Focal fatty sparing of the liver

Focal fatty sparing of the liver is the localized absence of increased intracellular hepatic fat, in a liver otherwise fatty in appearance i.e. diffuse hepatic steatosis. Recognition of this finding is important to prevent the erroneous belief that the region of sparing is itself a mass.

Epidemiology

While diffuse hepatic steatosis is considered common, affecting approximately 25% of the population, focal fatty sparing of the liver is considered a slightly less common pattern across multiple studies ^7-9.

Clinical presentation

Focal fatty sparing is per se an asymptomatic and benign phenomenon. However, the patient may be symptomatic from the abnormally increased fattiness of the remainder of the liver.

Important caveat: areas of focal fat sparing may be found adjacent to metastases (see below).

Pathology

Similar to its inverse pathological counterpart, focal fatty change, regions of focal fatty sparing are thought to have altered perfusion compared to the rest of the liver (also, known as liver third inflow). The cause of this is incompletely understood. In the context of metastases, this may be due to compression/invasion of portal venules by tumor ³.

Radiographic features

Focal fatty sparing typically has a geographic appearance and occurs in characteristic locations ^1,3:

• adjacent to the porta hepatis (segment 4)
• gallbladder fossa
• adjacent to the falciform ligament
• subcapsular parenchyma

Important features, along with location and echogenicity/density/intensity are ²:

• absence of mass effect
• absence of distortion of vessels that run through the region

When it occurs outside of these areas or has a nodular appearance, it may become problematic distinguishing it from a focal liver lesion, especially as regions of focal sparing may be seen around focal liver lesions ^2,3. 

Ultrasound

Liver with generalized steatosis demonstrates increased echogenicity ². The area(s) of focal fatty sparing will lack this increased echogenicity, and the reporter may erroneously believe these areas to be abnormal. 

Contrast-enhanced ultrasound

Focal fatty sparing has a varying appearance in the arterial phase with isoenhancement being most common, while rarely hyperenhancement can also be observed ⁵. In the portal venous and late phase the lesion will never demonstrate washout and should remain isoenhancing compared to the surrounding normal liver ⁶. 

CT

Liver with generalized steatosis demonstrates reduced liver attenuation on both precontrast and portal venous phase imaging. The area(s) of focal fatty sparing will lack this reduced liver attenuation, and the reporter may erroneously believe these areas to be abnormal. 

MRI

Requires both in- and out-of-phase imaging and contrast to adequately assess ¹.

Pseudolesions (focal sparing) are better seen on out-of-phase imaging, but otherwise, appear normal and similar to the rest of the liver on T2 and contrast-enhanced sequences ¹. Hepatobiliary contrast agents such as gadoxetate disodium can show greater delayed uptake and biliary excretion when compared to the fatty liver due to a greater concentration of functioning hepatocytes ⁴.  

The rest of the liver demonstrates:

• T1: hyperintense
• T2: mildly hyperintense
• IP/OP: signal drop out on the out-of-phase sequence

Treatment and prognosis

When focal sparing is idiopathic, and not related to a hepatic focal mass, then the prognosis is that of a patient with diffuse hepatic steatosis.

Differential diagnosis

Possible considerations include:

• hepatocellular carcinoma
• hepatic metastases