Segmental anatomy of the liver under the right diaphragmatic dome: evaluation with axial CT

Human liver territories: Think beyond the 8-segments scheme

Worldwide, compartmentalization of the human liver into portal venous territories today follows the eight-segments scheme credited to Couinaud. However, there are increasing reports of anatomical, radiological and surgical observations that contradict this concept. This paper presents a viewpoint that enhances understanding of these inconsistencies and can serve as a basis for customized liver interventions. Clin. Anat. 30:974-977, 2017. © 2017 Wiley Periodicals, Inc.

A study of the right intersectional plane (right portal scissura) of the liver based on virtual left hepatic trisectionectomy

BACKGROUND

Left hepatic trisectionectomy is a challenging procedure. For an anatomically correct resection, it is necessary to have understanding of the right intersectional plane; however, little is known on this issue. The purpose of this study was to investigate the 3D anatomy of the right intersectional plane and to enable safe and precise left trisectionectomy.

METHODS

A virtual left trisectionectomy was performed using 3D-processing software, in patients who underwent computed tomography. The reconstructed images were reviewed, and attention was paid to the extent of the right hepatic vein (RHV) exposure on the transected plane and the type of the inferior right hepatic vein (IRHV).

RESULTS

Of the 200 study patients, 109 (54.5 %) patients showed complete exposure of the RHV on the transected plane, whereas the remaining 91 exhibited partial exposure. In the 109 patients with complete exposure, 58 (53.2 %) patients had no IRHV and the remaining 51 had a small IRHV. None of the patients had a large IRHV. In contrast, of the 91 patients with partial exposure, only 10 (11.0 %) patients had no IRHV, 35 (38.5 %) had a small IRHV, and 46 (50.5 %) patients had a large IRHV. The incidence of IRHV types was significantly different between the two groups (P < 0.001).

CONCLUSIONS

The RHV does not always run along the right intersectional plane, i.e., the vein is not always fully exposed on the transected plane even after anatomically correct left trisectionectomy. The extent of the RHV exposure is closely related to the type of the IRHV.

Three-dimensional volumetry in 107 normal livers reveals clinically relevant inter-segment variation in size

BACKGROUND

The anatomic resection of Couinaud's segments is one of the key techniques in liver surgery. However, the territories and volumes of the eight segments are not adequately assessed based on portal branching.

METHODS

Three-dimensional (3D) perfusion-based volumetry was performed in 107 normal livers. Based on Couinaud classification, the portal branches were identified and the volumes of each segment were calculated. The relationships between branching patterns of the portal veins and segmental volumes were assessed.

RESULTS

In descending order of volume, median volumes of segments VIII, VII, IV, V, III, VI, II and I were recorded. Segment VIII was the largest, accounting for a median of 26.1% (range: 11.1-38.0%) of total liver volume (TLV), whereas segments II and III each represented <10% of TLV. In 69.2% of subjects, the portal branches of segment V diverged from the trunk of the branches of segment VIII. No relationship was found between branching type and segment volume.

CONCLUSIONS

The territories and volumes of Couinaud's segments vary among segments, as well as among individuals. Detailed 3D volumetry is useful for preoperative evaluations of the dissection line and of future liver remnant volume in anatomic segmentectomy.

Anatomy of the liver: an outline with three levels of complexity--a further step towards tailored territorial liver resections

The vascular anatomy of the liver can be described at three different levels of complexity according to the use that the description has to serve. The first--conventional--level corresponds to the traditional 8-segments scheme of Couinaud and serves as a common language between clinicians from different specialties to describe the location of focal hepatic lesions. The second--surgical--level, to be applied to anatomical liver resections and transplantations, takes into account the real branching of the major portal pedicles and of the hepatic veins. Radiological and surgical techniques exist nowadays to make full use of this anatomy, but this requires accepting that the Couinaud scheme is a simplification, and looking at the vascular architecture with an unprejudiced eye. The third--academic--level of complexity concerns the anatomist, and the need to offer a systematization that resolves the apparent contradictions between anatomical literature, radiological imaging, and surgical practice. Based on the real number of second-order portal branches that, although variable averages 20, we submit a system called the "1-2-20 concept", and suggest that it fits best the number of actual--as opposed to idealized--anatomical liver segments.

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.

Left inferior phrenic artery feeding hepatocellular carcinoma: angiographic anatomy using C-arm CT

OBJECTIVE

The left inferior phrenic artery (LIPA) is one of the common extrahepatic collateral arteries that supply hepatocellular carcinomas (HCCs). The purpose of this study is to describe the anatomy of the LIPA that supplies HCCs using C-arm CT in 23 patients.

CONCLUSION

The anteromedial limb of the ascending branch was present in 14 patients and accessory gastric branches were noted in 11 patients. The use of angiography and C-arm CT of the LIPA showed 26 tumor feeders in 23 patients. The feeders were seen in the anteromedial limb (n = 12), lateral limb (n = 9), anterior limb (n = 3), and descending branch (n = 2). The anteromedial limb of the ascending branch is a common tumor feeder of the LIPA and can supply HCCs located in the right liver dome. Gastric staining is also frequently depicted on LIPA angiography and should not be confused with tumor staining.

