Biliary system

preferable if the catheter tip is beyond the origin of the cystic artery to avoid injury to the gallbladder.

Portal venography This may be achieved by many routes (see the section on the portal vein). Within the liver the left portal vein passes anteriorly before turning left, and oblique views are therefore necessary to view this part of the vein. Before it turns left, a recurrent branch passes to the right to the quadrate lobe. This must not be mistaken for a right portal vein if the actual right vein is obstructed.

Hepatic venography This is achieved via the inferior vena cava (usually via a retrograde approach from the internal jugular vein in the neck) w i th catheterization of each of the three main hepatic veins in turn. The right vein may enter separately and the middle and left veins may be seen to unite as a common trunk. This retrograde approach may also be used to achieve radiographically directed hepatic venous pressure measurements or transjugular hepatic biopsy, and to create an artificial portosystemic shunt in patients w i th portal hypertension when a TIPS procedure (transjugular intrahepatic portosystemic shunt) is performed.

Venous drainage of the caudate lobe Because part of the caudate lobe is drained by small veins that enter the inferior vena cava directly, this lobe may be spared in diseases that occlude the main hepatic veins (e.g. the Budd-Chiari syndrome). As a result it may undergo hypertrophy which, if extensive, may itself obstruct the IVC.

Hepatic scintigraphy On anterior views in hepatic scintigraphy, an indentation is seen in the inferior border of the liver at the site of the gallbladder and another at the site of the ligamentum teres. A photon-poor area is seen above the latter at the attachment of the falciform ligament. A defect is seen superiorly at the site of the entry of the hepatic veins into the IVC, and another centrally at the porta hepatis. Superimposition of the breast and of the bowel (especially after barium studies) can cause false defects.

On lateral views the liver is seen as elliptical, w i th the renal fossa posteriorly and the gallbladder fossa anteriorly, and a central defect for the porta hepatis.

THE BILIARY SYSTEM (Figs 5.30 and 5.31) •

The gallbladder

The gallbladder is a pear-shaped sac attached to the extrahepatic bile ducts by the cystic duct. It is very variable in size but normally measures up to 10 cm in length and 3 cm in diameter. It is described as having a fundus, body and neck, and it hangs on its bed on the visceral surface of the liver w i th its neck lying superiorly and its fundus inferiorly A pouch called Hartmann's pouch on the ventral surface just proximal to the neck is seen when the

(a) In two thirds individuals the bile duct is anterior to the hepatic artery. (b) In one quarter the bile duct passes behind the artery. (c) In 12% the right hepatic artery arises from the SMA and passes behind the bile duct.

Fig. 5.31 Variation in the relationship of bile ducts, and hepatic artery at the porta hepatis.

gallbladder is dilated in disease, but is probably not a normal anatomical feature.

The gallbladder is covered by peritoneum on its fundus and inferior surface. It may have a mesentery and hang free from the inferior surface of the liver.

The mucosa lining the gallbladder is smooth except at the neck and the cystic duct, where it forms folds that are arranged spirally and called the valves of Heister.

The relations of the gallbladder These are as follows: • Anterosuperiorly: — The gallbladder bed of the liver — The fundus is related to the anterior abdominal wall at the point where the lateral edge of the right rectus muscle meets the ninth costal cartilage; and • Posteroinferiorly: — The neck: lesser omentum — The body: first part of the duodenum — The fundus: transverse colon.

Normal variants These are as follows: • The lumen may have a septum. • The fundus may be folded back on itself - known as the

Phrygian cap.

• Diverticula may occur anywhere, commonest at the fundus. • Location may be retrohepatic or suprahepatic. • Location may be intrahepatic (normal in the fetus up to 2 months). • Location may be left-sided: — if under the left lobe — if herniated through the epiploic foramen — as part of transposition of the viscera • Absent gallbladder - very rare (0.05%) associated w i th other congenital abnormalities. • Double gallbladder - even rarer (0.025%); if double they usually share a duct.

Arterial supply

The gallbladder is supplied by the cystic artery, a branch of the right hepatic artery, and by branches that supply it directly from the liver in the gallbladder bed.

