Phantom-Based Approach for Comparing Conventional and Optically Pumped Magnetometer Magnetoencephalography Systems

Abstract

Magnetoencephalography (MEG) is a vital tool for understanding neural dynamics, offering a noninvasive technique for measuring subtle magnetic field variations around the scalp generated by synchronized neuronal activity. Two prominent sensor technologies exist: the well-established superconducting quantum interference device (SQUID) and the more recent optically pumped magnetometer (OPM). Although many studies have compared these technologies using human-subject data in neuroscience and clinical studies, a direct hardware-level comparison using dry phantoms remains unexplored. This study presents a framework for comparing SQUID- with OPM-MEG systems in a controlled environment using a dry phantom that emulates neuronal activity, allowing strict control over physiological artifacts. Data were obtained from SQUID and OPM systems within the same shielded room, ensuring consistent environmental noise control and shielding conditions. Positioning the OPM sensors closer to the signal source resulted in a signal amplitude approximately 3-4 times larger than that detected by the SQUID-MEG system. However, the source localization error of the OPM-MEG system was approximately three times larger than that obtained by the SQUID-MEG system. The cause of the large source localization error was discussed in terms of sensor-to-source distance, sensor count, signal-noise ratio, and the spatial coverage provided by the sensor array of the source signal.

Keywords: dry phantom; magnetoencephalography; optically pumped magnetometers; superconducting quantum interference device.

Figure 7

Butterfly plot of recorded signals from SQUID- and OPM-MEG sensors during the activation of the triangular phantom coils (ECD 1–49): (a) SQUID-MEG system; (b) OPM-MEG system (closest position); (c) OPM-MEG system (lift-off position). The arrows indicate the analysis timing for ECD 49 as represented in Figure 9.

Figure 8

Noise spectra recorded by the SQUID magnetometers simultaneously with the phantom measurements using (a) SQUID-MEG system; (b) OPM-MEG system (closest position); (c) OPM-MEG system (lift-off position).

Figure 9

Magnetic field distribution from the 49th ECD recorded by each sensor configuration: (a) SQUID-MEG system; (b) OPM-MEG system (closest position); (c) OPM-MEG system (lift-off position). Small circles indicate the relative position of the sensors.

Figure 10

Source localization error defined as the displacement between the effectual and estimated ECD positions: (a) SQUID-MEG system; (b) OPM-MEG system (closest position); (c) OPM-MEG system (lift-off position).

Figure A1

Photograph and dimensions of the marker coil used in this study.

Figure A2

Sequence of current application to the marker coils for the experiments with (a) SQUID-MEG system and (b) OPM-MEG system. In the same way as in Figure 6b, t_osc is the reciprocal of the frequency of the oscillator of the current-driver circuit.

Figure 1

Configuration of the SQUID- and OPM-MEG systems in a single MSR: (a) schematic of the measurement setup; (b) photograph of the setup at NYUAD.

Figure 2

Noise spectra of the SQUID- and OPM-MEG systems.

Figure 3

Photograph and configuration of the dry-type MEG phantom: (a) closeup view of the triangular coil; (b) photograph of the dry-type MEG phantom; (c) schematic and dimensions of the phantom.

Figure 4

Experimental setup with the dry phantom: (a) SQUID-MEG system; (b) OPM-MEG system (closest position); (c) OPM-MEG system (lift-off position).

Figure 5

Positions of the sensors (circles), effectual ECDs (triangles), and center of the conductive sphere model (squares): (a) SQUID-MEG system; (b) OPM-MEG system (closest position); (c) OPM-MEG system (lift-off position).

Figure 6

A current-driver circuit for the dry phantom: (a) schematic diagram; (b) waveforms of the current sequentially applied to the phantom.

Figure 11

Selected 90-channel SQUID sensors for re-estimation. Circles, triangles, and squres indicate the positions of the sensors, effectual ECDs, and center of the conductive sphere model, respectively.

Figure 12

Dependence of the source localization error of the SQUID-MEG system on the number of sensors.

Figure 13

Source localization error defined derived using the data of 35 ECDs excluding the data from 14 ECDs located below the rim of the sensor array: (a) SQUID-MEG system; (b) OPM-MEG system (closest position); (c) OPM-MEG system (lift-off position).