CN112998752B - Fetal heart detection method based on fetal heart position guidance - Google Patents

Fetal heart detection method based on fetal heart position guidance Download PDF

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CN112998752B
CN112998752B CN202110194530.4A CN202110194530A CN112998752B CN 112998752 B CN112998752 B CN 112998752B CN 202110194530 A CN202110194530 A CN 202110194530A CN 112998752 B CN112998752 B CN 112998752B
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fetal heart
signal
transducer
measuring point
vector
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CN112998752A (en
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黄海
姚剑
林晓强
陆旭
耿晨歌
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Zhejiang Zhongming Health Technology Co ltd
Wenzhou Research Institute Of Zhejiang University
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Zhejiang Zhongming Health Technology Co ltd
Wenzhou Research Institute Of Zhejiang University
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    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
    • A61B8/02—Measuring pulse or heart rate
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
    • A61B8/48—Diagnostic techniques
    • A61B8/488—Diagnostic techniques involving Doppler signals
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B8/00—Diagnosis using ultrasonic, sonic or infrasonic waves
    • A61B8/52—Devices using data or image processing specially adapted for diagnosis using ultrasonic, sonic or infrasonic waves
    • A61B8/5215—Devices using data or image processing specially adapted for diagnosis using ultrasonic, sonic or infrasonic waves involving processing of medical diagnostic data

Abstract

The invention discloses a fetal heart detection method based on fetal heart position guidance, which comprises the following steps: the method comprises the steps of ultrasonic transducer design, fetal heart instrument ultrasonic probe design, fetal heart signal feature extraction algorithm design and fetal heart position guiding algorithm design. The fetal heart detection method can realize the fetal heart position searching function and give the fetal heart orientation information, can guide a pregnant woman without fetal heart measurement experience to find an accurate fetal heart measurement position, thereby obtaining an accurate fetal heart rate, solves the problems of large fetal heart rate counting error, fuzzy fetal heart sound and the like caused by inaccurate fetal heart measurement position, has a large fetal heart signal detection range, is accurate in fetal heart rate measurement and calculation, is simple to operate, and can meet the fetal heart detection requirement in a family scene.

Description

Fetal heart detection method based on fetal heart position guidance
Technical Field
The invention belongs to the technical field of fetal heart detection, and particularly relates to a fetal heart detection method based on fetal heart position guidance.
Background
With the improvement of the national living standard and the conception of prenatal and postnatal care, people have increasingly increased demands for health monitoring, and the health problems of pregnant women and fetuses become the focus of the society. Fetal heart monitoring is an important means for fetal health monitoring, and can effectively guarantee the health of a fetus and a pregnant woman; the traditional fetal heart monitoring mode cannot achieve autonomous and day-by-day continuous monitoring, and the household fetal heart monitor is suitable for operation and widely applied. At present, most of household fetal heart detectors adopt a single-transducer and single-frequency ultrasonic Doppler detection technology, and although the fetal heart detectors are convenient to use and low in price, the fetal heart detectors have the problems of small detection range, low detection sensitivity, large noise and the like. Meanwhile, pregnant women often lack professional clinical knowledge and often cannot find accurate fetal heart measurement positions in autonomous use, so that the problems of large fetal heart rate error, fuzzy fetal heart sound and the like in detection are caused, and effective fetal heart monitoring cannot be realized.
At present, a fetal heart detecting device based on ultrasonic Doppler generally has the problem of small fetal heart position detecting range, because ultrasonic waves with frequencies have strong directivity, ultrasonic signals are strong on a propagation path of the ultrasonic waves, and signals in other areas are weak. Therefore, in the fetal heart detection process, if an accurate fetal heart detection position cannot be found, the problems of large fetal heart rate detection error, fuzzy fetal heart sound and the like are easily caused; the detection range of the ultrasonic probe is much smaller than that of the abdomen of the pregnant woman, so that the ultrasonic probe needs to be moved continuously when the pregnant woman wants to find an accurate fetal heart detection position, and the detection process is time-consuming and labor-consuming and often returns without work due to the fact that the fetal heart position is not guided or prompted.
To solve the above problems, researchers at home and abroad have conducted a lot of research and practice. European Union patent publication No. WO9747242A1 proposes a fetal heart detection method for adjusting ultrasonic energy level, which increases the sensitivity of a probe to improve the detection range of the probe; although the method expands the fetal heart detection range to a certain extent, the improvement of the ultrasonic energy may affect the fetus, and has certain potential safety hazard. The document [ Smith L, set-Bororforos M, Thomenius K, et al.Total heart rate monitor with a de search area U.S. patent Application 12/649,735[ P ].2011-6-30] proposes a fetal heart detection method combining a plurality of transducers and an acoustic lens, so as to increase the effective detection range of an ultrasonic probe, expand the detection range of a single transducer by using the acoustic lens, respectively detect fetal hearts by using a plurality of transducers in a grouping mode, and select the best combination mode of the signal to noise ratio to detect fetal heart signals; although the method expands the effective detection range to a certain extent, the ultrasonic probe has too large volume due to more transducers, and the method does not meet the requirements of small size and portability of a household fetal heart monitor. Based on the same idea, chinese patent publication No. CN102499673A proposes a multi-chip ultrasonic transducer fetal heart detection method, which performs fetal heart signal acquisition by grouping methods respectively, and selects the chip group with the best signal quality for signal acquisition.
In the existing fetal heart detection methods, the problems of complex system, overlarge equipment volume, high cost and the like exist, and the requirements of a household fetal heart instrument are not met.
Disclosure of Invention
In view of the above, the invention provides a fetal heart detection method based on fetal heart position guidance, which can guide a pregnant woman without fetal heart measurement experience to find an accurate fetal heart measurement position, thereby obtaining an accurate fetal heart rate.
A fetal heart detection method based on fetal heart position guidance comprises the following steps:
(1) acquiring fetal heart signals of a current measuring point by using an ultrasonic probe of an ultrasonic Doppler fetal heart monitor, wherein the ultrasonic probe is provided with a transducer H5 for acquiring the fetal heart signals and four transducers H1-H4 for detecting the position, the transducer H5 is positioned at the central position of the probe, and the transducers H1-H4 are respectively positioned at the north, east, south and west positions of the transducer H5 on the plane of the probe;
(2) extracting feature information which can effectively reflect fetal heart distance information from fetal heart signals collected by the transducers H1-H4, wherein the feature information comprises periodic features, coefficient of variation features, effective energy ratio features and power spectrum entropy features, and then fusing the four groups of features through an optimal weighting method;
(3) according to the fetal heart signal characteristic value dis correspondingly fused with the transducers H1-H41～dis4Guiding the user to find the right by the fetal heart position guiding algorithmTo acquire a good quality fetal heart signal.
Further, the diameter of the transducer H5 is 18-22 mm, and the resonance frequency is 2.5-3.8 MHz; the diameters of the transducers H1-H4 are 8-10 mm, and the resonant frequency is 200-250 KHz.
Furthermore, 5-15-degree inclination angles are arranged between the transducers H1-H4 and the transducer H5, so that ultrasonic waves emitted by the transducers H1-H4 are diffused to the periphery of the probe, and the coverage area of a sound field of the probe is increased.
Further, the calculation method of the periodic characteristics comprises the following steps: firstly, calculating an autocorrelation function of a fetal heart signal by the following formula;
Figure BDA0002944767810000031
wherein: n is a radical of0Is the length of a section of fetal heart signal (generally 200-400 sampling points), n and m are natural numbers, x (n) is the signal value of the nth sampling point in the fetal heart signal, x (n + m) is the signal value of the (n + m) th sampling point in the fetal heart signal, Rxx(m) is the autocorrelation function value of x (n) and x (n + m);
the second peak in the autocorrelation function is then taken as the periodic characteristic of the fetal heart signal.
Further, the method for calculating the coefficient of variation characteristics includes: firstly, a fixed-size window is adopted to carry out sliding scanning on fetal heart signals, the standard deviation sigma and the mean value mu of the fetal heart signals in each window are calculated, and the formula C is used for calculating the standard deviation sigma and the mean value mu of the fetal heart signals in each windowvCalculating the variation coefficient of the core signal in each window as sigma/mu; then, the windows with the coefficient of variation larger than a given threshold are counted, and the coefficients of variation of the windows are averaged to finally serve as the coefficient of variation characteristics of the fetal heart signal.
Further, the method for calculating the effective energy ratio characteristic comprises the following steps: firstly, autocorrelation and Fourier transformation are carried out on fetal heart signals to obtain power spectrums of the signals, and then effective energy ratio characteristics P of the fetal heart signals are calculated according to the power spectrums through the following formulaval；
Figure BDA0002944767810000032
Wherein: e (f) is the energy corresponding to the frequency f in the fetal heart signal power spectrum, EsumIs the total energy of the fetal heart signal power spectrum.