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.

A new and simple practical plane dividing hepatic segment 2 and 3 of the liver: evaluation of its validity

Objective

The conventional method of dividing hepatic segment 2 (S2) and 3 (S3) is subjective and CT interpretation is unclear. The purpose of our study was to test the validity of our hypothesis that the actual plane dividing S2 and S3 is a vertical plane of equal distance from the S2 and S3 portal veins in clinical situations.

Materials and Methods

We prospectively performed thin-section iodized-oil CT immediately after segmental chemoembolization of S2 or S3 in 27 consecutive patients and measured the angle of intersegmental plane on sagittal multiplanar reformation (MPR) images to verify its vertical nature. Our hypothetical plane dividing S2 and S3 is vertical and equidistant from the S2 and S3 portal veins (vertical method). To clinically validate this, we retrospectively collected 102 patients with small solitary hepatocellular carcinomas (HCC) on S2 or S3 the segmental location of which was confirmed angiographically. Two reviewers predicted the segmental location of each tumor at CT using the vertical method independently in blind trials. The agreement between CT interpretation and angiographic results was analyzed with Kappa values. We also compared the vertical method with the horizontal one.

Results

In MPR images, the average angle of the intersegmental plane was slanted 15 degrees anteriorly from the vertical plane. In predicting the segmental location of small HCC with the vertical method, the Kappa value between CT interpretation and angiographic result was 0.838 for reviewer 1 and 0.756 for reviewer 2. Inter-observer agreement was 0.918. The vertical method was superior to the horizontal method for localization of HCC in the left lobe (p < 0.0001 for reviewers 1 and 2).

Conclusion

The proposed vertical plane equidistant from S2 and S3 portal vein is simple to use and useful for dividing S2 and S3 of the liver.

Reappraisal of Right Portal Segmental Ramification Based on 3-dimensional Volume Rendering of Computed Tomography During Arterial Portography

OBJECTIVE

To investigate and describe the segmental ramification patterns of the right portal vein (RPV) according to the Couinaud system.

MATERIALS AND METHODS

Between February 2004 and June 2005, 127 patients with hepatic tumors underwent computed tomography during arterial portography with a 16-slice multidetector computed tomography. The final analysis included 90 patients without RPV thrombosis or obvious vascular distortion. The ramification patterns of RPV were verified by 3-dimensional portograms using volume-rendering technique.

RESULTS

Seventy-five patients (83.3%) had bifurcation of the main portal vein, 12 (13.3%) had trifurcation, and 3 (3.3%) had the right posterior portal vein (RPPV) arising from main portal vein. A total of 5 segmental types and 3 subsegmental subgroups of RPV ramification patterns were clarified: type I, the classic ramification pattern with right anterior portal vein (RAPV) branching to S8/S5 and RPPV branching to S7/S6 (63; 70%); II, two separate segmental branches to S7 and S6 without a definite main stem of RPPV (18; 20%); III, "whisk-like" ramification pattern of RPV (2; 2.2%); IV, RAPV branching to S8 alone and RPPV to S5, S6, and S7, consecutively (5; 5.6%); and V, RPV first branching to S8/S5 and then to S7/S6 after a common path (2; 2.2%); subgroup a with dorsocranially directed branches arising from P8 and supplying S8 posterior to the right hepatic vein (28; 31.1%); subgroup b with RPPV branching to the dorsal part of S5 (11; 12.2%); and subgroup a + b, combination of the aforementioned 2 subgroups (45; 50%). In most patients, RAPV had dorsocranially directed branches posterior to the right hepatic vein (73; 81.1%), and RPPV gave off branches to the dorsal part of S5 (56; 62.2%).

CONCLUSIONS

Recognition of these ramification patterns could be helpful for more accurate anatomical resection of right hemiliver and preoperative planning, although some variants are present.

Liver segmentation in living liver transplant donors: comparison of semiautomatic and manual methods

PURPOSE

To compare the accuracy and repeatability of a semiautomatic segmentation algorithm with those of manual segmentation for determining liver volume in living liver transplant donors at magnetic resonance (MR) imaging.

MATERIALS AND METHODS

The institutional review board approved this retrospective study and waived the requirement for informed consent. The semiautomatic segmentation algorithm is based on geometric deformable models and the level-set technique. It entails (a) placing initialization circle(s) on each image section, (b) running the algorithm, (c) inspecting and possibly manually modifying the contours obtained with the segmentation algorithm, and (d) placing lines to separate the liver segments. For 18 living donors (eight men and 10 women; mean age, 34 years; age range, 25-46 years), two observers each performed two semiautomatic and two manual segmentations on contrast material-enhanced T1-weighted MR images. Each measurement was timed. Actual graft weight was measured during surgery. The time needed for manual and that needed for semiautomatic segmentation were compared. Accuracy and repeatability were evaluated with the Bland-Altman method.

