Optimal filter design for shielded and unshielded ambient noise reduction in fetal magnetocardiography.
The greatest impediment to extracting high-quality fetal signals from fetal magnetocardiography (fMCG) is environmental magnetic noise, which may have peak-to-peak intensity comparable to fetal QRS amplitude. Being an unstructured Gaussian signal with large disturbances at specific frequencies, ambient field noise can be reduced with hardware-based approaches and/or with software algorithms that digitally filter magnetocardiographic recordings. At present, no systematic evaluation of filters' performances on shielded and unshielded fMCG is available. We designed high-pass and low-pass Chebychev II-type filters with zero-phase and stable impulse response; the most commonly used band-pass filters were implemented combining high-pass and low-pass filters. The achieved ambient noise reduction in shielded and unshielded recordings was quantified, and the corresponding signal-to-noise ratio (SNR) and signal-to-distortion ratio (SDR) of the retrieved fetal signals was evaluated. The study regarded 66 fMCG datasets at different gestational ages (22-37 weeks). Since the spectral structures of shielded and unshielded magnetic noise were very similar, we concluded that the same filter setting might be applied to both conditions. Band-pass filters (1.0-100 Hz) and (2.0-100 Hz) provided the best combinations of fetal signal detection rates, SNR and SDR; however, the former should be preferred in the case of arrhythmic fetuses, which might present spectral components below 2 Hz.
Comani
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S
Department of Clinical Sciences and Bio-imaging, Chieti University, Italy. comani@itab.unich.it
Mantini
D
D
Alleva
G
G
Di Luzio
S
S
Romani
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GL
D000465:Algorithms; D001145:Arrhythmias, Cardiac; D005318:Fetal Heart / Q000473:pathology*; D005323:Fetal Monitoring / Q000379:methods*; D005865:Gestational Age; D006340:Heart Rate, Fetal; D006801:Humans; D008280:Magnetics; D015225:Magnetoencephalography / Q000295:instrumentation* / Q000379:methods*; D015233:Models, Statistical; D016011:Normal Distribution; D010363:Pattern Recognition, Automated; D011296:Prenatal Diagnosis; D012815:Signal Processing, Computer-Assisted; D012984:Software