Ultrasonic study of the normal and dilated biliary tree. The "shotgun" sign.

With the advent of real time scanners, segments of the normal biliary tree can now be seen regularly. Normally, the biliary junction is narrower than the portal vein or the portal division. With early dilatation of the bile ducts the diameters of the two systems tend to equalize. Thus, the presence of two parallel ducts with similar diameters is pathologic. As the dilatation of the biliary tree progresses, the portal system may become flattened, thus reversing the initial proportion between the diameter of the biliary junction and that of the biliary tree.