Helical computed tomography has become the radiologic tool of choice in the assessment and treatment of patients with urinary tract calculi (Hubert et al, 1997; Smith et al, 1999). However, the full potential of helical CT to differentiate among stone types by structure or radiodensity has yet to be realized. Most CT scans for stones are used simply to identify the existence of a stone and give some indication of its size and location. These scans are viewed using soft tissue windows, in order to look for other possible causes of the patient's pain, such as appendicitis, gallstones, and colonic diverticulitis. However, soft tissue windows do not show structure within the kidney stone: stones appear as bright white objects in these images. The potential for observing structure in stones (using viewing windows closer to those used to view bone) has not been assessed in clinical studies. Currently, only the average CT attenuation value of urinary tract calculi has been investigated as an indicator of stone composition (Nakada et al, 2000; Mostafavi et al, 1998; Kuwahara et al, 1984). The average CT attenuation value has been shown to be useful for distinguishing some stones (such as uric acid from calcium oxalate) but considerable overlap in CT attenuation between stone types exist.