Anatomy 721

Normal Anatomy

The liver is divided into the left and right lobe, and each lobe is divided into segments on the basis of its vascular anatomy and biliary drainage. The in-trahepatic bile ducts generally follow the internal hepatic segmental anatomy. In the left lobe, a left medial segment duct and a left lateral segment duct normally join to form the main left hepatic duct. The right hepatic duct branches near its origin at the common hepatic duct. Frequently, the right hepatic duct has a dorso-caudal branch, draining the posterior segment of the right lobe, and a ventro-cranial branch, draining the anterior segment of the right lobe. Ductal drainage of the caudate lobe is variable and may be related to the left or right ductal system. The left and right hepatic ducts unite just outside the liver to form the common hepatic duct, which is usually 3-4 cm in length. The common hepatic duct courses ventral-ly and inferiorly with the hepatic artery and the portal vein from the porta hepatis in the hepato-duodenal ligament. The common hepatic duct joins the cystic duct to form the common bile duct that averages 6-7 cm in length. The common bile duct is divided into suprapancreatic, intrapancre-atic, and ampullary segments, and enters the posterior-medial aspect of the second portion of the duodenum through a 1-2 cm long intramural tunnel terminating at the major duodenal papilla (Papilla of Vateri). The common bile duct in many cases joins the pancreatic duct in the duodenal wall, and has a short common channel. The sphincter of Oddi surrounds the common channel and the choledochal sphincter surrounds the common bile duct from its entrance into the duodenal wall to its junction with the pancreatic duct. Microscopically, the extrahepatic ducts are composed mainly of elastic fibers, and are sparse in muscle fibers. This explains their change in size in response to fluctuations in intraductal pressure.

The gallbladder is an elliptical organ that straddles the intersegmental plane between liver segments IV and V. The gallbladder is divided into four parts: the fundus, the body, the infundibulum, and the neck. Usually the organ is attached to the liver by the parietal peritoneum. When relaxed, the normal gallbladder is approximately 10 cm long, 3-5 cm in diameter and has a capacity of approximately 50 ml. The gallbladder wall is usually 2-3 mm thick and composed of columnar epithelium. Lymphatic drainage of the gallbladder descends around the bile duct and involves cystic and perichole-dochal lymph nodes. From the pericholedochal nodes the drainage continues to nodes found posterior to the pancreas, portal vein, and common hepatic artery, until finally the lymphatic flow reaches the lymph nodes of the interaortocaval region, celi-ac artery, and superior mesenteric artery. The gallbladder communicates with the common bile duct via the cystic duct, which is 2-4 cm in length, and contains tortuous folds known as the spiral valves of Heister near the neck of the gallbladder. The cystic duct usually joins the common hepatic duct approximately halfway between the porta hepatis and the ampulla ofVateri to form the common bile duct.

Biliary Tree and Gallbladder Anomalies

The primitive liver is composed of bipotential embryonic cells that can differentiate into either parenchymal or biliary cells. Biliary differentiation occurs when the embryonic cells are in contact with the mesenchyme surrounding the portal vein ramifications; this cell layer ultimately forms an epithelial cylinder called the ductal plate. Some segments of the cylindrical lumen form tubular structures that are gradually incorporated into the portal mesenchyme. This results in the portal triad acquiring a tubular bile duct surrounded by portal connective tissue. Abnormalities in ductal plate development can affect any level of the biliary tree, thereby explaining the diversity of congenital bile duct abnormalities [15].

Biliary tree malformations, with the exception of choledocal cysts, are often asymptomatic. Therefore the diagnosis is frequently not made until adulthood.

Choledochal Cyst and Cystic Dilatation of the Bile Duct

Choledochal cysts are anomalies of the biliary system characterized by dilatation of the extrahepatic or intrahepatic bile ducts. These anomalies are more frequent in females with a ratio of 4:1. In about 60% of cases the diagnosis is made in the first ten years of life, although choledochal cysts may become evident at any age (see Chap. 10,"MR Imaging of the Liver in Pediatric Patients", section 10.3.6, "Choledochal Cyst and Cystic Dilatation of the Bile Duct").

Choledochal cysts are considered to arise as a result of an anomalous junction of the pancreatic duct and common bile duct resulting in a long common channel [28]. This was first recognized in 1969 by Babbitt [4], who suggested that this long common channel allowed reflux of pancreatic enzymes into the common bile duct, with subsequent inflammation and weakening of the common bile duct wall with progressive dilatation.

Until recently, the Todani Classification [64] (Fig. 1) was used to categorize cystic dilatation of the biliary ducts. According to this classification, type I, the most common type, comprises cystic (iA), focal segmental (IB), and fusiform choledochal dilatations (IC); type II cysts are true di-verticula arising from the common bile duct; type III cysts, also known as choledochoceles, are dilatations of the intraduodenal portion of the common bile duct; type IV cysts are multiple intra-and extrahepatic cysts (IVA) or multiple extrahepatic cysts (IVB); and type V choledochal cysts, also known as Caroli's disease, are multifocal cystic or saccular dilated intrahepatic bile ducts that may diffusely involve the liver, or less commonly involve only the left segment of the liver.

