Anatomy of the liver segments
Robin Smithuis
Radiology Department of the Rijnland Hospital, Leiderdorp, the=20 Netherlands
Liver anatomy can be described using two different aspects: =
morphological=20
anatomy and functional anatomy.
The traditional morphological anatomy =
is=20
based on the external appearance of the liver and does not show the =
internal=20
features of vessels and biliary ducts branching, which are of obvious =
importance=20
in hepatic surgery.
C. Couinaud (1957) divided the liver into eight=20
functionally indepedent segments.
This classification will be =
presented here=20
with several illustrations.
Segmental anatomy
Couinaud classification
The Couinaud classification of liver anatomy divides the liver into =
eight=20
functionally indepedent segments.
Each segment has its own vascular =
inflow,=20
outflow and biliary drainage.
In the centre of each segment there is =
a branch=20
of the portal vein, hepatic artery and bile duct.
In the periphery of =
each=20
segment there is vascular outflow through the hepatic veins.
Right hepatic vein divides the right lobe into anterior and =
posterior=20
segments.
Middle hepatic vein divides the liver into right and =
left=20
lobes (or right and left hemiliver). This plane runs from the inferior =
vena cava=20
to the gallbladder fossa.
Left hepatic vein divides the left =
lobe into=20
a medial and lateral part.
Portal vein divides the liver into upper and lower =
segments.
The=20
left and right portal veins branch superiorly and inferiorly to project =
into the=20
center of each segment.
Because of this division into self-contained units, each segment can =
be=20
resected without damaging those remaining. For the liver to remain =
viable,=20
resections must proceed along the vessels that define the peripheries of =
these=20
segments. This means, that resection-lines parallel the hepatic =
veins,
The=20
centrally located portal veins, bile ducts, and hepatic arteries are=20
preserved.
Segments numbering
There are eight liver segments.
Segment 4 is sometimes divided =
into=20
segment 4a and 4b according to Bismuth.
The numbering of the segments =
is in a=20
clockwise manner (figure).
Segment 1 (caudate lobe) is located =
posteriorly.=20
It is not visible on a frontal view.
The illustrations above are schematic presentations of the =
liversegments.=20
In reality however the proportions are different.
On a normal =
frontal=20
view the segments 6 and 7 are not visible because they are located more=20
posteriorly.
The right border of the liver is formed by segment 5 =
and=20
8.
Although segment 4 is part of the left hemiliver, it is situated =
more to=20
the right.
Couinaud divided the liver into a functional left and right liver (in =
French=20
'gauche et droite foie') by a main portal scissurae containing the =
middle=20
hepatic vein. This is known as Cantlie's line.
Cantlie's line runs =
from the=20
middle of the gallbladder fossa anteriorly to the inferior vena cava=20
posteriorly.
On this illustration it looks as if the medial part of the left lobe =
is=20
separated from the lateral part by the falciform ligament. However it =
actually=20
is the left hepatic vein, that separates the medial part (segment 4) =
from the=20
lateral part (segments 2 and 3).
The left hepatic vein is located =
slightly to=20
the left of the falciform ligament.
Transverse anatomy
The far left figure is a transverse image through the superior liver=20
segments, that are divided by the hepatic veins.
The right figure shows a transverse image at the level of the left =
portal=20
vein.
At this level the left portal vein divides the left lobe of the =
liver=20
into the superior segments (2 and 4A) and the inferior segments (3 and=20
4B).
The left portal vein is at a higher level than the right portal=20
vein.
The image on the far left is at the level of the right portal vein. =
At this=20
level the right portal vein divides the right lobe of the liver into =
superior=20
segments (7 and 8) and the inferior segments (5 and 6).
The level of =
the=20
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=20 portal vein, only the inferior segments are seen (right=20 image).
Caudate lobe
The caudate lobe or segment 1 is located posteriorly.
The caudate =
lobe is=20
anatomically different from other lobes in that it often has direct =
connections=20
to the IVC through hepatic veins, that are separate from the main =
hepatic=20
veins.
The caudate lobe may be supplied by both right and left =
branches of=20
the portal vein.
On the left a patient with cirrhosis with extreme =
atrophy of=20
the right lobe, normal volume of the left lobe and hypertrophy of the =
caudate=20
lobe.
Due to a different blood supply the caudate lobe is spared from =
the=20
disease process and hypertrophied to compensate for the loss of normal=20
liverparenchyma.
Other Classifications and Variants
There are many other anatomical and functional descriptions of the =
liver=20
anatomy.
In the classical description the external appearance of the =
liver is=20
used to describe the anatomy.
However there are many differences =
between this=20
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.
Classical Anatomy
The classical description of the liver anatomy is based on the =
external=20
appearance.
On the diaphragmatic surface, the ligamentum falciforme =
divides=20
the liver into the right and left anatomic lobes, which are very =
different from=20
the functional right and left lobes (or right and left hemiliver). =
In this classical description, the quadrate lobe belongs to the right =
lobe of=20
the liver, but functionally it is part of left=20
lobe.
Bismuth's classification
This classification is very similar to the Couinaud classification, =
although=20
there are small differences. It is popular in the United States, while=20
Couinaud's classification is more popular in Asia and Europe. =
According to=20
Bismuth three hepatic veins divide the liver into four sectors, further =
divided=20
into segments.
These sectors are termed portal sectors as each is =
supplied=20
by a portal pedicle in the centre.
The separation line between =
sectors=20
contain a hepatic vein.
The hepatic veins and portal pedicels are=20
intertwined, as are the fingers of two hands.
The left portal scissura divides the left liver into two sectors: =
anterior=20
and posterior.
Left anterior sector consists of two segments: =
segment IV,=20
which is the quadrate lobe and segment III, which is anterior part of =
anatomical=20
left lobe.
These two segments are separated by the left hepatic =
fissure or=20
umbilical fissure.
Left posterior sector consists of only one =
segment II. It=20
is the posterior part of left lobe.
Variations
In the Couinaud classification little attention is given to the high=20
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=20
the individual segmental anatomy can now be obtained in a non-invasive =
manner=20
(2,3).
The significance of this anatomical insight lies in the =
planning of=20
anatomical resections, whereby the relationship between tumour and =
individual=20
segmental anatomy can be depicted in a three-dimensional format.
Three dimensional liver imaging is of most practical value if a =
resection of=20
one or more segments or sectors is considered, especially in the right=20
hemiliver.
In these cases, 3D liver imaging can demonstrate the =
precise=20
location of the scissuras to the surgeon=20
pre-operatively.