Dual Energy Computed Tomography (DECT) for Determination of Renal Calculi Composition - In-Vivo Analysis and In-Vitro Comparison with Qualitative Chemical Analysis - A Prospective Comparative Study at a Single Centre at SDM Medical College and Hospital – Dharwad, Karnataka

BACKGROUND Dual energy computed tomography (DECT) is a new method of computed tomography (CT) imaging which allows to determine stone composition in addition to assessing stone morphology. The purpose of this study was to evaluate the role of dual energy CT (DECT) preoperatively to assess the composition of urinary stones and to compare it with post-operative in vitro qualitative chemical analysis as reference standard. METHODS Forty patients (18 male and 22 female) who presented with symptoms of renal stones in the department of urology were included in the study. All 40 patients who were diagnosed to have renal stones clinically and by ultrasonography (USG) kidney, ureter and bladder (KUB) region were subjected to dual energy CT. The stone composition assessed in vivo using DECT preoperatively and in vitro by chemical analysis post operatively after stone extraction by surgical procedure. The results were compared by statistical analysis. Sensitivity, specificity and positive predictive value (PPV) were calculated and descriptive study done using Statistical Package for Social Sciences (SPSS) 20.00 version. Data was analysed by comparing it with correlative qualitative chemical analysis. RESULTS In our study, in vivo analysis using DECT showed most common type of stone was calcium oxalate seen in 20 cases compromising 50 % of total cases. Next common stone type was uric acid stone (22.5 %) followed by cysteine (17.5 %) and calcium hydroxyapatite (10 %) respectively. When the same stones were subjected to ex vivo chemical analysis, one of the calcium oxalate stone came out to be calcium phosphate and one of the cysteine stone came out to be mixed stone. Thus, out of 40 stones, 38 stones were found to have the same result in ex vivo chemical analysis as that of in vivo analysis by dual energy CT. Hence, accuracy of dual energy CT in diagnosis of renal stones was found to be 100 % with CI 91.19 % - 100 %. CONCLUSIONS With dual energy CT, it is possible to determine the composition of renal calculi in vivo non-invasively (with specificity of 100 % in our present study). Therefore, this helps in deciding the modality of treatment pre-operatively whether the stone is amenable to medical management (e.g., Uric acid stones) or requires extracorporeal shock wave lithotripsy (ESWL) or surgical intervention can be determined preoperatively. This helps to reduce the unnecessary financial burden and is found to be time saving. KEYWORDS Renal Calculus, Dual Energy CT, Stone Composition, Uric Acid Stones, Non-Uric Acid Stones, Attenuation, Hounsfield Units HU, Chemical Analysis of Stones.