15D.C. Madoff et al. (eds.), Venous Embolization of the Liver,DOI 10.1007/978-1-84882-122-4_3, © Springer-Verlag London Limited 2011

Portal and Hepatic Vein Anatomy

Yoji Kishi, Takuya Hashimoto, and Masatoshi Makuuchi

3

Evaluation of the vascular anatomy is mandatory in order to perform portal vein embolization (PVE) safely, which is followed by major hepatic resection. Recent advances in radiologic imaging allow for more precise evaluation of the intrahepatic vascular anatomy, including the tertiary and much peripheral branches. Anatomic liver resection is defined as a resection of the region perfused by specific portal vein branch(es). Both the portal vein and the hepatic vein anatomy indicate the hepatectomy procedures that may be achieved.

In this section, the following points are discussed: 1. General knowledge of the portal vein and the

hepatic vein anatomy necessary for assessing the indications for PVE and the safety of PVE.

2. Variations of portal and hepatic veins.

3.1 Relationship Between the Portal Vein and the Hepatic Vein

Liver segmentation is determined accordingly to the portal and the hepatic vein anatomy, and two types of liver segmentation have generally been used. Healey et al.1 proposed a segmentation under which each segment is bordered by the middle and the right hepatic veins and the umbilical portion of the left portal vein. Whereas Couinaud2 proposed a

M. Makuuchi (*)Japanese Red Cross Medical Center, 4-1-22 Hiroo, Shibuya-ku, Tokyo 150-8935, Japane-mail: makuuchi_masatoshi@med.jrc.or.jp

Abstract

Assessment of portal vein anatomy is mandatory to evaluate the indication or strategy of portal vein embolization. Recent advances in radiological imaging facilitate the precise assessment of intrahepatic tertiary or more peripheral tribu-taries of the intrahepatic vascular system. Not only the portal vein anatomy itself, but also the hepatic vein anatomy and anatomical relationship between the portal vein and hepatic vein should be accurately evaluated for the following reasons: First, sacrifice of hepatic vein tributaries may result in the congestion of large part of remnant liver after hepatic resection. Second, the intersegmental plane after major hepatectomy may not necessarily be straight but curved.

Keywords

Anatomy of the liver • Portal and hepatic vein anatomy • Hepatic vein anatomy • Portal vein embolization • Radiology of portal and hepatic vein anatomy

16 Y. Kishi et al.

segmentation defined by the anatomy of the portal vein, and sectors are separated by three major hepatic veins. In this chapter, Couinaud’s nomen-clature will be used.

Some authors have described that the sectors bor-dered by the three major hepatic veins do not neces-sarily coincide with the sectors defined as the regions perfused by portal vein branches. Hata et al.3 has recently reported that the right hepatic vein (RHV) tributaries and portal vein branches of segment (S) 6 frequently run parallel rather than perpendicular to each other. Another example is a well-developed mid-dle hepatic vein (MHV) tributary draining S6 found in as many as 23% of examined cases.4 In this type of variation, an extended left hepatectomy including the MHV5, and right paramedian sectoriectomy6 result in the congestion of a large part of the remnant right liver. These fact do not conflict with the conventional liver anatomy when the lobulated mammalian liver is considered7 (Fig. 3.1). Cho et al.8 reported that the dor-sal branch of the S8 portal vein distributes perfusion over the entire dorsal and cranial area of the right liver posterior to the RHV in 90% of cases. However, the location of the intersectorial lines on the surface of the liver varies. Imagine the cut surface of a solid pine-apple. As shown in Fig. 3.2, the intersectorial line on the surface differ by patients. This is easily perceived

RHV

MHV

LHV

Portal vein

Plane ofhepatic hilum

P6 P5

Fig. 3.1 Liver of mammals with lobulation. The portal vein runs parallel to the RHV tributaries in segment 6

Portal vein

Portal fissure

Hepatic veinIntersectorial lineon liver surface

Hepatichilum

Fig. 3.2 Scheme of intersectorial line defined as the border of portal vein branches in each sector

173 Portal and Hepatic Vein Anatomy

by observing where the falciform ligament is attached on the liver surface under CT scan.

Evaluation of the hepatic vascular anatomy based only on thick-slice axial CT images can result in an underestimation of variations. Atasoy and Ozyurek9 suggested that multi-row detector CT may reveal a higher incidence of variations compared to conven-tional axial thick-slice CT, whereas Savier et al.10 sug-gested that ultrasonography is more sensitive compared to CT for detecting the right-sided round ligament and emphasized the importance of evaluating the liver anatomy three-dimensionally.

