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 fluid-fluid 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. 

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: relatively hypointense compared to bile, though not as hypointense as formed calculi

• T1 C+ (Gd): no enhancement

• DWI/ADC: no diffusion restriction

Complications

• biliary colic ⁶

• cholecystitis

• acute cholangitis

• pancreatitis

Differential diagnosis

On ultrasound consider

• certain forms of artifact ⁷

  • ​secondary lobe artifact

  • section thickness artifact