A tuned SQUID amplifier for MRI based on a DOIT flux locked loop

We have developed a 4.2 K, flux locked, tuned d.c. SQUID amplifier to improve the SNR of a low field MRI system operating at 0.01 T (425 kHz). The flux locked loop, based on the Direct Offset Integration Technique, has a noise level of 2.6 /spl mu//spl Phi//sub 0/ Hz/sup -1/2/ and a slew rate of 1.6/spl times/10/sup 6/ /spl Phi//sub 0/ s/sup -1/ at 425 kHz when used with a commercially obtained SQUID. The high intrinsic Q-factor of the MRI pick-up coil is damped by the action of the loop and by an additional feedback circuit to provide imaging bandwidths of up to 10 kHz. We have developed a special low noise liquid helium cryostat so that the final system has a magnetic field resolution of 0.08 fT Hz/sup -1/. This receiver was used in a small scale MRI system to image non-conducting test objects and the human arm. The images show significant improvements in SNR over those obtained with an equivalent room temperature receiver.