A fully integrated whole head Helium OPM MEG: performance assessment compared to cryogenic MEG

Maxime Bonnet ^1,2 Denis Schwartz ^1,2 Tjerk Gutteling ^1,2 Sebastien Dalligault ² Etienne Labyt ^3*

• ¹ Centre d'Etude et de Recherche Multimodal et Pluridisciplinaire en Imagerie du Vivant (CERMEP), Bron, France
• ² Centre de Recherche en Neurosciences de Lyon, Bron, Auvergne-Rhone-Alpes, France
• ³ Mag4Health, Grenoble, France

Magnetoencephalography (MEG) is a neuro-imaging technique that measures neuronal activity at a millisecond scale. A few years ago, a new generation of MEG sensor has emerged: optically pumped magnetometers (OPM). The most common OPM use alkali atoms as sensing element. These alkali OPM sensors must be heated to around 150 °C, in contrast to classical MEG sensors (SQUID MEG) which need to be cooled down to -269°C. This article focuses on a new kind of OPM, using Helium-4 gas as sensing element, which solves some disadvantages of alkali OPM. 4 He-OPM sensors operate at room temperature, with negligible heat dissipation (10 mW) and thus do not need thermal insulation. They also offer a large dynamic range (±200 nT) and frequency bandwidth (2000 Hz). The main goal of this study is to characterize the performance of a whole-head MEG system based on 4 He OPM sensors ( 4 He OPM MEG). We first simulated different sensors configurations with three different numbers of channels and three different head sizes, from child to adult, in order to assess the signalto-noise ratio and the source reconstruction accuracy. Experimental testing was also performed using a phantom for simulating brain magnetic activity. Simulation and experiments show equivalent detection capability and localization accuracy on both MEG systems. These results illustrate the benefit of 4 He OPM sensors that operate at room temperature and are positioned closer to the scalp.