Accuracy of sonography for determining the number and size of gallbladder stones before and after lithotripsy

Department of Radiology, University of British Columbia, Vancouver, Canada.

Accurate knowledge of gallstone number and size is required both for selection of patients for treatment by lithotripsy and for the evaluation of therapeutic results. We measured the accuracy of sonography for use in counting and measuring gallstones and gallstone fragments in a three-part study. First, specimen gallstones of known size were examined in a water bath with four different sonographic units. Second, seven pigs with surgically implanted gallstones had sonography before and after lithotripsy. Finally, we compared the measurements made on the preoperative sonogram with postoperative surgical specimen measurements in 27 patients who underwent cholecystectomy. In vitro gallstone measurement errors were 1.0 mm or less with each of the four sonographic units. In the animal experiments, all prelithotripsy measurements were highly accurate (+/- 1.1 mm). The animal postlithotripsy measurements were similarly accurate in five of seven cases, but substantial errors were made in the remaining two cases. In precholecystectomy patients, the sonographic stone counts and measurements were accurate in all 16 patients who had fewer than six stones. In all 11 patients with six or more stones, we correctly determined with sonography that at least six stones were present, but in only four of 11 cases was the largest stone measured accurately (+/- 2.0 mm). We conclude that sonography can be used accurately to count and measure gallstones when fewer than six stones are present, can reliably determine when six or more stones are present, and, in many cases, can determine the size of the largest fragments remaining after lithotripsy.
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