Focal fatty sparing of the liver

Focal fatty sparing of the liver is athe localised absence of fatty changeincreased intracellular hepatic fat, in a liver otherwise, affects with fatty change (in appearance i.e. diffuse hepatic steatosis). Recognition of this finding is usefulimportant to prevent falsely thinkingthe erroneous belief that the region of sparing is itself a mass.

Epidemiology

To be added

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 infiltrationchange, regions of focal fatty sparing are thought to have differentaltered perfusion characteristics thancompared to the rest of the liver. The cause of this is incompletely understood but is thought to result again from alterations in perfusion. In the context of metastases, eitherthis may be due to compression or invasions/invasion of portal venules by the tumour ³.

Radiographic features

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

• adjacent to the porta hepatis (segment IV)
• 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 fatty changegeneralised steatosis demonstrates increased echogenicity and attenuation ². The echogenic wallsarea(s) of the portal veinsfocal fatty sparing will lack this increased echogenicity, and hepatic veins are lost, duethe reporter may erroneously believe these areas to the increased liver attenuationbe abnormal.

CT

Affected regions demonstrateLiver with generalised steatosis demonstrates reduced liver attenuation on both precontrast and portal venous phase imaging. It is important The area(s) of focal fatty sparing will lack this reduced liver attenuation, and the reporter may erroneously believe these areas to remember that the spleen enhances earlier than the liver (due to systemic arterial supply rather than predominantly portal) and thus arterial phase or very early portal venous phase scans should not be usedabnormal.

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. 

The rest of the liver demonstrates:

• T1: hyperintense
• T2: mildly hyperintense
• IP/OP: signal drop out inon 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 considererations include

• hepatocellular carcinoma (HCC)
• hepatic metastases

See also

• hepatic pseudolesion near the falciform ligament