Probabilistic Source Separation for Robust Fetal Electrocardiography

Blind source separation (BSS) techniques are widely used to extract signals of interest from a mixture with other signals, such as extracting fetal electrocardiogram (ECG) signals from noninvasive recordings on the maternal abdomen. These BSS techniques, however, typically lack possibilities to incorporate any prior knowledge on the mixing of the source signals. Particularly for fetal ECG signals, knowledge on the mixing is available based on the origin and propagation properties of these signals. In this paper, a novel source separation method is developed that combines the strengths and accuracy of BSS techniques with the robustness of an underlying physiological model of the fetal ECG. The method is developed within a probabilistic framework and yields an iterative convergence of the separation matrix towards a maximum a posteriori estimation, where in each iteration the latest estimate of the separation matrix is corrected towards a tradeoff between the BSS technique and the physiological model. The method is evaluated by comparing its performance with that of FastICA on both simulated and real multichannel fetal ECG recordings, demonstrating that the developed method outperforms FastICA in extracting the fetal ECG source signals. 1. Introduction Current fetal monitoring mainly relies on the cardiotocogram (CTG); the simultaneous registration of fetal heart rate; and uterine activity. Unfortunately, in many cases the information provided by the CTG is insufficient. In these cases, obstetricians have to rely on other sources of information or on their intuition and experience to make the optimal treatment plan. A valuable complementary source of information is provided by the fetal electrocardiogram (ECG) [1]. In clinical practice, the fetal ECG is measured during labor using an invasive electrode. The use of this electrode requires the fetal membranes to be ruptured and the cervix to be sufficiently dilated. An alternative method to obtain the fetal ECG makes use of electrodes placed on the abdomen of the mother [2]. Although these abdominal recordings are a promising candidate for use in fetal monitoring, their widespread use is impeded by the quality of the fetal ECG signals which is typically poor. Specifically, each signal recorded from the maternal abdomen consists of a mixture of signals, including the fetal ECG, maternal ECG, activity of abdominal muscles and uterus, and interferences from external sources. Several methods to extract the fetal ECG from such mixtures have been proposed in the literature [2–5]. Prominent among these methods are