Development of precise off-diagonal magnetoimpedance gradiometer for magnetocardiography

We have developed a precise off-diagonal magnetoimpedance (MI) gradiometer that can operate in an unshielded environment and at room temperature with 200 pT root-mean-square noise in a 100 Hz bandwidth. The MI sensor probe is compact and easy to handle. The achieved noise level corresponds approximately to the maximum magnetocardiography (MCG) signal reported so far. We have performed MCG measurements using the developed gradiometer system in an unshielded environment, and a real-time signal like MCG can be identified by the MI gradiometer when the distance between the sensor head and chest surface is less than 3 mm. However, the signal seems to be affected by the movement of the chest surface caused by the heartbeat. A peak magnetic signal of 100 pT (corresponding to conventional MCG) was observed when the sensor head was set 10 mm apart from the chest surface to avoid the influence of the chest movement. Under such conditions, the signal needed to be averaged over more than 50 cycles to identify the peak magnetic signal