of the gallbladder caused in most instances by obstruction
of the cystic
duct, usually by a gallstone and resulting in acute inflammation
of the gallbladder wall. Acute cholecystitis
(AC) complicates the course of symptomatic gallstones in 1020% of patients.
Sonography contributes substantially to the diagnosis of AC in many instances. Most sonographic signs are not typical but suggestive of AC. Gallstones are visualized in the large majority of patients. Gallbladder sludge is visualized commonly as nonshadowing echogenic but inhomogeneous material which tends to form a fluid-fluid level.
The main sonographic feature in AC is thickening of the gallbladder wall to more than 3 mm (Fig.1). The thickened wall in AC may present a multilayered aspect due to the visualization of a hypoechoic middle layer between two outer hyperechoic layers, which is related to the oedema and cellular infiltration of subserosa and submucosa (Fig.2). It is, however, well known that many conditions unrelated to gallbladder disease may cause thickening of the gallbladder wall. The most frequent are hepatitis, hypoalbuminaemia and ascites.
Sonographic Murphys sign is defined as the presence of maximal tenderness, elicited by direct pressure of the transducer over a sonographically located gallbladder.
Colour Doppler sonography of the cystic artery flow in the anterior wall of the gallbladder will demonstrate increased flow in patients with AC. However, the specificity of this finding is low as it may also be observed in gallbladder wall thickening caused by conditions other than AC.
Typical signs of AC on CT, besides the presence of stones in the gallbladder or in the cystic duct, are focal or diffuse thickening of the wall of the gallbladder (more than 3 mm of the wall of the noncontracted gallbladder), fluid in the gallbladder fossa, enlargement of the gallbladder and infiltration of the surrounding fat (Fig.3).
A low attenuation concentric band surrounding the gallbladder may indicate fluid accumulation in the gallbladder bed or oedema of the outer layers of the wall of the gallbladder.
The attenuation value of the contents of the gallbladder can be abnormally elevated in AC due to the presence of haemobilia. On contrast enhanced CT increase in attenuation of the gallbladder wall as well as increased contrast accumulation in the inflamed hyperaemic liver parenchyma adjacent to the gallbladder can be observed. Inflammatory reaction in the pericholecystic fatty tissue is a specific CT sign of AC. It is visible as streaky or band like soft tissue densities extending into the lipomatous tissue around the gallbladder.
Magnetic Resonance Imaging
MRI can demonstrate the same morphological features of AC as displayed on CT (Fig.4). On the T2-weighted images inflammatory pericholecystic changes will be visible as high signal linear and strand-like structures around the gallbladder. Gadolinium Gd -enhanced T1-weighted images will basically depict the same inflammatory changes of the gallbladder wall, pericholecystic fat and intrahepatic periportal tissues as those visible on CT.
Complications of AC include: empyema; gangrenous cholecystitis; perforation; pericholecystic abscess and bilioenteric fistula. Clinically there are few signs that allow detection of the occurrence of these complications in patients with AC. Radiological imaging can therefore contribute substantially to the differential diagnosis. Empyema or suppurative cholecystitis occurs more commonly in patients w are, however, better depicted on CT than on ultrasound (Fig.5). In particular the extent and the exact anatomical localization of the pericholecystic abscesses are well visualized on CT (Fig.6). CT is also better suited than ultrasound to correctly identify small pericholecystic gas collections and distinguish them from bowel gas as well as to detect a gallstone lying outside the lumen of the gallbladder. The CT features in complicated AC, such as diffuse or focal wall thickening as well as infiltratative changes in the pericholecystic lipomatous tissue, can be similar to the changes present in gallbladder carcinoma.