Gangrenous cholecystitis is another form of severe inflammation that results in gallbladder wall ischemia and necrosis. The diagnosis should be suggested when intraluminal membranes from sloughed gallbladder mucosa are seen on US. The gallbladder wall may appear striated and have irregular thickening. Often, a sonographic Murphy sign is absent in patients with gangrenous cholecystitis; CT findings are more specific. The absence of gallbladder wall enhancement is the most important feature and is indicative of ischemia (Fig. 2). Intraluminal membranes, hemorrhage, and gallbladder wall disruption are additional findings.

Xanthogranulomatous cholecystitis (XGC) is a rare form of chronic cholecystitis that resembles gallbladder carcinoma on imaging studies. The presumed etiology is intermittent cystic duct obstruction from stones. During episodes of obstruction, increased intraluminal pressure within the gallbladder causes intravasation of bile into the gallbladder wall. The intravasated bile causes a chronic, lipid-laden, inflammatory process. There is a significant overlap in CT features of XGC and gallbladder carcinoma. Both entities may demonstrate wall thickening, infiltration of the surrounding fat, hepatic invasion, and lymphadenopathy. XGC can be suggested when nodular foci are observed in the gallbladder wall (Fig. 3). Foci are hypoechoic on US and hypoattenuating on CT and represent foci of lipid-laden inflammation in the gallbladder wall. In most cases, this is difficult to differentiate prospectively from degeneration or necrosis that may be found in gallbladder carcinoma.

Adenomyomatous hyperplasia, also called adenomyomatosis, is a common benign tumor-like lesion of the gallbladder that may produce focal, segmental, or diffuse mural thickening. Pathologically, adenomyomatosis is characterized by hyperplasia of all layers of the gallbladder wall. Epithelial hypoplasia results in the formation of exaggerated invaginations called Rokitansky-Aschoff sinuses. Muscular hyperplasia thickens the gallbladder wall. Sonographically, adenomyomatous hyperplasia is characterized by echogenic foci that produce a characteristic reverberation artifact, commonly called ring-down artifact, emanating from the gallbladder wall. The ringdown artifact produces twinkle artifact on color Doppler (Fig. 4). Echogenic foci represent bile salts, cholesterol crystals, or small stones in Rokitansky-Aschoff sinuses. Focal or diffuse gallbladder wall thickening commonly accompanies the ring-down artifact. Focal adenomyomatosis is often localized in the gallbladder fundus. On MR, Rokitansky-Aschoff sinuses are best visualized on breath-hold T2-weighted MRI sequences as small, round foci of high T2 signal (Fig. 4) that in severe cases are visible throughout the gallbladder wall. This appearance is called the pearl-necklace sign. In cases with significant wall thickening, the finding of Rokitansky-Aschoff sinuses on MRI may be a helpful distinguishing feature from gallbladder carcinoma, which also may cause significant wall thickening.