History: 55 year old male with knee pain.
This is a classic example of bone infarcts, here called osteonecrosis of the knee, which on radiographs is represented as a well defined sclerotic lesion with serpentine margins. In our example, the articular surfaces are mildly distorted, more so on the tibial plateau, causing collapse.
On MRI, bone infarcts in the acute phase are dark on T1 weighted images and bright on T2 weighted images, representing edema. The double line sign is diagnostic of bone infarcts, which is seen on T2 weighted images as a high-signal-intensity line within a parallel rim of decreased signal intensity. Read more about the double line sign here.
The differential diagnosis here includes spontaneous osteonecrosis of the knee, which usually involves the medial femoral condyle and middle age and older females, and regional migratory osteoporosis, which you can read more about here.