Shotgun (Parallel Channel) Sign

In 1978, two ultrasonographic signs of biliary-tree obstruction were published. One was called the ‘parallel channel’ sign by Melvyn Conrad and colleagues ¹ (Parkland Memorial Hospital, Dallas), since the simultaneous appearance of dilated right or left main hepatic ducts and the accompanying portal vein branches resulted in an ultrasound image with parallel channels. The second was named the ‘shotgun’ sign by Francis Weill and coworkers ² , presumably with a double-barrel shotgun in mind, since it also described the visualization of side-by-side tubular structures. In this case, the main bile duct (denoting common hepatic and common bile duct) and the portal vein were the side-by-side structures (Figure 1). When the main bile duct was seen to be equal to or greater in size than the portal vein, this was deemed an early sign of biliary-tree obstruction. ‘Since it is now possible to visualize at least the proximal segment of the biliary tree with regularity, the diagnosis of early dilatation of the biliary tree is possible. Normally, the biliary junction is narrower than the portal vein or the portal division. With early dilatation the two diameters tend to equalize. Thus, the presence of two parallel channels with similar diameters (which we call the ‘shotgun’ sign) is pathologic.’ ² It was found to be a valuable criterion for differentiating obstructive from non-obstructive jaundice in a study of 100 cases ² . Figure 1 This transverse scan of the liver with the transducer angled cephalad shows the biliary junction and a proximal segment of the main bile duct (B) in front of the hilar division of the portal vein (P). The diameter of the bile duct is equal to that of the portal vein. (C = vena cava).’ Reprinted from Weill et a/.<xref ref-type="bibr" rid="ref79_2"> ² </xref> Ultrasonic study of the normal and dilated biliary tree. <italic>Radiology</italic> 1978, 127, 221–224, with permission of the RSNA https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003076568/34c29fd5-5662-4e8f-b58c-badd7fa5d724/content/fig79_1_B.jpg"/>