Benign liver lesions containing fat are common and include:
Hepatic steatosis,
which can be diffuse or focal,
is often seen.
The typical locations (periligamentous or periportal) and the absence of “mass effect” usually help in recognizing focal steatosis.
(Fig. 1) However,
multifocal nodular steatosis with patchy focal fat deposition may simulate metastatic disease at US or CT [4].
The use of GRE with in-phase and opposed-phase imaging allows a correct diagnosis.
(Fig. 2)
Adenoma is a benign,
encapsulated neoplasm that usually occurs in young women taking oral contraceptives.
Type I glycogen storage disease and use of anabolic steroids are also associated with increased risk.
Adenomas frequently haemorrhage and rarely become malignant.
Hepatocellular adenomas are composed of cords of hepatocytes,
which may be filled with glycogen and fat and uncommonly intra and intercellular manifests as macroscopic fat deposits.
Although only 7% of adenomas demonstrate lipid deposition in CT, 35-77% of adenomas show steatosis at chemical shift MR imaging [5,6].
(Fig. 3) Due to its hypervascular nature it usually present early enhancing during arterial phase and rapid washout in contrast-enhanced MRI [6].
Focal Nodular Hyperplasia (FNH) is the second most common benign hepatic lesion often discovered incidentally in young asymptomatic woman.
FNH is a well-circumscribed mass consisting of hepatocytes containing bile ducts and mononuclear inflammatory cells and exhibits benign behaviour.
Typically presents at MRI as a isointense or nearly isointense on T1- and T2-wheighted images with a intense enhancement in the arterial phase and rapid washout [7].
A central scar bright on T2 and delayed contrast enhancement is characteristic.
The presence of fat is extremely rare and is usually patchy in distribution [8].
(Fig. 4) Intratumoral steatosis,
better demonstrated at MR imaging,
may exist and can be associated to diffuse hepatic steatosis.
Lipomas of the liver are extremely uncommon and are constituted by mature adipose tissue.
They appear as homogeneous hyperechoic well circumscribed lesions at US and present characteristics of a fatty lesion at CT and MRI.
(Fig. 5)
Angiomyolipoma (AML) is a benign mesenchymal tumor composed of varying proportions of smooth muscle cells,
thick-walled blood vessels and mature adipose tissue.
They are frequent in kidneys but rare in liver and may occur as a solitary mass or associated with tuberous sclerosis [9].
(Fig. 6 ,
Fig. 7) AMLs can be histologically categorized into mixes,
lipomatous,
myomatous and angiomatous types [10].
AML typically demonstrate the fat component and prominent central vessels. AML presents as a hypodense lesion on unenhanced CT,
usually with areas of macroscopic fat with attenuation values less than -20HU.
The appearance at MR imaging is variable depending on the proportion of Intratumoral fat.
AML show early intense contrast enhancement that peaks later than that of a hepatocellular carcinoma (HCC) [11].
PEComa is a mesenchymal tumor composed of histologically and immunohistochemically distinctive perivascular epitheliod cells and some have malignant behavior.
PEComas often resemble AML or HCC,
not only in terms of imaging features but histological as well.
Imaging characteristics are variable but well-demarcated margins and hyperintensity on T2-weighted images and strong enhancement are common.
[12] The nodules may show variable fat component usually identified on chemical shift sequences.
(Fig. 8)
Cystic Teratomas of the liver is an extremely rare entity.
The majority of the denominated “hepatic teratomas” are intraperitoneal or retroperitoneal teratomas with liver “invasion”.
These encapsulated tumors arise from pluripotential cells and usually have components derived from all three germ layers.
The cystic mass often contains fat,
hair and calcifications.
The presence of fat,
fluid and calcification in a mass virtually indicates teratoma [13].
Adrenal Rest Tumor (ART) is an ectopic collection of adrenocortical cells in an extra-adrenal site. Hepatic ART are similar to adrenocortical tumors and the presence of fat is the most characteristic feature.
At imaging they are typically subcapsular demonstrating macroscopic fat and hypervascurarity [14].
Pseudolipoma of the Glisson Capsule,
also known as hepatic pseudolipoma,
refers to an encapsulated lesion containing degenerated fat that is enveloped by liver capsule.
A detached colonic epiploic appendix may be the origin of this lesion that may become attached to the liver capsule.
It appears as a well-circumscribed nodule on the liver surface with a center of fatty attenuation.
(Fig. 9)
Focal Fat adjacent to intrahepatic Inferior Vena Cava can be occasionally seen and is described as a normal variant.
Is more frequent in patients with chronic liver disease and can mimic a fat-containing liver lesion [15].
(Fig. 10)
Xanthomatous Lesions in Langerhans Cell Histiocytosis (LCH) in the liver are uncommon.
The liver lesions associated with LCH are usually seen in patients with extensive LCH and characteristically located in the periportal region.
Four stages have been decribed: proliferative,
granulomatous,
xanthomatous and fibrous [16].
Xanthomatous lesions have low attenuation at CT and display characteristics of fat at MR imaging.
(Fig. 11)
Malignant fat-containing liver lesions include:
Hepatocellular Carcinoma (HCC) is the most common primary hepatic malignant lesion and usually develops in a cirrhotic liver.
Fatty metamorphosis was found in up to 17% of HCCs,
usually diffuse-type in small lesions and patchy in larger tumors [17].
Macroscopic fat within HCC is well demonstrated on CT.
At MR imaging the fatty areas appear hyperintense on T1-weighted images and demonstrates loss of signal on chemical shift images.
However hyperintensity on T1-weighted images may be due to other factors as haemorrhage,
glycogen content,
clear cell formation and excessive copper/zinc accumulation [18].
(Fig. 12,
Fig. 13)
Liposarcoma is an uncommon malignant tumor accounting for 15% of all sarcomas.
Metastatic spread of retroperitoneal and extremity liposarcomas is common,
but liver is involved in only 10% of cases [19].
The majority of hepatic liposarcomas are metastatic although cases of primary hepatic liposarcomas have been reported [20].
Hepatic Metastases are common but usually do not contain fat.
Rare examples of metastases with focal fat exist.