Liver Anatomy and Cross-Sectional Imaging Techniques: A Practical Approach

Abstract In the past decade or two, there has been a significant change in the epidemiology of liver diseases, such as rise in the incidence of nonalcoholic steatohepatitis, good control of viral load in hepatitis B and C related liver diseases, and revolutionary changes in the treatment of hepatocellular carcinoma. Hence, monitoring of these diseases warrants effective noninvasive imaging techniques. Besides, organ transplantation has evolved to play a major role in the treatment of chronic liver diseases and acute liver failures. With the advent of better technology and new imaging sequences in cross-sectional imaging, there has been a dramatic change in the arena of liver imaging. Knowledge of these imaging modalities and effective application of the existing and new imaging techniques is essential to meet these changing clinical needs. This article aims at revisiting the liver anatomy from a practical stand point and touches upon the key cross-sectional imaging techniques of computed tomography and magnetic resonance imaging with recent advances.


Introduction
Epidemiological changes in the lifestyle over past two decades have led to an increase in the incidence of nonalcoholic fatty liver disease and in turn chronic liver parenchymal disease. Besides, there have been significant changes in the treatment of viral hepatitis C. Similarly, a variety of treatment options are now available for hepatocellular carcinoma (HCC) ranging from targeted drug therapy, immune modulation therapy, locoregional treatment, and radiation therapy. These have led to the development of several noninvasive tools for monitoring the progress of liver parenchymal disease and its response to medical management.
On the other hand, organ transplantation has proved to be a game changer in the treatment of decompensated liver diseases and chronic liver diseases with HCC (within operable criteria). Hence, thorough knowledge of hepatic segmen-tal, vascular, and biliary anatomy is the need of the hour to guide in the process of live donor liver transplantation and surgical resection of liver tumors.
Major technological advances have revolutionized crosssectional imaging, thus providing a new perspective to liver imaging and catering to all these varied causes.
This article aims at the following: • Revisiting the cross-sectional liver segmental, vascular, and biliary anatomy, with practical guide points from the perspective of surgical resection and transplantation. • Use of the right cross-sectional imaging technique for the liver in a unit equipped with a basic multidetector row computed tomography (CT) and 1.5-T magnetic resonance imaging (MRI), so as to get the best results from available resources. It is based on the guidelines laid down by liver imaging reporting and data system (LI-RADS v2018). 1 Keywords ► anatomy ► cross sectional ► imaging ► liver

Abstract
In the past decade or two, there has been a significant change in the epidemiology of liver diseases, such as rise in the incidence of nonalcoholic steatohepatitis, good control of viral load in hepatitis B and C related liver diseases, and revolutionary changes in the treatment of hepatocellular carcinoma. Hence, monitoring of these diseases warrants effective noninvasive imaging techniques. Besides, organ transplantation has evolved to play a major role in the treatment of chronic liver diseases and acute liver failures. With the advent of better technology and new imaging sequences in cross-sectional imaging, there has been a dramatic change in the arena of liver imaging. Knowledge of these imaging modalities and effective application of the existing and new imaging techniques is essential to meet these changing clinical needs. This article aims at revisiting the liver anatomy from a practical stand point and touches upon the key cross-sectional imaging techniques of computed tomography and magnetic resonance imaging with recent advances.
Anatomy of the Liver: from a Clinicoradiological Perspective

