Pseudotumors

1. Pathogenesis:

• The low-density region on CT emulates a low-attenuation metastasis.
   
• More common on either side of the falciform ligament, in the vicinity of the gallblader fossa, and in the posterior aspect of segment IV (left lobe).
   
• Associated with obesity, alcohol abuse, steroids, and hyperalimentation.

2. Radiographic findings:

• U/S: bandlike geographic areas of increased echogenicity in a lobar/segmental distribution (arrows).

• CT: no mass effect on adjacent vessels, patchy areas of decreased attenuation.
  
  
• MRI: can use "chemical shift" or "in-phase and out-of-phase" gradient-echo imaging to detect focal fat variation.  This method is both highly sensitive and specific in detecting fat and water in the same image voxel. On in-phase image, fat and water combine their signals.  On out-of-phase image, fat and water signals cancel out, thus resulting in reduced relative signal when both are present.

Fatty sparing

1. Pathogenesis:

• A normal liver looks hyperdense to a fatty infiltrated liver on noncontrast CT. In a fatty infiltrated liver, there are areas that are spared from the infiltration process, therefore, appearing "normal" or "hyperdense" to the surrounding fatty liver. These hyperdense areas can create an appearance of pseudomasses.
   
• Characteristic locations for sparing include the periportal regions, caudate lobe, and adjacent to the gallbladder fossa.

2. Radiographic findings:

• The lesion is hyperdense to the surrounding liver parenchyma.  The lesion is actually normal, non-infiltrated liver tissue.
  
  
• MRI: can use "chemical shift" or "in-phase and out-of-phase" gradient-echo imaging to detect focal fat variation.  This method is both highly sensitive and specific in detecting fat and water in the same image voxel. On in-phase image, fat and water combine their signals.  On out-of-phase image, fat and water signals cancel out (arrows), resulting in reduced relative signal when both are present. The image below is an example of chemial fat saturation.

  

Atypical regenerative nodules

1. Pathogenesis:  

• In chronic cirrhosis, some regenerative nodules may resemble masses (i.e. hepatocellular carcinoma) on US and MRI.
  
  

2. Radiologic Findings:

• US:  lesions may appear well-defined, homogeneous, and hypoechogenic.
  
  

• Biphasic CT:  lesions are often not visualized. 
  
  

• MRI:  they are often of increased signal on T1 weighting (due to accumulation of fat or glycogen) and reduced signal on T2-weighted images (due to accumulation of iron). 

NOTE:  malignant foci can develop within these nodules.  On T2-weighted MRI, these foci appear as high signals within the low signal nodules. Therefore, if there is heterogeneity within nodules, malignancy should be suspected.