A high-temperature rf SQUID system for magnetocardiography

A first-order axial electronic gradiometer having a baseline of 10 cm was constructed by assembling two YBa2Cu3O7-delta rf SQUID magnetometers with coplanar tank resonators, each having a white magnetic field resolution of about 20 fT HZ(-1/2) at 77 K. The gradiometer's near-field resolution was about 35 fT HZ(-1/2), including the Dewar flask's noise. A peak-to-peak noise level of 3 pT was obtained in the bandwidth 0.016-250 Hz. Magnetocardiographic (MCG) measurements were performed using this bandwidth. Measurements on human subjects have been conducted in a magnetically shielded room of moderate shielding factor. Using the signal either of the lower magnetometer or of the gradiometer, high-quality heart signal traces could be collected, which were suitable for diagnostic use. A team of physicians, assisted by two of the authors, used the equipment over 10 months to perform MCG measurements in a medical study of about 80 clinical patients with cardiac arrhythmia problems and healthy persons. The system's performance was stable over that whole period.