Sonographic "parallel channel" sign of biliary tree enlargement in mild to moderate obstructive jaundice.

The criteria for the diagnosis of minimal obstructive jaundice (i.e., bilirubin less than 11.0 mg/100 ml) using sonography have not been established. A reliable diagnosis of obstructive jaundice with mild dilation of the bile ducts can be made by sonography when the "parallel channel" sign is seen. This sign is due to the simultaneous imaging of the dilated right or left main hepatic duct and the adjacent contiguous main portal vein branch. A prospective study of 86 patients showed this to be a highly reliable means of differentiating obstructive from nonobstructive jaundice when minimal duct dilation is present.