JPH0634790B2 - Ultrasonic Doppler blood flow meter - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrasonic Doppler blood flow meter used for performing flow velocity measurement at an arbitrary position in a living body in the medical field.

2. Description of the Related Art In recent years, ultrasonic Doppler blood flowmeters have become popular in the medical field such as the heart and circulatory organs. This ultrasonic Doppler blood flow meter is based on the principle that the ultrasonic waves transmitted into the living body undergo frequency shift due to the Doppler effect that occurs when the ultrasonic waves are reflected by a moving object such as blood flow, and are compatible with blood flow velocity. By displaying the Doppler shift frequency, the blood flow velocity distribution in the living body can be easily observed from the body surface.

Hereinafter, a conventional ultrasonic Doppler blood flow meter will be described with reference to FIG. In FIG. 5, reference numeral 101 denotes an ultrasonic wave transmitting / receiving means (hereinafter referred to as a probe) which transmits an ultrasonic pulse into the living body through the ultrasonic wave transmitting / receiving surface 101a and receives an echo signal reflected by a difference in acoustic impedance.
Generally, it is composed of a piezoelectric material. A drive circuit 102 drives the probe 101 by generating a drive voltage for generating an ultrasonic pulse transmitted from the probe 101 at the frequency of an external clock and the timing of an external trigger, and 103.
Is a transmission timing circuit that gives a timing at which the drive circuit 102 generates a drive voltage as a trigger, 104 is a phase detector that phase-detects the echo signal received by the probe 101, and 105 is phase detection of the transmission signal and the echo signal of the drive circuit 102. A reference signal generation circuit that serves as a reference for the frequency and phase of the reference signal when phase detection is performed by the detector 104. Reference numeral 106 denotes a gate signal at a time corresponding to the propagation time of ultrasonic waves to the transmission / reception surface of the probe 101 and the target site. A gate signal generation circuit for generation 107, an analog switch for passing the detection signal phase-detected by the phase detector 104 during the section of the gate signal generated by the gate signal generation circuit 106, and a reference numeral 108 for integrating the detection signal passed through the analog switch 107. Then
An integrating circuit for obtaining a Doppler shift signal by repeating integration for each transmission / reception of an ultrasonic pulse, 110 is an integrating circuit 1
Prior to 08 performing integration, a sample hold circuit that holds the integrated result for resetting until the next integration result is obtained, 111 is several hundred Hertz or less from the Doppler shift signal obtained by the integration circuit 108. A high pass filter for removing signals, 112 is a high pass filter 111
The frequency analyzer analyzes the frequency of the Doppler shift signal that has passed through, and 113 is a display unit that displays the result of the frequency analyzer 112.

Next, the operation will be described with reference to the time chart shown in FIG. The transmission timing circuit 103 generates a trigger signal T at a fixed or arbitrary interval ((a) in the same figure) and supplies it to the drive circuit 102. Drive circuit 1
03 is a trigger signal T for driving the probe 101 with a pulse TX
The probe 101 drives the ultrasonic pulse in the living body. The ultrasonic pulse propagates in the living body and is reflected by the parts with different acoustic impedances.
It reaches the probe 101 again and is received as an echo signal E ((c) in the same figure). The echo signal E is the probe 101
The echo from the moving reflector is obtained as the delay time t _d from t _{0 of the} trigger signal T corresponding to the round-trip propagation time of the ultrasonic wave to the point where the ultrasonic pulse transmitted from the transmission / reception surface 101a is reflected. Since the round-trip propagation time of the signal always changes, t _d also changes. The amount of change in propagation time that changes during the trigger signal T is Δt _d , the angular frequency of the echo signal is ω,
Assuming that the time from the time t0 when the ultrasonic pulse is transmitted is t and the echo signal strength is A, 1 obtained by the n-th transmission and reception
The echo signal E from each reflector is expressed by the following equation.

E = Acosω {t- (td + nΔtd)} (1) where (td + nΔtd) represents the delay time of the echo signal in the n-th transmission / reception.

