The NIH Director writes on his blog about the new wearable Magnetoencephalography (MEG) scanner
Francis S. Collins @NIHDirector (2018-03-27)
"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
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.
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)
(Includes magnetic field cancellation with coils)
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/
"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."