Acute disseminated encephalomyelitis (ADEM), as the name would suggest, is featured by a monophasic acute inflammation and demyelination of white matter typically following a recent (1-2 weeks prior) viral infection or vaccination^1-3.

MRI is far more sensitive than CT and demonstrates lesions characteristic of demyelination: 

• T2: regions of high signal, with surrounding edema, typically situated in subcortical locations; the thalami and brainstem can also be involved
• T1 C+ (Gd): punctate, ring or arc enhancement (open ring sign) is often demonstrated along the leading edge of inflammation; absence of enhancement does not exclude the diagnosis
• DWI: there can be peripherally restricted diffusion; the center of the lesion, although high on T2 and low on T1, does not have increased restriction on DWI (cf. cerebral abscess), nor does it demonstrate absent signal on DWI as one would expect from a cyst; this is due to increase in extracellular water in the region of demyelination