Development of Peak-to-Peak Voltage Detector-Type MI Gradiometer for Magnetocardiography

We have developed a high-performance magnetoimpedance (MI) gradiometer with a peak-to-peak voltage detector by synchronized switching (shortened: pk-pk VD-type MI gradiometer) for biomagnetic signal measurement in an unshielded environment at room temperature. This newly developed MI gradiometer detects both the positive peak and the negative peak of the induced voltage waveforms in pick-up coils for enhancing sensitivity and reducing noise level. In this paper, we have investigated the sensitivity and noise level of this new MI gradiometer. The noise level we achieved here is lower than 2 pT/Hz(1/2) in an unshielded environment for the frequency range of 1-100 Hz. Meanwhile, we have also demonstrated the pk-pk VD-type MI gradiometer for magnetocardiography (MCG) measurements in an unshielded environment and at room temperature with simultaneous measurements of electrocardiography. We have achieved the real-time measurements of R peak in MCG wave using pk-pk VD-type MI gradiometer with an amplitude of 100 pT, when the distance between the MI sensor and the chest surface is 10 mm, in sitting position. Moreover, a clear R wave and T wave of MCG are observed via pk-pk VD-type MI gradiometer, in averaging over six and eight cycles. The room temperature measurements of R wave and T wave in MCG waveform by averaging processing over 10 s (10-15 cycles) without any shielding equipment will contribute to the diagnosis of heart disease.