2 Liver

The liver is the largest gland as well as the largest single organ in the human body, weighing 1500–1800 g (accounting for about 2.3–3% of body weight). Blood, primarily from the splanchnic region, flows through the liver, where numerous substances are recovered and metabolized. Apart from breaking down and synthesizing substances, the liver is a key element in storage pathways (glycogen, fat). It either releases these metabolites directly into the bloodstream or excretes them via the bile (1500 mL/day). In addition, the liver is characterized by its organ-specific macrophages (Kupffer cells), which are an integral part of the human macrophage system (reticuloendothelial system, RES). In summary, this organ plays a vital role not only in metabolic detoxification but also in the synthesis de novo and storage of proteins, sugar molecules, and fat as well as in the secretion of digestive juices.

In order to carry out these varied functions, the anatomy of the liver is based on the socalled hepatic lobules with their functional unit, the hepatic acinus, according to Rappaport (Fig. 2.1). Splanchnic blood flowing through a branch of the portal vein passes through three successive metabolic zones where the substances transported by the blood are metabolized. All products synthesized by the liver are passed into the bloodstream through the central vein or excreted via a countercurrent mechanism into the bile ducts that parallel the terminal portal branches (Fig. 2.2).

Fig. 2.1 Hepatic lobule and acinus according to Kiernan and Rappaport. The gray hepatic lobule of Kiernan is characterized by its central vein and the peripheral portal areas; the colored hepatic acinus of Rappaport has a terminal branch of the portal vein in its center while the central veins (of the hepatic lobule) are on its periphery, and in between there are three distinct metabolic zones: zone 1 is rich and zone 3 poor in nutrients.

Fig. 2.2 Schematic diagram of a hepatic sinusoid together with a portal triad. In the sinusoids, blood from the terminal branch of the portal vein (joined in the portal triad by the artery and the bile duct) flows through the three successive metabolic zones into the central vein. Excretion into the bile ducts paralleling the branches of the portal vein is by countercurrent principle.

The liver is located in the right hypochondrium, protected by the ribs (Fig. 2.3). Its general shape is that of a pyramid, with the base pointing to the right side of the body. The average transverse diameter is 25–30 cm, the cephalocaudal length is 12–20 cm, and the normal anteroposterior diameter is 6–10 cm. The superior aspect (diaphragmatic surface) borders the diaphragm and is composed of a fixed and a mobile part; parts of the anterior aspect are in contact with the abdominal wall. The inferior surface of the liver points in a posterocaudal direction; on the left and in the middle the inferior aspect is in contact with the gastric wall, while on the lateral right side it borders the hepatic flexure of the colon. The gallbladder is attached to the inferior surface of the liver in the midclavicular line. The right dorsal aspect of the liver is anterior to the upper pole of the right kidney. The porta hepatis is situated in the middle of the inferior surface, anterior to the caudate lobe (see below) and the dorsal vena cava (Fig. 2.4); the porta hepatis contains the hepatic artery proper and its two branches, the left and right hepatic artery, the extrahepatic bile duct, lymphatic vessels, and nerves.

Fig. 2.3 Topography. Location of the liver in the right hypochondrium, protected by the lower right ribs; note the relationship with the diaphragm, right kidney, major retroperitoneal vessels, colon, and stomach.

Fig. 2.4 Schematic anatomy of the porta hepatis. Aorta and vena cava are posterior; the portal vein is on the right, coursing obliquely and anteriorly to the vena cava. The portal vein is posterior to the hepatic artery and medial to the extrahepatic bile duct.

From a macroscopic anatomical point of view, the liver is subdivided into a larger right lobe (representing the base of the pyramid) and a smaller left hepatic lobe (the apex of the pyramid), with the falciform ligament (containing the round ligament or ligamentum teres) dividing the two lobes (Fig. 2.5). In terms of its microanatomy, the liver is made up of eight functionally separate segments that follow the vascular tree (Fig. 2.6; Table 2.1). If circumscribed masses or pathological findings can be correlated to a specific segment, this is extremely helpful in preoperative and intraoperative diagnosis as well as for possible resectability or any planned interventional angiographic procedures. However, since anatomical variants are numerous and not particularly infrequent, segmental correlation is not always certain and thus may be of only limited value in some cases.

Fig. 2.12 The caudate lobe often has an own runoff into the caval vein; this may be the explanation for the hypertrophy in Budd–Chiari syndrome and veno-occlusive disease. Accessory finding: small hemangioma segment I (cursors).

