Bile Ducts

Anatomy

The bile ducts are generally divided into the intrahepatic and extrahepatic portions. The intrahepatic ducts run in the portal triads with the portal veins and hepatic arteries. The right and left hepatic ducts are anterior to the adjacent portal veins. The peripheral intrahepatic ducts run parallel and adjacent to the hepatic arteries and portal veins, but the relative anterior and posterior relationship of the three structures is more variable than the extrahepatic ducts.

The extrahepatic portion of the bile ducts includes the common hepatic duct, the common bile duct, and a portion of the central right and left ducts. The common hepatic duct is the segment located above the cystic duct insertion and the common bile duct is the segment below. In most cases the cystic duct insertion is not seen, making it impossible to distinguish the common hepatic from the common bile duct. Therefore many people simply refer to the duct as the common duct and divide it subjectively into the proximal, mid, and distal segments. Fig. 4-1A shows the relationship of the extrahepatic bile ducts to the hepatic artery and the portal vein. At the porta hepatis the proximal common duct runs anterior to the right and main portal vein and the right hepatic artery (see Fig. 4-1B ). The mid duct runs posterior to the duodenum. In situations in which the cystic duct insertion is visible, it is seen posterior to the common duct (see Fig. 4-1C ). Inferiorly, the distal common duct enters the head of the pancreas and travels along the posterior-most aspect of the pancreatic head (see Fig. 4-1D ).

The hepatic artery arises from the celiac axis and travels in the hepatoduodenal ligament anterior to the portal vein and medial to the common duct (see Fig. 4-1E ). On transverse views of the porta hepatis, this configuration produces the Mickey Mouse appearance, with the head being the portal vein, the ear to the patient's left being the artery, and the ear to the right being the bile duct (see Fig. 4-1F ). The relationship of the bile duct and hepatic artery can be remembered by noting that the bile duct comes from the liver (a right-sided structure) and the hepatic artery arises from the aorta (a left-sided structure). The right hepatic artery passes between the common duct and the portal vein in approximately 85% to 90% of patients (see Fig. 4-1A and B ). Approximately 10% to 15% of individuals have a normal variant in which the artery passes anterior to the common duct ( Fig. 4-2A ). In a small percentage of patients, two arteries are seen anterior and/or posterior to the duct (see Fig. 4-2B and C ). This can be due to either two branches of the right hepatic artery or the right hepatic artery and the cystic artery.

Compared with the common duct, the hepatic artery is relatively curved, and so it is difficult to display more than 2 to 3 cm of its long axis in any plane. In addition, the hepatic artery maintains a relatively similar diameter throughout its course. Finally, the hepatic artery may cause an extrinsic impression on the bile duct and/or the portal vein. By contrast, the common duct is relatively straight, has a diameter that varies along its course, and does not produce an impression on adjacent vessels (see Figs. 4-1 and 4-2 , Table 4-1 ).

A replaced right hepatic artery arising from the superior mesenteric artery is a common normal variant that alters the anatomy of the porta hepatis. As described in Chapter 3 , a replaced or accessory right hepatic artery runs between the inferior vena cava and the portal vein and is situated on the right lateral aspect of the portal vein. It can be distinguished from the bile duct by following it to its origin or by Doppler analysis (see Chapter 3 ). Another potentially confusing anatomic variant in this area is a cystic duct that inserts unusually low. In such cases the cystic and common hepatic ducts travel in a common sheath and appear as parallel tubular structures before they join to form the common bile duct ( Fig. 4-3 ). Occasionally a tortuous gallbladder neck simulates the appearance of the proximal common duct ( Fig. 4-4 ). Careful scanning in multiple obliquities usually reveals the continuity of a tortuous neck with the rest of the gallbladder. In addition, it is not possible for a tortuous gallbladder neck to elongate to the same extent as the common duct.

Technique

The proximal common duct is usually best seen by placing the patient in a left lateral decubitus or left posterior oblique position and by scanning from a right subcostal approach during a deep inspiration. Because of its size and ease of visualization, the portal vein is a valuable landmark for the common duct. As the portal vein and bile duct exit from the liver, they separate from each other, with the portal vein heading toward the left and the bile duct heading more inferiorly and eventually bending slightly toward the right (see Fig. 4-1A ). Therefore if the main portal vein can first be imaged in its long axis, the mid and distal bile duct can then be visualized by rotating the transducer slightly in a clockwise direction and moving the inferior aspect of the probe slightly to the patient's right.

Most pathological processes affecting the bile duct occur distally. The distal common duct is located in the posterior and right lateral aspect of the head of the pancreas. From an anterior epigastric approach, the superior mesenteric vein can usually be seen in a longitudinal plane, running posterior to the body of the pancreas. Angling the transducer to the patient's right will then visualize the pancreatic head, and eventually the bile duct. If overlying bowel gas is a problem, pressure can be applied with the transducer to push the gas out of the way. In some cases it is necessary to have the patient drink water to displace the gas out of the stomach and the duodenum. Changing the patient from a supine to an upright position is also occasionally useful. Despite all these maneuvers, the anterior approach is often not useful in visualizing the distal bile duct. In such cases, a right lateral or anterolateral approach with the patient in a left posterior oblique position frequently allows the distal common duct to be visualized in a semicoronal plane. It also can be useful to position the patient in a left posterior oblique or left lateral decubitus position, so that the gallbladder is directly over the head of the pancreas. This allows the gallbladder to be used as an acoustic window ( Fig. 4-5 ). Another technique sometimes used to deal with overlying gastric or duodenal gas is to place the patient in a right lateral decubitus position until the gas moves out of these structures. The patient can then be moved into a supine or left lateral decubitus position to reimage the bile duct.