3.2 Necessary Knowledge for Volumetric Analysis

Because the indication for PVE is determined based on the remnant liver volume, volumetry is mandatory before and after PVE. The recent advances in imag-ing techniques have allowed for the use of the volume-rendering method to precisely evaluate the volume of each segment of the liver based on calculations of the volume of the region fed by a selected portal vein branch. Among the several softwares, recently released SYNAPSE VINCENT® (FUJIFILM, Co., Ltd., Tokyo, Japan) is a 3D volume analyzer easy to manipulate and enables quick assessment of intrahe-patic vascular structures and segmental volumes.11 The border between any two adjacent segments is not a two-dimensional flat plane, as can be seen by the demarcation line that appears after inflow occlusion of the branches of the liver to be resected (Fig. 3.3). Fischer et al.12 compared the results of volumetry per-formed by a volume-rendering method based on the portal vein branching pattern and by calculating each segment delineated by two-dimensional plane. The results showed a large dissociation, especially of S5, S7, and S8. Unfortunately, there is limited availabil-ity of such special software for volumetry based on volume-rendering method, and a more popular method resently progressed based on manual delineation of each sector is employed. Usually, the border along the plane of the umbilical fissure, main portal fissure, and the right portal fissure indicate the border of each sector. Basically, the umbilical fissure is the plane in which the umbilical portion of the left portal vein and the cranial portion of the fissural vein (Fig. 3.4) run. And the left, the main and the right portal fissure are the planes in which the left, the middle and the

right hepatic veins, run, respectively. These vascular structures, however, may not be recognized in all CT slices. Ideally, the borderline between the portal vein branches on both sides of the sectors recognized in each slice of CT images should be delineated. However, the borderline between S3 and S4 is sometimes difficult to define because the portal vein branches originate from the umbilical portion and usually run along the umbili-cal fissures, which are often defined as S3 branches.

Fig. 3.3 Demarcation line that appeared after ligation of the left hepatic artery and portal vein. Although this line approxi-mately coincides with Rex’s line, it is not straight

Scissural vein

LHVP8

P

A

cm

R L

Fig. 3.4 Scissural vein that drains the cranial part of segment 4 and conjugates into the root of the middle hepatic vein is indi-cated by an arrow. This vein usually runs along or left to the same plane as the umbilical fissure and indicates the border between segments 3 and 4. Further, if this vein is preserved dur-ing an extended right hepatectomy, a large area of congestion of segment 4 can be prevented. P8 indicates a portal vein branch of segment 8

18 Y. Kishi et al.

This becomes an issue when embolization of the S4 portal veins is indicated. It is still controversial as to whether the portal vein branches of S4 should be embolized before a right lobectomy (S4-8 resection). Nagino et al.13 first reported the PVE technique for the right liver and S4, and observed a significant degree of hypertrophy of S2 and S3. Similar results have been reported by Kishi et al.14 In contrast, Capussotti et al. showed comparable hypertrophy of S2/S3 after right PVE with or without additional embolization of the S4 branches.15 In practice, embolization of the S4 branches with preservation of the S2 and S3 branches is complicated because multiple branches ramify from the umbilical portion, and it is difficult to completely embolize all of these branches. However, this may be overcome by evaluating several different projections using C-arm CT.16 However, it is enough to embolize the major branches (superior and inferior branches of S4), actually.

The main trunk of the left, the middle and the right hepatic veins usually runs along the intersectorial plane in the cranial portion of the liver. Under caudal CT slices, however, they are not observed as one main trunk running along the same plane, but rather as several peripheral branches running within each sector. The border between S4 and the right paramedian sector is easily recognized because the gallbladder bed is a good indicator (Fig. 3.5). The border between S5 and S6 on the other hand is sometimes difficult to define because there is no definitive indicator; therefore, the peripheral

portal vein branches of these two segments should be recognized in order to determine the border for the preoperative evaluation and the ultrasound guided staining technique should be used during hepatectomy. Even in the cranial portion, the RHV does not indicate an unequivocal plane between the right paramedian and the lateral sectors (right portal fissure). In 10–60% of cases, the common trunk of the RHV is short and the confluence of two to three branches of equal sizes is located near the root of the RHV (Fig. 3.6).2,17,18

3.3 Portal Vein Variations

Incidence of portal vein anatomic variations at the level of the hepatic hilum is much lower than that at the bile duct or hepatic artery. A common portal vein variation is the lack of the main trunk of the right portal vein, that is, trifurcation of the right paramedian branch, right lat-eral branch, and left main trunk, or the right lateral branch independently originating from the main portal vein. These types of variations are not contraindica-tions for PVE. However, close attention must be paid so that coils or embolization materials do not protrude or migrate into the left portal vein during the right PVE.

