In most cases, CT scan is not used to detect gallstones, but this imaging test does have its uses in the biliary system. First of all, the entire main duct can be seen using CT scan because unlike ultrasound, air in the GI tract does not interfere with CT. High-speed CT with computer-assisted reformatting capabilities allows the radiologist to move quickly through numerous images. The ability of CT to find stones in the common bile duct approximates ultrasound. In general, CT scan is a better test for more complicated problems, although it may be used together with ultrasound.