Using a Limited Number of Sensors in MEG or the Feasibility of Partial Head Coverage OPM MEG

Since the fields produced by the brain are very weak, the neuroimaging technique magnetoencephalography (MEG) requires magnetometers with high sensitivity. Until recently, the only suitable sensor was the SQUID magnetometer, but in the last few years optically pumped magnetometers (OPMs) with comparable sensitivity have emerged. These sensors have the advantage that they do not need cooling with a cryogenic liquid. As OPMs are still expensive, many laboratories perform OPM MEG with only partial coverage of the head. We, therefore, examined whether a limited number of magnetometers are sufficient for MEG measuring a focal activity such as the auditory evoked field. In more detail, we checked how the number of sensors affects the spatial resolution of the source localization method in MEG. To calculate the optimal sensor layout with a limited number of sensors we used the geometry of a whole-head SQUID MEG system. The optimal sensor layout was determined with the limited lead selection (LLS) algorithm. Additionally, we showed that this algorithm can be used to extend the measurement area for better visualization or to correct faulty channels if we have prior measurements with an extended set of sensors.