Hounsfield unit density in the determination of urinary stone composition

Objectives. Noncontrast computed tomography (NCCT) has emerged as the diagn ostic study of choice in the evaluation of acute flank pain, Recent in vitr o studies have suggested that NCCT can be used to predict the composition o f urinary stones on the basis of differences in radiodensity (measured in H ounsfield units, HUs). We sought to determine whether the analysis of in vi vo urinary stones seen on NCCT could predict their composition. Methods. Between March 1997 and August 1999, 100 pure stones from patients seen at the Wilford Hall Medical Center in San Antonio, Texas were submitte d for analysis. All had been visualized by NCCT before stone passage or ret rieval. A General Electric High-Speed Advantage CT scanner evaluated most o f these patients by a "flank pain protocol" (ie, helical technique with bre ath-holding at 120 kV, 200 mA with 5 mm collimation). Each scan was interpr eted by one of two staff radiologists who measured the HUs for each stone. A statistical comparison was made between the stone composition and radiode nsity. To allow for subsequently observed increases in radiodensity with in creasing stone size regardless of composition, the HU value was divided by each stone's largest transverse diameter in millimeters to give the HU dens ity. A statistical comparison was then made between stone composition and H U density. Results. No significant difference was noted between the HU values of calci um oxalate and calcium phosphate stones, and thus they were analyzed collec tively as "calcium stones." When the HU values of calcium (n = 87), uric ac id (n = 7), struvite (n = 4), and cystine (n = 2) stones were compared, the overlap of ranges precluded accurate identification, and the mean HU value s were not significantly different from one another, There was less overlap noted when comparing the HU densities of the stones studied, and no noncal cium stone had an HU density greater than 76 HU/mm. Using one-way analysis of variance, significant differences were noted between the mean HU density of calcium (105 +/- 43) and uric acid (50 +/- 24) stones (P = 0.006). A tr end toward significance was found between the mean HU density of the calciu m and struvite stones (53 +/- 28, P = 0.073). No significant differences we re found among the other stones. Conclusions. HU density compared with the HU value alone better characteriz ed differences in radiodensities among urinary stones; calcium stones can b e distinguished from uric acid stones on the basis of this value. However, neither the HU density nor the mean HU value was able to identify urinary s tones in vivo. UROLOGY 58: 170-173, 2001. (C) 2001, Elsevier Science Inc.