Serial magnetic resonance imaging findings of Rasmussen's encephalitis--case report.

A 12-year-old boy presented with Rasmussen's encephalitis. Serial magnetic resonance (MR) imaging one year after the onset of seizure showed no abnormality. T2-weighted imaging detected a high intensity lesion in the left frontal cortex when the frequency of seizures increased 11 months later. MR imaging showed the lesion rapidly spread into the white matter, then gradually regressed after biopsy and immunoglobulin therapy, and mild focal atrophy of the left frontal lobe in spite of recurrence of seizures 3 months after the therapy. Histological examination of the biopsy sample showed the characteristic findings of Rasmussen's encephalitis. Five months after the biopsy, another new high intensity lesion was detected next to the previous one, although his seizures were well controlled at that time. Although the extent of the high intensity lesion did not correlate with the frequency of seizures, such sequential changes in MR imaging appearance seemed to reflect the course of this disease.