Prof. Dr. Holger Strunk |
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Lebersegmentanatomie |
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Übersichten | ||||||||||||||||||||||||||||||
Limitations and Pitfalls of Couinaud`s Segmentation of the Liver in Transaxial ImagingINTRODUCTIONThe segmental anatomy of the human liver has become a matter of increasing interest to the radiologist especially in view of the need for an accurate preoperative localization of focal hepatic lesions. Presently, the type of surgical resection chosen depends largely on the segmental localization of the hepatic lesion (1,2). Procedures for delineating segmental and subsegmental anatomy on ultrasonography (US), computed tomographic (CT) and magnetic resonance (MR) images have, therefore, been the subject of several studies (3-14) during the past decade. Essentially, these procedures are based on the concept of three vertical planes that divide the liver into four segments and of a transverse scissura that further subdivides the segments into two subsegments each (2, 4, 6, 11). Although convenient for daily radiologic practice, use of this concept is highly questionable from an anatomic point of view (15, 16). Radiologists also have recently expressed skepticism and have described observations that are incompatible with this concept (13, 17-19). In this article we first want to give an overview of the different classical concepts. In a second part, we also want to delineate the limitations of these methods. Because this article deals only with segmental anatomy, extrahepatic variations of bifurcating patterns of the portal vein, arterial blood supply or biliary drainage is of no concern (20-29). DESCRIPTIVE LIVER ANATOMYThe classical descriptive anatomy nomenclature is based on external landmarks visible on the surface of the liver. It distinguishes four lobes: right, left, caudate and quadrate. The right and left lobes are separated by the falciform ligament on the anterior surface of the liver. On the inferior and posterior surfaces, an H-shaped group of fissures and fossae delimits the four lobes (Fig. 1). This classical anatomic concept however does not reflect the functional anatomy, which is considered far more important for hepatic surgeons.
Modern hepatic surgeons and radiologists use a nomenclature based essentially on the internal vascular and biliary architecture of the organ. In Europe and Japan the nomenclature most commonly used by surgeons and radiologists is based on the description introduced by Couinaud (30) and Bismuth (31). In American and British publications the terminology proposed by Goldsmith and Woodburne (32) was generally employed, however Couinauds classification has recently also permeated the English North American radiology and surgery literature (6, 33-35). Couinaud`s system distinguishes two lobes and four segments (Fig. 2). Each segment has a branch (or a group of branches) of the portal vein at its center and a hepatic vein at its periphery. Each of these afferent territories ( called: portal compartments pars pro toto by Couinaud and Bismuth) is separated from the others by planes corresponding to portal scissures which allow surgical removal of parenchyma without impairing the blood supply of the remaining liver tissue. In the last official edition of the International Anatomical Nomenclature (36) and in the latest published edition (37), a posterior and anterior segment are attributed to the right lobe, and a medial and lateral segment to the left lobe. Each of these segments is further divided into an antero-inferior and a postero-superior segment (or subsegment for English native speakers). Thus the liver is divided into eight segments, both longitudinally along the hepatic veins and transversely through the right and left portal pedicles. Seen from in front, they are numbered clockwise from I to VIII, with 1-4 in the left lobe and 5-8 in the right (Fig. 2). Segment 1 is the caudate or Spiegel lobe.
When a cross-sectional imaging technique is used, such as US, CT or MR imaging, lines drawn from the inferior vena cava straight through to the three hepatic veins coincide with the boundaries between segments—the longitudinal scissurae (3, 11). Thus, it is believed that the longitudinal divisions can be clearly delineated on transverse images, since they run perpendicular to the axis of the scan (6). COUINAUD´S CLASSIFICATIONUsing cross-sectional imaging, the following anatomic landmarks are used to divide the liver into segments:
-) hepatic veins When 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 (Figs. 2,3).
-) portal veins The 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) (Fig. 4) (7).
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 6 (Fig. 6).
Segment 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 ligaments The 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 (Figs. 5b, 8).
