Comparison Between Conventional SQUID Based and Novel OPM Based Measuring Systems in MEG

Magnetoencephalography (MEG) is a neuroimaging technique for measuring magnetic signals in vicinity of the head. With various source localization algorithms we can estimate the excited areas inside cortex. Standard MEG devices use SQUID-based channels which, despite their very good signal-to-noise ratio (SNR) have numerous drawbacks. As an alternative to these, commercial optically pumped magnetometers (OPMs) have recently developed to such a degree, that they are suitable for measuring magnetic fields in MEG. In this work we present measurements of the brain auditory evoked fields (AEF) with a system of 15 OPM sensors, that can detect both radial and tangential components of the magnetic field. These results are compared to the results obtained with the SQUID system. However, the quantitative comparison of two MEG systems is not trivial. We present a method for comparing two MEG system, which operates on the principle of minimum norm estimate (MNE) source localization algorithm. We show, that performance of the MEG system consisting of a small number of OPM sensors is slightly worse, but still comparable to results of the complete standard squid system with 125 gradiometers.