Active Compensation for OPM-MEG Inside a Two-Layer Magnetically Shielded Room

Measuring brain responses by magnetoencephalography (MEG) is currently in a hardware development phase since a new type of easy-to-use sensor became available. These optically pumped magnetometers (OPM) have many advantages in comparison to the long-employed SQUID sensors, but the requirements for magnetic shielding are more demanding. One is the need to compensate for field fluctuations below 1 Hz, which are a consequence of the decreasing efficiency of magnetic shielding towards lower frequencies. Here a cost effective and simple active compensation is developed, which uses a Helmholtz-like compensation coil together with a simple digital Proportional-Integral (PI) controller. The signal from a single OPM is used as the control loop input variable. An attenuation of 21 dB is achieved in z-direction at 100mHz enabling linear operation of the OPMs. Linear operation is then investigated by measuring MEG while a subject is listening to tones. The experiment is performed twice with active compensation OFF and ON and higher auditory M100 amplitudes are observed for the ON condition.