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 (Figs. 7). LIMITATIONS OF COUINAUD`S CONCEPTAlthough the above outlined concept may be correct in some patients, from a morphologic point of view it is questionable (15, 42). In particular, the anatomists Platzer and Maurer (15) pointed out that the variability of segmental bounderies is too large to render any scheme viable. Fasel et al. (16), who also used an anatomic approach, confirmed that the vertical planes that intersect the trunks of the hepatic veins do not correspond to the presumed intersegmental boundaries. Radiologists, as well, have recently published observations that call into question the routine radiologic methods currently used for delineation of the segmental anatomy of the liver. Nelson et al (6) evaluated the preoperative subsegmental localization of focal hepatic lesions with the use of CT during arterial portography and concluded that the CT findings of localization disagreed with the extent observed at surgical resection in 11 of 36 (31 %) lesions. Soyer et al. (8) reported a discrepancy in eight of 36 (22 %) cases in an analogous study with two-dimensional images obtained at CT during arterial portography; they concluded that indirect landmarks are not reliable for the correct delineation of portal venous segments and subsegments. Before we move on, an important difference needs to be noted: no intrahepatic anastomoses exist between the portal, arterial, and biliary structures of adjacent segments, whereas the hepatic veins have large and numerous intrahepatic anastomoses. In addition several segments are bordered by the same hepatic vein, and the hepatic veins have abundant anatomic variations as shown in several studies (43, 44). As the portal, arterial, and biliary systems are grouped together in vasculobiliary sheaths (30), interruption of the elements within these sheaths will inevitably lead to depriving the region of its arterial and portal blood supply and also to the creation of bile stasis or leakage that result from a lack of intrahepatic anastomoses between the portal, arterial, and biliary structures of adjacent segments. Thus, the portal venous anatomy is the critical factor in candidates for systematic subsegmental hepatectomy. This is confirmed by a study of Rieker et al. (19), who performed biphasic helical CT scans on patients evaluated for liver resection. During the first evaluation, all liver lesions were localized in the conventional way using the planes of the three major hepatic veins and the portal trunks as segmental boundaries. In a second review, all lesions were attributed to the nearest peripheral portal branches. The path and the segmental attribution of the portal branches were analysed. Evaluations were performed using an interactive cine mode as well as three-dimensional reconstructions. 20 of 126 (16%) liver lesions had a different segmental location if the individual anatomy of the peripheral portal branch was used instead of the conventional technique (Fig. 9). These different locations were due to the path of the portal trunks or the path of the peripheral portal branches crossing the planes of the major hepatic veins. It remains however unclear, if these differences when known preoperative, would have changed the surgical approach.
One of the greatest differences occured in the right dome of the liver, concerning the boundary between segment 7 and segment 8 (Fig. 9). Other major discrepancies occured in determinating the boundary between right and left hemiliver and between segment 6 and segment 7, the so-called "transverse scissura". -) boundary between segment 7 and segment 8 In 1994, van Leuwen et al. (13) by using three-dimensional spiral CT reconstructions stated, that in the vast majority of patients, the right scissura did not coincide with a coronal plane through the right hepatic vein. In the cranial part of the liver the average angle of the right scissura was 58.4° tilted posteriorly relative to the coronal plane, whereas in the caudal part of the liver, the average angle of the right scissura was only 2.8° tilted anteriorly relative to the coronal plane, suggesting a closer relationship between right hepatic vein and right scissura in the caudal part of the liver. However the range of the angle of the inferior part of the right scissura was very large: from 65° anterior to 50° posterior to the coronal plane. In addition, Ohashi et al. (18) observed discrepancies between the imaginary intersegmental boundaries and the margins of the areas actually enhanced with contrast material on axial CT arteriograms obtained with selective injections, especially in the region under the right side of the diaphragm. Lines drawn from the IVC straight through the middle or right hepatic vein did not correspond to the boundaries between the right and left lobes, which is the main (middle) longitudinal scissura, or the boundary between the anterior and posterior sectors (segments 8 and 7), which is the right longitudinal scissura (Fig. 10).
The angle between the line drawn through the right hepatic vein and the margin of the area enhanced by injection into the anterior branch (A8) and the posterior branch (A7) was 43.9° +- 14.0 on average (range, 10°-70°), that is the right longitudinal scissura was angled posteriorly 43.9° on average (18) (Figs. 11, 12).
In patients with cirrhosis the right scissura was angled posteriorly to the line in all patients evaluated (range, 10°-70°). Consequently, segment 8 was revealed to be fan-shaped beyond the hepatic vein / IVC lines and peripheral on the axial images. In three-dimensional perspective with mental reconstruction from the axial images, segment 8 was revealed to overhang the right hepatic vein (18) (Figs. 9-12). Nelson et al. (6) reported that a lesion located high under the hemidiaphragm may be difficult to localize to a specific segment because of the absence of landmarks at this level. This area consisted of both lobes in 54% of cases, only the right lobe in 41 %, and only the left lobe in 5% (18). Furthermore, the area enhanced by injection into the posterior branch or A7 was not seen above the level of the confluence of hepatic veins. These results indicate that the right lobe is more frequently associated with the area under the right side of the diaphragm than is the left lobe and that segment 7 is not demonstrated in the area at all (Fig. 9). These results also correlate with those reported by others (13, 19, 45), who found that, in most patients, the region just below the diaphragm was supplied by posteriorly directed branches of the right anterior portal vein. In 93 % of patients, the dorsal branch of segment 8 gave rise to dorsally directed branches posterior to the right hepatic vein (45) (Fig. 11). -) differentiation of right and left hemiliver Even for the differentiation of right and left hemiliver, discrepancies were noted: the angle between the line drawn through the middle hepatic vein and the margin of the area enhanced by injection into the RHA (Figs. 11, 13a), and/or the LHA (Fig 13b) was –16.2° +- 16.8 on average (range, -53° to 28°), that is the main longitudinal scissura was angled anteriorly 16.2° on average (18).
