Focal Fat Sparing

Figure 1: US image of the liver shows focal masslike area of hypoechogenicity of the left lobe posterior to the left portal vein branch. Note high echogenicity of the background liver, suggesting fatty change.

Figure 2 & 3: In-phase and out-of-phase MR images show liver signal intensity drop in the chemical shift imaging confirming diffuse fatty liver. The abnormality in the left lobe liver does not change between the two phases, suggesting a focal area of fat sparing.

Facts: Fatty Liver

• Most common abnormality of the liver seen on cross-sectional imaging
• Common patterns: diffuse fat accumulation, diffuse fat accumulation with focal sparing, and focal fat accumulation in an otherwise normal liver
• Unusual patterns may mimick neoplasm, inflammation or vascular conditions
• Pathology: triglyceride acculation within cytoplasm of hepatocytes
• Term "fatty liver" is preferred over "fatty infiltration of the liver" because triglyceride accumulation occurs within hepatocytes but rarely other cell types. Infiltration of fat into parenchymal does not occur

Imaging Findings and Sensitivity/Specificity

• US: 1) Liver echo greater than renal cortex and spleen with attenuation of sound wave, 2) loss of definition of diaphragm, 3) poor delineation of intrahepatic architecture (to avoid false-positive diagnosis, all three findings should be fulfilled).  Sensitivity 60-100%. Specificity 77-95%.
• CT: Liver attenuation 10 HU less than that of spleen, or less than 40 HU. Sensitivity 43-95%. Specificity 90%.
• MRI: Signal intensity loss on opposed-phase images in comparison with in-phase images. Sensitivity 81%. Specificity 100%.

Patterns

1. Diffuse deposition: most common
2. Focal deposition and focal sparing: characteristically in specific areas (adjacent to falciform ligament or ligamentum venosum, porta hepatis, in GB fossa). Suggestive findings of fatty pseudolesions rather than true masses are:
1. Fat content
2. Characteristic location
3. Absence of mass effect on vessels and other liver structures
4. Geographic configuration (not round or oval)
5. Poorly delineated margin
6. Contrast enhancement similar to or less than that or normal liver parenchyma
3. Multifocal deposition
4. Perivascular deposition
5. Subcapsular deposition

Reference:


Hamer OW, Aguirre DA, Casola G, et al. Fatty liver: imaging patterns and pitfalls. Radiographics 2006;26: 1637-1653.