Compensation System for Biomagnetic Measurements with Optically Pumped Magnetometers inside a Magnetically Shielded Room

Abstract

Magnetography with superconducting quantum interference device (SQUID) sensor arrays is a well-established technique for measuring subtle magnetic fields generated by physiological phenomena in the human body. Unfortunately, the SQUID-based systems have some limitations related to the need to cool them down with liquid helium. The room-temperature alternatives for SQUIDs are optically pumped magnetometers (OPM) operating in spin exchange relaxation-free (SERF) regime, which require a very low ambient magnetic field. The most common two-layer magnetically shielded rooms (MSR) with residual magnetic field of 50 nT may not be sufficiently magnetically attenuated and additional compensation of external magnetic field is required. A cost-efficient compensation system based on square Helmholtz coils was designed and successfully used for preliminary measurements with commercially available zero-field OPM. The presented setup can reduce the static ambient magnetic field inside a magnetically shielded room, which improves the usability of OPMs by providing a proper environment for them to operate, independent of initial conditions in MSR.

Magnetic Field Mapping and Correction for Moving OP-MEG

“…Here we discussed the change in the OPM recordings from movement while the OPMs are operating within their dynamic range of ±5.56 nT [ 19 ]. We are fortunate that the central part of our room meets these specifications after degaussing the inner mu-metal layer, but for other rooms or different ranges of movement, we envisage that real-time field correction may be required to keep the OPMs within their operating range.…”

“…It is partially for this reason that alpha (8)(9)(10)(11)(12)(13)(14)(15), beta (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30) and gamma (>30 Hz) activity has successfully been recorded with OP-MEG during movement [9], [10], [15], theta (4-8 Hz) has been recorded while the participant was unconstrained [16], but delta and infra-slow waves (<4 Hz) remain a topic of future research. A number of methods have been suggested for minimizing changes in the background magnetic field during an OP-MEG experiment, with the most successful involving the placement of electromagnetic coils around the participant [13], [17]- [19]. The currents in these coils can be adjusted to minimize the magnetic field within the volume around the head, meaning that when the participant moves, the change in field is minimal [9], [17].…”

“…A number of methods have been suggested for minimizing changes in the background magnetic field during an OP-MEG experiment, with the most successful involving the placement of electromagnetic coils around the participant [ 13 ], [ 17 ]–[ 19 ]. The currents in these coils can be adjusted to minimize the magnetic field within the volume around the head, meaning that when the participant moves, the change in field is minimal [ 9 ], [ 17 ].…”

Noise Analysis in Pre-Amplifier Circuits Associated to Highly Sensitive Optically-Pumped Magnetometers for Geomagnetic Applications

“…The Rabi frequency Ω is proportional to the RF magnetic field. From Equation (11), when the RF power (the Rabi frequency) increases, the signal amplitude reaches a maximum value. In this case, the sensitivity of the magnetometer can be optimized.…”

“…Compared to the SQUID (Superconducting Quantum Interference Device) magnetometers, the OPMs are able to operate under room temperature, to provide a high resolution including a bandwidth from DC (direct current) to several hundred/thousand Hz [1][2][3]. Thus, the OPMs are widely used in geophysical explorations, aerospace developments, information receiving, bio-magnetism researches, etc., [4][5][6][7][8][9][10][11].…”