High-Tc SQUID gradiometer system for magnetocardiography in an unshielded environment

We set up an electronic gradiometer for magnetocardiography (MCG) in an unshielded environment. The electronically balanced gradiometer consists of high-temperature radio-frequency superconducting quantum interference device (rf SQUID) magnetometers. The rf SQUID magnetometers are arranged to form the electronic first-order, four-vector gradiometer, or second-order gradiometer. The output of the MCG signal was filtered by a band pass (1–30 Hz) and power line filters. We can reduce the noise to ∼1 pT Hz−1/2 at 1 Hz for second-order or four-vector gradiometers. Two-dimensional MCG imaging is demonstrated. The results are discussed with data measured in the moderate magnetically shielded environment.