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Magnetoencephalography
Magnetoencephalography (MEG) is a functional neuroimaging technique for mapping brain activity by recording magnetic fields produced by electrical currents occurring naturally in the brain, using very sensitive magnetometers. Arrays of SQUIDs (superconducting quantum interference devices) are currently the most common magnetometer, while the SERF (spin exchange relaxation-free) magnetometer is being investigated for future machines. Applications of MEG include basic research into perceptual and cognitive brain processes, localizing regions affected by pathology before surgical removal, determining the function of various parts of the brain, and neurofeedback. This can be applied in a clinical setting to find locations of abnormalities as well as in an experimental setting to simply measure brain activity〔Carlson, Neil R. (2013). ''Physiology of Behavior.'' Upper Saddle River, NJ: Pearson Education Inc.〕
== History of MEG ==
MEG signals were first measured by University of Illinois physicist David Cohen in 1968,〔Cohen D. "Magnetoencephalography: evidence of magnetic fields produced by alpha rhythm currents." ''Science'' 1968;161:784-6〕 before the availability of the SQUID, using a copper induction coil as the detector. To reduce the magnetic background noise, the measurements were made in a magnetically shielded room. The coil detector was barely sensitive enough, resulting in poor, noisy MEG measurements that were difficult to use. Later, Cohen built a better shielded room at MIT, and used one of the first SQUID detectors, just developed by James E. Zimmerman, a researcher at Ford Motor Company,〔Zimmerman, J.E., Theine, P., and Harding, J.T. "Design and operation of stable rf-biased superconducting point-contact quantum devices, etc." ''Journal of Applied Physics'' 1970; 41:1572-1580.〕 to again measure MEG signals.〔Cohen D. Magnetoencephalography: detection of the brain's electrical activity with a superconducting magnetometer. ''Science'' 1972;175:664-66〕 This time the signals were almost as clear as those of EEG. This stimulated the interest of physicists who had been looking for uses of SQUIDs. Subsequent to this, various types of spontaneous and evoked MEGs began to be measured.
At first, a single SQUID detector was used to successively measure the magnetic field at a number of points around the subject’s head. This was cumbersome, and, in the 1980s, MEG manufacturers began to arrange multiple sensors into arrays to cover a larger area of the head. Present-day MEG arrays are set in helmet-shaped dewar that typically contain 300 sensors, covering most of the head. In this way, MEGs of a subject or patient can now be accumulated rapidly and efficiently.
Recently it has been demonstrated that MEG could work with a chip-scale atomic magnetometer (CSAM) ()
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