Source localization using event related beamformer of magnetoencephalography for interictal spikes in pediatric neocortical epilepsy

Purpose: We studied the relationship between source localization of interictal discharges using an event-related beamformer (ERB), equivalent current dipole analysis and the ictal onset zone on intracranial video EEG (IVEEG). Methods: We acquired interictal MEG data using a whole-head 151channel gradiometer system in 35 children with intractable neocortical epilepsy. We visually identified the earliest peak of each spike, by examining the raw MEG recordings using a band pass filter of 15-70 Hz. We used a spatiotemporal beamforming method to estimate the spatial distribution of source power in individual interictal spikes over the whole brain. The resulting volumetric source power images of individual spikes were then averaged and the results displayed on the patient´s MRI. We compared the results to the localization obtained using the equivalent current dipole model and to the ictal onset zones on IVEEG. Results: Thirty-one patients had a single MEG dipole cluster. Four patients had more than 2 MEG spike source (MEGSS) clusters. Twenty-three patients showed focal ERB, including a single focal ERB 16 (46%) patients, and more than 2 ERBs in 7 (20%) patients. The Euclidean distance between the centroid of the MEGSS cluster and the ERB was less than 2 cm in 27 (77%) patients. The distance between the centroid of the MEGSS cluster and ERB was less than 2 cm in 21/23 patients with focal ERB (mean, 1.19cm; SD, 0.65cm ). ERB was localized within the ictal onset zone gyral in 24 (69 %) patients, regional concordance was seen in 8 (23 %) patients and discordant in 3 (8 %) patients. A focal ERB was associated with concordant seizure localization (p=0.02) while all three patients with discordant IVEEG and ERB activation had multiple ERB foci. Maximum ERB was included in the resection margin in 28 (80 %) patients. In 23 patients with focal ERB, the ERB area was included in the resection margin in 22 (95 %) patients. A favorable surgical outcome was obtained in 17 (74 %) with a- focal ERB. Conclusion: ERB for interictal MEG spikes highly corresponded to the center of the MEGSS clusters and ictal onset zones localized on IVEEG. The frequency analysis of interictal MEG spikes may correlate with a subset of the epileptogenic high frequency oscillations.