What is the normal appearance of the liver on CT and MRI?
The liver is the largest organ in the abdomen and has attenuation of 50 to 70 Hounsfield units (HU) on CT, which on average is about 10 HU greater than that of the spleen. On MRI, the liver has high T1-weighted signal intensity relative to spleen secondary to abundant intracellular protein and paramagnetic substances ( Figure 28-1, A ), whereas the liver has low T2-weighted signal intensity relative to spleen ( Figure 28-1, B ). The normal contour of the liver is smooth, and the liver parenchyma normally homogeneously enhances.
Following the intravenous administration of contrast material, sequential phases of enhancement of the liver are seen as follows:
- • The early arterial (or hepatic arterial) phase of enhancement occurs ≈10 to 20 seconds after contrast administration, when arteries will enhance but portal and hepatic veins do not enhance. This phase is useful for delineation of arterial anatomy prior to therapeutic intervention.
- • The late arterial (or portal inflow) phase of enhancement occurs ≈25 to 35 seconds after contrast administration, when arteries enhance and portal veins begin to enhance, but hepatic veins do not enhance. In this phase, the spleen has heterogeneous enhancement, the kidneys are usually in the corticomedullary phase of enhancement, and hypervascular lesions are seen to best effect ( Figure 28-1, C ).
- • The hepatic venous (or portal venous or parenchymal) phase of enhancement occurs ≈50 to 90 seconds after contrast administration, when hepatic veins enhance. In this phase, the spleen has homogeneous enhancement, the kidneys may be in the corticomedullary or nephrographic phase of enhancement, the liver parenchyma homogeneously enhances, hypovascular lesions are seen to best effect, and hypervascular lesions may have washout of enhancement relative to liver parenchyma ( Figure 28-1, D ).
- • The delayed (or excretory or equilibrium) phase of enhancement occurs ≈>120 to 180 seconds after contrast administration, when liver parenchyma and vessels decrease in enhancement, while the renal collecting systems increasingly opacify with excreted intravenous contrast material. This phase is useful to more definitely characterize certain hepatic lesions, including hemangioma (which has persistent delayed phase enhancement), FNH (which has delayed phase central scar enhancement), and cholangiocarcinoma (which often has delayed phase enhancement of fibrous stroma).
- • The hepatobiliary phase of enhancement is subsequently visualized on MRI if a hepatobiliary MRI contrast agent was intravenously administered, using T1-weighted MR images typically acquired >20 minutes after contrast administration. In this phase, the liver parenchyma homogeneously retains contrast material and appears hyperintense relative to skeletal muscle, the hepatic vessels appear hypointense relative to liver parenchyma, and the biliary tree, gallbladder, and renal collecting systems (given the ≈1 : 1 ratio of hepatobiliary : renal excretion of gadoxetate disodium) become opacified with excreted intravenous contrast material