Determination of the Chemical Composition of Urinary Calculi by Computerized Tomography

Abstract

Clinical management of [human] renal calculi would be aided if a direct in vivo determination of stone chemical composition could be made. The possibility of obtaining this information was investigated by a quantitative analysis of the computerized tomography [CT] scan images of 80 urinary calculi. By using an appropriately calibrated CT scanner the differentiation of stone chemical composition can be made on the basis of 3 parameters, i.e., absolute CT value at a single X-ray energy, the difference between CT values measured at 2 different X-ray energies and CT value-frequency histograms (pixel patterns) of the stones. Uric acid stones were differentiated from all other stones at a significance level of P < 0.001. Cystine was differentiated from calcium oxalate and brushite at the same significance level. Using pixel patterns cystine and struvite were separated from each other correctly with 70% accuracy. Struvite stones of low or moderate calcium phosphate content were identified correctly with 80% accuracy. Struvite stones of high calcium phosphate content could not be differentiated from calcium oxalate or brushite. Calcium oxalate and brushite could not be separated. The minimum stone size that allowed chemical identification was established for each stone type. All urinary calculi examined were visible on CT scan regardless of chemical composition or size.