MRCP sequences are highly sensitive and accurate in depicting cholecystolithiasis and can outperform ultrasound and computed tomography (Kelekis and Semelka 1996). Gallstones generally present as intraluminal, signal-void, round or facetted structures on both T1- and T2-weighted images. High signal intensity of the gallstones on T1- and T2-weighted sequences is not uncommon. A gallstone can be differentiated from a gallbladder polyp by the lack of enhancement on T1-weighted post-gadolinium images.
Acute inflammation of the gallbladder is caused by obstruction of the cystic duct (e.g., by cystic duct stones) in 80–95% of patients. Findings that are indicative of acute cholecystitis on post-gadolinium T1-weighted images are (Kelekis and Semelka 1996; Motohara et al. 2003):
- increased wall enhancement,
- transient increased enhancement of adjacent liver parenchyma on immediate post-gadolinium images,
- increased thickness of the gallbladder wall (>3 mm).
Findings on T2-weighted images that are helpful to establish the diagnosis are - presence of gallstones, - presence of pericholecystic fluid, - presence of intramural abscess as hyperintense focus in the gallbladder wall, - increased wall thickness.
Periportal high signal intensity may be observed but is a non-specific finding.
Acute acalculous cholecystitis makes up about 5–15% of all acute cholecystitis cases. It can be caused by depressed motility (e.g., in patients with severe trauma/ surgery, burns, shock, anesthesia, diabetes mellitus), by decreased blood flow in the cystic artery due to extrinsic obstruction or embolization, or by bacterial infection (Kelekis and Semelka 1996; Motohara et al. 2003).
Chronic cholecystitis is more common than acute cholecystitis. In chronic cholecystitis mural gadolinium enhancement is mild and most prominent on delayed post-gadolinium images. Pericholecystic enhancement is minimal or absent (Kelekis and Semelka 1996). The wall of the gallbladder may calcify, resulting in porcelain gallbladder. On MR images, calcifications may appear as signal void foci.
Calculi in the biliary ducts, although less frequent than in the gallbladder, are the most common cause of extrahepatic obstructive jaundice. MRCP is a non-invasive technique that is ideally suited for detecting bile duct stones owing to the high contrast of calculi as intraluminal low signal intensity or signal-void structures against high signal intensity bile.
At MRI, ductal biliary stones typically have a rounded or oval-shaped configuration with a meniscus of fluid above their proximal edge.
On thin-section source images, stones consistently appear as signal void foci, and can be detected at sizes as small as 2 mm in dilated and non-dilated ducts (Motohara et al. 2003; Holzknecht et al. 1998).