A more recent classification scheme, the Miyano Classification [47] (Fig. 2), now classifies chole-dochal cysts according to an association with the common channel. The disease is thus classified as:

1) associated with the common pancreatic bile duct: A (cystic), B (with intrahepatic biliary tree dilatation), C (fusiform),

2) without common pancreatic bile duct association: D (diverticula), E (sub-stenosis of the papilla with distal dilatation), F (intrahepatic dilatations - Caroli's disease). Choledochal cysts typically vary from 1-10 cm, in size, although larger forms exist which may contain 5-10 l of bile. Histopathologically, the wall is usually thickened by inflammation and fibrosis, and is stained with bile; islets of cylindrical or columnar epithelium and intestinal metaplasia may be present.

The classic clinical triad of pain, a mass in the right upper abdominal quadrant and jaundice oc

Todani Classification Biliary Tree
Fig. 1. Todani Classification of cystic dilatation of the biliary ducts
Fig. 2. Miyano Classification of cystic dilatation of the biliary ducts

curs in fewer than a third of patients with chole-dochal cysts. In newborns and infants, obstructive jaundice is the most common presentation, while in older children and adults the signs and symptoms are those of ascending cholangitis [65].

From the surgeon's point of view, road mapping is important for planning the operative approach to choledochal cysts. Information on the anomalous arrangement of the pancreaticobiliary duct system and the morphology of the duct is extremely important for determining the appropriate surgical procedure. Surgeons are concerned about the exact location of the pancreatic duct, the site of entry in the duodenum, and the length of the common channel.

At US these malformations appear as anechoic, hypoechoic fusiform or cystic lesions in the region of the porta hepatis, with communication to the biliary tree (Fig. 3). The gallbladder is always wellseparated from the cyst. Often it is possible to observe intraluminal sludge or stones within the biliary tree. Color-Doppler US shows the absence of vascularization within the cyst (Fig. 4). Unfortunately, the precise extent of cystic dilatation of the biliary system, the relationship of the cyst to the gallbladder and pancreatic duct, and the angle and site of the junction with the duodenum may be difficult to assess using US [36].

ERCP is frequently used to study the biliary tree in patients with choledochal cysts. However, it

Fig. 3a, b. Choledochal cyst. US scans of choledochal cysts show an anechoic fusiform lesion (a, arrowhead and a cystic lesion (b, asterisk) in the region of the liver hilum

Fig. 3a, b. Choledochal cyst. US scans of choledochal cysts show an anechoic fusiform lesion (a, arrowhead and a cystic lesion (b, asterisk) in the region of the liver hilum

Fig. 4. Choledochal cyst. On Color Doppler US the choledochal cyst does not show any vascularization

is a diagnostic procedure that demands technical expertise and a number of safety measures, and it is not without complications. These include development of sepsis in an obstructed system, perforation of the viscera pancreatitis, and overdose of contrast agent.

At present, CT cholangiography (CTC) and increasingly, MRC, are effective non-invasive imaging alternatives to invasive cholangiography and ERCP, particularly in adult patients. MRC in particular appears to offer similar information to ER-CP without the potential complications [45]. Both MRC and CTC offer comparable performance in establishing a diagnosis of choledochal cyst in pe-diatric patients [36].

MRC is based on imaging of stationary fluids on heavily T2-weighted pulse sequences. Since bile and pancreatic secretions have high signal intensity, choledochal cysts are characterized by a hyperin-tense tubular, fusiform or cystic structure (Fig. 5).

For the detection of intraductal stones CTC seems to be slightly superior to MRC. This could be related to the fact that the signal within the biliary tree is generated by the long T2 of bile. Since the T2 is shortened by protein plugs and stones, these may be more difficult to see on MRC compared to CTC. Nevertheless, when MRC is used, 2D reconstruction appears to be superior to 3D reconstruction for the detection of intraductal stones (Fig. 6).

Caroli's Disease

Caroli's disease, also known as communicating cavernous ectasia of the biliary tree, is a rare congenital disorder characterized by non-obstructive, saccular or fusiform dilatations of the intrahepatic bile ducts. It corresponds to type V of the Todani Classification and to type F of the Miyano Classification and occurs with equal frequency in males and females.

Two types of the disease have been described: the real, so-called "pure" type, and the more common type associated with congenital hepatic fibro-sis. The less common pure form is characterized by saccular, digitate, or moniliform estasia of the intrahepatic bile ducts, with no other histologic abnormalities. The abnormalities typically predominate in one segment, usually in the left lobe, and may be diffuse or localized. The ectasias communicate freely with the bile ducts, promoting stasis and sludge formation, which can lead to lithia-sis and cholangitis. Biliary infection and stones account for the usual presenting symptoms of fever and abdominal pain. Cholangiocarcinoma develops in approximately 5-10% of cases [46].

