Gallbladder or biliary sludge, also known as biliary sand, biliary sediment, or thick bile, is a mixture of particulate matter and bile. On ultrasound, it is normally seen as a fluid-fluid level in the gallbladder, 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 are tiny (<3 mm) calculi, which are 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.

Diagnosis

Gallbladder sludge is typically diagnosed on ultrasound, whereas biliary microlithiasis requires microscopic examination for its diagnosis ⁹.

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 ^8,9:

• 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 echogenicity ⁹, layering on the posterior wall, and frequently forming fluid-fluid level with anechoic bile above it. It moves slowly with changes in patient position ⁹. Sludge does not cause shadowing unless associated with gallstones ^2,9

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

Treatment and prognosis

Gallbladder sludge can ⁹

• complete resolve (40%)

• cyclical resolution and precipitation (40%)

• progress to cholelithiasis (20%)

Patients can be treated medically with ursodeoxycholic acid and/or cholecystokinin with good effect ⁹.

Complications

• biliary colic ^6,9

• cholecystitis ⁹

• pancreatitis ⁹

• acute cholangitis

Differential diagnosis

On ultrasound consider

• certain forms of artifact ⁷

  • ​secondary lobe artifact

  • section thickness artifact