Further, the calculation method of the power spectrum entropy characteristics comprises the following steps: firstly, performing autocorrelation and Fourier transform on fetal heart signals to obtain a power spectrum of the signals, dividing the whole power spectrum into a plurality of frequency sections by taking 2Hz as unit interval width, and then calculating the effective energy ratio of each frequency section according to the following formula;
Figure BDA0002944767810000041
wherein: e (f) is the energy corresponding to the frequency f in the fetal heart signal power spectrum, EsumTotal energy of the power spectrum of fetal heart signals, EiDenotes the ith frequency bin, piThe effective energy ratio of the ith frequency zone;
finally, the power spectrum entropy characteristic H of the fetal heart signal is calculated according to the following formulaSE；
Figure BDA0002944767810000042
Further, the specific implementation process of the tire center position guidance algorithm in the step (3) is as follows:
3.1 establishing a plane rectangular coordinate system by taking the transducer H5 as the center, and establishing a tire center signal characteristic value dis1～dis4Corresponding feature vector in coordinate system
Figure BDA0002944767810000043
And then the synthetic orientation vector is calculated by means of vector superposition
Figure BDA0002944767810000044
And determining the vector
Figure BDA0002944767810000045
Included angle with positive direction of x-axis in coordinate system
Figure BDA0002944767810000046
3.2 according to the included angle
Figure BDA0002944767810000047
Determining the direction of the fetal heart position relative to the current measuring point, guiding a user to move the probe and acquiring a fetal heart signal of the next measuring point, specifically:
when in use
Figure BDA0002944767810000048
The position of the fetal heart is positioned in the east direction of the current measuring point;
when in use
Figure BDA0002944767810000049
The position of the fetal heart is located in the northeast direction of the current measuring point;
when in use
Figure BDA00029447678100000410
The position of the fetal heart is located in the due north direction of the current measuring point;
when in use
Figure BDA00029447678100000411
The position of the fetal heart is located in the northwest direction of the current measuring point;
when in use
Figure BDA00029447678100000412
The position of the fetal heart is positioned in the positive west direction of the current measuring point;
when the temperature is higher than the set temperature
Figure BDA00029447678100000413
The position of the fetal heart is positioned in the southwest direction of the current measuring point;
when in use
Figure BDA00029447678100000414
The position of the fetal heart is positioned in the south-to-south direction of the current measuring point;
when in use
Figure BDA00029447678100000415
The position of the fetal heart is positioned in the southeast direction of the current measuring point;
3.3 calculating and obtaining the corresponding fused fetal heart signal characteristic value dis of the transducers H1-H4 of the next measuring point according to the step (2)1'～dis4' and establishing a characteristic value dis of the signal of the fetal heart1'～dis4' feature vectors corresponding in a coordinate System
Figure BDA00029447678100000416
Obtaining a comprehensive characteristic vector by fusing the characteristic vectors of the front and the rear measuring points
Figure BDA00029447678100000417
And determining the vector
Figure BDA00029447678100000418
Included angle with positive direction of x-axis in coordinate system
Figure BDA00029447678100000419
3.4 according to the included angle
Figure BDA00029447678100000420
Determining the direction of the fetal heart position relative to the current measuring point, the positioning standard and the included angle
Figure BDA00029447678100000421
And correspondingly, guiding the user to move the probe and collect the fetal heart signals of the next measuring point, and evaluating the quality of the fetal heart signals collected by the transducer H5 of the next measuring point: if the quality reaches the standard, stopping detection; and if the quality does not reach the standard, returning to execute the step 3.3.
Further, the feature vectors in step 3.1
Figure BDA0002944767810000051
Is represented as follows:
Figure BDA0002944767810000052
orientation vector
Figure BDA0002944767810000053
If vector
Figure BDA0002944767810000054
In the first quadrant of the coordinate system, then
Figure BDA0002944767810000055
If vector
Figure BDA0002944767810000056
In the second quadrant of the coordinate system, then
Figure BDA0002944767810000057
If vector
Figure BDA0002944767810000058
In the third quadrant of the coordinate system, then
Figure BDA0002944767810000059
If vector
Figure BDA00029447678100000510
In the fourth quadrant of the coordinate system, then
Figure BDA00029447678100000511
Further, the feature vectors are integrated in step 3.3
Figure BDA00029447678100000512
Is expressed by calculation ofThe following:
Figure BDA00029447678100000513
Figure BDA00029447678100000514
if vector
Figure BDA00029447678100000515
In the first quadrant of the coordinate system, then
Figure BDA00029447678100000516
If vector
Figure BDA00029447678100000517
In the second quadrant of the coordinate system, then
Figure BDA00029447678100000518
If vector
Figure BDA00029447678100000519
In the third quadrant of the coordinate system, then
Figure BDA00029447678100000520
If vector
Figure BDA0002944767810000061
In the fourth quadrant of the coordinate system, then
Figure BDA0002944767810000062
Wherein: phi is a unit ofn-1The rotation angle of the current measuring point relative to the ultrasonic probe of the previous measuring point is shown.
Based on the technical scheme, the fetal heart detection method can realize the function of fetal heart position search and give fetal heart position information, and can guide pregnant women without fetal heart measurement experience to find out an accurate fetal heart measurement position, so that an accurate fetal heart rate is obtained, and the problems of large fetal heart rate counting error, fuzzy fetal heart sound and the like caused by inaccurate fetal heart measurement position are solved.
Drawings
Fig. 1 is a schematic diagram of the acoustic field of a circular piston transducer.
Fig. 2 is a piezoelectric transducer sound field beam diagram.
FIG. 3 is a schematic view of radial and thickness coupled vibration of a piezoelectric transducer.
Fig. 4 is a schematic structural diagram of an ultrasonic transducer correlation experiment.
Fig. 5(a) is a schematic diagram of the test results of the fetal heart signal acquisition transducer.
Fig. 5(b) is a schematic diagram showing the test results of the azimuth detection transducer.
Fig. 6 is a schematic diagram of transducer distribution for an ultrasound probe of the present invention.
FIG. 7 is a schematic view of orientation detecting transducer fetal heart detection.
Fig. 8 is a schematic structural diagram of an ultrasonic probe of the present invention.
Fig. 9 is a schematic view of the sound field of the ultrasonic probe of the present invention.
FIG. 10 is a schematic view of the sound field coverage of each depth probe.
FIG. 11(a) is a schematic diagram of the time domain waveform of the fetal heart signal at a fetal heart distance of 0cm and its autocorrelation function.
FIG. 11(b) is a schematic diagram of the time-domain waveform of the fetal heart signal at a fetal heart distance of 6cm and its autocorrelation function.
FIG. 12(a) is a graph showing the time domain waveform of fetal heart signals at a fetal heart distance of 2cm and the variation coefficient thereof.
FIG. 12(b) is a graph showing the time domain waveform of the fetal heart signal at a fetal heart distance of 8cm and the variation coefficient thereof.
Fig. 13(a) and 13(b) are power spectrograms of fetal heart signals at a distance of 2cm and 8cm from the fetal heart, respectively.
Fig. 14 is a schematic structural diagram of a device for simulating a fetal heart test.
FIG. 15 is a graph of periodic features based on an autocorrelation function as a function of tire center distance.
FIG. 16 is a graph of coefficient of variation characteristics as a function of tire center distance.
FIG. 17 is a graph of the effective energy fraction characteristic as a function of the tire center distance.
FIG. 18 is a graph of power spectrum entropy characteristics as a function of tire center distance.
Fig. 19 is a graph of the tire center distance characteristic dis as a function of the tire center distance.
FIG. 20 is a schematic view of the azimuthal distribution of transducers.
FIG. 21 is a simplified schematic view of the orientation of the fetal heart.
FIG. 22 is a schematic view of a method for calculating the orientation of the fetal heart at a single measurement point.
Fig. 23 is a schematic view of calculation of the fetal heart orientation.
FIG. 24 is a schematic of feature vectors at two locations.
Fig. 25 is a schematic view of an ultrasonic probe placement point setting distribution.
Fig. 26 is a schematic flow chart of the fetal heart position guidance strategy.
Detailed Description
In order to more specifically describe the present invention, the following detailed description is provided for the technical solution of the present invention with reference to the accompanying drawings and the specific embodiments.
The invention provides a fetal heart detection method based on fetal heart position guidance, which mainly realizes the following two core functions:
1. the fetal heart meter needs to accurately detect fetal heart signals of fetal heart beats for calculating the fetal heart rate and playing fetal heart sounds.
2. If the fetal heart instrument cannot acquire effective fetal cardiac signals due to poor detection position, the pregnant woman needs to be guided to move the fetal heart instrument in a larger range to find a proper fetal heart measurement position.
