On-scalp MEG

On-scalp MEG is the system-level opportunity created by small cryogen-free magnetometers (OPMs and, potentially, high-Tc SQUIDs): placing sensors directly on the head instead of inside a rigid one-size dewar. The gain is geometric, because the magnetic field from a cortical source falls roughly as 1/r^2 to 1/r^3, so moving from about 20 mm to a few mm of standoff increases amplitude and, crucially, preserves the higher spatial frequencies that survive to the sensor, tightening source localisation and better separating nearby sources.

Realising this gain demands solving problems a fixed dewar never had: co-registering many independently placed sensors to the head and to anatomy, correcting for the fact that subjects can now move, handling a limited dynamic range when the head moves within residual fields, and rejecting interference without the spatial oversampling of a 300-channel SQUID array. On-scalp MEG is thus less a finished modality than a design target that OPM hardware is progressively approaching.