Magnetoencephalography (MEG) with optical/atomic magnetometers (SERF magnetometers)


It is suggested to refer to the section  "Alkali atom optical pumping and optical transitions".



Classic SERF atomic magnetometer


An atomic magnetometer of superior sensitivity is the spin-exchange-relaxation-free (SERF) magnetometer. It is noted that it requires zero ambient magnetic field. Its principle of operation is described below and in Figure 1. 


A droplet of alkali metal e.g. rubidium in a glass chamber is heated to produce a vapor (gaseous phase). A laser beam of circularly polarized light tuned at the D1 line aligns the spins of the rubidium atoms via optical pumping in the direction of propagation of the light (pump beam). The polarization fraction of the spins and by extension of the atoms depends among others on the pump power and the atom density.


Upon the effect of a magnetic field perpendicular to the pump beam, the spins are rotated and reorient. As a result, the refraction index of the gas changes. A beam of linearly polarized light, termed the probe beam, which is typically perpendicular to the pump beam, is used to detect these events by the rotation of its polarization, which constitutes the magneto-optical phenomenon termed the Faraday effect/rotation. The rotation of the polarization is linked to the differential behavior of the right and left circularly polarized components which compose a linearly polarized beam. It is considered that the spin polarization is imprinted on the light polarization. The rotation is proportional to the magnetic field strength and therefore can be used to measure it. 


The sensitivity of an atomic magnetometer operating with density n in a volume V due to shot noise is:

where γ is the gyromagnetic ratio, t is the measurement time and T2 is the relaxation or decoherence time of the atomic population. Relaxation is due among others to collisions with chamber walls and inter-population collisions. Collisions are linked to spin-exchange or spin destruction. By using conditions such as a high-density population, it is possible to remove spin-exchange relaxation. Magnetometers which operate in these conditions are termed spin-exchange-relaxation-free (SERF) magnetometers. It is noted that polarization is conducted optically using circularly polarized light and detection is also performed optically or magneto-optically using the Faraday effect/rotation (all-optical magnetometer).






SERF optical magnetometer for Magnetoencephalography (MEG) by Sandia Labs


An atomic SERF magnetometer for Magnetoencephalography (MEG) was developed by Sandia National Laboratories in 2010 (ref.). Figure 1 describing the principle of operation of this magnetometer is derived from the above reference. Following the publication of the development of the sensor (Colombo et al 2016), Sandia Labs reported the generation of a 20-channel MEG system using SERF optical magnetometers and a magnetically shielded chamber (Borna et al 2017, archiv link). A press release by Sandia Labs is found at this link.



Figure 1: Principle of operation of a SERF magnetometer (image from "Atomic Magnetometer for Human Magnetoencephalograpy" by SANDIA Labs )





A portable Magnetoencephalography helmet/scanner with SERF optical magnetometers


In 2018, Boto et al published in Nature their study of a MEG portable helmet with SERF magnetometers (PMC link). It is noted that a zero ambient magnetic field regime is required. This study was cited in the NIH’s director blog (ref.) (Figure 2). A presentation of the authors is found at this link. Information on the sensor is available at the QuSpin site and also at the publication by Shah et Wakai 2013.




Figure 2: NIH Facebook post referring to NIH's director article on a wearable magnetoencephalography (MEG) scanner




The NIH Director writes on his blog about the portable/wearable Magnetoencephalography (MEG) scanner


Francis S. Collins @NIHDirector (2018-03-27) on Twitter:
"A #brain scanner that looks like a futuristic cross between a helmet and hockey mask is pushing functional brain imaging into the future. Find out how on my blog. #NIH"






Earth’s magnetic field cancellation used in the study of the portable MEG with optical magnetometers



As optical magnetometers require a zero ambient magnetic field, the measurement using the portable/wearable MEG scanner of the study Boto et al 2018 (PMC link) is conducted in a magnetically-shielded room. However, there is a remnant magnetic field of about 25nT which is spatially inhomogeneous. A bi-planar coil setting is used for cancellation of this magnetic field.
In order to have a zero ambient magnetic field in a 40 × 40 × 40 cm3 volume surrounding the head, coils were used to generate fields that are equal and opposite to the remnant Earth's magnetic field. Coils were placed on two 1.6 × 1.6 m^2 planes on either side of the subject with 1.5 m separation (left panel of post image). The design of the coils can be seen in Figure 2.
Three coils generated spatially uniform fields (Bx, By and Bz) while two additional coils were used to remove the dominant field variations (dB(x)/dz and dB(z) /dz). Four reference OPM sensors were coupled to the coils in a feedback loop to null the residual magnetic field.
Another protocol can be found in the study Shah et Wakai 2013 describing the sensor of this portable MEG scanner.




DARPA's Program "Atomic Magnetometer for Biological Imaging In Earth’s Native Terrain (AMBIIENT)" - Magnetoencephalography and Magnetocardiography


"The AMBIIENT program is challenging the research community to devise new types of magnetic gradiometers that can detect picoTesla- and femtoTesla magnetic signatures out in the open, without shielding and with whatever the ambient magnetic field environment might be. To do so will require researchers to, in Lutwak’s words, “exploit novel atomic physics techniques and architectures to directly measure extremely tiny gradients in magnetic fields without having to compare the difference between absolute field measurements from two sensors separated along a baseline.” One physics-based approach AMBIIENT performers are likely to pursue is to monitor changes in the polarization or other measureable features of a small laser beam as it passes through vapor cells hosting atoms that respond in laser-beam-altering ways to even femtoTesla magnetic fields."

"Traditionally, measuring small magnetic signals in ambient environments has relied on pairs of high-performance sensors separated by a baseline distance and then measuring the small field-strength differences between the two sensors,” said Robert Lutwak, AMBIIENT’s program manager in DARPA’s Microsystems Technology Office. “This gradiometric technique has worked well for applications in geophysical surveying and unexploded ordnance detection,” Lutwak added, “but due to the combination of the sensors’ limited dynamic range and the natural spatial variation of the background signals, this approach falls several orders of magnitude short of being able to detect biological magnetic signals.”