In vivo and in vitro ultrasonic characteristics of gallstones were compared with stone size and composition with the aim of understanding the basis for these features. In vivo and in vitro ultrasonic features of the same stone were identical. Acoustic shadowing was not due to stone type, radiodensity, or calcium content. However, all stones larger than 4 mm in diameter produced a distinct sonic shadow. Gallbladder sludge produced internal echoes without a sonic shadow. These echoes shifted slowly when the patient was reexamined in a different position. Four of seven cholesterol stones containing more than 88% cholesterol floated and produced a sonic shadow without internal echoes or with an area of internal echoes within the gallbladder at a distance from the posterior wall. This last feature may identify patients that are good candidates for a trial of gallstone dissolution with chenodeoxycholic acid.
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