Segmental Anatomy of Liver
Understanding of the lobar and segmental classification of the liver is of immense help in the preoperative evaluation and planning in the treatment of benign and malignant liver tumors. Another application of this knowledge is for living donor liver transplantation in adult and pediatric patients, in which the volume of liver graft and remnant liver volume are of utmost and primary importance for donor selection. There are multiple systems used for liver segmentation. The most commonly used and accepted standard is the Couinaud system, wherein the liver is divided into eight segments based on three portal scissura (right, middle, and left hepatic veins). This division is based on the orientation of hepatic veins and the level of portal vein bifurcation. 1 Each segment is an independent functional unit having its own function, inflow, and outflow of blood, and biliary and lymphatic drainage. 2 An imaginary line, namely, Cantlie's line, traverses from the gall bladder fossa to the left of the inferior vena cava (IVC) dividing the liver into right and left hemi-livers. Alternatively, the middle portal scissura also divides the liver into two halves.
For practical purposes, to uniformly define and classify surgical hepatectomies, the Committee of the International Hepato-Pancreato-Biliary Association recommended a new terminology for liver segmentation. 3 As per this committee, the first-order division is the right and left hemi-liver. So the right hemi-liver consists of segments V to VIII and the left hemi-liver consists of segments II to IV. Resection of these hemi-livers is termed right or left hepatectomy.
The second-order division is separation of two hemilivers into the following sections: anterior (segments V and VIII) and posterior (segments VI and VII) sections for the right hemi-liver and medial (segments IVa and IVb) and lateral (segments II and III) sections for the left hemi-liver.
The right hepatic vein divides the right hemi-liver into anterior and posterior sections. Segment V is an anterior segment of the right hemi-liver anterior to the right hepatic vein, below the portal vein bifurcation. Segment VIII is an anterior segment of the right hemi-liver anterior to the right hepatic vein above the portal vein bifurcation. The posterior segment above the portal vein bifurcation and posterior to the right hepatic vein is segment VII and below the portal vein bifurcation is segment VI.
The left medial section is between the middle hepatic vein and the fissure for ligamentum teres. It comprises segments IVa and IVb, above and below the portal vein bifurcation plane, respectively. The left lateral section lies medial to the fissure for the ligamentum teres, with segment II above and segment III below the portal vein bifurcation plane (►Table 1 and ►Fig. 1).
Resection of these sections is sectionectomy. When resection includes the right hemi-liver and adjoining left medial section, it is termed extended right hepatectomy or right trisectionectomy. Similarly, resection of the left lobe and right anterior section is termed extended left hepatectomy or left trisectionectomy. Knowledge of these terminologies is important to provide functional liver remnant on CT, in patients with liver masses.
In the third-order division, the sections are classified into many types. Surgical resection of a segment is termed segmentectomy. When it involves two segments, it is termed bisegmentectomy. An optional second-order division based on the portal vein is sectors. The corresponding term for resection of these sectors is sectorectomy. In the right hemiliver, the anterior and posterior sections are synonymous with the anterior and posterior sectors. In the left hemi-liver, the left medial sectors include segments III and IV and the lateral sector includes only segment II.
The caudate lobe or Couinaud segment I has a unique anatomy and vascular supply/drainage and is not a part of the right or left hemi-liver. The anatomy of this segment is as follows. Anteriorly, it is bounded by the fissure for ligamentum venous, posteriorly by the IVC, and inferiorly by the foramen of Winslow. This segment has an arterial and venous supply usually from the right hepatic artery and the right or both branches of the portal vein, with multiple small veins draining directly into the IVC. 4

Vascular Anatomy of Liver Porta Hepatis
It is the hilum of the liver through which the portal vein, hepatic artery, common hepatic duct (CHD), nerves, and lymphatics enter the liver.

Arterial Anatomy of Liver
Preoperative evaluation of hepatic arterial anatomy and variations is important to prevent arterial injury during laparoscopic or open laparotomies for liver surgery. Besides, any minimally invasive surgery is going to offer a limited view of the patient's anatomy, which makes preoperative imaging essential for identification of anatomy and potential variations. For liver transplantation, the variations in the arterial anatomy define single or multiple arterial anastomoses for the graft liver in the recipient. Approximately 20 to 25% of blood supply to the liver is by the common hepatic artery (CHA). It is commonly seen as a branch of the celiac trunk. The celiac trunk arises from the abdominal aorta at the D12-L1 vertebral level. After few centimeters of its origin, it commonly trifurcates into the CHA, left gastric artery, and splenic artery. The CHA, after giving off the gastroduodenal artery, continues as the hepatic artery proper, which usually lies anterior to the portal vein and medial to the bile duct. It divides into the right and left branches supplying the respective lobes of the liver. Sometimes the middle hepatic artery may also be present supplying segment IV of the liver. Variations in the hepatic artery are seen in 20% of cases. 5 Common variations in the anatomy of the hepatic artery are given by the Michel classification system (►Table 2, ►Fig. 2). Hiatt further modified the Michel classification system and added another variation of secluded origin of the CHA from the abdominal aorta. 6

Portal Venous Anatomy
The portal vein is formed by a confluence of the splenic vein and the superior mesenteric vein, posterior to the neck of the pancreas. It travels cranially and to the right in an oblique fashion to the porta hepatis of the liver, where it divides into two branches: right and left. At the level of the porta, the main portal vein lies posterior to the hepatic artery proper and bile duct. The right portal vein has a horizontal course within the liver. It gives off anterior and posterior branches supplying segments V, VIII, and VI, VII. The left branch of the portal vein has an oblique cranial course in the left hemiliver, supplying segments II to IV. A normal portal venous anatomy is seen in 60 to 80% of the population at CT angiography. 7,8 Multiple variations exist; Cheng classified them into four common types (►Table 3, ►Fig. 3).
Another type is the segment IV branch coming off as a separate branch of the right portal vein.
Practical implication of this anatomy is for planning of liver resections and in pretransplant evaluation of liver donors. When there is crossover of the portal venous supply, that is, segmental portal veins of the right lobe from the left portal vein and the segment IV portal branch from the right  portal vein, the donor liver anatomy is unacceptable for donation.