Venous drainage

Blood from the gallbladder drains via small veins to the liver from the gallbladder bed. Sometimes a cystic vein is formed that drains to the portal vein.

Bile ducts (Figs 5.30-5.36) Intrahepatic bile ducts follow the same pattern of distribution as the hepatic arteries. As w i th the arteries, a knowledge of the segmental anatomy is especially important in assessment prior to segmentectomy. Bile ducts from segments unite to form right and left hepatic ducts (with a normal diameter of up to 3 mm each), which in turn unite outside the liver in the porta hepatis to form the common hepatic duct. This duct measures up to 7 mm in diameter and lies anterior to the portal vein in the free edge of the lesser omentum, w i th the hepatic artery on its left side.

Fig. 5.32 Variation in relationship of hepatic artery and portal vein.

The common hepatic duct is joined after a variable distance (usually 3.5 cm) by the cystic duct (usually 3.5 cm long) to form the common bile duct.

The common duct has a supraduodenal third where it lies in the free edge of the lesser omentum, w i th the hepatic artery on its left and the portal vein posteriorly. It also has a retroduodenal third where it passes behind the first part of the duodenum w i th the gastroduodenal artery, and a pancreatic third where it passes behind the pancreas in a tunnel or groove towards the midpoint of the second part of the duodenum. The common bile duct usually unites w i th the pancreatic duct in a common dilated terminal ampulla (of Vater) and empties into the duodenum at the papilla 8-10 cm from the pylorus. The ampulla is surrounded by a circular layer of involuntary muscle known as the sphincter of Oddi. This protrudes slightly into the duodenum, aiding identification at endoscopy.

Variation in the biliary ducts The commonest anatomical arrangement of the bile ducts is shown in Figures 5.30 and 5.31. Some variation in the intrahepatic ducts occurs in over 40% of cases. These vary more often than the extrahepatic ducts. • Accessory hepatic ducts may arise in the liver, particularly in the right lobe, and join the right hepatic duct, the main hepatic duct, the common bile duct, the cystic duct, or rarely the gallbladder. • The right and left ducts may fail to unite, giving rise to a

'double' hepatic duct.

• The cystic duct may: — Be absent so that the gallbladder joins the hepatic duct — Join the common hepatic duct on its left rather than its right side (passing posteriorly) — Join the right hepatic duct, or — Join the hepatic duct anywhere between the porta hepatis and the duodenum. Where the cystic duct is long it passes close to and usually behind the hepatic duct. Compression of the hepatic duct by calculi in such a cystic duct results in a syndrome known as Mirrizzi's syndrome. • The common bile duct and the pancreatic duct may have separate openings close to each other in up to 40% of cases and widely separate from each other in 4% of cases. • The papilla may be positioned as far proximally as the stomach or as far distally as the third part of the duodenum.

Radiological features of the biliary system

Plain films of the abdomen The normal gallbladder or biliary system are not visible. Gas may be seen in the extrahepatic ducts in the elderly where the ampullary tone is low, after sphincterotomy, or after surgical anastomosis of bile ducts to small bowel (choledochojejunostomy).

Ultrasound examination of the biliary system (Fig. 5.34) Intrahepatic bile ducts are tiny structures that run with portal vein radicles. They can often be detected on higherresolution scanning and normally measure less than 2 mm, or less than 40% calibre of the adjacent portal venous branch. Anything larger is abnormal and indicates biliary obstruction. The union of the right and left hepatic ducts can be seen in the porta hepatis. The right hepatic artery can be seen to cross between the main portal vein and the common duct at the porta (Fig. 5.35). The upper limit of normal for diameter of the hepatic duct varies w i th age. The mean diameter is 3 mm at age 20 and 6.5 in asymptomatic subjects over 75 years. However, up to 8 mm is considered normal in older subjects, and up to 10 mm is normal in the postcholecystectomy case. A simple rule of thumb is that

Fig. 5.34 Ultrasound of porta hepatis: usual configuration of portal vein, bile duct and hepatic artery.

the upper limit of normal is approximately one-tenth of the age of the subject in millimetres - 4 mm at age 40, 7 mm at age 70, etc.

