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Definition:
- Relative paucity of fat in solid mass compared to that of background reference tissue (e.g. fatty liver).
Usage:
- Applies, in fatty livers, to solid masses that unequivocally contain less fat than background liver.
- Does not apply to cysts, hemangiomas, confluent fibrosis
- In patients at risk for HCC, fat sparing in a solid mass is an ancillary feature favoring malignancy.
- Radiologists at their discretion may apply lesional fat sparing to upgrade category of solid masses (up to LR-4)
- MRI is more sensitive and specific for detection of lesional fat sparing than CT
- At MRI, a mass may be characterized as having lesional fat sparing if:
- the liver shows signal loss on out-of-phase (OP) compared to in-phase (IP) gradient echo images or on fat-suppressed compared to non-fat-supressed images (i.e., the liver is fatty) AND
- compared to background liver, the mass shows less signal loss on OP compared to IP gradient echo images or on fat-suppressed compared to non-fat-suppressed images (i.e. the mass has lower fractional fat content than liver).
- At CT, a mass may be characterized as having lesional fat sparing if:
- the liver attenuation measures ≤ 40 Hounsfield units (HU) (on unenhanced images or enhanced images) or ≥ 10 HU less than that of spleen (on enhanced images) (i.e., the liver is fatty) AND
- the mass has greater attenuation than the liver (i.e., the mass has lower fractional fat content than liver).
- Do not characterize as lesional fat sparing
Potential pitfalls and challenges:
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