Hepatic Venous Anatomy
Normally, there are three hepatic veins draining into the IVC. The left hepatic vein lies in the left portal scissura and drains segments II and III. The middle hepatic vein lies in the middle portal scissura and drains segment IV, most of the times. It also receives the branches from segments V and VIII. 9 The right hepatic vein is the largest and drains segments V to VIII most times. Single or multiple separate venous channels drain the caudate lobe, directly into the IVC. The middle and left hepatic veins join to form a common trunk in 60 to 95% of cases. [10][11][12] Hepatic venous variations are more common in women than in men. 13 A common variant is the presence of the accessory right inferior hepatic vein, which drains into the middle of the retrohepatic IVC. 10 Multiple individual hepatic veins draining separately into the IVC are also seen as a variation (►Fig. 4).
The presence of the right inferior hepatic vein and separate veins draining into the IVC implies an increase in the number of venous anastomoses in liver transplant recipients. If the insertion of the accessory vein is close to the right hepatic-IVC confluence, then a common venous channel can be created and single anastomoses with the recipient IVC can be made. Besides accessory hepatic veins with a size of 4 mm or more have to be anastomosed with the recipient IVC to    avoid venous congestion in the graft. 10 Thus, information of the size and number of accessory or separate hepatic veins is of practical importance to the transplant surgeon.

Biliary Anatomy
Bile formed from hepatocytes is drained by the biliary system into the duodenum. The bile ducts are classified into extra-and intrahepatic ducts. Multiple biliary canaliculi from a particular segment join to form segmental bile ducts, which join to form sectional ducts. 14 So the right anterior sectional duct drains segments V and VIII, and the right posterior sectional duct drains segments VI and VII. The left medial sectional duct drains segment IV, and the left lateral sectional duct drains segments II and III. Normally, the right anterior and posterior ducts join to form the right hepatic duct and the left medial and lateral ducts join to form the left hepatic duct. Both the right and left hepatic ducts join to form a CHD. The cystic duct joins the CHD to form the common bile duct (CBD), which eventually drains into the second part of the duodenum. The cystic duct, CHD, and CBD are extrahepatic ducts.
Practical application of the knowledge of the biliary anatomy is essential for radiologists. Preoperative evaluation of the biliary anatomy prior to laparoscopy and laparotomy for treatment of benign and malignant lesions of the hepatobiliary system is essential. Variations of the biliary system are common occurrence and if not identified on imaging may result in injury with bile leak and peritonitis during liver resection surgeries and in living donor liver transplantations. These may also result in unexpected hepaticojejunal anastomoses in recipients.
The Huang classification of the biliary anatomy is a commonly used system. 15 It is described in ►Table 4 and ►Fig. 5.
Similarly, based on insertion of the segment IV duct onto the left hepatic duct, the right hepatic or sectoral duct, cystic duct, and CHD, variations of the left biliary anatomy have also been described by Huang et al. 15 These variations are classified into B1 to B4 depending on their insertion onto the left hepatic duct, biliary confluence, right hepatic duct, and CHD.
Cystic duct insertion variations are also common, few of the common ones being the following: • Low insertion into the distal CBD.
• Parallel course of the cystic duct with CHD.

