Liver Anatomy and Ultrasound Notes

Anatomy and Physiology of the Liver

• The liver is a large intraperitoneal organ located primarily in the right upper quadrant (RUQ) with relationships to other upper abdominal structures.

• Key anatomical relationships:

  • The fundus of the stomach lies posterior and lateral to the left lobe of the liver; the remainder of the stomach sits inferior to the liver.

  • The body of the pancreas lies inferior to the left lobe of the liver.

• Overall position:

  • Occupies the right hypochondrium, most of the epigastrium, and part of the left hypochondrium extending to the mammillary line.

  • Lies inferior to the diaphragm.

• Quadrants for orientation (anatomic/sonographic): RUQ, RLQ, LUQ, LLQ.

• Diaphragmatic relation: the liver is apposed to the diaphragm.

Lobes of the Liver

• Classical lobation: Right lobe, Left lobe, and Caudate lobe.

• Additional anatomical features present on ultrasound:

  • Right lobe is the largest and contains anterior and posterior segments.

  • Left lobe is smaller; division into medial and lateral segments.

  • Caudate lobe lies posterior to the left lobe, between the ligamentum venosum and the IVC.

• Corollary structures adjacent to lobes include:

  • Gallbladder lies near the right lobe in the gallbladder fossa.

  • Common bile duct (CBD) and portal vein run in close proximity to the liver.

  • Hepatic artery and portal vein enter through the porta hepatis; bile ducts exit via the CBD.

Fissures, Ligaments, and Bare Area

• Fissures (evident as echogenic lines on ultrasound due to collagen/fat):

  • Main lobar fissure: separates right and left lobes; identifiable between the gallbladder (GB) neck and the portal vein junction.

  • Right intersegmental fissure: divides right lobe into anterior and posterior segments; landmark is the right hepatic vein.

  • Left intersegmental fissure: divides left lobe into medial and lateral segments; landmarks include the left hepatic vein, left portal vein, falciform ligament, and ligamentum teres.

  • Ligamentum venosum: separates caudate lobe from the left lobe; remnant of the ductus venosus; landmarked near the left lobe and IVC.

• Ligaments of the liver:

  • Falciform ligament: connects liver to the anterior abdominal wall from the umbilicus to the diaphragm; contains the ligamentum teres; helps attach the liver to the anterior abdominal wall.

  • Ligamentum teres (round ligament): remnant of the umbilical vein; runs from the umbilicus to the left portal vein; lies within the falciform ligament; can recanalize with portal hypertension (extrahepatic portion).

  • Coronary ligaments and triangular ligaments (right and left): attach the liver to the diaphragm; create a bare area on the posterior/inferior surface where peritoneum is absent.

• Bare area:

  • Large triangular region on the diaphragmatic surface where the liver rests directly on the diaphragm, not covered by peritoneum (Glisson capsule covers the rest of the liver).

• Glisson capsule:

  • Peritoneal covering surrounding most of the liver; provides a capsule for vascular structures.

Vascular Anatomy and the Portal Triad

• Portal triad: encased in collagenous sheaths with echogenic walls. It consists of:

  • Portal vein (PV)

  • Hepatic artery (HA)

  • Common bile duct (CBD)

• The portal triad is found at the porta hepatis; the CBD, hepatic artery, and portal vein are key inflow/outflow structures for hepatic function.

• Portal venous system:

  • Formed by the superior mesenteric vein (SMV) and splenic vein; the splenic vein drains spleen, IMV, stomach, esophageal, pancreas and gallbladder (cystic) veins.

  • The main portal vein (MPV) enters the liver and divides into left and right branches.

  • Hepatopetal flow: flow toward the liver; portal veins carry blood from the bowel toward the liver.

  • Diameter and portal hypertension: an MPV diameter of
    ext{MPV diameter} > 13\,\text{mm}
    implies portal hypertension.

• Hepatic artery:

  • Carries oxygenated blood from the aorta via the celiac trunk (axis) which branches into:

  • Splenic artery

  • Common hepatic artery

  • Left gastric artery

  • Proper hepatic artery runs parallel to the main portal vein (anterior and to the left) and tends to have a low-resistance waveform (hepatopetal flow).

  • Variations exist in about 45% of individuals, including:

  • Replaced right hepatic artery arising from the superior mesenteric artery (SMA)

  • Replaced left hepatic artery arising from the left gastric artery

  • Replaced common hepatic artery arising from SMA

• Hepatic veins:

  • Drain the liver and return blood to the IVC.

  • Main hepatic veins: right hepatic vein, middle hepatic vein, left hepatic vein; drain between lobes/segments.

  • The hepatic veins are relatively non-echogenic walls compared to portal veins.

  • The “bunny sign” is a sonographic reference to the branching pattern of hepatic veins.

Couinaud Segments and Intrahepatic Anatomy

• Couinaud’s hepatic segments (8 total) form the functional units of the liver; subdivisions are vasculature-driven and surgically resectable.

