Abstracts

Rationale: MEG can be helpful in the presurgical localization of epileptogenic zones. However, candidates for epilepsy surgery may have had prior placement of a vagal nerve stimulator (VNS). The presence of a VNS causes very high magnetic noise in the MEG signals, obscuring spontaneous magnetic brain activity. Beamformer analysis can diminish both nearby and distant magnetic noise sources, and may make MEG analysis possible in patients with a VNS.Methods: We studied 4 children and adults who were presurgical epilepsy candidates and who each had prior VNS placement. We recorded 275 channels MEG and 23 channels simultaneous scalp EEG in standard 10-20 placement. We examined raw signals of MEG, simultaneous bipolar EEG, and MEG processed with a beamformer analysis, synthetic aperture magnetometry (SAM), at 323K intracranial virtual sensor locations. We evaluated a statistical measure of increased kurtosis, SAMg2 that reflects increased spikiness at each virtual sensor location. We compared location, morphology, and timing of epileptiform discharges seen in the EEG signals with spikes detected in the virtual sensors by SAMg2 (VS-spikes). Results: The patients had 13 to 90 epileptiform discharges recorded and 25 to 193 VS-spikes recorded. Peak SAMg2 VS-spikes were multiple locations in same lobe as EEG and had shorter duration. VS-spikes were detected just prior to and during discharges detected by EEG. Conclusions: Beamformer analysis allows analysis and localization of MEG signals obscured by VNS.