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

Francis S. Collins @NIHDirector (2018-03-27)
https://twitter.com/NIHDirector/status/978623744053784576
"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"

 

NIH on Facebook: https://www.facebook.com/nih.gov/posts/10156009236911830

 

 

 

 

MEG wearable scanner: measurement of the brain magnetic field with an array of optically pumped magnetometers (OPM)


Use of properties of alkali metals like Rubidium and a 795 nm circularly polarized laser beam

https://www.nature.com/articles/nature26147

 

Notes:

1. Cancel out the Earth magnetic field using a magnetically-shielded room and also coils near the head of the subject. You will now have a zero-magnetic field.
2. Attach the sensors which consist of a 3 mm-side cube filled with Rubidium (Rb) in gas form.
3. Excite the Rb atoms with a 795 nm circularly polarized laser beam, tuned to the D1 transition of Rubidium (cf. sodium D lines in the solar spectrum). In a zero-magnetic field setting, this will induce alignment of the spins with the direction of the laser beam.
4. The changing electrical activity of the brain produces a magnetic field. 
5. In the presence of a magnetic field, the Rb spins will start Larmor precession and they will no longer be aligned with the laser beam.
6. We know how automatic doors work with optical sensors. If the door starts closing and you wave your hand where a beam is present, you disrupt the light beam and the door reopens. In the Rb example, the spin now gets in the way of the laser beam and it somehow obstructs the light passage (cf. attached image). This obstruction can be measured by placing an optical sensor (e.g. photodiode) in the path similar to the door example. The change in the signal will be proportional to the magnetic field of the brain.

 

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Earth magnetic field cancellation used in the study of the MEG wearable with optically pumped magnetometers


https://www.nature.com/articles/nature26147

The measurement is conducted in a magnetically-shielded room. However, there is a remnant magnetic field of about 25nT which is spatially inhomogeneous.

Excerpt: "To ameliorate this problem, we constructed a set of bi-planar electromagnetic coils designed to generate fields equal and opposite to the remnant Earth’s field, thereby cancelling it out. The coils were designed [25,26] on two 1.6 × 1.6 m^2 planes, placed either side of the subject with a 1.5 m separation (Fig. 3a). 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). In this way, unlike standard field-nulling technologies (for example, tri-axial Helmholtz coils), our system can account for spatial variation of the field over a 40 × 40 × 40 cm^3 volume of interest enclosing the head. Furthermore, we were able to cancel all components of the field vector using coils confined to just two planes, hence retaining easy access to the subject. Four reference OPM sensors were coupled to the coils in a feedback loop to null the residual static field in the volume of interest. We achieved a 15-fold reduction in the remnant Earth’s field and a 35-fold reduction in the dominant field gradient (Fig. 3b)

 

 

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Publication on the sensor used for the wearable MEG

(Includes magnetic field cancellation with coils)

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3971838/

 

Excerpt: "The frequency response of the AM is given by the square root of a Lorentzian function, s(f)= s0/[1+(2 π f T2)2]1/2, where the width is determined by the inverse of the spin coherence time, T2. The 3-dB bandwidth of our AM was roughly 100 Hz, corresponding to a T2 ~ 2 ms."

 

Reference link: https://quspin.com/

 

 

 

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

 

DARPA FB link

 

"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.”