Gallbladder sludge, also known as biliary sand, biliary sediment, or thick bile, is a mixture of particulate matter and bile, normally seen as a fluid-fluid level in the gallbladder on ultrasound, corresponding to the precipitate of bile solutes.
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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.
Factors that may contribute to gallbladder sludge include 8:
pregnancy - hormonal changes during pregnancy raise cholesterol levels in bile, causing sludge accumulation
rapid weight loss increases cholesterol secretion into bile, leading to sludge formation
diabetes
critical illness
organ transplantation
total parenteral nutrition
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
Complications
Differential diagnosis
On ultrasound consider
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certain forms of artifact 7
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