Understanding the intrahepatic anatomy is crucial to perform liver resections and, in particular, parenchymal-sparing resections. The Couinaud’s liver segmentation system is based on the identification of the three hepatic veins and the plane passing by the portal vein bifurcation. Nowadays, Couinaud’s classification is widely used clinically, because it is best adapted for surgery and has become essential in localizing and monitoring various intrahepatic lesions.

As above-mentioned, Couinaud’s portal segmentation is entirely different from the historically defined two hemilivers based on external landmarks and is also partially different from Healey’s arteriobiliary segmen-tation. According to Couinaud’s descriptions, the right, middle and left hepatic veins divide the liver into four sectors (called suprahepatic segmentation by Couinaud), each of which is supplied by a portal pedicle that consists of a branch of the hepatic artery, portal vein and bile duct.

The middle hepatic vein runs in the main portal scissura (midplane of the liver) which separates the liver into the right and the left hemiliver. The main portal scissura moves forward from the gallbladder fossa anteriorly to the left of the suprahepatic IVC posteriorly, and in clinical practice, these external landmarks may be used as external demarcation line between the functional right and left hemiliver. Both the right and left hemilivers are further separated into sectors by the right and left portal scissura holding the right and left hepatic veins separately.

In the right hemiliver, the right portal scissura divides the right hemiliver into the right anterior sector (right paramedian sector) and the right posterior sector (right lateral sector). It is noteworthy that in the right hemiliver, Healey’s liver sections which he defined as segments are accurately the same as Couinaud’s sectors. In the left hemiliver, the left portal scissura divides the left liver into the anterior sector (left medial sector or left paramedian sector) and the posterior sector (left posterior sector or left lateral sector).

The anterior sector consists of segments 4 and 3, and the posterior sector only includes segment 2. However, in the left hemiliver, Healey’s liver sections which he defined as segments are not the same as Couinaud’s sectors. In the right hemiliver, as Healey’s sections are precisely the same as Couinaud’s sectors, the right anterior sector (section) can be further subdivided into segment 8 superiorly and segment 5 inferiorly. The right posterior sector (Healey’s section) is also further subdivided into segment 7 superiorly and segment 6 inferiorly.

In the left hemiliver, Healey’s sections are not the same as Couinaud’s sectors. The Healey’s left medial section locates between the main portal scissura and the falciform ligament, and it is comprised only of segment 4, which can further be subdivided into segment 4A superiorly and segment 4B inferiorly, while the Healey’s left lateral section is comprised of segments 2 and 3, being divided by the left hepatic vein which runs in the left portal scissura.

For the Couinaud’s left medial sector, it is comprised of segments 3 and 4, locating between the middle hepatic vein running in the main portal scissura and the left hepatic vein running in the left portal scissura. The falciform ligament and the umbilical fissure separate segment 4 from segment 3. The Couinaud’s left lateral sector, which is located within the left territory of the left hepatic vein, is comprised only of segment 2. The caudate lobe is defined as segment 1 in both the Couinaud’s portal and the Healey’s arteriobiliary segmentation systems. This segment is surrounded by the major vascular structures, with the retrohepatic posteriorly, the main portal pedicle inferiorly and the hepatocaval confluence superiorly. Its inflow vasculature originates from both the right and the left portal pedicles, and its biliary drainage exists as a similar pattern. Its venous drainage directly enters into the retrohepatic IVC.