Cross Sectional Imaging of Liver Multiphasic Computed Tomography
A good imaging technique is essential in evaluating liver pathology. This is particularly true in a cirrhotic liver, wherein fibrotic, inflammatory, and regenerative changes can alter the hepatic morphology and parenchymal perfusion/outflow, thus reducing conspicuity of the regenerative and dysplastic nodules and, more so, malignant liver masses. This dictum holds true even for evaluation of the anatomy and variants in a prospective liver donor.
It is recommended to include postcontrast late arterial, portal venous, equilibrium phases with unenhanced phase, being optional for complete evaluation of the liver. According to the Liver Imaging Reporting and Data System (LI-RADS), arterial phase hyperenhancement is an important major criterion in the characterization of HCC, without which a lesion cannot be characterized as LI-RADS 5. Lesion de-enhancement on venous phases and enhancing capsule/pseudocapsule also form a part of the major criteria of LI-RADS.
Although the liver CT protocol assessment criteria are not provided by the OPTN or LIRADS guidelines, an important benchmark recommended in the literature is that the liver should be enhanced by a minimum of 50 HU in the portal venous phase. To image the liver in these various blood pool phases, rapid intravenous contrast injection of 4 to 5 mL/s and weight-based contrast dose of 1.5 mL/kg body weight is thus recommended. These form the basis of multiphasic CT imaging of the liver. 1,16,17 The protocol followed by our institution, in a suspected or known chronic liver disease, is based on the above guidelines and is as given in ►Table 5.
The postcontrast phases are obtained at the following time intervals after the start of contrast injection, as described in ►Table 6.

Table 4 Huang's classification of variations in right-sided biliary anatomy
Huang type

A1
The right posterior duct drains into the right anterior hepatic ducts and joins the left hepatic duct to form the common hepatic duct (CHD)

A2
The right posterior hepatic duct, right anterior hepatic ducts, and left hepatic duct join each other in a trifurcation and form the CHD

A3
The right posterior hepatic duct drains into the left hepatic duct and then joins the right anterior hepatic duct to form the CHD

A4
The right anterior hepatic duct drains into the left hepatic duct and then joins the right posterior hepatic duct to form the CHD (right posterior duct drains into the CHD)

A5
The right anterior hepatic duct drains into the left hepatic duct and forms the CHD. The right posterior hepatic duct drains into the cystic duct

Importance of Dual-Energy CT
Dual-energy CT (DECT) is based on CT data acquisition at two different energy spectra. It is meant to overcome the shortcoming of contrast-enhanced CT to rely purely on Hounsfield units. 18 Every vendor has adopted a different method of applying this technique for clinical use. These are either prospective or retrospective. Prospective techniques include rotate-rotate technique (Toshiba), dual source and twin beam techniques (Siemens), and rapid kilovoltage peak (kVp) switching (General Electrical). Retrospective techniques include sandwich detector (Philips) and photon counter (Siemens). These are depicted in ►Fig. 6. Rapid kVp switching technique (by General Electrical company) utilizes a single X-ray tube with fast kVp switching with energy separation at the detector level (gemstone scintillator detector). By the virtue of this technique, material decomposition can be done based on the energy-dependent   attenuation profile of specific materials. 19 Post acquisition of the late arterial phase in gemstone scintillator detector gemstone spectral imaging (GSI) mode, two to three simple mouse clicks can give subtracted iodine-only images, which improve the visualization of arterial enhancing lesions in a background nodular liver. Another prospective advantage of this technique has been reported in posttreatment analysis of HCC, when treated with molecular targeted drug therapy, which reduces the angiogenesis within the tumor, while not causing a significant change in the size of the lesion. Quantification of tissue iodine on the iodine density images can serve as an image biomarker of tumor viability in these cases. 20,21 Multiphasic MR Imaging of the Liver Over the past decades, a higher magnetic field strength and multichannel coil arrays have enabled faster MRI scanning and improved image quality. Besides, there is development of newer sequences such as MRI-PDFF (magnetic resonance imaging-proton density fat fraction) with multi-echo chemical-shift-encoded sequence for fat quantification, 22 multiecho gradient-echo sequence for quantification of iron overload, and MR elastography (MRE) for evaluation of liver fibrosis and cirrhosis. 23 This has led to recognition of MRI as a robust and comprehensive clinical tool in liver imaging.
The protocol for multiphasic MRI of the liver (followed at our institute) is as described in ►Table 7.
In addition, for MRE, a modified 2D phase-contrast gradient recalled echo (GRE) pulse sequence is used to obtain four axial images from the largest transverse dimension of the liver.
An important difference between 1.5-and 3-T MRI is the capacity of acquiring high-quality thin-section postgadolinium T1-weighted 3D gradient-echo sequences on 3 T, most clinically relevant for the detection and characterization of small hypervascular malignant diseases.