A part of segment IV was fed by the right hepatic artery in 2 / 18 Patients (18 %) (46). Even in patients with hepatic deformity caused by cirrhosis, in whom the left lobe is usually enlarged, the main scissura was angled anteriorly to the line (14.7° on average; range, -28° to 50°) (18). -) "transverse scissura" Pronounced discrepancy between the radiologically determined segmental and subsegmental anatomy and the actual anatomic territories in the central regions of the liver - that is, in the region of the so-called "transverse scissura" – were also noted (13). This however was almost to be expected, because the portal venous territories show especially increased undulations in this transitional region. In no patients could a definite transverse scissura, devoid of portal segmental branches, be seen in the right anterior or posterior sectors (13). These results illustrate that a division of the right hemiliver in an anterosuperior sector and a posteroinferior sector along the angled plane of the individually defined right scissura could be appreciated in all patients studied. Further subdivision into smaller portal units, or segments, is fairly arbitrary, because each trunk gives off an array of portal branches, each supplying an individual portal segment. Downey (17) called attention to the fact that the scissurae may curve, undulate, or even interdigitate within the liver. These radiologic observations accord well with the results obtained in the study by Fasel et al. (47). They examined vascular casts of the liver with helical CT. Liver segmental and subsegmental anatomy were determined on the CT scans according to customary radiologic practice guidelines and compared with authentic anatomic territories seen at anatomic examinations (Fig. 14). As a result, an average of 17.3 % +- 6.5 of the hepatic area visualized on axial CT scans was attributed to an incorrect subsegment. For the central zones, this error amounted to 51.6 % +- 19.9. Expressed in absolute numbers, the error amounted to 40 mm on axial CT scans. The boundaries betwwen the subsegments proved to be complex, undulating scissurae rather than simple, flat planes (Fig. 15). A uniform transverse plane, which is the assumption of the radiologic concept, could not be observed anatomically for either the right or the left hemiliver. The vertical scissurae also were observed not to be simple planes; rather they were undulating, irregular boundaries.
The above stated divergences remain too high to permit reliable determination of segmental and subsegmental anatomy. With current radiologic procedures based on indirect landmarks it is therefor not possible to determine the authentic segmental and subsegmental anatomy of the liver. Every concept of flat planes that delimit the portal venous territories is an oversimplification that is not in agreement with anatomic reality. True segmental and subsegmental determination will be possible only with methods that account for the actual anatomy of the portal venous tree, including the smallest peripheral branches. This can be done by evaluation of the overlapping transverse slices in an interactive cine-mode or by performing 3D-rendering. For patients in whom central lesions are to be resected, or in whom one or more segments in the right hemiliver are considered for anatomic resection 3D-reconstructions can help to plan a resection tailored to the individual segmental anatomy (8, 48). CONCLUSIONThe implications for radiologic practice depend on the clinical circumstances. If a radiologist is concerned with preoperative localization of hepatic lesions, tumor must be located relative to the avascular planes between the different portal territories. In our experience, the "easiest" way to do this, is a meticulous evaluation of the overlapping transverse slices in an interactive cine-mode on a workstation or the CT-unit. This allows the appreciation of the individual portal segmentation and the location of the scissurae in most patients. 3D-reconstructions often are useful to show the exact relation between lesions and individual vascular anatomy on one image instead of demonstrating a multitude of images, but they are time-consuming and not necessarily needed for exact localization. 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Radiology 1998; 206: 151-15648. Bennett W, Bova J, Petty L, Martin E. Preoperative 3D rendering of MR imaging in liver metastases. J Comput Assist Tomogr 1991; 15: 979-984 | ||||||||||||||||||||||||||||||||
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Comparison of the anatomy
shown in (b) with that shown in (a)
demonstrates the discrepancy between the
rigid radiologic concept and the more
complex anatomic reality. The numbers
indicate the subsegments designations.
(Fasel et al. Radiology 1998; 206: 151)