In vitro investigation of the origin of echoes with biliary sludge.

To investigate the origin of echoes from "biliary sludge," concentrated bile and bile from 5 patients with ultrasound findings of biliary sludge were examined in a tissue-equivalent phantom before and after filtration through progressively smaller pore sizes. Filtration converted echogenic bile to echo-free bile. Examination of the filtration residue by light microscopy established that the source of echoes in biliary sludge was particles, predominantly pigment granules, with lesser amounts of cholesterol crystals. Partial chemical characterization by determination of vulnerability to different solvents verified that the sludge was mainly calcium bilirubinate.