Abstracts

Rationale: Localizing the generating sources of interictal epileptiform spikes observed in EEG recordings of patients with refractory epilepsy provides useful information for epilepsy surgery evaluation. The generation of interictal spikes is a complex phenomenon, and propagation of activity from the source to remote cortical regions can occur within milliseconds. As a consequence, a common problem in the EEG source imaging (ESI) procedure is the selection of the time points or time periods of the spike in order to localize the origin and not the spread of the epileptic activity. In this study, we evaluate a Bayesian ESI technique for distributed sources using different time periods corresponding with interictal spike activity.Methods: We evaluated the multiple volumetric sparse priors (MSVP) approach using averaged interictal spikes in six patients who were successfully treated with surgery. Three different time periods were chosen for inversion: (i) a period starting 50 ms before the spike peak till 50% of the spike peak during the rising phase of the spike, (ii) a period starting 50 ms before the spike peak till the spike peak and (iii) a period containing the full spike time period starting 50 ms before the spike peak till 230 ms after the spike peak. To identify the origin of the spike activity, the source with the maximum energy from 50 ms before the spike peak till 50% of the spike peak was chosen for each of the considered time periods. We compared the MSVP technique with inversions using the LORETA approach implemented in the CARTOOL software and an equivalent current dipole (ECD) approach. For these approaches, we evaluated the obtained sources at the spike peaks and at 50% of the peaks. The resected zones in each of the patients, extracted from post-operative MR images, were used as verification basis.Results: We found equal or smaller distances to the resection border for the MSVP approach using the full time period of the spike activity compared to the LORETA and ECD techniques (See Fig. 1). This means that distances were found smaller than 15 mm, with robust results for all the patients. We also found that the distances to the resection border for the MSVP inversions of the full spike time periods were generally lower compared to the MSVP inversions of the time periods before the spike. The results for patient 1 are presented in Fig.2. In the first row, we show the resected zone. In the second row, the results for the CARTOOL software and ECD solutions are given. In the third row we show the results of the MSVP technique using the full spike time window.Conclusions: We presented an ESI technique to localize interictal spike activity by introducing multiple volumetric regions. This technique uses a time period of the interictal spike activity to localize the generating sources. Using averaged interictal spike data of six patients, the findings suggest that our approach is potentially useful to delineate the irritative zone in addition to other distributed approaches such as those implemented in the CARTOOL software and ECD modeling techniques.