The main content of the invention comprises: the method comprises the steps of ultrasonic transducer design, fetal heart instrument ultrasonic probe design, fetal heart signal feature extraction algorithm design and fetal heart position guiding algorithm design.
(1) Ultrasonic transducer design
According to the ultrasonic Doppler fetal heart rate signal detection principle, in the fetal heart rate signal detection process, an ultrasonic transducer generates mechanical vibration under the action of excitation voltage and emits ultrasonic waves to the abdomen of a pregnant woman, the ultrasonic waves encounter the fetal heart which moves in a reciprocating mode in the transmission process and accordingly generate ultrasonic echo signals containing Doppler frequency shift, the ultrasonic transducer receives the ultrasonic echo and converts the ultrasonic echo into electric signals, and finally the system extracts fetal heart rate signals through software and hardware processing. Therefore, whether the fetal heart is located in the sound field of the ultrasonic transducer is the key factor for detecting the Doppler fetal heart signal, and the distribution characteristics of the ultrasonic sound field emitted by the ultrasonic transducer are the key factors for determining the detection range and the detection sensitivity of the fetal heart monitor system.
The ultrasonic transducer adopted by the fetal heart monitor is a disc-shaped piezoelectric transducer, and common vibration modes of the ultrasonic transducer comprise a radial vibration mode and a thickness (axial) vibration mode. The piezoelectric transducer working in the radial vibration mode does telescopic vibration along the radial direction, and the emitted sound field of the piezoelectric transducer is parallel to the radial direction and vertical to the thickness direction, so that the piezoelectric transducer is difficult to be directly used for detecting fetal heart signals in the depth direction. Correspondingly, the sound field emitted by the piezoelectric transducer working in the thickness vibration mode is parallel to the thickness direction and perpendicular to the upper surface and the lower surface of the piezoelectric transducer, so that the piezoelectric transducer working in the thickness vibration mode is generally adopted as an ultrasonic probe to detect fetal heart signals in the existing fetal heart monitor system.
The vibration state of the piezoelectric transducer in the thickness vibration mode can be regarded as piston vibration, and is also called a circular piston transducer. As shown in fig. 1, an ultrasonic sound field emitted by the circular piston transducer can be divided into a near field region and a far field region, a section near the ultrasonic transducer is called the near field region, ultrasonic beams in the near field region are propagated in parallel, ultrasonic energy is concentrated, the length of the ultrasonic beams is N, and ultrasonic waves in the far field region are diffused outwards at a certain angle θ.
In practical ultrasound detection, the length of the near field region is usually calculated by the following formula:
Figure BDA0002944767810000081
in the formula: n is the near field length, D is the diameter of the piezoelectric transducer, f is the operating frequency of the piezoelectric transducer, and c is the ultrasonic propagation velocity.
In the far field region of the ultrasonic beam, the ultrasonic wave begins to diffuse outward in θ, which is also known as the half-diffusion angle. As shown in fig. 2, the main energy of the ultrasonic sound field is concentrated on the main lobe, and the occupation ratio of the side lobe is small. An acute angle theta is typically used0And half power spread angle theta-3dBThe directivity of the sound field is described.
Acute angle theta0Is the angle between the first minima appearing on both sides of the main lobe, half-power spread angle theta-3dBAlso called beam width, which is the angle between the two sides of the main lobe beam when the power attenuation is half of the center, so the half power spread angle theta is generally used-3dBThe beam width of the ultrasonic wave is described. Theta-3dBHalf of which is the half diffusion angle theta described above. For a disc-shaped piezoelectric transducer, the beam width Θ is-3dBComprises the following steps:
Figure BDA0002944767810000082
by combining the formula 1.1 and the formula 1.2, in the fetal heart signal detection process, since the propagation medium of the ultrasonic wave is relatively fixed, and the propagation speed c of the ultrasonic wave can be regarded as a constant, the larger the diameter D and the higher the resonant frequency f of the piezoelectric transducer are, the longer the near field region N of the ultrasonic sound field emitted by the piezoelectric transducer is, and the longer the beam width Θ is-3dBThe smaller, the narrow beam sound field. Correspondingly, the smaller the diameter D, the shorter the near field region of the sound field emitted by the piezoelectric transducer with the lower resonance frequency f, the larger the beam width, and the wide beam sound field.
In fetal heart signal detection, the piezoelectric transducer with a narrow beam sound field can obtain a stronger ultrasonic echo signal, so that the accuracy of fetal heart rate calculation and the definition of fetal heart sounds are ensured, but the detection range is small, and the method is not suitable for large-range fetal heart position detection. The piezoelectric transducer with the wide wave beam sound field can enlarge the detection range of fetal heart signals, but ultrasonic energy of the piezoelectric transducer is dispersed, the signal quality of the detected fetal heart signals is low, and the accuracy of fetal heart rate calculation cannot be ensured. Therefore, if the two core functions of the invention are to be satisfied simultaneously, the ultrasonic transducers of the wide and narrow two different beam sound fields are respectively used on the probe of the fetal heart monitor, and the detection sensitivity and the detection range of the fetal heart monitor are considered. According to the functional requirements of the fetal heart instrument, the fetal heart signal acquisition transducer and the azimuth detection transducer are respectively designed.
The fetal heart signal acquisition transducer is used for acquiring fetal heart signals at a proper measuring position to calculate fetal heart rate and broadcast fetal heart sounds, the fetal heart signals need higher signal quality, and therefore the narrow-beam ultrasonic transducer is adopted, the piezoelectric transducer with larger diameter and higher resonance frequency is selected, the piezoelectric transducer works in a thickness vibration mode, and the narrow-beam ultrasonic transducer is also a main implementation scheme of the ultrasonic probe of the current fetal heart instrument product, and therefore the detailed description is omitted.
The azimuth detection transducer is used for detecting the azimuth of the fetal heart in a larger range, the detected signal is used for judging the azimuth of the fetal heart, the transducer is required to have a larger sound field range, and the fetal heart needs to be ensured to be positioned in the sound field range, so that a wide-beam ultrasonic transducer needs to be adopted, and a piezoelectric transducer with a smaller diameter and a lower working frequency is selected according to the sound field distribution theory based on the thickness vibration mode. However, in practical applications, it is difficult to effectively expand the sound field width by using the thickness vibration mode.
Working in thickness vibration mode:
Nt＝ft×H (1.3)
in the formula: n is a radical oftIs the frequency constant, f, of the piezoelectric transducer in the thickness directiontH is the thickness of the disc-shaped piezoelectric transducer, which is the resonance frequency in the thickness direction thereof.
Operating in radial vibration mode:
Np＝fr×D (1.4)
in the formula: n is a radical ofpIs the frequency constant, f, of the piezoelectric transducer in the direction of the diameterrIs its radial resonant frequency.
As can be seen from equation 1.3, for a piezoelectric transducer operating in a thickness vibration mode, a lower resonance frequency results in a thicker piezoelectric transducer, which increases the manufacturing cost of the transducer, and is also not advantageous for miniaturization of the transducer. For PZT-5H piezoelectric material commonly used for ultrasonic transducers, the thickness of piezoelectric transducer with a resonant frequency of 1MHz has reached 2 mm. Thus, the thickness resonance frequency of a disc-shaped piezoelectric transducer supplied by the piezoelectric transducer manufacturer is typically 1MHz at the lowest. Compared with the mature piezoelectric transducer product catalog, the minimum diameter corresponding to the lowest frequency of 1MHz is 10mm, if the smaller resonance frequency and diameter are needed, the mold opening customization is needed, the cost and the process difficulty are higher, and the piezoelectric transducer is not suitable for the household fetal heart instrument product, therefore, the resonance frequency of 1MHz is selected under the thickness vibration mode, the sound field width obtained by the piezoelectric transducer with the diameter of 10mm is the maximum, and if the average propagation speed c of ultrasonic waves at the soft tissues of the human body is 1540m/s, the ultrasonic wave propagation speed of the ultrasonic waves at the soft tissues of the human body is:
the near field length N of the sound field is calculated from equation 1.1:
Figure BDA0002944767810000101
the half-spread angle θ of the sound field is calculated from equation 1.2:
Figure BDA0002944767810000102
clinical researches show that the distance between the ultrasonic probe and the heart of the fetus is about 6-15 cm, and the calculated distance can indicate that the coverage diameter of the sound field at the depth of 15cm is about 3.16cm, so that a larger coverage range of the sound field can not be formed obviously, and the coverage range of the sound field is further reduced along with the reduction of the depth of the heart of the fetus.
