The test of choice is ultrasound. It is quick, noninvasive, and accurate for the diagnosis of gallstones and acute cholecystitis. When present, signs of acute cholecystitis such as pericholecystic fluid and a thickened gallbladder wall can easily be seen on ultrasound. CAT scan often does not show gallstones if the density of the stones is similar to that of bile. HIDA scan is usually reserved for patients in whom ultrasound is negative but suspicion of gallbladder disease is high. MRI is expensive and not studied for the diagnosis of stones. ERCP is usually done to rule out common duct stones.