Reproducibility of EEG‐MEG fusion source analysis of interictal spikes: Relevance in presurgical evaluation of epilepsy

Abstract

Fusion of electroencephalography (EEG) and magnetoencephalography (MEG) data using maximum entropy on the mean method (MEM-fusion) takes advantage of the complementarities between EEG and MEG to improve localization accuracy. Simulation studies demonstrated MEM-fusion to be robust especially in noisy conditions such as single spike source localizations (SSSL). Our objective was to assess the reliability of SSSL using MEM-fusion on clinical data. We proposed to cluster SSSL results to find the most reliable and consistent source map from the reconstructed sources, the so-called consensus map. Thirty-four types of interictal epileptic discharges (IEDs) were analyzed from 26 patients with well-defined epileptogenic focus. SSSLs were performed on EEG, MEG, and fusion data and consensus maps were estimated using hierarchical clustering. Qualitative (spike-to-spike reproducibility rate, SSR) and quantitative (localization error and spatial dispersion) assessments were performed using the epileptogenic focus as clinical reference. Fusion SSSL provided significantly better results than EEG or MEG alone. Fusion found at least one cluster concordant with the clinical reference in all cases. This concordant cluster was always the one involving the highest number of spikes. Fusion yielded highest reproducibility (SSR EEG = 55%, MEG = 71%, fusion = 90%) and lowest localization error. Also, using only few channels from either modality (21EEG + 272MEG or 54EEG + 25MEG) was sufficient to reach accurate fusion. MEM-fusion with consensus map approach provides an objective way of finding the most reliable and concordant generators of IEDs. We, therefore, suggest the pertinence of SSSL using MEM-fusion as a valuable clinical tool for presurgical evaluation of epilepsy.

Transcranial direct current stimulation over the sensory‐motor regions inhibits gamma synchrony

“…To this purpose, we took advantage of MEG. This technique allows measuring cortical activity noninvasively, with high temporal resolution, with better spatial resolution than EEG (Chowdhury et al, ; Hedrich, Pellegrino, Kobayashi, Lina, & Grova, ; Larsen et al, ; Pellegrino et al, ; Pellegrino, Tomasevic, Herz, Larsen, & Siebner, ; von Ellenrieder et al, ). This technique is also especially suitable in the context of off‐line tDCS as it does not require the application of electrodes on the scalp and has no impact on tDCS montage (Pellegrino et al, ).…”

Fast oscillations >40Hz localize the epileptogenic zone: an electrical source imaging study using high-density electroencephalography

“…In clinical practice most studies assessed the concordance of the source with the assumed EZ at the lobar level [Duez et al, 2019; Rampp et al, 2019], or at best at the sub-lobar level [Heers et al, 2016]. We evaluated our findings using different quantitative validation metrics, several of them being similar to the ones proposed in Pellegrino et al [2018] and Chowdhury et al [2018] in order to assess the quality of our results as objectively as possible regarding the source map maxima, its spatial extent, and the spatial overlap with the resection cavity. The originality of our proposed approach was also to consider an accurate delineation of the resection cavity as our reference for the evaluation of source localization results, whereas such a comparison was more qualitative in other studies [Abdallah et al, 2017].…”

“…In order to take into account reliability and reproducibility between epileptic discharges to be localized (IEDs or FOs), we applied the concept of consensus maps of source localization [Chowdhury et al, 2018]. In order to estimate these consensus maps for every patient, we first applied cMEM or wMEM to generate source maps of all single events, FO or spike events.…”

“…This consensus approach was considered instead of simple averaging of the event followed by one source localization, in order to enhance reliability between source localization results and reducing the influence of more noisy maps. We previously carefully demonstrated the robustness of this approach when considering either EEG source imaging, MEG source imaging or EEG-MEG fusion source imaging [Chowdhury et al, 2018].…”

“…Final maps were thresholded at 50% of the maximum reconstructed intensity for both visualization and statistical purposes. We previously demonstrated that MEM source localization results provide maps with high contrast, therefore results on the spatial extent are relatively stable within a large range of detection threshold, as opposed to other standard source localization techniques [Chowdhury et al, 2018; Pellegrino et al, 2018; Pellegrino et al, 2019]. In order to assess the success or failure of the localization, we considered the surgical cavity as a reference.…”