ADEM

This young woman presented with progressive left hemiparesis after having had a viral infection a week earlier. This MRI demonstrates bilateral asymmetric lesions with open ring enhancement  characteristic of demyelination. Note that restricted diffusion is not seen centrally (usually seen in cerebral abscesses but at the advancing rim of demyelination. She was treated with steroids, and rapidly improved.

Followup three weeks later demonstrate the initially larger lesions to have reduced in size and others, which were small on the original scan to have progressed, but no new lesions having appeared.

Findings are consistent with ADEM.  Although monophasic (compared to multiple sclerosis)  not all lesions mature at the same rate within the illness.