Hepatic Variants


Technical Aspects

Diagnostic imaging of the hepatobiliary system, with multidetector computed tomography (MDCT) and magnetic resonance imaging (MRI), plays a major role in hepatobiliary surgery, helping to choose the best therapeutic approach, reduce complications, and identify the anatomy requiring special attention at surgery.

Anatomic variants of the biliary and hepatic vascular anatomy are common; they dictate the surgical technique and also may predict the risk for postsurgical complications, in both the case of complex surgeries, such as liver transplantation, and of more common procedures, such as laparoscopic cholecystectomy.

MDCT and MRI, especially when hepatobiliary contrast agents are used (e.g., gadolinium ethoxybenzyl diethylenetriamine pentaacetic acid [Gd-EOB-DTPA], gadobenate dimeglumine [Gd-BOPTA], mangafodipir trisodium), clearly visualize both biliary and arterial anatomic variants, with a high degree of correlation with intraoperative cholangiography and digital subtraction angiography.

Pros and Cons

Contrast-enhanced MDCT and MRI are noninvasive techniques that permit angiographic and parenchymal evaluation of the liver.

MDCT and MR angiography have shown excellent correlation with catheter angiography but are devoid of its invasiveness and many of its complications. Moreover, the radiation burden is reduced in MDCT when compared with catheter angiography and absent in the case of MRI.

Because of the availability of biliary excreted contrast agents, cholangiography now can be performed in a noninvasive way by both MDCT and MRI. Currently, MDCT cholangiography, owing to the higher spatial resolution, allows better visualization of second-order bile ducts than MR cholangiography.

MDCT, MR angiography, and MR cholangiopancreatography (MRCP) protocols, used at our institution, are summarized in Tables 44-1 to 44-3 . Raw data, obtained from MDCT and MRI are postprocessed to maximize the information they can provide; and multiplanar reformatted, three-dimensional reconstruction, maximum intensity projection (MIP), and volume rendering images are obtained.

TABLE 44-1
Multidetector Computed Tomography Angiography Scanning Protocol
Protocol Hepatic Arterial Phase Venous Phase
Range Entire liver Entire liver
Scan delay 20-25 sec after start of bolus injection 60-65 sec
Empirical bolus tracking Automatically triggered at 125 HU in aorta at the celiac artery level
Pitch 1-1.5 1-1.5
Slice thickness 1-2 mm 2-5 mm
Kilovoltage peak 120-140 120-140
Milliamperes 200-280 200-280
Image reconstruction thickness 1-2 mm and 50% overlap 2-5 mm and 50% overlap
HU, Hounsfield units.

TABLE 44-2
Magnetic Resonance Angiography Protocol
Protocol Hepatic Arterial Phase Venous Phase Delayed Venous Phase
Scan delay 15-18 sec 60 sec 180 sec
TR/TE Minimum/ 15 ms Minimum/ 15 ms Minimum/ 15 ms
Flip angle 100 degrees 100 degrees 100 degrees
Field of view 400 mm 400 mm 400 mm
Effective section 2-4 mm 2-4 mm 2-4 mm
Matrix 160 × 256 160 × 256 160 × 256
TR/TE, Repetition time/echo time.

TABLE 44-3
Magnetic Resonance Cholangiopancreatography Protocol
Protocol T2-Weighted MRCP 3D SPGR
Scan delay None Gd-BOPTA: 60 min
Gd-EOB-DTPA: 20 min
TR/TE 2800-3300/900-1100 ms 6.5/2.1 ms
Flip angle 0 degrees 15 degrees
Field of view 400 mm 400 mm
Effective section 60 mm 2.4 mm
Orientation Coronal oblique Axial and coronal
Matrix 160 × 256 160 × 256
Gd-BOPTA, Gadobenate dimeglumine; Gd-EOB-DTPA , gadolinium ethoxybenzyl diethylenetriamine pentaacetic acid; MRCP, magnetic resonance cholangiopancreatography; 3D SPGR, three-dimensional spoiled gradient recalled echo; TR/TE, repetition time/echo time.

Controversies

Whether MDCT or MRI is the better modality by which to assess hepatic variants remains unclear. Both methods have undergone rapid improvement, and each bears inherent advantages and disadvantages. Currently, the choice of a specific modality over the other is largely dictated by institutional preferences.

