JPH0221259B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an ultrasonic diagnostic apparatus, and particularly to an ultrasonic diagnostic apparatus that displays tomographic images.

BACKGROUND TECHNOLOGY Ultrasonic diagnostic equipment usually employs the pulse echo method, and this type of ultrasound diagnostic equipment includes a probe that transmits and receives an ultrasonic pulse beam into the subject, and A tomographic image is displayed based on the echo signal. In other words, in the pulse-echo method, an ultrasonic pulse beam is emitted from a probe into the subject, the reflected waves from various depths are detected, and the intensity is displayed on a CRT as a function of depth. Since reflection of ultrasonic waves occurs at parts of the subject where acoustic impedance changes, the pulse echo method allows for image display of interfaces between organs and the like within the subject. Therefore, although the pulse echo method allows obtaining morphological information of a subject, there is a problem in that it is difficult to qualitatively diagnose living tissues.

Therefore, research is being conducted to measure the attenuation coefficient of living tissues and use this for qualitative diagnosis of living tissues.
(Computed Tomography) has been proposed.
The principle of ultrasonic CT is to transmit ultrasonic waves into the subject from many different directions, perform reconstruction calculations based on the intensity of these transmitted ultrasonic waves, and obtain the attenuation coefficient of the living tissue. however,
This ultrasound CT uses the principle of detecting transmitted ultrasound waves, so if there are substances in the subject that are difficult for ultrasound to pass through, such as bone or air, The drawback was that the attenuation coefficient could not be measured.

OBJECTS OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and its purpose is to provide an ultrasonic diagnostic apparatus that can easily measure the attenuation coefficient of a subject from reflected echo signals.

Configuration of the Invention In order to achieve the above object, the present invention provides an ultrasonic diagnostic apparatus that transmits and receives an ultrasonic pulse beam to and from a subject and displays a tomographic image based on a reflected echo signal from the subject. a bandpass filter and sample circuit for extracting a specific frequency component from the detection circuit; a logarithmic converter for logarithmically converting the signal from the detection circuit via the bandpass filter; a first delay circuit that outputs a delayed signal delayed by a time interval of , and subtraction to obtain a difference signal by inputting the signal output from the logarithmic converter and the delayed signal output from the first delay circuit. A feedback circuit consisting of a second delay circuit and an attenuator inputs the difference signal to an adder circuit provided between the input and output circuits, calculates the average value of the difference signals, and calculates the reflection coefficient inside the subject. an averaging circuit that outputs an attenuation signal with reduced statistical fluctuations, and an attenuation coefficient of the ultrasonic wave for a specific frequency component is measured by the attenuation signal output from the averaging circuit. do.

Embodiments Hereinafter, preferred embodiments of the present invention will be described based on the drawings.

First, the principle of the present invention will be explained. The attenuation coefficient α of ultrasonic waves is a function of the frequency f, which can be expressed as α(f)
It is expressed as Among the reflected echo signals from the object, those reflected from the part far from the probe have a longer propagation path within the object, so they are much larger than the echo signals from near the probe. Attenuation of sound waves increases. Furthermore, since the attenuation coefficient is a function of frequency, not only the intensity of the ultrasound wave decreases, but also the spectrum of the reflected echo signal changes. The present invention utilizes this property,
This method focuses on a specific frequency in the spectrum and measures the attenuation coefficient for the specific frequency based on its changes.

FIG. 1 shows a block circuit according to the principles of the present invention.

The probe 10 is excited by an oscillator 12 to emit an ultrasonic pulse beam to a subject (not shown), and a reflected echo from the subject is received by the probe 10 . A bandpass filter 14 is connected to the probe 10, and the bandpass filter 14 can extract a specific frequency component from the reflected echo signal. Note that here, the reflected echo signal is obtained as a function of time t, and in the frequency band of ultrasound used in ultrasound diagnostic equipment, the speed of sound is considered to be constant regardless of frequency.
If this speed of sound is represented by C, then the position x where the reflected echo occurs and the time t when the probe detects the reflected echo have the following relationship.

x=Ct/2...(1) Therefore, in the following explanation, the position x within the subject and the time t may be treated equally.

Impulse response h of the bandpass filter 14
(t) shall be given by the following equation.

h(t)=a(t)cos(2πf ₁ t)...(2) In equation (2), f ₁ is the center frequency of the bandpass filter 14, and a(t) is the envelope of the impulse response. means. It is assumed that the bandpass filter 14 is a causal system and can be regarded as a(t)=0(t<0) 0(t> _t0 )...(3).

