Focal fatty infiltration of the liver can have three different patterns: geographic pattern, focal fatty infiltration, or generalized diffuse involvement. 
The geographic pattern is characterized by a sharp interface between the normal and fatty liver that is not in a lobar distribution. In addition, the vessels are not displaced and there is no mass effect. A tumor generally does not have a sharp border with normal liver tissue. 
Focal fatty infiltration may mimic a malignancy; however, there is no mass effect and no displacement of vessels. The margin may be either sharply or poorly defined. It may be wedge shaped, segmental, patchy, or nodular. A tumor usually has an infiltrative pattern and has mass effect on the vessels. Focal fatty infiltration can change rapidly with either resolution or improvement seen on CT. [2,3] Focal fatty infiltration commonly involves the anteromedial segment of the left lobe of the liver. 
The generalized pattern is identified in patients with systemic disease. It can be seen in patients with diabetes or alcoholism, those who are pregnant, and those who have had steroid, chemotherapy, or hyperalimentation administration as well as other etiologies.  Focal sparing of the liver can also occur where there is minimal normal liver in a generalized pattern of fatty infiltration. Common locations for focal sparing occur anterior to the right portal vein, around the gallbladder, in the medial segment of the left lobe of the liver, in a subcapsular location,  and adjacent to interlobar fissures.  Focal sparing can also have a "finger-like" appearance and can resemble metastatic disease.
On ultrasound, focal fatty lesions often present as hyperechoic areas with "angulated, geo-metric, or finger-like boundaries between normal and fatty tissue."  Metastases are often round. It is often difficult to delineate focal normal tissue in a diffusely fatty liver.
On CT scans, focal fatty areas are of low attenuation on unenhanced CT. The area will enhance after contrast but will remain lower density than normal liver. Usually the areas are sharply delineated with geometric or straight borders. In the patchy form, illdefined borders can be seen. Post-contrast scans show minimal enhancement of the fatty areas. [2,3]
On magnetic resonance imaging (MRI), differentiation of focal fatty infiltration on routine spin-echo sequence is difficult. Phase contrast MRI is sensitive in detecting focal fatty infiltration.  Chemical shift imaging allows differentiation between areas of normal liver and fatty change. When "opposed phase" or "out of phase" sequences are used, focal fat is decreased relative to normal tissue and thus easily distinguished. 
While focal fatty infiltration may be difficult to differentiate from metastatic disease in a patient with malignancy, its typical location and imaging characteristics can be useful in distinguishing between the two entities.
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Verhille R, Marchal G, Baert AL, et al. Focal fatty infiltration of the liver associated with important mass effect. J Belge Radiol. 1994;77:10-12.
Debaere C, Rigauts H, Laukens P. Transient focal fatty liver infiltration mimicking liver metastasis. J Belge Radiol. 1998, 81:174-175.
Wang SS, Chiang JH, Tsair YT, et al. Focal hepatic fatty infiltration as a cause of pseudotumors: ultrasonographic patterns and clinical differentiation. J Clin Ultra-sound. l990;18:401-409.
Gariballa SE,Woods DJ, Davidson AR. Case report: focal fatty liver infiltration causing mass effect. Clin Radiol. 1997;52:630-632.
Hirohashi S, Ueda K, Uchida H, et al. Nondiffuse fatty change of the liver: discerning pseudotumor on MR images enhanced with ferumoxides: initial observation. Radiology. 2000;217:415-420.
Marla R. Hersh, From the Department of Radiology at the H. Lee Moffitt Cancer Center & Research Institute, Tampa, Fla.
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