2011 DETECTION OF URIC ACID STONES USING LOW AND CONVENTIONAL-DOSE COMPUTED TOMOGRAPHY

INTRODUCTION AND OBJECTIVES: Non-contrast computed tomography (CT) is the gold standard for evaluating urinary calculi, but exposes the patient to significant radiation exposure. Although lowdose CT protocols have been shown to have excellent detection for dense calcium oxalate ureteral stones, the ability of low-dose CT to diagnose less dense uric acid stones has not been well characterized. The purpose of this study is to determine the sensitivity and specificity of low-dose CT for detection of uric acid ureteral stones. METHODS: Twenty-seven, 3-7 mm uric acid stones were obtained from a reference laboratory and prospectively placed into 14 ureters arranged in 50 random configurations (523 total scanned stones). The intact urinary tracts were placed into a male cadaver (BMI 27.1 kg/m2) and CT imaging was performed at conventional dose 140 milliampere-second (mAs) and low dose 70, 50, 30, 15, 7.5, and 5 mAs settings, while holding all other imaging parameters constant. CT images were reviewed in a blinded fashion by a single radiologist and stone detection was reported. Sensitivity and specificity were compared between different settings. RESULTS: Overall sensitivity and specificity were 89 and 91%, respectively. Imaging using 140, 70, 50, 30, 15, 7.5, and 5 mAs settings resulted in 97%, 97%, 96%, 93%, 83%, 83% and 70% sensitivity, and 92%, 92%, 91%, 89%, 88%, 91% and 94% specificity, respectively. False negatives were more frequent among 3 mm stones compared to 5 and 7 mm stones (p = 0.01). False negatives for 3 mm stones occured 0.5%, 0.5%, 1.1%, 1.1%, 4.4%, 3.8%, 5.5% at 140, 70, 50, 30, 15, 7.5, and 5 mAs. CONCLUSIONS: Both low and conventional-dose CT scans demonstrate excellent sensitivity and specificity for the detection of uric acid ureteral stones. Detection of 3 mm uric acid stones is compromised at extremely low-dose (< 30 mAs) settings. To maximize uric acid stone detection sensitivity and specificity, a low-dose 30 mAs protocol should be utilized. Uric acid stone detection appears to be decreased compared to the more dense calcium oxalate stones particularly with smaller stone sizes. (Table presented).