Development of Active Shielding-Type MI Gradiometer and Application for Magnetocardiography

We have developed an active magnetic shielding system for a magnetoimpedance (MI) gradiometer, in which a reference-type MI element output is fed back to a sensor head with a feedback coil to reduce the effect of environmental magnetic noise around the sensor head, and applied the device to magnetocardiography (MCG). This type of sensor reduced the 60 Hz noise related to the power source line by -40 dB and can operate in an unshielded environment and at room temperature with a noise amplitude of 100 pT. We simultaneously measured the electrocardiography (ECG) and MCG signals at a point on the chest surface near the pit of the stomach. In the output of the developed MI gradiometer, negative magnetic peaks were observed after average processing over 25 cycles, and these signals corresponded to the characteristics of the ECG waveform. The amplitude of the sharp magnetic peak corresponding to the R peak in the ECG was approximately 60 pT. This result corresponded to the value obtained by a low-Tc superconducting quantum interference device. Consequently, we considered the active magnetic shielding system to be effective for a very weak magnetic signal such as MCG.