Analysis of ictal magnetoencephalography using gradient magnetic-field topography (GMFT) in patients with neocortical epilepsy

•Gradient magnetic-field topography (GMFT) is useful for ictal magnetoencephalography (MEG).•GMFT delineates the area of ictal MEG onset (ictal GMFT) at the gyral-unit level.•The ictal GMFT area is concordant with the ictal onset zone of intracranial electroencephalography.

ObjectiveWe aimed to validate the usefulness of gradient magnetic-field topography (GMFT) for analysis of ictal magnetoencephalography (MEG) in patients with neocortical epilepsy.MethodsWe identified 13 patients presenting with an ictal event during preoperative MEG. We applied equivalent current dipole (ECD) estimation and GMFT to detect and localize the ictal MEG onset, and compared these methods with the ictal onset zone (IOZ) derived from chronic intracranial electroencephalography. The surgical resection areas and outcomes were also evaluated.ResultsGMFT detected and localized the ictal MEG onset in all patients, whereas ECD estimation showed localized ECDs in only 2. The delineation of GMFT was concordant with the IOZ at the gyral-unit level in 10 of 12 patients (83.3%). The detectability and precision of delineation of ictal MEG activity by GMFT were significantly superior to those of ECD (p < 0.05 and p < 0.01, respectively). Complete resection of the IOZ in the concordant group provided seizure freedom in 3 patients, whereas seizures remained in 9 patients who had incomplete resections.ConclusionsBecause of its higher spatial resolution, GMFT of ictal MEG is superior to conventional ECD estimation in patients with neocortical epilepsy.SignificanceIctal MEG study is a useful tool to estimate the seizure onset in patients with neocortical epilepsy.