Measurement of Human Auditory Evoked Fields Using a Flexible Multichannel Optically Pumped Magnetometer MEG System

Xin Zhang and others, from the Suzhou Institute of Biomedical Engineering and Technology of the Chinese Academy of Sciences, University of Science and Technology of China, Jihua Laboratory in Foshan, Guangdong Province, and Guoke Medical Technology Development Co., Ltd. in Jinan, Shandong Province, published in the journal “J. Integr. Neurosci.” in 2024.

Background

Magnetoencephalography (MEG) is a non-invasive imaging technique that can directly measure the external magnetic fields generated by the synchronized activation of pyramidal neurons in the brain. Optically Pumped Magnetometers (OPM) demonstrate great potential in MEG-based functional neuroimaging due to their low cost, lack of need for cryogenics, mobility, and user-friendly custom designs. However, traditional MEG systems are bulky, complex, and heavy, limiting experimental flexibility and making them unsuitable for studying children, infants, and subjects with limited mobility. Therefore, developing a wearable system for acquiring brain activity data is highly necessary.

Against this background, the optically pumped magnetometer is a promising device that can operate independently at room temperature and meets the basic technical requirements of wearable brain imaging in terms of weight, brain coverage, and sensitivity. Moreover, the small size, light weight, and wearable characteristics of the OPM allow it to be combined with other modalities, such as Functional Near-Infrared Spectroscopy (fNIRS) or Electroencephalography (EEG).

Research Motivation and Goals

The goal of this study is to measure human auditory evoked fields (AEFs) using a multichannel optically pumped magnetometer MEG system in a magnetically shielded room. To better address motion artifacts and head shape variability in traditional MEG systems, this study developed a flexible helmet and designed a dual-plane coil system with background field and gradient elimination. In addition, the study explored the performance of combining this OPM MEG system with an EEG system.

Source of the Paper and Research Institutions

This research was completed by Xin Zhang et al., from the Suzhou Institute of Biomedical Engineering and Technology, Nanjing University Suzhou Affiliated Hospital, and others. The paper was published on April 30, 2024, in the journal “J. Integr. Neurosci.”

Research Process and Methods

Process Overview

The research process is divided into multiple parts, including: 1. Construction and design of the multichannel optically pumped magnetometer MEG system. 2. Experimental design and data acquisition. 3. Data preprocessing and analysis. 4. Result presentation and discussion.

System Construction and Design

The multichannel optically pumped magnetometer MEG system consists of 15 commercial OPM sensors (Gen-2.0 QZFM, QuSpin Inc., Louisville, Colorado, USA) mounted on an adjustable flexible helmet. The helmet is made of elastic plastic straps, adjustable to fit different head shapes. The sensors are fixed with 3D-printed brackets, allowing easy adjustment of their positions. Additionally, the system operates in a custom magnetically shielded room (MSR) fitted with a dual-plane coil system to eliminate residual magnetic fields.

Experimental Design and Data Acquisition

The experiment involved three subjects aged 28 to 35 years old (two males and one female), approved by the ethics committee of Suzhou Hospital. The experimental task was designed to verify the stability and robustness of the magnetic field elimination system and measure the auditory evoked fields by playing pure tone auditory stimuli to the left ear of the subjects.

Specific Experimental Process

1. Validation of the stability and robustness of the magnetic field elimination system.
2. Experimental design for measuring auditory evoked fields, involving 300 trials, of which 80% were valid trials.
3. Comparative trials for independent system and hybrid system.

Data Processing and Analysis

Data processing was performed using Matlab and the FieldTrip toolbox, including filtering, baseline correction, data segmentation, principal component analysis, and independent component analysis.

Research Results

Magnetic Field Elimination System

The experiment demonstrated the stability and effectiveness of the magnetic field elimination system within a 30 cm cubic range around the head, with residual static magnetic fields of about 2 nT and a residual field gradient of less than 6 nT/m.

Measurement of Auditory Evoked Fields

The auditory evoked fields were successfully measured using the OPM MEG system, and the three subjects showed similar response patterns, indicating that the system could effectively capture auditory evoked signals. These signals peaked at about 50 ms and 100 ms, corresponding to mid-latency and long-latency auditory evoked fields.

Source Localization

Using the Linearly Constrained Minimum Variance (LCMV) algorithm, the sources were localized near the primary auditory cortex in the temporal lobe.

Comparison of EEG and MEG Hybrid System

The experimental results indicated no significant difference in data quality between the hybrid system and the independent systems. The hybrid system could synchronously measure MEG and EEG signals, and the MEG signals showed better resistance to motion artifacts.

Conclusion and Value

This study proposes a novel OPM MEG system that achieves high-quality measurement of auditory evoked fields through a flexible helmet design and field elimination system. Additionally, it explores the feasibility of combining OPM with an EEG system, demonstrating the potential of their integration in neuroimaging. The advantages of this system include adaptability to different head shapes and sizes and high-quality data acquisition even with head movements, expected to expand the application range of MEG measurements and provide a reference for constructing hybrid MEG/EEG systems.

This research provides significant scientific and technical references for the future development of flexible, wearable brain imaging devices.