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. 

Terminology

The term biliary microlithiasis is occasionally used as a synonym for sludge, however this is not strictly correct. Microlithiasis refers to the tiny calculi (<3 mm) undetectable on normal transabdominal ultrasound. Sludge may include these microliths in its composition, but this is only one element of a variable mixture of crystals, proteinaceous debris, lysed cells and mucin ⁵.

Pathology

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

Radiographic features

Ultrasound

Gallbladder sludge appears as a low amplitude homogeneous echoes, layering on the posterior wall, and frequently forming fluid-fluid level with anechoic bile above it. 

• it moves slowly with changes in patient position
• sludge does not cause shadowing unless associated with gallstones ²

Sludge can get compacted forming a mass-like lesion referred to as tumefactive sludge ⁴: 

• hypoechogenic well defined intraluminal mass
• no posterior acoustic shadowing
• no internal vascularity at color Doppler
• mobility is not always demonstrated, therefore, raising differentials with polyps or gallbladder carcinoma 

MRI

• T1: high-signal
• T2: iso- to mild hyperintensity 
• T1 C+ (Gd): no enhancement
• DWI/ADC: no diffusion restriction