Objective: To evaluate whether diffusion mapping could lateralize intractable seizures in mesial temporal lobe epilepsy (MTLE) patients.

Background: Animal seizure models show acute postictal depression of the apparent diffusion coefficient of water (ADCw), interictal normalization, then chronic elevation.

Methods: The hippocampal plane was imaged with five diffusion weightings along each axis. Three orthogonal ADCw maps were averaged to produce an isotropic ADCw map.

Results: In all eight MTLE patients, ADCw was elevated by a mean of 10+/-3% (p<0.01, paired t-test) interictally in the ipsilateral hippocampus, where side of seizure focus was determined electrographically with corroboration by volumetric MRI studies. Measured ADCw values in phantoms and five normal brains agree with published values.

Conclusions: Brain tissue with interictally increased ADCw may represent an epileptogenic region with neuronal loss, gliosis, and expanded extracellular space (hippocampal sclerosis). Thus, diffusion mapping may confirm seizure lateralization.