If the reflected echo signal to be input to the bandpass filter 14 is represented by P(τ), the output signal b(t) of the bandpass filter 14 can be expressed as follows using equation (2).

b(t)=∫ ^t _t-t0 p(τ)a(t-τ) cos{2πf ₁ (t-τ)}dτ...(4) Here, p(τ)×a(t-τ) When the result of Fourier transform of τ is expressed as A(f, t)e ^j 〓 ^{(f, t)} , A(f, t) e ^j 〓 ^{(f, t)} becomes as follows.

A(f,t)e ^j 〓 ^(f,t) =∫ ^t _t-t0 p(τ) a(t-τ)e ^-2 〓 ^jf 〓dτ ……(5) Therefore, formula (5) can be written as When used, equation (4) is transformed as follows.

b(t)=A(f ₁ , t) cos {2πf ₁ t+ξ(f ₁ , t)}
...(6) Here, ξ(f ₁ , t) is a phase term. This(6)
The meaning of the expression is as follows. That is, the envelope A of the output signal of the bandpass filter 14
(f, t) is the absolute value at frequency f ₁ of the Fourier spectrum of the function p(τ)×a(t-τ), in other words, the output signal A( f ₁ , t) gives the absolute value of the frequency f ₁ component included in the signal extracted by multiplying the reflected echo signal P(τ) by the window function a(t-τ).

Next, we will explain how the attenuation coefficient α(f) is related to A(f ₁ , t), that is, the output signal of the detection circuit 16, and in order to find the attenuation coefficient α(f) from A(f ₁ , t). We will explain the means for this.

As mentioned above, since the attenuation coefficient α(f) is a function of the frequency f, the waveform of the reflected echo signal depends on the depth of the reflector. If the reflection coefficient at position x in the subject is r(x), then the reflected echo signal Px(τ) reflected from this position x is as follows.

Px(τ)=r(x)∫ ^∞ _-∞ S(f)e ^-2 〓(f) ^x e ^-2 〓 ^jf(2x/C) e ² 〓 ^jf 〓df ……(7) Here, S (f) is the Fourier transform of the ultrasonic pulse waveform incident on the subject. Using this equation (7), the reflected echo signal P(τ) from the subject becomes as follows.

P(τ)=∫ ^∞ ₀ Px(τ)dx...(8) Next, consider the signal obtained by multiplying the window function a(t-τ) by equation (8).

Fig. 2 shows various waveforms according to the principle of the present invention, and in Fig. 2, the horizontal axis is the time τ (or position x in the subject) at which the reflected echo was detected.
The time τ and the position x have the relationship expressed by equation (1) as described above.

In Figure 2a, the reflection coefficient r inside the object
(x) is shown, and the reflection coefficient r(x) is shown as an impulse train because it exists only at the interface where the acoustic impedance changes within the subject. Further, in FIG. 2b, the reflected echo signal P(τ) expressed by equation (8) is shown, and in FIG. 2c, the envelope a of the impulse response of the bandpass filter 14 is shown.
(t-τ) is shown, and further, in FIG. ing.

As is clear from Fig. 2, the window function a(t-τ)
If the bandpass filter 14 is selected so that the shape is close to a rectangle, then P(τ)・a(t−τ) is such that the position x is in the range of C(t−t ₀ )/2<x<Ct/2. It can be approximated to a reflected echo from a reflector. That is, becomes. When we Fourier transform this equation (9) with respect to τ, we get is obtained.

Then, the output signal of the detection circuit 16 becomes the absolute value of the value when f=f ₁ in equation (10). Therefore, It is expressed as

A logarithmic amplifier 18 is connected to the detection circuit 16, and the logarithmic amplifier 18 logarithmically transforms the output signal from the detection circuit 16. Therefore, the output signal Al (f ₁ , t) of the logarithmic amplifier 18 is obtained by logarithmically transforming the above equation (11), and Al (f ₁ , t) = lnA (f ₁ , t) = ln | S (f ₁ )｜−α(f ₁ )tC+ε(t ₀ , t, f ₁ )
...(12) can be expressed as. However, in equation (12),
The logarithm of the integral term in equation (11) is expressed as ε(t ₀ , t, f ₁ ).