The more common form presents in childhood with abnormalities related to hepatic fibrosis and portal hypertension. In contrast to congenital hepatic fibrosis that arises due to the abnormal development of small interlobular bile ducts, the

Mri Having Problem

Fig. 5a-g. Choledochal cyst type IC. On heavily T2-weighted sequences (a-f), the choledochal cyst appears as a fusiform, markedly hyperintense structure (arrowheads). The cyst shows reduction in calibre (c-f) just above the union with the Wirsung duct (arrow). 3D MR cholangiography (g) shows the ductal dilatation that involves the right hepatic duct (arrowhead). Note the normal calibre (arrow) below the conjunction between the choledochal cyst and Wirsung duct

Fig. 6a, b. Choledochal cyst. 2D MR-cholangiography in coronal orientation (a) demonstrates a choledochal cyst and well-defined, round, hypointense intraductal structures (arrowheads) that correspond to stones. On 3D MR-cholangiography (b) the number and size of stones (arrowhead) is less accurately defined

b more common form of Caroli's disease affects the large intrahepatic bile ducts. Histologically, intrahepatic bile duct ectasia and proliferation are associated with severe periportal fibroses. Compared with "pure" Caroli's disease, biliary duct dilatation is less marked in this form and cholangitis and biliary stone formation are usually absent. However severe problems may arise due to liver failure or complications associated with portal hypertension.

Proposed mechanisms for bile duct malformation include abnormal growth of the developing biliary epithelium and supporting connective tissue, and a lack of normal involution of the ductal plates that surround the portal tracts, resulting in epithelium-lined cysts surrounding the portal triads [34]. In the complex form, the genetic factor that causes the arrest in ductal plate remodeling seems to act not only during the early phase of embryogenesis, but also later during development of the more slender intrahepatic bile ducts and interlobular ducts. This results in the development of hepatic fibrosis at a more peripheral level of the biliary tree. Both forms of Caroli's disease can occur in combination with kidney abnormalities such as infantile polycystic kidney disease and medullary sponge kidney as well as other types of choledochal cysts. If all levels of the biliary tree are involved, features of both congenital hepatic fibro-sis and Caroli's disease are present. This condition is often termed Caroli's syndrome [34,41,46].

Macroscopically, the intrahepatic cystic dilatations are round or lanceolate, from a few millimeters up to 5 cm in size, and may be separated by stretches of essentially normal duct. Microscopically, the dilated ducts frequently show chronic in flammation, and varying degrees of fibrosis and hyperplasia. Patients typically suffer from bouts or recurrent fever and pain, and jaundice may occur when sludge or stones block the common bile duct.

In the pure form of Caroli's disease, cholan-giography reveals multiple communicating sacculi of the intrahepatic biliary tree. Stones are common and appear as filling defects. Bile duct strictures and wall irregularities may occur as a consequence of recurrent cholangitis. A similar appearance may be observed with magnetic resonance cholan-giopancreatography (MRCP).

On US and CT examinations, the sacculi appear as well-defined intrahepatic cystic anechoic and hypodense areas, respectively (Fig. 7, 8). Color Doppler US usually reveals a characteristic "dot sign" related to the presence of portal vein branches at the periphery of the bile duct dilatation. The presence of stones and/or sludge may increase echogenecity and density, and the content may appear heterogeneous. Demonstration of communication between sacculi and the bile duct is important in distinguishing Caroli's disease from poli-cystic liver disease, but this is not easy to detect when the disease is in the early stages [46].

On MR imaging, the sacculi appear as homogeneously hypointense areas on T1-weighted images and as homogeneously hyperintense areas on T2-weighted images (Fig. 9). The signal intensity is generally homogeneous but may appear heterogeneous if stones, sludge, or phlogistic material are present within the sacculi and bile ducts (Fig. 10). In this latter situation, the surrounding liver parenchyma may also show changes in signal intensity [3]. MRCP is a valid tool for demonstrating

Fig. 7. Caroli disease. On US, round and tubular, hypoechoic structures (arrowheads) are visible
Caroli Disease
Fig. 9a, b. Caroli disease. On pre-contrast Tl-weighted GRE (a) and T2-weighted TSE (b) images, diffuse intrahepatic round lesions (arrows), corresponding to intrahepatic sacculi, are homogeneously hypointense and homogeneously hyperintense, respectively
Fig. 10a, b. Caroli disease. Pre-contrast Tl-weighted GRE (a) and T2-weighted TSE (b) images show heterogeneity within the sacculi (arrowheads) due to the presence of intraductal stones
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