8. Source localization of the seizure onset zone from ictal EEG and MEG data

Introduction Surgical treatment of drug-resistant epilepsy relies on the identification of the seizure onset zone (SOZ) and often requires intracranial EEG (iEEG). We have developed a new approach for non-invasive electric and magnetic source imaging of the SOZ from ictal electroencephalography (EEG) and magnetoencephalography (MEG) recordings, using the wavelet-based Maximum Entropy on the Mean (wMEM) method. Taking as reference the clinical localization of the SOZ defined on the basis of iEEG or lesion topography and considering seizures recorded during simultaneous EEG-MEG scans, we compared: (1) ictal EEG source imaging vs ictal MEG source imaging; (2) Ictal source imaging vs interictal source imaging. Methods Among the patients undergoing simultaneous EEG-MEG (56 EEG channels and 275 MEG sensors), 13 had at least one seizure. A total of 46 MEG or EEG seizures were analyzed. wMEM was applied around seizure onset, centered on the frequency band showing the strongest power change. Principal component analysis applied to spatio-temporal reconstructed wMEM sources (0.4–1 s around seizure onset) identified the main spatial pattern of ictal oscillations. Qualitative sublobar concordance and quantitative measures of distance and spatial overlaps were estimated to compare EEG and MEG, ictal and interictal source imaging. Results Both ictal EEG and ictal MEG source imaging showed a good concordance with the clinical Seizure Onset Zone, ranging between 64% for EEG to 90% for MEG. Ictal MEG performed slightly better than ictal EEG, localizing sources closer to the clinical-SOZ ( p = 0.012) and to interictal MEG source imaging ( p = 0.040). The concordance and distance from the clinical-SOZ was not significantly different between interictal and ictal source imaging. Conclusions wMEM allows non-invasive localization of the SOZ from ictal EEG and MEG. EEG ictal source imaging might be sufficient in the daily clinical practice, but for the most challenging cases ictal MEG can provide more accurate results. Ictal source imaging can be a useful tool during presurgical evaluation of drug-resistant epilepsy patients and can guide iEEG implantation and brain surgery.