Liver

colon can be distinguished from small intestinal loops by the presence of solid faecal matter and gas, its larger calibre, its relatively featureless smooth wall, and by following its sigmoid course from descending colon to rectum. CT colonography is a technique in which the colon is insufflated w i th air to distend it. The colon can then be followed from rectum to caecum on sequential thin CT cuts to screen the mucosa for polyps.

THE LIVER (Figs 5.25-5.27) The liver, the largest organ in the body, is found in the right upper quadrant of the abdomen. It is relatively much larger in the fetus and child. The liver assumes the shape of the cavity it occupies. It has two surfaces, the diaphragmatic surface and the visceral surface.

The diaphragmatic surface is smooth and flat posteriorly and has a smooth, rounded upper surface w i th a large dome for the right hemidiaphragm and a smaller dome for the left hemidiaphragm. A depression between these marks the site of the central tendon and the overlying heart.

The diaphragmatic surface ends anteriorly in the inferior border of the liver. This lies at the costal margin laterally to within about 4 cm from the midline, the site of the gallbladder notch. Medial to this, the inferior border ascends less obliquely than the costal margin and lies below it as it crosses the midline to meet the costal margin of the left side at approximately the eighth costal cartilage. The lateral extent of the left lobe is variable: it may extend only to the midline or may surround the stomach or spleen to reach the left lateral abdominal wall.

In addition to the notch for the gallbladder, the inferior border is marked by a notch for the ligamentum teres. This ligament is the obliterated remnant of the left umbilical vein, which carries blood from the placenta to the fetus. It is also known as the round ligament. It passes, w i th small

paraumbilical veins, from the umbilicus to the inferior border of the liver in the free edge of a crescentic fold of peritoneum called the falciform ligament (meaning 'sickleshaped'). This meets the liver just to the right of the midline. The site of attachment of the falciform ligament is used as an anterior marker of the sagittal plane of division of the liver into traditional anatomical left and right lobes.

The posteroinferior, or visceral, surface of the liver is marked by an H-shaped arrangement of structures (see Fig. 5.25b). The crossbar of the H is made by the horizontal hilum of the liver called the porta hepatis. This is the entry site of the right and left hepatic arteries and portal veins, and also the exit of the right and left hepatic ducts. There are also autonomic nerves and lymph vessels.

The gallbladder in its bed, together w i th the IVC in a deep groove or tunnel, forms the right vertical part of the H. These are separated by the caudate process.

The left vertical part of the H is formed by the ligamentum teres and the ligamentum venosum. The ligamentum teres runs to its attachment to the left portal vein in the left extremity of the porta hepatis. This is continuous w i th the fissure for the ligamentum venosum. This is a deep fissure lined by peritoneum, w i th the obliterated remnant of the ductus venosum (the ligamentum venosum) at its base. The ductus venosum shunts blood from the left umbilical vein to the IVC in the fetus, bypassing the liver. At the upper end of the fissure the ligamentum venosum curves laterally to attach to either the left hepatic vein or the IVC.

The visceral surface of the liver lies in contact w i t h, and is slightly moulded by: • The oesophagus, stomach and lesser omentum on the left; • The pancreas (through the lesser omentum) and the duodenum in the midline; and • The right kidney and adrenal and the hepatic flexure of the colon on the right.

The peritoneal attachments of the liver determine the distribution of free gas and of fluid in this part of the peritoneal cavity and are discussed in the section on the peritoneum.

Old lobar anatomy

Traditional anatomic teaching divides the liver into a large right and a small left lobe. These are divided anteriorly by the attachment for the falciform ligament, and on the visceral surface by the grooves for the ligamenta teres and venosum. Two further lobes are described: the caudate lobe posteriorly between the IVC and the fissure for the l i gamentum venosum, and the quadrate lobe anteroinferiorly between the gallbladder bed and the fissure for the ligamentum teres. These lobes are part of the conventional right lobe. The caudate lobe is attached to the remainder of the right lobe by the caudate process. This division of the lobes bears no relationship to the functional structure of the liver and has been replaced by new International Anatomical Terminology. However, the old terminology is still in frequent use (see Table 5.3 for a comparison between old and new terminologies).

