The concentric ring sign is a pathognomonic sign for a subacute haematoma on MRI.
A subacute haematoma may show 3 characteristic layers of signal intensity:
- a thin peripheral rim of low signal intensity on all pulse sequences corresponding to haemosiderin.
- an inner peripheral high-signal intensity zone on T1-weighted imaging due to methaemoglobin.
- the central component of the haematoma has variable signal intensity based on the amounts of seroma, deoxyhaemoglobin, and methaemoglobin present.
This appearance of the periphery of a fluid collection on T1-weighted imaging, known as the concentric ring sign, which is pathognomonic for a subacute haematoma.