The echo signal E is quadrature-detected by the phase detector 104 by a reference signal R ((d) in the figure) described later. The reference signal R has the same angular frequency ω as the ultrasonic pulse and the echo signal E to be transmitted, and although not shown, the amplitudes of the trigger signals T having a phase difference of 90 ° with respect to the time t ₀ are constant. Na V _X
And V _Y , which are represented by the following equation.

In the phase detector 104, the signals of the following equations are obtained by multiplying the equations (1) and V _X and V _Y of the equation (2).

Two signals C of the formula (3) obtained by the phase detector 104
Integrating at _t 1 ~t ₂ sections, ON the analog switch 107 integrator circuit 108 (FIG. (E)) gate signals generated by the gate signal generating circuit 106 G (FIG. (F)). In the signal C ((e) in the figure) shown by the equation (3), the phase term is {−ω (td + nΔtd)} and {2ωt−ω (t
d + nΔtd)}, but the former is a DC signal because it does not include the time parameter t, and the latter is a high-frequency signal having a frequency twice the transmission frequency ω. , The latter component disappears, and the Doppler shift signals X and Y obtained at the integration end time t ₂ are as follows.

k is a constant determined by the circuit constant of the integrating circuit 108, and t _g is the time width of the gate signal G. The integration section of the integration circuit 108 is adapted to the time t _{1 to} t _{2 at} which the echo signal from the target site in the living body determined by the gate signal G is received, and the integration result X received at the n-th time by integration is X. , Y, if the echo signal between t _{1 and} t ₂ is from a plurality of reflectors, all of the information is added and included ((g) in the same figure). The voltages of the integration results X and Y are held as indicated by broken lines until the next (n + 1) th integration result is obtained by the next sample and hold circuit 110 ((h) in the figure). The integrating circuit 108 is reset when the holding of the sample hold 110 is completed. The integration result obtained as described above is a DC voltage as can be seen from the equation (4) in the case of transmission and reception only once, but by repeating the transmission and reception at every cycle T of the trigger signal T, Increases and X and Y change while maintaining the phase difference of 90 °. This is a quadrature Doppler signal, where X is a real part signal and Y is an imaginary part signal. X and Y are obtained as discrete information for each cycle T, and n is transmitted from the nth transmission / reception.
Amount of delay time Δt _d that changes to the interval T of the + 1st transmission / reception
And the velocity v of the reflector in the living body have the following relationship.

Δtd = 2vTcosθ / C (5a) Here, C is the speed of sound in the living body, and θ is the angle between the moving direction of the reflecting object and the ultrasonic wave traveling direction.

Doppler shift frequency fd of Doppler shift signals X and Y
Can be obtained by substituting the equation (5a) into the phase term (ωnΔtd + ωtd) of the equation (4). In the phase term of the equation (4), ωtd is a fixed phase that does not have a parameter indicating time, and is irrelevant to the Doppler shift frequency fd. Therefore, among the phase terms of the equation (4), the term that affects X and Y is ωnΔtd, and ω = 2πf
Substituting equation (5a) and equation (5a) yields equation (5b) below.

ωnΔtd = 2π (2vfcosθ / C) nT (5b) where f is the reference signal frequency (generally equal to the transmitted ultrasonic pulse frequency).

In general, the phase term of a sine wave is composed of angular frequency and time. In the case of equation (5b), 2π and (2vfcosθ) / C correspond to angular frequency, and nT corresponds to time. Therefore, (2
Since vfcosθ) / C corresponds to the Doppler shift frequency fd, the Doppler shift frequency fd of the Doppler shift signals X and Y is expressed by the following equation.

fd = (2vfcosθ) / C (6) In the living body, when the echo signal from the blood flow is captured, the amplitude of the Doppler shift signal is very weak because the reflection intensity of the blood flow is small, and Since the blood flow velocity is high, the shift frequency f _d becomes high. In a living tissue such as an internal organ, the reflection intensity A is large, and the movement is slow due to body movement or the like, so that the shift frequency f _d becomes very low. By passing this through the high-pass filter 111, it is possible to obtain only the Doppler shift signal from the blood flow having a small amplitude and a high frequency. Removing the Doppler shift signal from the living tissue is a very important part for expanding the dynamic range of the frequency analyzer 112, and is generally set to 100 to 1 kHz. Frequency analyzer 112
Then, frequency analysis is performed on the Doppler shift signals X and Y, the direction of blood flow is obtained from the phase relationship between the two signals, and the blood flow pattern is displayed on the display unit 113.