Segmental liver anatomy

(Figs 5.26 and 5.27 and Table 5.3) The liver is comprised of two functionally independent right and left lobes, defined by the arterial distribution. Each is supplied by the right and left portal veins and the right or left hepatic arteries, and each drained by the right or left hepatic duct. The plane of division between these lobes is called the principal plane. There is no external marking of this plane on the anterosuperior surface, but it lies parallel to and about 4 cm to the right of the attachment of the falciform ligament. On the visceral surface the principal plane is defined by the IVC superiorly and the gallbladder bed inferiorly The hepatic veins do not run w i th the structures of the portal triad (portal vein, hepatic artery and bile duct) but are intersegmental, draining portions of adjacent segments. The middle hepatic vein lies in the principal plane and drains from both lobes.

In current terminology, the left lobe includes the caudate lobe (which lies between the IVC and the fissure for the ligamentum venosum) and most of the quadrate lobe (which lies between the gallbladder bed and the fissure for the ligamentum teres). This is defined by the distribution of the left hepatic artery. The right hepatic artery supplies a variable portion of the quadrate lobe.

In 5-10% of females and rarely in males, the lower border of the right lobe, a little to the right of the gallbladder, may project downwards for a considerable distance as a broad tongue-like or bulbous process called Reidel's lobe. This is not a true lobe.

Further subdivision into segments is based on branches of the right and left hepatic arteries. Knowledge of these segments is essential in assessment of the location and extent of hepatic pathology prior to liver surgery, because this is performed in segmental fashion and the distribution of disease determines whether lesions are resectable. Segments are numbered in the Couinaud system in a clockwise direction starting at the caudate lobe. The caudate lobe is segment I. Segments II and I II are the furthest left, divided by the left hepatic vein from segment IV. The left portal vein separates segment II above from segment I II below. Segment IV lies between the left hepatic vein and the middle hepatic vein. It is divided into segment IVa above and IVb below by the left portal vein. The right lobe has four segments, divided by the right hepatic vein into anteromedial and posterolateral divisions and by the plane of the right branch of the portal vein into superior and inferior sections. These four segments are numbered in a clockwise fashion from anterior inferomedial: V, V I, V II and V I II (see Fig. 5.22). The segments may also be named descriptively according to their location, e.g. posterior

Table 5.3 Surgical and traditional anatomical terminology

Anatomical division

Surgical/ functional division Right lobe Left lobe Caudate lobe

Right (part of) liver Left (part of) liver Posterior (part of) liver

Right lateral division Right medial division Left medial division Left lateral division Right caudate Left caudate

Seg VII (posterior superior) Seg VIII (anterior superior) Seg IVa (medial superior) Seg II (lateral superior) Segment I

Seg VI (posterior inferior) Seg V (anterior inferior) Seg IVb (medial inferior) Seg III (lateral inferior)

segment (caudate), right posterior lateral, posterior medial, anterior lateral and anterior medial segments, and left medial superior, medial inferior and lateral segments (see Table 5.3).

The functional subunit of the liver is the microscopic lobule, which has a central vein and, in spaces between the lobules, portal canals or triads, each w i th a branch of the hepatic artery, portal vein and bile duct.

The old anatomical description of the liver as having a large right lobe and small left lobe separated by the falciform ligament defines the anatomical left lobe as consisting only of segments II and I I I.

The arterial supply of the liver

The hepatic artery, one of the three branches of the coeliac trunk, supplies the right gastric and gastroduodenal arteries before approaching the liver in the free edge of the lesser omentum, anterior to the portal vein and medial to the bile duct. It divides into approximately equal-sized right and left hepatic arteries before entering the liver at the porta hepatis.

Variations in arterial supply to the liver These are common and important to recognize prior to surgery or to hepatic arterial interventional procedures. Tiny anastomoses exist between the vascular territories. Accessory arteries, where they exist, provide a vital arterial contribution to the segments they supply. The hepatic artery normally passes anterior to the portal vein and posterior to the common hepatic duct. Hepatic arteries originating from the SMA pass posterior to the portal vein and lateral to the common bile duct. This is an important variation to recognize at surgery as the bile duct could be clamped inadvertently.

Accessory branches may exist in addition to the normal origin of the hepatic artery from the coeliac trunk. A ll or part of either hepatic artery may be replaced by a vessel originating in a different manner from the norm. The following variations exist: • The entire liver is supplied by a replaced hepatic artery in 2.5%. • The entire right lobe is supplied by a replaced right hepatic artery in 10%. • Part of the right lobe is supplied by an accessory right hepatic artery in 6%. • The left hepatic artery is replaced by the left gastric artery in 12%. • Accessory left hepatic arteries arise from the left gastric in 13%. • The common hepatic artery may divide early or trifurcate w i th the gastroduodenal artery. • The hepatic artery may arise separately from the aorta rather than from the coeliac trunk.

