Recording fetal and adult magnetocardiograms using high-temperature superconducting quantum interference device gradiometers

In this paper, we analyze the influence of the superconducting quantum interference device (SQUID) gradiometer baseline on the recording of magnetocardiographic measurements. The magnetometers consist of high-temperature superconducting radio-frequency SQUIDs fabricated from YBaCuO thin films' and a substrate resonator which serves as tank circuit. The gradiometers are formed using two or three such magnetometers with individual readouts in electronic difference. We have compared the measurement results using a magnetometer and first- and second-order gradiometers with different baselines. In a standard magnetically shielded room, we found not only an increasing signal-to-noise ratio in adult magnetocardiographic measurements, but also a decreasing distortion of the magnetic field map with increasing baseline of the gradiometer. Using a first-order gradiometer with an ultralong baseline of 18 cm, we have successfully measured the heart signal of a fetus in real time.