Figure 4. Staphylococcus aureus septic arthritis of the left hip. The patient presented with left hip pain and fever. The initial film (a) was normal, with no evidence of effusion. 6 weeks later (b) there is periarticular osteoporosis and cortical destruction of the femoral head in keeping with septic arthritis. (c,d) MRI confirms septic arthritis with a tense joint effusion in the left hip and thickened synovium with T₂ weighted hyperintensity on these STIR images (straight arrows). There is also bone marrow oedema in the roof of the acetabulum (curved arrow) and femoral head. (e,f) The same sections on T₁ weighted post gadolinium images show intense enhancement of the synovium. There is also increased signal from the marrow of the acetabulum, suggesting early ostemyelitis.