Interest of tri-axial rubidium and helium optically pumped magnetometers for on-scalp magnetoencephalography recording of interictal epileptiform discharges

Cryogenic magnetoencephalography (MEG) enhances the presurgical assessment of refractory focal epilepsy (RFE). Optically pumped magnetometers (OPMs) are cryogen-free sensors that enable on-scalp MEG recordings. Here, we investigate the interest of tri-axial OPMs (87Rb (Rb-OPM) and 4He gas (He-OPM)) for the detection of interictal epileptiform discharges (IEDs). IEDs were recorded simultaneously with 4 tri-axial Rb- and 4 tri-axial He-OPMs in a child with RFE. IEDs were identified visually, isolated from magnetic background noise using independent component analysis (ICA), and the orientation of magnetic field generated by the IEDs was reconstructed at each sensor location. Most IEDs (>1,000) were detectable by both He- and Rb-OPM recordings. IEDs were isolated by ICA and the resulting magnetic field oriented mostly tangential to the scalp in Rb-OPMs and radial in He-OPMs. Likely due to differences in sensor locations, the IED amplitude was higher with Rb-OPMs. This case study shows comparable ability of Rb-OPMs and He-OPMs to detect IEDs and the substantial benefits of triaxial OPMs to detect IEDs from different sensor locations. Tri-axial OPMs allow to maximize spatial brain sampling for IEDs detection with a limited number of sensors. ### Competing Interest Statement N.H. and M.J.B. hold founding equity in Cerca Magnetics Limited, a spin-out company whose aim is to commercialize aspects of OPM-MEG technology based on QuSpin's Rb-OPMs. E. L. and A.P-L hold founding equity in Mag4Health SAS, a French startup company, which is developing and commercializing MEG systems based on He-OPM technology. The remaining authors have no conflicts of interest. ### Funding Statement O.F. is supported by the Fonds pour la formation a la recherche dans l industrie et l agriculture (FRIA, Fonds de la Recherche Scientifique (FRS-FNRS), Brussels, Belgium). P.C. is supported by the Fonds Erasme (Convention Alzheimer, Brussels, Belgium). X.D.T. is Clinical Researcher at the FRS-FNRS. The OPM-MEG project at the Hopital Universitaire de Bruxelles and Universite libre de Bruxelles is financially supported by the Fonds Erasme (Projet de Recherche Clinique et Convention Les Voies du Savoir 2) and by the FRS-FNRS (Credit de Recherche: J.0043.20F, Credit Equipement : U.N013.21F). The MAGIC OPM MEG project at CEA Tech Lille, Amiens and Lille University Hospitals and Universite de Picardie Jules Verne and Universite de Lille was co-funded by European Union with the European Funding of Regional Development and the Region Hauts de France. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: This study was approved by the institutional Ethics Committee (Hopital Erasme, Reference: P2019/426). The patient and her legal representative gave written informed consent prior to the inclusion. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Data are available upon reasonable request to the corresponding author and after approval of institutional (Hôpital universitaire de Bruxelles & Université libre de Bruxelles) authorities. * EEG : Electroencephalogram He-OPM : Helium-based optically pumped magnetometer ICA : Independent component analysis IED : Interictal epileptiform discharge MEG : Magnetoencephalography MSR : Magnetic shielded room OPM : Optically pumped magnetometer Rb-OPM : Rubidium-based optically pumped magnetometer RFE : Refractory focal epilepsy SNR : Signal-to-noise ratio SQUID : Superconducting quantum interference device