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).

Liver Parenchyma Assessment

Liver ultrasound assessment requires careful scanning of all segments while evaluating the echotexture, echogenicity, margins, and contour. There are no strict rules on how to scan the liver, but rather a suggestion of a systematic approach in order to provide a concise description of liver anatomy and ensure that the whole liver and associated vascular and biliary structures are assessed completely.

Start by scanning in the longitudinal scan (LS) plane within the epigastrium, showing segments I and II of the liver, the aorta, and the IVC. Ensure that you sweep from right to left of these liver segments to assess for any focal lesions, contour abnormalities, or biliary tree dilatation. When sweeping left (towards the right side of the patient), the IVC will be more clearly visible, with the caudate lobe well defined between the IVC and the ligamentum venosum (Figure 3.21a); when sweeping right (towards the left side of the patient) the aorta will appear running posteriorly to the left lobe (Figure 3.21b) (see Videos 3.1a and b). Of note are the possibility of accessory lobes (normal anatomical variants), particularly of segment II, which can extend to the left upper quadrant and around the spleen (Figure 3.17). 

 Next, bearing in mind that the LHV represents the boundary between segments II/III and IV, turn the probe into the transverse scan (TS) plane and then sweep up and down, ensuring that these liver segments are also visualised in the orthogonal plane. By doing so segment IVA will be visualised in the most cranial plane next to segment II, and segment IVB will be visualised just below, next to segment III (Figure 3.22). Segment IV should also be imaged in LS view (Figures 3.21 and 3.23) (Videos 3.2 and 3.3). Be aware of the appearance of the ligamentum teres in the TS plane, since it may mimic a hyperechoic focal lesion, and keep in mind that all structures need to be visualised in at least two planes (Figure 3.5) (Video 3.4). 

  The right liver lobe includes segments V–VIII, which should be assessed in both the LS and TS planes via a subcostal and intercostal approach, as shown in Figures 3.24–3.26 (Video 3.5). Start again by keeping the probe in the epigastric region in a TS view. By angling upwards, you will visualise the most cranial liver segments (from left to right of the patient will be segment II, IV, VIII) and you will image part of the heart, eventually excluding or highlighting the presence of a pericardial effusion. Then, remaining in the subcostal scanning position, turn the probe oblique (rotating anticlockwise) and slowly angle downwards, making small adjustments as required. By doing so you will visualise the confluence of the three hepatic veins and the IVC from two slightly different angles, with the oblique scan favouring the visualisation of the right hepatic vein (Figure 3.24) (Video 3.5). Maintaining the same probe position and slowly moving downwards, you will visualise first the GB and left branch of the PV and then, eventually turning the patient left side down and making small adjustments, you will visualise the PV crossing and ‘dividing’ the liver in a cranial and caudal region (Figure 3.25) (Videos 3.6 and 3.7). Bear in mind that according to body habitus, when sweeping downwards the PV might appear before the GB or viceversa. Representative images of the right lobe should also be obtained intercostally, moving the probe from one intercostal space to another angling upwards and downwards in order to have a complete view from each angle (Video 3.5). A representative image of the liver with the right kidney is important to allow comparison of its echogenicity to the cortex of the right kidney to diagnose or exclude steatosis. This image can be obtained in the LS plane, starting along the mid‐clavicular line and sweeping outwards (laterally) until the kidney is visualised, or intercostally in case of bowel gas interference (Figure 3.26) (Video 3.8).   

Gallbladder and Common Bile Duct

 The GB is assessed in the subcostal position, in two orthogonal planes, and also intercostally. As a first approach, place the transducer on the anterior abdominal wall along the mid‐clavicular line, adjusting its position until the GB is located. Ask the patient to take a deep breath to lower the diaphragm and push the liver downwards below the costal margin; this will facilitate GB visualisation. It is essential to image the GB in its entire long axis and to angle the transducer so that it is also imaged transversally. The longitudinal intercostal approach will complete the GB visualisation, also offering an alternative to a sometimes challenging subcostal view in case of bowel gas interposition (Figure 3.27) (Video 3.9). In other circumstances, especially in the presence of narrow intercostal spaces, an intercostal approach might not be ideal. The CBD is best visualised with the patient supine or slightly turned with the left side down. Start with the probe obliquely positioned in the epigastrium, in line with the anatomical plane of the CBD. Sweep subcostally and outwards until you see the image of the portal triad (Figure 3.28). This may require some fine adjustments of the probe position (Video 3.10). The CBD is usually measured longitudinally where the HA intersects the CBD and PV; nevertheless, if the CBD shows some size variations it should be measured at the level of its maximal calibre (Figure 3.29). As for the GB, there may be occasions, owing to bowel gas, in which the CBD is better visualised in the anterior intercostal plane (Figure 3.30). 

