JPS6113165B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an apparatus for measuring the flow velocity and direction of flow of blood, etc., using ultrasonic waves.

[Background of the invention]

An electroacoustic transducer is driven by a burst-shaped sine wave pulse train repeated at a predetermined period to transmit pulsed ultrasonic waves to a moving object, and the amount of Doppler shift that occurs in the reflected waves is detected to determine the object's moving speed. The method for measuring is generally called the pulse Doppler method, and is shown in, for example, the Acoustical Society of Japan, Vol. 29, No. 6 (1973), pp. 351-352. In the pulse Doppler method, the round trip time of the sound wave from when the transmitted pulse reaches the object until the reflected wave is detected is determined by applying a time gate to the received signal to extract the reflected wave from the object being measured. The location of the speed measurement can be determined by using the following method.

In the device shown in the above-mentioned known example, the amount of Doppler shift is detected by mixing the extracted received signal and a single auxiliary signal (reference wave). In other words, when a reference wave with the same frequency as the frequency f ₀ of the carrier wave of the transmitted signal is mixed with the extracted received signal, the frequency difference between the two (Doppler frequency) corresponding to the amount of Doppler shift caused by the movement of the object will decrease. A frequency component appears, and by passing the mixed wave through a low-pass filter, this low frequency component is extracted and the amount of Doppler shift is detected based on its frequency. Although this method can detect the absolute value of the Doppler shift, it has the disadvantage that it cannot distinguish the direction of the shift, that is, the direction in which the object is moving.

Therefore, a proposal has been made to detect the speed of movement of an object, including the direction of movement. In one example, two reference waves having a phase difference of 90° from each other are prepared, and each of these reference waves is mixed with a received signal of a reflected wave that has undergone a Doppler shift of ωd.
Pass each through a low-pass filter and cosω
Obtain a mixed waveform output of dt and sinωdt. A 90° phase shifter adds a 90° phase delay to the cosωdt waveform.
The phase shifter output is sinωdt when ωd>0, and - when ωd<0.
sinωdt. Therefore, if we take the sum and difference between the phase shifter output and the mixed wave output called sinωdt, when ωd>0, an oscillation waveform with a Doppler frequency appears on the sum side, and ω
When d<0, a vibration waveform with a Doppler frequency appears on the difference side, so the speed can be measured by frequency measurement, and the direction of movement of the object can be determined depending on whether the output waveform appears on the sum or difference side. According to this improved pulsed Doppler method,
Two systems of mixing sections are required, and a phase shifter is also required, which complicates the device.

Note that the pulse signal using the single reference wave mentioned above
By sacrificing the range of Doppler shift amount that can be detected by the Doppler method, that is, the speed detection range, it is possible to detect both the speed and the direction of movement. To explain this, first, according to the above-mentioned pulse Doppler method, if the pulse repetition period of the transmitted signal is _T2 , the range of the amount of Doppler shift that can be detected is essentially up to 1/ _2T2 . That is, the spectrum of the transmitted signal has secondary wave components of f ₀ ±n/T ₂ (n=1, 2, . . . ) on the left and right sides of the center frequency component of carrier frequency f ₀ . Therefore, for example, a received signal that has undergone a Doppler shift f _d will also _{have f 0 +} f _d ±n/T ₂ (n=1,
2...) sub-wave components. Therefore, the mixed wave of the received signal and the reference wave of frequency f ₀ has a component whose frequency is |(f ₀ +f _d )−f ₀ |=|f _d |, as well as |
(f ₀ +f _d +n/T ₀ )−f ₀ ｜=｜f _d ±n/T ₂ ｜(n
= 1, 2...). Therefore, in order to extract only the component |f _d | from these components, a low-pass filter with a cutoff frequency of 1/2T ₂ is used to obtain the absolute value of the amount of Doppler shift |f _d |. When the amount of Doppler shift |f _d | exceeds 1/ _2T2 , the component becomes |f _d | instead of | in the low-pass filter.
Since the component of f _d −1/T ₂ | is extracted, the correct amount of Doppler shift |f _d | cannot be obtained. In other words, the detection range of the Doppler shift amount by the pulse Doppler method is up to 1/ _2T2 .

