Cross-Axis projection error in optically pumped magnetometers and its implication for magnetoencephalography systems

Area of Science:

• Physics
• Biomedical Engineering
• Quantum Sensing

Background:

• Optically pumped magnetometers (OPMs) are crucial for magnetoencephalography (MEG), operating in the spin-exchange-relaxation-free (SERF) regime.
• OPMs typically use alkali atoms like ⁸⁷Rb and lasers to detect magnetic fields perpendicular to optical propagation.
• The SERF regime enhances OPM sensitivity but has limitations regarding remnant static magnetic fields (up to ±5 nT).

Purpose of the Study:

• To investigate the impact of multi-axis magnetic signals on OPMs operating within SERF criteria.
• To identify and characterize deterministic errors, termed cross-axis projection errors (CAPE), in OPM output.
• To assess the consequences of CAPE on the accuracy of OPM-based MEG systems.

Main Methods:

• Simulations were performed to model OPM behavior under various magnetic field conditions.
• Experimental validation was conducted to confirm simulation findings regarding error introduction.
• Analysis focused on the contribution of perpendicular magnetic field components to the OPM signal.

Main Results:

• Multi-axis magnetic signals in small remnant static magnetic fields introduce significant error terms in OPM output.
• These errors, identified as CAPE, cause substantial amplitude and phase distortions in the OPM signal.
• Simulations demonstrated that CAPE can degrade localization and calibration accuracy in OPM-MEG systems.

Conclusions:

• Cross-axis projection errors (CAPE) are a significant deterministic error source in OPMs operating in the SERF regime.
• Understanding and mitigating CAPE is essential for improving the reliability and accuracy of OPM-based MEG.
• Further research is needed to develop strategies for correcting or minimizing CAPE in practical OPM-MEG applications.