A dependent layer of variable nonshadowing echogenicity in the gallbladder is characteristic of sludge, which often contains stones (Figure 4).

Attempts also should be made to evaluate the presence of a sonographic Murphy’s sign – tenderness to direct palpation over the gallbladder with the ultrasound probe. The sonographic Murphy’s sign is a useful but imperfect sign with an overall accuracy of 87.2%, sensitivity of 63%, specificity of 93.6%, positive predictive value of 72.5%, and negative predictive value of 90.5%.

The presence of pericholecystic fluid should also be noted. This appears as an anechoic stripe of fluid along the dependent surface or in the gallbladder wall.

Gallbladder wall thickening occurs in the normal postprandial state as well as in a variety of disease entities, including cirrhosis, hypoalbuminemia, congestive heart failure, HIV disease, pancreatitis, and renal failure.

Acalculous cholecystitis is a rare disease but should be considered when the gallbladder is enlarged; the patient appears toxic and has certain underlying medical conditions such as diabetes.

Biliary duct obstruction. Biliary duct obstruction caused by stones, pancreatic pathology (e.g. mass), or stricture is detected measuring a CBD larger than 6-7 mm. Sonographically, the CBD appears as an anechoic tubular structure in the main portal triad, anterior to and following the course of the main portal vein (Figure 5).

Variants and mimics. Several normal anatomic variants may be noted. Indentations may produce septations of the lumen and can be mistaken for gallstones (Figure 6). Shadowing does not usually occur with septations. A fold of the gallbladder fundus is termed a Phrygian cap.

Biliary polyps can be mistaken for gallstones but may be distinguished from stones because they do not move with a change in positioning (Figure 7).

Hepatic cysts also can be mistaken for the gallbladder. Hepatic cysts have sharp margins, no internal echoes, and increased posterior acoustic enhancement.

Pearls and Pitfalls

Some final pearls and pitfalls include the following:

• The gallbladder is a mobile organ; remember to change patient positioning and/or probe placement to find the organ of interest.
• Distinguish gallstones from polyps and septations or folds by always scanning through the whole organ and in both longitudinal and transverse planes.
• During the biliary exam, use color Doppler to help distinguish nonvascular from vascular structures.
• Ultrasound findings must be interpreted in the context of the clinical presentation; findings suggestive of acute cholecystitis (e.g., gallstone or thickened wall) may be present in patients in a nondiseased state.
• The common bile duct can be dilated in the absence of pathology in older patients and postcholecystectomy patients.
• Measure the anterior wall of the gallbladder. The posterior wall may appear artificially thickened because of acoustic enhancement or artifact from bowel gas.