Reducing crosstalk in optically-pumped magnetometer arrays

Abstract

Magnetoencephalography (MEG) measures small magnetic fields (femtotesla to picotesla) generated by neuronal currents within the brain and has led to insights into neural activity at millisecond time scales (Supek and Aine 2014). Most MEG systems are based on low-temperature superconducting quantum interference devices (SQUIDs). The SQUIDs are housed in a helmet-shaped liquid helium dewar with roughly 2 cm of space between the SQUIDs and the inside surface of the helmet. Since the magnetic field falls off quickly with distance from the source, e.g. 1/r 3 for magnetic dipoles, the distance between the sensors and scalp is important for the quality of the MEG images. This problem is compounded by the fixed positions of the low-temperature SQUIDs cannot account for differences in a patient's head shape and size. On-scalp sensors, such as optically-pumped magnetometers (OPMs) and arrays of high-temperature SQUIDs (Zhang et al 1993, Dammers et al 2014, Pfeiffer et al 2019), can be arranged in geometries conformal to the head, independent of the size and shape. Recent studies simulating whole-head MEG systems with on-scalp sensors promise greatly increased spatial resolution compared to the current SQUID-based MEG systems (Luessi et al 2014, Boto et al 2016, Iivanainen et al 2017).In the OPMs used here, 87 Rb atoms are confined in a microfabricated vapor cell (Liew et al 2004) made from silicon and glass. Light at 795 nm, on resonance with the D1 transition in 87 Rb, is circularly polarized and used to create a spin-polarization in the 87 Rb atoms. The transmitted light power is altered as a function of the magnetic field at the location of the vapor cell. A detailed description of the sensor geometry can be found in Sheng et al (2017). In order to reach high field sensitivities, the OPMs are operated in the regime where decoherence from spin-exchange collisions is suppressed: at very low ambient magnetic fields and high 87 Rb densities (Happer and Tang 1973, Allred et al 2002). Nitrogen at a density of roughly 1 amagat is used as a buffer gas to inhibit 87 Rb collisions with the cell walls as well as a quenching gas to limit radiation trapping, both of which are necessary to increase the spin-coherence time at high density. The zero-field resonance is measured by monitoring the transmission of light from a single laser beam through the vapor cell (Dupont-Roc et al 1969). This creates a symmetric resonance lineshape centered around zero magnetic field. A small magnetic modulation field of a few kilohertz and about 100 nanotesla is applied in a direction perpendicular to the laser beam. Phase-sensitive detection of the transmission signal at this modulation frequency yields a dispersive resonance as the magnetic

A conformal array of microfabricated optically-pumped first-order gradiometers for magnetoencephalography

“…However, as the distance between magnetometers decreases, the modulating coils from one sensor start to influence the measurements of nearby sensors [27]. To address this problem in hardware, we have developed more sophisticated feedback coils than a simple Helmholtz geometry and have reduced the crosstalk from 8% to 0.8% at the closest possible sensor spacing of 1.25 cm and from 2% to 0.1% between the two magnetometers that constitute a gradiometer with 2 cm spacing [28]. This has not yet been implemented in this array but is a crucial next step in advancing OPM-based MEG technology.…”

Pragmatic spatial sampling for wearable MEG arrays

“…This problem is static (if the sensors do not move relative to one another) and deterministic but assumes that a suitable calibration procedure is in place to correct for these issues 48 . Additionally the use of custom designed on-board coils can reduce the impact of this issue 49 .…”

Spherical harmonic based noise rejection and neuronal sampling with multi-axis OPMs

“…Furthermore, the average sensor spacings are increased from 22mm (for single axis only) to 32mm for triaxial. Increasing the distance between sensors by 50% will have the added benefit of reducing the effects of sensor cross-talk (Nardelli et al, 2019) by a factor > 2.…”