Next, in order to obtain the difference between the output signal from the logarithmic amplifier 18 and a signal obtained by delaying this output signal by a delay time d, a delay circuit 20 and a subtraction circuit 22 are provided, and the output G ₀ ( f ₁ , t)
is, G ₀ (f ₁ , t) = Al (f ₁ , t-d) - Al (f ₁ , t) = α (f ₁ )・d・C-ε (t ₀ , t, f ₁ ) + ε (t ₀ , t-d, f ₁ ) ...(13). By this equation (13), S in equation (12)
(f ₁ ) can be eliminated, and the output signal G ₀ (f ₁ , t) from the subtraction circuit 22 is the attenuation coefficient α (f ₁ ) at the frequency f ₁ , the delay time d of the delay circuit 20 and the sound speed C and add {−ε(t ₀ ,
t, f ₁ )+ε(t ₀ , t−d, f ₁ )} is added.

Therefore, this error term {−ε(t ₀ , t, f ₁ )+ε
(t ₀ , t−d, f ₁ )} In other words, focus on the integral term of equation (11). The reflection coefficient r(x+Ct/2) in the integral term should be regarded as a random variable. That is, since the reflection coefficient r(x) within the subject varies due to respiratory movement of the living body, movement due to pulse membranes, movement due to blood flow, etc., it cannot be considered to have a fixed value. Therefore, considering the case where the statistical properties of the subject can be considered uniform within the range indicated by the delay time d, ε(t ₀ , t, f ₁ ) and ε(t ₀ , t−d, f ₁ ) have the same expected value μ. Therefore, if we take the expected value of the error term {-ε(t ₀ , t, f ₁ )+ε(t ₀ , t-d, f ₁ )}, we get E{-ε(t ₀ , t, f ₁ )+ε (t ₀ , t−d, f ₁ )} =E{−ε(t ₀ ,t, f ₁ )} +E{ε(t ₀ ,t−d, f ₁ )} =−μ+μ =0. That is, the error term {−ε(t ₀ , t, f ₁ )+
The average value of ε(t ₀ , t−d, f ₁ )} is 0.

Therefore, in the above equation (13), if we take the expected values on both sides, that is, if we find the average value of the output signal from the subtraction circuit 22, then the error term {−ε(t ₀ , t,
f ₁ )+ε(t ₀ , t−d, f ₁ )} approaches 0, so α
It is understood that (f ₁ )・d・C is required.

Therefore, in order to average the output signals from the subtraction circuit 22 and obtain the average value of the output signal G ₀ (f ₁ , t),
An averaging circuit is provided, which consists of a summing circuit 24, an attenuator 26 and a delay circuit 28. Note that the delay time of the delay circuit 28 is set equal to the repetition frequency of the oscillator 12, and the delay time of the attenuator 28 is set equal to the repetition frequency of the oscillator 12.
Changing the weight determined by 6 corresponds to changing the average number of times.

In the above manner, the average value of the output signal G ₀ (f ₁ , t) from the subtraction circuit 22 is obtained, and since the delay time d and the sound velocity v are known, the attenuation is performed based on the above equation (13). The coefficient α(f ₁ ) can be found,
By appropriately correcting this, the attenuation coefficient α(f ₁ ) at the frequency f ₁ of the subject will be displayed on the display device 30.

In the above explanation, for the sake of simplicity, it is assumed that the attenuation coefficient α (f ₁ ) is constant within the subject, but as can be easily understood from the above explanation, the attenuation coefficient α
(f) does not need to be uniform throughout the subject; it is sufficient that the attenuation coefficient α(f) can be approximated to be constant within the delay time d of the delay circuit 20.

As described above, according to the principles of the present invention, it is possible to easily measure the attenuation coefficient of a subject based on the reflected echo signal, and thereby to display an image of the attenuation coefficient for a specific frequency within the subject. Can be done.

Next, FIG. 3 shows a block circuit of an ultrasonic diagnostic apparatus according to an embodiment of the present invention. In the block circuit _of FIG. ₃ , the attenuation coefficient α (f ₁ ), α(f ₂ ) can be measured.

That is, in the block circuit of FIG.
6-1, 16-2, logarithmic amplifier 18-1, 18-
2. Delay circuits 20-1, 20-2, subtraction circuit 22
-1, 22-2, addition circuits 24-1, 24-2,
Attenuators 26-1, 26-2, and delay circuit 28-
1 and 28-2 are provided with two systems for specific frequencies f ₁ and f ₂ , respectively.
Attenuation coefficient α(f ₁ ) for specific frequencies f ₁ , f ₂ of the species,
It becomes possible to measure α(f ₂ ).

