JPH058373B2 - - Google Patents

Description

Detailed Description of the Invention Background of the Invention A Technical Field The present invention improves an ultrasonic measuring device that transmits ultrasonic waves to an object and receives reflected ultrasonic waves from inside the object to measure acoustic characteristics inside the object. In particular, the present invention relates to an ultrasonic measurement device that measures changes in frequency due to ultrasonic propagation inside an object and provides information regarding acoustic characteristics such as ultrasonic attenuation of the object.

B. Prior art and its problems Ultrasonic measurement technology is currently used in metal flaw detection, fish detection,
It is used in a wide range of fields such as medical diagnosis. Among these, recent developments in medical ultrasonic tomography devices are remarkable. Ultrasonic tomography equipment basically uses the pulse-echo method, which takes advantage of the phenomenon in which ultrasound pulses transmitted into a living body are reflected at boundaries of different acoustic impedances inside the living body.
This reflected wave (echo) is received and a tomographic image is displayed using the so-called B-mode method.

Therefore, although this echo contains information such as the attenuation of the ultrasound inside the living body, acoustic impedance, and speed of sound, the only information currently being used is the amplitude of the echo.

Specifically, assuming that the speed of sound in the living body is constant, the attenuation due to ultrasound propagation in the living body is measured using the so-called STC circuit (Sensitivity Time Control) or TGC circuit.
Luminance modulation is performed using an echo amplitude value arbitrarily corrected by a circuit called a time gain control circuit, and the result is simply displayed as a tomographic image on a cathode ray tube. Therefore, the obtained tomographic image is only a qualitative image of the two-dimensional distribution of acoustic impedance discontinuities inside the living body, and morphological information regarding the position and shape of the living tissue is inevitably used. It is the center of However, at present, information such as ultrasound attenuation information, which is a characteristic of living tissues, has not been measured.

Several attempts to obtain attenuation information of biological tissues have been reported. However, as will be described in detail later, the echo waveform contains two pieces of information: attenuation due to propagation in living tissue and reflection intensity at different boundaries of acoustic impedance, both of which are unknown. Therefore, it must be said that it is currently extremely difficult to strictly distinguish and identify these two effects.

Assuming that the reflected intensity does not depend on the ultrasound frequency, if ultrasound echoes are transmitted and received in the same area using two or more multiple frequencies and the sound pressure ratio of each frequency component of the echoes is measured, It becomes possible to obtain the attenuation coefficient without the influence of the reflection intensity. Such an assumption holds true in the case of an interface that has a sufficiently large spread compared to the wavelength of the ultrasonic wave, such as a flat reflector. However, in actual living tissues, there are often scatterers of wavelengths or less, so this assumption does not necessarily hold true when considering the entire living tissue.

Furthermore, assuming that the reflection intensity is approximately constant in a certain part of the living tissue, the ratio of the echo sound pressures before and after that part of the tissue is considered to be directly proportional to the attenuation coefficient. Additionally, an attempt has been reported to assume a frequency-dependent relationship of reflection intensity, transmit and receive ultrasonic echoes in the same area using three or more types of frequencies, and calculate the attenuation coefficient from the sound pressure of each frequency component of the echoes. ing.

As described above, in each case, a method is used in which certain assumptions are made regarding the reflected intensity, and the attenuation coefficient is separated and measured by transmitting and receiving ultrasonic waves having one or more frequency components.

In addition, several attempts have been reported to measure the degree of attenuation that combines absorption and reflection (scattering) information (Japanese Patent Laid-Open Nos. 58-24824 and 1982-179745). Furthermore, some attempts to measure the frequency dependence of the attenuation degree have been reported (Japanese Patent Application Laid-Open No. 57-89858,
139326, Japanese Unexamined Patent Publication No. 57-209040).

These reports basically analyze the frequency of echo signals in areas of interest in biological tissue using well-known Fourier analysis, and show the resulting frequency spectrum together with conventional tomographic images. be. Furthermore, the difference between the spectra at two points within the region of interest can be displayed to provide information on the frequency dependence of acoustic characteristics (attenuation) within the region of interest. In any case, the information obtained is a spectrum, and compared to conventional image information such as tomograms, it is possible to diagnose tissue properties by capturing changes in the shape of these spectra.
It must be said that this is difficult in practice, and it can also be assumed that it is difficult to capture the spectrum distribution with respect to the spread of the tissue in real time.

