The Couinaud classification of liver anatomy divides the liver into eight functionally indepedent segments.
Each segment has its own vascular inflow, outflow and biliary drainage.
In the centre of each segment there is a branch of the portal vein, hepatic artery and bile duct.
In the periphery of each segment there is vascular outflow through the hepatic veins.
Right hepatic vein divides the right lobe into anterior and posterior segments.
Middle hepatic vein divides the liver into right and left lobes (or right and left hemiliver). This plane runs from the inferior vena cava to the gallbladder fossa.
Left hepatic vein divides the left lobe into a medial and lateral part.
Portal vein divides the liver into upper and lower segments.
The left and right portal veins branch superiorly and inferiorly to project into the center of each segment.
Because of this division into self-contained units, each segment can be resected without damaging those remaining.
For the liver to remain viable, resections must proceed along the vessels that define the peripheries of these segments.
This means, that resection-lines parallel the hepatic veins,
The centrally located portal veins, bile ducts, and hepatic arteries are preserved.
There are eight liver segments.
Segment 4 is sometimes divided into segment 4a and 4b according to Bismuth.
The numbering of the segments is in a clockwise manner (figure).
Segment 1 (caudate lobe) is located posteriorly. It is not visible on a frontal view.
The illustrations above are schematic presentations of the liversegments.
In reality however the proportions are different.
On a normal frontal view the segments 6 and 7 are not visible because they are located more posteriorly.
The right border of the liver is formed by segment 5 and 8.
Although segment 4 is part of the left hemiliver, it is situated more to the right.
Couinaud divided the liver into a functional left and right liver (in French 'gauche et droite foie') by a main portal scissurae containing the middle hepatic vein. This is known as Cantlie's line.
Cantlie's line runs from the middle of the gallbladder fossa anteriorly to the inferior vena cava posteriorly.
On this illustration it looks as if the medial part of the left lobe is separated from the lateral part by the falciform ligament.
However it actually is the left hepatic vein, that separates the medial part (segment 4) from the lateral part (segments 2 and 3).
The left hepatic vein is located slightly to the left of the falciform ligament.
The far left figure is a transverse image through the superior liver segments, that are divided by the hepatic veins.
The right figure shows a transverse image at the level of the left portal vein.
At this level the left portal vein divides the left lobe of the liver into the superior segments (2 and 4A) and the inferior segments (3 and 4B).
The left portal vein is at a higher level than the right portal vein.
The image on the far left is at the level of the right portal vein.
At this level the right portal vein divides the right lobe of the liver into superior segments (7 and 8) and the inferior segments (5 and 6).
The level of the right portal vein is inferior to the level of the left portal vein.
At the level of the splenic vein, which is below the level of the right portal vein, only the inferior segments are seen (right image).
The caudate lobe or segment 1 is located posteriorly.
The caudate lobe is anatomically different from other lobes in that it often has direct connections to the IVC through hepatic veins, that are separate from the main hepatic veins.
The caudate lobe may be supplied by both right and left branches of the portal vein.
On the left a patient with cirrhosis with extreme atrophy of the right lobe, normal volume of the left lobe and hypertrophy of the caudate lobe.
Due to a different blood supply the caudate lobe is spared from the disease process and hypertrophied to compensate for the loss of normal liverparenchyma.
Other Classifications and Variants
There are many other anatomical and functional descriptions of the liver anatomy.
In the classical description the external appearance of the liver is used to describe the anatomy.
However there are many differences between this classical model and the fuctional models, as popularized by Couinaud and Bismuth.
A more detailed discussion of the various models is given in reference 4.
The classical description of the liver anatomy is based on the external appearance.
On the diaphragmatic surface, the ligamentum falciforme divides the liver into the right and left anatomic lobes, which are very different from the functional right and left lobes (or right and left hemiliver).
In this classical description, the quadrate lobe belongs to the right lobe of the liver, but functionally it is part of left lobe.
This classification is very similar to the Couinaud classification, although there are small differences.
It is popular in the United States, while Couinaud's classification is more popular in Asia and Europe.
According to Bismuth three hepatic veins divide the liver into four sectors, further divided into segments.
These sectors are termed portal sectors as each is supplied by a portal pedicle in the centre.
The separation line between sectors contain a hepatic vein.
The hepatic veins and portal pedicels are intertwined, as are the fingers of two hands.
The left portal scissura divides the left liver into two sectors: anterior and posterior.
Left anterior sector consists of two segments: segment IV, which is the quadrate lobe and segment III, which is anterior part of anatomical left lobe.
These two segments are separated by the left hepatic fissure or umbilical fissure.
Left posterior sector consists of only one segment II. It is the posterior part of left lobe.
In the Couinaud classification little attention is given to the high prevalence of anatomical variations which occur, especially in the right hemiliver.
Using volumetric acquisition techniques, such as magnetic resonance imaging or spiral computed tomography scanning, detailed insight into the individual segmental anatomy can now be obtained in a non-invasive manner (2,3).
The significance of this anatomical insight lies in the planning of anatomical resections, whereby the relationship between tumour and individual segmental anatomy can be depicted in a three-dimensional format.
Three dimensional liver imaging is of most practical value if a resection of one or more segments or sectors is considered, especially in the right hemiliver.
In these cases, 3D liver imaging can demonstrate the precise location of the scissuras to the surgeon pre-operatively.