On-scalp magnetoencephalography based on optically pumped magnetometers to investigate temporal lobe epilepsy

Cryogenic magnetoencephalography (MEG) has a lower yield in temporal lobe epilepsy (TLE) than in extra-TLE (ETLE). The advent of optically pumped magnetometers (OPMs) might change this thanks to on-scalp MEG, which allows sensors to be placed closer to the brain and the design of bespoke sensor arrays to target specific brain regions. This study aims to investigate the detection and localization accuracy of interictal epileptiform discharges (IEDs) using on-scalp MEG in TLE and the added-value of face-OPMs for temporal IED detection/localization. Eleven patients underwent a 1-h MEG recording with OPMs placed both on the scalp (flexible cap, scalp-OPMs) and on the face (3D-printed glass-like structure, face-OPMs). Nine patients also underwent cryogenic MEG. IEDs were visually detected, averaged and localized using distributed source reconstruction. On-scalp MEG IED amplitude and signal-to-noise (SNR) were assessed and compared with cryogenic MEG when more than 10 IEDs were detected. Neural sources with and without face-OPMs were compared. The correlation between face- and scalp-OPMs was assessed. A mean of 13 IEDs/patient was detected using on-scalp MEG (mean amplitude: 3.3pT, mean SNR: 9.4) and localized in the (medial, anterior, basal, lateral or posterior) temporal lobe. Three patients had >10 IEDs in on-scalp and cryogenic MEG signals with amplitude and SNR that were either higher or similar for the on-scalp MEG recording compared with cryogenic MEG, and sources were separated by 8-11 mm. In two other patients, on-scalp MEG source locations were confirmed by gold-standard methods (surgical resection cavity, n=1; stereo-electroencephalography, n=1). Face-OPMs had a clear added-value (i.e., IED detection and localization) in one patient with antero-medial TLE. Face-OPM signals were correlated with scalp-OPM signals in most patients, showing that face-OPMs recorded brain activity. This study shows that on-scalp MEG is able to detect and localize IEDs in TLE and to discriminate irritative zones from different key (medial, anterior, basal, lateral or postero-lateral) temporal areas with similar or enhanced SNR than cryogenic MEG. Face-OPMs have a clear added-value in patients with anterior/medial TLE and increase the spatial coverage of the temporal lobe. This study paves the way for the future use of on-scalp MEG in patients with refractory TLE and patients with other brain disorders affecting the temporal lobe such as Alzheimer disease.