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Traditionally the only way to detect the extremely small magnetic fields generated by the brain (a billion times smaller than the Earth's magnetic field) was to exploit superconducting sensors known as SQUIDs. SQUIDs must be kept at cryogenic temperatures (-269 °C), and this means that sensors in a conventional MEG system must be immersed in liquid helium. This, in turn, makes scanners heavy, cumbersome and very expensive. This has proven a significant barrier to widespread uptake of MEG.
Recent breakthroughs in physics have enabled the development and fabrication of OPMs that exploit the quantum properties of alkali atoms to measure very small magnetic fields. OPM sensitivity rivals that of superconducting devices, but OPMs do not require cryogenic cooling. Moreover, their size is no larger than a Lego brick. OPMs thus provide the perfect building block for our integrated wearable MEG system.
Because the magnetic fields measured by MEG are so small, the scanner needs to be sited in a well-controlled, low magnetic field environment. Conventional MEG systems are housed inside a magnetically-shielded room (MSR) which comprises multiple layers of high permeability material known as MuMetal®, as well as a single layer of highly conductive material such as copper. This acts to reduce magnetic interference (i.e. magnetic fields that vary over time), however in conventionally designed rooms, the presence of the metal means that there is a residual static magnetic field. If OPMs are to be used and participants allowed to move freely, this field must be removed.
Recent developments have enabled the construction of new types of MSR, with built-in degaussing to remove residual magnetisation and reduce the static magnetic field; parallel work has enabled the design and construction of electromagnetic coils which allow high fidelity control of residual fields and their variation over space inside the MSR. All of this combined has led to an exceptional magnetic environment with fields below 1 nT; a shielding factor of over 50,000. This shielding facilitates the perfect environment for an OPM-MEG system.
Given OPMs and shielding, OPM-MEG requires a solution to the non-trivial problem of integrating an array of OPMs into a wearable helmet; ensuring that the array is optimised to characterise the magnetic fields generated by the brain, and allowing it to “talk” to the shielding to provide the best MEG data possible.
The Cerca OPM-MEG System uses a unique helmet design to place 64 sensors as close as possible to the scalp surface, and an advanced array design which ensures effective cancellation of magnetic fields of no interest arising from interference sources. Published data show that a prototype system based on the Cerca design can outperform even the best cryogenic systems available when measuring neural oscillations and functional connectivity. Further, a person can wear our helmet and move freely whilst the brain is scanned. Helmets can be made to fit anyone, meaning it is possible to scan adults, or babies, using the same system. The ergonomic chair and helmet mean participants can easily be scanned in 30 minute or longer sessions on the Cerca System.
The Cerca OPM-MEG System is the world’s most advanced brain scanner – an integrated lightweight, ergonomic and wearable device that will offer an unparalleled window on human brain function, in health and disease.
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