AJR 2005; 184:1103-1110
© American Roentgen Ray Society
Fat-Containing Lesions of the Liver: Cross-Sectional Imaging Findings with Emphasis on MRI
Ceyla Basaran1,
Musturay Karcaaltincaba1,
Deniz Akata1,
Nevzat Karabulut2,
Devrim Akinci1,
Mustafa Ozmen1 and
Okan Akhan1
1 Department of Radiology, Hacettepe University School of Medicine, Sihhiye,
Ankara 06100, Turkey.
2 Department of Radiology, Pamukkale University Hospital, Denizli, Turkey.
Received June 7, 2004;
accepted after revision September 22, 2004.
Address correspondence to M. Karcaaltincaba
(musturayk{at}yahoo.com).
Abstract
OBJECTIVE. The purpose of this pictorial essay is to identify
different types of liver lesions that contain fat. Cross-sectional imaging
findings of fat- or lipid-containing lesions can help in characterizing focal
liver lesions. We searched our archive retrospectively and reviewed the
literature for fat-containing liver lesions and identified 16 different
types.
CONCLUSION. These lesions can contain macroscopic fat (i.e.,
angiomyolipoma, lipoma, liposarcoma, hydatid cyst, lipopeliosis, adrenal rest
tumor, pseudolipoma, hepatic teratoma, pericaval fat, extramedullary
hematopoiesis, and metastases) or intracellular lipid (i.e., focal steatosis,
adenoma, focal nodular hyperplasia, regenerative nodules, and hepatocellular
carcinoma). CT, MRI, and sonographic findings of these lesions can help in
characterization by allowing specific diagnosis or narrowing the differential
diagnosis of liver lesions.
Introduction
Cross-sectional imaging findings of fat- or lipid-containing lesions
can help in characterizing focal liver lesions. We searched our archive
retrospectively and reviewed the literature for fat-containing liver lesions
and identified 16 different types.
Liver lesions can contain macroscopic fat or intracellular lipid
(Table 1). Macroscopic
fat-containing liver lesions include angiomyolipoma, lipoma, liposarcoma,
hydatid cyst, lipopeliosis, adrenal rest tumor of the liver, pseudolipoma of a
Glisson capsule, hepatic teratoma, fat adjacent to intrahepatic inferior vena
cava (pericaval fat), extramedullary hematopoiesis, and metastases.
Macroscopic fat-containing lesions can be easily characterized on CT and MRI
by negative Hounsfield values and hyperintensity on T1- and T2-weighted images
and signal loss on fat-saturated MR images, respectively. Fat droplets can be
seen in hydatid cysts. Intracellular lipid-containing lesions include focal
hepatic steatosis, hepatic adenoma, focal nodular hyperplasia (FNH),
regenerative nodules, and hepatocellular carcinoma (HCC). Intracellular
lipid-containing lesions can be characterized objectively by chemical shift
MRI techniques (in- and out-of-phase T1-weighted gradient-echo images) and
dynamic gadolinium-enhanced studies.
Sonography is currently the first screening method for focal hepatic
lesions, but sonographic findings of many hepatic nodules are nonspecific. Fat
generally produces high echogenicity when present in nodular lesions on
hepatic sonographic screening. However, hyperechogenicity is also
characteristic of some non-fat-containing lesions such as cavernous
hemangioma. Because hyperechoic liver nodules cannot be characterized on
sonography, subsequent examination using CT, conventional MRI, or even
fine-needle aspiration cytology is necessary in symptomatic or oncology
patients. The CT characteristics of some nodules with fatty components can
also be nonspecific, because of the lack of sufficient lipid pixels. Chemical
shift gradient-echo imaging is a readily available MRI technique that can help
to determine with certainty whether a given hyperechoic nodule contains fat
[1].
