Role of multichannel magnetoencephalography in the evaluation of ablative seizure surgery candidates

Magnetic source imaging (MSI) was performed on 30 ablative epilepsy surgery candidates. The technique involved high resolution multiplaner MRI images with lipid fiducials attached to the patient’s head to define a head-based 3D coordinate space. Magnetoencephalography (MEG) was performed after digitizing the same fiducial points. A 37-channel magnetometer obtained data at two to five sites over each hemisphere. MEG epileptiform data were obtained with and without EEG triggering. A single equivalent current dipole model was used to determine orientation and location of a dipole generator whose surface isocontour map most closely fits the measured dipolar data for each event. The MEG data were then transformed to MRI images for source localization. In five of seven cases with ictal anterior temporal lobe foci, MSI data were localized to the same temporal lobe but did not provide additional spatial data. In 10 of 11 cases with convexity foci, MSI provided additional spatial localizing data. MSI did not verify depth electrode localization in one anterior temporal-orbital frontal and three orbitofrontal cases. In seven of eight cases in which depth EEG recordings were nonlocalizing, MSI provided insufficient localizing data. MSI appears to provide additional spatial localizing data in most cases with a convexity epileptic focus.