Presentation
Painful swelling of the left knee with tenderness and fever.
Patient Data
There is a lobulated lytic lesion of the tibial metaphysis, oriented along the long axis of the tibia with a thin surrounding rim of reactive sclerosis.
The MRI sequences demonstrate a well-defined lobulated intramedullary cystic lesion of low signal on T1, high signal on PD fat sat/T2 with restricted diffusion centrally, and high signal T1 and low signal PD fat sat/T2 with enhancement peripherally "penumbra sign". Surrounding bone marrow edema extending to the epiphysis and proximal diaphysis as well as to the surrounding soft tissues.
Normal appearance of the menisci, cruciate and collateral ligaments.
No pathological joint effusion.
Case Discussion
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.