Magnetoencephalography (MEG)

A present flowing in a wire sets up a magnetic area which flows around the current vector. Therefore the synchronized flow of currents along dendrites of about 50 000 cortical pyramidal cells all oriented in the similar direction is sufficient to set up a measurable, if weak, magnetic field. This is measured in MEG magnetoencephalography using an array of magnetometers called superconducting quantum interference devices (SQUIDS) that surround the head. The MEG signal is generated mostly by the flow of intracellular currents in dendrites generated through synaptic activity. Because cortical activity generates an area of order 10^-13 tesla (T) compared with the Earth’s magnetic field of 3.1 * 10^-5 T. MEG must be done in a magnetically shielded environment. SQUIDs are sensitive to magnetic fields as small as 10^-18 T.

Figure: Principle of magnetoencephalography. (a) A linear current sets up a concentric cylindrical magnetic field with clockwise polarity for current entering the plane of the paper (corkscrew rule). (b) Magnetic field generated by current flow along radially oriented pyramidal cell dendrites due to synaptic input.  The current vector (thick arrow) points in the direction of intracellular local circuit current flow.

A main benefit of magnetoencephalography over electroencephalography is its temporal resolution of better than 1 ms, that is comparable to intracranial electrodes. Also, magnetic fields are less distorted through skull anatomy than electric fields that give MEG a better spatial resolution.