Diagnosis: PVNS, Focal Pigmented Villonodular Synovitis
Findings:T1 weighted, T2 weighted and proton density images demonstrate focal hypointense mass in the anterior knee joint.
Discussion:Hemosiderin in this lesion accounts for its hypointense appearance on MRI sequences. The lesion is locally aggressive and may result in erosions. Diffuse and focal variants of the lesion are recognized. In the diffuse form, the entire synovium of a large joint is involved. Focal form can present as a joint lesion or as a tendon mass, where it is usually called giant cell tumor of tendon sheath. There is no either sex predilection, and knee joint is the most commonly involved site.