Magnetoencephalography (MEG) is a relatively new neurophysiological technique that allows the measurement of the brain's weak magnetic fields. MEG localization of the irritative zone showed excellent agreement with invasive electrical recordings could clarify the spatial relationship between the irritative zone and structural lesions, and finally attribute epileptic activity to lobar subcompartments in temporal lobe and extratemporal epilepsies. In temporal lobe epilepsy, MEG is helpful to differentiate between patients with mesial, lateral, and diffuse seizure onsets. In extratemporal epilepsy, MEG provides unique, and often decisive information in nonlesional cases and patients with large lesions, where it helps to guide the placement of subdural grid electrodes. Furthermore, MEG can clarify the spatial relationship between epileptic tissue and eloquent cortex. MEG also contributes to the clinical decision process in patients with various special epileptic conditions, including brain tumors, vascular malformations, cortical dysplasias, tuberous sclerosis, Landau–Kleffner syndrome, malignant rolandic–sylvian epilepsy syndrome, and west syndrome. MEG proved to be extremely useful for the reevaluation of patients with persistent seizures after lesionectomy or prior epilepsy surgery, because MEG signals are less distorted than electroencephalography (EEG) signals by prior craniotomies. MEG seems to be more sensitive than scalp EEG for the detection of epileptic discharges arising from lateral neocortex as 3.5-4 cm2 of synchronized activity seem to be sufficient to produce a detectable MEG signal as opposed to the 6-10 cm2 proposed for the scalp EEG.
Hamer, H. M., & Katsarou, N. (2003). Noninvasive EEG in the definition of the irritative zone. Handbook of Clinical Neurophysiology. https://doi.org/10.1016/S1567-4231(03)03002-8