Liver Segments on CT Scan

Overview

This lecture explains how to identify the different liver segments on a CT scan, including anatomical landmarks, segment numbering, and the significance of each segment for clinical practice.

Liver Segment Anatomy

• The liver is divided into segments based on blood supply and drainage, not just surface morphology.
• The three hepatic veins (left, middle, right) divide the liver into four sections: left lateral, left medial, right medial/anterior, and right lateral/posterior.
• Each section is further split into superior and inferior segments by the portal vein and its branches.

Segment Numbering and Identification

• Segments are numbered in a clockwise manner starting from the left superior lateral section (segment 2).
• Segment 1 (caudate lobe) is posterior and medial, functioning independently from other segments.
• Segments 2 & 3 are left lateral; segment 2 is superior, 3 is inferior.
• Segment 4 (left medial) is split into 4a (superior) and 4b (inferior).
• Segments 5–8 are on the right: 5 (anterior, inferior), 6 (posterior, inferior), 7 (posterior, superior), 8 (anterior, superior).

CT Scan Identification Steps

• Confirm liver location on the right side and scan through the parenchyma for abnormalities.
• Identify the inferior vena cava (IVC) posterior to the liver, noting hepatic veins draining into it.
• Use hepatic veins as landmarks to separate the four primary liver sections.
• Identify the portal vein (anterior to IVC) and its right and left branches to distinguish superior/inferior segments.
• Use the falciform ligament as an optional surgical marker between segments 3 and 4, but rely on hepatic veins for anatomical accuracy.
• Segment 1 (caudate) is medial to the IVC and should be evaluated for size, especially in cirrhosis.

Clinical Relevance

• Each segment acts as a functional unit with independent blood, venous, and biliary drainage.
• Surgical removal or preservation of a segment is possible because of this independence.
• Enlargement of the caudate lobe can indicate liver disease.

Key Terms & Definitions

• Hepatic Vein — Veins draining blood from liver to IVC, used as anatomical dividers.
• Portal Vein — Vessel bringing blood from digestive organs to the liver, splits into right and left branches.
• Caudate Lobe (Segment 1) — Posterior liver segment, functions independently.
• Falciform Ligament — Anatomical structure sometimes used to separate segments 3 and 4.
• Riedel’s lobe — Normal downward extension of segment 6.

Action Items / Next Steps

• Review color-coded liver segment diagrams for visual understanding.
• Practice identifying liver segments on multiple CT scans for repetition.
• Observe the caudate lobe’s size in cases of cirrhosis during scan reviews.