Location, Shape, and Borders

The liver is the largest parenchymal organ in the abdominal cavity. It is located below the diaphragm, extending from the right hypochondrium to the epigastrium, usually reaching the left subcostal edge (Figure 3.1). It has a smooth, dome‐shaped diaphragmatic surface and a visceral, more irregular one, moulded by the adjacent organs and indented by the left, right, and interlobar fissures (Figure 3.2). Normally during respiration, the liver moves following the diaphragm. This movement is important and can be increased with deep inspiration to optimise liver visualisation during ultrasound imaging in a subcostal view. The magnitude of these excursions depends on the individual’s lung capacity as well as body habitus and the mechanical properties of the thoracic wall (obesity and some structural diseases of the musculature or bone of the thoracic wall reduce this oscillation). Owing to the liver’s high anatomical variability, it is generally accepted to compare its size to the right kidney to gauge whether it is enlarged, normal, or atrophic, rather than taking an exact measurement of its diameter [1]. A subcostal maximal length of 16 cm taken in the mid‐clavicular is considered the upper limit of normal.

The suspensory system of the liver is constituted by ligaments that are seen on ultrasound as hyperechoic linear structures of different widths that fix the liver to the diaphragm, abdominal wall, and adjacent organs. Other ligaments envelope vascular and biliary structures and provide useful landmarks for the description of the complex liver structure. More specifically, the falciform ligament connects the dorsal surface of the liver to the diaphragm and to the anterior abdominal wall dividing the liver into the anatomic right and left lobes. Its free margin continues with the remnant of the obliterated umbilical vein, which is known as the ligamentum teres (or round ligament) and runs along the ventral surface of the liver, coming into direct contact with the left branch of the portal vein (PV) (Figure 3.5). The obliterated remnants of the ductus venosus constitute the venous ligament, which is seen on ultrasound as a thin hyperechoic line that surrounds the parenchyma adjacent to the retrohepatic inferior vena cava (IVC) defining the borders of the caudate lobe (Figure 3.6). The lesser omentum attaches the liver to the lesser curvature of the stomach through the hepatogastric ligament and to the duodenum through the hepatoduodenal ligament. The latter envelopes the portal triad running from the porta hepatis to the very first portion of the duodenum. It contains the liver lymphatics and is often the site of lymphadenopathies that can be seen in infective, inflammatory, or neoplastic liver diseases.

In normal conditions, the liver has smooth margins and regular contour, the echotexture is homogeneous, and the echogenicity is almost equal to or slightly brighter than the cortex of the right kidney (Figure 3.3). The liver is enveloped within the fibrous Glisson’s capsule, which contains sensitive nerve endings supplied by the phrenic nerve. The capsule can be barely seen on ultrasound as a hyperechoic line that permeates the liver in direct contact with the peritoneum and is therefore more easily distinguished when there is ascites (Figure 3.4).

Gallbladder and Biliary Tree

The gallbladder (GB) is a pear‐shaped structure located in the GB fossa along the inferior surface of the right liver lobe, lateral to the second portion of the duodenum and anterior to the right kidney (Figure 3.7). Its position is variable according to the patient’s body habitus [2]. Four anatomical variants are described and should be borne in mind, since anatomical landmarks and GB positioning might vary considerably:

• Hypersthenic body habitus: the diaphragm, liver, GB, and stomach tend to lie high in the abdomen and the ultrasound examination is often limited due to the presence of overlying bowel gas and food residue.
  
• Sthenic: the liver and GB lie as expected in the right upper quadrant and the GB has an oblique position.
  
• Hyposthenic: the liver and GB lie lower, often in the lumbar region, and the GB is more vertically oriented.
  
• Asthenic (extremely hyposthenic): the liver and GB might lie as low as in the right iliac fossa and the GB is vertically oriented [2].

The biliary tree can be divided into intrahepatic and extrahepatic segments. The intrahepatic ducts run across the liver from the periphery to the liver hilum, converging in larger ducts, and are in tight anatomical connection with the hepatic arterial supply and the portal venous system. In proximity to the liver hilum, the cystic duct that drains bile from the GB joins the main hepatic duct to form the common bile duct (CBD). The CBD terminates with the pancreatic duct at the ampulla of Vater within the second portion of the duodenum.

In normal physiological conditions, the CBD is the only biliary duct that can be clearly seen as a thin tubular structure with echogenic walls that in the majority of cases runs anteriorly and parallel to the PV at the level of the hepatic hilum (Figure 3.8). However, the anatomical relationship of the biliary ducts and the portal vessels may vary along their course, and usually the peripheral biliary ducts (which are only clearly visible when dilated or significantly thickened) run posteriorly to the PV (Figures 3.9 and 3.10).

The CBD measures between a minimum of 2–3 mm and an upper limit of 6–7 mm. Larger calibres are observed, especially post cholecystectomy and with age, where it is generally accepted that the calibre may increase by 1 mm each decade after 70 years [3].

Liver Vascular Anatomy

The PV is formed by the confluence of the superior mesenteric vein and the splenic vein, draining the blood of the whole digestive system and spleen (Figure 3.11). Under physiological conditions the portal venous system delivers 75% of the total hepatic inflow, whereas the hepatic artery (HA) is responsible for the remaining 25%. It is important to keep in mind the physiology and pathophysiology of the hepatic blood inflow, since during the progression of liver disease, especially when cirrhosis and portal hypertension develop, the portal venous inflow is reduced while the arterial hepatic inflow is increased (See Chapter 8). The PV can be recognised on ultrasound as a tubular structure with a variable normal calibre of approximately 8–12.5 mm, with thick echogenic walls that enters the liver together with the HA at the level of the hepatic hilum. It is followed by the HA and the biliary system in its whole intrahepatic course and for a short portion in its extrahepatic tract at the level of the porta hepatis, where it is contained within the hepatoduodenal ligament. Upon entering the liver, the PV and HA divide into the left and right branches, with further divisions providing the blood supply to each of the eight main liver segments (Figure 3.12). At the periphery of the liver lobules the arterial and venous blood mix and enter the sinusoids, terminating finally in the central veins that converge to form the right (RHV), middle (MHV), and left hepatic veins (LHV) that finally drain into the IVC (Figure 3.13). It is of note that the caudate lobe is drained independently by a main or multiple small pericaval veins. Its independent venous drainage system is the reason why the caudate lobe typically hypertrophies in advanced chronic liver disease. In Budd–Chiari syndrome, this compensatory mechanism is even more pronounced, since while the main three hepatic veins are obstructed, the pericaval ones often remain patent, leading to an abnormally hypertrophied caudate lobe (See Chapter 11).

Vascular Segments of the Liver

Based on the divisions of the portal and hepatic veins, the liver may be divided into eight segments, as first suggested by the French surgeon Claude Couinaud in 1957 (Figure 3.14) [4]. This classification relies on the fact that each of these segments has its own individual blood supply and might be resected without jeopardising the viability of other segments. In this classification, the liver segments II and III are situated to the left of the LHV and falciform ligament, and the left branch of the PV (LPV) divides them into segment II (above the PV) and segment III (below the LPV). Segment IV is situated between the LHV and the MHV and the LPV divides them into segment IVA (above the LPV) and segment IVB (below the LPV). Segments V and VIII are located between the MHV and RHV, whereas segments VI and VII represent most lateral segments situated to the right of the RHV. The right branch of the RPV divides segment V (caudal) from VIII (cranial) and segment VI (caudal) from VII (cranial) (Figure 3.15). On the dorsal, central part of the liver, between the IVC and the venous ligament, lies the caudate lobe that corresponds to segment I (Figures 3.6 and 3.12c).