Predicting urinary stone composition based on single-energy noncontrast computed tomography: the challenge of cystine.

OBJECTIVE

To study several measurements from a single-energy noncontrast computed tomography (NCCT) that may distinguish calcium oxalate, uric acid, and cystine stones.

METHODS

Patients with pure urinary stones who had at least 1 single-energy NCCT before the stone composition analysis from January 2008 to December 2012 were enrolled in this study. The analyzed data comprised stone size, volume, core Hounsfield unit (HU), periphery HU, absolute and relative HU differences between core and periphery, and HU density. After these measurements, an NCCT bone window was subjectively evaluated to study the homogeneity of each stone from core to periphery. The Spearman correlation test was used to determine the correlation between HU values and stone size and volume for each group.

RESULTS

A total of 113 patients were found with pure urinary stones who also had a corresponding NCCT. There were 36, 47, and 30 patients in the calcium oxalate, uric acid, and cystine groups, respectively. The core HU, periphery HU, absolute and relative HU differences, and HU density were significantly different among the 3 groups (P<.001). Stone size and volume had a positive correlation with core and periphery HUs only for calcium oxalate and cystine stones. The subjective evaluation of the urinary calculi revealed a different pattern for each stone composition.

CONCLUSION

Single-energy NCCT may predict calcium oxalate stones with a high degree of accuracy. There is an overlap in radiographic profiles of cystine and uric acid stones, making a definitive differentiation more challenging.