The differential diagnosis of a fat-containing liver mass is usually limited to masses of hepatocellular origin. Fat-containing liver lesions of nonhepatocellular origin are less common and may be identified by ancillary imaging or clinical features.
- Fat-Containing Hepatocellular Lesions
Hepatocellular adenomas (HCA)
HCAs are uncommon benign tumors, usually encountered in women with a history of oral contraceptive use,
athletes taking anabolic steroids,
or patients with metabolic diseases.
The main complications of HCA are hemorrhage,
which typically occurs in tumors larger than 5 cm,
and malignant transformation,
which occurs in approximately 5% of cases.
Histologically,
this tumor corresponds to a benign proliferation of normal hepatocytes without portal tracts.
HCA is a heterogeneous tumor with three distinct molecular subtypes and a fourth,
as yet-uncharacterized subtype.
The two most common sub-types are HNF1 α-mutated adenoma and inflammatory adenoma,
both of which can contain intracellular fat.
Inflammatory HCAs typically arise in steatotic livers.
MRI most often demonstrates a lesion with iso-signal intensity on T1-weighted sequences,
and marked high signal intensity on T2-weighted images due to the dilated sinusoids.
Arterial phase enhancement is intense and persists into the portal and delayed phases.
Intra-voxel fat has been observed in 11–20% of inflammatory HCAs;
When present,
in out of phase sequences,
it has mild loss of signal.
In contrast to inflammatory HCA,
HCAs inactivated for hepatocyte nuclear factor–1α typically show homogeneous,
diffuse intratumoral fat ,
mild hyperintense T2 signal,
less-avid arterial enhancement that does not persist on images acquired in later phases,
and a nonsteatotic background liver.
Focal nodular hyperplasia (FNH)
FNH is the second most common benign hepatic lesion following hemangioma.
It is usually discovered incidentally and is most frequent in females.
Histologically,
FNH consists of hepatocyte nodules that are circumscribed by fibrous septa containing bile ducts and mononuclear inflammatory cells.
At CT and MR imaging,
FNH demonstrates brisk,
intense,
homogeneous contrast enhancement.
A characteristic feature is a bright,
central scar on T2 weighted images.
The presence of fat in FNH is uncommon and is usually patchy in distribution.
Intratumoral steatosis is shown by a signal drop on out-of-phase T1-weighted images ( Fig. 1 ).
This does not always equate to an atypical appearance on MRI if the other criteria are brought together.
It can,
however,
lead to difficulties in diagnosis,
especially in distinguish FNH from inflammatory hepatic adenoma.
The use of hepatocyte-specific contrast agents can help in differential diagnosis.
Steatosis
Hepatic steatosis corresponds to the accumulation of triglycerides in hepatocytes secondary to the altered metabolism of free fatty acids and it usually results from nonalcoholic or alcoholic liver disease.
It can be diffuse,
focal,
or multifocal.
Focal hypersteatosis can be mistaken for a focal lesion on sonography or CT.
Chemical shift imaging shows fat present in the hepatocytes with a signal drop on the out-of-phase sequence.
This finding,
together with characteristic locations (periportal region and IVth hepatic segment),
lack of mass effect,
and parallel enhancement gradient to that of the adjacent parenchyma,
allows a certain diagnosis.
Multifocal steatosis may mimic metastatic disease; however uniform loss in signal intensity at opposed-phase imaging is helpful in exluding metastases.
Hepatocellular carcinoma (HCC)
HCC is the commonest primary hepatic malignant neoplasm that commonly develops in a cirrhotic liver.
This lesion produces a variable signal on T1- and T2-weighted sequences,
with hypervascularity in the arterial phase and washout in the portal and/or delayed phase. It is estimated that 17%–35% of hepatocellular carcinomas contain fat.
Fat deposition is usually patchy compared to the uniform fat deposition in adenomas.
( Fig. 2 ).
Fat is more common in well-differentiated or moderately differentiated HCC.
The pattern is more typically diffuse in smaller lesions and patchy in larger lesions.
HCC with fatty change appears hyperintense on T1-weighted images,
displays mild contrast-enhancement compared to non fatty HCC and demonstrates signal intensity drop on chemical shift images.
- Fat-Containing Nonhepatocellular Lesions
Angiomyolipoma (AML)
It is a benign lesion.
Histologically,
it is composed of smooth muscle cells,
vessels,
and adipocytes
Hepatic AMLs are usually solitary,
altough,
in patients with tuberous sclerosis,
they often coexist with renal AMLs.
Typically,
the lesion is non-homogeneous,
with variable high signal intensity on T2 weighted sequences,
high signal intensity on T1-weighted sequences,
and a signal drop on fat-suppressed images ( Fig. 3 ).
The angiomatous component typically results in avid arterial enhancement that peaks later than that of a hepatocellular carcinoma (HCC).
Dynamic contrast-enhanced CT or MR images obtained during the early phase of enhancement may be useful in discriminating between AML and fat-containing HCC.
The fatty areas of AMLs are well vascularized and enhance early,
whereas steatotic foci in HCC are relatively avascular and have less contrast enhancement.
Lipoma
Hepatic lipomas are extremely uncommon.
Histologically,
these lesions consist of mature adipose tissue.
At US,
they appear as well-circumscribed,
uniformly hyperechoic lesions.
Pure fat attenuation of −20 HU or less is measured on CT.
On MRI,
lipomas are isointense to subcutaneous fat on all sequences,
with homogeneous loss of signal noted on fat-saturated images ( Fig. 4 ),
and they are circumscribed by etching artifact on opposed-phase GRE T1-weighted MRI.
Hydatid cyst
Involvement of the liver in hydatid disease is common (75% of cases).
This lesion can be asymptomatic or,
in case of infection or rupture, can be painful.
Its imaging features include intraluminal debris corresponding to hydatid sand (the snowflake sign); multivesicular,
multiseptated lesions in which daughter cysts partly or completely fill the mother cyst,
resembling a honeycomb; detached membranes (the water lily sign) and rim calcification.
The presence of a fat droplets or a fat-fluid level within the hydatid cyst is thought to be an indirect sign of communication with the biliary tree,
which may be longstanding ( Fig. 5 ).
When present,
fat droplets or a fat-fluid level within a liver lesion distinguish hydatid disease from other fat-containing liver lesions.
Hepatic Adrenal Rest Tumors
Adrenal rest tumor is an ectopic collection of adrenocortical cells in an extra-adrenal site. This tumor may be nonfunctional or hormonally active and manifesting as an endocrine syndrome.
On imaging,
hepatic adrenal rest tumors are heterogeneous,
hypervascular lesions that show fat and soft-tissue components,
making them difficult to distinguish from more common fat-containing tumors,
such as HCC,
HCA,
and AML.
Metastases of an Extrahepatic Primary Tumor
Fat is usually not present in liver metastases.
However,
metastatic lesions of the liver, may contain fat if the primary tumor has a fat component,
as occurs with fat-containing metastatic liposarcomas.
Liposarcoma is an uncommon malignant mesenchymal tumor. Metastatic spread of retroperitoneal and extremity liposarcomas is relatively common,
but the liver is involved in only 10% of cases ( Fig. 6 ).
Most hepatic liposarcomas are metastatic,
although isolated cases of primary hepatic liposarcomas have been reported.
Cystic Teratoma
True liver cystic teratomas are exceptionally rare.
Teratomas are benign,
encapsulated tumors arising from pluripotential cells.
They frequently have components derived from all three germ layers.
Tissue heterogeneity is shown on imaging by the combined presence of fat,
calcification,
and fluid,
with a possible Rokitansky nodule and lack of enhancement.