If you were to drill a hole in the bottom of a hanging bathtub where you could insert your head, then it would look a bit like a modern magnetoencephalography (MEG) machine. These are less well known than MRI machines, but at their root, they do something similar: allow other people to peer inside of your head.
Current MEG devices are so bulky because they are SQUIDs. That is, they contain superconducting quantum sensors designed to measure extremely small magnetic fluctuations near your head. These fields are triggered when neurons fire and serve as an important signal of neural activity.
Since the early 2000s, a new type of magnetometer has emerged called a SERF magnetometer, or OPM, for optically pumped magnetometer. These devices use lasers and vapor cells to take measurements, and have long had the potential to be much smaller and less bulky than SQUIDs, making them better suited for measurements of the head. But only now have researchers actually figured out how to reinvent a complex OPM sensor as a small, modular unit, suitable for replacing a SQUID. Researchers from the University of Sussex described their new development in a preprint released on June 10.
“For us, it’s very significant,” said Peter Krüger, the senior coauthor, in an interview with Motherboard. “Building something from scratch that can be scaled up—this is a shift of approach.”
There are currently only a few hundred MEG devices around the world, mostly used for research purposes, according to Krüger. Each MEG might contain several hundred separate SQUID detectors. These are placed into a dense spatial array such that they measure the magnetic signal over a broad portion of the head. Scientists can then use this signal to map out the activity of neurons in the brain.
“That’s a game changer”
A central limitation for SQUIDs is that they must be cooled to extremely low temperatures. They must be placed in a cryogenic environment, typically with a substantial supply of an ultracold substance like liquid helium. For this reason, they cannot be situated very close to the head, which limits their sensitivity to brain signals. “The magnetic signals that come from the neuron activity inside the brain drop off very quickly with distance,” Krüger said.
In contrast, the researchers have designed their modular OPMs so that they will be shrinkable and placeable much closer to each other and to the head. “The difference between having a sensor only a millimeter away, like we can do with these new quantum sensors, from the conventional superconducting sensors that are centimetres away, is gigantic,” Krüger said. “That's a game changer.”
For most of their history, OPM sensors have mainly been tabletop laboratory devices that are unsuitable for applications like MEG. OPM sensors must integrate numerous components to work. They function by shining a laser through a gaseous vapor and measuring the amount of light that passes through and hits a photodetector. The strength of this light correlates with the magnetic field strength, allowing researchers to infer the latter.
“A lot of people up to this point in research have been concentrating on getting single sensors and improving single sensor performance,” said Thomas Coussens, the paper’s first author, to Motherboard. “But no one's really been focusing on the problem from the beginning as a multi-sensor array issue, where, from the outset, you're thinking, how do I make this array scalable?”
The core components of the OPM are the laser, vapor cell, and photodetector. The researchers’ new sensor integrates all these components into a small cubic module of about four centimeters per side. “We deliberately made a choice to make it small enough that building an array is still possible, but big enough so that we can still improve on each component,” Krüger said.
The modules are designed to share resources, such as their laser source. The components of each module are also designed with a plug-and-play capability similar to a desktop personal computer. This fosters the ability of different teams to specialize on different components
The researchers note that a company called QuSpin began commercially offering single OPM sensors in 2016. These enabled researchers to begin to attempt to create arrays of OPM sensors. However, each QuSpin sensor functions like a separate system. Krüger realized that “we should build the system, from scratch, modular so that everything is scalable, everything is based on semiconductor technology, so that if you have one, then you can have many.”
In their paper, the researchers demonstrate the operation of both one sensor and two sensor configurations. With one sensor, they proved the sensitivity of their module by detecting the change in brain waves of a human participant, which occurred when they closed their eyes. In an experiment with two sensors, they separately showed that they could combine the measurements of each sensor to reduce background signal noise.
One of the biggest challenges of modularizing their sensors was temperature management. The gaseous vapor in each module must be heated to over 100 degrees celsius for optimal sensitivity. However, the module must still be capable of being placed close to a person’s head. “We need to insulate these cells within the housing quite effectively, but also retain the capability of being able to switch out the cells to test new cell types,” Coussens said.
The researchers note that their modular sensors will have applications across many other areas besides MEG. New diagnostics are needed for the development of new types of batteries, for example. Batteries dissipate small amounts of energy over time, generating tiny currents that can be mapped with OPM sensors, allowing the battery functioning to be better understood. Krüger says that much of his grant funding comes from that area.
His overarching vision is to continue to shrink and modularize each OPM until they can build a magnetic camera, a system composed of many individual pixel sensors that maps magnetics fields in great detail, similarly to how a camera maps light on your phone. These quantum sensors will serve to diagnose batteries, but perhaps more compellingly, brains too.