Furthermore, in general, in living tissue, the attenuation coefficient α(f) is proportional to the frequency, and the constant of proportionality of the attenuation coefficient to the frequency is an effective parameter for tissue diagnosis. Therefore, if this constant of proportionality is β, the constant of proportionality β is β={α(f ₁ )−α(f ₂ )}/(f ₁ −f ₂ )
...(14) The proportionality constant β can be found from the frequencies f ₁ and f ₂ and the damping coefficients α(f ₁ ) and α(f ₂ ).

Therefore, according to the block circuit of FIG. 3, it is possible to display the attenuation coefficients α(f ₁ ), α(f ₂ ), the proportionality constant β, and a normal pulse echo image. The block circuit shown in FIG. 3 will be explained in detail below.

In FIG. 3, a transmission signal from an oscillator 12 is supplied to a probe 10 via a drive circuit 32, and the probe 10 transmits an ultrasonic pulse beam to a subject.
The reflected echo from the subject 34 is received by the probe 10 and converted into an electrical signal. The reflected echo signal from the probe 10 is transmitted to a high frequency amplifier 36, a detection circuit 38 and a video amplifier 40.
After a predetermined amplification effect is performed at , the signal is supplied to a display device (CRT) 30 via a multiplexer 42.
The display device 30 displays a normal B-mode or M-mode tomographic image.

A scan control circuit 44 is provided to mechanically or electrically scan the ultrasonic pulse beam emitted from the probe 10, and a sweep circuit 46 is provided to control the sweep of the display device 30. A sweep circuit 46 provides a sweep trigger signal 104 to the display device 30 based on a scan position signal 100 from the scan control circuit 44 and a synchronization signal 102 from the oscillator 12 . Furthermore, the synchronizing signal 102 from the oscillator 12 is used not only as a synchronizing signal for the sweep circuit 46, but also as a synchronizing signal for each part of the apparatus.

The embodiment is characterized in that the attenuation coefficients α(f ₁ ) and α(f ₂ ) with respect to frequencies f ₁ and f ₂ are measured, and for this purpose, the reflected echo signal from the probe 10 is After being subjected to a predetermined amplification effect by the high frequency amplifier 48, the center frequencies f ₁ and f ₂ differ from each other.
The signal is supplied to bandpass filters 14-1 and 14-2 having the following characteristics. Then, the damping coefficients α(f ₁ ) and α(f ₂ ) for the frequencies f ₁ and f ₂ are determined by the same action as in the block circuit of FIG. 1 described above.

Note that the amplitude of the reflected echo signal decreases over time due to attenuation within the subject 34. Therefore, in order to prevent the amplitude of the input signals to the bandpass filters 14-1 and 14-2 from fluctuating greatly, it is necessary to increase the gain of the high-frequency amplifier 48 over time.
is provided, and the high frequency amplifier 48 can be controlled by the signal generator 50.

Further, a subtraction circuit 52-1 is provided between the logarithmic amplifier 18-1 and the subtraction circuit 22-1, and similarly, a subtraction circuit 52-1 is provided between the logarithmic amplifier 18-2 and the subtraction circuit 22-2. 2 are provided, and variations in the gain of the high frequency amplifier 48 can be corrected by the subtraction circuits 52-1 and 52-2. That is,
The logarithm of the gain of the high frequency amplifier 48 is added to the output signals from the logarithmic amplifiers 18-1 and 18-2, and the gain control signal from the signal generator 50 is logarithmically converted by the logarithmic amplifier 54, and this is subtracted. Circuit 52-
By subtracting by 1,52-2, it is possible to correct the variation in the gain of the high frequency amplifier 48.

Further, the delay times of the delay circuits 28-1 and 28-2 are set equal to the repetition period of the oscillator 12.

