Reduced Dose Computed Tomography: The Effects of Voltage Reduction on Density Measurements of Urolithiasis.

Abstract

CT is a widely used imaging modality in the diagnosis of urolithiasis but subjects patients to ionizing radiation. Reduced dose protocols have recently gained wide acceptance. Stone density measurements have been used to predict composition and help guide treatment in standard dose CT (sCT) but not in reduced dose CT (RdCT). We aimed to compare density measurements obtained through RdCT vs sCT and to determine if there is a correlation between stone composition. A total of 201 patients undergoing evaluation for renal colic were prospectively recruited, whereby each subject underwent an sCT (120 kVp) followed immediately by an RdCT (80 or 100 kVp). All calculi <5 mm were excluded to prevent volume averaging and distortion to Hounsfield units measurements. The iliac bone cortex was utilized as an internal control. We also collected stone compositional analyses. In total 36 stones were identified. When the same calculi were evaluated with 120 kVp vs 80 kVp, the RdCT revealed a higher HU (n = 18, 1214 ± 520 vs 1007 ± 307, p < 0.005). Statistical difference was not achieved when 120 kVp was compared with 100 kVp scans (p = 0.151). The percentage differences were 10.6 ± 21 and 1.4 ± 15, respectively. Measurements of the iliac crest mirrored the findings of calculi, with density measurements in 80 kVp being statistically higher than those obtained through 120 kVp. A total of 41 stone analyses were reviewed in accordance with variable CT dosages. When stratified by CT kVp and stone composition, RdCTs trended toward wider density ranges than sCT. Density measurements of urolithiasis in RdCT, achieved by voltage reduction, corresponds to a paradoxical increase in HU. Although HU can be used as a predictor for stone composition, there can be a wide deviation in measured density, and this can be further magnified in the setting of dose reduction.