Computed tomography and composition of renal calculi.

The attenuation values of computed tomography were studied in 50 recovered renal calculi of more than 10 mm in diameter, in the hope of determining the composition of calculi in situ. The attenuation value of various calculi (mean +/- s.d.) in a 5 mm slice with a maximal rectangular region of interest was as follows (Hounsfield units); mixed calcium oxalate and phosphate 1,555 +/- 193 (n = 22), magnesium ammonium phosphate 1,285 +/- 284 (n = 18), calcium oxalate 1,690 (n = 1), calcium phosphate 1,400 (n = 2), cystine 757 +/- 114 (n = 5) and uric acid 480 (n = 2). Attenuation values ranging from 500 to 1,600 overlapped for various calculi, except those composed of uric acid calculi. There was no correlation between the attenuation value and the mineral content such as calcium or magnesium per unit weight of calculus. The mineral content per unit volume seemed to be attributable to the attenuation value. From the present results we conclude that the determination of calculous composition by the attenuation value is possible only for oxalate calculi of more than 1,600 and uric acid calculi of less than 500 attenuation value, provided that the proper CT slice location, the region of interest and the appropriate calculus size can be determined.