Computed Tomography Evaluation of Urinary Stones Densities Compared to in Vitro Analysis of Its Chemical Composition

Abstract

Purpose: The aim of the study was to compare between the urinary stones of different chemical composition and their radio-densities (Computed Tomography attenuation values or Housefield Units). The prediction of the stone type would in turn lead to better selection of the interventional modalities. Materials & Methods: A retrospective review was performed for patients who underwent pretreatment Non Contrast Computed Tomography scan for urinary stones. When measuring stone density in Hounsfield unit (HU) on Computed Tomography, a Standard Deviation (SD) was calculated for the measured area of interest that contained several pixels and a standardized area of interest of 0.026 cm2, equivalent to 25 pixels, was used. Determination of chemical constituents of stones/fragments was done using Fourier Transform Infrared Spectroscopy (FT-IR spectroscopy). Our laboratory report indicated stones of mixed composition by listing the components in rank order with quantification of their presence and we compared the Hounsfield density of the stones with the chemical findings. Results: The chemical composition of uric acid, mixed oxalate and calcium oxalate monohydrate stones was accurately identified based on the absolute Computed Tomography value. The mean Housefield Unit (HU) density for uric acid stone was 459±80, which was considerably lower than those of other stones. Mixed oxalate calculi could be distinguished from uric acid, calcium oxalate monohydrate and apatite stones by the absolute Computed Tomography value (the mean Housefield Unit density was 777±224). Moreover, calcium oxalate monohydrate stones were easily distinguished from all stones using the absolute Computed Tomography value (the mean Housefield Unit density was 1158±156) except when compared to apatite containing stones, which were not commonly encountered. The difference of Computed Tomography value, among the above-mentioned stones, was statistically significant (p<0.001). Conclusion: This study demonstrated that Computed Tomography scanning could predict the chemical composition of urinary stones. The Hounsfield density was a convenient radiographic measure that correlated well with the chemical composition. A significant correlation between the stone size and Housefield Unit values was also demonstrated.