Magnetoencephalography (MEG) measures small magnetic fields outside the head that are generated by electrical activity in the brain. The magnitude of these magnetic fields is of the order of femtotesla (10-15 T), which can be sensed by Magnetometers; in the case of MEG these are Superconducting Quantum Interference Devices, SQUID Magnetometers, in the helmet of the scanner. Hence, MEG is a non-invasive imaging mechanism, and all the magnetic activity in the scanning environment is generated by the participant’s brain activity.
A schematic of the MEG sensor structure is shown in Figure 1.4. As the image shows, the Signal Detection Coils (in YNiC's case SQUID Magnetometers) are arranged around the helmet of the dewar, with the Reference Detection Coils just above them (a combination of SQUID magnetometers and gradiometers). There are then a layer of SQUID Amplifiers, which are directly beneath the Liquid Helium Reservoir. Surrounding all this are thermal shields and an outer casing. The signals that are amplified by the SQUIDS are then pased on to the Remote Card cage, then on to the DAS (Data Acquisition System).
The sizes of the magnetic fields produced by the brain are tiny in comparison to the magnetic fields that we are exposed to in everyday life which are of the order of tens of microtesla (10-6 T), our hearts generate a field in the order of tens of nanotesla (10-9 T) and a car moving will generate a magnetic field that is still of the order of femtotesla when the field is recorded 1 mile away from the car. MEG scans are therefore performed within a magnetically shielded room to isolate the scanner from environmental noise. The magnetically shielded room is constructed from a special metal called mu-metal which is highly effective at screening magnetic fields.