Low-T_c direct current superconducting quantum interference device magnetometer-based 36-channel magnetocardiography system in a magnetically shielded room

We constructed a 36-channel magnetocardiography (MCG) system based on low-T_c direct current (DC) superconducting quantum interference device (SQUID) magnetometers operated inside a magnetically shielded room (MSR). Weakly damped SQUID magnetometers with large Steward–McCumber parameter β_c (β_c ≈ 5), which could directly connect to the operational amplifier without any additional feedback circuit, were used to simplify the readout electronics. With a flux-to-voltage transfer coefficient ∂V/∂Ø larger than 420 μV/Ø₀, the SQUID magnetometers had a white noise level of about 5.5 fT·Hz^-1/2 when operated in MSR. 36 sensing magnetometers and 15 reference magnetometers were employed to realize software gradiometer configurations. The coverage area of the 36 sensing magnetometers is 210× 210 mm². MCG measurements with a high signal-to-noise ratio of 40 dB were done successfully using the developed system.