Real-time Noise Reduction: 4D Neuroimaging 2500WH System

The 2500WH MEG system by 4D Neuroimaging has magnetometer detectors that rely on real-time noise reduction in addition to shielding of a magnetically shielded room. Theoretically, the set of reference magnetometers and gradiometers, distant from the recording array, are sufficient to remove environmental noise through first order spatial variation within the shielded room. However, in magnetically shielded rooms, environmental magnetic fields in the center of the room are minimized while high order spatial variations are increased. The proportion of the high order field components that are remove from the MEG data depends on the correlation of the high order components with the low order components as well as the configuration of the active noise reduction system. In addition to reducing environmental noise, the real-time noise reduction hardware/software introduces noise into MEG signals. The actual performance of the noise reduction system depends on operational characteristics of the hardware/software as well as environmental noise characteristic of the MEG installation site. We examined the procedures used to calculate reference channel weight factors. Second, we assessed the performance characteristics of the noise reduction hardware/software including input-output response characteristics. Third, we consider briefly the implication of the noise reduction performance on the MEG data for typical MEG applications, (signal averaged evoked responses, epileptic spike activity, and power spectral analysis).