Juvenile dermatomyositis is a autoimmune inflammatory conditions involving muscles.
The juvenile form has a tendancy to be more severe and is less associated with an underlying malignancy than the adult form.
Auto-antibodies are relatively specific for the condition -- the specific elevated muscle specific antibodies being anti-RNa and anti-Mi2.
This case has the characteristic findings of diffuse high signal throughout affected muscles on T2/TIRM sequences.