RESULTS

Mean interaction time was reduced from 25 minutes with manual segmentation to 5 minutes with semiautomatic segmentation. The mean total time for the semiautomatic process was 7 minutes 20 seconds. Differences between the actual volume and the estimated volume ranged from -223 to +123 mL for manual segmentation and from -214 to +86 mL for semiautomatic segmentation. The 95% limits of agreement for the ratio of actual graft volume to estimated graft volume were 0.686 and 1.601 for semiautomatic segmentation and 0.651 and 1.957 for manual segmentation. Semiautomatic segmentation improved estimation in 15 of 18 cases. Inter- and intraobserver repeatability was higher with semiautomatic segmentation.

CONCLUSION

Use of the semiautomatic segmentation algorithm substantially reduces the time needed for volumetric measurement of liver segments while improving both accuracy and repeatability.

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.

Hepatic MR imaging with a dynamic contrast-enhanced isotropic volumetric interpolated breath-hold examination: feasibility, reproducibility, and technical quality

PURPOSE

To evaluate the feasibility, reproducibility, and technical quality of a dynamic contrast material-enhanced isotropic three-dimensional (3D) volumetric interpolated breath-hold hepatic magnetic resonance (MR) imaging examination.

MATERIALS AND METHODS

Fifty patients underwent 3D spoiled gradient-echo imaging (4.2/1.8 [repetition time msec/echo time msec]; flip angle, 12 degrees; interpolation in three directions; intermittent fat saturation; pixel size </= 2.5 mm in all dimensions) before and dynamically after administration of gadopentetate dimeglumine, with the first enhanced acquisition timed for hepatic arterial dominance by using a test bolus of contrast material. Qualitative and quantitative measures of image quality were determined. Patterns of arterial and venous anatomy were assessed. Ten patients (20%) underwent repeat imaging within 6 months, and reproducibility was evaluated.

RESULTS

Hepatic contrast-to-noise ratios for nonenhanced and arterial, portal venous, and equilibrium phase studies averaged 13.0 +/- 12.6 (SD), 17.4 +/- 11.8, 30.4 +/- 16.2, and 28.6 +/- 21.1, respectively. During arterial phase, the liver enhanced a mean of 29% of the maximal enhancement as measured during portal venous phase. Hepatic vascular anatomic variants were comparable in distribution to those cited in published articles. Repeat studies were not significantly different in image quality when compared with original studies.

CONCLUSION

High-quality arterial phase 3D volumetric interpolated breath-hold images can be obtained reliably and reproducibly when timed by using a test dose of contrast material.

Anatomy of the right anterosuperior area (segment 8) of the liver: evaluation with helical CT during arterial portography

PURPOSE

To evaluate the segmental anatomy of the right anterosuperior area (segment 8) of the liver by using helical computed tomography during arterial portography (CTAP).

MATERIALS AND METHODS

Twenty-seven patients without lesions at segment 8 underwent helical CTAP. Three-dimensional portograms were reconstructed to verify the course of the portal veins. The number of subsegmental branches, in addition to the branching point and the distribution in segment 8, was assessed.

RESULTS

In 25 (93%) patients, the dorsal branch of segment 8 gave rise to dorsally directed branches posterior to the right hepatic vein. In only four (25%) of 16 patients in whom the medial branch of segment 8 arose near the porta hepatis, the long paracaval portal branch of the caudate lobe extended upward above the interval between the middle and right hepatic veins.

CONCLUSION

In most of the patients, the dorsal branches of segment 8 supplied the dorsocranial area of the right lobe posterior to the right hepatic vein. The paracaval portion of the caudate lobe was limited to below the interval between the middle and right hepatic veins in the majority of patients who showed medial branches of segment 8 arising near the porta hepatis. Recognition of this vascular anatomy is clinically important for preoperative evaluation of hepatic tumors in segment 8 because it may contribute to a safer surgical approach.

The watershed between right and left hepatic artery territories: findings on CT scans after transcatheter oily chemoembolization of hepatic tumors. A preliminary report

The goals of this study were to delineate the boundary between the right and left hepatic artery territories by computed tomography (CT), to compare this boundary with the middle scissura of Couinaud's segmental anatomy, and to discuss the clinical implications of these findings. The 18 patients who underwent transcatheter oily chemoembolization (TOCE) of liver tumors via the right or left hepatic artery, were examined with an immediate postembolization CT scan. We measured the orientation of the watershed line between the right and left hepatic artery territories and the orientation of the middle scissura on other available sectional imaging modalities, and then compared the middle scissura with the arterial watershed line. A part of segment IV was fed by the right hepatic artery in two of 18 (11.1%) patients. Moreover, one of 4 segment IV lesions was embolized via right hepatic artery infusion. Thus, in 11.1% of cases there was no coincidence between the arterial watershed line and the middle scissura. Some segment IV lesions may be fed and therefore embolized only via right hepatic artery infusion in TOCE for liver tumor.