Furthermore, a method called a spectral color method is well known as one of the display methods for tomographic images of different frequencies obtained by multifrequency tomography. For example, pages 195-196 of ``Invisible Information Imaging Co., Ltd.'' edited by the Television Society, and Proceedings of the 28th Research Presentation of the Japanese Society of Ultrasonics in Medicine (November 1975).
28-27 (pages 47-48) and US Pat. No. 4,228,804. This method attempts to capture the frequency dependence of attenuation information of biological tissues using different frequencies. However, as will be explained in detail later, this method simply displays tomographic images for each frequency obtained using multiple frequencies, and this method does not allow for the thickness of the attenuating medium (tissue), Depending on the distance from the probe to the target medium (tissue) to be measured and the influence of other tissues between the target tissue and the probe, even tissues with originally the same attenuation may be measured as different amounts of attenuation. It faces the fundamental problem of

On the other hand, as a method of measuring information regarding attenuation, the spectral distribution of the echo signal from the object to be measured is analyzed without directly using its amplitude or intensity, the center frequency is determined from this as a function of distance, and its differential coefficient or difference is A method for measuring attenuation information from coefficients has been reported (Japanese Patent Application Laid-Open No. 1989-1999)
−179745). In this report, the attenuation slope is calculated based on the assumption that the spectrum of the transmitted ultrasonic wave has a Gaussian distribution and that the echo signal also maintains a Gaussian distribution.

In addition, based on the same assumption as above, without analyzing the spectral distribution of the echo signal, a quantity corresponding to the frequency (zero crossing density: Zero Crossing
A method has been reported in which the density is measured and this value is displayed as a function of distance. (1983 USA
It is shown on pages 95-116 of "ULTRASONIC IMAGING" Vol. 5, No. 2, published by ACADEMIC PRESS. ) This method suffers from the same problems as the spectral color method mentioned above.

Purpose of the invention The present invention overcomes the drawbacks of the prior art,
An object of the present invention is to provide an ultrasonic measurement device that can comprehensively display information regarding attenuation and reflection scattering accompanying ultrasonic propagation by approximately measuring the frequency components of an ultrasonic echo signal from an object to be measured. purpose.

According to the present invention, an ultrasonic measurement device that measures the acoustic characteristics of an object by transmitting an ultrasonic pulse to the object to be measured and detecting an ultrasonic echo signal reflected from the object to be measured is provided. A receiving means for receiving an ultrasonic echo signal reflected from an object, a zero-crossing density detecting means for detecting a zero-cross density of the received ultrasonic echo signal, and a difference value of the detected zero-crossing density for each unit echo reception time. It includes a difference value detection means and a display means for displaying the difference value as a visible image.

The difference value detection means includes means for externally setting a unit echo reception time for taking a difference value, and difference value averaging means for taking an average of the difference values for each unit echo reception time for a plurality of echo signals.

The display means includes a color display means that displays different colors corresponding to the positive and negative signs of the difference value.

Specific Description and Effects of the Invention The present invention will be described in detail below with reference to Examples.

FIG. 1 illustrates the principle of the invention. Echo signal 100 due to ultrasonic waves transmitted from the probe 3 to the object to be measured 1, reflected therein and received by the probe 3
is the A obtained with a conventional pulse-echo device.
This is a mode signal that has already undergone logarithmic amplification and STC correction. This A mode signal 10
The spectrum for a certain portion of 0, for example 101 and 102 frequencies, is given as curves 111 and 112. This spectrum is generally obtained by multiplying the A-mode signal before logarithmic amplification by, for example, a Hamming window and then performing Fourier transformation.

Now, the echo signal 100 from the object to be measured 1, such as a living body, is received by the probe 3 under the influence of attenuation due to propagation and emission and scattering at the echo generating section. In other words, the received A-mode signal contains a mixture of the above-mentioned attenuation information and reflection/scattering information, but as already mentioned, it is currently impossible to strictly separate these two pieces of information. It's very difficult.

However, to date, it has been confirmed to some extent through animal experiments and non-clinical experiments using biological tissue pieces that the attenuation in the soft tissue of a living body is approximately proportional to the frequency. That is, the damping coefficient α
() is expressed by the following formula.

α()=α ₀ · ⁿ n=1 to 2 Although the reflection/scattering coefficient is not very clear, it is said that it is proportional to the 1st to 2nd power of the frequency.

