Determination Of Chemical Composition Of Upper Urinary Calculi By Noncontrast Helical Computed Tomography

Objective: The aim of this study was to determine whether the analysis of upper urinary stones seen on noncontrast helical computed tomography could predict their composition. Methods: Between July 2009 and January 2010,one hundred and sixty urinary calculi were obtained by URL/PCNL extraction or surgical removal.They were visualized on noncontrast helical computed tomography before stone removal.A General Electic, high-speed CT/iCT scanner evaluated these patients at 120KV, 320mA, and 0.6∶1 pitch, with 5mm collimation. The largest diameter in soft-tissue window, the average CT-attenuation value using soft-tissue window or bone window was determined on each scan. All stones were analyzed using Infrared spectroscopy.Statistical analyses were performed to find the ranges of attenuation values for different stone minerals. Results: When 160 urinary calculi were imaged using noncontrast helical CT in soft-tissue window, the average CT value ranges from 123 to 1668 HU. The 53 pure stones were separated into four groups and included 27 calcium oxalate monohydrate, 15 uric acid, 9 hydroxyapatite, 2 L-cystine stones. The 107 mixed stones included sereval components. In the analysis of pure stones, the mean HU of the stones composed of calcium oxalate monohydrate, uric acid and hydroxyapatite were 986±162HU, 485±63HU and 907±182HU, respectively. The analyses showed that the uric acid calculi could be distinguished from calcium oxalate monohydrate and hydroxyapatite stones (P<0.05 for both). A statistically significant difference (P<0.05) was found between pure uric acid stones (485±63HU) and other stones (879±201HU) by the average CT-attenuation value in soft-tissue window. Using the HU value cutoff of 550 or lower, the sensitivity was 93.3%, specificity 98.6%, positive predictive value 87.5%, and negative predictive value 99.3% that a stone would be primarily pure uric acid.No statistically significant difference (P=0.85) was noted between calcium oxalate monohydrate (973±116HU) and hydroxyapatite (933±131HU) calculi by measuring the average CT-attenuation value derived from the region of interest in bone window. Conclusions: tial clinical study demonstrated that we were able to differentiatiate between pure uric acid and other stones using the average CT-attenuation value in soft-tissue window of urinary calculi from noncontrast helical computed tomography.An average CT-attenuation value in soft-tissue window of urinary calculi lower than 550 HU was highly suggestive of a pure uric acid stone, and the positive predictive value was 87.5%.Stone groups of calcium oxalate monohydrate and hydroxyapatite could not be diffrentiated using noncontrast helical CT.