Imaging of cortical spreading depression by EIT: implications for localization of epileptic foci

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Abstract
Severe epileptics may require curative neurosurgery. Sometimes focus localization requires recording with electrodes inserted deep into the brain, which may cause death or permanent neurological damage. Since epileptic seizures are associated with marked changes in cerebral impedance, the authors propose that electric impedance tomography (EIT) with sub-dural electrodes (inserted between the brain and skull) could provide a superior and less dangerous method for the localization of epileptic foci. The purpose of the experiments reported here was to determine whether EIT could be used to localize the origin of seizure activity. In terms of impedance characteristics, an appropriate model is cortical spreading depression in the animal brain. Six rabbits were anaesthetized and paralysed and the brain exposed. EIT images and DC potentials were recorded from an array of 16 electrodes on the brain during cortical spreading depression induced by a DC stimulus. Cortical spreading depression could be localized by EIT with an accuracy of 8.7%+or-6.4% (mean +or-SD) of electrode array diameter. The errors in localization appeared to be distributed randomly. In a phantom of similar geometry, the error was 5% after correction for a systematic component. Results are sufficiently encouraging that the authors intend to extend this study to human patients.

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