MRI features of a Brodie abscess of the tibial metaphysis with a "penumbra sign".

The penumbra sign is seen as a rim of vascularized granulation tissue surrounding a bone abscess cavity. It elicits a higher signal intensity than the cavity itself on T1, hypointense on T2 with enhancement on postcontrast sequences. The penumbra sign can help distinguish acute osteomyelitis from other benign or malignant lesions.