High Tc Superconducting Shielded Biomagnetometer System

SQUID sensor array configurations for magnetoencephalography applications

“…Environmental noise cancellation can be accomplished by hardware means (shielded rooms, active shielding) or by synthetic means either using references or directly from the measured signal. Hardware noise cancellation methods were described in detail [12,[40][41][42][43][44][45], and this section will emphasize the synthetic methods. Synthetic sensors can be realized by the linear combination of primary sensors with either reference sensors or other primary sensors.…”

“…The magnetometer traces correspond to three orthogonal components and indicate that at 0.1 Hz the noise magnitude is about 1 nT rms Hz −1/2 and the intersect of the magnetometer low frequency noise with the sensor white noise level (about 5 f T rms Hz −1/2 ) is at about 10 Hz (at some sites the onset of magnetometer low frequency noise can be as high as 70 Hz). This result indicates that it is not possible to use bare magnetometers when the system is positioned in conventional MEG shielded rooms (nonetheless, magnetometers can be used in high temperature whole body superconducting shields with about 10 8 low frequency attenuation [40] or in high quality µ-metal rooms with nearly 10 5 passive and more than 10 7 active shielding at low frequencies [41]).…”

“…The difference between the thin dashed line (magnetometers in noiseless case) and the heavy solid line (synthetic third-order gradiometers in the presence of noise or radial gradiometers with 5 cm baseline in the absence of noise) is the basis for occasional claims that the magnetometers can resolve smaller dipoles than the gradiometers. However, even in the noiseless situation, the advantage of magnetometers over gradiometers is very small and since in practice the assumption of perfect shielding is not realizable (except within special high performance shields [40,41]), the gradiometers consistently exhibit a better performance. The performance of planar gradiometers in a noiseless environment is also shown in figure 34 by a thin dash-dot-dash line.…”