History: middle aged male with knee pain. 

This is a case of lipohemarthrosis beautifully demonstrated on MRI. Lipohemarthrosis is simply the presence of fat and blood in a joint effusion (see a large knee joint effusion here), and it typically is caused by a fracture at the articular surface of a joint that extends into the bone marrow, allowing fat to leak into the joint space. MRI is the ideal imaging modality to diagnose lipohemarthrosis with its capabilities of identifying fluid with differing signal parameters. Fat will float to the non-dependent portion of the knee, which in the supine patient being imaged in a magnet will be around the patella, as seen on the second two images above. The hemarthrosis portion of the effusion will further layer out into serum and cellular components (somewhat like a centrifuge), giving the fluid-fluid level seen above. For an excellent resource on lipohemarthrosis of all joints, read Dr. Gentili’s site here.