Scalp attached tangential magnetoencephalography using tunnel magneto-resistive sensors

Abstract

Non-invasive human brain functional imaging with millisecond resolution can be achieved only with magnetoencephalography (MEG) and electroencephalography (EEG). MEG has better spatial resolution than EEG because signal distortion due to inhomogeneous head conductivity is negligible in MEG but serious in EEG. However, this advantage has been practically limited by the necessary setback distances between the sensors and scalp, because the Dewar vessel containing liquid helium for superconducting quantum interference devices (SQUIDs) requires a thick vacuum wall. Latest developments of high critical temperature (high-Tc) SQUIDs or optically pumped magnetometers have allowed closer placement of MEG sensors to the scalp. Here we introduce the use of tunnel magneto-resistive (TMR) sensors for scalp-attached MEG. Improvement of TMR sensitivity with magnetic flux concentrators enabled scalp-tangential MEG at 2.6 mm above the scalp, to target the largest signal component produced by the neural current below. In a healthy subject, our single-channel TMR-MEG system clearly demonstrated the N20m, the initial cortical component of the somatosensory evoked response after median nerve stimulation. Multisite measurement confirmed a spatially and temporally steep peak of N20m, immediately above the source at a latency around 20 ms, indicating a new approach to non-invasive functional brain imaging with millimeter and millisecond resolutions.

Development of Magnetocardiograph without Magnetically Shielded Room Using High-Detectivity TMR Sensors

“…Therefore, magnetic field shielding by software is crucially important. Gradiometers are widely used to reduce noise disturbance [ 17 , 20 ]. While SSS requires a large number of sensors for the calculation process according to the degree of calculation ( , ), a gradiometer has advantages in that it needs only two sensors, and the configuration of the system is simple and easy to handle.…”

“…In fact, the quadrupole has been observed in the magnetocardiogram of patients with Brugada syndrome [ 8 ], which suggests a higher spatial resolution brings a better diagnostic accuracy. In addition, it has been reported that brain magnetic fields, which are far smaller than heart magnetic fields, have been measured with scalp attached magnetoencephalography with TMR sensors [ 17 ]. Thus, TMR sensors, which are approachable to the signal source, are adequate for magnetocardiographs.…”

A Spherical Coil Array for the Calibration of Whole-Head Magnetoencephalograph Systems

“…In contrast to SQUID sensors, sensor arrays of "on-scalp" MEG systems that use non-cryogenic magnetic flux sensors, such as optically pumped magnetometers (OPMs) [22][23][24][25] or magnetoresistive (MR) sensors [26,27], have recently gained increasing attention. Importantly, the sensor arrays of on-scalp MEG systems have small distortions because they are not exposed to extremely low temperatures.…”

Neuromorphic applications in medicine

“…Because of these advancements, researchers have further investigated the potential for spintronic devices for neuromorphic computing. Kanno et al discusses how magnetic tunnel junctions can be adapted to behave similarly to neurons and synapses [133], and how differing magnetic textures, like those mentioned regarding tunable magnetic materials, can emulate functioning neurons [134,135]. Scientists believe that the first integrations of spintronics in neuromorphic hardware will likely be the digital magnetic memories they can provide, which has a proven history in conventional circuit design.…”