In this way, the propagation of pulse echoes in living organisms, etc. undergoes attenuation, reflection, and scattering proportional to the first to the second power of the frequency. Therefore, among the echo signals 100, the echoes 102 that have returned from deep within the living body, etc.
In general, the spectrum is often deformed compared to the shallow echo 101. That is, as shown in the figure, the approximate center frequency shifts downward from ₀ to ₀ ', and the amplitude of each frequency is often attenuated. Therefore, by taking the difference between the two, the difference spectrum 113 gives the frequency dependence that integrates the attenuation, reflection, and scattering of the object to be measured 1 during this period.

The present invention does not perform such spectral analysis and display it as in conventional methods, but rather attempts to intuitively develop quantities corresponding to such spectral changes as an image. That is, the zero-crossing density in a unit section of the echo signal 100 is measured, and the variation (differential or difference) of this zero-crossing density is modulated in brightness to create an image.

This zero crossing density gives the number of zero crossings per unit interval 200, that is, per unit time, as shown in Figure 2. For example, if it is 60 times/10 microseconds, the average interval is 6 times/1 microsecond, or about 0.17 microseconds. It crosses zero at Therefore, the period is 0.33 microseconds, which is twice this, and when converted into a frequency, it is approximately 3MHz.

If this frequency were to shift to 2.9MHz, the zero crossing density would change to 58 times/10 microseconds. In other words, the zero-crossing density is a quantity proportional to the frequency of the echo signal, and can be said to roughly correspond to the center frequency on the spectrum.

FIG. 3 shows one embodiment of the present invention, and the circuit excluding the dotted line frame 30 shows the components of a conventional pulse echo B-mode device, which is well known, so a detailed explanation will not be provided here. Omitted.

The ultrasonic waves emitted from the probe 3 driven by the transmitting circuit 2 are partially reflected inside the object to be measured 1, and are received by the probe 3 as an ultrasonic echo signal. At this time, the probe 3 is scanned in the azimuth direction by the scanning section 12 under the control of the control circuit 11. The received ultrasonic echo signal passes through the receiving circuit 4 and is logarithmically amplified by the logarithmic amplifier circuit 5. The signal is further subjected to STC correction by the STC circuit 6 and outputted as the echo signal 100 described above. Note that in this drawing, signals and leads carrying them are indicated by the same reference numerals.

The echo signal 100 is detected by the detection circuit 7,
It is stored in the memory 8 in the form of a digital signal. This is later read out from the memory 8 under the control of the control circuit 11 and passed through the video amplifier 9.
It is displayed as a visible tomographic image of the object to be measured 1 on a display unit 10 such as a CRT.

Incidentally, the output of the STC circuit 6 is input to the zero-cross density difference measuring section 30. Its input is on the one hand transmitted to the pixel setter 2 by means of a delay circuit 13.
The signal is divided into a signal delayed by a predetermined time according to the designation of 0, and a signal without a direct time delay on the other hand. It is measured in In other words, it is sufficient to consider that the section 200 shown in FIG. 2 corresponds to the delay time.

For details, see the echo signal 100 as shown in Figure 4.
The echo 100a in the lower row is delayed by the time τ corresponding to the interval 200 with respect to the echo 100 in the upper row. A point 110 of the echo signal 100
is now at position 110, 200 behind. Therefore, if the echo signals are divided from 110 at intervals of delay time τ, they become 120, 130, 140, . . . . Looking at the correspondence between the echo 100 in the upper row and the delayed echo 100a in the lower row, 130 and 120, 140 and 130
...correspond to each other. Therefore, if we take the difference between the zero-crossing density ρ ^100a/120 of the 120th interval of the 100a echo signal and the zero-crossing density ρ ^100/130 of the 130th interval of the 100th echo signal, this will eventually become about the original echo signal 100. This means that the difference between the zero cross density ρ ^100/120 in the section 120 and the zero cross density ρ ^100/130 in the section 130 is measured. After all, this corresponds to calculating a shift in the approximate center frequency of the echo signal 100.

The zero-crossing density of the echo signal 100 is sequentially measured, and this output is input to the next addition memories 16 and 17, where the zero-crossing density value is stored for each delay time. In other words, as shown in Figure 5, each
61, 162, 163... and 171, 172,
Zero crossing density values of 173... are stored in memories 16 and 17. In principle, each difference value is measured by the subtractor 18 using these values,
i.e. 171-161, 172-162, 17
3-163..., this value is stored in the memory 19.
The output of the memory 19, which is this difference value, may be input to the video amplifier 9, and the output may be developed as an image on the display unit 10 by performing brightness modulation.