Normal Anatomy

Anatomic variants of the biliary, hepatic arterial, hepatic venous, and portal venous anatomy are common. Classic biliary and hepatic arterial anatomy is found in only 58% and 55% of the population, respectively. To understand anatomic variants, a brief description of normal anatomy is provided.

Liver

The liver ( Figure 44-1 ) is a large, wedge-shaped parenchymal organ that occupies most of the right hypochondrium and epigastrium and extends to the left epigastrium with its narrow end. The falciform ligament, the ligamentum teres, and the ligamentum venosum divide the liver into a large right lobe and a smaller left lobe. This anatomic description does not correlate with functional hepatic anatomy and is therefore inadequate for interventional radiology and surgery. Rather, functional anatomy of the liver is based on the vascular and biliary territories ( Figure 44-2 ). Cantlie's line, running on a coronal oblique plane oriented 75 degrees toward the left, from the middle of the gallbladder to the left side of the inferior vena cava (IVC), divides the liver into right and left. Grossly, this yields the right liver and the left liver, which are two separate functional units, with independent vascular inflows and outflows and autonomous biliary drainage. The middle hepatic vein (MHV) lies along the cranial continuation of Cantlie's line. The right hepatic vein (RHV), MHV, and left hepatic vein (LHV) divide the liver into four sectors, each one supplied by an independent portal pedicle. They are the posterolateral and anteromedial sectors in the right liver and the posterior and anterior sectors in the left liver. The posterior sector is undivided and constitutes segment II. The anterior sector is divided by the umbilical fissure into a medial segment (IV) and a lateral segment (III). A transverse plane at the level of the main portal bifurcation divides the posterolateral sector into a posterior (VII) and an anterior segment (VI), the anteromedial sector into an anterior (V), and a posterior segment (VIII) and subdivides segment IV into a posterior (IVa) and an anterior segment (IVb). The caudate lobe constitutes segment I, or the Spigel lobe, which, owing to its autonomous vascularization, is considered separate from the others.

Figure 44-1, Drawing of normal anatomy of the liver. CBD, Common bile duct; CD, cystic duct; CHD, common hepatic duct; HA, hepatic artery; IVC, inferior vena cava; LHA, left hepatic artery; LHD, left hepatic duct; LHV, left hepatic vein; LPV, left portal vein; MHV, middle hepatic vein; PV, portal vein; RHA, right hepatic artery; RHD, right hepatic duct; RHV, right hepatic vein; RPV, right portal vein.

Figure 44-2, Hepatic segmental anatomy. A, Anatomic drawing of the various liver segments. Segmental anatomy of the liver in the coronal ( B1 to B5 ) and axial ( C1 to C4 ) planes from multidetector computed tomography and magnetic resonance imaging, respectively. D, Corresponding color-coded three-dimensional liver segmental reconstruction from a liver donor computed tomography examination.

An ultrasound image of normal liver presents a homogeneous pattern of low-level echoes. Vessels and biliary ducts are anechoic ( Figure 44-3 ).

Figure 44-3, Ultrasound images of normal liver. A and B, Normal hepatic parenchyma presents as a homogeneous pattern of low-level echoes. Vessels and bile ducts appear anechoic. Right portal vein is indicated by the arrow.

On unenhanced MDCT, the liver exhibits homogeneous intermediate attenuation (50 to 75 Hounsfield units), similar to that of the spleen ( Figure 44-4 ). Vessels and biliary ducts are hypodense. During contrast-enhanced imaging, liver attenuation values progressively increase, with peak enhancement occurring during the portal and hepatic venous phases of enhancement. Maximal arterial enhancement occurs during hepatic arterial phase, usually around 30 seconds after the start of contrast injection; the portal and hepatic veins maximally enhance at approximately 70 seconds after contrast injection. Biliary ducts are unopacified, unless contrast media with biliary excretion are administered.

Figure 44-4, Computed tomography (CT) image of normal liver. On unenhanced multidetector CT, the liver exhibits homogeneous intermediate attenuation and appears similar to the spleen.

