Gallbladder sludge

Gallbladder sludge (also known as biliary sand, biliary sediment, or thick bile) is a mixture of particulate matter and bile, normally seen as a liquid-liquid level in the  gallbladder on ultrasound, corresponding to the precipitate of bile solutes ¹. 

These precipitates consist of cholesterol monohydrate crystals, calcium bilirubinate granules, calcium salts and mucus secreted by the gallbladder ¹. 

On ultrasound, gallbladder sludge appears as a low amplitude homogeneous echoes, layering on the posterior wall. It moves slowly with changes in patient position.

Small amounts of gallbladder sludge may mimic gallstones (so called "tumefactive sludge") but sludge does not cause shadowing, unless associated with gallstones ². Large amounts of tumefactive sludge may mimic a mass.