Abstract
We developed a two-dimensional gradiometer that detects the gradient of a magnetic field in two orthogonal directions to measure the biomagnetic signal in an unshielded environment. We based the gradiometer on a low-Tc superconducting quantum interference device (SQUID) and wire-wound pickup coil. The gradiometer we developed detects both the axial-second-order and planar-first-order gradient of a magnetic field. The experimental results revealed that its noise-reduction ratio (NRR) was 54 dB from 0.5 to 49 Hz and 14 dB (5 times) larger than that of the axial-second-order gradiometer. Moreover, by using the new gradiometer, we obtained a clear magnetocardiography (MCG) waveform in real time without averaging under an unshielded environment (noise level: 3.8 nT/√Hz at 1 Hz; 150 pT/√Hz at 10 Hz).
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