Do you know the "penumbra sign"? Can you describe it?
The penumbra sign represents a rim of vascularized granulation tissue around a bone abscess cavity that has a higher T1 signal intensity than the cavity itself.
An irregular yet well defined intra-medullary non-expanding lesion of abnormal signal intensity is seen involving the meta-diaphysis of the lower tibia. It has a small projection heading towards the growth plate (arrows).
The lesion shows the classic penumbra sign.