Adaptive frequency dependent gradiometry applied to SQUID magnetocardiography

In the case of magnetocardiography (MCG), gradiometry is a commonly used method to reduce environmental noise. Here, the balance of the gradiometer is one of the main parameters determining the achievable noise reduction. In earlier work, we reported the use of frequency dependent gradiometer coefficients to effectively balance a gradiometer consisting of signal and reference sensors. In this work, the advantages of adaptive determination of these coefficients during a measurement without perturbing the MCG signal are investigated. MCG measurements were recorded with first and second order high temperature SQUID gradiometers inside and outside magnetic shielding. It will be shown that the heart signal is not distorted by applying our software noise reduction technique. Furthermore, by recalculation of gradiometer coefficients during the MCG measurement, an improved signal-to-noise ratio is achieved. The optimum repetition time for coefficient recalculation is determined.