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Conspicuity of Renal Calculi at Unenhanced CT: Effects of Calculus Composition and Size and CT Technique¹

¹ From the Department of Radiology, Albany Medical College, NY (M.E.T., M.E.M., F.N.T.); and Department of Radiology, Duke University School of Medicine, Durham, NC (M.E.M., D.M.D., M.A.K.). Received January 2, 2001; revision requested February 16; final revision received February 26, 2002; accepted March 25. Address correspondence to M.E.T., Department of Radiology, University of Pittsburgh Medical Center, 200 Lothrop St, Pittsburgh, PA 15213 (e-mail: tublinme@msx.upmc.edu).

PURPOSE: To determine the effects of calculus size, composition, and technique (kilovolt and milliampere settings) on the conspicuity of renal calculi at unenhanced helical computed tomography (CT).

MATERIALS AND METHODS: The authors performed unenhanced CT of a phantom containing 188 renal calculi of varying size and chemical composition (brushite, cystine, struvite, weddellite, whewellite, and uric acid) at 24 combinations of four kilovolt (80–140 kV) and six milliampere (200–300 mA) levels. Two radiologists, who were unaware of the location and number of calculi, reviewed the CT images and recorded where stones were detected. These observations were compared with the known positions of calculi to generate true-positive and false-positive rates. Logistic regression analysis was performed to investigate the effects of stone size, composition, and technique and to generate probability estimates of detection. Interobserver agreement was estimated with statistics.

RESULTS: Interobserver agreement was high: the mean value for the two observers was 0.86. The conspicuity of stone fragments increased with increasing kilovolt and milliampere levels for all stone types. At the highest settings (140 kV and 300 mA), the detection threshold size (ie, the size of calculus that had a 50% probability of being detected) ranged from 0.81 mm + 0.03 (weddellite) to 1.3 mm + 0.1 (uric acid). Detection threshold size for each type of calculus increased up to 1.17-fold at lower kilovolt settings and up to 1.08-fold at lower milliampere settings.

CONCLUSION: The conspicuity of small renal calculi at CT increases with higher kilovolt and milliampere settings, with higher kilovolts being particularly important. Small uric acid calculi may be imperceptible, even with maximal CT technique.

© RSNA, 2002

Index terms: Computed tomography (CT), experimental studies, 81.12111, 81.12115 • Experimental study • Kidney, calculi, 81.811 • Phantoms

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