New Method for Beat-to-Beat Fetal Heart Rate Measurement Using Doppler Ultrasound Signal

Figures

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  Figure 1. An example of the one-second direct electrocardiogram (FECG) with Doppler ultrasound signals from two transducers (US1, US2) attached to maternal abdomen in different position relative to a fetal heart. In the US signal the cardiac phases are observed: opening and closing the crescent—Ao and Ac, atrioventricular valve—Mo and Mc, atrial wall contraction—Atc and walls heart chambers—Vc.

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  Figure 2. Doppler ultrasound signal (US) with separated components relating to the fetal heart wall (USW) and valve (USV) movements. Corresponding envelopes (ENVW and ENVV) are shown together with an envelope of full signal (ENV).

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  Figure 3. Block diagram generally describing the procedure of subsequent signal processing stages and methods for final evaluation of the results obtained.

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  Figure 4. Doppler ultrasound signal processing. (A) original signal after demodulation; (B) signal after band-pass filtering 100 ÷ 300 Hz (wall movements) and two envelopes determined using 50 Hz low-pass filter and the Hilbert transform; (C) signal after band-pass filtering 300 ÷ 600 Hz (valve movements) and two envelopes as in B.

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  Figure 5. Idea of a trapezoidal window applied for the prediction function prediction function (PF). The upper graph (A) shows the autocorrelation function with PF, whose center is determined by the last correctly determined value of the Ti−1 interval. The lower graph (B) shows that correction of autocorrelation function (AF) by trapezoidal PF which enables determination of the correct periodicity value (τ2) and thus the instantaneous measurement Fj, despite the distortions in the US signal, which gives false AF maximum (τ1) before PF application.

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  Figure 6. General block diagram of the D1 algorithm for determining the instantaneous value of fetal heart rate (Ti interval) based on a raw Doppler US signal.

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  Figure 7. General block diagram of the D2 algorithm for determining the fetal heart rate based on a raw US signal.

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  Figure 8. Idea of searching a starting point within the analysis of the envelope signal (A) for segmentation of instantaneous Fj values using the D2 algorithm based on RMS function (B).

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  Figure 9. Operation of matching algorithm in the segmentation method based on values of instantaneous periodicity measurements Fj (A). Three middle graphs (B–D) show the principle of determining the correction parameter. For seven most recently determined intervals, matching to instantaneous measurement values is tested. If the average difference between Fj values and the corresponding values of the intervals Ti decreases when the intervals are shifted by γ or −γ (repetition step for measurements Fj), then the length of the determined interval is corrected by ε = γ/5 or −ε = γ/5, respectively (E).

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  Figure 10. Synchronization procedure of the fetal heart rate signals: FHR_E—obtained from the reference direct FECG, FHR_U—determined by one of the methods based on a raw Doppler US signal and FHR_CTG—acquired directly from a fetal monitor output.