Intracellular Lipid-Containing Liver Lesions
Focal Hepatic Steatosis
Fatty change in the liver can result from excessive triglyceride
deposition, and it may be uniform, patchy, or focal. Focal hepatic steatosis
may mimic the appearance of hyperechogenic mass lesions such as hemangioma,
angiolipoma, lipoma, or metastasis on sonography. Recognition of diffuse
hepatic steatosis on CT requires liver attenuation to be 8-10 H lower than
that of the spleen on unenhanced images. Focal hepatic steatosis of the liver
may present as focal lesions
[2]; is common in the medial
segment of the left lobe of the liver, adjacent to the falciform ligament,
central tip of segment IV, and, less commonly, along the gallbladder; and can
be multifocal. Sometimes varying degrees of hepatic steatosis of the liver can
occur, and focal hypodense areas (more fatty) can be present in diffusely
steatotic liver on CT (Figs.
1A,
1B, and
1C).
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Fig. 1B. —50-year-old woman with breast cancer. Axial in-phase
(B) and out-of-phase (C) images show greater signal drop of
lesion in C (arrow, C), consistent with hypersteatosis
(more fatty) compared with diffusely fatty infiltrated liver.
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Fig. 1C. —50-year-old woman with breast cancer. Axial in-phase
(B) and out-of-phase (C) images show greater signal drop of
lesion in C (arrow, C), consistent with hypersteatosis
(more fatty) compared with diffusely fatty infiltrated liver.
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MRI is particularly effective in evaluating focal hepatic steatosis.
Out-of-phase T1-weighted gradient-echo imaging is a highly accurate technique
to distinguish focal hepatic steatosis from neoplastic masses. Focal hepatic
steatosis is isointense or hyperintense to liver on in-phase images and loses
signal homogeneously on out-of-phase images, which is highly diagnostic for
focal steatosis (Figs. 2A,
2B, and
2C). The morphology of focal
hepatic steatosis most often permits distinction from fat within tumors, such
as HCC, adenoma, angiomyolipoma, or lipoma. Focal hepatic steatosis usually
has angular, wedge-shaped margins that are usually relatively well defined and
appear isointense to liver on gadolinium-enhanced T1-weighted MR images
[3]. Diagnosis is also
suggested by lack of mass effect on vessels or the biliary system. At times,
blood vessels traversing the focal steatotic areas are identified.
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Fig. 2B. —45-year-old woman who had prior ovarian cancer surgery and
multiple liver lesions. Axial T1-weighted in-phase (B) and out-of-phase
(C) images show multiple lesions with signal drop on opposed-phase
images consistent with focal fatty infiltrations. MR images were obtained on
0.5-T MRI system with TEs of 14 and 6 for in- and out-of-phase images,
respectively.
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Fig. 2C. —45-year-old woman who had prior ovarian cancer surgery and
multiple liver lesions. Axial T1-weighted in-phase (B) and out-of-phase
(C) images show multiple lesions with signal drop on opposed-phase
images consistent with focal fatty infiltrations. MR images were obtained on
0.5-T MRI system with TEs of 14 and 6 for in- and out-of-phase images,
respectively.
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HCC
The histologic pattern, the degree of tumor differentiation, the amount of
fibrosis, the presence of internal necrosis or hemorrhage, and the
intracellular content of glycogen, fat, or metal ions greatly affect the
radiologic appearance of the HCC. These factors affect particularly the MRI
appearance, which may range from hypointense to iso- or hyperintense on
T1-weighted images and from hypointense to hyperintense on T2-weighted images
[4]. HCC occasionally contains
fat (Figs. 3A,
3B,
3C, and
3D). HCC with fatty
metamorphosis was found in up to 17% of the lesions. Some investigators have
suggested that fatty metamorphosis is the principal cause of hyperintensity on
T1-weighted images of some HCCs and that this finding can help in establishing
the diagnosis [4,
5], but other causes of
hyperintensity on T1-weighted images with conventional spin-echo sequences can
be present such as content of glycogen, subacute hemorrhage, clear cell
formation, and excessive copper accumulation
[5].
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Fig. 3C. —78-year-old man with cirrhosis. Arterial (C) and
delayed (D) phase axial gadolinium-enhanced MR images show
hypervascularity and contrast washout of hepatocellular carcinoma
(arrow), respectively.
