A novel method for prediction of stone composition: the average and difference of Hounsfield units and their cut-off values

The purpose of the study was to investigate the predictive value of stone measurements by including a novel method on non-contrast computed tomography (NCCT) images for stone composition. We retrospectively evaluated patients who had stone analysis, NCCT images, and underwent percutaneous nephrolithotomy between 2013 and 2016. Patient characteristics, stone measurements on NCCT images, and stone analysis results were evaluated. Hounsfield unit (HU) values (maximum (HUmax), minimum (HUmin), and average (HUave) of HU values) were investigated on NCCT images. HUdiff was calculated as the difference between the HUmax and the HUmin values. Patients were divided into seven stone groups and data were compared. Then patients were separately divided into two groups according to mineral complexity (mono-mineral and multi-mineral groups) and calcium-based (calcium and other stone groups) evaluation. In the study, 115 patients were evaluated. Age, gender, HUmin, HUmax, and HUave were significantly different between the stone groups. HUdiff and HUave were found to be 341.5 HU (AUC = 0.719, p = 0.017) and 1051.5 HU (AUC = 0.701, p = 0.029) as cut-off, respectively. Seventy of 72 > 341.5 HUdiff patients and 64 of 67 > 1051.5 HUave patients had multi-mineral stones (p = 0.001, OR 9.26, and p = 0.028, OR 4.27), respectively. In multivariate analysis, > 341.5 HUdiff rate was significantly higher in multi-mineral and calcium stone groups; HUave was also significantly higher in the calcium stone group. HUdiff and HUave were significant predictors of mineral complexity. HUdiff of < 341.5 HU showed 81.8% sensitivity and 67.2% specificity for identification of mono-mineral stones.