• Segments and their typical labeling:

  • I: Caudate lobe

  • II: Left lateral superior segment

  • III: Left lateral inferior segment

  • IVa: Left medial superior segment

  • IVb: Left medial inferior segment (quadrate segment)

  • V: Right anterior inferior segment

  • VI: Right posterior inferior segment

  • VII: Right posterior superior segment

  • VIII: Right anterior superior segment

• Intrahepatic vasculature organization:

  • Intrasegmental vessels: portal veins course through the center of each segment and are encased by Glisson’s capsule, giving echogenic walls on ultrasound.

  • Intersegmental vessels: hepatic veins separating segments/lobes and coursing between lobes/segments; their walls are non-echogenic.

  • Major branches of the portal veins run centrally within segments; the ascending portion of the left portal vein runs in the left intersegmental fissure.

• Practical use:

  • These segments guide surgical planning and localization of masses or cysts.

  • The quadrate lobe is an archaic term for part of the left lobe; MHV and LHV divide it.

Liver Segments: Landmarks and Visualization

• Segmentation can be identified by:

  • Right hepatic vein (RHV) defining the right intersegmental fissure (anterior vs posterior right segments)

  • Middle hepatic vein (MHV) and left hepatic vein (LHV) defining other segment boundaries

  • Ligamentous landmarks: falciform ligament, ligamentum teres, ligamentum venosum

  • Main lobar fissure separating right and left lobes

  • Gallbladder fossa and portal vein (as reference points)

• Key sonographic notes:

  • The right lobe comprises anterior and posterior segments (V, VIII, VI, VII etc., depending on view).

  • The left lobe comprises medial and lateral segments (IVa/IVb and II/III).

  • The caudate lobe (I) lies posteriorly near the IVC and the ligamentum venosum.

Contour, Variants, and Pathologic Anomalies

• Normal variations and anomalies discussed:

  • Riedel’s lobe: inferior projection of the right lobe; more commonly seen in women.

  • Extremely small left lobe, deep costal impressions, complete atrophy of the left lobe (potential left portal vein compression).

  • Transverse, saddle-shaped liver with a relatively large left lobe; tongue-like projections; deep renal impressions; diaphragmatic grooves.

• Agenesis and positional anomalies:

  • Agenesis of the liver is incompatible with life in reported cases; hypertrophy of remaining lobes may occur.

  • Situs inversus: liver located on the left with spleen on the right; diaphragmatic hernia or omphalocele may allow liver tissue to herniate.

• Accessory fissures:

  • True accessory fissures are uncommon; caused by infolding of peritoneum. Inferior accessory hepatic fissure extends from the right portal vein to the inferior surface of the right lobe.

• Vascular anomalies:

  • Hepatic artery variations occur in about 45% of people, including replaced left/right hepatic arteries and replaced common hepatic arteries arising from SMA or other arteries.

  • Portal venous anatomy variations are less common but include atresias, strictures, and obstructing valves.

  • Variations in the hepatic veins' branching are common; an accessory vein may drain the superoanterior segment of the right lobe into the middle hepatic vein or right hepatic vein.

Functional Aspects and Clinical Correlates

• Primary functions of the liver:

  • Detoxes and processes blood; metabolizes nutrients; produces bile; destroys old red blood cells and forms bilirubin; synthesizes plasma proteins (albumin, others) and enzymes (AST/ALT, alkaline phosphatase).

  • Important metabolic roles include cholesterol processing and production of metabolic proteins.

• Liver function tests (LFTs):

  • AST (aspartate aminotransferase): elevated with acute hepatitis and cirrhosis.

  • ALT (alanine aminotransferase): more specific for liver injury than AST.

  • LDH (lactic dehydrogenase): elevated in several tissues including liver.

  • Alkaline phosphatase (ALP): elevated with intra- or extrahepatic obstruction and with certain carcinomas.

  • Bilirubin (indirect/direct/total): bilirubin metabolism and excretion assessment.

  • Prothrombin time (PT): reflects clotting function; often prolonged with hepatic dysfunction.

  • Albumin and globulins: synthetic function of the liver; decreases in chronic liver disease.

• Clinical patterns (pathology):

  • Obstructive disease: mild AST/ALT rise with a marked ALP and bilirubin rise; often due to ductal compression or duct obstruction.

  • Hepatocellular disease: marked AST/ALT elevation with relatively milder ALP rise; treated medically.

• Liver Doppler (conceptual):

  • Portal hypertension assessment via Doppler; normal portal flow is hepatopetal (toward the liver).

  • Doppler studies interrogate hepatic artery, portal vein, and hepatic veins to assess perfusion and venous return.

Sonographic Evaluation Protocol and Technique

• Patient preparation:

  • NPO for at least 6–8 hours; morning exams preferred.

• Equipment and settings:

  • Transducer: 2.5–4 MHz curvilinear or sector, or 3–5 MHz curvilinear; adjust depth and gain accordingly.

  • Time gain compensation (TGC) and overall gain adjusted to achieve uniform echogenicity.

• Patient positioning:

  • Supine, but oblique and/or decubitus positions as needed for optimal views.