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Fig. 3D. —78-year-old man with cirrhosis. Arterial (C) and
delayed (D) phase axial gadolinium-enhanced MR images show
hypervascularity and contrast washout of hepatocellular carcinoma
(arrow), respectively.
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In characterizing HCC for fat content, chemical shift imaging has been
reported to be very useful in the detection of lipomatous nodules in cirrhotic
liver [3]. However, benign
regenerative nodules can also contain fat.
FNH
FNH is a well-circumscribed mass, lacking a true capsule, and is
characterized by a central scar. The typical MRI appearance is usually
isointense or nearly isointense on both T1-weighted and T2-weighted images.
The central scar appears hypointense on T1-weighted images and hyperintense on
T2-weighted images. The lesion shows intense enhancement, with the central
scar being unenhanced in the arterial phase of gadolinium-enhanced dynamic
MRI. In the portal phase, the lesion undergoes rapid washout of contrast
material, becoming isointense to liver. The central scar may show delayed
enhancement [4]. In a recent
study, the authors reported markedly heterogeneous FNH due to extensive
intralesional fat [6].
Previously, investigators reported intratumoral fat depicted on MR images of
FNH, and they considered it as an exaggerated expression of this patient's
native hepatic disease characterized by fatty liver
[7].
Hepatocellular Adenoma
Hepatocellular adenoma is an uncommon primary benign tumor. Oral
contraceptives and androgen steroid therapy have been identified as definitive
causes. Adenoma presents as a solitary lesion in most cases and is typically a
well-circumscribed tumor. Histologically, adenoma is composed of cords of
hepatocytes, which contain increased amounts of glycogen and sometimes fat.
The content of glycogen and fat is the main element responsible for the
hyperintensity of adenoma on T1-weighted images. Nevertheless, areas of
internal subacute hemorrhage are markedly hyperintense on T1-weighted images.
Chemical shift imaging (Figs.
4A,
4B, and
4C) can confirm fat content by
showing a decrease in tumor signal intensity on opposed-phase images
[4]. On dynamic
contrast-enhanced MRI, the lesion shows early enhancement during the arterial
phase, with rapid washout accounting for its typical hypervascular nature
[8].
Macroscopic Fat-Containing Liver Lesions
Angiomyolipoma
Hepatic angiomyolipoma is a rare tumor, which may occur as a solitary mass
or as an associated finding with tuberous sclerosis
[9].
Angiomyolipoma is a benign mesenchymal tumor, with a mixture of mature fat,
smooth muscle, and thick-walled blood vessels. It occurs frequently in the
kidney but rarely in liver (Fig.
5). Patients usually have no symptoms, and most of these tumors
are found incidentally on routine sonographic studies. The accuracy of
preoperative diagnosis is very low as a result of variable imaging appearances
due to the varying content of the three components and the rarity of the
lesion. The fat component of angiomyolipoma varies between 10% and 90%
[9].
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Fig. 5. —29-year-old woman with tuberous sclerosis. Axial CT image
shows fat-containing liver lesion (arrow) consistent with hepatic
angiomyolipoma. Note bilateral multiple renal angiomyolipomas.
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In one study, angiomyolipoma (11/12 lesions) appeared as a hypodense lesion
on unenhanced CT scans and markedly enhanced on the arterial phase with
central vascular opacification in eight lesions
[9]. On the portal venous
phase, eight lesions remained enhancing with central vessels seen in six
lesions. We suggest that presence of central vessels within the lesions may be
a characteristic feature of angiomyolipoma. The feeding blood vessels can be
seen in other hypervascular lesions such as HCC and FNH, but the vessels in
those cases usually are located in the periphery of the lesions. MRI is also
an important diagnostic technique that allows fat suppression and multiphase
dynamic contrast-enhanced scanning. The lesions have various signal
intensities from slight to strong hyperintensity on fast spin-echo T2-weighted
images because of the different proportion of smooth muscle and vessels.
