What is the normal segmental anatomy of the liver, and how are hepatic lobes and segments identified on cross sectional imaging?
The liver is subdivided into right, left, and caudate lobes. The right hepatic lobe is further subdivided into anterior and posterior segments, and the left hepatic lobe is further subdivided into medial and lateral segments. Whereas the hepatic veins are located between segments of the liver (i.e., are intersegmental in location), the portal veins, hepatic arteries, and bile ducts are located within segments of the liver (i.e., are intrasegmental in location). Superiorly, the middle hepatic vein delineates the boundary between the left and right hepatic lobes, the right hepatic vein delineates the boundary between the anterior and posterior segments, and the left hepatic vein delineates the boundary between the medial and lateral segments. Inferiorly, the interlobar fissure, which is in the plane of the gallbladder fossa, delineates the boundary between the left and right hepatic lobes, and the fissure for the falciform ligament (or left intersegmental fissure) delineates the boundary between the medial and lateral segments.
The Bismuth-Couinaud system is often used to provide a segmental nomenclature for localizing focal hepatic lesions. In this system, vertically oriented planes along the right, middle, and left hepatic veins are maintained and further divided by a horizontal plane through the right and left portal veins to divide the liver into 9 “segments” comprised of the caudate lobe and 8 hepatic subsegments.
Segment I is the caudate lobe of the liver, which is located posterior to the fissure for the ligamentum venosum, and which is functionally an autonomous part of the liver. It has a separate blood supply, venous drainage, and biliary drainage. Segments II, IVa, VIII, and VII are the lateral, medial, anterior, and posterior segments of the liver, respectively, located above the plane of the portal vein. Segments III, IVb, V, and VI are the lateral, medial, anterior, and posterior segments of the liver, respectively, located below the plane of the portal vein