Utility of Hepatobiliary Contrast Agents
Hepatobiliary contrast agents carry the dual properties of acting like blood pool gadolinium-based contrast agents during the first pass and early phases of contrast enhancement, followed by the characteristic delayed or hepatobiliary enhancement (►Fig. 7). In the initial phases after injection, their contrast enhancement is dominated by the distribution in vessels and the extracellular space, like in conventional gadolinium-based contrast agents. However, unlike other contrast agents, these contrast agents are then taken up by functional hepatocytes. As a result, there is a persistent enhancement in hepatocytes that allows for identifying lesions of hepatocellular origin.
In addition, biliary excretion of this contrast allows for both identifying lesions that contain biliary canaliculi and enhancing the biliary excretion system. 24 Lesion de-enhancement described in the hepatobiliary phase of MRI is one of the ancillary features, favoring malignancy in LI-RADS documentation. 2 Gadobenate dimeglumine and gadoxetate disodium are the commercially available hepatobiliary contrast agents. Whereas 5% of gadobenate dimeglumine is taken up by the liver, gadoxetate disodium is more hepatospecific and $50% of this contrast is excreted through the hepatobiliary system. However, the latter is available only in Europe (commercially known as Primovist) and in United States (commercially known as Eovist). Delayed hepatobiliary phase with gadoxetate is obtained 20 minutes postcontrast injection, whereas with gadobenate, the hepatobiliary phase is obtained at 45 to 60 minutes.
Several studies have shown that MRI with a hepatobiliary contrast material is more sensitive than MRI with an extracellular contrast material in the detection of HCC, probably because MRI with a hepatobiliary contrast material displays "hepatobiliary phase (HBP) hypointense nodules without arterial phase hyperenhancement (APHE)," which often progress to HCC. 25,26

Magnetic Resonance Elastography
This MRI technique quantitatively evaluates tissue stiffness by propagating low-frequency mechanical waves through the liver. It is performed using an MRI safe passive driver that is applied to the right upper abdomen and lower chest overlying the right lobe of the liver while the patient is being scanned in the MRI scanner. The MRE sequence is performed with four short breath holds and completed within 1 to 2 minutes, without the need for intravenous contrast injection. 27 An active driver generates low-frequency mechanical waves (typically at 60 Hz), which are conducted to the passive driver through a long plastic tube. The passive driver vibrates and produces shear waves that are propagated across the liver. The wavelength of the propagating shear wave is directly proportional to the stiffness of the liver, that is, the stiffer the liver, the longer the wavelength.
By applying an inversion algorithm to the raw data, elastograms or stiffness maps that depict tissue stiffness are generated. Elastograms may be displayed in grayscale or color scale. The result is obtained by placing the region of interest (ROI) on the processed images (►Fig. 8). It is measured in kilopascals (kPa). Hence, this technique requires an additional investment in elastography-specific hardware  and software, with the MRI scanner, limiting its availability to centers dedicated to hepatobiliary surgery and liver transplantation.

MRI Quantification of Fat Content of Liver
MR is sensitive to signal from protons in mobile water and triglycerides. The difference in resonant frequency of these different molecules forms the basis of chemical shift imaging, and can quantify the differential amounts of fat and water in tissues. This technique is the basic principle for PDFF technique.
PDFF is defined as the ratio of the density of mobile protons from triglycerides and the total density of protons from mobile triglycerides and mobile water. It is expressed as an absolute percentage and ranges from 0 to 100%. This technique is well correlated with a histological assessment of hepatic steatosis, which is expressed as the percentage of cells containing intracellular droplets of fat. 28 It is a single breath hold technique ($15-20 seconds) and results are in the form of images, in which the ROI can be placed on the liver and the value thus obtained equates fat content in percentage (as shown in ►Fig. 9). MR Quantification of Iron Content of the Liver T2 represents the time constant of the intrinsic decay of the transverse magnetization from spin-spin interactions, while T2 Ã also incorporates effects from local magnetic field inhomogeneities. The superparamagnetic effect of ferritin and hemosiderin creates a local susceptibility-induced contribution to the local magnetic field that results in faster decay of the transverse magnetization. This decay and hence iron content can be detected and quantified using GRE sequences due to their increased sensitivity to magnetic susceptibility. Recent techniques for iron quantification in clinical practice are with the use of single breath hold multi-echo gradient sequence, whereby the R2 Ã values of the liver and in turn T2 Ã (1/R2 Ã ) values can be obtained (►Fig. 10).
In multiple studies, R2 Ã has been demonstrated to have a linear correlation with biopsy-determined liver iron content, which makes R2 Ã relaxometry a reliable technique for noninvasive liver iron overload quantification. 29

Conclusion
Thorough knowledge of liver anatomy and optimal crosssectional imaging techniques is essential for proper interpretation of liver pathologies and to provide effective guidance in patient treatment and care. Liver Anatomy and Cross-Sectional Imaging Techniques Garde, Bhute 99