A Novel Ultrasonic Doppler Fetal Heart Rate Detection System Using Windowed Digital Demodulation

Fetal heart rate (FHR) is an essential indicator of fetal well-being. The ultrasonic Doppler FHR detector is widely used to monitor the fetus's health due to its advantages of non-invasion, high sensitivity, and good directivity. However, the existing commercial Doppler FHR detector primarily uses an analog multiplier for demodulation, which has limited functions and insufficient sensitivity. The analog demodulation is performed only on a single-sideband signal(in-phase or quadrature) (IQ), which is impossible to derive the fetal heart movement's vector velocity. Moreover, repetitive recognition of the mitral valve's positive and negative motion easily causes the measured FHR twice the actual one (FHR doubling). Although the traditional digital demodulation FHR detector can obtain in-phase and quadrature demodulation signals, it needs a high-level field-programmable gate array chip and therefore cannot meet low-power consumption requirements in battery-powered situations. Herein we proposed a novel digital demodulation FHR algorithm using time-domain windowing. Then, we applied the algorithm into a low-power complex programmable logic device to achieve low-cost acquisition of vector velocity and suppress FHR doubling and finally designed a hand-held, noninvasive digital Doppler FHR detection system for continuous perinatal detection. In this work, the comparison curve between the TREND curve generated by the simulator and the measured curve of our FHR detector was obtained. The comparative experiment between the commercial FM-3A FHR detector and our proposed FHR detector was performed, and the relative error and standard error of the obtained FHR data were used to evaluate the two FHR detectors' detection accuracy. The results showed that our FHR detector showed excellent detection accuracy at an accuracy rate of 98%, which was much higher than the average accuracy of the FM-3A FHR detector and indicated our proposed digital FHR detector showed excellent detection performance. It means that the proposed digital windowed FHR demodulation algorithm is practical, the multi-FHR monitoring system is feasible, and our proposed FHR detector has high detection accuracy, effectively suppresses FHR doubling, and meets the requirements of Chinese national standards. It provides an alternative scheme towards the low-cost hand-held digital demodulation FHR detection application.