Split fat sign | Radiology Signs

Answer:

The split fat sign is a thin rim of preserved fat surrounding a fusiform soft tissue mass, seen as high T1 signal on MRI or as an echogenic border on high-resolution ultrasound, indicating that the mass lies within the intermuscular neurovascular bundle. It is one of the classic features of a peripheral nerve sheath tumour, alongside the entering and exiting nerve sign, the fascicular sign of multiple small ring-like structures within the lesion, and the target sign of a low-signal centre with a high-signal rim on T2-weighted images. Together these establish that the lesion is neurogenic, which is often more useful than trying to distinguish schwannoma from neurofibroma; a schwannoma is typically eccentric to the nerve and can be shelled out, while a neurofibroma is fusiform and intimately involves the fascicles. The critical question is malignant transformation, which should be suspected with rapid growth, size above about 5 cm, ill-defined margins, heterogeneous signal with necrosis, perilesional oedema, loss of the target sign, high uptake on FDG PET and low apparent diffusion coefficient values; in neurofibromatosis type 1 this must be actively excluded. The sign is not unique to nerve sheath tumours and can be produced by any slow-growing mass within the intermuscular fat plane, including a lipoma or a vascular malformation.

Why is it called so?

The mass appears to split the intermuscular fat, leaving a thin layer of fat capping each end of the lesion.

Pathophysiology

Peripheral nerves travel with arteries and veins in the intermuscular fat plane, surrounded by loose areolar tissue. A tumour arising from the nerve sheath expands slowly and concentrically within that plane, so instead of infiltrating and destroying the surrounding fat it displaces and compresses it, leaving a residual rim, most conspicuous at the proximal and distal poles of the fusiform mass where the fat is pushed ahead of the growing lesion. Aggressive tumours that invade rather than displace obliterate this fat, which is why loss of the sign in a previously stable lesion is one of the features that raises concern for malignant peripheral nerve sheath tumour.

Alternative names: Split fat plane sign

Other associated named signs: Target sign, the fascicular sign and the entering and exiting nerve sign of neurogenic tumours

References

1. Yun JS, Lee MH, Lee SM, Lee JS, Kim HJ, Lee SJ, et al. Peripheral nerve sheath tumor: differentiation of malignant from benign tumors with conventional and diffusion-weighted MRI. Eur Radiol. 2021;31(3):1548-1557. PMID: 32894357.