AJR:184, April 2005 1103 AJR 2005 184:1103���1110 0361���803X/05/1844���1103 �� American Roentgen Ray Society Abdominal Imaging Basaran et al. Imaging of Fat-Containing Liver Lesions Pictorial Essay Ceyla Basaran1 Musturay Karcaaltincaba1 Deniz Akata1 Nevzat Karabulut2 Devrim Akinci1 Mustafa Ozmen1 Okan Akhan1 Basaran C, Karcaaltincaba M, Deniz A, et al. Received June 7, 2004 accepted after revision September 22, 2004. 1Department of Radiology, Hacettepe University School of Medicine, Sihhiye, Ankara 06100, Turkey. Address correspondence to M. Karcaaltincaba (musturayk@yahoo.com). 2Department of Radiology, Pamukkale University Hospital, Denizli, Turkey. Fat-Containing Lesions of the Liver: Cross-Sectional Imaging Findings with Emphasis on MRI OBJECTIVE. The purpose of this pictorial essay is to identify different types of liver le- sions 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, li- poma, liposarcoma, hydatid cyst, lipopeliosis, adrenal rest tumor, pseudolipoma, hepatic ter- atoma, 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. ross-sectional imaging findings of fat- or lipid-containing lesions can help in characterizing focal liver lesions. We searched our ar- chive retrospectively and reviewed the litera- ture 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 angiomyoli- poma, lipoma, liposarcoma, hydatid cyst, li- popeliosis, adrenal rest tumor of the liver, pseudolipoma of a Glisson capsule, hepatic ter- atoma, fat adjacent to intrahepatic inferior vena cava (pericaval fat), extramedullary hemato- poiesis, and metastases. Macroscopic fat-con- taining 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, re- spectively. Fat droplets can be seen in hydatid cysts. Intracellular lipid-containing lesions in- clude 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 gadolin- ium-enhanced studies. Sonography is currently the first screen- ing method for focal hepatic lesions, but sonographic findings of many hepatic nod- ules are nonspecific. Fat generally pro- duces high echogenicity when present in nodular lesions on hepatic sonographic screening. However, hyperechogenicity is also characteristic of some non���fat-con- taining lesions such as cavernous hemangi- oma. Because hyperechoic liver nodules cannot be characterized on sonography, subsequent examination using CT, conven- tional 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 gradi- ent-echo imaging is a readily available MRI technique that can help to determine with certainty whether a given hyperechoic nod- ule contains fat [1]. C

Basaran et al. 1104 AJR:184, April 2005 Intracellular Lipid-Containing Liver Lesions Focal Hepatic Steatosis Fatty change in the liver can result from ex- cessive triglyceride deposition, and it may be uniform, patchy, or focal. Focal hepatic ste- atosis may mimic the appearance of hypere- chogenic mass lesions such as hemangioma, angiolipoma, lipoma, or metastasis on sonog- raphy. Recognition of diffuse hepatic steato- sis 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, cen- tral tip of segment IV, and, less commonly, along the gallbladder and can be multifocal. Sometimes varying degrees of hepatic steato- sis of the liver can occur, and focal hypodense areas (more fatty) can be present in diffusely steatotic liver on CT (Fig. 1). 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 he- patic 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 (Fig. 2). The morphology of focal hepatic ste- atosis most often permits distinction from fat within tumors, such as HCC, adenoma, angi- omyolipoma, or lipoma. Focal hepatic steato- sis usually has angular, wedge-shaped margins that are usually relatively well de- fined and appear isointense to liver on gado- linium-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. 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 ap- pearance of the HCC. These factors affect Fig. 1.���50-year-old woman with breast cancer. A, Axial CT image shows diffuse fatty infiltration of liver and hypodense lesion (arrow) suspicious for metastasis in segment IV. B and C, 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 dif- fusely fatty infiltrated liver. B A C