Although GB ultrasound assessment can be easily carried out, there are a few important tips and pitfalls that should be kept in mind. Before starting the scan, check if the patient has undergone a cholecystectomy that might be associated with CBD dilatation, or endoscopic retrograde cholangiopancreatography requiring Oddi’s sphincterotomy that usually leads to aerobilia. Both these findings after these procedures should be considered paraphysiological, unless the clinical picture and laboratory results suggest that an underlying pathological obstructive/infective process is also present.

If the GB cannot be visualised, consider an ectopic position and check lower down within the pelvis. Always be sure that the patient has truly fasted for at least six hours, since a contracted GB can mimic cholecystitis and in general does not allow correct evaluation of its wall thickness and content, thus carrying a risk of missed pathology. Some congenital variants can make its visualisation or assessment difficult (see Chapter 6). It is important to keep in mind that the main interlobar fissure is a useful landmark to identify the GB fossa. Follow the fissure from the right branch of the PV to its other extremity that ends with the GB fossa (Figure 3.7) [6]. This will help to locate and identify the GB, especially when it is small or contracted. Be aware of the presence of near‐field artefact, which may obscure the anterior wall of the GB; the proximity of the duodenum may bulge against the posterior wall, mimicking pathology or limiting its correct assessment. Changing the patient’s position may also help to move the duodenum’s position and content and improve GB visualisation (Figure 3.31). Partial volume artefact can mimic the presence of sludge that accumulates in the infundibulum or a ‘side‐lobe artefact’ with posterior acoustic shadowing can give a false image of a stone lying in the neck of the GB. As previously mentioned, changing angle and plane of insonation will clarify the actual presence or absence of underlying pathology and confirm if the sonographic findings are artefactual.

Portal Vein, Hepatic Artery, and Hepatic Veins

The PV is best assessed in the intercostal position, typically when scanning through the right liver segments. When scanning intercostally be sure to visualise the hepatic hilum and the main PV (the main trunk will appear as the more posterior structure) (Figure 3.32a). Alternatively the main PV can be visualised maintaining a subcostal oblique approach (Figure 3.32b). In order to visualise the origin of the main extrahepatic PV, start by searching the splenic vein with a transverse subcostal epigastric view. The splenic vein will be seen lying below the pancreas and above the superior mesenteric artery in its transverse section (Figure 3.33a). Once the splenic vein is visualised, slowly turn the probe clockwise approximately 90° in order to visualise the superior mesenteric vein (Figure 3.33b). Next turn the probe anticlockwise by 45°, maintaining an oblique subcostal position. By doing so the confluence of the splenic vein and superior mesenteric vein will be seen forming the extrahepatic PV entering the liver (Figure 3.33c) (Video 3.11). It is important to remember that there may be subjective variability in the visualisation of hepatic structures, especially of the vessels, according to the angle of insonation. Therefore, it is advised to follow the mentioned landmarks moving slowly, since small movements of the probe can lead to considerable changes in ultrasound imaging. 

The PV calibre is measured at the hepatic hilum at the crossing with the HA, where a diameter up to 12.5–13 mm is considered normal (Figure 3.34).

The HA has echogenic walls, it runs anteriorly to the PV and posteriorly to the CBD, and its normal calibre at the hepatic hilum measures up to 3 mm in diameter (Figure 3.34). The hepatic veins have thinner and less echogenic walls [8] and have a straighter and linear course compared to the portal venous system. Although the measurement of the hepatic veins is usually not performed on a routine basis, the cut‐off value of their calibre is approximately 8 mm, measured at about 2–3 cm from their confluence into the IVC [9]. It should be kept in mind that in lean subjects both IVC and hepatic veins may be more ectatic. When performing a liver ultrasound scan it is important to keep in mind that the echogenicity of both hepatic veins and PV walls changes according to the angle between the ultrasound beam and the vascular wall. The more acute is the angle of insonation, the closer it is to being parallel to the longitudinal axis of the vessel. Therefore, despite there being clear differences between the thick perivascular collagen of the portal venous system and the thin walls of the hepatic veins (Figure 3.35), if the angle of insonation is low between the ultrasound beam and the PV walls, these could appear very thin or even not be visible. On the other hand, if the angle of insonation with the hepatic veins is close to 90°, the walls will appear thick and echogenic. It is always important to keep in mind this physical principle, remembering the anatomical landmarks and tracing the vessels to their origin: the PV to the hepatic hilum and the hepatic veins to their confluence into the IVC.

