220 — Moving magnetoencephalography towards real-world applications with a wearable system

Boto, Holmes, Leggett & Roberts et al (10.1038/nature26147)

Read on 28 March 2018
#neuroscience  #methods  #imagery  #imaging  #MEG  #quantum  #rubidium  #MEG  #superconductor  #laser 

Current state-of-the-art magnetoencephalography (MEG) scanners use liquid helium to cool the ultra-sensitive magnetic-field sensors: If these sensors weren’t kept so cold, they would lose their femtotesla sensitivity.

But liquid helium is nothing to play around with: It’s immensely dangerous if it comes in contact with your body, and it takes a lot of energy to keep it cold. As a result of this, MEGs are large, stationary scanners that look a lot like a vertical MRI. When you’re undergoing a scan, you cannot move your head.

This isn’t a huge deal for many studies, but when we try to understand how the brain works… Well, the brain is used to moving around as you walk or turn your head or do whatever it is you do. And so even if the measurements are perfect, it’s a hard sell to convince anyone that your visuospatial experience is the same with your head tied into a scanner as it is while moving around freely. Oh, and good luck getting a young child to sit still in a scanner.

But what if you could switch out the superconductive, supercooled sensors for equally sensitive but less finicky quantum sensors?!

In an Optically Pumped Magnetometer (OPM), Hot rubidium vapor is spin-polarized by a laser which shines through the Rb. If the sample is exposed to a magnetic field perpendicular to the laserbeam, however, the spin of the rubidium is disrupted in a predictable way, and the vapor blocks a greater percentage of the laser light. This means that even tiny changes to the surrounding magnetic field are detectable using OPMs — perfect for MEG-style recordings.

Unlike supercooled superconductor sensors, OPMs are warm (less than 200ºC) and can be mounted, with insulation, directly to a helmet. This means that a subject can wear the MEG like a (big wiry) hat, rather than being required to sit perfectly still in a stationary scanner.

There are still a few nanoteslas of the Earth’s magnetic field to worry about when wearing this heaset: Unlike a stationary conventional MEG, this machine suffers from the slight variations of the Earth’s magnetic field when you move your head through space. To counteract this, two giant walls of magnetic coils counteract the remnants of that nanotesla magnetic field, a few feet to either side of the user’s head. So… This MEG isn’t totally portable just yet.