Beware very early common duct obstruction, before the duct becomes obviously dilated. The duct may be mildly dilated at the lower end, just proximal to a
stone. Likewise intermittent obstruction by a small stone at the lower end of the duct may be nondilated by the time the scan is performed.
A significant ultrasound feature in the absence of any other identifiable findings is that of thickening of the wall of the bile duct. This represents an
inflammatory process in the duct wall, which may be found in patients with small stones in a nondilated duct, but is also associated with sclerosing
cholangitis. It is sometimes technically difficult in some patients (particularly those with diffuse liver disease) to work out whether a tubular structure on
ultrasound represents a dilated duct or a blood vessel. Colour Doppler will differentiate the dilated bile duct from a branch of hepatic artery or portal
vein (Fig. below).
Dilated biliary ducts do not demonstrate flow on colour Doppler, differentiating them from portal
vessels. and Originally suspected as a dilated biliary tree, colour Doppler demonstrates that the ‘extra tubes’ are, in fact,
dilated intrahepatic arteries in a patient with end-stage chronic liver disease with reversed portal venous flow.