Electrophysiology · reads
SQUID-based (cryogenic) MEG is a helmet of superconducting sensors kept near -270 °C by liquid helium; in 2018 commercial systems held about 300 sensors in such a cooled container. It reads the extremely weak magnetic fields outside the head, about 10⁻¹⁵ to 10⁻¹¹ tesla, produced by neuronal electrical activity, and from them infers the underlying currents, mainly postsynaptic currents in aligned cortical pyramidal cells, so the neural source is estimated indirectly. It is usually paired with MRI.
SQUID-based MEG is the recording of the weak magnetic fields outside the head that are generated by the electrical activity of neurons. The sensor is a superconducting quantum interference device (SQUID): a small superconducting loop interrupted by two weak links, the Josephson junctions. At very low temperature a SQUID is extremely sensitive to changes in magnetic field, of the order of 10⁻¹⁵ T, and its output is converted to a digital signal. Neuromagnetic fields are extremely weak, about 10⁻¹⁵ to 10⁻¹¹ T, and must be detected against environmental magnetic noise from external electrical and magnetic equipment of about 10⁻⁷ T and above. In 2018, commercial systems held about 300 sensors in a cryogenic vessel, the dewar, containing liquid helium; flux transformers couple the neuromagnetic field to the SQUIDs. Liquid helium, near -270 °C, keeps the sensors superconducting. The sensor array is a helmet that gives whole-head coverage. The recorded fields cross the dura, skull and scalp relatively unaltered and carry millisecond-accurate information about neuronal currents. MEG primarily reflects net intracellular dendritic postsynaptic currents in aligned cortical pyramidal cells. It is usually combined with MRI to give magnetic source imaging.
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