although 80-kVp image quality was lower (p < 0.0001), the overall acceptability was similar. Of 42 urinary findings, 40 were detected on 80-kVp images (< 2-mm calyceal calculus and tiny foci of collecting system gas were missed in one patient each, both large patients). Of 137 extraurinary findings, 130 were detected on 80-kVp images (no findings of high clinical significance were missed). Low tube voltage (80 kVp) during excretory phase CT urography is feasible, with improved urinary system opacification, acceptable image quality, and lower radiation dose.