COUINAUD´S CLASSIFICATIONUsing cross-sectional imaging, the following anatomic landmarks are used to divide the liver into segments:

hepatic veins (cranial) portal system gallbladder (caudal) falciforme ligament lig. venosum

hepatic veinsWhen seen with an oblique coronal subxiphoid view, the three hepatic veins form a "W," with its base on the IVC (Fig. 3). The left and middle hepatic veins join the left anterior part of the IVC. The hepatic veins separate the following segments: the left hepatic vein separates segment 2 from segment 4; the middle hepatic vein separates segment 4 from segments 5 and 8; and the right hepatic vein separates the anteriorly situated segments 5 and 8 from the more posteriorly situated segments 6 and 7

portal veinsThe main portal vein divides into right and left branches. The right portal vein has an anterior branch that lies centrally within the anterior segment of the right lobe and a posterior branch that lies centrally within the posterior segment of the right lobe. The left portal vein initially courses anterior to the caudate lobe. The ascending branch of the left portal vein then travels anteriorly in the left intersegmental fissure to divide the medial and lateral segments of the left lobe.The segmental branches of the portal vein (each one of which leads into a segment) can be outlined in the form of two "H's" turned sideways, one for the left lobe (segments 1-4), and one for the right lobe (segments 5-8)

Left lobe:The H for the left lobe, sonographically best visualized with an oblique, upwardly tilted subxiphoid view (also called recurrent oblique view by Weill (38)), is formed by the left portal vein, the branch entering segment 2, the umbilical portion of the left portal vein, and the branches to segments 3 and 4 (Figs. 4, 5). To this recumbent H are attached two ligaments, the ligamentum venosum (also called the lesser omentum or the hepatogastric ligament) and the falciform ligament. The ligamentum venosum separates segment 1 from segment 2.Segment 1 (caudate lobe) is bordered posteriorly by the IVC, laterally by the ligamentum venosum, and anteriorly by the left portal vein (Fig. 5). Unlike the other segments of the liver, it may receive branches of the left and right portal veins. The portal veins to segment 1 are usually small and are seen sonographically in approx. 17% of examinations. The caudate lobe has one or more hepatic veins that drain directly into the IVC, separately from the three main hepatic veins (39, 40). The hepatic vein for segment 1 can be seen in 12% of subjects. This special vascularization is a distinctive characteristic of segment 1.The portal vein leading to segment 2 is a linear continuation of the left portal vein, completing the lower horizontal limb of the H (Figs. 4, 5). Segmental branches to segments 3 and 4 form the other horizontal limb. Segments 2 and 3 are thus located to the left of the umbilical portion of the left portal vein, the ligamentum venosum, and the falciform ligament. Segment 4 (the quadrate lobe) is situated around the

right anterior limb of the portal venous H, to the right of the umbilical portion of the left portal vein and the falciform ligament. Segment 4 is separated from segments 5 and 8 by the middle hepatic vein and by the main fissure (a line between the neck of the gallbladder and the right portal vein) (41) (Figs. 2,3). It is separated from segment 1 by the left portal vein (Figs. 4,5). In a study performed by Lafortune et al. 1991 (7) the vascular anatomy of the left lobe was constant in all but two subjects (98% ) and in both of the livers dissected for this purpose. In two subjects, the umbilical portion of the left portal vein was situated to the left of the falciform ligament so that the falciform ligament was no longer aligned with it. Segments 2 and 3 were each supplied by two portal vein branches in five and seven patients, respectively. Segment 4, often the largest of the left lobar segments, received up to seven portal venous branches (mean, four branches); 57 (57%) received three to four branches. Except for the branch that is part of the H mentioned earlier, the branches for segment 4 originated from the umbilical portion of the left portal vein in a fan-shaped fashion.

Right lobe:In most patients, the portal branches of the right anterior trunk have a paired appearance, with one of the paired branches located parallel and anterior to the other. Also, in most patients, the area posterior to the right hepatic vein and just below the diaphragm is supplied by posteriorly directed branches of the right anterior trunk (13). The right posterior trunk also shows a consistent pattern of ramification. In most patients, the posterior trunk is directed dorsolaterally, more or less in the horizontal plane, with only caudally directed branches along the first two thirds of its length. In nine of 26 patients, the first caudally directed branch supplies an area anterior to the right hepatic vein (13).The right portal vein follows an oblique or vertical course, directed anteriorly (Figs. 4, 6). On sagittal oblique sonograms the branches leading to the segments of the right lobe of the liver are also distributed in the shape of a side-ways H (Fig. 6). The right portal vein forms the crossbar of the H. The branches to segments 5 and 8 (anterior segment) form the upper limb of the H, while the branches to segments 6 and 7 (posterior segment) form its lower portion. The branches of segments 6 and 7 are more obliquely oriented, and for sonographic visualization the transducer should be rotated slightly upward for segment 7 and downward in the direction of the right kidney for segment 6Segment 5 is bordered medially by the gallbladder and the middle hepatic vein and latero-dorsal by the right hepatic vein. The right portal vein separates segment 5 from segment 8. Segment 8 is separated from segment 7 by the right hepatic vein, from segment 5 by the main right portal vein, and from segment 4 by the middle hepatic vein.The basic H for segments 5-8 was found in all subjects and dissected livers (7). In 94 (94%) subjects, segments 5 and 8 were supplied by two or three venous branches that emerged at right angles or in a "V" shape from the right portal vein. In six (6% ) subjects, there was one or four segmental branches. The origin of segmental branches was rarely symmetric. Segments 6 and 7 received two portal venous branches in 14 (14 %) and two (2 %) subjects, respectively.

gallbladder and ligamentsThe liver ist covered by a thin connective tissue layer called Glisson’s capsule. At the porta hepatis the main portal vein, the proper hepatic artery, and the common bile duct are contained within investing peritoneal folds known as the hepatoduodenal ligament.

The falciform ligament ( Fig. 7) conducts the umbilical vein to the liver during fetal development. After birth, the umbilical vein atrophies, forming the ligamentum teres. The ligamentum venosum carries the obliterated ductus venosus, which until birth shunts blood from the umbilical vein to the inferior vena cava

The falciform ligament is seen between the umbilical portion of the left portal vein (30) and the outer surface of the liver. As outlined above it separates segment 3 from segment 4.The right and left lobes are separated by the main hepatic fissure (Cantlie-line), a line connecting the gallbladder and the left side of the IVC