Dynamic contrast-enhanced imaging on MRI is similar to that of CT with the
central vessels seen in the lesions
[9,
10].
Metastatic Liver Disease
Metastatic liver disease is one of the most common problems in oncology
patients. Liver metastases generally represent the histotype of the primary
neoplasm. Fat-containing primary tumors such as teratoma, liposarcoma, Wilms'
tumor, and renal cell carcinoma that are high in signal intensity on
T1-weighted images can metastasize to the liver
[3,
4].
Lipoma and Liposarcoma
Hepatic lipomas are rarer than angiomyolipomas and can occur sporadically.
They are homogeneous and circumscribed and show fat attenuation on CT and do
not enhance after IV administration of contrast material
[2,
11]
(Fig. 6). On MRI, lipomas can
be multiple and appear as fatty tumors that are hyperintense on T1-weighted
images and hypointense on fat-suppressed T2-weighted images
[3].
Liposarcoma is a rare mesenchymal malignant tumor, which usually originates
in the retroperitoneum and the extremities. Primary liver liposarcoma is
extremely rare, and what is believed to be the first reported case was
published in 1987. Sonography of liver liposarcoma showed a poorly defined,
lobulated, infiltrating echogenic tumor with shadowing and heterogeneity
secondary to areas of hemorrhage and necrosis. A low-attenuation mass of fat
density was confirmed on CT
[12].
Lipopeliosis
Peliosis hepatis is characterized by dilated, blood-filled sinusoids.
However, lipopeliosis is described as another distinct peliosis-like lesion
that occurs in a transplanted steatotic liver after centrilobular hepatocyte
injury and necrosis [13,
14]. Lipopeliosis is an
unusual liver lesion in which sinusoids become engorged by fat globules.
Although lipopeliosis is seen in the setting of necrosis of fatty hepatocytes
in the transplanted liver, any fatty condition of the liver with a
superimposed ischemic injury may result in a similar lesion
[13,
14]. To our knowledge, CT
findings of lipopeliosis are first described in our patient
(Fig. 7).
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Fig. 7. —45-year-old woman with acute leukemia. Axial CT image shows
fat-containing lesion (arrows) in right lobe extending to caudate
lobe that was not present on CT 1 year ago. Attenuation measurement of lesion
revealed -32 H. Biopsy of lesion revealed hepatic necrosis with fatty
replacement consistent with lipopeliosis.
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Hepatic Hydatid Cyst
The liver is the organ most commonly affected by hydatid cysts. Mendez
Montero et al. [15] reported
fat-fluid levels inside hydatid cysts in two patients in the form of either
fat-fluid levels or fat droplets (Fig.
8). They identified a large perforation in the cyst wall
communicating with a main biliary radicle on both sonography and CT in two of
the cysts, and they suggested that fat droplets inside the hydatid cysts are
derived from the lipid elements in bile
[15]. Also, omentopexy changes
seen after hydatid cyst surgery can mimic fatty liver lesions
(Fig. 9).
Focal Fat Adjacent to the Intrahepatic Inferior Vena Cava
Focal fat can be seen adjacent to the intrahepatic inferior vena cava on CT
(Figs. 10A, and
10B). This lesion can be a
normal variant, which is more frequently seen in patients with chronic liver
disease and can mimic a fat-containing liver lesion
[16].
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Fig. 10B. —45-year-old man with abdominal pain who underwent CT
examination. Sagittal reformatted image shows protrusion of apical portion of
pericaval fat into inferior vena cava lumen (arrow).
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Miscellaneous
Also, adrenal rest tumor of the liver, pseudolipoma of a Glisson capsule,
and hepatic teratoma can contain fat. Recently, focal intrahepatic
extramedullary hematopoiesis has been reported to contain fat
[17].
Conclusion
Fat can be present in a variety of benign and malignant liver lesions. The
presence of fat can allow specific diagnosis or narrow the differential
diagnosis of liver lesions. Preferred imaging methods for the diagnosis of fat
content are shown in Table
1.
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Abstract
Introduction
