Liver segmentation. Except for the caudate lobe (segment I), the liver is divided into right and left livers based on the primary (1) division of the portal triad into right and left branches, the plane between the right and left livers being the main portal fissure (1) in which the middle hepatic vein lies. On the visceral surface, this plane is demarcated by the right sagittal fissure. The plane is demarcated on the diaphragmatic surface by an imaginary line (Cantlie line- Cantlie, 1898) running from the notch for the fundus of the gallbladder to the IVC. The right and left livers are subdivided vertically into medial and lateral sectors by the right portal (2) and left portal (umbilical) (3) fissures, in which the right and left hepatic veins lie. The left portal fissure is demarcated externally by the falciform ligament and the left sagittal fissure. The right portal fissure has no external demarcation. Each division receives a secondary (2) branch of the portal triad (a portal pedicle). [Note: the medial sector of the left liver is part of the right anatomical lobe; the lateral division of the left liver is the same as the left anatomical lobe.] A transverse plane at the level of the horizontal parts of the right and left branches of the portal triad (4) subdivides three of the four sectors (all but the left medial sector), creating six hepatic segments receiving tertiary branches. The left medial sector is also counted as a hepatic segment, so that the main part of the liver has seven segments (segments II through VIII, numbered clockwise). The caudate lobe (segment I, bringing the total number of segments to eight) is supplied by branches of both right and left divisions and drained by its own minor hepatic veins. Each segment thus has its own intrasegmental blood supply and biliary drainage. The hepatic veins are intersegmental, draining the portions of the multiple segments adjacent to them.
Traditional gross anatomy divided the liver into four lobes based on surface features. The falciform ligament is visible on the front (anterior side) of the liver. This divides the liver into a left anatomical lobe, and a right anatomical lobe.
If the liver is flipped over, to look at it from behind (the visceral surface), there are two additional lobes between the right and left. These are the caudate lobe (the more superior) and the quadrate lobe (the more inferior).
From behind, the lobes are divided up by the ligamentum venosum and ligamentum teres (anything left of these is the left lobe), the transverse fissure (or porta hepatis) divides the caudate from the quadrate lobe, and the right sagittal fossa, which the inferior vena cava runs over, separates these two lobes from the right lobe.
Each of the lobes is made up of lobules; a vein goes from the centre, which then joins to the hepatic vein to carry blood out from the liver.
On the surface of the lobules, there are ducts, veins and arteries that carry fluids to and from them.
Correspondence between anatomic lobes and Couinaud segments
Segment* Couinaud segments
Lateral 2, 3
Medial 4a, 4b
Right 5, 6, 7, 8
* or lobe, in the case of the caudate lobe
Each number in the list corresponds to one in the table. 1. Caudate
2. Superior subsegment of the lateral segment
3. Inferior subsegment of the lateral segment
4a. Superior subsegment of the medial segment
4b. Inferior subsegment of the medial segment
5. Inferior subsegment of the anterior segment
6. Inferior subsegment of the posterior segment
7. Superior subsegment of the posterior segment
8. Superior subsegment of the anterior segment
MHV, middle hepatic vein; LHV, left hepatic vein; IVC, inferior vena cava; RPV, right portal vein; LPV, left portal vein; FL, falciform
Imaging Landmarks: CT and MR Imaging Computed tomography and MR are traditionally considered as cross-sectional axial imaging modalities, and the concept of three vertical planes that divide the liver into four sectors and of a transverse scissura that further subdivides the sectors into two segments each, is applied to localize liver lesions (Bismuth 1982; Couinaud 1957; Soyer et al. 1994). The hepatic veins separate the following segments:the left hepatic vein separates segment 2 from segment 4; the middle hepatic vein separates segment 4 from segments 5 and 8; and the right hepatic vein separates the anteriorly situated segments 5 and 8 from the more posteriorly situated segments 6 and 7. The main portal vein divides into right and left branches. The right portal vein has an anterior branch that lies centrally within the anterior segment of the right lobe and a posterior branch that lies centrally within the posterior segment of the right lobe. As the portal vein divides into the sectorial and the segmental branches very close to the hepatic hilum, the plane where the segmental branches originate can be approximated to the plane passing by the main portal bifurcation: in each sector the inferior segments (5 and 6) will lie caudal to the portal bifurcation, and the superior segments (7 and 8 ) will be cranial to it. Therefore in the right anterior sector segment 5 will be below and segment 8 above, and in the posterior sector segment 6 below and segment 7 above. The left portal vein initially courses anterior to the caudate lobe and describes a smooth arch from the main bifurcation to the round ligament. All liver tissue consisting of the concavity of the arch and the middle hepatic vein will be segment 4. On the convexity of the arch, on the left side (the left lobe of the descriptive anatomy) the distal part of the left hepatic vein will separate segment 2 (posteriorly and superiorly) from segment 3 (more anteriorly and inferiorly).