From the above analysis, it is known that the piezoelectric transducer operating in the thickness vibration mode is difficult to satisfy the requirement of the fetal heart monitor orientation detection transducer for the sound field coverage area, because the resonant frequency in the thickness direction is generally high, and if the piezoelectric transducer is operated in the radial vibration mode, the resonant frequency is often low. As can be seen from equation 1.4, the resonant frequency of the radial vibration of the piezoelectric material depends on the diameter of the piezoelectric material. Book (notebook)The invention uses PZT-5H series piezoelectric ceramics, the radial frequency constant of which is NPAbout 2000m Hz. For the piezoelectric transducer with the diameter of 10mm and the thickness of 2mm, the radial resonance frequency is calculated to be about 200KHz and far lower than the thickness resonance frequency by 1 MHz. However, the radial vibration mode has a low resonance frequency, but the generated sound field is parallel to the radial direction, and cannot be directly used for detecting the tire center in the depth direction.
Therefore, the two common vibration modes of the piezoelectric transducer are difficult to meet the design requirements of the orientation detection sensor of the fetal heart monitor. The invention is based on the characteristic that the piezoelectric transducer has low resonant frequency in a radial vibration mode, combines the sound field distribution characteristics in a thickness vibration mode, and adopts a radial and thickness coupling vibration mode to design the transducer for detecting the orientation of the fetal heart instrument.
At present, the vibration analysis of the wafer-shaped piezoelectric transducer is generally processed approximately based on a one-dimensional vibration theory, and under the one-dimensional vibration theory, the piezoelectric transducer can be regarded as working under a single vibration mode, such as a pure thickness vibration mode and a pure radial vibration mode. This approximation is reasonable when the piezoelectric transducer has a large difference in thickness and radius, as in the fetal heart signal acquisition transducer of the present invention, which is generally selected as a thin disc having a ratio of thickness H to radius R of less than 0.1, which can be considered as a pure thickness or a pure radial one-dimensional vibration. When the thickness and diameter of the piezoelectric transducer are not sufficiently different, the piezoelectric transducer cannot be considered to operate in a single vibration mode. As shown in fig. 3, due to the mechanical stress, the piezoelectric transducer will have strong vibration coupling in operation, the radial direction and the thickness direction will vibrate simultaneously, and the coupling strength of the vibration increases with the increase of the thickness-to-diameter ratio, i.e. the coupling vibration mode of the piezoelectric transducer.
In practical application, a thick wafer piezoelectric transducer with a thickness not much different from the diameter is used, the polarization direction and the excitation direction of the thick wafer piezoelectric transducer are parallel to the thickness direction, the piezoelectric transducer is made to perform stretching vibration along the radial direction by controlling the excitation frequency, and the mechanical stress generated by the radial vibration can cause the vibration along the thickness direction, so that the sound field radiation along the thickness direction of the piezoelectric transducer is realized near the radial resonance frequency with lower frequency.
In summary, for the fetal heart direction detecting transducer, it is necessary to select a piezoelectric transducer with a small diameter and a large thickness and excite the piezoelectric transducer to operate in a low-frequency coupled vibration mode to obtain a low-frequency wide-beam sound field. For the fetal heart signal acquisition transducer, the piezoelectric transducer with larger diameter and higher resonance frequency is selected, so that the signal quality of fetal heart detection is improved.
The fetal heart direction detection transducer adopts a thick wafer piezoelectric transducer with the diameter of 10mm and the thickness of about 2mm, and the resonant frequency of radial vibration of the transducer is about 200 KHz. The near-field length N of the emitted sound beam is calculated to be about 0.32cm, the near-field length N can be basically ignored in actual fetal heart measurement, the half-power diffusion angle theta is about 23.6 degrees, and compared with the half-diffusion angle of 4.6 degrees in the thickness vibration mode, the beam width of the sound field is effectively expanded by adopting the radial and thickness coupling vibration mode.
The fetal heart signal acquisition sensor adopts a piezoelectric transducer with the diameter of 20mm and the resonant frequency of about 3MHz in the thickness direction, the N of a near field area is about 19.5cm, and the normal range of the fetal heart to an ultrasonic probe during fetal heart detection is about 6-15 cm, so that the fetal heart can be considered to be always in the near field area with concentrated energy in actual measurement, the fetal heart has high detection sensitivity, and the fetal heart signal acquisition sensor is suitable for signal acquisition after being positioned to the fetal heart.
Through designing an ultrasonic transducer correlation test experiment, the sound field distribution characteristics of the fetal heart signal acquisition transducer and the fetal heart direction detection transducer are tested. The designed ultrasonic transducer correlation experiment can not obtain accurate results as a professional hydrophone device, but can also preliminarily reflect the sound field distribution characteristics of the transducer.
As shown in fig. 4, two transducers with the same parameters are used for the correlation test in the experiment, a signal generator is used for exciting an ultrasonic transmitting transducer to transmit ultrasonic waves from one side of a water tank to the other side of the water tank, an ultrasonic receiving transducer is used for receiving the ultrasonic waves from the other side of the water tank and displaying the ultrasonic waves through an oscilloscope, the intensity of a sound field transmitted by the ultrasonic transmitting transducer can be indirectly obtained by observing the output waveform of the oscilloscope, the intensity of the sound field at different positions can be detected by moving the receiving transducer, and therefore the distribution condition of the sound field transmitted by the transmitting transducer is obtained. In the experiment, the excitation voltage of two different groups of transducers is set to be 5V, the distance between the two transducers is 12cm, and the movable range of the receiving transducer is 20 cm.
The designed experimental detection process is as follows: the ultrasonic transmitting transducer is arranged at the center point of one side of the water tank and is excited to continuously transmit ultrasonic waves, and the ultrasonic transmitting transducer moves towards the left side and the right side from the center point at a distance of 1cm each time at the other side of the water tank and records the voltage received at each position, so that the sound field distribution condition of the ultrasonic transmitting transducer can be indirectly detected.
The test results of the fetal heart signal acquisition transducer and the fetal heart position detection transducer are respectively shown in fig. 5(a) and 5(b), wherein in the two figures, the abscissa is a negative number to represent the leftward movement, and the positive number represents the rightward movement. According to comparison of test results, under the same excitation voltage, the highest voltage received by the fetal heart signal acquisition transducer is 1.56V at the central point, and is far greater than 0.72V at the central point of the fetal heart position detection transducer, but the signal cannot be received basically within 2cm around the central point along with the high transverse attenuation speed, and the characteristic of the fetal heart signal acquisition transducer is consistent with the characteristic of energy concentration of the fetal heart position detection transducer in a near field region. For the fetal heart direction detecting transducer, although the received voltage is relatively low, the attenuation speed to the left and the right is slower, which indicates that the sound field emitted by the fetal heart direction detecting transducer has a larger beam width, and this is consistent with the conclusion obtained by the above theoretical analysis.
The experiment verifies that the sound field distribution characteristics of the fetal heart signal acquisition transducer and the azimuth detection transducer designed by the invention are consistent with the result obtained by theoretical analysis, and the design requirement of the system is met.
(2) Fetal heart monitor ultrasonic probe design
The fetal heart signal acquisition transducer can acquire fetal heart signals with high signal quality at an accurate fetal heart measuring position, and the fetal heart direction detection transducer has a larger fetal heart position detection range. If the doppler shift signal detected by a single ultrasonic transducer is adopted, generally, only information such as fetal heart rate and fetal heart signal strength can be extracted, but the orientation information of the fetal heart relative to the ultrasonic probe cannot be obtained. Therefore, a plurality of ultrasonic transducers can be used to perform orientation detection of the fetal heart in a particular arrangement. In summary, the ultrasound probe containing multi-directional information should at least include a fetal heart signal acquisition transducer and a plurality of position detection transducers.
The ultrasonic probe designed by the invention is shown in figure 6, wherein the No. 1 transducer is a fetal heart signal acquisition transducer, is positioned in the center of the probe, has the working frequency of 3MHz and the diameter of 20mm, and is used for positioning the effective fetal heart signal acquisition behind the fetal heart; no. 1 ~ 4 transducer is child heart position detection transducer, and its operating frequency is 200KHz, and the diameter is 10mm, is located four different positions, detects the child heart signal in different positions.
As shown in fig. 7, the fetal heart position detecting transducer mainly uses the sound field diffused to the outside of the probe for fetal heart signal detection, while the sound field diffused to the center of the probe has no substantial effect, because in actual detection, if the fetal heart is in this area, the fetal heart signal collecting transducer located at the center of the probe can detect the fetal heart signal without activating the fetal heart position detecting transducer. Therefore, in order to fully utilize the part of the sound field, the invention sets a certain inclination angle for the orientation detection transducer, so that the ultrasonic wave emitted by the orientation detection transducer is diffused to the periphery of the probe, the sound field coverage area of the probe is increased, and the structure is shown in fig. 8.
Fig. 9 shows an ultrasonic sound field of an ultrasonic probe designed by the invention, a fetal heart signal acquisition transducer utilizes a near field region of the sound field to perform fetal heart detection, and ultrasonic sound beams emitted by the transducer propagate forwards in parallel and are concentrated in energy. The fetal heart orientation detection sensor has small near field region of sound field, large diffusion angle and larger beam width, and the half diffusion angle of the sound field is theta, and the outward inclination angle is beta, so that the beta can be obtained by geometric relation1＝β2Therefore, the outward diffusion angle of the fetal heart position detection transducer after the inclination angle is added is theta + beta, and the fetal heart position detection transducer can effectively expandThe detection range of a large ultrasonic probe.