Rare but important variations associated with the indication for PVE and hepatectomy are the right-sided round ligament or the complete absence of the right or the left main trunk (horizontal por-tion) of the portal vein.

The right-sided round ligament was first described by Matsumoto19 in 1986. The incidence is reported to be 0.1–0.7%.20 Intrahepatic portal vein anatomy with a

LHV

MHV

RHV

Fig. 3.6 Short common trunk of the right hepatic vein (RHV). The RHV does not run along the plane of the right portal fissure, even in the cranial portion

V5

P

A

cm

R L

Fig. 3.5 In the caudal portion of the liver, the middle hepatic vein does not consist of one intersegmental branch, but only the branches running within segments 4 and 5 are recognized. The gallbladder bed, however, indicates the main portal fissure. V5 indicates a tributary of the middle hepatic vein that drains segment 5

193 Portal and Hepatic Vein Anatomy

right-sided round ligament is divided into two types. In the first type, the right lateral branch ramifies first, with the left main trunk extending to the left side giving rise to S2 and S3 branches. From the umbilical portion, ramifies the right paramedian branches. No apparent branch of S4 is observed at the left side of MHV. In the second type, the left portal vein (S2 and S3 branches) ramifies first, with the right main trunk giving rise to the right lateral branch ending at the umbilical portion, where the right paramedian branches ramify.21 Special attention must be paid to associated variations, such as the absence of the umbilical portion of the left portal vein or the horizontal part of the left hepatic duct.

Complete absence of the left main trunk (horizontal portion) or the right main trunk of the portal vein is another rare variation that would usually be a contraindi-cation for hemihepatectomy. However, using the trans-parenchymal division technique or the portal pedicle preserving the main arcade of the portal vein and transec-tion of the liver parenchyma along the MHV under inflow occlusion, right or left hemihepatectomy is pos-sible. But in our vast experiences of hepatic resections, we have never encountered the anomaly. Absence of the left portal vein was initially observed by Couinaud2 in 1 out of 103 specimens and recently by Fraser-Hill et al.22 in 7 out of 18,550 patients (0.04%) who had undergone abdominal ultrasonography. In Couinaud’s case, after the ramification of S6 and S7 branches, the main portal vein entered the hepatic parenchyma and extended as usual into the right anterior branch. Then, it headed for the left liver, curving toward the cranial side and running along the umbilical fissure in the opposite direction after the ramification of S3 and S4 branches, and ended up as an S2 branch. Complete absence of the right portal vein is rarer, and was observed by Fraser-Hill et al.22 in 4 out of 18,550 (0.02%) ultrasonography studies. In these four cases, the right liver was atrophic and was supplied only by small branches arising from the main and the left por-tal veins, although anatomy of RHV of MHV was not described in this report.22

3.4 Hepatic Vein Variations

3.4.1 Left Hepatic Vein

The main trunk of the left hepatic vein (LHV) runs along the intersegmental plane of S2 and S3, draining these two segments. However, there usually exists a fissural vein that runs along the umbilical fissure and

conjugates into the root of the LHV. This vein drains the cranial portion of S4. This means that an extended right hepatectomy including the MHV will at least not result in the congestion of the cranial portion of S4 if the vein can be preserved.

3.4.2 Middle Hepatic Vein

The MHV runs along the main portal fissure and is the drainage vein for S4 and the right paramedian sector. The volume of S4 that drains via the tributaries of the MHV is usually small due to the scissural vein that joins with the left hepatic vein. Even if the drainage vein of the cranial part of S4 joins with the MHV near its confluence to the inferior vena cava, the cranial part of S4 may be preserved in most cases because an extended right hepatectomy is most frequently indi-cated in cases with tumor invading the hepatic hilum or MHV. Therefore, this S4 drainage vein can be pre-served and complete resection of S4 (right lobectomy) rarely becomes necessary.

A large portion of the right paramedian sector on the other hand, is drained via the tributaries of the MHV, and in some cases, there is a long and thick MHV tributary draining a portion of S6 (Fig. 3.7).5,6 Therefore, sacrifice of the MHV in an extended left hepatectomy may result in the congestion of a large part of the right anterior sector and S6. The volumes of the areas congested after sacrificing the MHV must be pre-estimated because intrahepatic venous communi-cation (in the peripheral MHV and right hepatic vein and/or IRHV) exists in only about 20% of the cases,23 and these congested area would not function normally without the MHV.

