Using LabVIEW and Ni hardware to accurately and safely measure fetal heart rate
challenge: design a low-power optical fetal heart rate monitor to avoid damage to the fetus caused by ultrasound
solution: use NI LabVIEW software and Ni hardware design, and use advanced digital signal processing technology to design a fetal heart rate monitor
"using LabVIEW, we have successfully realized digital synchronous detection and adaptive filtering technology"
fetal heart rate (FHR) detection is a main method for judging the health status of the fetus before birth and helping to identify potential dangers such as fetal hypoxia or compression. The purpose of early detection is to reduce fetal incidence rate and mortality
at present, Doppler ultrasound is the most commonly used way to detect fetal heart rate, and the standard prenatal fetal health test is fetal no load test (NST). These tests are usually completed in hospitals with continuous wave instruments
although the current ultrasonic fetal heart rate detector has been greatly improved, the price has been continuously reduced, and the volume is more compact, we still need accurate sensor calibration and certain professional knowledge to operate the detector correctly. In addition, such instruments are quite sensitive to movement, and the safety problems that may be caused by fetal long-term exposure to ultrasound have not been determined yet. Therefore, the use of the detector is now limited to short-term testing
another method of measuring fetal heart rate is fetal electrocardiogram (FECG), but its steps are more complex and less practical. Moreover, there is no commercial non-invasive FECG device on the market at present
recently, someone proposed an optical method that is still in the research stage. This method uses halogen lamp or tungsten filament lamp as the light source and realizes detection through photomultiplier. However, these technologies are expensive, require high light intensity, and are difficult to achieve due to the limitations of instrument size and power consumption
optical fetal heart rate detection system
our research team proposed a low-power optical technology based on photoelectric blood vessel plethysmography (PPG) signal to detect fetal heart rate noninvasively. PPG signal is produced by light modulated by blood pulse. Doctors or technicians irradiate the abdomen of pregnant women with LED lights (less than 68 MW), and the light beam is modulated through the blood circulation of the mother and fetus. The maximum light wavelength that can be penetrated is 890 nm. The mixed signal can be analyzed by using the adaptive filter obtained by digital signal processing, and the PPG of the pregnant woman's index finger is used as the reference input
the optical fetal heart rate (ofhr) detection system is developed by using the uniaxial compressive strength view graphical system design software and Ni hardware, which is called the load received by lab per unit area. In the ofhr system, SNR decreases with the decrease of incident power; The excitation signal is the modulated beam. The system can implement synchronous detection, and the software subroutine in LabVIEW uses Ni 9474 digital output module to generate modulation frequency at the counter end
at the receiver, low noise amplification and synchronous detection ensure that useful information is saved with minimum noise power. The 24 bit Ni usb-9239 analog-to-digital converter (ADC) reduces the impact of quantization noise. Once the digitization is completed, the fetal PPG is extracted from the mixed signal after the signal is processed by adaptive noise canceller (ANC) technology
connect the fetal probe (main signal) to the abdomen of the pregnant woman with a belt, and keep the ir-led at a distance of 4 cm from the photoelectric detector. Connect the reference probe to the mother's index finger. Since the selected ir-led can only emit a maximum power of 68 MW, the working optical power of the ofhr system is set to be less than 87 MW specified by the International Commission on non ionizing radiation protection (ICNIRP). In order to modulate ir-led, 725 Hz modulation signal is generated by software subroutine and connected to LED driver via Ni 9474 counter (Fig. 1). In Figure 1, the diffuse reflected light from the abdomen of a pregnant woman is measured by a low-noise photodetector and expressed in the form of I (M1, f), where M1 and f represent the influence of the mother's abdomen and fetus on the signal, respectively
Figure 1: the hardware module in the ofhr system block diagram is realized by LabVIEW program.
low noise (6 nv/hz1/2) transimpedance amplifier converts current into voltage. The reference probe (connected to the mother's index finger) consists of an ir-led and a solid-state photodiode with a built-in preamplifier. The signal from this probe is expressed as I (M2); M2 represents the mother's influence on the signal. This channel does not need to be detected synchronously, because the photoelectric vascular volume map of the index finger has a high signal-to-noise ratio (SNR)
ni usb-9239 24 bit resolution data acquisition module synchronously collects signals from two probes at a rate of 5.5 kHz. Demodulation, signal filtering, and signal estimation are performed in the digital domain. The software includes modulation signal generation, synchronous detection algorithm, down sampling, high pass filtering, and adaptive noise cancellation (ANC) algorithm
the design team adopts LabVIEW to realize the whole algorithm and some instruments. After the preprocessing and application of ANC algorithm, LabVIEW will display the results of fetal signal and fetal heart rate
figure 2A shows the laboratory prototype and graphical user interface of the ofhr system, and gives the PPG of the pregnant woman's index finger (upper), abdominal PPG (middle), and the estimated PPG of the fetus (lower)
figure 2a:ofhr prototype
figure 2B shows three optional displays, including digital synchronous or phase-locked amplifier (LIA), adaptive noise canceller (ANC), and heart rhythm trajectory. The first two displays can be used to assist development, and the third display is used to indicate the value of the relative time of fetal heart rate. Users can observe the data or save it for further analysis
figure 2B: after the development of the graphical user interface of the ofhr system, we tested the functionality of the system based on a total of 24 groups of data from six clinical subjects ranging from 35 to 39 weeks of pregnancy, and the data were provided by the medical center of National University of Malaysia. All fetuses participating in this study were examined by obstetricians to be in a healthy state, and there were no complications at birth
in the study, we obtained a correlation coefficient of 0.97 between optical and ultrasonic fetal heart rate (P value is less than 0.001), and the maximum error is 4%. Clinical results show that the closer the probe is to fetal tissue (not limited to brain or hip), the better the signal quality and detection accuracy
the research team has developed a new ofhr detection system using low-cost, low-power IR lamps and commercially available silicon detectors. By using LabVIEW, we can quickly and easily realize digital synchronous detection and adaptive filtering technology. Compared with the standard measurement method (Doppler ultrasound), the accuracy of our fetal heart rate results is higher. Based on the novelty of the scheme, we are currently applying for a patent for its commercial use
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