Liver Anatomy, Segmental Classification (Couinaud), and Perfusion

Liver Anatomy, Segmental Classification (Couinaud), and Perfusion

The liver is the largest internal organ and the primary metabolic hub of the human body, situated in the right hypochondrium and epigastrium of the abdomen. For the ARDMS Abdomen specialty exam, a sonographer must master the liver's gross lobar divisions, its complex ligamentous support structures, the functional segmental classification (Couinaud system), and its unique dual-perfusion hemodynamics.

Gross Lobar Anatomy

Traditionally, the liver is divided into four anatomical lobes: the right lobe, the left lobe, the caudate lobe, and the quadrate lobe. However, in modern sonography and surgical practice, physiological divisions based on vascular supply are utilized:

1. Right Lobe: The largest lobe, occupying the right hypochondrium. It is separated from the left lobe by the main lobar fissure and middle hepatic vein.
2. Left Lobe: Lies in the epigastrium and left hypochondrium. It is divided into medial and lateral segments.
3. Caudate Lobe: Located on the posterior-superior surface of the liver, bounded anteriorly by the ligamentum venosum and posteriorly by the inferior vena cava (IVC).
4. Quadrate Lobe: Historically identified on the inferior surface of the left lobe, functionally categorized as the medial segment of the left lobe (Segment IV).

The primary sonographic boundary dividing the right and left lobes is the Main Lobar Fissure (MLF). On ultrasound, the MLF appears as a thin, hyperechoic line extending from the right portal vein to the gallbladder fossa. The middle hepatic vein lies within this fissure. Physiologically, this division is known as Cantlie’s Line, running from the gallbladder fossa to the IVC.

Ligaments and Fissures

The liver is anchored within the peritoneal cavity by several reflective, hyperechoic peritoneal folds and ligamentous remnants of fetal circulation:

• Falciform Ligament: A broad, sickle-shaped fold of peritoneum that attaches the liver to the anterior abdominal wall and diaphragm. While it anatomically divides the left and right lobes on the anterior surface, it does not represent the physiological boundary.
• Ligamentum Teres (Round Ligament): The fibrous remnant of the fetal umbilical vein. It runs along the free edge of the falciform ligament. Sonographically, it appears as a bright, echogenic triangle or round structure in the transverse plane, separating the left medial segment (Segment IV) from the left lateral segments (Segments II and III). In the setting of severe portal hypertension, the ligamentum teres can recanalize to allow collateral flow (hepatofugal shunt).
• Ligamentum Venosum: The fibrous remnant of the fetal ductus venosus, which shunted oxygenated blood from the umbilical vein directly to the IVC, bypassing the liver. It separates the caudate lobe posteriorly from the left lateral lobe anteriorly, appearing as a thin, hyperechoic horizontal line.
• Coronary Ligaments and Triangular Ligaments: Peritoneal reflections that anchor the posterior liver to the diaphragm, leaving a portion of the posterior right lobe uncovered by peritoneum—this is the bare area of the liver, where fluid collections (like ascites) cannot accumulate.

Couinaud Segmental Classification

The Couinaud classification system is the gold standard for describing functional hepatic anatomy, dividing the liver into eight segments. Each segment functions as an independent unit, possessing its own vascular inflow (portal vein branch and hepatic artery branch) and biliary drainage path.

The division is defined by:

• Vertical Plane Boundaries: The three main hepatic veins (right, middle, left) run between the segments (interlobar/intersegmental).
  • The Left Hepatic Vein divides the left lobe into lateral and medial segments.
  • The Middle Hepatic Vein divides the liver into right and left lobes (along the main lobar fissure).
  • The Right Hepatic Vein divides the right lobe into anterior and posterior segments.
• Horizontal Plane Boundaries: Established by the transverse plane of the left and right portal vein branches (intralobar/intrasegmental).

The Caudate Lobe (Segment I) Exception

Segment I is highly unique and frequently tested on the ARDMS Abdomen exam. It is located on the posterior surface of the liver, posterior to the ligamentum venosum and anterior to the IVC. Unlike the other seven segments, Segment I receives dual vascular supply from both the right and left portal veins and hepatic arteries. Furthermore, its venous outflow drains directly into the IVC via small emissary veins rather than the three main hepatic veins. Consequently, in cases of hepatic vein obstruction (Budd-Chiari syndrome), the caudate lobe is spared and undergoes compensatory hypertrophy, while the rest of the liver parenchyma undergoes atrophy and congestion.

Normal Liver Perfusion and Doppler Waveforms

The liver receives a dual blood supply, resulting in a complex hemodynamical profile:

1. Portal Vein (PV): Provides 70% to 80% of the blood volume entering the liver. However, because this blood has already passed through the gastrointestinal tract, it is partially deoxygenated, supplying only 50% of the liver's oxygen requirements. The main portal vein is formed by the union of the splenic vein and the superior mesenteric vein (SMV) posterior to the pancreatic neck (confluence). Normal portal flow is hepatopetal (directed toward the liver), demonstrating a continuous, low-velocity waveform with mild respiratory fluctuations. Normal peak velocities range from 15 to 30 cm/s. Hepatofugal flow (away from the liver) is always pathological, indicating portal hypertension.
2. Hepatic Artery (HA): Supplies 20% to 30% of liver blood volume but provides 50% of the liver's oxygen supply. The common hepatic artery arises from the celiac axis and branches into the proper hepatic artery at the porta hepatis. The normal hepatic artery exhibits a low-resistance waveform characterized by a rapid systolic upstroke and continuous, forward flow throughout diastole. The normal Resistive Index (RI) is between 0.55 and 0.70. An elevated RI (>0.70) is seen in transplant rejection, acute hepatitis, or cirrhosis, while a low RI (<0.55) or tardus-parvus waveform suggests proximal stenosis (e.g., hepatic artery stenosis).
3. Hepatic Veins (HVs): Drain the liver parenchyma into the IVC. HVs lie interlobar and intersegmental. Normal hepatic vein flow is triphasic and pulsatile, reflecting the pressure changes within the right atrium of the heart:
   • S Wave: Anterograde flow toward the IVC/heart, corresponding to ventricular systole (atrium is filling).
   • D Wave: Anterograde flow toward the IVC/heart, corresponding to ventricular diastole (tricuspid valve opens, blood rushes into ventricle).
   • A Wave: A brief period of retrograde (reversed) flow away from the heart, corresponding to right atrial contraction (atrial kick pushing blood back toward the liver).
   • V Wave: A transition point between the S and D waves.
   • Loss of triphasic pulsatility (e.g., a monophasic or flat waveform) indicates parenchymal compression from cirrhosis, fatty infiltration, tumor compression, or Budd-Chiari syndrome. Conversely, increased pulsatility may indicate right-sided congestive heart failure or tricuspid regurgitation.