Fig. 5.28 Portal vein: segmental anatomy. Usual configuration (> 90% cases). ,

Portal vein (Fig. 5.28) The portal system is much less prone to anatomical variation than the hepatic artery. It normally forms posterior to the neck of the pancreas by the union of the superior mesenteric vein (SMV) and the splenic vein at the level of the L1/L2 disc space. It ascends anterior to the IVC and passes to the right in the posterior aspect of the free edge of the lesser omentum. It runs posterior to the bile duct and the hepatic artery to the porta hepatis. At the porta it divides into right and left branches to supply the right and left lobes. The usual configuration of the portal venous system in the liver is shown in Figure 5.28.

In most individuals the inferior mesenteric vein joins the splenic vein close to the confluence of SMV and splenic vein. In approximately 40% it drains into the SMV. In approximately one-third of individuals the inferior mesenteric vein joins w i th the SMV and splenic vein in a confluence. Very rarely the portal vein is double, caused by non-union of the superior mesenteric and splenic veins (cf. section on portal system).

Venous drainage of the liver (Fig. 5.29) The liver is drained by hepatic veins, which drain upwards and backwards to the IVC without an extrahepatic course. (These veins also assist in the stabilization of the liver.) The distribution of the hepatic veins differs from that of the hepatic artery, the portal vein and the bile ducts. Right, middle and left hepatic veins drain corresponding thirds of the liver. The middle hepatic vein lies in the principal plane and may unite w i th the left hepatic vein and have a common final course to the IVC. A lower group of small veins

Fig. 5.29 Ultrasound of liver.

(a) Transverse image showing confluence of hepatic veins. In this case, two middle hepatic veins join close to the confluence.

1. IVC 2. Left hepatic vein 3. Middle hepatic veins 4. Right hepatic vein

(b) Sagittal section through the left lobe of the liver to show gastro-oesophageal junction.

1. Left lobe of liver 3. Aorta 2. Gastro-oesophageal junction 4. Spine

(c) Sagittal section of IVC in liver.

1. IVC 3. Right lobe of liver 2. Right renal artery

drain directly to the IVC from the lower parts of the right and caudate lobes. Hepatic veins have no valves.

Lymphatic drainage of the liver

Lymph from the deep lymphatics of the liver drains in the connective tissues of the portal triads and along hepatic veins. Lymphatics accompany the portal vessels and ducts draining to nodes in the porta hepatis, to hepatic nodes along the hepatic vessels and to ducts in the lesser omentum. From here lymph drains via retropyloric nodes to the coeliac nodes and thence to the cisterna chyli. There is also a superficial network of lymphatics under the capsule of the liver. The anterior parts of the diaphragmatic and visceral surface of the liver drain to the deep lymphatics. The posterior part of the visceral and diaphragmatic surfaces drain toward the bare area of the liver. From here, lymph drains into phrenic lymph nodes, or joins deep lymphatics that run w i th the hepatic veins towards the IVC. These lymphatics pass through the diaphragm w i th the IVC and drain into posterior mediastinal lymph nodes.

Radiological features of the liver

Plain films of the abdomen (see Fig. 5.1) The margins of the liver are visible where they are outlined by fat; thus the lateral border can be seen where it is flanked by properitoneal fat. Intraperitoneal and omental fat lie near the inferior border. This border is best seen in the obese, whose visceral surface tends to be less steep and more tangential to the beam on AP views. A ir in the lungs outlines the pleura and diaphragm over the upper limit of the liver, but gas in the stomach may give a misleading impression of the position of the inferior border of the liver, as the stomach may pass posterior to this border.

CT and MRI (see Figs 5.2-5.4, 5.10 and 5.11; 5.56-5.60) The relationship of the liver to the diaphragm, pleura and lungs can be seen on CT. The relations of the visceral surface can be seen on lower cuts, that is, the lesser curve and stomach on the left, the pancreas and duodenum posteriorly in the midline, and the right kidney and adrenal on the right.

