Hylan G-F 20 (Synvisc) is an FDA-approved hyaluronate derivative which is administered by injection into osteoarthritic joints for the treatment of osteoarthritis. Preclinical investigations have demonstrated that Hylan G-F 20 can favorably affect chondrocyte metabolism and recent clinical studies [Ann Rheum Dis. 2004 May;63(5):478-82; Osteoarthritis Cartilage. 2005 Mar;13(3):216-24] provide support for a disease-modifying effect of Hylan G-F 20 therapy. This study will utilize dGEMRIC imaging of patients with knee OA to assess the effects of Hylan G-F 20 therapy on cartilage in vivo over time. Delayed Gadolinium Enhanced MRI of the Cartilage (dGEMRIC) permits a quantitative assessment of GAG distribution within intact cartilage in vivo. Since dGEMRIC is noninvasive, the technique may be useful to non-invasively monitor the influence of various treatments on hyaline cartilage.