Now, change the frequency of the reference wave to f ₀ (f ₀ −
1/4T ₂ ), the low frequency component of the mixed wave of the Doppler-shifted received signal and this reference wave has a frequency of |(f ₀ + f _d ) - (f ₀ - 1/4T ₂ ) |=|f _d +
The component becomes 1/4T ₂ | (f ₀ ±f _d ±n/T ₂ )−(f ₀ −1/4T
₂ ) |=|f _d +n/T ₂ +1/4T ₂ |(n=1, 2...). In this case, the frequency of the output through the low-pass filter with a cutoff frequency of 1/2T ₂ is |f _d + 1/4T ₂ |, so if this output frequency is, for example, zero, the value of the Doppler shift f _d It can be seen that the value of _{f d when the output frequency is 1/4T 2 is} zero, and when the output frequency is 1/2T ₂ , f _d is +1/4T ₂ . When f _d exceeds 1/4T ₂ , the low-pass filter reduces |f _d +1/
Since the component |f _d −1/T ₂ +1/4T ₂ | is extracted instead of the component of 4T ₂ |, the correct value of f _d cannot be obtained. In other words, by setting the frequency of the reference wave to (f _d -1/4T ₂ ), the direction of the Doppler shift, that is, the direction of movement, can also be detected, but the detectable range of the shift amount is halved to 1/4T ₂ Resulting in.

[Purpose of the invention]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a flow velocity measuring device that has a simple device configuration and is capable of measuring velocity including the direction of movement without halving the measurable velocity range.

[Summary of the invention]

A feature of the present invention is that it is equipped with a bandpass means (filter) having a predetermined passband, and after passing the received signal extracted by a time gate through this filter, it is mixed with an auxiliary signal (reference wave) to produce a frequency difference component between the two. The point is to detect.

The passband of the filter is preferably a band corresponding to one component of the spectrum of the transmitted signal (bandwidth is T ₂ ^-1 with respect to the repetition period T ₂ of the transmitted signal).

[Embodiments of the invention]

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a configuration diagram of a portion (hereinafter referred to as a wave transmitting portion) that transmits ultrasonic waves of an apparatus according to the present invention. 1st
In the figure, 1 is the signal generator, usually 2~5MHz
Generates a sine wave (frequency is f ₀ ). 2 is a normal gate circuit made of diodes, etc., and by opening and closing the gate with a pulse train 5 synchronized with the output 4 of a signal generator generated by a pulse generator 3 consisting of a counting circuit and a logic circuit, a sine wave pulse train shown in 6 is generated. occurs. where the pulse width
T ₁ is measured at an integer multiple of the period of the signal 4 and has a time length of several μs with respect to the distance resolution of the observation site. In addition, T ₂ of signal 6 corresponds to the distance where unnecessary signals are mixed and the maximum measurable speed, and as mentioned above, the speed up to 1/2T ₂ in the amount of Doppler frequency movement is the conventional distance CsT ₂ /2 (Cs: sound speed) It becomes possible to measure the interference of unnecessary signals (described in detail later). After sufficiently amplifying such a signal 6 with an amplifier 7, it is transmitted into the body using an electroacoustic transducer 8 made of PZT or the like. The ultrasonic waves thus transmitted are reflected by blood cells and the like and are detected by a piezoelectric receiver 9 made of PZT or the like in the wave receiving section shown in FIG. This received signal is received with a delay of the sound wave propagation time τ as shown in FIG. 310. Here, τ _A and τ _B correspond to the propagation time to blood cells A and B, respectively. Such a received signal 10 is sufficiently amplified by an amplifier 11 and then applied to a gate circuit 12. This gate circuit 12 has almost the same configuration as the gate circuit shown in _FIG. ) is generated by a delay circuit 14 such as a shift register, and is used as an opening/closing signal for the gate circuit 12. The operation of the gate circuit 12 configured in this way is such that when τ=τ _a is set, only the reflected wave from the object A is passed, as shown in FIG. 3, 15. Strictly speaking, reflected signals from objects located further away from object A by an integral multiple of CsT ₂ /2 (Cs: speed of sound) will also pass through the gate 12, and these are unnecessary signals. This will result in contamination. However, in general, such a reflected signal from a long distance becomes a weak signal due to attenuation during sound wave propagation.

FIG. 4a shows the frequency spectrum of the above-mentioned transmission signal, with the frequency f ₀ of the carrier wave (output wave of the signal generator 1) as the center frequency component, and f ₀ ±n/T ₂ (n =1, 2...) sub-wave components. On the other hand, if the object A is stationary, the received signal that has passed through the gate circuit 12 is
As shown in Figure a, when the object A is moving, the entire frequency spectrum will shift horizontally (strictly speaking, the frequency will be multiplied by a constant) due to the Doppler effect in response to the speed of movement. Therefore, in this embodiment, we focus on one component of the spectrum of the transmitted signal, for example, the component at the frequency f _T =f ₀ +1/T ₂ shown in FIG. Detect the amount of movement. That is, the second
FIG. 17 shows a filter having a passband width of 1/T ₂ centered on the frequency f _T . Figure 4b shows the frequency f of the frequency spectrum of the transmitted and received signals.
This is an enlarged view of the vicinity of _T , and the dotted line indicates the spectrum of the transmitted signal. The solid line indicates the spectrum of the received signal, which is shifted by f _d with respect to each component of the transmitted signal due to the Doppler effect. The selection characteristics of the filter 17 are as shown in 16 in Fig. 4b, and the signal passing through the filter 17 is as shown in Fig. 4C.
become that way. That is _, unless the amount of Doppler shift |f _d | exceeds 1/ _2T2 , one component of the received signal (f _T
+f _d ) appears at the output of the filter 17, and other received signal components are blocked. Therefore, the output of this filter 17 and a signal of frequency f _c obtained from the signal generator 19 are mixed in a mixer 18,
A low-pass filter 20 extracts only low frequency components of the mixed wave. The frequency of its output is |f _T +f _d −f _C |
becomes. _The value of f _{C is} set outside the passband of the filter 17 as _shown in _{FIG .} Just do it. Then, when the Doppler shift f _d is zero, the frequency of the extracted output is |f _T - f
_C |, and if f _T - f _C > 0, when f _d is positive (positive Doppler shift), the frequency of the extracted output will be on the high frequency side, and when f _d is negative (negative Doppler shift), the frequency of the extracted output will be on the high frequency side. is shifted toward the lower frequency side by |f _d |. Therefore, the Doppler movement amount |f _d | and its direction can be determined from the frequency of the output of the low-pass filter 20.

