Wednesday, November 15, 2006

Definition of the Couinaud Segments

Couinaud Liver Segments

The Couinaud classification divides the liver into 8 independent segments each of which has its own vascular inflow, outflow, and biliary drainage. Because of this division into self-contained units, each 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. In general, this means resection lines parallel the hepatic veins while preserving the portal veins, bile ducts, and hepatic arteries that provide vascular inflow and biliary drainage through the center of the segment. [Gazelle]

Resecting only specific liver segments is especially useful in patients with hepatocellular carcinoma. Fifty to 75% of these patients have underlying liver cirrhosis and poor liver reserve. The surgical challenge in these patients is to resect enough liver to allow complete tumor resection while retaining all possible non-tumorous liver to prevent further loss of liver function. In support of the benefit of resections along segmental boundaries, MacIntosh has reported an operative mortality of 0-16% with segment based resections compared to mortalities of 20-60% in patients receiving traditional lobectomies or non-segment based wedge resections. [MacIntosh].

Liver Vessels Related to Surface Contours

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Fig 4-1 MIP views of vessels related to surface contours
with the gallbladder, ivc, and falciform ligament labeled.
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The 3-dimensional Maximum Intensity Projections (MIP) shown above outline the vessels in white while showing the liver parenchyma in grey. This allows us to "see through" the parenchyma and relate surface contours to the underlying vessels. These MIP views show the portal vein entering the liver hilum. The additional components of the portal triad, the hepatic artery and bile ducts, are below the resolution of the scan. The portal veins (along with the hepatic artery and the bile duct) project into the center of the Couinaud segments. In most cases, the vascular outflow for each segment is provided by the 3 hepatic veins at its periphery, however accessory hepatic veins are common.

Plane of the Right Hepatic Vein

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Fig 4-2 MIP view of the liver (left panel) showing plane of right hepatic vein as it courses to the IVC. Shaded-Surface projection (right panel) showing, how the plane of the right hepatic vein provides the vertical division of the right liver lobe into anterior and posterior segment groups.
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The intersegmental plane defined by the right hepatic vein subdivides the right lobe of the liver into anterior (V and VIII) and posterior (VI and VII) divisions. Note that the right lateral border of the liver contour is formed by segments VIII (superiorly) and V (inferiorly) as the liver viewed in situ. Segments VI and VII lie posterior to VIII and V respectively. Shown above are a maximum intensity projection and surface projection of the liver indicating the location and significance of the right hepatic vein plane. Both projections are rotated 30 degrees to illustrate the positions of segments VI and VII which actually lie posterior to V and VIII and not lateral to them as as they often appear in the literature. As will be explained later, the anterior and posterior divisions are further subdivided by a plane defined by the right portal vein. [VanLeeuwen, Makuuchi, Masselot, Nakamura].

Plane of the Middle Hepatic Vein

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Fig 4-3 MIP and shaded surface views showing that the course of the middle hepatic vein falls roughly along a palne extending from the gallbladder fossa and the IVC. This plane divides the right hepatic lobe from the left hepatic lobe.
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In both the traditional and the Couinaud classifications, the plane defined by the middle hepatic vein subdivides the liver into the true right and left lobes. A standard right or left lobectomy requires division along the plane of the middle hepatic vein. Segments IVa and IVb lie to the left of the plane while segments V and VIII lie to the right with VIII being superior to V. In the movie, the gallbladder is slightly brighter than the rest of the plane. Because the plane of the middle hepatic vein usually intersects the gallbladder fossa, Cantlie's line (the projection on the liver surface of a plane between the gallbladder and IVC) is generally a valid line of division between the right and left lobes. However, it is the vasculature that determines the true boundary.

Plane of the Umbilical Fissure

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Fig 4-4 MIP and shaded-surface views showing that the plane of the umbilical fissure divides the left lobe into medial and lateral portions which need to be further divided to qualify as independent Couinaud segments.
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The umbilical plane divides the left lobe of the liver into medial (segment IV) and lateral (segments II and III) divisions. This division is the only vertically oriented plane of division that is not defined by the hepatic veins. It can be defined on the surface of the liver by its associated landmarks. It extends from the umbilical fissure anteriorly through the ligamentum venosum along the lateral aspect of the caudate lobe. Structures within the plane of the umbilical fissure include the falciform ligament, ligamentum venosum, and the ligamentum teres. The ligamentum venosum and the ligamentum teres are remnants of the ductus venosus and umbilical vein respectively [Schneck].