In fig. 9, MN is the detection dead zone width of the ultrasonic probe, and the size of MN depends on the gap between the two ultrasonic transducers and the near-field length of the fetal heart position detection transducer. When beta is theta, the sound beam boundary MP of the fetal heart orientation detection transducer is parallel to the sound beam boundary NQ of the fetal heart instrument signal acquisition transducer, and the width PQ of a blind zone at the detection depth H is MN; and if beta>θ, then PQ results>MN, namely the detection blind area of the ultrasonic probe at the moment becomes larger along with the increase of the detection depth. Therefore, although the detection range of the ultrasonic probe can be expanded by setting the inclination angle, the value of the inclination angle should not exceed the half diffusion angle of the ultrasonic sound field, namely, beta is less than or equal to theta, so that the outward diffusion angle of the ultrasonic sound field of the ultrasonic probe can be up to 2 theta. If influence of factors such as the near field length is neglected, let R be the radius of the ultrasonic probe, and the coverage radius of the sound field of the ultrasonic probe at the depth H is RHThen, there are:
RH≈H×tan(θ+β)+R0 (2.8)
in the formula: r0Is the radius of the ultrasound probe.
Since the width of the blind zone of the sound field of the probe is small and can be ignored in the actual fetal heart signal detection, the sound field coverage surface of the probe at each depth is as shown in fig. 10. Since the normal range of the fetal heart distance from the ultrasonic probe is about 6-15 cm, 5cm, 10cm and 15cm are taken as examples here. In FIG. 10, 4a is the transverse cross-section of the sound beam at a depth of 5cm below the probe, 4b is the transverse cross-section of the sound beam at a depth of 10cm below the probe, and 4c is the transverse cross-section of the sound beam at a depth of 15cm below the probe.
The sound field calculation of the fetal heart direction detection transducer in the prior art shows that the near field length N of a sound beam emitted by the fetal heart direction detection transducer is about 0.32cm, the near field length N can be basically ignored in the actual fetal heart measurement, the half power diffusion angle theta is about 23.6 degrees, if the inclination angle beta is 10 degrees, the sound field coverage radius corresponding to the ultrasonic probe at different depths can be obtained according to a formula 2.8, the result is shown in a table 1, the calculation result can be easily found, and the ultrasonic probe designed by the invention can effectively enlarge the fetal heart detection range and lay a foundation for the fetal heart position guiding.
TABLE 1
Figure BDA0002944767810000141
(3) Fetal heart signal feature extraction algorithm design
The energy of the received ultrasonic signals of the fetal heart at different transverse distances in the ultrasonic sound field of the position detection transducer is reduced along with the increase of the transverse distance from the transducer, so that the energy of the ultrasonic echo signals reflected by the fetal heart is also gradually reduced along with the increase of the transverse distance from the transducer. The ultrasonic probe designed by the invention comprises 4 azimuth detection transducers at different positions, and in the actual fetal heart detection process, the transverse distances between the 4 azimuth detection transducers and the fetal heart are different, so that the energy of ultrasonic echo signals reflected by the fetal heart and received by each transducer is different. Therefore, the present invention extracts the feature quantity that can effectively reflect the fetal heart distance information from the fetal heart signal.
(ii) periodic features based on autocorrelation function
The autocorrelation function of a signal refers to the degree of correlation between the signal and itself at different times, and has a length of N0The autocorrelation function of the digital signal x (n) of (a) is calculated as follows:
Figure BDA0002944767810000142
in the formula: rxx(m) is the autocorrelation function of the signal x (n) after a delay m.
FIGS. 11(a) and 11(b) are time domain waveforms of fetal heart signals acquired by the position detecting transducer at different fetal heart distances and their autocorrelation functions at the same depth of the fetal heart, where FIG. 11(a) is at a fetal heart distance of 0cm and FIG. 11(b) is at a fetal heart distance of 6 cm. Comparing and finding that the autocorrelation function in fig. 11(a) has obvious peak values, and the peak values are large, and the peak values appear periodically, and the period of the peak values is the same as the period of the fetal heart signal; the autocorrelation function in fig. 11(b) varies more gradually without a distinct peak. Therefore, the difference between the autocorrelation function peaks at different fetal heart distances can be utilized to extract the periodic characteristics based on the autocorrelation function so as to represent the difference of the periodicity of the fetal heart signals acquired at different fetal heart distances.
Let the period of the fetal heart signal be t0When m is t0At integer multiple of RxxThe maximum will be taken as shown in the following equation:
Figure BDA0002944767810000151
in the formula: a is a positive integer. As can be seen from the above equation, a is the maximum peak of the autocorrelation function when a is equal to 0, and is also the mean square value of the signal, i.e., the energy of the signal, but this value cannot characterize the periodicity characteristics of the signal. And a is the second largest peak of the autocorrelation function when a is 1, which can reflect the periodic component in the signal and characterize the periodic characteristics of the signal, so the second largest peak of the autocorrelation function can be extracted as the periodic characteristics of the fetal heart signal, which is expressed as follows:
Figure BDA0002944767810000152
in the formula: per is the periodic feature based on the autocorrelation function extracted by the present invention,
Figure BDA0002944767810000153
the second largest peak in the autocorrelation function.
Variation coefficient characteristics
It is considered that as the fetal heart distance increases, the signal-to-noise ratio of the fetal heart signal detected by the transducer decreases, with the magnitude of the effective signal component approaching that of the noise signal. The noise signal and the fetal heart signal have larger difference in statistical characteristics, so the method extracts the characteristics based on the difference of the statistical characteristics of the fetal heart signal.
The coefficient of variation, also called "dispersion coefficient", is a kind of normalized measure commonly used for measuring the dispersion degree of probability distribution, and is the ratio of standard deviation to average value (average value is not 0) in mathematical expression, and its specific expression is as follows:
Figure BDA0002944767810000154
in the formula: cvThe coefficient of variation is σ, the standard deviation, and μ is the mean.
The coefficient of variation is often applied to the field of signal processing, and when the amplitude of a signal changes less (such as noise), the standard deviation of the signal is smaller, and the coefficient of variation is also smaller; when the amplitude of the signal changes significantly (such as a deterministic signal), the standard deviation of the signal is larger and the coefficient of variation is also larger. And the coefficient of variation has the characteristic of normalization, thus reducing the interference caused by the difference of the data value range and expanding the application range of the method.
The present invention applies the coefficient of variation to analyze fetal heart signals, and fig. 12(a) and 12(b) show the time domain waveform of fetal heart signals at different fetal heart distances and the coefficient of variation curve thereof at the same fetal heart depth, where fig. 12(a) is at a fetal heart distance of 2cm, and fig. 12(b) is at a fetal heart distance of 8 cm. It can be observed from the graph that when the amplitude of the fetal heart signal changes obviously, the value of the coefficient of variation is larger and has an obvious peak value; when the amplitude of the fetal heart signal changes more slowly, the numerical value of the coefficient of variation is smaller, but the peak value also exists; the value of the coefficient of variation at the beat-to-beat gap, which can be considered as a noise signal, is small with no distinct peaks. Comparing the variation coefficient curves of the fetal heart signals at different fetal heart distances can find that the amplitude of the variation coefficient of the fetal heart signals in fig. 12(a) is obviously changed and the variation coefficient is large; in fig. 12(b), the variation coefficient of the fetal heart signal has a gentle change in amplitude and a small variation coefficient. The variation coefficients of the fetal heart signals can be different at different fetal heart distances, so that the variation coefficients of the fetal heart signals can be extracted to represent the difference of the probability distribution discrete degrees of the fetal heart signals acquired at different fetal heart distances.
In summary, the present invention extracts the peak value of the variation coefficient of fetal heart signals, and takes the average value as the characteristic, and the calculation formula is as follows:
Figure BDA0002944767810000161
in the formula: cvmaxIs the peak value of the coefficient of variation, k is the number of the peak values of the coefficient of variation, ε is the threshold value of the peak value of the coefficient of variation (for removing small peaks), Cv0Is a coefficient of variation feature of the present invention.
Third, the effective energy ratio characteristic
In fetal heart signals collected by the azimuth detection transducer, the signal frequency reflecting fetal heart movement is mainly concentrated in a frequency range of 10-30 Hz, and along with the increase of the fetal heart distance, the energy in the main frequency range is gradually reduced, and the energy of noise signals cannot be obviously changed. Therefore, the invention defines the energy of the signal reflecting the fetal heart movement in the main frequency band as the effective energy of the fetal heart signal, and redefines the ratio of the effective energy to the total energy of the signal as the effective energy ratio, and the calculation formula is as follows:
Figure BDA0002944767810000162
in the formula: p is a radical ofvalEffective energy ratio of fetal heart signals, EsumE (f) is the energy of different frequency components of the signal.
