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MEG

The MEG scanner when empty The MEG suite houses YNiC's 4D Neuroimaging Magnes 3600 Whole Head 248 Channel MEG scanner. In this scanner a participant sits in the hydraulic chair which is then raised so that their head rests inside the helmet of the scanner. Introduction to MEG

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 (10-15T), which can be sensed by Superconducting Quantum Interference Devices (SQUIDs) 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.

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), our hearts generate a field in the order of tens of nanotesla (10-9) 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 μ-metal which is highly effective at screening magnetic fields.

The magnetic fields measured with MEG are derived from synaptic activity. Neurones connect via synapses, which are chemically mediated junctions between two nerve cells. When neurones are active, the flow of neurotransmitter chemicals changes the electrical current into the recipient neurone, and affects the cell's electrical potential. This is referred to as a change in the Post Synaptic Potential (PSP), and can be excitatory (EPSP) or inhibitory (IPSP).

The summation of the neural currents produced by neural activity is what is indirectly observed using MEG. MEG measures the magnetic field associated with this electrical activity. However, SQUIDs are unable to sense the magnetic fields from such electrical activity in just one neurone. Approximately 50000 adjacent neurones need to be active at the same time to generate a collective magnetic field that SQUIDs are able to detect.