Excised urinary calculi were subjected to computed tomographic (CT) It has been found that even "radiolucent" calculi have considerably higher CT attenuation values than neoplasms, blood clots, or fungus balls, and that CT can distinguish such calculi accurately from the latter entities.To date, however, no extensive data have been published on the exact CT characteristics of various types of urinary calculi, nor has there been a rigorous effort to define the ability of CT to distinguish among groups of stones with different compositions.To address these questions, we performed CT scanning of excised urinary calculi in vitro. The CT density characteristics of the stones were analyzed and correlated with their chemical composition. Materials and MethodsThirty-five excised calculi were obtained from kidneys, ureters, and bladder; there were nine uric acid, eight struvite (mixed MgNH4PO4 and calcium phosphate), five cystine, two brushite (calcium hydrogen phosphate), and six calcium oxalate stones. Five calculi were of mixed composition:four calcium oxalate-uric acid and one calcium phosphate-cystine. The calculi were analyzed by crystallographic methods [6] using polarization microscopy, x-ray diffractometry, and infrared spectroscopy. The stones were selected after a preliminary analysis that left them largely intact in order to preserve their size and integrity. At the conclusion of the study, all stones were cut in cross section, permitting analysis of each stone in its entirety.The calculi were studied with an EMI 7070 CT scanner. Each stone was placed in a water bath and scanned at 120 kV and 90 mA using a 3-sec scan time. Each stone was scanned as close to its estimated "equator" as possible; section thickness was 2 mm. (Calibration of the scanner with air and water phantoms revealed mean values of -997 and -14.7 H, respectively.) Scanning and analysis of the images were performed in a blind fashion: Stone composition was not revealed until after the scan analysis was completed. CT density was measured by indicating a circular region of interest with the cursor, which was placed around the stone and contracted so that the entire stone image was included. A histogram was drawn of the frequency of pixel densities contained within the region of interest ( fig. 1). The value of the Downloaded from www.ajronline.org by 54.245.13.81 on 05/12/18 from IP address 54.245.13.81.

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