RF-SQUID to DC-SQUID upgrade of a 28-channel magnetoencephalography (MEG) system

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Abstract
A Dornier 28-channel magnetoencephalography (MEG) system equipped with RF-SQUIDs and control electronics made by CTF (Port Coquitlam, BC, Canada) was upgraded by replacing the RF-SQUIDs with DC-SQUIDs developed at the University of Jena, Germany. New preamplifiers were designed that emulate the function of the system's original RF amplifiers. This allowed the continuing use of the system's control electronics without any modification. Retaining these control devices was instrumental for preserving the system's unique ability to derive software-defined gradiometer measurements from its magnetometer signals, and also saved a considerable investment of control software tailored to the characteristics of these devices. The performance of the upgraded system compares favourably to that of other multichannel instruments; its software gradiometer channels typically attain an overall sensitivity of (an improvement by a factor of three over the original RF system) and suppress residual noise inside the shielded room by 40 to 60 dB. Also described are the procedures used to calibrate the system, to eliminate crosstalk between neighbouring channels, and to determine the coefficients for optimum suppression of ambient noise by the software gradiometer.