Matched-filter template generation via spatial filtering: application to fetal biomagnetic recordings.

We have developed a two-step procedure for signal processing of fetal biomagnetic recordings that removes cardiac interference and noise. First, a modified matched filter (MF) is applied to remove maternal cardiac interference; then, a simple signal space projection (SSP) is applied to remove noise. The key difference between our MF and a conventional one is that the interference template and the template scaling are derived from a signal that has been spatially filtered to isolate the interference, rather than from the raw signal. Unlike conventional MFs, ours is able to separate maternal and fetal cardiac complexes, even when they have similar morphology and overlap strongly. When followed by a SSP that preserves only the signal subspace, the noise is reduced to a low level.