Problems to be Solved by the Invention However, in contrast to the trigger signal T shown in FIG. 6 (a), the echo signal E from the biological tissue which seems to be almost stationary as shown in FIG. 6 (b). Very strong part b,
It is roughly divided into a weak portion a from the blood flow that is constantly moving. In order to measure the blood flow velocity, it is necessary to amplify the weak echo signal portion a from the blood flow with a large gain. If only the weak echo signal from the blood flow is to be amplified, the integrating circuit 108 will include only the AC component. Real part signal X _a and imaginary part signal Y with
_a is obtained with an amplitude up to the amplitude limit V of the integrator circuit (see (c) in the figure). Further, when the setting of the measurement point is in the strong echo signal portion b from the biological tissue, the phase difference between the reference signal R and the echo signal E is always constant or changes very slowly. At the output, a DC component or an AC component of the real part signal Xd and the imaginary part signal Yb having a very slow change appears ((d) in the figure).
reference). In actual clinical practice, the ultrasonic beam transmitted from the probe 101 has a divergence. Therefore, when a blood vessel that is thinner than the beam diameter is captured, the blood flow and the echo signal from the living tissue are often present at the same time. The wall filter 111 is used to remove the influence of the living tissue. However, when a relatively shallow measurement point is observed, for example, the real part signal Xa and the imaginary part signal Ya are obtained in a form of being superimposed on the real part signal Xb and the imaginary part signal Yb, and become like Xc and Yc (see FIG. (See (e)). However, due to the amplitude limit V of the integrating circuit 108, the shaded portion causes clipping and the waveform is distorted. When the DC component is small and does not cause saturation, the DC component can be removed by the conventional bypass filter 111. However, when measuring the blood flow in a relatively shallow carotid artery or a deep thin blood vessel, ultrasonic waves are used. Due to the divergence of the beam, the echo signal from the living tissue around the blood vessel is particularly strong and the DC component is particularly increased.
A phenomenon occurs in which 08 is saturated and the Doppler shift signal disappears. Therefore, the high-pass filter 111 cannot obtain the Doppler shift signal even if this DC component is removed, and an unnecessary frequency component is generated in the result of the frequency analysis, or the analysis result is interrupted. Further, when either of the Doppler shift signals X and Y is saturated, a phenomenon occurs in which information regarding the direction of blood flow cannot be obtained, which is a diagnostic problem.

Therefore, the present invention is to solve the above-mentioned problems of the prior art, and removes the DC component generated by the echo signal of the living tissue from each of the real part output and the imaginary part output output from the integrating circuit, It is an object of the present invention to provide an ultrasonic Doppler blood flow meter capable of performing frequency analysis of only blood flow and obtaining reliable blood flow information.

Means for Solving the Problems And the technical means of the present invention for solving the above problems are to transmit an ultrasonic pulse in a living body and receive an echo signal reflected by a plurality of reflectors in the living body. An ultrasonic transmitting / receiving means, a drive circuit for generating an ultrasonic pulse to be transmitted by the ultrasonic transmitting / receiving means, and a transmission timing signal circuit for giving a timing at which the drive circuit generates an ultrasonic pulse,
A phase detector for phase-detecting the echo signal obtained by the ultrasonic wave transmitting / receiving means, and a reference signal generating circuit serving as a reference for the frequency and phase of the reference signal when phase-detecting the transmission signal of the drive circuit and the echo signal. A gate signal generating circuit that generates a gate signal at a time corresponding to an echo signal from a target portion, and an analog switch that controls opening and closing of the detection signal phase-detected by the phase detector with the gate signal, An integration circuit that integrates the detection signal that has passed through the analog switch only in the section of the gate signal, and obtains a Doppler shift signal by repeating the integration for each transmission / reception of the ultrasonic pulse, and obtained by this integration circuit. And a direct current feedback circuit for negatively feeding back the direct current component or the ultra low frequency component of the obtained Doppler shift signal to the integration circuit.