At different fetal heart distances, the effective fetal heart signal energy has difference, but the noise energy has no obvious difference, so that the effective energy ratio is different. Therefore, the invention extracts the energy efficiency energy ratio characteristic of the fetal heart signal to represent the fetal heart distance.
Power spectrum entropy characteristics
The power spectrum entropy characterizes the spectrum structure of a time series and is a measure of the uncertainty of a signal in the frequency domain. When the frequency components of the signals are simple and the energy of the signals is concentrated, the corresponding power spectrum entropy is small, and the uncertainty and the complexity of the signals are small; conversely, when the energy of the signal is distributed more uniformly in the frequency domain, the power spectrum entropy is larger, which indicates that the uncertainty and complexity of the signal are larger. Therefore, the power spectrum entropy reflects the energy distribution situation on the signal frequency domain.
Fig. 13(a) and 13(b) show power spectra of fetal heart signals at different fetal heart distances, and it can be found that the distribution of the energy of the fetal heart signals acquired at different fetal heart distances in the frequency domain is also different. The distribution of the fetal heart signal energy in the frequency band of 10 to 30Hz is more concentrated (concentrated near a certain frequency) in FIG. 13(a), while the energy of the fetal heart signal at 8cm is more dispersed in FIG. 13(b), and the main frequency is not prominent. Therefore, the power spectrum entropy features can be extracted to reflect the distribution characteristics of fetal heart signal energy on the frequency domain at different fetal heart distances.
The power spectrum entropy is calculated as follows: firstly, the signal energy ratio of each frequency band component in the whole frequency band needs to be calculated, and the calculation formula is as follows:
Figure BDA0002944767810000171
in the formula: p is a radical offIs f1～f2The energy of the signal in the frequency band (width of 2Hz) is proportional.
Then, according to the signal energy proportion of each frequency band, calculating power spectrum entropy:
Figure BDA0002944767810000172
in the formula: hSEFor power spectrum entropy, the smaller the value, the more concentrated the energy distribution of the fetal heart signal in the frequency domain.
And (3) acquiring fetal heart signals at different fetal heart distances through a simulated fetal heart experiment, and analyzing and calculating the fetal heart signals to verify the effectiveness of the four types of characteristic quantities in reflecting the fetal heart distances.
The device for simulating the fetal heart test is shown in fig. 14, a signal sent by a function signal generator drives a vibration exciter to do telescopic motion through a power amplifier, and a thin wire drives a stainless steel ball to do up-and-down motion so as to simulate the beating of the fetal heart. Clinical studies have shown that the overall diameter of a normal fetal heart over 16 weeks can reach 10mm, so stainless steel pellets of 10mm diameter are selected for the experiment. The amplitude and the frequency of the small ball vibration can be controlled by controlling the amplitude and the frequency of the output signal of the function signal generator so as to simulate the beating intensity and the fetal heart rate of the fetal heart. The water tank filled with water simulates the uterus environment of a pregnant woman, and the inner wall of the water tank is adhered with a sound absorption material so as to reduce the interference of the reflection of ultrasonic waves on the inner wall of the water tank on the experimental measurement result; the azimuth detection transducer is placed on the support, the surface of the azimuth detection transducer is coated with coupling agent, and the azimuth detection transducer is tightly attached to the bottom surface of the water tank so as to measure fetal heart signals. The experiment mainly researches the difference of characteristic values of fetal heart signals acquired by a single azimuth detection transducer at different fetal heart positions, so that the experiment is provided with the single azimuth detection transducer to acquire the fetal heart signals, L is the transverse distance between the transducer and a simulated fetal heart, and h is the longitudinal distance between the transducer and the simulated fetal heart to simulate the depth of the fetal heart.
In the experiment, the position of the fetal heart is kept unchanged, the azimuth detection transducer is transversely moved to simulate different fetal heart distances, and L is respectively set to be 0cm, 2cm, 4cm, 6cm, 8cm and 10 cm. Clinical research shows that the depth of the fetal heart in the actual fetal heart detection process is about 6-15 cm, so the fetal heart depth h in the experiment is respectively set to be 5cm, 10cm and 15 cm.
The effectiveness of the four characteristic values is verified by repeated experiments and analysis and processing of experimental data, and the experimental result is as follows: table 2 shows the relationship between the periodic characteristics based on the autocorrelation function and the fetal heart distance at different fetal heart depths, and the curve is shown in fig. 15; table 3 shows the relationship between the variation coefficient characteristics and the tire center distance at different tire center depths, and the curve is shown in FIG. 16; table 4 shows the relationship between the effective energy ratio characteristic and the center distance at different depths of the center of the tire, and the curve is shown in fig. 17; table 5 shows the relationship between the power spectrum entropy characteristics and the fetal heart distance at different fetal heart depths, and the curve is shown in fig. 18. The four characteristic values extracted by the method are all reduced along with the increase of the fetal heart distance, have good monotonicity and can effectively reflect the fetal heart distance.
TABLE 2
Figure BDA0002944767810000181
TABLE 3
Figure BDA0002944767810000182
Figure BDA0002944767810000191
TABLE 4
Figure BDA0002944767810000192
TABLE 5
Figure BDA0002944767810000193
The experimental results show that the four types of features extracted by the method can effectively reflect the fetal heart distance, but the single feature has instability and has the problem of poor sensitivity when the fetal heart distance is larger or smaller. Therefore, the four characteristics can be fused, and each characteristic is respectively weighted, so that the characteristic capable of effectively representing the fetal heart distance is obtained.
The invention utilizes an optimal weighting method to perform characteristic value fusion, the optimal weighting method is a characteristic fusion algorithm which utilizes a minimum mean square error criterion to determine the weight of each characteristic value, and the principle is as follows:
Figure BDA0002944767810000194
in the formula: t isiFor the value of the i-th characteristic,
Figure BDA0002944767810000195
for the weighted eigenvalues, wiIs the ith special characterThe weight of the eigenvalues. The mean square error of the eigenvalues is then:
Figure BDA0002944767810000196
in the formula:
Figure BDA0002944767810000197
is the mean square error of the ith eigenvalue,
Figure BDA0002944767810000198
is the mean square error of the weighted eigenvalues. The minimum value of the above formula is obtained under the condition that the weight sum is 1, and the obtained optimal weight is as follows:
Figure BDA0002944767810000199
according to the principle of the optimal weighting method, the smaller the variance of the characteristic value is, the more stable the characteristic value is, the larger the weight is, and the requirement is met. Therefore, the weight of each characteristic value can be determined according to the variance of each characteristic value in repeated experiments, and the characteristic value fused by the method can represent the fetal heart distance more stably and effectively. Through calculation of experimental data of repeated experiments, the variance of the four characteristic values extracted by the invention is shown in table 6, the weight of each characteristic value can be obtained by calculation according to the formula 3.11, and finally the weight of each characteristic value is shown in table 7.
TABLE 6
Figure BDA0002944767810000201
TABLE 7
Figure BDA0002944767810000202
The fused fetal heart signal characteristic value can effectively reflect the size of the fetal heart distance, and is named as the fetal heart distance characteristic dis, and the calculation formula is as follows:
dis＝0.2×per0+0.4×Cv0+0.25×pval+0.15×HSE (3.12)
the relationship of the center distance characteristic dis to the center distance at different center depths is shown in table 8, and the graph thereof is shown in fig. 19. From the figure, the characteristic value has better monotonicity and sensitivity, and better linearity, so that the size of the tire center distance can be stably characterized.
TABLE 8
Figure BDA0002944767810000203
(4) Fetal heart position guiding algorithm design
The ultrasonic probe designed by the invention comprises 4 azimuth detection transducers at different positions, the distances between the azimuth detection transducers and the position of the fetal heart are different in the actual fetal heart detection process, and if the information reflecting the azimuth of the fetal heart can be extracted from the difference of the distances between the azimuth detection transducers and the position of the fetal heart, a user can be further guided to find out a proper fetal heart measurement position. The invention comprehensively considers the difference between the characteristic values of the fetal heart signals acquired by each azimuth detection transducer, calculates the fetal heart azimuth information and further provides a fetal heart position guiding strategy.
To simplify the description of the azimuth, the azimuth coordinates are established based on the ultrasonic probe, as shown in fig. 20, the directions represented by the 4 azimuth detection transducers are always unchanged, wherein the transducer No. 1 represents the north direction, the transducer No. 2 represents the east direction, the transducer No. 3 represents the south direction, and the transducer No. 4 represents the west direction, which are based on the ultrasonic probe.
