Stone characteristics on noncontrast computed tomography: establishing definitive patterns to discriminate calcium and uric acid compositions.

ABSTRACT

OBJECTIVE: To evaluate if different stone compositions can be distinguished by variations in Hounsfield unit (HU) patterns from stone core to periphery. MATERIALS AND METHODS: We retrospectively reviewed our stone registry searching for pure uric acid (UA) and calcium oxalate (CaOx) stones. Inclusion criteria comprised pure calculi; noncontrast computed tomography before stone analysis; and stone size ≥4 mm. A single urologist reviewed all images (abdominal/bone windows). Absolute HU attenuation was measured in the center (core) and edges (periphery) of each stone. HU density (HU/size) and stone volume were calculated. Categorical variables were compared with Chi-square/Fisher exact test; continuous with analysis of variance/Student t test. Spearman's test was used correlate HU values and stone dimensions. A P <.05 was considered significant. RESULTS: Included were 47 UA and 36 CaOx stones. CaOx cohort had higher HU values for all parameters (P <.001). HU variation (core-periphery) was more homogeneous in the UA group (-25%) than in the CaOx group (-52%). Stone dimensions had a significant positive correlation with HU values in CaOx cohort. In the UA group, stone volume and periphery HU had significant positive correlation. On bone window, CaOx stones had a heterogeneous pattern with a higher attenuation core with decreasing attenuation toward the periphery; UA calculi had a more homogeneous constant frosted white cloudy appearance. CONCLUSION: Stone size does not affect HU for UA stone, but has a positive correlation with CaOx stone. UA stones can be differentiated from CaOx by evaluating variation in HU from core to periphery and by subjective evaluation of homogeneity in bone windows.