Hounsfield units on computed tomography predict calcium stone subtype composition.

INTRODUCTION: Hounsfield unit (HU) determination of urinary stones on noncontrast computed tomography (NCCT) has been shown to predict stone composition. However, no in vivo studies have attempted to radiographically separate the various calcium stone compositions. We investigate the efficacy of HU measurement on NCCT to determine if it can differentiate the various calcium stone subtypes.

PATIENTS AND METHODS: Of the 684 patients who had undergone ureteroscopy at our institution from 1/2003 to 10/2007, 100 were identified with a documented NCCT, a chemical stone analysis and a stone size >5 mm but <2 cm.

RESULTS: Stone compositions were categorized as 100-80% calcium oxalate monohydrate (CaOMH) (n = 24), <80-60% CaOMH (n = 21), <60-50% CaOMH (n = 11) calcium oxalate dihydrate (CaODH) (n = 16), apatite (n = 9), brushite (n = 4), cystine (n = 2) and uric acid (n = 13). Mean HU were 879 +/- 230, 769 +/- 295, 717 +/- 304, and 517 +/- 203 for the 100-80% CaOMH, <80-60% CaOMH, <60-50% CaOMH and CaODH groups, respectively. The average HU for the apatite, brushite, cystine and uric acid groups were 844 +/- 346, 1,123 +/- 254, 550 +/- 74 and 338 +/- 145, respectively. The CaOMH groups together had a significantly higher HU than the CaODH group (p < 0.05) and a significantly lower HU than the brushite group (p < 0.05).

CONCLUSIONS: HU measurement of urinary stones on NCCT may be used to separate some calcium stone subtypes, specifically CaOMH and CaODH. This information may be useful in counseling patients on treatment options for patients requiring intervention.