13.1 Liver Anatomy, Variants & Examination Technique

The liver is the largest solid organ in the abdomen and, in practice, the structure most often interrogated within the 75-question Abdomen section, so precise anatomic localization is a core exam skill. (ARRT weights Abdomen as a whole and does not publish per-organ counts.)

Segmental Anatomy: The Couinaud System

The Couinaud classification divides the liver into eight functionally independent segments (numbered I through VIII), each with its own branch of the portal triad (portal vein, hepatic artery, and bile duct) and its own hepatic venous drainage. Segment I is the caudate lobe. The hepatic veins are the key sonographic dividers:

• The middle hepatic vein (MHV) separates the right and left lobes. Its projected plane, Cantlie's line, runs from the gallbladder fossa to the inferior vena cava (IVC).
• The right hepatic vein (RHV) divides the right lobe into anterior (segments V and VIII) and posterior (segments VI and VII) sectors.
• The left hepatic vein (LHV) divides the left lobe into medial (segment IV) and lateral (segments II and III) segments.
• Portal vein branches run within segments, while hepatic veins run between them — a relationship that lets you localize any focal lesion.

The main lobar fissure appears as a thin echogenic line extending from the gallbladder neck to the right portal vein and marks the same right-versus-left boundary as the MHV.

Ligaments and Accessory Lobes

The ligamentum teres (round ligament) is the obliterated remnant of the fetal umbilical vein. It sits within the free edge of the falciform ligament, appears as a bright echogenic focus separating the medial and lateral segments of the left lobe, and is important clinically because it can recanalize in severe portal hypertension. The ligamentum venosum, the remnant of the ductus venosus, forms the echogenic boundary between the caudate lobe and the left lobe.

The quadrate lobe is the anatomic region bounded by the gallbladder fossa and the ligamentum teres; functionally it belongs to the medial segment of the left lobe (segment IV). The caudate lobe is unique: it receives portal branches from both the right and left systems and drains directly into the IVC through small emissary veins. Because of this independent drainage, the caudate lobe is characteristically spared — and may hypertrophy — in Budd-Chiari syndrome when the main hepatic veins are occluded.

Blood Supply and Physiology

The liver has a dual blood supply: the portal vein delivers roughly 75% of hepatic blood flow (nutrient-rich, low-oxygen venous blood from the gut and spleen), while the hepatic artery supplies the remaining 25% of flow but about half of the oxygen. Together with the bile duct they form the portal triad, enclosed in the echogenic fibrofatty Glisson's sheath; the liver surface itself is covered by Glisson's capsule except at the bare area, a posteromedial region in direct contact with the diaphragm where peritoneum is absent. The liver synthesizes albumin and clotting factors, stores glycogen, produces bile, and detoxifies metabolites — which is why advanced disease lowers albumin and prolongs the prothrombin time. On a transverse subcostal view at the diaphragm, the three hepatic veins converging on the IVC produce the classic 'bunny' or 'reindeer' sign, a landmark for identifying each vein.

Normal Variants You Must Recognize

• Riedel's lobe: a benign, tongue-like inferior extension of the right lobe (segments V/VI), more common in women. It can extend below the right kidney and is frequently misread as hepatomegaly or a mass; the giveaway is normal, continuous parenchyma and normal echotexture.
• Papillary process of the caudate lobe: an inferior caudate projection that can appear rounded and separate on transverse views, mimicking a periportal lymph node or pancreatic mass. Scanning through in two planes shows its continuity with the caudate lobe.
• Diaphragmatic slips: pseudofissures caused by muscular slips of the diaphragm indenting the superior liver surface, sometimes mistaken for subcapsular lesions.

Normal Echotexture

Normal hepatic parenchyma is homogeneous with mid-level echoes. Standard comparisons:

Examination Technique

• Preparation: fast 6-8 hours to reduce bowel gas and keep the gallbladder distended for the combined right upper quadrant study.
• Transducer: curvilinear array, typically 2-5 MHz; harmonics help in obese patients.
• Approach: subcostal sweeps in deep inspiration plus intercostal windows along the right midclavicular and anterior axillary lines; roll the patient into the left lateral decubitus position to drop the liver below the costal margin.
• Respiration: suspended deep inspiration displaces the liver inferiorly and opens intercostal windows.
• Measurements: longitudinal right-lobe span at the midclavicular line — above roughly 15-17 cm suggests hepatomegaly; portal vein diameter should not exceed about 13 mm in quiet respiration.

Clinical indications include elevated liver function tests (ALT, AST, alkaline phosphatase, bilirubin), hepatomegaly, jaundice, right upper quadrant pain, and hepatocellular carcinoma surveillance every 6 months (with serum alpha-fetoprotein) in patients with cirrhosis or chronic hepatitis B.

Documentation and Normal Doppler

A complete study documents longitudinal and transverse sweeps through both lobes, the hepatic vein confluence, the portal vein at the porta hepatis, the caudate lobe with the IVC, and a liver-versus-right-kidney echogenicity comparison. Normal Doppler baselines worth memorizing: main portal vein flow is hepatopetal (toward the liver), continuous with gentle respiratory undulation, at a peak velocity of roughly 15-40 cm/s (mean about 15-30 cm/s); the hepatic artery shows a low-resistance arterial tracing with continuous diastolic flow; and hepatic veins show triphasic pulsatility transmitted from the right atrium. Measuring the portal vein at the point where it crosses the IVC keeps exams reproducible.