[Segmental anatomy of the liver in computed tomography: do we localize the lesion accurately?]

Patient specific anatomy: the new area of anatomy based on computer science illustrated on liver

BACKGROUND

Over the past century, medical imaging has brought a new revolution: internal anatomy of a patient could be seen without any invasive technique. This revolution has highlighted the two main limits of current anatomy: the anatomical description is physician dependent, and the average anatomy is more and more frequently insufficient to describe anatomical variations. These drawbacks can sometimes be so important that they create mistakes but they can be overcome through the use of 3D patient-specific surgical anatomy.

METHODS

In this article, we propose to illustrate such improvement of standard anatomy on liver. We first propose a general scheme allowing to easily compare the four main liver anatomical descriptions by Takasaki, Goldsmith and Woodburne, Bismuth and Couinaud. From this general scheme we propose four rules to apply in order to correct these initial anatomical definitions. Application of these rules allows to correct usual vascular topological mistakes of standard anatomy. We finally validate such correction on a database of 20 clinical cases compared to the 111 clinical cases of a Couinaud article.

RESULTS

Out of the 20 images of the database, we note a revealing difference in 14 cases (70%) on at least one important branch of the portal network. Only six cases (30%) do not present a revealing difference between both labellings. We also show that the right portal fissure location on our 20 cases defined between segment V and VI of our anatomical definition is well correlated with the real position described by Couinaud on 111 cases, knowing that the theoretical position was only found in 46 cases out of 111, i.e., 41.44% of cases with the non-corrected Couinaud definition.

CONCLUSIONS

We have proposed a new anatomical segmentation of the liver based on four main rules to apply in order to correct topological errors of the four main standard segmentations. Our validation clearly illustrates that this new definition corrects the large amount of mistakes created by the current standard definitions, increased by physician interpretation that can vary from one case to another.

Liver segmentation: practical tips

The liver segmentation system, described by Couinaud, is based on the identification of the three hepatic veins and the plane passing by the portal vein bifurcation. Nowadays, Couinaud's description is the most widely used classification since it is better suited for surgery and more accurate for the localisation and monitoring of intra-parenchymal lesions. Knowledge of the anatomy of the portal and venous system is therefore essential, as is knowledge of the variants resulting from changes occurring during the embryological development of the vitelline and umbilical veins. In this paper, the authors propose a straightforward systematisation of the liver in six steps using several additional anatomical points of reference. These points of reference are simple and quickly identifiable in any radiological examination with section imaging, in order to avoid any mistakes in daily practice. In fact, accurate description impacts on many diagnostic and therapeutic applications in interventional radiology and surgery. This description will allow better preparation for biopsy, portal vein embolisation, transjugular intrahepatic portosystemic shunt, tumour resection or partial hepatectomy for transplantation. Such advance planning will reduce intra- and postoperative difficulties and complications.

3D imaging-guided individualized hepatic vein typing: Application to liver tumor resection

AIM: To conduct 3D imaging-guided individualized typing of hepatic veins to provide a basis for digitalized liver segment resection.

METHODS: Fifty hospitalized liver cancer patients who underwent 64-row spiral CT preoperatively were included. 3D reconstruction of the liver was performed to observe the variation of reconstructed hepatic veins and typing them.

RESULTS: Hepatic veins were overall classified into two types: A (27/50, 54%) and B (23/50, 46%). The right hepatic vein were classified into four types: A: (36/50, 72%), B (10/50, 20%), C (2/50, 4%), and D (2/50, 4%). The right hepatic vein was classified into four subtypes: I (17/50, 34%), II (28/50, 56%), III (3/50, 6%), and IV (4/50, 8%). The middle hepatic vein was classified into three types: A (39/50, 78%), B (10/50, 20%), and C (1/50, 2%). The left hepatic vein was classified into two types: A (41/50, 82%) and B (9/50, 18%).

CONCLUSION: 3D imaging-guided individualized hepatic vein typing can help retain more normal liver tissue and reduce the occurrence of postoperative complications in liver surgery.

Concepts for Liver Segment Classification: Neither Old Ones nor New Ones, but a Comprehensive One

Concepts dealing with the subdivision of the human liver into independent vascular and biliary territories are applied routinely in radiological, surgical, and gastroenterological practice. Despite Couinaud's widely used eight-segments scheme, opinions on the issue differ considerably between authors. The aim of this article is to illustrate the scientific basis for understanding and harmonizing inconsistencies between seemingly contradictory observations. Possible clinical implications are addressed.

Liver segments: an anatomical rationale for explaining inconsistencies with Couinaud's eight-segment concept

BACKGROUND AND PURPOSE

An increasing number of surgical and radiological observations call Couinaud's concept of eight liver segments into question and such inconsistencies are commonly explained with anatomical variations. This paper was intended to demonstrate that, beyond variability, another anatomical principle may allow to understand supposedly differing concepts on liver segmentation.

MATERIALS AND METHODS

The study was performed on 25 portal vein casts scanned by helical CT. The branches of the right and left portal vein and their corresponding territories were determined both anatomically and mathematically (MEVIS LiverAnalyzer, MEVISLab).

RESULTS

The number of branches coming-off the right and left portal vein was never 8, but many more (mean number 20, range 9-44). Different combinations of these branches and their respective territories, carried out in this study, yielded larger entities and supposedly contradictory subdivisions (including Couinaud's eight segments), without calling upon anatomical variability.

CONCLUSIONS

We suggest the human liver to be considered as corresponding to 1 portal venous territory at the level of the portal vein, to 2 territories at the level of the right and left branch of the portal vein, and to 20 at the level of the rami of the right and left branch. This "1-2-20-concept" is a rationale for reconciling apparent discrepancies with the eight-segment concept. On a pragmatic level, in cases in which imaging or surgical observations do not fit with Couinaud's scheme, we propose clinicians not to autonomically conclude to the presence of an anatomical variation, but to become aware of the presence of an average of 20 (and not 8) second-order portal venous territories within the human liver.

Reclassification of segmental anatomy of liver

Radiologists have become increasingly interested in segmental anatomy of human liver, especially in view of the need for an accurate preoperative localization of focal hepatic lesions and preoperative appraisal of living donor liver transplantation. The method proposed by Couinaud in 1964 to divide the liver into 8 surgically relevant segments longitudinally along the hepatic veins and transversely through the right and left portal pedicles, has been widely accepted. Although this method may be used in some patients, from a morphologic point of view, it is questionable because of the variation in segmental anatomy of human liver. In this review, reclassification of segmental anatomy of liver is described.

[Anatomy of the liver: what you need to know]

A precise knowledge of arterial, portal, hepatic and biliary anatomical variations is mandatory when a liver intervention is planned. However, only certain variations must be searched when a precise intervention is planned. The basic liver anatomy as well as the most relevant malformations will be precised.

Limitations and pitfalls of Couinaud's segmentation of the liver in transaxial Imaging

The 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. In this review article first an overview of the different classical concepts for delineating segmental and subsegmental anatomy on US, transaxial CT, and MR images is given. Essentially, these procedures are based on Couinaud's 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. In a second part, the limitations of these methods are delineated and discussed with the conclusion that if exact preoperative localization of hepatic lesions is needed, tumor must be located relative to the avascular planes between the different portal territories.