Table 2.2 Assessment of liver size (longitudinal view in right midclavicular line)

Extrinsic Criteria

Size—depends on the patient’s build (Table 2.2)

●Thick-set: 80 mm

●Regular build: 120 mm

●Asthenic: 150 mm

●Caudate lobe: ca. 50 × 20 mm

●Left hepatic lobe < right lobe

Shape

●Left hepatic lobe (viewed anteriorly to the aorta): convex/concave

●Right hepatic lobe (viewed laterally right): biconvex

Surface

● Smooth

Contour

●Sharply angled configuration, 30 ° (asthenic)

●<45 ° (pyknic)

Anomalies

●Situs inversus

●Accessory lobe

Extrinsic criteria ( 2.1; Table 2.3). The size of the liver, its superior and inferior aspect, its shape, its caudal edge, and any normal variants are assessed on the basis of the following extrinsic criteria.

The size of the liver is determined along the midclavicular line and is given in centimeters; to a large extent this parameter depends on the build of the patient (Table 2.2). In addition, the dimensions of the caudate lobe and the distribution of mass between left and right hepatic lobe have to be assessed.

Usually the surface of the liver is smooth; during routine studies its appearance is better visualized at the inferior aspect of the liver since, because of the physics of the ultrasound beam, the image is blurred in the near field. Another alternative is to switch to a higherfrequency probe to assess the ventral surface.

The shape takes into account the distribution of mass between the right lobe and the left as well as the caudate lobe; in addition, the inferior aspect of the organ is assessed in lon-

gitudinal views, with the left lobe being imaged in the midline anterior to the aorta (the anterior aspect is convex and the posterior one concave), while the right hepatic lobe is scanned in the right axillary line (biconcave).

The inferior contour of the liver is examined, too; for the left lobe it is sharply angulated at 30–45 °, while the lateral edge of the right hepatic lobe is blunt (60 °).

Normal variants are uncommon and, apart from a variant position (situs inversus), the sonographer has to keep accessory lobes in mind (so-called Riedel lobe) (Fig. 2.10,

Fig. 2.11).

Intrinsic criteria (Table 2.4). The intrinsic criteria deal with the actual parenchymal echotexture of the liver, organ-induced attenuation of the ultrasound beam, and in particular the demonstration of and changes in the venous vessels and portal triads with portal vein, bile duct, branches of the hepatic artery, and the lymphatics.

The echotexture of the hepatic parenchyma ( 2.2) is made up of numerous individual echoes. Assessment of these individual echoes has to consider size (graininess), brightness, and uniformity (homogeneity) as well as distribution and position (density). The parenchyma of a normal liver will present as finely dispersed, moderately bright, homogeneously and uniformly distributed individual echoes of moderate echogenicity. With normal presettings of the equipment, no relative amplification or attenuation will be seen: both criteria are highly dependent on the type of equipment used and the presets (time gain compensation, TGC).

The hepatic veins ( 2.3 and 2.4) converge on the so-called venous star (Fig. 2.8, Fig. 2.13), which terminates at the vena cava; they drain the blood from the periphery of the liver, run in straight lines between the segments with sharply angled branching, are smoothly lined, and do not demonstrate any wall echoes. The blood flow within the hepatic

d Usually, the surface of the liver is smooth (most often best assessed at the inferior aspect).

g Localized changes in the surface due to scarring after bypass surgery with subsegmentation in the left lobe.

j Circumscribed changes in the surfaces bulging into the gallbladder wall due to a hemangioma.

e Inflammatory disorders will result in diffuse changes in the entire surface of the liver with an irregular wavy contour, here in chronic autoimmune hepatitis.

h Scar at the inferior aspect of segment VI after partial liver resection, filled with fluid; ascites? bile?

k Gastric cancer metastasis infiltrating and penetrating into the diaphragm with concomitant pleural effusion.

f Irregular nodular hepatic surface in cirrhosis; imaging is facilitated by the presence of ascites.

i Intrahepatic masses can bulge the surface of the liver and indent adjacent organs: here, a unilocular cyst at the inferior aspect of segment III indenting the gallbladder wall.

l Cancerous impression at the inferior aspect of segment VI of the right hepatic lobe with central pit and bulging wall.