The IVC can be seen in a tunnel or groove near the dome of the liver posteriorly, w i th the hepatic veins draining into it. The structures at the porta hepatis and the gallbladder can also be seen. The positions of the lobes and of segments are deduced from a knowledge of their relationship to these structures and their blood supply. The vessels that divide the liver into segments do not run in straight lines and must be identified on sequential axial images in order to assign the segments correctly. This is somewhat easier on MRI, where the images can be obtained in sagittal and sagittal oblique planes along the hepatic veins. CT images that have been acquired by continuous helical scanning can also be reformatted from their three-dimensional volume data sets and displayed in various planes. This is useful for hepatic surgical planning.

The caudate lobe is connected to the remainder of the right lobe by the caudate process, which can be seen passing between the IVC and the portal vein. A projection of the caudate lobe to the left of the portal vein, called the papillary process of the caudate lobe, may mimic a porta hepatis mass on CT if its continuity w i th the liver is not appreciated. Fat within fissures, including incomplete accessory fissures on the smooth diaphragmatic surface, may also mimic a mass. Prominent diaphragmatic muscle slips may indent the soft surface of the liver but can be identified by their characteristic location, and are usually multiple. Fat covers the diaphragmatic slips and this also helps to distinguish this variant from masses.

The hepatic veins lie between the following segments: • The middle hepatic vein lies between the right and left lobes. • The left hepatic vein lies between medial and lateral segments of the left lobe. • The right hepatic vein lies between anterior and posterior segments of the right lobe.

Ultrasound (Fig. 5.29) As a solid organ, the liver is particularly suitable for ultrasound examination (Fig. 5.29) and it is seldom covered by gas-containing bowel. Its smooth contour and soft structure can be appreciated as it moves up and down w i th breathing. The liver is used as an acoustic window for visualization of other structures, including the right kidney and adrenal gland, the gallbladder and the pancreas. Vessels and bile ducts of the liver are particularly well seen on ultrasound studies (see section on Biliary tree). Blood flow can be studied using colour flow Doppler, and the direction and velocity of flow in the portal vein can be evaluated w i th pulsed wave Doppler. Anatomical features are as for CT and MRI.

Magnetic resonance imaging On MRI the liver is of equal signal intensity to the pancreas and higher on T1- and lower on T2-weighted images than the spleen. Normal hepatic vessels are seen as areas of signal void on standard imaging. The major hepatic veins and the secondary branches of the portal veins are visible. Hepatic arteries are less well seen unless intravenous contrast is given. The intrahepatic biliary radicals are seen on fluid-sensitive imaging in MR cholangiography (see section on Biliary tree) On T2-weighted images the ligamentum venosum and the ligamentum teres are of low intensity but the fat within their fissures is of high intensity. As w i th CT and ultrasound, the hepatic veins and these fissures aid in identification of segments and lobes of the liver.

Hepatic arteriography (see Fig. 5.12) This is achieved via the aorta and the coeliac trunk w i th greater selectivity if the contrast agent is injected distal to the origin of the gastroduodenal artery. The frequency of variation in the arteries may make injection of the superior mesenteric and left gastric arteries also necessary.

MR and CT angiography can also produce excellent images of the coeliac trunk and SMA. These are acquired after the injection of gadolinium or iodine contrast agents intravenously as a bolus, after several seconds to allow the contrast to pass into the arterial system. In both techniques axially acquired data can be displayed in any plane. With MR, angiographic images can also be acquired without contrast using flow-sensitive imaging techniques.

CT angioportography (CTAP) In this technique CT is performed approximately 60 seconds after selective injection of the superior mesenteric artery, w i th images acquired in the portal venous phase. This technique was more commonly performed preoperatively before the more universal availability and vastly improved capabilities of MRI, and was used primarily to identify liver tumours or metastases in patients being considered for resection. In approximately 10% of individuals portal perfusion defects (areas of non-enhancement) in segments I (caudate), IV (gallbladder bed) and around the falciform ligament can be seen on CTAP. These have been shown to be produced by aberrant venous drainage directly into subsegmental hepatic parenchyma. These three sites are also typical areas for focal fatty infiltration, or focal sparing in cases of diffuse fatty infiltration of the liver.

Embolization of the hepatic artery This is sometimes undertaken in symptomatic tumours of the liver. Embolization of the hepatic artery branches, which are end arteries, or arteries w i th only tiny anastomoses, does not result in infarction because of the dual supply to the liver, w i th the portal venous system also bringing blood to the liver. Radiographic demonstration of the patency of the portal system is essential prior to embolization. When the right lobe is being embolized it is