Multi-channel HTS rf SQUID gradiometer system recording fetal and adult magnetocardiograms

We report on experiments with a multi-channel HTS radio-frequency (rf) SQUID gradiometer for recording fetal and adult magnetocardiograms. Four sensing SQUID magnetometers and two common reference SQUID magnetometers form a 4 channel electronic gradiometer system of either first or second order. The magnetometers consist of HTS step edge SQUIDs and flux concentrators fabricated from YBaCuO thin films, with dielectric substrate resonators serving as tank circuits. With a washer area of 18 mm in diameter, all six magnetometers reached a field sensitivity of 20-30 fT/√Hz. Each gradiometer channel is formed using two or three such magnetometers with individual readouts in electronic difference. The dc and rf crosstalk between any channel pair was measured. In ordinary operation we did not find any noise contribution from neighboring channels, even though the resonant frequencies of the resonators are closely spaced. In a standard magnetically shielded room, using a first-order gradiometer configuration with an ultra-long baseline of about 20 cm, we demonstrated 4 channel real-time heart signal recordings of a fetus in the 33rd week of gestation.