Biliary sludge is a mixture of cholesterol crystals and liquid crystalline droplets. It may appear hypoechoic to hyperechoic on ultrasound depending on the calcium content. Sludge will typically layer in a dependent fashion within the gallbladder (longitudinal and axial images shown) and may also contain hyperechoic stones intermixed. Sludge is often a sign of biliary stasis, which may be a transient benign process or may lead to acalculous cholecystitis. If the gallbladder is completely filled with sludge, the gallbladder may not be differentiated from the liver on ultrasound, a process referred to as hepatization.
