ED1-5-INV
Recent progress in high-Tc SQUID-based on-scalp MEG
Dec.1 16:40-17:05 (Tokyo Time)
Department of Microtechnology and Nanoscience – MC2, Chalmers University of Technology, Gothenburg, Sweden1
Department of Neuroscience and Biomedical Engineering, Aalto University, Aalto, Finland2
NatMEG, Department of Clinical Neuroscience, Karolinska Institute, Stockholm, Sweden3
Clinical Neurophysiology, Karolinska University Hospital, Stockholm, Sweden4
Center of Functionally Integrative Neuroscience (CFIN), Aarhus University, Aarhus, Denmark5
MedTech West and the Inst. of Neuro. and Physio., Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden6
Chalmers Industriteknik, Gothenburg, Sweden7
In the growing field of on-scalp magnetoencephalography (MEG), the neuromagnetic field arising from brain activity is non-invasively sampled using sensors that can be flexibly placed in close proximity to the subject's head. Because of the higher signal magnitudes available to on-scalp sensors, simulations predict that a full-head on-scalp MEG system can extract more information about brain activity than a conventional MEG system based on low critical temperature (low-Tc) superconducting quantum interference devices (SQUIDs) — even with slightly higher noise levels.
On the way towards such a full-head system, we have built a 7-channel on-scalp MEG system based on high critical temperature (high-Tc) SQUIDs – see Fig. 1. The system houses seven single layer YBa2Cu3O7-x (YBCO) SQUID magnetometers with a 9.2 mm × 8.6 mm directly coupled pickup loop and bicrystal grain boundary junctions. The magnetic field noise levels of the magnetometers in the system are 50–130 fT/√Hz. The sensors are arranged in a dense (2 mm edge-to-edge) hexagonal head-aligned array, which allows for high spatial sampling and a small sensor-to-head standoff distance of 1–3 mm. Using direct injection of current into the SQUID loop as a feedback method, we achieve both low feedback flux crosstalk and reliable temperature-independent sensor calibration.
We present MEG measurements with our system that demonstrate the feasibility of our approach [2] and show that source localization of brain activity is possible. Furthermore, neural activity patterns obtained from stimulating five phalanges of the right hand and the first-ever on-scalp MEG recordings on an epilepsy patient indicate that our on-scalp MEG system allows retrieval of information unavailable to conventional MEG.
Finally, we present the ongoing development of improved sensors for a 21-channel system. We investigate both replacing the grain boundary junctions with the more flexible and cheaper grooved Dayem bridge junctions, as well as using multi-layer flux transformers. Our best single layer grooved Dayem bridge junction-based magnetometer achieves 63 fT/√Hz, while our best bicrystal grain boundary junction-based magnetometer with a 10 mm × 10 mm inductively coupled flux transformer achieves 11 fT/√Hz.
References:
[1] Schneiderman. J. Neurosci. Methods 222, 42–46 (2014). [2] Pfeiffer et al. IEEE T. Bio-Med. Eng. 67, 1483–1489 (2020). [3] Ruffieux et al. Supercond. Sci. Technol. 30, 054006 (2017). [4] Ruffieux et al. Supercond. Sci. Technol. 33, 025007 (2020). [5] Pfeiffer et al. NeuroImage 212, 116686 (2020). [6] Andersen et al. NeuroImage 221, 117157 (2020). [7] Westin et al. Clin. Neurophysiol. 131, 1711–1720 (2020). [8] Trabaldo et al. Nano Lett. 19, 3, 1902-1907 (2019). [9] Trabaldo et al. Appl. Phys. Lett 116, 132601 (2020).
Fig. 1: The 7-channel on-scalp MEG system. a) The cryostat with the sensors behind the vacuum window on the left, the cryostat body in the center, and the breakout box on the right. b) Closeup of the magnetometers. c) Illustration of the head-aligned array. d) Subject during an on-scalp MEG recording.
Keywords: magnetoencephalography (MEG), on-scalp MEG, high-Tc SQUID, MEG system
