CN1095356C - Mutual correlation-spectrum figure method in supersonic blood current measurement - Google Patents

Abstract

The invention relates to a "cross-correlation-spectrogram" method in ultrasonic blood flow measurement. In the method, a pulse signal is generated by an ultrasonic transmitting circuit, an electric pulse signal is generated and applied to a controller, and the echo signal is amplified by a radio frequency amplifying circuit. The cross-correlation calculator calculates the cross-correlation coefficient of the echo signals between two transmissions, and finally, the time-domain complex envelope function is formed by the correlation function, and the dynamic power of the blood flow signal is calculated by the spectrum analyzer. The method of the invention solves the problem of frequency spectrum aliasing in the measurement of high-speed blood flow that has plagued people for a long time, and can be widely used in medicine and industry.

Description

"Cross-correlation-spectrogram" method in ultrasonic blood flow measurement

The invention relates to a "cross-correlation-spectrogram" method in ultrasonic blood flow measurement, which belongs to the technical field of physical measurement.

Ultrasonic measurement of human blood flow has a wide range of clinical applications. Traditional ultrasonic blood flow measurement is designed based on the Doppler principle. It solves the problem of non-invasive measurement of human blood flow to a certain extent. However, the traditional Doppler method has two inherent defects: (1) the maximum measurable flow velocity and the maximum detectable depth are mutually restricted; (2) the distance resolution and the velocity resolution are mutually restricted. Therefore, when the measured flow rate is large, there will be aliasing phenomenon on the output spectrum, which will bring doubts to clinical diagnosis. How to solve the spectrum aliasing in high-speed blood flow measurement has always been a problem that people pay close attention to.

In the existing ultrasonic Doppler blood flow measurement, the measurement result is output in the form of a dynamic power spectrum (see FIG. 1 ). The abscissa in the figure is the time axis; the ordinate is the frequency offset, which is equivalent to the flow velocity; the gray scale in the spectrogram represents the energy of the echo signal generated by an object with a certain flow velocity at a specific moment. The part above the abscissa in the spectrogram represents forward flow (ie, the direction of flow velocity points to the transducer); the spectrogram below the abscissa represents reverse flow (ie, the direction of flow velocity is away from the transducer). When the measured blood flow velocity is relatively high, the spectral folding phenomenon will occur. For example, the higher forward flow section will fold into the reverse flow area, as shown in Figure 2. When a higher velocity of blood flow occurs, severe spectrum folding will cause spectral aliasing, as shown in Figure 3. This will bring difficulties to clinical diagnosis.

Spectral aliasing is an inherent problem in Doppler flow measurement systems. To solve this problem, a method of "cross-correlation" blood flow measurement was proposed. This approach can somewhat resolve aliasing in velocity measurements. However, the results measured by the ordinary "cross-correlation" method can only provide the average flow rate, but cannot provide a spectral display. Such a result is not in line with the habits of doctors, nor is it convenient to use in clinical disease diagnosis.

The purpose of the present invention is to develop a method of "cross-correlation-spectrogram" in ultrasonic blood flow measurement, to solve the problem of frequency spectrum aliasing in high-speed blood flow measurement that has plagued people for a long time, and to more accurately measure human blood flow (especially high-speed blood flow).

The "cross-correlation-spectrogram" method in ultrasonic blood flow measurement of the present invention comprises the following steps:

(1) The electric pulse signal is generated by the ultrasonic transmitting circuit and applied to the transducer to excite the piezoelectric crystal, which is converted into ultrasonic waves and injected into the human body;

(2) The weak echo signal is amplified by the radio frequency amplifier circuit;

(3) Calculate the cross-correlation function of the echo signal between the two transmissions by the cross-correlation calculator, and thus find the maximum value of the cross-correlation function and the corresponding time displacement;

(4) The time-domain complex envelope function is formed by the maximum value and displacement of the cross-correlation function, and the dynamic power spectrum of the blood flow signal is calculated by a spectrum analyzer.

The "cross-correlation-spectrogram" method in ultrasonic blood flow measurement proposed by the present invention is a new anti-aliasing blood flow measurement method. The new method, called "cross-correlation-spectrogram," solves the long-standing problem of spectral aliasing in high-speed blood flow measurements. The instrument designed based on this new detection principle can be used to measure human blood flow (especially high-speed blood flow) more accurately. This type of instrument can also be used in the measurement of the fluid in the pipeline in the field of industrial production and the measurement of the speed of some moving objects.

