Muscle Fat Infiltration

Muscle fat infiltration is the presence or accumulation of adipose tissue or lipid within skeletal muscle regions. On imaging, the term can refer to fat located between muscle groups, within a muscle between fascicles, or, depending on the method, lipid associated more closely with muscle fibers. Because studies use different anatomical definitions, the exact compartment should be specified when possible .¹

Magnetic resonance imaging and computed tomography can assess muscle fat infiltration, while magnetic resonance spectroscopy can characterize some intramuscular lipid pools more specifically. Quantitative MRI methods such as Dixon techniques can estimate the fraction of signal attributable to fat. These approaches describe tissue composition rather than merely the amount or cross-sectional area of muscle .¹

Muscle fat infiltration overlaps with the term myosteatosis, but the two are not always used identically. Myosteatosis is often a broader label for abnormal fat accumulation in skeletal muscle and may be inferred from reduced muscle attenuation on CT or increased fat fraction on MRI. Since terminology and measurement thresholds vary, reported values should be interpreted according to the imaging method and the fat compartment being measured .¹