In order to increase the measurement accuracy of this zero cross density value, that is, to reduce the statistical recoil,
As shown in FIG. 6, the scanning direction (azimuth direction) 50
Average about 0. In other words, the average zero-crossing density for multiple scanning lines is calculated. For the depth direction 502, the delay time is 20
Since the zero crossing density is measured for each area divided by 0, the number of scanning lines to be averaged is selected so that the distance 200a is approximately the same in the scanning direction 500. In this way, the zero cross density for the pixels 300 can be measured on average.
For this purpose, in the addition memories 16 and 17, the zero cross detection unit 14
The zero cross densities measured in steps 1 and 15 are sequentially averaged and stored in the memory. When the zero crossing densities from the last scan line of the scan lines to be averaged are measured and averaged in the addition memories 16 and 17, the outputs of the addition memories 16 and 17 are input to the subtraction unit 18, and the difference is calculated as described above. The value is measured and entered into memory 19. Similar measurements are performed for the subsequent scanning lines, and the zero cross density difference value Δρ _ij is finally stored in the memory 19 for each pixel as shown in FIG.

The contents of the memory 19 are developed as an image on the display section 10. As for the display method, the images may be displayed on one screen side by side with the conventional B-mode image, or they may be displayed alternately. Furthermore, various methods can be considered, such as displaying the B-mode image and the image according to the present invention in an overlapping manner by modulating the brightness of each image in a different color. Furthermore, it is convenient if the pixel size can be changed externally using the pixel setter 20 depending on the purpose.

The above explanation is based on the general phenomenon that the zero-crossing density difference value Δρ _ij takes a positive value, that is, the center frequency of the echo signal shifts toward the lower side as it propagates. However, if there is a large scatterer in the middle of the propagation, and the scattering intensity is proportional to the 1st to 2nd power of the frequency, the higher the frequency, the greater the scattering will be. In this case, if the rate at which high frequency components increase due to this scattering is greater than the frequency reduction of the center frequency due to propagation attenuation, the center frequency may shift toward the higher side as a result. For example, Ultrasonic from Academic Press.
SW of Imaging Vol. 5, pp. 95-116 (1983)
This development is also described in the report by Flax et al.

Therefore, by changing the display depending on the sign of the difference value Δρ _ij , it is possible to qualitatively understand from the displayed image that such development has occurred. For example, if the difference value is positive, it is displayed in a blue color. However, if the value is negative, by displaying it in red, you can immediately determine whether the center frequency is shifting to a lower or higher direction, and you can also determine whether the center frequency is shifting to a lower or higher direction. It would be even more advantageous if the size of the image could also be determined.

In reality, however, the sound field of an ultrasonic beam transmitted from the ultrasonic probe 3 toward, for example, water with extremely low attenuation often changes depending on the aperture and center frequency of the probe 3. Are known. The sound field is approximately as shown in Figure 8A, and the intensity on the central axis is determined by the distance from the ultrasound probe 3
It changes as shown in FIG. 8B. Note that the vertical axis of B indicates the relative intensity I _x /I ₀ at distance X with respect to the maximum intensity I ₀ . As is clear from this figure, a fairly complex sound field (near field) is formed near the probe 3. On the other hand, the distant sound field is called the far sound field and forms a nearly spherical wave sound field.

Practical pulse-echo B-mode devices often use a near-field sound field. Therefore, when performing some quantitative measurement using the echo signal strength, such as measuring the attenuation coefficient, it is necessary to calibrate this sound field characteristic so that the measured value does not depend on the sound field characteristics of the equipment used. It is extremely important to set a value.

In the case of the present invention, since measurement is not performed using echo signal strength, the above-described calibration of sound field characteristics is not necessary. However, as the sound field changes, probe 3
It is predicted that the spectrum will change depending on the distance from the center, and that the center frequency will change, and there are also reports of this. Therefore, in the present invention, the change in the center frequency due to the change in the sound field is measured in advance by the following method, and the measured value is calibrated using this value.