The gallbladder can be visualized between the liver and the right kidney. Its wall thickness varies w i th the degree of distension. The size of the gallbladder varies also between the fed and the fasting states, but can be up to 10 cm long and 3 cm wide. The position of the fundus can be as low as the pelvis. The normal wall is seen as a thin echogenic line less than 3 mm thick.

The spiral valves of Heister in the gallbladder neck and the cystic duct may cast acoustic shadows that must not be confused w i th calculi.

The hepatic and common bile ducts can sometimes be visualized throughout their course, but part of the common bile duct is often obscured by gas as it passes posterior to the first part of the duodenum.

Contrast examinations (Fig. 5.35) A variety of contrast examinations of the gallbladder and biliary tree are possible, depending on the clinical circumstances. Some, such as intravenous cholangiography and oral cholecystography, have been replaced by ultrasound. Others such as intraoperative, T-tube, transhepatic and retrograde cholangiography (ERCP) are still in use (see Fig. 5.37), although MR cholangiography has replaced ERCP for first-line investigation of the intra- and extrahepatic biliary trees, w i th ERCP now largely reserved for interventional procedures. Preoperative MR cholangiography has also replaced intraoperative cholangiography to look for stones in the bile ducts.

Anatomical points to note during contrast studies These include:

• Some intrahepatic ducts, particularly those to the left lobe, have an anterior course and to f i ll these in transhepatic cholangiography, for example, a patient may need to be turned from a supine to a prone position

Fig. 5.35 ERCP study of the pancreaticobiliary tree.

to visualize them. To view anteriorly or posteriorly directed ducts oblique views are taken. The ducts to the quadrate lobe (segment IV) and to the inferior segment of the left lobe (segment III) are most commonly used as afferent ducts to bypass hilar biliary tumours, and their patency should be established in preoperative assessment of these patients. • The ducts most commonly punctured during percutaneous transhepatic cholangiography are the ducts of segments V and VI on the right and the duct of segment I II on the left, as these are the most accessible (see Fig. 4.24a). • The gallbladder is directed anteroinferiorly and to the right and the cystic duct usually joins the hepatic duct on its right side. These facts dictate patient manoeuvres to f i ll these organs. • The extrahepatic bile ducts are anterior to the vertebral column and oblique views are necessary to project them away from the bones on contrast studies.

MR cholangiography (Fig. 5.36) Excellent images of the biliary and pancreatic systems are obtained w i th fluid-sensitive MR imaging techniques.

Fig. 5.36 MR cholangiopancreaticogram. Coronally-acquired HASTE images demonstrate parts of the bile duct and pancreatic duct. This is a water-sensitive sequence in which fluid is bright.

MR cholangiography capitalizes on the long T2 relaxation time of static fluid. Heavily T2-weighted fat-suppressed sequences produce images where static f l u id in the bile ducts and pancreatic duct is hyperintense and background signal is suppressed. Axially acquired images can be postprocessed into projection images that resemble direct cholangiograms produced by ERCP, or coronal or sagittal images can be acquired directly to demonstrate sections of the ducts. This technique has replaced ERCP as a first-line investigation of the biliary tree.

CT of the biliary tree The intrahepatic bile ducts are seen running w i th the portal veins. These are distinguished from one another by enhancing the veins w i th intravenous contrast agents. Ducts are usually anterior to the portal vein radicles.

On CT (see Figs 5.2 and 5.11) the position of the gallbladder is variable. The position of the neck is constant at the porta, defining the quadrate lobe anteromedially. The body and fundus may lie transversely, closely applied to the visceral surface of the liver, or may hang inferiorly The common bile duct is also seen at the level of the porta anterior to the portal vein.

On lower-level CT slices the common bile duct may be seen in a groove or a tunnel posterior to the head of the pancreas. The body of the gallbladder is seen anterior to the duodenum. In CT cholangiography, images may be obtained after the administration of oral cholangiographic agents the night before, or 60 minutes after the infusion of intravenous cholangiographic agents. The bile ducts are then identified as contrast-filled structures. The intrahepatic ducts are best seen with the intravenous agent; the gallbladder, cystic duct and common bile duct are well seen w i th the oral agents.