Gradient magnetic-oscillation topography depicts high-frequency oscillations on the brain surface related to the epileptic focus in intractable epilepsy with lesions

This study aimed to evaluate high-frequency oscillations (HFOs), a potential biomarker of epileptic activity, using magnetoencephalography (MEG). We developed a new spatial filter, termed “gradient magnetic-oscillation topography” (GMOT), to visualize color-coded topographies on the brain surface using a spectrogram calculated at each sensor level. We enrolled 15 patients with lesions and intractable neocortical epilepsy who underwent focal resection surgery and preoperative MEG. We compared GMOT findings with those of the equivalent current dipole (ECD) and resected area based on intracranial video-electroencephalography (IVEEG) or intraoperative electrocorticography (ECoG) with respect to seizure outcomes. We also calculated the proportion of the power of each sensor relative to the entire head at each frequency band. GMOT successfully visualized the high-power gradient magnetic field at the fast ripple band (FR band, 201–330 Hz) and detected the highest power at the FR band around the lesion in 13 of the 15 patients. The resected area included both clustered ECDs and the highest power in the FR band on GMOT in 9 of the 15 patients. The resected area had a statistically higher proportion of power as the frequency band increased in the HFO group (p < 0.01). We visualized high power in the FR band of the brain surface. The areas of highest power in the FR band were correlated with epileptic focus based on IVEEG and intraoperative ECoG.