SQUID gradiometry for magnetocardiography using different noise cancellation techniques

Magnetocardiographic (MCG) measurements in unshielded environment require efficient noise cancellation techniques. We have applied two software gradiometry methods to analyze the time series of signal and reference data recorded outside magnetic shielding with high temperature superconducting quantum interference device (HTS SQUID) based gradiometers. One method uses adaptive frequency dependent gradiometer coefficients determined in the Fourier domain to subtract the reference from the signal data. The other method combines recently developed techniques for nonlinear projection with properties of the wavelet transform to extract noise in state space. The analyzed MCG data sets showed improved signal-to-noise ratios for both methods as compared to the data recorded with the electronic gradiometer. In this way, it is possible to increase the bandwidth from 130 Hz for our electronic gradiometer to 250 Hz without using any additional filtering.