Plane of the Left Hepatic Vein

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Fig 4-5 MIP and shaded-surface views showing that the plane of the left hepatic vein subdivides the lateral portion of the left hepatic lobe.
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The significance of the left hepatic vein plane is somewhat controversial [Gazelle]. Some authors have shown it to coincide with the umbilical fissure [Soyer]. In reality the left hepatic vein courses to the lateral to the umbilical fissure. Some authors have claimed that the true division between segments II and III is formed by the transverse plane of the left portal vein [Pagani, Soyer, Gupta, and Nelson]. Most investigators feel that the plane defined by the left hepatic vein is a true intersegmental boundary and is not the same as the plane of the umbilical fissure [Lafortune, Gazelle, Bismuth, Mukai, Lunderquist, and Ger]. Here we define the plane of the left hepatic vein as the boundary between segments II and III. In actual practice, when a lesion occurs within the lateral segment of the left lobe, both Couinaud segments II and III are usually removed based on the plane formed by the umbilical fissure (left lateral segmentectomy). Note that because the plane of the left hepatic vein is oblique, it forms a division between segments III anteriorly and segment II posteriorly.

Plane of the Portal Vein

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Fig 4-6 MIP and shaded surface views showing the plane of the main intrahepatic portal vein. Note, this plane (arrow on surface view) is roughly horizontal but in some cases may be angled as shown in the in the MIP dataset. The dotted line indicates how this plane is sometimes used to divide segment IV into superior (IVa) and inferior (IVb) divisions.
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In Couinaud nomenclature, the plane defined by the right branch of the portal vein divides the anterior and posterior divisions of the right liver superiorly and inferiorly, thus dividing the right lobe into 4 segments (V-VIII). The medial segment of the left lobe can also be divided into two segments by the plane of the portal vein (IVa and IVb)[Bismuth]. While the portal vein plane has often been portrayed as transverse [Soyer], it may be oblique since the left branch runs superiorly and the right branch runs inferiorly. In addition to forming an oblque transverse plane between segments, the left and right portal veins branch superiorly and inferiorly to project into the center of each segment.

Segment 1: The Caudate Lobe

Segment 1 (caudate lobe)
Fig 4-7 Posterior shaded surface view of the liver magnified to show the relation of the caudate with the IVC medially and the fissure for the ligamentum venosum laterally.
The most unique of the Couinaud segments is segment I which corresponds to the caudate lobe (also known as the Spigel lobe). It is located on the posterior surface of the liver adjacent to segment IV. Its medial and lateral boundaries are defined by the IVC and ligamentum venosum respectively.

Segment I is different than the other segments in that its portal inflow is derived from the left and right branches of the portal vein, and it often has its own short hepatic veins connecting directly to the IVC. The vessels of the caudate lobe are rarely seen on CT because they are small. Because of the extensive crossing of vessels and its position relative to the porta hepatis and IVC, segment I is not often resected. However, several examples exist in the literature [Lerut, Yamamoto].

Couinaud Segments: the Complete Picture

Axial mip of the liver
Fig 4-8 Definition of the segments is shown with maximum intensity projections from an inferior view of the entire 3D data set much as seen on the standard CT scan except that the "see through" nature of 3D data allows us to see the courses of the portal and hepatic veins in one view.
In the projections of the entire 3D data set below, you are looking through the entire liver, and the course of the portal and hepatic veins are seen in a single view (left panel). The right panel maps the peripheries of the Couinaud segments with dotted lines. Note that the portal branches supply the middle of the segments while the hepatic veins drain the periphery of the segments. Note also that segments I, II, VI, and VII are mostly posterior and hidden from the surgeon's view in the operating room. Before surgical resection, focal liver lesions must be defined in relation to the deep vessels and the segmental anatomy. This is easily, and quickly accomplished using intraoperative ultrasound. (See next section).

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Surface Projection Movie of the 8 Segments

Fig 4-9 Surface Projection of the liver
showing the 8 Liver Segments as defined by the underlying vascular planes. Note that the plane of the portal vein subdivides segment IV into two parts (IVa and IVb).
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Anterior and Posterior view of 8 segments

Fig 4-10a Anterior and posterior view of liver showing 3-dimensional reconstructions of helical CT scan data in shaded surface projections which have been segmented according to the Couinaud classification.
Superior and Inferiof view of the 8 segments
Fig 4-10b Shaded-Surface 3D reconstructions of the liver segments viewed in the transverse plane at the level of the rostral part of the liver and inferiorly from the caudal surface.
The Couinaud segments and their corresponding traditional nomenclature are summarized above. The corresponding images of the liver surface are for visualization of the relative positions of the segments. It is important to note that the view marked "anterior" is actually rotated about 30 degrees to the patients left so that the posterior segments of the right lobe (VI/VII) can be seen. In an AP view, usually only the anterior segments (V and VIII) of the right lobe are seen since they form the lateral border of the liver contour in a true frontal view.
I) caudate/Spigel lobe
II) left posterolateral segment
III) left anterolateral segment
IVa) left superomedial segment
IVb) left inferomedial segment
V) right anteroinferior segment
VI) right posteroinferior segment
VII) right posterosuperior segment
VIII) right anterosuperior segment

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