At MRI, the signal intensity of the normal liver varies by sequence. It presents hyperintense to the spleen on T1-weighted images, hypointense on T2-weighted images, and is always homogeneous ( Figure 44-5 ).

Figure 44-5, Magnetic resonance imaging (MRI) images of normal liver. On MRI, normal hepatic tissue presents hyperintense to the spleen on out-of-phase (A) and in-phase (B) T1-weighted images, is hypointense on T2-weighted images (C), and enhances homogeneously after administration of nonspecific gadolinium-based contrast media. In the arterial dominant phase of contrast enhancement, hepatic veins are unopacified (D), whereas contrast medium is detectable in the liver parenchyma and portal vein (inset). During the portal (E) and late (F) phases of contrast enhancement, both hepatic veins and parenchyma are enhanced.

Normal liver enhances homogeneously and transiently after administration of nonspecific gadolinium (Gd)-based contrast media, such as gadopentetate dimeglumine (Gd-DTPA). In contrast, hepatic-specific contrast agents are taken up by liver cells. Reticuloendothelial system–specific contrast agents, composed of iron microparticles, are taken up by Kupffer cells, with resultant reduction of the signal intensity of the liver on T2*-weighted images. Hepatocyte-specific contrast agents include gadoxetic acid (Gd-EOB-DTPA) and gadobenate dimeglumine (Gd-BOPTA), which are selectively taken up by hepatocytes without being metabolized, with resultant progressive increased signal intensity on delayed T1-weighted images and subsequent excretion into, and enhancement of, the biliary system. Biliary excretion accounts for approximately 50% of the administered Gd-EOB-DTPA dose, compared to 5% for Gd-BOPTA. It should be noted that Gd-EOB-DTPA is administered at one-quarter the recommended clinical dose (0.025 mmol/kg vs. 0.1 mmol/kg) compared to nonspecific gadolinium contrast agents because of its relatively high relaxivity; this low clinical dose of Gd-EOB-DTPA may lead to less conspicuous vascular enhancement during arterial and venous phases, as well as to bolus injection/acquisition timing errors because of the small injection volume.

Hepatic Arterial Anatomy

The classic hepatic arterial anatomy, characterized by the proper hepatic artery dividing into right and left hepatic arteries, is observed in approximately 55% of the population ( Figure 44-6 ). The Michel classification of hepatic arterial variant anatomy is illustrated in Table 44-4 .

Figure 44-6, Normal hepatic arterial anatomy. Axial maximum intensity projection image shows the normal anatomy of the hepatic artery. CHA, Common hepatic artery; LHA, left hepatic artery; RHA, right hepatic artery; SA, splenic artery; Seg. IV HA, segment IV hepatic artery.

TABLE 44-4
Hepatic Arterial Variants According to Michel's Classification
Type Frequency (%) Description
I 55 RHA, MHA, LHA arise from CHA
II 10 RHA, MHA, and LHA from CHA; replaced LHA from LGA
III 11 RHA and MHA from CHA, replaced RHA from SMA
IV 1 Replaced RHA and LHA
V 8 RHA, MHA, LHA arise from CHA; accessory LHA from LGA
VI 7 RHA, MHA, LHA arise from CHA; accessory RHA
VII 1 Accessory RHA and LHA
VIII 4 Replaced RHA and accessory LHA or replaced LHA and accessory RHA
IX 4.5 Entire hepatic trunk from SMA
X 0.5 Entire hepatic trunk from LGA
CHA, Common hepatic artery; LGA, left gastric artery; LHA, left hepatic artery; MHA, middle hepatic artery; RHA, right hepatic artery; SMA, superior mesenteric artery.

Hepatic Venous Anatomy

Classic hepatic venous anatomy is characterized by three main hepatic veins—LHV, MHV, and RHV—draining into the IVC. The LHV drains segments II and III; the MHV drains segments IV, V, and VIII; and the RHV drains segments V, VI, and VII. The MHV and LHV join to form a common trunk in approximately 60% of the population ( Figure 44-7 ).

Figure 44-7, Hepatic vein confluence on multidetector maximum intensity projection coronal image. LHV, Left hepatic vein; MHV, middle hepatic vein; RHV, right hepatic vein.

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