Since the user cannot move the ultrasound probe in a very accurate direction during use, the fetal heart orientation is simplified herein to 8 main directions, as shown in fig. 21, dividing the 360 ° orientation equally into 8 directions, each direction ranging from 45 °, and being symmetric about the x-axis and the y-axis, respectively.
4.1 fetal heart orientation calculation method
In order to extract information capable of effectively reflecting the fetal heart direction from the fetal heart distance features acquired by the 4 direction detection transducers, the invention firstly provides a fetal heart direction calculation method based on a single measuring point, and then improves the method by utilizing the dynamic measurement idea, and provides a fetal heart direction calculation method based on multiple measuring points, thereby greatly improving the accuracy of fetal heart direction calculation.
The fetal heart orientation calculation method of the single measuring point comprises the steps of firstly establishing an ultrasonic probe space coordinate system, constructing space characteristic vectors on the basis of fetal heart distance characteristics acquired by 4 orientation detection transducers at the current positions, and extracting fetal heart orientation information by utilizing the space vector synthesis of each characteristic vector. In order to fully utilize the spatial information of 4 sets of position fetal heart signals, the invention constructs a characteristic vector on the basis of the fetal heart distance characteristic, as shown in fig. 22, a plane rectangular coordinate system is established by taking a fetal heart signal acquisition transducer (transducer No. 5) as the center, and the direction detection transducers (transducers No. 1 to 4) distributed around the plane rectangular coordinate system construct a vector in a coordinate system according to the relative positions and the characteristic values of the direction fetal heart signals. The unit vectors corresponding to No. 1-4 azimuth detection transducers in the coordinate system are respectively
Figure BDA0002944767810000211
And
Figure BDA0002944767810000212
which respectively represent the positive y-axis direction, the positive x-axis direction, the negative y-axis direction, and the negative x-axis direction. The characteristic values of fetal heart signals collected by No. 1-4 azimuth detection transducers are respectively dis1、dis2、dis3And dis4Then, the corresponding eigenvalue vector is:
Figure BDA0002944767810000213
Figure BDA0002944767810000214
Figure BDA0002944767810000215
Figure BDA0002944767810000216
in order to comprehensively consider the contribution degrees of different orientation vectors, the invention obtains the synthetic orientation vector through vector superposition calculation
Figure BDA0002944767810000221
The calculation formula is as follows:
Figure BDA0002944767810000222
substituting the coordinate values of the vector quantity can obtain:
Figure BDA0002944767810000223
Figure BDA0002944767810000224
by the angle between it and the positive direction of the x-axis
Figure BDA0002944767810000225
Indicating the direction of the position of the fetal heart, as shown in fig. 23.
To accurately obtain an included angle
Figure BDA0002944767810000226
Value of obtaining a vector
Figure BDA0002944767810000227
After the coordinates are obtained, the quadrant in which the vector is located is calculated, and the vector can be determined according to the quadrant in which the vector is located
Figure BDA0002944767810000228
Included angle with positive direction of x-axis
Figure BDA0002944767810000229
I.e. the orientation of the fetal heart.
A first quadrant:
Figure BDA00029447678100002210
a second quadrant:
Figure BDA00029447678100002211
and a third quadrant:
Figure BDA00029447678100002212
and a fourth quadrant:
Figure BDA00029447678100002213
the calculation of the fetal heart orientation is as follows:
Figure BDA0002944767810000231
in the formula: dir1Is the calculation result of the fetal heart orientation calculation method of the single measuring point.
The method is a fetal heart position calculation method based on a single measuring point, but in the actual use process, the data of the single measuring point is easily influenced by inconsistent coupling degree of the ultrasonic probe and the abdomen of the pregnant woman, so that misjudgment is easily caused. In addition, in the actual process of guiding the position of the fetal heart, data of a plurality of measuring points exist, and if the data before and after movement can be integrated to calculate the position of the fetal heart, the probability of misjudgment of the position of the fetal heart caused by inconsistent coupling degrees can be effectively reduced.
FIG. 24 shows the feature vectors at two positions, the feature vector at position 1 being
Figure BDA0002944767810000232
Figure BDA0002944767810000233
The calculated azimuth angle of the fetal heart is
Figure BDA0002944767810000234
The feature vectors at position 2 are respectively
Figure BDA0002944767810000235
Figure BDA0002944767810000236
The calculated azimuth angle of the fetal heart is
Figure BDA0002944767810000237
Position 1 to position 2 by a distance ln-1. Because the ultrasonic probe can rotate to a certain degree in the process of moving the ultrasonic probe of the fetal heart monitor, the characteristic vectors at the positions 1 and 2 have an included angle phin-1(the angle is smaller). Combining the feature vectors at positions 1 and 2, the final result of the feature vectors is:
Figure BDA0002944767810000238
calculating the sum of the eigenvectors by using the eigenvectors, and writing the angle between the sum of the eigenvectors and the positive direction of the x axis as thetanAnd calculating to obtain theta according to the quadrant in which the theta is positionednThe calculation formula is as follows:
a first quadrant:
Figure BDA0002944767810000239
a second quadrant:
Figure BDA0002944767810000241
and a third quadrant:
Figure BDA0002944767810000242
and a fourth quadrant:
Figure BDA0002944767810000243
the calculation of the fetal heart orientation is as follows:
Figure BDA0002944767810000244
in the formula: dir2The method is a calculation result of the fetal heart orientation calculation method of the multiple measuring points.
In order to verify the effective line of the fetal heart orientation calculation method, fetal heart signals of all orientation detection transducers are acquired at different fetal heart positions, the fetal heart orientation is calculated by using a single-measuring-point and multi-measuring-point method, and the fetal heart orientation is compared with the actual fetal heart orientation to verify whether the calculation result is correct.
Experimental apparatus as shown in fig. 14, when the ultrasonic probe designed by the present invention is used in an experiment, the experimental measurement points (i.e. the placement points of the ultrasonic probe) are respectively set at 4cm, 6cm and 8cm (the distance is the distance between the center of the whole ultrasonic probe and the position of the fetal heart) from the position of the fetal heart in 8 main directions as shown in fig. 25, and 24 measurement points are set in total. In the experiment process, the direction of the ultrasonic probe is kept unchanged, and the experiment is repeated at the positions of the fetal heart depth h of 5cm, 10cm and 15cm respectively.
Tables 9, 10 and 11 show the experimental data at the fetal heart depths of 5cm, 10cm and 15cm and the fetal heart orientation calculation results. Dis in table1、dis2、dis3And dis4Respectively 1-4 azimuth detection transductionThe calculation result 1 of the characteristic value of the fetal heart signal collected by the device is the calculation result of the fetal heart orientation calculation method of a single measuring point, and the calculation result 2 of the fetal heart orientation calculation method of multiple measuring points (the current measuring point and the adjacent measuring point in the same direction are used for calculation, the rotation angle is 10 degrees), wherein the bold result is an error result.
TABLE 9
Figure BDA0002944767810000251
Watch 10
Figure BDA0002944767810000252
Figure BDA0002944767810000261
TABLE 11
Figure BDA0002944767810000262
Figure BDA0002944767810000271
According to the experimental result, the accuracy of the method for calculating the fetal heart orientation at the single measuring point is 86.1%, the accuracy of the method for calculating the fetal heart orientation at the multiple measuring points is 97.2%, and the accuracy of the method for calculating the fetal heart orientation at the multiple measuring points is greatly improved, so that the method for calculating the fetal heart orientation at the multiple measuring points is used for calculating the fetal heart orientation.
4.2 fetal Heart position guidance strategy
On the basis of the fetal heart orientation calculation method, the invention further provides a fetal heart position guiding strategy to guide a user to find a proper fetal heart measuring position. The strategy calculates the fetal heart position by using the fetal heart data before and after moving in the guiding process, gives the fetal heart position indication, and finally finds a proper fetal heart measuring position.
As shown in fig. 26, the specific process of the fetal heart position guidance strategy of the present invention is as follows:
1) the fetal heart signal detection starts;
2) judging whether the fetal heart signal acquisition transducer detects a fetal heart signal or not, if so, calculating and displaying the fetal heart rate, and ending the process; if not, go to step 3);
3) starting 4 direction detection transducers, collecting and storing direction fetal heart signals;
4) judging whether the group of fetal heart signals are the 1 st group of signals, if so, going to the step 5), and if not, going to the step 6);
5) calculating the fetal heart orientation by using a fetal heart orientation calculation method of a single measuring point, giving a fetal heart orientation indication, and going to step 7);
6) calculating the fetal heart position by using the latest two groups of position fetal heart signals and a fetal heart position calculation method of multiple measuring points, giving a fetal heart position indication, and going to step 7);
7) the user is guided to move about 2cm in that direction and to step 2).
Because the distance that the user removed the fetus-voice meter is difficult to control in the use, and the great many uncertain factors that bring of removal distance, therefore the step length location that moves in this strategy is about 2 cm.
