Magnetoencephalography and its usefulness in epilepsy surgery

About 10% to 20% of all epileptic patients ultimately suffer from medically intractable epileptic seizures. Epilepsy surgery has been considered as a useful treatment option for these patients. A successful outcome from surgery is a seizure-free state without imposition of neurological deficit. Thus, the goals of presurgical workup include the delineation of the epileptogenic zone and the identification of the nearby brain area with eloquent function. Noninvasive evaluation consists of intensive video-EEG monitoring, magnetic resonance imaging, single photon emission tomography, positron emission tomography, magnetic resonance spectroscopy, and neuropsychological test. For those patients with no convergent localization information, invasive EEG recordings with subdural or depth electrodes are indicated. However, these procedures carry significant risk. Magnetoencephalography (MEG) is a totally noninvasive tool to measure the cerebral magnetic fields generated by intraneuronal currents. The advent of whole-scalp MEG systems facilitates simultaneous measurement of the entire brain activities. MEG has been used to localize the irritative focus and surrounding brain areas with eloquent function, such as sensorimotor cortex. Thus, MEG offers a noninvasive evaluation to help the planning of invasive recordings and surgical treatment. In this article, we will review the basics of MEG and then discuss its applications in the evaluation for epilepsy surgery. Illustrated figures were obtained in our own patients measured with a whole-scalp neuromagnetometer (VectorviewTM, 4-D Neuroimaging).