That is, as shown in FIG. 9, a tank 1000 is filled with degassed water 1100, and a flat plate (stainless steel or the like) 1002 having a sufficiently large area compared to the area of the probe 3 is provided as a standard reflector in the degassed water. . Echo signal 1 from this standard reflector 1002
Zero cross density of 200 between probe 3 and reflector 10
02 as a function of distance. This becomes, for example, a waveform 1300 in FIG. This waveform 1
300 data are stored in advance in the calibration memory 22 of the device.
(FIG. 3), and the measured value is calibrated by dividing it by this standard zero-crossing density value, and the final value is displayed on the display unit 10.
All of these operations are performed using memory 19 in FIG.

As described above, the present invention measures the difference in zero-crossing density of echo signals with a relatively simple configuration without performing the complicated and time-consuming process of spectrum analysis of echo signals, which is required by conventional devices. It is possible to approximately measure changes in the center frequency that could not be obtained with the conventional method.

Furthermore, since the measurement according to the present invention is not based on the amplitude or intensity of the echo signal as in the past, the measurement can be performed on a signal after logarithmically amplifying the echo signal and performing STC correction. Therefore, the dynamic range of the measurement system may be small, and since the measured values obtained are simply counted values, the memory capacity or resolution (number of bits) can be relatively small for practical use. It also has the advantage that measurements can be fully processed in real time.

Although this embodiment has been described with respect to linear scanning, it goes without saying that the present invention can be effectively applied to other types of contact compound scanning and sector scanning.

Specific Effects of the Invention As described above, according to the present invention, the zero cross density is used as a value representing the change in the spectrum of an echo signal due to the attenuation due to the propagation of ultrasound waves in the object to be measured, especially the living tissue, and the attenuation due to reflection and scattering. By developing the differential or difference value as an image,
Changes in the center frequency of echo signals due to biological tissue, which could not be obtained with conventional tomographic images, can be intuitively grasped in real time on the displayed image.

[Brief explanation of the drawing]

1 and 2 are explanatory diagrams explaining the basic principle of the present invention, FIG. 3 is a block diagram showing an embodiment of the ultrasonic measuring device according to the present invention, and FIGS. The explanatory diagrams used to explain the operation of the embodiment shown in the figure, FIGS. 8 to 10, are explanatory diagrams showing a method of calibrating the ultrasound probe in the embodiment of FIG. 3. Explanation of symbols of main parts, 3...Ultrasonic probe,
5... Logarithmic amplifier circuit, 8, 19... Memory, 9...
...Video amplifier, 10...Display unit, 11...Control circuit, 13...Delay circuit, 14, 15...
... Zero cross detection section, 16, 17 ... Addition memory, 18 ... Subtraction section, 20 ... Pixel setter, 22
...Calibration memory, 30...Difference measuring section.

Claims (1)

[Claims] 1. An ultrasonic measurement device that measures acoustic characteristics of an object by transmitting ultrasonic pulses to the object and detecting ultrasonic echo signals reflected from the object. , the apparatus includes: a receiving means for receiving an ultrasonic echo signal reflected from the object to be measured; a first zero-crossing density for receiving the ultrasonic echo signal from the receiving means and detecting a zero-crossing density of the echo signal. a detection means; a first averaging means for adding and averaging the zero-crossing densities detected by the first zero-crossing density detecting means for a plurality of echo signals; a delay means for providing a delay; a second zero cross density detection means for receiving an ultrasonic echo signal from the delay means and detecting a zero cross density of the echo signal; and a zero cross detected by the second zero cross density detection means. a second averaging means that adds and averages the densities of a plurality of echo signals; and a unit echo, the difference value of each zero cross density averaged by the first averaging means and the second averaging means. A difference value detecting means for detecting each reception time; and a display means for displaying the difference value as a visible image, the difference value being displayed in different colors corresponding to the positive or negative sign of the difference value. 1. An ultrasonic measurement device comprising: display means. 2. The ultrasonic measurement device according to claim 1, characterized in that the device includes setting means for externally setting a unit echo reception time in at least the delay circuit and the difference value detection means. Ultrasonic measuring device. 3. The ultrasonic measuring device according to claim 1, characterized in that the device has a pixel setting means for externally setting the size of the pixel of the difference value according to the purpose. Device. 4. In the ultrasonic measuring device according to claim 1, the device has a storage means for storing calibration data measured in advance in a standard medium, and the difference value detecting means An ultrasonic measurement device characterized in that the detected difference value is calibrated using the calibration data and displayed on the display means.