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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).

View larger version (73K): [in a new window]   Figure 1a. (a) Schematic diagram of phantom construction. (b) Transverse CT image of renal phantom shows stone clusters (arrows).

View larger version (124K): [in a new window]   Figure 1b. (a) Schematic diagram of phantom construction. (b) Transverse CT image of renal phantom shows stone clusters (arrows).

View larger version (32K): [in a new window]   Figure 2. Graph indicates visibility of renal calculi of various diameters imaged at 140 kV and 200 mA. A logistic regression model was used to produce probability estimates for the detection of stones of various sizes. The 95% CI of the threshold visibility of each type of stone (ie, the stone size with a 50% probability of being seen) is indicated by the horizontal lines. The model was based on six independent parameters: stone type, stone size, kilovolt setting, milliampere setting, stone position, and clustering. Br = brushite, Cy = cystine, St = struvite, Ur = uric acid, We = weddellite, Wh = whewellite.

View larger version (29K): [in a new window]   Figure 3. Graph indicates visibility threshold sizes of renal calculi at various combinations of kilovolt and milliampere settings. A logistic regression model was used to produce estimates of the 50% visibility threshold sizes for all five types of calculi at each combination of CT settings. Because the model indicated that the effects of kilovolt and milliampere levels were uniform among the five types of calculi, the threshold sizes of the five calculi types were averaged, normalized to the threshold sizes at the 140 kV and 300 mA combination, and then plotted as points to highlight the general effects of kilovolts and milliamperes.

View larger version (26K): [in a new window]   Figure 4. False-positive observations at various kilovolt and milliampere settings. False-positive observations are those in which the recorded position did not correspond to the position of an actual calculus. The number of false-positive observations seen in the entire phantom with a given combination of CT settings was divided by the total number of notations (ie, both true and false), and this ratio was expressed as a percentage. = 300 mA, = 280 mA, = 260 mA, = 240 mA, = 220 mA, = 200 mA.