Acute cholecystitis is inflammation of the gallbladder, in 95% of cases caused by a gallstone impacted in the cystic duct. Classic sonographic findings include gallstones, gallbladder wall thickening (>3 mm), pericholecystic fluid, and a positive sonographic Murphy sign. Acalculous cholecystitis (without stones) occurs in critically ill patients and carries higher morbidity. Gangrenous cholecystitis may show intraluminal membranes, asymmetric wall thickening, and absent Murphy sign (due to necrosis of the gallbladder wall).