a. Field of the Invention
This invention relates to a method of separating a signal from a mixture of two signals where a distorted version of one of the signal is available. It is particularly useful for the estimation of a fetal electrocardiogram (FECG) using signals recorded at the surface of the body. The FECG signal reflects the electrical activity of the fetal heart. It contains information on the health status of the fetus and therefore, an early diagnosis of any cardiac defects before delivery increases the effectiveness of the appropriate treatment. There are several technical problems associated with the extraction of FECG from signals recorded at the abdominal surface which result in a poor signal quality. The main sources of possible noise include the maternal ECG, the maternal electromyogram EMG, (50 or 60) Hz power line interference, baseline wandering and random electronic noise. To simplify the problem, it is assumed that state of the art low noise electronic amplifiers with high common mode rejection ratio are used to eliminate the power line interference and electronic random noise. The EMG noise can also be reduced, but not necessarily eliminated, with the use of classical low pass filtering techniques. Therefore, this invention provides a method of suppressing the maternal electrocardiogram (MECG) to extract the FECG.
b. Related Art
The problem of detecting and separating a desired biomedical signal corrupted by other periodic interference and random noise signals is of extreme importance in medicine. Examples include His Purkinje System Electrogram (HPSE) described by H. Al-Nashash, S. Kelly S. and D. Taylor in “Beat-to-Beat detection of His-Purkinje system signals using adaptive filters”, Med & Biol. Eng. & Comp., vol. 26, pp. 117-125,1988; and Ventricular Late Potentials (VLP)° and the Diaphragmatic Electromyogram (EMGdi) described by H. Al-Nashash, S. Kelly S. and D. Taylor in “Noninvasive beat-to-beat detection of ventricular late potentials”, Med. & Biol. Eng. & Comp., vol. 27, pp. 64-68,1989.
There have been different methods proposed for such extraction including adaptive filters for example in B. Widrow et al., “Adaptive noise canceling: Principle and application,” Proc. IEEE, vol. 63, pp. 1692-1716, Dec. 1975 or E. R. Ferrara and B. Widrow, “Fetal electrocardiogram enhancement by time-sequenced adaptive filtering.” IEEE Trans Biomed Eng., vol. 29, pp. 451-460, 1982., correlation techniques, for example S. Abbond, A. Alaluf, S. Finav, and D. Sadeh, “Real time abdominal fetal ECG recording using hardware correlator,” Comput. Biol. Med., vol. 22, pp. 32-335, 1992, singular-value decomposition (SVD) for example in D. Callaerts, B. D. Moor, J. Vandewalle, and W. Sansen, “Comparison of SVD methods to extract the fetal electrocardiogram from coetaneous electrode signals,” Med Biol. Eng. Comput., vol. 28, pp. 217-224, 1990, blind source separation, for example, V. Zarzoso and A. Nandiin “Noninvasive fetal electrocardiogram extraction: Blind separation versus adaptive noise cancellation”, IEEE Trans. Biomed Eng., vol. 48, no. 1, pp. 12-18, 2001; or L. De Laflaauwer. B. De Moor and J. Vandewalle, “Fetal Electrocardiograph Extraction by Source Subspace Separation”, Proc. IEEE SP/ATHOS Workshop on HOS, Girona, Spain, pp. 134-138, 1995, and wavelet transform, for example, A. Khamene and S. Negahdaripour. “A New Method for the Extraction of Fetal ECG from the Composite Abdominal Signal”, IEEE Trans. Biomed. Eng., vol. 47, no. 4, pp. 507-516, 2000 to mention a few. Despite the reported successes of these methods, they are still not used clinically at a large scale.
Other prior art techniques include U.S. Pat. No. 5,042,499 which describes a fetal heart rate monitor which is used to monitor weak fetal electrocardiogram signals in the presence of strong interfering material ECG complexes, general random background muscle noise, and 60 Hz power line noise. This invention uses an electrocardiographic adaptive cancellation process to cancel the maternal ECG component from an abdominal ECG signal recorded from a pregnant subject's abdomen signal.
U.S. Pat. No 5,372,139 describes a method for suppressing a maternal electrocardiogram signal from a fetal electrocardiogram signal obtained with invasive and non-invasive techniques using an almost pure maternal electrocardiogram signal as a trigger. The MECG suppression is achieved by using a substantially pure MECG trace as a trigger signal to calculate an MECG complex template which is subtracted from the composite signal using the MECG trace to align the MECG complex template with each MECG complex (16) in the composite signal.
U.S. Pat. No 6,751,498 describes an apparatus and method for non-invasive, passive fetal heart monitoring. An apparatus and method for fetal heart and maternal heart and uterine monitoring is described which acquires waveforms indicative of the mother's heart beat from sensors located at or near the mother's chest, and waveforms indicative of the combined maternal and fetal heart beats from abdominal sensors located on the mother's abdomen, lower back, or both. The signals from the abdominal sensors are divided into a plurality of channels. An adaptive signal processing filter (ASPF) algorithm or other suitable algorithm is then used to cancel the estimated maternal waveform from each channel derived from the abdominal sensors. The system then selects from the resulting waveforms at least one waveform to serve as the reference fetal waveform. The reference waveform is then processed against the other abdominal waveforms preferably using the ASPF algorithm again to form an enhanced fetal signal that is a representation of the fetus's electrocardiogram.
U.S. Pat. No 5,123,420 describes a method and apparatus for processing heart rate traces in a fetal monitor. A fetal monitor is disclosed which is capable of recording the heart rate trace, preferably the beat-to-beat heart rate trace, of a fetus and a second heart rate trace of the mother or of a second fetus. Coincidence between the heart rate traces is detected by means of a direct or indirect comparison of the two traces and comparison of the difference with a predefined or adaptive limit and a warning signal is generated if coincidence is detected.