Abstracts

Rationale: The most important aspect of successful epilepsy surgery is the accurate and complete identification of the ictal onset region. Research has shown that one of the best ways to define the extent of the ictal onset region is to use stereotactic placement of depth electrodes (sEEG). The disadvantage to the use of sEEG is that the contacts have limited sensitivity beyond the immediate area they are implanted which could lead to being unable to localize seizure onset. The present study investigates the combination of sEEG with whole head MEG as a tool to aid epileptologists in the identification of the ictal region.Methods: Three patients who underwent sEEG monitoring in the EMU at Dell Children’s Medical Center were selected to participate. The age range was from 14 months to 15 years of age. A stereotactic frame was attached to the patient and a T1-wieghted MRI was done to get stereotactic coordinates for depth electrode placement. Following implantation of the sEEG electrodes, a CT scan was done to identify the locations of each electrode contact. The CT was coregistered with the patient’s structural MRI and the three dimension coordinates of the electrode placements were identified. A three dimensional model of the cortex was generated with an automated algorithm to identify grey matter. A high resolution boundary element model (BEM) was generated from the patient’s MRI for dipole localization of electromagnetic cortical fields. For each patient, three or four depth electrodes were placed with 12 to 38 contacts per patient. The sEEG was digitized at 1000 Hz and filtered from 1 to 35 Hz. The MEG scan was done on a 306 channel magnetometer/gradiometer system (Electa Neuromag Triux). The cortical electromagnetic brain activity was digitized at 1000 Hz with a filter setting of .03 to 330 Hz. The acquired signal was processed using temporospatial source separation (MaxFilter) methods for noise reduction. The filtered data was reviewed by an epileptologist using the sEEG and MEG waveforms and source estimates of epileptiform activity were projected onto the 3D model of the brain. An equivalent current dipole source estimate of interictal epileptiform discharges was computed and projected onto the patient’s MRI and the 3D model generated from the MRI.Results: There were no seizures identified during the MEG scans. Interictal discharges were clearly easier to identify in the sEEG, relative to the MEG. When the discharges were seen in the MEG, as well as the sEEG, it was possible to localize the discharges. In these cases, the discharge localization estimates correlated with the sEEG localization and the region of the surgical site.Conclusions: This study shows that it is possible to combine sEEG and MEG. There were no adverse events related to getting a MEG scan after the implantation of the depth electrodes. Additionally, the MEG was able to localize interictal discharges seen in the depth electrodes and the source estimates correlated with the surgical decision.