Hepatic Variants

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.

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

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.

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 ).

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 .

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 ).