Reduction of Magnetic Noise Originating from a Cryocooler of a Magnetoencephalography System Using Mobile Reference Sensors

The use of helium recycling systems for magnetoencephalography (MEG) has rapidly expanded in recent years. Placing the cryocooler close to the MEG cryostat is expected to aid in achieving a high helium-recycling efficiency. However, the cryocooler typically introduces a large amount of magnetic interference in the MEG system owing to mechanical vibration, and electric and magnetic noise. In this study, we proposed a mobile reference sensor method using fluxgate magnetometers and vibration meters to reduce the noise originating from the cryocooler. We placed the MEG cryostat, which included superconducting quantum interference device (SQUID)-based MEG sensors and reference magnetometers, and a pulse-tube cryocooler inside a magnetically shielded room. We recorded the vibration and magnetic noise during the operation of the cryocooler using the vibration meters and fluxgate magnetometers placed on the cryocooler and reference SQUID magnetometers as the reference noise data. A noise reduction algorithm named time-shift principal component analysis (TSPCA) was applied to the recorded MEG sensor data by employing the reference noise data. A noise reduction ratio of –39 dB was achieved without temporal and spatial distortion of the MEG signal using an MEG phantom. Thus, we conclude that the mobile reference sensor method with the TSPCA algorithm is an effective technique for reducing the magnetic noise originating from the cryocooler.