A practical approach to the characterization of focal hepatic lesions by computed tomography (CT) and magnetic resonance (Mr) has traditionally focused on the distribution of conventional iodine and gadolinium (Gd) chelates to extracellular fluid spaces. This approach facilitates both the detection and characterization of lesions, the latter relying upon the recognition of distinctive patterns of enhancement of liver lesions. On the basis of their handling of these contrast agents, lesions are broadly classified as being hypervascular, hypo-vascular or as having delayed persistent enhancement (See Chap. 3, "Contrast Agents for Liver

Imaging", section 3.3,"Radiologic Classification of Focal Liver Lesions on MRI").

Hypovascular lesions include clearly avascular lesions such as cysts and abscesses, and also tumors that demonstrate only marginal peripheral enhancement, if any, compared to the surrounding liver parenchyma. The majority of metastases to the liver are hypovascular lesions from primary sources such as the colon, lung, and pancreas. The few primary sites from which hypervascular liver metastases derive include renal, thyroid, neuroendocrine (pancreatic islet cell), and sarcoma. Breast and melanoma metastases to the liver may be vascular, but generally less so than those of other tumors. Imaging of hypovascular tumors at dynamic SonoVue-enhanced ultrasound (US), iodine-enhanced CT or dynamic Gd-enhanced MR relies on imaging the liver before and during peak enhancement in order to maximize liver-lesion contrast before the equilibrium phases are reached.

The group of hypervascular liver lesions includes a wide number of lesion types, ranging from physio-pathologic to pathologic. Among these are benign and malignant lesions of both hepatocellular and non-hepatocellular origin. Enhancement characteristics can help narrow a differential diagnosis or, in many cases, achieve a definitive diagnosis.

Other lesions may be classified as demonstrating delayed persistent enhancement. This group of lesions principally comprises capillary and cavernous forms of hepatic hemangioma, although cholangiocellular carcinoma (CCC) also falls into this category, especially when an abundant fibrotic reaction is present. Additionally, some metastases from colon cancer may have a fibrotic component and thus appear with delayed central enhancement.

On hepatobiliary phase images after Gd-BOP-TA, Gd-EOB-DTPA and Mn-DPDP, hepatocellular lesions may appear iso-, hyper- or hypointense compared to the normal liver parenchyma, de pending on whether the lesion has preserved hepatocellular activity, modified pathways of biliary excretion, or poor or absent hepatocyte activity, respectively. Similarly, hepatocellular lesions on reticuloendothelial phase images after administration of superparamagnetic iron oxide (SPIO) agents may appear iso-, hypo- or hyperintense. Iso- and hypointensity reflect a signal drop due to the presence of abundant and functioning Kupffer cells. Conversely, lesion hyperintensity reflects scarce or absent Kupffer cell activity.

Non-hepatocellular lesions such as metastases and hemangiomas generally appear hypo- and hy-perintense, respectively, on hepatobiliary phase and reticuloendothelial phase images. (For a description of the characterization of liver lesions by unenhanced and contrast-enhanced MR imaging using different contrast agents see also: Chap. 3, section 3.3,"Radiologic Classification of Focal Liver Lesions on MRI").

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