Operation According to the present invention, the echo signal is quadrature-detected, frequency-analyzed and displayed in accordance with the same route as the above-mentioned conventional example. Here, negative feedback is applied to the integrator circuit by the DC component of the Doppler shift signal of the integration result, and the integrator circuit functions as a high-pass filter that blocks the extremely low frequency region. Therefore, it is possible to remove only the influence of living tissue and to select the frequency that does not adversely affect the detection of the Doppler shift from the blood flow, without integrating the DC component, only the AC component that is the Doppler shift signal. Can be output to the subsequent circuits. In this way, the DC or infra-low frequency component of the orthogonal output generated by the echo signal from the living tissue can be removed, and only the information on the blood flow direction can be obtained.

Embodiment Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block circuit diagram of an ultrasonic Doppler blood flow meter in one embodiment of the present invention. The ultrasonic Doppler blood flow meter of the present invention is a DC and ultra low frequency component of the quadrature signal output by the integration circuit or sample hold circuit in the conventional ultrasonic Doppler blood flow meter via an analog switch to the input of the integration circuit. DC feedback circuit 9 for negative feedback
It is equipped with.

That is, reference numeral 1 denotes an ultrasonic wave transmission / reception means (hereinafter referred to as a probe) that transmits an ultrasonic wave pulse into the living body through the ultrasonic wave transmission / reception surface 1a and receives an echo signal reflected by a difference in acoustic impedance, and is generally a piezoelectric material. It is composed of The reference numeral 2 generates a drive voltage for generating the ultrasonic pulse transmitted from the probe 1 at the frequency of the external clock and the timing of the external trigger, and the drive circuit 3 for driving the probe 1 and the drive circuit 2 for the 3 generate the drive voltage. A transmission timing circuit for giving timing as a trigger, 4 is a phase detector for phase-detecting echo signals from a plurality of reflectors received by the probe 1, and 5 is a phase detector 4 for phase-transmitting transmission signals and echo signals of the drive circuit 2. A reference signal generation circuit serving as a reference for the frequency and phase of the reference signal at the time of detection, 6 is a gate for generating a gate signal at a time corresponding to the propagation time of the ultrasonic wave to the transmission / reception surface 1a of the probe 1 and the target site. The signal generation circuit 7 passes the detection signal phase-detected by the phase detector 4 and the DC feedback voltage through the section of the gate signal generated by the gate signal generation circuit 7. The analog switch, 8 integrates the detection signal that has passed through the analog switch 7, and obtains a Doppler shift signal by repeating the integration every time an ultrasonic pulse is transmitted and received. 10 is an integration circuit before integration circuit 8 performs integration. A sample and hold circuit that holds the integrated result for resetting until the next integrated result is obtained. Reference numeral 9 is an analog circuit for the DC component of the Doppler shift signal output by the integrator circuit or sample and hold circuit 10 or an ultralow frequency signal. A direct current feedback circuit that negatively feeds back to the integrating circuit 8 via the switch 7, 11 is a high-pass filter that removes a signal of several hundreds Hertz or less from the Doppler shift signal obtained by the integrating circuit 8, and 12 is a Doppler that has passed through the high-pass filter 11. A frequency analyzer for frequency-analyzing the shift signal, and 13 is a display unit for displaying the result of the frequency analyzer 12. That.

Next, the operation of the above embodiment will be described. Also in the above-mentioned embodiment, the echo signal is quadrature-detected, frequency-analyzed and displayed according to the path explained in the above-mentioned conventional example. Negative feedback is applied to the integrating circuit 8 by the DC components of the real part new X and the imaginary part signal Y after integration, and they function as a high-pass filter in the extremely low frequency region.