The foregoing description of the embodiments is provided to enable one of ordinary skill in the art to make and use the invention, and it is to be understood that other modifications of the embodiments, and the generic principles defined herein may be applied to other embodiments without the use of inventive faculty, as will be readily apparent to those skilled in the art. Therefore, the present invention is not limited to the above embodiments, and those skilled in the art should make improvements and modifications to the present invention based on the disclosure of the present invention within the protection scope of the present invention.

Claims (8)

1. A fetal heart detection method based on fetal heart position guidance comprises the following steps:
(1) acquiring fetal heart signals of a current measuring point by using an ultrasonic probe of an ultrasonic Doppler fetal heart monitor, wherein the ultrasonic probe is provided with a transducer H5 for acquiring the fetal heart signals and four transducers H1-H4 for detecting the position, the transducer H5 is positioned at the central position of the probe, and the transducers H1-H4 are respectively positioned at the north, east, south and west positions of the transducer H5 on the plane of the probe;
the diameter of the transducer H5 is 18-22 mm, and the resonance frequency is 2.5-3.8 MHz; the diameters of the transducers H1-H4 are 8-10 mm, and the resonant frequency is 200-250 KHz;
(2) extracting feature information which can effectively reflect fetal heart distance information from fetal heart signals acquired by transducers H1-H4, wherein the feature information comprises periodic features, coefficient of variation features, effective energy ratio features and power spectrum entropy features, and fusing the features by an optimal weighting method;
(3) according to the fetal heart signal characteristic value dis correspondingly fused with the transducers H1-H41～dis4The method guides a user to find a proper fetal heart measuring position through a fetal heart position guiding algorithm so as to acquire a high-quality fetal heart signal, and specifically comprises the following steps:
3.1 establishing a plane rectangular coordinate system by taking the transducer H5 as the center, and establishing a tire center signal characteristic value dis1～dis4Corresponding feature vector in coordinate system
Figure FDA0003546093440000011
And then the synthetic orientation vector is calculated by means of vector superposition
Figure FDA0003546093440000012
And determining the vector
Figure FDA0003546093440000013
Included angle with positive direction of x-axis in coordinate system
Figure FDA0003546093440000014
3.2 based on the clampCorner
Figure FDA0003546093440000015
Determining the direction of the fetal heart position relative to the current measuring point, guiding a user to move the probe and acquiring a fetal heart signal of the next measuring point, specifically:
when in use
Figure FDA0003546093440000016
The position of the fetal heart is positioned in the east-righting direction of the current measuring point;
when in use
Figure FDA0003546093440000017
The position of the fetal heart is located in the northeast direction of the current measuring point;
when in use
Figure FDA0003546093440000018
The position of the fetal heart is located in the due north direction of the current measuring point;
when in use
Figure FDA0003546093440000019
The position of the fetal heart is located in the northwest direction of the current measuring point;
when in use
Figure FDA00035460934400000110
The position of the fetal heart is positioned in the positive west direction of the current measuring point;
when in use
Figure FDA00035460934400000111
The position of the fetal heart is positioned in the southwest direction of the current measuring point;
when in use
Figure FDA00035460934400000112
The position of the fetal heart is positioned in the south-to-south direction of the current measuring point;
when in use
Figure FDA00035460934400000113
The position of the fetal heart is positioned in the southeast direction of the current measuring point;
3.3 calculating and obtaining the corresponding fused fetal heart signal characteristic value dis of the transducers H1-H4 of the next measuring point according to the step (2)1'～dis4' and establishing a characteristic value dis of the signal of the fetal heart1'～dis4' feature vectors corresponding in a coordinate System
Figure FDA0003546093440000021
Obtaining a comprehensive characteristic vector by fusing the characteristic vectors of the front and the rear measuring points
Figure FDA0003546093440000022
And determining a synthetic feature vector
Figure FDA0003546093440000023
Included angle with positive direction of x-axis in coordinate system
Figure FDA0003546093440000024
3.4 according to the included angle
Figure FDA0003546093440000025
Determining the direction of the fetal heart position relative to the current measuring point, the positioning standard and the included angle
Figure FDA0003546093440000026
And correspondingly, guiding the user to move the probe and collect the fetal heart signals of the next measuring point, and evaluating the quality of the fetal heart signals collected by the transducer H5 of the next measuring point: if the quality reaches the standard, stopping detection; and if the quality does not reach the standard, returning to execute the step 3.3.
2. The fetal heart detection method of claim 1, wherein: an inclination angle of 5-15 degrees is arranged between the transducers H1-H4 and the transducer H5, so that ultrasonic waves emitted by the transducers H1-H4 are diffused to the periphery of the probe, and the coverage area of a sound field of the probe is increased.
3. The fetal heart detection method of claim 1, wherein: the periodic characteristic calculation method comprises the following steps: firstly, calculating an autocorrelation function of a fetal heart signal by the following formula;
Figure FDA0003546093440000027
wherein: n is a radical of0Is the length of a segment of fetal heart signal, n and m are natural numbers, x (n) is the signal value of the nth sampling point in the fetal heart signal, x (n + m) is the signal value of the n + m sampling point in the fetal heart signal, Rxx(m) is the autocorrelation function value of x (n) and x (n + m);
the second peak in the autocorrelation function is then taken as the periodic characteristic of the fetal heart signal.
4. The fetal heart detection method of claim 1, wherein: the method for calculating the coefficient of variation characteristics comprises the following steps: firstly, a fixed-size window is adopted to carry out sliding scanning on fetal heart signals, the standard deviation sigma and the mean value mu of the fetal heart signals in each window are calculated, and the formula C is used for calculating the standard deviation sigma and the mean value mu of the fetal heart signals in each windowvCalculating the variation coefficient of the core signal in each window as sigma/mu; then, the windows with the coefficient of variation larger than a given threshold are counted, and the coefficients of variation of the windows are averaged to finally serve as the coefficient of variation characteristics of the fetal heart signal.
5. The fetal heart detecting method according to claim 1, characterized in that: the method for calculating the effective energy ratio characteristic comprises the following steps: firstly, autocorrelation and Fourier transformation are carried out on fetal heart signals to obtain power spectrums of the signals, and then effective energy ratio characteristics P of the fetal heart signals are calculated according to the power spectrums through the following formulaval；
Figure FDA0003546093440000028
Wherein: e (f) is the energy corresponding to the frequency f in the fetal heart signal power spectrum, EsumIs the total energy of the power spectrum of the fetal heart signal.
6. The fetal heart detection method of claim 1, wherein: the calculation method of the power spectrum entropy characteristics comprises the following steps: firstly, performing autocorrelation and Fourier transform on fetal heart signals to obtain a power spectrum of the signals, dividing the whole power spectrum into a plurality of frequency sections by taking 2Hz as unit interval width, and then calculating the effective energy ratio of each frequency section according to the following formula;
Figure FDA0003546093440000031
wherein: e (f) is the energy corresponding to the frequency f in the fetal heart signal power spectrum, EsumTotal energy of the power spectrum of fetal heart signals, EiDenotes the ith frequency bin, piThe effective energy ratio of the ith frequency zone;
finally, the power spectrum entropy characteristic H of the fetal heart signal is calculated according to the following formulaSE；
Figure FDA0003546093440000032
7. The fetal heart detection method of claim 1, wherein: the feature vector
Figure FDA0003546093440000033
Is represented as follows:
Figure FDA0003546093440000034
orientation vector
Figure FDA0003546093440000035
If vector
Figure FDA0003546093440000036
In the first quadrant of the coordinate system, then
Figure FDA0003546093440000037
If vector
Figure FDA0003546093440000038
In the second quadrant of the coordinate system, then
Figure FDA0003546093440000039
If vector
Figure FDA00035460934400000310
In the third quadrant of the coordinate system, then
Figure FDA00035460934400000311
If vector
Figure FDA00035460934400000312
In the fourth quadrant of the coordinate system, then
Figure FDA00035460934400000313
8. The fetal heart detection method of claim 1, wherein: the synthetic feature vector
Figure FDA00035460934400000314
Is expressed as follows:
Figure FDA00035460934400000315
Figure FDA0003546093440000041
if vector
Figure FDA0003546093440000042
In the first quadrant of the coordinate system, then
Figure FDA0003546093440000043
If vector
Figure FDA0003546093440000044
In the second quadrant of the coordinate system, then
Figure FDA0003546093440000045
If vector
Figure FDA0003546093440000046
In the third quadrant of the coordinate system, then
Figure FDA0003546093440000047
If vector
Figure FDA0003546093440000048
In the fourth quadrant of the coordinate system, then
Figure FDA0003546093440000049
Wherein: phi is an-1The rotation angle of the current measuring point relative to the ultrasonic probe of the previous measuring point is shown.
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