The segmental anatomy of the liver is based on the vascular supply and drainage of the parenchyma. Until recently, classification of the liver lobes and sub-segments in the international literature was not uniform. Whereas British and American publications tended to follow the terminology of Goldsmith and Woodburne , in Europe and Japan the most common nomenclature used by radiologists and surgeons was based on the description of Couinaud and Bismuth [3,5].According to the terminology of Goldsmith and Woodburne , the liver comprises two liver lobes (left and right) separated by the middle hepatic vein, and four segments. While the right liver lobe consists of the right anterior and right posterior segments separated by the right hepatic vein, the left lobe is subdivided into the left medial and left lateral segments separated by the left hepatic vein.
Conversely, the classification of Couinaud and Bismuth [3, 5] describes a liver that is divided into a left and a right liver or hemiliver and sub-divided into four sectors and eight segments. Each of these eight segments is independent in terms of blood supply and biliary drainage. This allows for individual resection of a given segment without harm or alteration to the circulation of the remaining liver parenchyma. The left and right liver according to Couinaud and Bismuth correspond directly to the hepatic lobes described by Goldsmith and Woodburne . The right hemiliver is further subdivided into a right paramedian sector and a right lateral sector consisting of two anterior
inferior and two posterior superior segments, respectively. Similarly, the left hemiliver comprises a left paramedian sector and a left lateral sector which are further sub-divided into anterior inferior and posterior superior segments.
The eight liver segments described by Couinaud and Bismuth [3, 5] are numbered clockwise based on a frontal view of the liver beginning with the posterior superior segment of the left parame-dian sector, which corresponds to the caudate lobe, and ending with segment VIII which is consistent with the posterior superior segment of the right paramedian sector . Today, the most common classification of liver segments is based on the Couinaud and Bismuth classification [3,5].
Anatomically, the borders of the liver segments are well-defined, but show a wavy-shaped course . However, in clinical routine, segmentation of the liver on cross-sectional CT and MR imaging is sharply demarcated and usually based on certain landmarks that define the underlying borders (Fig. 20). Based on these landmarks, the ICV is considered the center point for liver segmentation. A line from the ICV to the middle hepatic vein and the gallbladder separates the left and right hemilivers and the corresponding liver segments V/VIII and I/IVa, b, respectively. Whereas the axis between the ICV and right hepatic vein corresponds to the border between liver segments VI/VII and V/VIII, the line between the ICV, the left hepatic vein and the falciform ligament separates liver segments IVa and IVb from segments II and III.
Whereas liver segments VII, VIII, I, IVa and II are located at the posterior aspect of the imaged abdominal situs and above the level of the left and right main portal vein, segments VI, V, IVb and III are located inferior to the level of the main portal veins at the anterior aspect of the liver. Segment I corresponds to the caudate lobe.
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