The magnetic signals produced by the brain are quite small (on the order of 100 fT/Hz1/2, about 10 orders of magnitude smaller than the earth’s magnetic field), and are traditionally detected using state-of-the-art superconducting quantum interference device (SQUID) magnetometers. The disadvantage of SQUID magnetometers is that they require cryogenic cooling to temperatures of about -450 degrees F. This often requires sophisticated, expensive, and bulky liquid helium vessels, making any portable application challenging.
Recent advances in alkali-vapor based magnetometry have enabled the development of sensors which rival (and even surpass) the sensitivity of state-of-the-art SQUID magnetometers, allowing observation of magnetic fields created by the brain and heart without requiring any cryogenic cooling. Prior work has involved sensors formed from alkali-vapors contained in blown glass vapor cells with dimensions on the order of several cm.
In the last few years, researchers at the National Institute of Standards and Technology (NIST) have made great strides in miniaturizing alkali-vapor based atomic magnetometers to form mm scale sensors, enabling much higher spatial resolution than their larger counterparts. Now, working together with researchers at Physikalisch-Technische Bundesanstalt (PTB), Berlin, the groups have used these tiny magnetometers to demonstrate detection of MEG signals. In addition to higher spatial resolution, the small magnetometers can be placed closer to the source than SQUIDs, enhancing signal amplitude. Using these micro-magnetometers, the researchers were able to monitor spontaneous α oscillations triggered by opening and closing of a subject’s eye. They were also able to monitor the response of the brain to electrical stimuli.
While the demonstrated signal-to-noise ratio is not quite as high as obtained with cryogenically cooled SQUID magnetometers, there is room for several orders of magnitude improvement. The small size of the sensors would allow the user to place a large array of such sensors around the head to obtain a detailed spatial map of MEG signals. Eventually, microfabricated atomic magnetometers may entirely replace SQUIDs for MEG applications. The lack of cryogenic cooling represents a major advantage of atomic magnetometers over SQUIDs for any portable application. One can envision incorporating these microfabricated magnetometers for direct brain-computer interface, opening up new possibilities in technology, ranging from speechless communication to direct control of prosthetic or robotic devices.
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