Magnetoencephalography for brain function imaging - OPM_MEG

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A New Study Comparing 4He OPM to SQUIDS Reaches New Exciting Results
MagnetoEncephaloGraphy (MEG) is a non-invasive functional imaging technique that provides direct measurement of neuronal activity at a millisecond time scale. Conventional MEG systems use superconducting quantum interference devices (SQUIDs) that require very low temperatures achieved by liquid helium. This leads to high costs, environmental impact, and experimental limitations. A new generation of MEG sensors, optically pumped magnetometers (OPMs), is emerging. Alkali-based OPMs were developed first, with good sensitivity and the potential to increase the signal power of neuromagnetic activity recording. However, they require heating, have limited bandwidth and small dynamic range, and need optimised shielded rooms. Recently, MAG4Health developed a 4He OPM-MEG system, which operates at room temperature, and has no noticeable heat dissipation, large resonance linewidth, large dynamic range, and large frequency bandwidth. MAG4Health has made considerable progress in sensitivity, and the 4He-OPMs now reach better than 45 fT/√Hz on two of the three axes.

A new study has compared the performance of conventional, cryogenic SQUID-MEG and newly developed wearable room-temperature 4He-OPM sensors using a somatosensory and visual stimulation paradigm. The classical SQUID-MEG recordings were made using a 275 SQUID-based axial gradiometers MEG system. In contrast, 4He-OPMs sensors were used to measure the brain's magnetic field along three axes, with a dynamic range of ±250 nT. The subject was seated with a conformable headset housing 96 possible positions for the 4He-OPM sensors. For the somatosensory experiment, the 4 remaining 4He-OPM sensors were located in LC11, LC13, LC 31 and LC33 locations around the somatosensory area, while for the visual experiment, the 4 remaining 4He-OPM sensors were located in LO11, LO31, RO11 and RO31 locations around the primary visual area. The signal was sampled at 11 kHz.

Results show that the 4He-OPM sensors show high similarity in time courses obtained compared with SQUID-MEG, and pick up changes in oscillatory brain dynamics across the human oscillatory neural range. However, the percentage signal change from baseline is lower for 4He-OPM sensors compared to SQUID-MEG, especially in the higher oscillatory (gamma) range, possibly due to muscle activity caused by the cabling of the sensors. The study concludes that 4He-Opm’s show very similar results to SQUID-MEG systems by taking advantage of a shorter distance to the brain, despite having a lower sensitivity.

https://brainbox-neuro.com/news/2023/a-new-study-comparing-4he-opm-to-squids-reaches-new-results

paper:
A New Generation of OPM for High Dynamic and Large Bandwidth MEG: The 4He OPMs—First Applications in Healthy Volunteers
Abstract:
MagnetoEncephaloGraphy (MEG) provides a measure of electrical activity in the brain at a millisecond time scale. From these signals, one can non-invasively derive the dynamics of brain activity. Conventional MEG systems (SQUID-MEG) use very low temperatures to achieve the necessary sensitivity. This leads to severe experimental and economical limitations. A new generation of MEG sensors is emerging: the optically pumped magnetometers (OPM). In OPM, an atomic gas enclosed in a glass cell is traversed by a laser beam whose modulation depends on the local magnetic field. MAG4Health is developing OPMs using Helium gas (4He-OPM). They operate at room temperature with a large dynamic range and a large frequency bandwidth and output natively a 3D vectorial measure of the magnetic field. In this study, five 4He-OPMs were compared to a classical SQUID-MEG system in a group of 18 volunteers to evaluate their experimental performances. Considering that the 4He-OPMs operate at real room temperature and can be placed directly on the head, our assumption was that 4He-OPMs would provide a reliable recording of physiological magnetic brain activity. Indeed, the results showed that the 4He-OPMs showed very similar results to the classical SQUID-MEG system by taking advantage of a shorter distance to the brain, despite having a lower sensitivity.
https://www.mdpi.com/1424-8220/23/5/2801

see also
An integrated full-head OPM-MEG system based on 128 zero-field sensors
Compact optically-pumped magnetometers (OPMs) are now commercially available with noise floors reaching 10 fT/Hz1/2. However, to be used effectively for magnetoencephalography (MEG), dense arrays of these sensors are required to operate as an integrated turn-key system. In this study, we present the HEDscan, a 128-sensor OPM MEG system by FieldLine Medical, and evaluate its sensor performance with regard to bandwidth, linearity, and crosstalk. We report results from cross-validation studies with conventional cryogenic MEG, the Magnes 3,600 WH Biomagnetometer by 4-D Neuroimaging. Our results show high signal amplitudes captured by the OPM-MEG system during a standard auditory paradigm, where short tones at 1000 Hz were presented to the left ear of six healthy adult volunteers. We validate these findings through an event-related beamformer analysis, which is in line with existing literature results.
https://www.frontiersin.org/articles/10.3389/fnins.2023.1190310/full

being hailed as the biggest advance since the MRI