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Background and Purpose. Rasmussen's encephalitis (RE) is a rare condition of unknown cause characterized by intractable seizures, progressive hemiparesis, mental impairment, and inflammatory histological findings in the cortex. The primary diagnosis is based on biopsy to confirm the typical clinical, electroencephalography, and brain imaging findings. The main objective of this study was to compare simultaneous structural and functional neuroimages in RE. Methods. Concomitant magnetic resonance imaging and 2-deoxy-2-[18F]-fluoro-D-glucose positron emission tomography data from the authors' series of 5 children and 8 patients described in the literature were analyzed and compared. Results. Typical early findings of RE include metabolic abnormalities (hypermetabolism and hypometabolism) starting in the frontal or temporal regions or, occasionally, involving the whole hemisphere. Focal abnormalities of cerebral glucose metabolism indicate lesions sooner and depict their extent better than concomitant magnetic resonance images. The major structural abnormality remains unilateral; however, contralateral frontal lobe hypometabolism or crossed cerebellar diaschisis can be a finding of this disease. Basal ganglia involvement and whole hemispheric metabolic abnormality appear typically only after several months of disease onset. Conclusion. Concomitant structural and functional neuroimaging provide possibly complementary information in the early noninvasive workup of RE and may facilitate the early diagnosis of this rare disorder. [ABSTRACT FROM AUTHOR]