• History and laboratory review:

  • Check order, prior imaging, current lab values (LFTs, bilirubin, albumin), and prior imaging studies.

  • Questions to ask: prior abdominal surgery, symptoms (pain, nausea), last oral intake, etc.

• Assessment criteria for the liver (proficiency protocol):

  • Size: measure superior–inferior in the longitudinal plane; typical liver size ~15 cm (approximate).

  • Anteroposterior dimension in the longitudinal plane; overall liver width.

  • Parenchymal texture: homogeneous and smooth on a normal study.

  • Echogenicity: liver is typically more echogenic than the right kidney and less echogenic than the pancreas.

  • Vascular structures and ligaments: identify hepatic veins, main lobar fissure, and fissures; Morison’s pouch should be evaluated for fluid (right subhepatic space between liver and right kidney).

  • Subphrenic space: check for abscess formation between the liver and diaphragm.

• Normal sonographic appearance expectations:

  • Normal liver is homogeneous; right lobe may appear slightly more echogenic than the left kidney cortex.

  • The portal veins have echogenic walls; hepatic veins have relatively non-echogenic walls.

  • The porta hepatis area should reveal the portal vein, proper hepatic artery, and CBD in a triad arrangement.

• Normal measurements and signs of abnormality:

  • Hepatomegaly: superior–inferior dimension >
    15\,\text{cm}
    on longitudinal view.

  • Decreased echogenicity or heterogeneity may indicate steatosis, edema, or focal lesions.

Practical Anatomy Review and Landmarks for Localization

• Surface landmarks and views helpful in ultrasound:

  • Falciform ligament, ligamentum teres, and ligamentum venosum help delineate segments.

  • Gallbladder fossa and main lobar fissure are useful for separating right and left lobes.

  • Bare area and coronary ligaments define diaphragmatic contact points and the diaphragmatic surface.

  • Right and left triangular ligaments, coronary ligaments, and the falciform ligament frame the liver on the diaphragmatic surface.

• Typical sectional anatomy summary (for quick orientation):

  • Right lobe: anterior and posterior segments; contains RHV as a major divider

  • Left lobe: medial and lateral segments; LHV divides medial vs lateral portions

  • Caudate lobe: posterior to left lobe; near IVC; separated by ligamentum venosum; often spared from disease due to emissary venous drainage directly into IVC

• Important clinical correlations:

  • Portal hypertension assessment often relies on portal venous diameter, hepatic vein flow patterns, and collateral formation on Doppler.

  • Riedel’s lobe and other variations can mimic pathology if misinterpreted; awareness reduces misdiagnosis.

  • Accessory fissures and aberrant arteries can complicate planning for interventions; knowledge of variations is critical for surgical planning.

Quick Reference: Key Measurements and Concepts (LaTeX-friendly)

• Portal vein diameter and portal HTN indicator:
  ext{if } ext{MPV diameter} > 13\,
  \text{mm} \Rightarrow \text{portal hypertension}

• Hepatomegaly threshold:
  ext{Superior–inferior liver dimension} > 15\,\text{cm}

• Functional segmentation (Couinaud) mapping:

  • I: Caudate lobe

  • II: Left lateral superior

  • III: Left lateral inferior

  • IVa: Left medial superior

  • IVb: Left medial inferior (quadrate)

  • V: Right anterior inferior

  • VI: Right posterior inferior

  • VII: Right posterior superior

  • VIII: Right anterior superior

• Portal triad components in the porta hepatis:
  {\,\text{Portal vein},\ \text{Hepatic artery},\ \text{CBD}\,}\;.

References to Study Prompts and Toward Mastery

• Recall the basic relationships in RUQ anatomy and how the left lobe is related to the stomach and pancreas.

• Be able to name the three lobes on the surface (Right, Left, Caudate) and identify the fissures that separate them.

• Understand the ligaments and their clinical significance (e.g., falciform ligament containing the ligamentum teres, recanalization in portal hypertension).

• Explain the portal venous system’s origin (SMV and splenic vein), flow direction (hepatopetal), and the significance of portal vein diameter in portal HTN.

• Differentiate between hepatic veins and portal veins on ultrasound in terms of echogenicity and flow direction.

• Memorize Couinaud segments and the anatomical landmarks used to identify them on imaging.

• Recognize common anatomic variants (Riedel’s lobe, accessory fissures) and vascular anomalies (replaced hepatic arteries, portal venous variations).

• Apply the basic liver protocol for ultrasound evaluation, including patient preparation, transducer choice, views, and metrics.

Questions to Test Your Understanding

• What is portal hypertension, and which ultrasound finding can help indicate it?

• Name the vessels that are interrogated during a liver Doppler exam.

• Which liver segments correspond to the right anterior and right posterior divisions in Couinaud’s system?

• How do the hepatic veins differ sonographically from the portal veins?

• Where is the caudate lobe located in relation to the IVC and ligamentum venosum?

• What are the key landmarks you would use to identify the main lobar fissure on ultrasound?

• What is the clinical significance of the ligamentum teres recanalization?

• What measurements define hepatomegaly and portal hypertension on ultrasound?