The following is a liver assessment check‐list with probe position for segment visualisation:

• Segments I–III and segment IVA/IVB: Longitudinal and transverse subcostal epigastric view.
• Confluence Segments IVA and IVB and confluence of hepatic veins: mainly transverse subcostal epigastric view but also longitudinal subcostal view to focus on the left hepatic vein draining into the IVC.
• Segments V and VI right subcostal transverse view and right intercostal longitudinal view.
• Segment VII and VIII: epigastric/right subcostal view and right intercostal longitudinal view.
• GB and CBD measurement at porta hepatis: right intercostal longitudinal view or transverse epigastric and transverse/oblique right subcostal view.

Doppler Studies of the Liver

A more in‐depth assessment of the PV may be needed, and this is usually traced from the confluence of the superior mesenteric vein and splenic vein forming the main PV, as previously described. This should then be traced to its intrahepatic division into the main right and left lobes of the liver (Video 3.6). These vessels should be assessed with greyscale, colour, and spectral Doppler flow (see Chapter 1). Portal venous flow is usually monophasic or mildly phasic and hepatopetal in direction (Figure 3.36). Portal flow velocity may be influenced by obstruction and increased intrahepatic resistance, particularly in the presence of PV thrombosis and cirrhosis or severe heart failure/increased central venous pressure. In the latter case, the portal venous flow is typically pulsatile (see Chapters 8 and 12). 

The confluence of the hepatic veins in the subcostal oblique view provides a good starting point to visualise their patency. From there the LHV, MHV, and RHV can be visualised along their entirety to the IVC with B‐mode and colour Doppler. These should also be assessed with spectral Doppler, best in the longitudinal plane, primarily to assess the flow phasicity (Figure 3.37) [9]. An assessment of the HA is usually not performed for routine clinical purposes. However, there has been much work assessing the Doppler flow of the HA in research studies for chronic liver disease [10–12]. The HA is typically visualised at the porta hepatis and in the longitudinal plane. A spectral Doppler trace allows measurement of the velocities and resistive index (Figure 3.38). Note that there are anatomical variants of the origin of the hepatic artery, which can be difficult to ascertain on occasion especially with ultrasound.

The Spleen

The spleen is a very important organ in liver disease, since it is linked anatomically and physiologically to the liver through the portal venous system. Its increase in size is described as one of the hallmarks of significant portal hypertension in cirrhosis (see Chapter 8), although extrahepatic causes of splenomegaly should always be borne in mind. Therefore, when performing a liver ultrasound scan, the size of the spleen must always be described, highlighting both longitudinal and anteroposterior diameters (respective cut‐offs ≤13 cm and ≤4 cm). However, splenic parenchymal evaluation is also very important in addition to the splenic size, where textural differences can be seen with different pathological conditions ranging from infectious diseases, haematological and immune/inflammatory‐related pathologies as well as congenital and acquired vascular malformations. Moreover, splenic parenchyma can also be the site of benign and primary or secondary malignancies manifesting as discrete focal lesions. Of note, accessory spleens (splenunculi) are relatively common (Figure 3.39). These are typically rounded or ovoid small lesions with similar splenic echotexture and vascularity. They are a result of the incomplete fusion of splenic tissue into a single organ during its embryonal development and thus can occur anywhere along the splenic bed. Occasionally, these can be difficult to distinguish from peritoneal or renal lesions and thus may require contrast ultrasound or further imaging for confirmation.The spleen is assessed intercostally through the left lateral intercostal spaces with the patient lying supine. Keep in mind that in some cases the spleen is more posterior and difficult to visualise entirely (Figure 3.39). In case of splenomegaly, the spleen can be assessed with a longitudinal subcostal scan; alternatively, in case of a normal‐sized spleen the intercostal approach remains the best option. Imaging may be improved sometimes by turning the patient right side down, although sometimes bowel loops and the lung might interfere, leading to suboptimal imaging.

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).