Intrinsic Criteria

Parenchymal echotexture

●Homogeneous, finely dispersed, me- dium-sized, moderately bright, individual echoes

Conduction

● Normal

Hepatic veins

●Drain between segments

●Common junction at the vena cava (“venous star”)

●Run straight, with angulated branching

●No wall echoes

●Typical signal on duplex scanning

Portal vein branches

●Originate at the hepatoduodenal ligament (bifurcation of the portal vein) and course in the center of the segments

●Periportal encasement

●Sufficient duplex signal for analysis

Hepatic artery

● Parallel the branches of the portal vein

Bile ducts

● Parallel the branches of the portal vein

Lymphatics

●Parallel the branches of the portal vein

●At this time without clinical significance

●Pathological lymphadenopathy at the porta hepatis

Dynamic Criteria

Respiratory motion

●During inspiration 2–3 fingerbreadths or 4–6 cm (~2 inches) caudad

Consistency

●Squeezable like a wet sponge

●Returns to its original shape once the pressure is released

Tenderness

● None

Example

●Abscess, adhesions

●Infiltrative changes, cirrhosis

●Capsular tension

Dynamic criteria (Table 2.5). The dynamic criteria take into account the respiratory motion of the liver, its consistency, and its (possible) tenderness on palpation.

During inspiration the liver moves caudad by 2–3 fingerbreadths or 4–6 cm (ca. 2 inches). The liver has a soft consistency and the palpating finger (e. g., of the left lobe anterior to the aorta/spine) can squeeze it like a wet sponge; when the pressure is removed it will return to

its original shape. The consistency of the liver can also be assessed around the superior aspect of the left lobe where the diaphragm passes on the pulsations of the adjacent heart. Normally the liver should be free of pain and palpation should not elicit any tenderness.

Supplementary findings. Any pathological findings in other organs and structures of the abdomen must also be noted, for instance in

the gallbladder (wall thickening, visible blood vessels within the wall), spleen (splenomegaly), pancreas (cysts, pancreatitis), and portal vessels (varicosities, collaterals), as well as any free intra-abdominal fluid (amount, type) (Table 2.5).

Table 2.6 is a summary of the criteria used in sonographic assessment of the liver.

Venous Flow Pattern

There is a distinct difference between the flow patterns in the hepatic and the portal veins (Fig. 2.15).

Hepatic veins. Here, blood flow is characterized by a physiological triple-peak/tripha- sic curve resembling the pulse curve in the jugular vein; it mirrors the pressure situation in the right heart.

●Phase I: first part of the blood flow is toward the heart; at the latter, it corresponds to the mechanical displacement of the valve plane (systolic phase)

●Phase II: second part of the blood flow is toward the heart; at the latter, it corresponds to the opening of the atrioventricular valve (diastolic phase)

●Phase III: regurgitation (back flow) is from the right atrium; it results from the contraction of the right atrium and the bulging of the tricuspid valve

Measurements are taken at all three major hepatic veins. The flow pathology concerns the make-up of various phases (increased regurgitation in right ventricular failure, vena cava engorgement, tricuspid insufficiency) or the number of phases (biphasic pattern or monophasic pattern in cirrhosis of the liver). A ribbon-like flow profile, solely back flow, or no flow at all are found in Budd–Chiari syndrome or veno-occlusive disease (VOD).

The arterial blood flow accounts for 20% of the entire blood flow of the liver in a healthy fasting individual. The remaining 80% flows through the portal vein.

This ratio can be changed by physiological influences (postprandial fall of the arterial flow) as well as pathological influences (e. g., increased arterial flow in portal hypertension, cirrhosis, tumor infiltration, missing decrease of postprandial arterial flow in patients with cirrhosis).

The blood flow in the hepatic artery proper resembles the curve of a parenchymal arterial vessel with a low peripheral resistance

(RI< 0.75); The peak flow rate (Vmax) is 40 cm/s. A lower RI followed by an increase in velocity can be seen in an intrahepatic arteriovenous fistula or shunt, e. g., in hereditary hemorrhagic telangiectasia (Osler’s disease of the liver).

A high RI can indicate a disruption of arterial circulation downstream as seen especially in graft rejection; after transplantation the hepatic artery must therefore be monitored closely, using Doppler sonography.

The arterial vascularization of the liver can be determined in the hepatic artery proper

Chronic liver congestion leads to a rigid, engorged hepatic vein

Inflammatory changes leading to the appearance of contour irregularities of the hepatic veins

In malignancies, vascular disturbances may be influenced in different ways: perivascular infiltration, impression, displacement, and vascular infiltration

a Rigid engorged hepatic vein.

d Pitting and scarred indentations in inflammatory disease.

g Rarefied hepatic vein, sonographic imaging not possible because of the parenchymal changes.

j Lymphoma infiltration and covering of the portal branch, dislocation of the hepatic vein.