RF-SQUID TO DC-SQUID UPGRADE OF A 28-CHANNEL MAGNETOENCEPHALOGRAPHY (MEG) SYSTEM

A Dornier 28-channel magnetoencephalography (MEG) system equipped with RF-SQUIDs and control electronics made by CTF (Port Coquitlam, BC, Ca nada) was upgraded by replacing the RF-SQUIDs with DC-SQUIDs developed at the University of Jena, Germany. New preamplifiers were designed t hat 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 fo r preserving the system's unique ability to derive software-defined gr adiometer measurements from its magnetometer signals, and also saved a considerable investment of control software tailored to the character istics of these devices. The performance of the upgraded system compar es favourably to that of other multichannel instruments; its software gradiometer channels typically attain an overall sensitivity of 9 fT H z(-1/2) (an improvement by a factor of three over the original RF syst em) and suppress residual noise inside the shielded room by 40 to 60 d B. Also described are the procedures used to calibrate the system, to eliminate crosstalk between neighbouring channels, and to determine th e coefficients for optimum suppression of ambient noise by the softwar e gradiometer.