A signal-to-noise chart for designing planar high-T_c SQUID gradiometers for magnetocardiographs

To design practical planar high-T_c SQUID gradiometers for magnetocardiographs, a signal-to-noise (S/N) chart that will enable the gradiometers to detect P-waves was developed. The signal was estimated as the gradient field generated by the mean current dipole of P-waves of 20 healthy volunteers. The current dipoles were obtained by using a conventional magnetocardiograph comprising low-T_c SQUID gradiometers. On the other hand, the noise was estimated from studies on previously reported gradiometers based on the limit of intrinsic gradient field noise. The obtained S/N chart indicates that a gradiometer with a baseline longer than 12 mm is necessary for detecting P-waves without averaging. The reliability of the S/N chart was confirmed by detecting actual P-waves with 100-beat averaging by using a fabricated planar high-T_c SQUID gradiometer with a baseline of 6.75 mm.