-Common patterns include diffuse fat accumulation, diffuse fat accumulation with focal sparing, and focal fat accumulation in an otherwise normal liver.
-Unusual patterns that may cause diagnostic confusion by mimicking neoplastic, inflammatory, or vascular conditions include multinodular and perivascular accumulation.
CAUSES:
-Most common: Alcohol, insulin resistance, obesity, hyperlipidemia.
-Common: Viral infection (hep. B.,C.), drug use.
-Rare: rapid weight loss, surgery ( jejuno-ileal bypass), parenteral nutrition, starvation, radiotherapy.
-Congenital: Storage disorders(Glycogen storage disorder, 1-Antitrypsin deficiency, Wilson disease, Hemochromatosis), cystic fibrosis.
Characterized histologically by triglyceride accumulation within the cytoplasm of hepatocytes.
Hepatocytes in the center of the lobule (near the central vein) are particularly vulnerable to metabolic stress and tend to accumulate lipid earlier than those in the periphery. Therefore steatosis tends to be most pronounced histologically in the zone around the central veins and less pronounced in zones around the portal triads.
Steatosis may progress to steatohepatitis (with inflammation, cell injury, or fibrosis accompanying steatosis) and then cirrhosis. There are data that suggest that the coexistence of steatosis with other liver diseases, such as viral hepatitis, increases the risk of disease progression
The accumulation of discrete triglyceride droplets in hepatocytes and, rarely, in
other cell types. Infiltration of fat into the parenchyma does not occur. The term fatty liver is
more accurate than fatty infiltration.
The prevalence of fatty liver in the general population is about 15%, but it is higher among those
who consume large quantities ( 60 g per day) of alcohol (45%), those with hyperlipidemia (50%) or obesity (body mass index, 30 kg/m2 ) (75%), and those with both obesity and high alcohol consumption (95%).
US
The echogenicity of the normal liver equals or minimally exceeds that of the renal cortex or spleen. Intrahepatic vessels are sharply demarcated, and posterior aspects of the liver are well depicted. Fatty liver may be diagnosed if liver echogenicity exceeds that of renal cortex and spleen and there is attenuation of the ultrasound wave, loss of definition of the diaphragm, and poor delineation of the intrahepatic architecture. To avoid false-positive interpretations, fatty liver should not be considered present if only one or two of these criteria are fulfilled.
normal 
fatty
fattyCT
At unenhanced CT, the normal liver has slightly greater attenuation than the spleen and blood,and intrahepatic vessels are visible as relatively hypoattenuated structures. Fatty liver can be diagnosed if the attenuation of the liver is atleast 10 HU less than that of the spleen or if the attenuation of the liver is less than 40 HU. In severe cases of fatty liver, intrahepatic vessels may appear hyperattenuated relative to the fat-containing liver tissue. Fatty liver can be diagnosed at contrast-enhanced CT if absolute attenuation is
less than 40 HU, but this threshold has limited sensitivity.
normal
fattyMR
PATTERNS:
Diffuse fat deposition:
Its the most frequently encountered pattern. Liver involvement usually is homogeneous.
Focal Deposition and Focal Sparing:
Slightly less common patterns are focal fat deposition and diffuse fat deposition with focal sparing. In these patterns, focal fat deposition or focal at sparing characteristically occurs in specific areas (eg, adjacent to the falciform ligament or
ligamentum venosum, in the porta hepatis, and in the gallbladder fossa). This distribution is
not yet fully understood but has been attributed to variant venous circulation, such as anomalous gastric venous drainage. Focal fat deposition adjacent to insulinoma metastases also has been reported and is thought to be due to local insulin effects on hepatocyte triglyceride synthesis and accumulation.
focal depositeImaging findings suggestive of fatty pseudolesions rather than true masses include the following: fat content, location in areas characteristic of fat deposition or sparing, absence of a mass effect on vessels and other liver structures, a geographic configuration rather than a round or oval shape, poorly delineated margins, and contrast enhancement that is similar to or less than that of the normal liver parenchyma.
focal depositeInvolved areas usually are relatively small, but occasionally there may be confluent
heterogeneous regions of focal deposition and sparing that span large areas of the liver.
Multifocal Deposition:
An uncommon pattern is multifocal fat deposition. In this pattern, multiple fat foci are scattered
in atypical locations throughout the liver. The foci may be round or oval and closely mimic true nodules. For this purpose, chemical shift GRE imaging (MRI) is more reliable than
CT or US.
Perivascular Deposition:
This pattern is characterized by halos of fat that surround the hepatic veins, the portal veins, or both hepatic and portal veins. The configuration is tramlike or tubular for vessels with a course in the imaging plane and ringlike or round for vessels with a course perpendicular to the imaging plane. The pathogenesis of perivascular fat deposition in the liver is unknown.
perivenous fat accumulation
periportal fat accumulationSubcapsular Deposition:
In patients with renal failure and insulin-dependent diabetes, insulin may be added to the peritoneal dialysate during kidney dialysis. This route of insulin administration exposes subcapsular hepatocytes to a higher concentration of insulin than that to which the remainder of the liver is exposed. Since insulin promotes the esterification of free fatty acids into triglycerides, the peritoneal administration of insulin results in a subcapsular pattern of fat deposition, which may be manifested as discrete fat nodules or a confluent peripheral region of fat.
Primary Lesions and Hypervascular Metastases
In general, the differentiation of focal or multifocal fat accumulations from primary hepatic lesions (eg, hepatocellular carcinoma, hepatic adenoma, and focal nodular hyperplasia) or from
hypervascular metastases in the liver is not problematic because these lesions exert a mass effect, tend to show vivid or heterogeneous enhancement after contrast agent administration, and may contain areas of necrosis or hemorrhage. Infiltrative hepatocellular carcinoma is a notable exception; on CT images, this tumor may exert a minimal mass effect, show
little evidence of necrosis, show the same degree of enhancement as the normal liver parenchyma, and closely resemble heterogeneous fat deposition. The correct diagnosis is usually possible with MR imaging.
infiltrative HCC native
infiltrative HCC portal phaseHypovascular Metastases and Lymphoma
The clinical manifestations and imaging features such as lesion morphology, location, and microscopic fat content usually permit a correct diagnosis. Chemical shift GRE imaging may be necessary to assess the amount of intralesional fat.
Perfusion Anomalies
Are visible only during the arterial and portal venous phases after contrast agent administration. They are not detectable on unenhanced images or equilibrium phase images.
Periportal Abnormalities
The US- and CT-based differential diagnosis of periportal fat deposition is broad and includes
edema, inflammation, hemorrhage, and lymphatic dilatation. Edema, inflammation, and lymphatic dilatation tend to affect the portal triads symmetrically. Hemorrhage characteristically involves the portal triads asymmetrically and may be associated with laceration or other signs of injury.
SUMMARY
Fatty liver is a common imaging finding, with a prevalence of 15%–95%, depending on the population. The most common imaging pattern is diffuse and relatively homogeneous fat deposition. Less common patterns include focal deposition, diffuse deposition with focal sparing, multifocal deposition, perivascular deposition, and subcapsular deposition. These patterns may
mimic neoplastic, inflammatory, or vascular conditions. Assessment of the lesion fat content, location, morphologic features, contrast enhancement, and mass effect usually permits a correct diagnosis. Chemical shift GRE imaging is more reliable than US or CT for assessing intralesional fat and may be necessary when findings are equivocal.
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