Focal fatty sparing of the liver is the localised 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.
To be added
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).
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. The cause of this is incompletely understood. In the context of metastases, this may be due to compression/invasion of portal venules by tumour 3.
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.
Liver with generalised 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.
Liver with generalised 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.
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 Eovist 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.
Possible considererations include
- 1. Tom WW, Yeh BM, Cheng JC et-al. Hepatic pseudotumor due to nodular fatty sparing: the diagnostic role of opposed-phase MRI. AJR Am J Roentgenol. 2004;183 (3): 721-4. AJR Am J Roentgenol (full text) - Pubmed citation
- 2. Jamal MH, Rayment JH, Meguerditchian A et-al. Impact of the Sentinel Node Frozen Section Result on the Probability of Additional Nodal Metastases as Predicted by the MSKCC Nomogram in Breast Cancer. 2010;doi:10.1093/jjco/hyq225 - Pubmed citation
- 3. Chung JJ, Kim MJ, Kim JH et-al. Fat sparing of surrounding liver from metastasis in patients with fatty liver: MR imaging with histopathologic correlation. AJR Am J Roentgenol. 2003;180 (5): 1347-50. AJR Am J Roentgenol (full text) - Pubmed citation
- 4. Yoo KD, Jun DW. [Focal fatty sparing of the liver]. (2014) The Korean journal of gastroenterology = Taehan Sohwagi Hakhoe chi. 63 (6): 382-5. Pubmed