Abstract
Recent advances in magnetic sensing has made on-scalp magnetoencephalography (MEG) possible. In particular, optically-pumped magnetometers (OPMs) have reached sensitivity levels that enable their use in MEG. In contrast to the SQUID sensors used in current MEG systems, OPMs do not require cryogenic cooling and can thus be placed within millimetres from the head, enabling the construction of sensor arrays that conform to the shape of an individual’s head. To properly estimate the location of neural sources within the brain, one must accurately know the position and orientation of sensors in relation to the head. With the adaptable on-scalp MEG sensor arrays, this coregistration becomes more challenging than in current SQUID-based MEG systems that use rigid sensor arrays. Here, we used simulations to quantify how accurately one needs to know the position and orientation of sensors in an on-scalp MEG system. The effects that different types of localisation errors have on forward modelling and source estimates obtained by minimum-norm estimation, dipole fitting, and beamforming are detailed. We found that sensor position errors generally have a larger effect than orientation errors and that these errors affect the localisation accuracy of superficial sources the most. To obtain similar or higher accuracy than with current SQUID-based MEG systems, RMS sensor position and orientation errors should be < 4,hbox {mm} and < 10^circ, respectively.
Highlights
Magnetoencephalography (MEG) is a non-invasive functional neuroimaging method for investigating electric neuronal activity inside the living human brain (Hämäläinen et al 1993; Hansen et al 2010)
The relative error of opticallypumped magnetometers (OPMs) array topographies due to miscoregistration is shown in Table 2 and Fig. 3
This error is largest in superficial areas, which is to be expected as these areas are closer to the sensors
Summary
Magnetoencephalography (MEG) is a non-invasive functional neuroimaging method for investigating electric neuronal activity inside the living human brain (Hämäläinen et al 1993; Hansen et al 2010). MEG functions by measuring the magnetic field produced by neural currents in the brain using sensors positioned around the head. The MEG signal is complementary to that of electroencephalography (EEG), in which the potential distribution caused by neural activity is measured using electrodes placed on the scalp. The magnetometer employed for MEG has almost exclusively been the low-Tc superconducting quantum interference device (SQUID). These sensors require a cryogenic temperature that is typically attained by immersing SQUIDs. Handling Editor: Seppo P.
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