Detection and modeling of the cardiac magnetic field using high-T _c SQUID gradiometers

Single channel MCG systems were used to probe the magnetic field signal from the heart activity in an unshielded or magnetically moderate shielded environment. Using the FFT of magnetometers with the Hanning window overlapped, we optimized the parameters required for the subtraction circuit to minimize the environmental noises in the frequency ranges of interest. Using the optimized parameters, low pass and notch filters, we reduced the environmental noise level much lower than the signal of the heart activity. Hence, we can perform MCG measurements in a magnetically unshielded environment or moderate shielded environment. It was found that a signal to noise ratio of 17 for four-vector gradiometer and 8 for 1^st order gradiometer were achieved in a moderate shielded environment. The four-vector gradiometer further improve the two times the sensitivity inside the moderate shielded room compared with that of the 1^st order gradiometer. To solve the inverse problem we simulated the magnetic field data with the current-dipoles model and obtained the optimized moving orbits of current dipoles. The current density mapping of the circulating current was derived. A comparison between the measured data and simulated results is given.