Osteonecrosis of the femoral condyle

MRI of a young woman on prednisone for inflammatory bowel disease who developed knee pain due to osteonecrosis of the femoral condyle without collapse (marrow infarction).
(Top panel) Sagittal proton density MRI of the knee shows a serpentine "double-line sign" (concentric high and low signal at the periphery) (arrow) that represents hypervascular granulation tissue and is pathognomonic of osteonecrosis.
(Bottom panel) Coronal inversion recovery image (a fat saturation MRI sequence) again demonstrates serpentine intramedullary pattern of decreased signal intensity.