FIGS. 3 (a) and 3 (c) show an example of specific circuits for the analog switch 7, the integrating circuit 8, the sample-hold circuit 10, and the DC feedback circuit 9 of the embodiment shown in FIG. 3B and 3D show signal waveforms of the terminals in FIGS. 3A and 3C. The circuit of FIG. 7A or FIG. 7C requires one system for each of the real part signal X and the imaginary part signal Y, but one of them is shown in the figure. The circuits of both (a) and (c) in the figure have the same effect. First, as a first embodiment, the operation of the circuit of FIG. 1A will be described with reference to FIG. The integrating circuit 8 in FIG. ₁ has an inverting amplifier OP ₁ having an amplification degree of −A ₁ , a resistor R ₁ , and a capacitor C _0.
The input terminals include E _{i to} which the output of the phase wave phase shifter 4 (see FIG. 1) is applied and E f ₀ to which the DC feedback voltage of the DC feedback circuit 9 is applied, and the signal at each terminal is an analog switch. 7 is added just before, and the analog switch 7 is O
When N, it is integrated. Analog switch 7 is integrated only during the time t _g to ON t ₁ ~t ₂ section of the gate signal, to the input terminal (the output of the integration circuit) Ef _i of DC feedback circuit 9 is a waveform as shown can get. The sample-hold circuit 10 has a gain of −A, and when the integration is completed, samples from t _{2 at} time tω and holds it after t ₃ . After the hold, the integrator circuit sets the analog switch 7'to RE before the next transmission / reception.
The ST signal is turned on by the ST signal in the interval of t ₃ to ₄ to discharge C ₀ and reset. At the output terminal E ₀ of the sample hold circuit 10, a Doppler signal discretized with a cycle T appears as shown in the figure, and the next high pass filter 11 (first
See figure). On the other hand, the DC feedback circuit 9 forms a low-pass filter with the inverting amplifier OP ₃ , the capacitor Cf, and the resistor Rf, and extracts DC and ultra-low frequency components from the output signal of the integrator circuit added to Ef _i to extract the resistance. Negative feedback is provided via R ₂ . The inverting amplifier OP ₂ has a gain of −Af and is for inverting the phase. Therefore, the lower the frequency of the output of the integrating circuit, the larger the feedback amount, and the smaller the integration result. The input / output characteristics of the above circuit are obtained as follows.

(7) includes a DC feedback voltage ef ₀ applied to the voltage e _i and the input terminal Ef ₀ of the phase detector output in the gate time t _g applied to the input terminal E _i, Table et relationships of the integration circuit output ef _i (8) is the input E of the DC feedback circuit 9
The integrated output ef _i added to f _i and the DC feedback voltage ef ₀ are represented. In the equation (8), ω _d is the frequency of the Doppler shift signal output to the integrating circuit, and (tgK +
tw) / T is the duty ratio of the period T and tw of the Doppler shift signal discretely existing, and K contained here is such that the integration section from t ₁ to t ₂ reaches ef _i . Since it is in a transient state, it is a number of 1 or less for correcting the duty ratio in this section. From the equations (7) and (8), the gain A _V (A _V = ef _i / e _i ) of the circuit of FIG. 3 (a) is as follows.

Further, the −3 dB low frequency cutoff frequency f _c of the equation (9) is as shown by the following equation.

f _c is sufficiently allowed to pass through the Doppler shift signals by the blood flow, and to select the frequency that can block the Doppler shift signals by the body movement, the ultrasonic pulse frequency is about 2~7MHz general ultrasonic pulses Doppler blood flow meter, 50-500
It is appropriate to select Hz. Next, as a second embodiment, the operation of the circuit shown in FIG. 9C will be described with reference to FIG. The main configuration and operation are the same as those of the first embodiment shown in FIG. 9A, but in the first embodiment, the DC feedback circuit 9 integrates from the output of the integrating circuit via the analog switch 7. In contrast to the input to the circuit, the present embodiment shown in FIG. 3C is different in that the output from the sample hold circuit 10 is added to the integrating circuit. Further, the inverting amplifier OP ₂ used in the first embodiment is unnecessary in this embodiment because the sample hold circuit 10 also has this function.
The input / output characteristics of this circuit are obtained as follows.

(11), (12), respectively (7), (8) corresponds to the formula, from the above equation, the gain _A V _(A V _{= e} 0 of the circuit of FIG. 3 (c) ~ e _i ) is shown by the following equation.

Further, the −3 dB low frequency cutoff frequency f _c of the equation (13) is expressed by the following equation.