If the volume of the remnant liver without congestion is estimated to be small, reconstruction of the MHV should be considered. Ideally, an accurate evaluation of the volumes of the areas expected to be congested should be performed using the volume-rendering method (the volume of liver parenchyma fed by a specific portal vein branch or drained by a specific hepatic vein branch estimated). Approximate estimation is also possible by using standard CT axial images, as reported by Hwang et al.24 In brief, the tributaries of the MHV and RHV draining the right paramedian sector are identified in each slice, and an imaginary line is drawn midway between the MHV tributaries and the RHV tributaries. The area confined between this imaginary line and the main portal fissure is defined as the area of congestion,

20 Y. Kishi et al.

and stacked images of congestion areas are used to cal-culate the expected congestion volume.

An approximate assessment of the congestion area may be much easier intraoperatively using Doppler ultrasonography. Under this method, the MHV trunk or tributaries are clamped during transection of the liver parenchyma which is then followed by clamping of the hepatic artery. This results in surface discolor-ation of the congested region, and Doppler ultrasonog-raphy would show no signal of the MHV tributaries or hepatofugal flow of the portal vein branches in the congested region. If the congested region shown by this technique is large, reconstruction of the MHV or its tributaries must be considered.23

3.4.3 Right Hepatic Vein

It should be noted that the RHV is the main drainage vein of the right lateral sector. As mentioned previ-ously, the RHV does not always mark an unequivocal distinction between the paramedian and the lateral sec-tors of the right hemiliver,25 especially in cases with thick IRHV or MRHV. IRHV or MRHV is reported to exist in approximately 60–70% of cases.2,26 and in 20–25% of these cases, they are thick and easily recog-nized by ultrasonography27, and the RHV is relatively less developed.13 Depending on the location of the tumor (e.g., tumor invading the root of three major hepatic veins) and remnant liver volume, an extended left trisectionectomy with resection of the RHV may be an option.28

3.4.4 Inferior and Middle Right Hepatic Vein

In cases with a well-developed inferior right hepatic vein (IRHV) or middle right hepatic vein (MRHV), the main RHV does not descend in the caudal direc-tion but just drains the cranial portion of the right liver (Fig. 3.8).26 In addition, the IRHV or MRHV runs within the dorsal part of S6 or S7, respectively, which is different from the plane in which the ordinal RHV runs.25

MHV

LHV

RHV

a

b

Fig. 3.7 (a) Middle hepatic vein (MHV) tributary draining seg-ments 5 and 6. The region drained by each tributary is calculated by volume rendering software. (b) Area drained by this MHV tributary is shown

MHV

LHVRHV

MRHV

IRHV

Fig. 3.8 Three-dimensional hepatic vein anatomy with well-developed middle right hepatic vein (MRHV) and inferior right hepatic vein (IRHV). Note that the right hepatic vein (RHV) does not descend in the caudal direction and drains only the cra-nial portion of the right liver

213 Portal and Hepatic Vein Anatomy

3.4.5 Caudate Vein Branches (S7/S1 Border)

In cases when resection of the caudate lobe is required, it is usually difficult to recognize the border between the right liver and the caudate lobe. A recent study aim-ing to identify the hepatic veins draining the caudate lobe in cadaveric livers showed that the caudate pro-cess hepatic vein that appears without exception by opening the space between the paracaval portion and IVC runs along the boundary between the regions fed by the portal vein branches of the caudate process and the right liver.29

3.5 Conclusion

Indications for hepatic resection vary with the type of the tumor. For example, hepatocellular carcinoma is usually encapsulated, and therefore tumors compress-ing the major hepatic vein or IVC may easily be dis-sected, whereas adenocarcinoma such as metastatic cancer from the colon and rectum or intrahepatic cholangiocarcinoma may not be dissected from the adjacent hepatic veins, and the vein must usually be sacrificed. In the latter cases, the volume of the would be congested area must be considered. Therefore, not only the portal vein anatomy but also the hepatic vein anatomy should be evaluated.

Furthermore, major hepatic resection is frequently required in cases with tumor invasion into the hepatic hilum such as hilar cholangiocarcinoma or intrahepatic cholangiocarcinoma, or a large tumor compressing major vascular structures. The branches to be pre-served should be evaluated precisely in each case according to the pattern and location of tumor infiltra-tion, in order to determine and carry out safe hepatic resection.

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