Description of drawings:

Figure 1 is a normal blood flow spectrum.

Figure 2 is a spectrum folding phenomenon.

Figure 3 is the spectrogram aliasing phenomenon.

Fig. 4 is a functional block diagram of the method of the present invention.

Fig. 5 is a schematic diagram of the cross-correlation operation.

Fig. 6 and Fig. 7 are the principle of the anti-aliasing spectrogram obtained by using the method of the present invention.

Figure 8 is an embodiment of the method of the present invention.

Figure 9 is a transmission circuit diagram.

Fig. 10 is a circuit diagram of radio frequency amplifier.

Fig. 11 is a circuit diagram of a cross-correlation operator.

Below in conjunction with accompanying drawing, introduce the content of the present invention in detail.

In Fig. 4-Fig. 10, 1 is a transducer, 2 is a skin, 3 is an ultrasonic beam, 4 is a blood vessel, and 5 is a body tissue scatterer.

Shown in Fig. 4 is the functional block diagram of the inventive method, and the working principle of each main step is as follows:

(1) Transmitting circuit:

In the pulse wave ultrasonic blood flow measurement system, the ultrasonic transducer is a single piezoelectric crystal. The electric pulse generated by the transmitting circuit excites the piezoelectric crystal, which is converted into ultrasonic wave and injected into the human body. The reflected waves caused by the internal organs are in turn converted into electrical signals by piezoelectric crystals and enter the receiving circuit.

(2) RF amplifier circuit:

Since the echo signal is very weak, further information extraction needs to be amplified. The RF amplifier in the figure must have a certain gain and bandwidth.

(3) Cross-correlation calculator:

The cross-correlation calculator is used to detect the time displacement of the echo signal generated by the same moving target during the two transmissions. See Figure 5 for its principle.

It can be seen from Figure 5 that, assuming that the moving object has moved a distance Δd in the time interval T between two transmissions, a corresponding time difference Δt will appear in the echo signal. Simple derivation can get: $\mathrm{\Δt}=\frac{2\mathrm{\Δd}}{c}----\left(1\right)$

where c is the propagation speed of ultrasound in the human body.

If the echo signals after two transmissions are used for cross-correlation calculation, the cross-correlation function will have a maximum value when τ=Δt. In other words, by detecting the moment when the maximum value of the cross-correlation function appears, Δt can be obtained. Δd can be obtained from Δt. Then divide the displacement Δd by the time T to get the axial velocity V of the moving object. $v=\frac{\mathrm{\Δd}}{T}----\left(2\right)$

The average velocity of moving objects at a specified depth is obtained by time-domain cross-correlation method, but this result is different from the spectrogram used by doctors in clinic.

(4) Spectrum Analyzer:

The result of the output of the cross-correlator is the time difference between the appearance of the target in the echo signal between the two transmissions, and this time difference reflects the speed of the object's motion.

In order to display this motion information in spectrogram format, a time-domain complex envelope signal can be constructed by the following method, and then the spectrogram can be obtained through a spectrum analyzer.

The constructed time-domain complex envelope signal is:

x(n)=A(n)e ^j(n) (3)

The amplitude function A(n) and phase function (n) in the formula are respectively $A\left(\mathrm{no}\right)=\sqrt{R_{\max }^{\left(\mathrm{no}\right)}\left({\mathrm{\τ}}_{\max }^{\left(\mathrm{no}\right)}\right)}----\left(4\right)$

是第n次发射与第(n+1)次发射后回波信号的互相关函数出现最大值时的位移时间； 是在位移量为 时的自相关函数值。In the formula

is the displacement time when the cross-correlation function of the echo signal after the nth emission and the (n+1)th emission appears the maximum value; is at a displacement of The value of the autocorrelation function when .

It can be proved that performing Fourier transform on the time-domain signal formed by formula (3) and calculating its power spectrum is the traditional Doppler method to obtain the blood flow signal spectrogram.

(5) Spectrum display:

The spectrogram obtained in step (4) can be displayed on the monitor in the traditional way. The displayed format is the same as the example given in Figure 1, that is, the abscissa is the time axis; the ordinate is the frequency deviation, which is equivalent to the flow velocity; the gray scale in the spectrogram represents the echo produced by an object with a certain flow velocity at a specific moment. energy of the wave signal.

