A high-performance compact magnetic shield for optically pumped magnetometer-based magnetoencephalography

The rapid development of the optically pumped magnetometer (OPM) has offered a much more flexible method for magnetoencephalography (MEG). Without using liquid helium and its associated dewar device in the OPM detectors, the large and expensive magnetically shielded room (MSR) for traditional MEG systems could be replaced by a compact shield. In the present work, an economic and compact cylindrical shield was designed and built to meet the low-field working requirement of the OPM in detecting human brain neuronal activities. The performance of the compact shield was evaluated and further compared with that of a commercial MSR. Our results showed that the residual magnetic fields and background noise of the compact shield were lower than or comparable to those of the MSR. The remnant field in the shield is found to be 4.2 nT, a factor of 13 000 smaller than the geomagnetic field which is applied to the transverse direction of the shield, and the longitudinal shielding factors measured using a known alternating-current magnetic field are approximately 191, 205, and 3130 at 0.1 Hz, 1 Hz, and 10 Hz, respectively; in addition, the evoked dynamic waveforms in the human auditory cortex that were recorded separately in these two shields demonstrated consistency. Our findings suggested that a compact shield is feasible for OPM-based MEG applications with high performance and low cost.