 Retrospective magnetic resonance imaging MRI studies in the chronic stages of epilepsy have shown that patients with PR are more likely to have focal structural lesions in the brain, and in particular to have signs of damage to the hippocampi. For example, there are retrospective data suggesting that a decreased hippocampal N-acetylaspartate NAAcreatine ratio measured by magnetic resonance spectroscopy MRS and hippocampal atrophy determined by hippocampal volumetry correlate with PR. However, it is not clear whether these findings reflect the underlying pathophysiology of PR, or simply reflect the effects of chronic seizures and chronic drug treatment on the brain. 