Compared with the traditional Doppler blood flow measurement, the "cross-correlation-spectrogram" method proposed by the patent of the present invention can not only provide the user with the same spectrogram display, but more importantly, the highest flow rate that can be measured by this method is not Bound.

分成两步到达，这样就相当于将采样率PRF提高了一倍。如果将

分得更细，就相当于得到了更高的采样率。用这样的方法自然就可以避免在频谱中出现混叠现象。即使在流速比较高的情况下，也不例外(参见图6和图7)。The highest flow rate in the spectrogram display is related to the sampling rate of the time domain data being analyzed. In traditional methods, the sampling rate is equal to the repetition frequency PRF of the transmitted pulse. Aliasing occurs in the spectrum once the higher flow velocity produces a frequency offset of more than half of the PRF. When using the "cross-correlation-spectrogram" method, if the flow rate is relatively high, the interpolation method can be used to

Arriving in two steps is equivalent to doubling the sampling rate PRF. if will

A finer division is equivalent to a higher sampling rate. In this way, aliasing in the frequency spectrum can naturally be avoided. This is true even at relatively high flow rates (see Figures 6 and 7).

An embodiment of the present invention is introduced below, as shown in FIG. 8 .

In this embodiment, an ordinary microcomputer is used as the basic hardware platform. The emission, detection and analysis of signals are all realized by self-made hardware. Corresponding to the above five steps, the method adopted in this embodiment is given.

(1) Transmitting circuit (see Figure 9)

The transmitting circuit consists of two triodes, two VMOS tubes and a transformer. The two complementary emission pulse signals generated by the system control circuit are respectively sent to the two triodes, which control the switch of the VMOS tube after level conversion. The two VMOS transistors are turned on alternately, and the formed emission pulse is coupled to the transducer chip through the transformer.

(2) RF amplifier circuit (see Figure 10)

The radio frequency amplifier selects AD600, which is a wide-band, low-noise radio frequency amplifier.

The input stage of the amplifier is connected in series with an L, C resonant circuit, so as to select the signal related to the transmission frequency in a targeted manner.

The output signal of the preamplifier is sent to the relevant arithmetic unit.

(3) Cross-correlation calculator (see Figure 11)

The cross-correlation operator in this embodiment selects TMC2023 digital cross-correlator. It is a 64-bit digital correlation operation dedicated chip. The input of the chip is the amplified ultrasonic echo signal f(n), and the output is the autocorrelation function R(N) of the echo signal after two transmissions.

The output of the cross-correlation operator passes through the detection circuit of the maximum correlation value to obtain the maximum cross-correlation value and the corresponding time delay

(4) Spectrum Analyzer:

The spectrum analyzer is a commercial TMS320C25 high-speed signal processing card. This is a TMS320C25-centric signal processing card. Compiling a program with TMS320C25 assembly language can realize 256-point complex FFT within 5ms.

The whole system is built on a common PC platform. Various control parameters necessary for system work can be controlled through a man-machine interactive interface. The spectrogram is displayed on the monitor of the PC.

Claims (1)

1. A "cross-correlation-spectrogram" method in ultrasonic blood flow measurement, characterized in that the method comprises the following steps: (1) The electric pulse signal is generated by the ultrasonic transmitting circuit and applied to the transducer to excite the piezoelectric crystal, which is converted into ultrasonic waves and injected into the human body; (2) The weak echo signal is amplified by the radio frequency amplifier circuit; (3) Calculate the cross-correlation function of the echo signal between the two transmissions by the cross-correlation calculator, and thus find the maximum value of the cross-correlation function and the corresponding time displacement; (4) The time-domain complex envelope function is formed by the maximum value of the cross-correlation function and the displacement through the spectrum analyzer, and the constructed time-domain complex envelope signal is: x(n)=A(n)e ^j(n) (1) The amplitude function A(n) and phase function (n) in the formula are respectively $A\left(\mathrm{no}\right)=\sqrt{R_{\max }^{\left(\mathrm{no}\right)}\left({\mathrm{\τ}}_{\max }^{\left(\mathrm{no}\right)}\right)}----\left(2\right)$ In the formula

is the displacement time when the cross-correlation function of the echo signal after the nth emission and the (n+1)th emission appears the maximum value;

is at a displacement of

The value of the autocorrelation function when The time-domain signal formed by the above formula (1) is subjected to Fourier transform and its power spectrum is obtained, that is, the blood flow signal spectrogram is obtained, and the dynamic power spectrum of the blood flow signal is calculated accordingly.

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