The output signals from the adder circuits 24-1 and 24-2 are transmitted to amplifiers 56-1 and 56-2, respectively.
receives a predetermined amplification or attenuation effect at the amplifier 5.
Attenuation coefficient signal 106-1 from amplifier 6-1 indicates attenuation coefficient α(f ₁ ), and similarly, attenuation coefficient signal 106-2 from amplifier 56-2 indicates attenuation coefficient α(f ₂ ).
This will show the following. the attenuation coefficient signal 106-1,
106-2 is supplied to the display device 30 via the multiplexer 42, and the display device 30 displays the attenuation coefficients α(f ₁ ) and α(f ₂ ). Further, the attenuation coefficient signals 106-1 and 106-2 are transmitted to the system subtraction circuit 58.
A proportionality constant β of the attenuation coefficient with respect to frequency is determined by the system subtraction circuit 58 and the amplifier 60 connected to the system subtraction circuit 58. That is, α(f ₁ )−α(f ₂ ) is calculated by the system subtraction circuit 58.
is determined, and the gain of the amplifier 60 is 1/(f ₁ −
f ₂ ), it is possible to obtain the proportionality constant β based on the above equation (14). Then, the signal of the proportionality constant β obtained by the system subtraction circuit 58 and the amplifier 60 is sent to the multiplexer 42.
is supplied to the display device 30 via the
0 indicates the proportionality constant β.

In the embodiment, since the multiplexer 42 is connected before the display device 30, the multiplexer 42 controls the damping coefficient α(f ₁ ) for the frequency f ₁ , the damping coefficient α(f ₂ ) for the frequency f ₂ , and the proportional One or more of the constant β and the normal B mode or M mode tomographic images can be appropriately selected and displayed on the display device 30 at the same time.

As described above, according to the embodiment, the damping coefficients α(f _{1 ) and} α(f ₂ ) for two types of frequencies, namely f 1 and f ₂
can be measured, and furthermore, the proportionality constant β of the damping coefficient with respect to frequency can be measured. Therefore, it is possible to display the attenuation coefficient distribution and the distribution of the slope of the attenuation coefficient with respect to frequency, and it is possible to provide information effective for diagnosis of a lesion.

In the above embodiment, two systems of blocks are provided to measure the attenuation coefficient, and two types of frequencies are used.
The damping coefficients α(f ₁ ) and α(f ₂ ) for f _{1 and} f 2 were found, but it is necessary to provide three or more systems of blocks to measure the damping coefficients and measure the damping coefficients for three or more types of frequencies. is also possible.

Effects of the Invention As described above, according to the present invention, it is possible to easily measure the attenuation coefficient of a subject from a reflected echo signal. Therefore, the attenuation coefficient of the subject can be measured and this can be effectively used for qualitative diagnosis of living tissue.

[Brief explanation of drawings]

FIG. 1 is a block circuit diagram according to the principle of the present invention;
FIG. 2 is a waveform diagram according to the principle of the present invention, and FIG. 3 is a block circuit diagram of an ultrasonic diagnostic apparatus according to an embodiment of the present invention. 14... Band pass filter, 16... Detection circuit, 18... Logarithmic amplifier, 20... Delay circuit, 2
2... Subtraction circuit, 24... Addition circuit, 26... Attenuator, 28... Delay circuit, 34... Subject, 58
...System subtraction circuit.

Claims (1)

[Scope of Claims] 1. In an ultrasonic diagnostic apparatus that transmits and receives an ultrasonic pulse beam to a subject and displays a tomographic image based on a reflected echo signal from the subject, a band for extracting a specific frequency component from the reflected echo signal. a pass filter and a detection circuit; a logarithmic converter that logarithmically converts the signal from the detection circuit via the bandpass filter; and a delay circuit that inputs the signal output from the logarithm converter and delays it by a predetermined time interval. a first delay circuit that outputs a signal; a signal output from the logarithmic converter and the first delay circuit;
A subtraction circuit receives the delayed signal output from the delay circuit and obtains a difference signal, and a feedback circuit comprising a second delay circuit and an attenuator sends the difference signal to an addition circuit provided between the input and output. and an averaging circuit that calculates the average value of the difference signals and outputs an attenuation signal in which statistical fluctuations in the reflection coefficient within the subject are reduced, the attenuation signal output from the averaging circuit. An ultrasonic diagnostic device that measures an attenuation coefficient of ultrasonic waves for a specific frequency component. 2. The device according to claim 1, wherein a plurality of systems of the bandpass filter, the detection circuit, the logarithmic converter, and the subtraction circuit are provided for each of a plurality of specific frequency components, and a system subtraction circuit that calculates the difference between signals from each system. An ultrasonic diagnostic apparatus comprising: measuring attenuation coefficients for a plurality of specific frequency components using each system, and measuring a proportionality constant of the attenuation coefficients for the frequency components using the system subtraction circuit.