The determination of f _c is the same as in FIG. 3 (a), and FIG.
Shows the frequency characteristics of the circuits of FIGS. 3 (a) and 3 (c). As shown in the figure, by selecting the frequency f _c that can remove only the influence of the living tissue and does not adversely affect the detection of the Doppler shift from the bloodstream, the Doppler shift can be achieved without integrating the DC component. It is possible to output only the AC component that is the transfer signal to the subsequent circuits. The Doppler shift signal appearing in the quadrature signals X and Y from which the direct current component has been removed in this manner is not affected even when echo signals from the living tissue are simultaneously received. FIG. 4 shows output waveforms of the integrating circuit 6 according to the embodiment of the present invention and the conventional example. The solid line shows the embodiment of the present invention, the broken line shows the conventional one, and the shaded portion shows the waveform removed due to saturation. As shown in the figure, by implementing the present invention, the amplitude limit is expanded, and since there is no influence of the DC voltage, it is possible to measure even a very thin blood vessel, the integrating circuit 8 does not cause saturation, and the Doppler bias does not occur. The amplitude of the transfer signal can make maximum use of the amplitude limit V.

As described above, according to the ultrasonic Doppler blood flow meter of the present invention, the DC component or the ultra low frequency component of the Doppler shift signal obtained by the integrating circuit is negatively fed back by the DC feedback circuit. Therefore, even if an echo signal that does not change with time from living tissue is captured at the same time as the echo signal from the blood flow with a simple circuit configuration, it is possible to perform frequency analysis of only the blood flow, and the blood flow on the display can be analyzed. It is possible to obtain accurate information on the direction of the blood flow because the flow pattern is not displayed except for the blood flow.

[Brief description of drawings]

FIG. 1 is a block circuit diagram showing an ultrasonic Doppler blood injection meter in one embodiment of the present invention, FIG. 2 is a time chart showing the operation of the blood flow meter, and FIG. A concrete circuit diagram in one embodiment of the main part, FIG. 3 (b) is a time chart showing the operation of the circuit of FIG. 3 (a), and FIG. 3 (c) is another main part of the blood flow meter. A specific circuit diagram in the embodiment, FIG. 3 (d) is a time chart showing the operation of the circuit of FIG. 3 (c), and FIG. 3 (e) is FIG. 3 (a).
FIG. 4 is a diagram showing the frequency characteristics of the circuit of FIG. 4C, FIG. 4 is a diagram comparing the present invention and a conventional integrated waveform, FIG. 5 is a block circuit diagram of a conventional ultrasonic Doppler blood flow meter, and FIG. It is a figure which shows the integrated waveform obtained using the ultrasonic Doppler blood flow meter of FIG. 1 ... Probe (ultrasonic wave transmitting / receiving means), 2 ... Driving circuit, 4 ... Phase detector, 5 ... Reference signal generating circuit, 6 ...
… Gate signal generation circuit, 7… Analog switch, 8…
… Integrator circuit, 9 …… DC feedback circuit, 10 …… Sample hold circuit, 11 …… High pass filter, 12 …… Frequency analyzer, 13 …… Display section.

Claims (1)

[Claims] 1. An ultrasonic wave transmitting / receiving means for transmitting an ultrasonic wave pulse in a living body and receiving echo signals reflected by a plurality of reflectors in a living body, and an ultrasonic wave pulse to be transmitted by this ultrasonic wave transmitting / receiving means. A drive circuit, a transmission timing signal circuit for giving a timing at which the drive circuit generates an ultrasonic pulse, a phase detector for phase-detecting an echo signal obtained by the ultrasonic wave transmitting / receiving means, a transmission signal of the drive circuit, and A reference signal generation circuit that serves as a reference for the frequency and phase of the reference signal when phase-detecting the echo signal, a gate signal generation circuit that generates a gate signal at a time corresponding to the echo signal from the target site, and An analog switch that controls the opening and closing of the detection signal phase-detected by the phase detector with the gate signal, and passes through the analog switch only in the section of the gate signal. The detection signal is integrated, and an integration circuit that obtains a Doppler shift signal by repeating the integration for each transmission / reception of the ultrasonic pulse, and a DC component of the Doppler shift signal obtained by this integration circuit or an ultra-low An ultrasonic toppler blood flow meter, comprising: a DC feedback circuit that negatively feeds back a frequency component to the integration circuit.