Fatty Liver

General characteristics

Fatty liver or steatosis is defined as accumulation of triglycerides within hepatocytes.

Aetiology

The causes of hepatic steatosis include alcohol abuse, diabetes mellitus, obesity, malnutrition, and exposure to toxins. Fatty degeneration may present as diffuse uniform or patchy, focal, or with spared foci of normal liver. At times focal fatty infiltration or geographic regions of normal liver within fatty liver (fat sparing) may mimic the appearance of mass lesions.

Imaging

Out-of-phase gradient-echo (TE = 2.1) imaging is a highly accurate MRI technique to examine for fatty liver and to distinguish focal fat from neoplastic masses. Fat in substantial amounts has high signal intensity on in-phase T1-weighted images due to its short T1.Comparing outof- phase (TE = 2.1 ms) to in-phase (TE = 4.2 ms) gradientecho images, the presence of fatty metamorphosis results in signal loss.

An area that is isointense or hyperintense to surrounding background parenchyma on in-phase images and loses signal homogeneously on out-of-phase images is highly diagnostic for focal fatty infiltration. Focal fat usually has angular, wedge-shaped margins that are relatively well-defined.

Masses that contain fat usually have a rounded configuration. Common locations for focal fat are adjacent to the ligamentum teres, the central tip of segment IV; less commonly it appears along the gallbladder. On out-of-phase images focal normal liver in the setting of diffuse fatty infiltration (focal sparing) appears as a focus of high signal intensity on a background of liver of diminished signal intensity. The central tip of segment IV is a common location for focal fatty sparing. Liver metastases in the setting of fatty liver may demonstrate peritumoral fat sparing due to compression of this circumferential liver (Mitchell 1992).

Nonalcoholic fatty liver disease (NAFLD)

Nonalcoholic fatty liver disease (NAFLD) is related to obesity and type 2 diabetes. NAFLD is one of the causes of cryptogenic cirrhosis. Cirrhosis related to obesity and NAFLD are risk factors for HCC. Nonalcoholic steatohepatitis (NASH) occupies a middle position in the range of NAFLD and represents an intermediate stage of fatty liver damage.



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