The absolute value of the amount of Doppler shift that can be measured at this time is 1/ _2T2 , that is, this device can measure speeds in the range from -1/ _2T2 to +1/ _2T2 in terms of Doppler shift amount. . Also, the part added to detect the direction of Doppler shift is filter 17.
This is particularly advantageous when constructing a device that simultaneously measures the velocity of objects at multiple positions (various τ).

In addition, in the above embodiment, the center frequency f ₀ of the transmission signal
We have described an example configuration in which we focus on the component of f _T = f ₀ + 1/T ₁ and measure the Doppler shift of the received signal component corresponding to this component, but which component of the transmitted signal spectrum You can also focus on However, the receiving section shown in FIG. 2 often has some kind of electrical circuit connection with the signal generator 1 shown in FIG.
In addition to the extracted received signal, the gate 12 also receives the frequency f ₀
unnecessary signals are mixed in. Therefore, if the component of the transmitted signal to be focused on is f ₀ , the passband of the filter 17 is from f ₀ -1/2T ₂ to f ₀ +1/2T ₂ , and the above unnecessary signal f ₀ is passed through the filter 17. It will pass. On the other hand, when the component of the transmitted signal of interest is one component of a sub-wave different from f ₀ as in the above embodiment, the filter 17 can block this unnecessary signal having the frequency f ₀ . Therefore, it is preferable that the frequency component f _T to be focused on is a sub-wave component different from f _0. In this case, there is no need to strictly prevent coupling between the signal generator 1 and the receiving part, so the device configuration becomes significantly easier.

〔Effect of the invention〕

As described above, according to the present invention, the measurable speed range is the same as that of the conventional method by using a simple filter, and it is also possible to measure the speed including the direction of movement of the object.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram illustrating the configuration of a wave transmitting section that generates and transmits a sine wave pulse of the present invention and the waveforms of each section, and FIG. 2 is a configuration of a wave receiving section that processes reflected signals of the present invention. FIG. 3 is an explanatory diagram showing the signal waveforms of each part in the wave receiving section, and FIG. 4 is an explanatory diagram explaining the frequency spectra of the gate passing signal and the filter passing signal. Here, 1 and 9 are signal generators, 2 and 12 are gate circuits, 3 is a pulse generator, 7 and 11 are amplifiers, and 8
is an electroacoustic transducer, 9 is a similar receiver, 14 is a delay circuit, 17 is a filter, 18 is a mixer, 20
is a low pass filter.

Claims (1)

[Scope of Claims] 1. A transmitting means for transmitting an ultrasonic pulse using a transmission signal of a burst-shaped sine wave pulse train repeated at a predetermined period; a receiving means for receiving a reflected wave generated by the ultrasonic pulse; Extracting means for allowing the received signal from the receiving means to pass only during a period with a desired time delay from the transmitted signal, thereby extracting a received signal of a reflected wave from a moving object in an area of a desired distance. have,
The apparatus for measuring the flow velocity and direction of the moving object from the extracted signal includes band-pass means for passing only signals in a predetermined frequency band among the extracted received signals, A flow velocity measuring device characterized in that the amount and direction of the Doppler shift of the reflected wave are measured by measuring the difference between the frequency of one component of the spectrum of the received signal and a specific frequency. 2. The flow velocity measuring device according to claim 1, wherein the bandpass means has a bandwidth of approximately T ₂ ^-1 with respect to a repetition period T ₂ of the transmitted signal. 3. The band-passing means passes a frequency band centered on one component of the secondary wave different from the center frequency component of the transmitted signal, and passes an unnecessary signal having the same frequency as the center frequency of the transmitted signal. 2. The flow rate measuring device according to claim 1, wherein passage of the flow rate is prevented.