Brodie abscess is a form of subacute osteomyelitis manifesting as a focal lytic lesion that can contain a sequestrum and be surrounded by bone sclerosis. It usually involves long bone metaphysis. Periosteal reaction may be absent.

The "penumbra sign" is a thin layer of vascularized granulation tissue that lines the cavity of the abscess and appears slightly hyperintense in T1-weighted images with strongly and rapidly enhance after contrast.

Case courtesy :  Dr.ssa Laura Braccaioli