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.
The Falciform ligament divides the left lobe into a medial- segment IV and a lateral part - segment II and III.
The 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.
The illustrations above are schematic presentations of the liversegments.
In reality however the proportions are different.
On a normal frontal view the segments VI and VII are hardly visible because they are located more posteriorly.
The right border of the liver is mainly formed by segment V and VIII.
Although segment IV is part of the left hemiliver, it is situated more to the right.
Couinaud divided the liver into a functional left and right liver 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.
There are eight liver segments.
Segment IV is sometimes divided into segment IVa and IVb according to Bismuth.
The numbering of the segments is in a clockwise manner.
Segment I (the caudate lobe) is located posteriorly.
It is not visible on a frontal view.
This figure is a transverse image through the superior liver segments, that are divided by the right and middle hepatic veins and the falciform ligament.
This is a transverse image at the level of the left portal vein.
At this level the left portal vein divides the left lobe into the superior segments (II and IVa) and the inferior segments (III and IVb).
The left portal vein is at a higher level than the right portal vein.
This image 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 (VII and VIII) and the inferior segments (V and VI).
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 visible.
How to separate liver segments on cross sectional imaging
Left lobe: lateral(II/III) vs medial segment (IVA/B)
Extrapolate a line along the falciform ligament superiorly to the confluence of the left and middle hep veins at the IVC (blue line).
Left vs Right lobe: IVA/B vs V/VIII
Extrapolate a line from the gallbladder fossa superiorly along the middle hep vein to the IVC (red line).
Right lobe: anterior (V/VIII) vs posterior segment (VI/VII)
Extrapolate a line along the right hepatic vein from the IVC inferiorly to the lateral liver margin (green line).
The caudate lobe or segment I 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.
This CT-image is of a patient with liver 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.
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 the 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 MRI or CT 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.