Liver Ultrasound Anatomy Overview

Overview

This lecture covers the normal ultrasound anatomy of the liver, including grayscale characteristics, vascular supply, anatomical segmentation, and key measurements.

Liver Lobes and Segments

• The liver is divided into right, left, and caudate lobes.
• The main lobar fissure, passing through the gallbladder fossa, separates the right and left lobes.
• The right lobe divides into anterior and posterior segments.
• The left intersegmental fissure divides the left lobe into medial and lateral segments.
• The caudate lobe is posterior, bordered by the inferior vena cava (IVC) and ligamentum venosum fissure.
• Reidel’s lobe is a tongue-like extension of the right lobe's inferior tip, often seen in thin women.

Vascular Anatomy of the Liver

• There are three hepatic veins: right, middle, and left.
• The right hepatic vein divides the right lobe into anterior and posterior segments.
• The middle hepatic vein separates the right lobe's anterior segment from the left lobe's medial segment.
• The left hepatic vein divides the left lobe into medial and lateral segments.
• Hepatic veins are intersegmental/interlobar (run between segments/lobes), while portal veins are intrasegmental (within segments).
• The portal vein forms from the union of intestine, spleen, pancreas, and gallbladder veins and splits into right and left branches at the porta hepatis.
• Portal veins are seen as bright echogenic-walled structures on ultrasound; hepatic veins lack echogenic borders.

Key Anatomic Landmarks

• The ligamentum venosum fissure is the anterior border of the caudate lobe; IVC is the posterior border.
• The ligamentum teres separates medial and lateral left lobe segments and appears as an echogenic structure.

Segmental (Couinaud) Anatomy

• The liver contains 8 segments, each with its own vascular and biliary supply.
• Segments are determined by hepatic and portal vein branches.
• Segment 1 (caudate lobe) may receive blood from both portal vein branches and drains directly to the IVC.

Grayscale Echogenicity

• Liver is homogeneous with fine, low-level echoes.
• Liver is hypoechoic or isoechoic to the pancreas, hyperechoic or isoechoic to the right kidney, and always hypoechoic to the spleen.
• Left lobe enlargement wrapping around the spleen is called "beaver tail" and may mimic hematoma but can be distinguished by vessel visualization and Doppler.

Liver Size Measurements

• Liver length is measured at the mid-clavicular line.
• Normal average liver length: 14 ± 1.7 cm; men average: 14.5 ± 1.7 cm, women: 13.5 ± 1.7 cm.
• Liver length ≥16 cm at mid-clavicular line is considered enlarged.

Key Terms & Definitions

• Hyperechoic — produces more echoes; appears brighter on ultrasound.
• Hypoechoic — produces fewer echoes; appears darker on ultrasound.
• Isoechoic — similar in echogenicity to surrounding tissue.
• Intersegmental — between liver segments (e.g., hepatic veins).
• Intrasegmental — within a liver segment (e.g., portal veins).
• Couinaud Anatomy — universal liver segmentation system based on portal and hepatic vein anatomy.

Action Items / Next Steps

• Review the segmental anatomy and Couinaud classification.
• Practice identifying portal and hepatic veins and liver landmarks on ultrasound images.
• Memorize normal liver size thresholds for exams.