JPS60188842A - Ultrasonic probe - Google Patents

Abstract

PURPOSE:To reduce an influence exerted on a received wave of a transmitting sound field by providing the first layer for executing a mutual conversion of an electric signal and an ultrasonic signal, and the second layer containing a medium having a sound speed which is higher than the average sound speed of an object to be measured in the side of the face opposed to the object to be measured. CONSTITUTION:A packing material 2 is stuck on the rear of an ultrasonic vibrator 10, and in front of said vibrator, and an interposed medium 20 for compressing a short distance sound field, whose sound speed is higher than that of a biological 1 is provided by placing it between sound matching layers 4, 6 functioning as a 1/4 wavelength matching layer used for raising sensitivity and improving responsiveness. Also, as for the medium 20, that which satisfies such conditions as its sound speed is higher enough than that of the biological 1, the attenuation of an ultrasonic wave is small, an acoustic impedance is about middle between the vibrator 10 and the biological 1, the density is small, etc. is used. In this way, a short-distance sound file limit distance is shortened, and a long-distance sound field can be formed in the biological 1 substantially.

Description

[Detailed Description of the Invention] ■ Background of the Invention A, Technical Field The present invention transmits ultrasonic waves to an object, receives reflected ultrasonic waves from inside the object, and quantitatively determines sound characteristics inside the object. In particular, the present invention relates to the improvement of ultrasonic probes that measure ultrasonic waves, and in particular provides a transmitting sound field in an object to be measured that does not have an adverse effect on received waves caused by the near-field sound field formed during transmission in ultrasonic probes. Ultrasonic probe (related)

B. Prior art and its problems Ultrasonic d11] detection technology is currently used in a wide range of fields, including metal flaw detection, fish detection, and medical diagnosis. Among these, recent developments in medical ultrasonic tomography devices are remarkable. In principle, ultrasonic tomography devices use the noxious echo method, which takes advantage of the phenomenon that ultrasonic pulses transmitted into a living body are reflected at boundaries of different acoustic impedances inside the living body, and detects the reflected waves ( It receives echoes (echoes) and displays tomographic images using the so-called B-mode method.

Therefore, this echo contains information on the attenuation of ultrasound waves inside the living body.
Although information such as acoustic impedance and speed of sound is included, 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 a so-called STC circuit (5-ensi
Tivity Time Control) or TGC circuit (Time Ga1n Control)
The image is simply displayed as a tomographic image on a cathode ray tube by performing brightness modulation using an echo amplitude value that has been arbitrarily corrected using a circuit called . 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, measurement of ultrasonic attenuation level-4, etc., which is a characteristic of living tissue, has not been carried out.

Several attempts to obtain attenuation information of biological tissue have been reported (%Publication 5"6-57820゜Unexamined Japanese Patent Publication No. 1983-1989)
179745, % opening 57-550).

However, when putting these methods into practical use, the influence of the transmitted sound field of the ultrasonic probe used for measurement must be taken into consideration. In other words, it is well known that the sound field formed by the wave transmitted from the probe changes with its propagation distance, and for example, the sound field transmitted by a planar circular vibrator can be approximately plotted as shown in Figure 1 (G). ing. That is, a near sound field (Fresnel zone) and a far sound field (Fraunhofer zone) are entirely formed by the relationship between the aperture D (diameter) of the planar circular probe and the wavelength λ of the ultrasonic wave. Figure 1: Relative intensity on the central axis of the sound field (relative intensity VI normalized by maximum intensity I)
It is well known that O,> changes as shown in Figure 1 (B) (Physical Principles of U published by Academic Press, USA).
LTRASONICDLAGNO8IS 54th negative). The intensity pattern depending on the propagation distance of the transmitted sound field still changes in a complicated manner as shown in FIG. 1(C). Therefore, if you use the received signal from the object to be measured as is,
It is clear that (no matter what signal processing method is used) the influence of the above-mentioned transmitting sound field cannot be avoided. A method has been reported in which the distance between the object and the measurement area of the object to be measured is always kept constant (4? 1987-2)
482'4). However, this method is a so-called water immersion method that requires a medium such as water between the probe and the object to be measured, and in order to maintain a constant distance between the probe and the object to be measured, the probe must be moved each time the measurement area is changed. need to be moved mechanically. Therefore, from a practical point of view, the water immersion method has the drawbacks of complicated operation, the need for mechanical equipment for moving the probe, and unnecessarily long testing time.

On the other hand, in order to calibrate (correct) changes in the transmitted sound field, the reflected received waves at each propagation distance (depth) from the reference medium are stored in advance as correction information in memory, and the reflected received waves from the 1+11 constant objects are A method has been reported in which waves are corrected using the correction information corresponding to their depth (distance) (Japanese Patent Laid-Open No. 58-55850). In that case, the question is what is appropriate as a reference medium. This report states that it is convenient to use water or physiological saline as the reference medium, and to use as the reference reflector a sphere or a substantially flat body made of aluminum, silastic, etc., with an uneven surface. As a practical matter, it seems difficult at present to obtain a clear answer as to what is specifically an ideal reference medium and what is an inferior reference reflector. Also, experimental confirmation is unclear.

There are other reports on sound field correction using IL quasi-medium (IEEE TRANSACTIONS ON
5ONIC8AND ULTRASONIC8 No. 30
Ma on pages 26-36 of Volume No. 1 (January 1'983)
tthew O'Donne11's "Quantita
tive Volume 13ack scatter
ImagingJ).

In this report, we used an underwater flat plate as a reference medium and compared the energy changes of backscattered waves (received waves) at each depth with the energy changes of received waves from near the surface of a gelatin phantom, which is close to a living body. It is said that they match. However, when compared accurately, there are differences between the two, and although it is true that the influence of the sound field is less than when no correction is applied, there is still a problem in quantitative terms.

As mentioned above, there have been several reports on eliminating the influence of the sound field transmitted by the probe, but at present no method that is satisfactory from a practical or quantitative standpoint has yet been proposed.

■・ Purpose of the Invention The present invention eliminates the drawbacks of the prior art, and improves the ultrasonic technology that minimizes the influence of the transmitting sound field of the probe on the received waves, especially the influence of the near-field sound field. The purpose is to provide a sonic probe.

According to the present invention, an ultrasonic probe that transmits an ultrasonic noggle to an i++1+ constant object and receives an ultrasonic echo signal reflected from the object to be measured is capable of transmitting an a first layer comprising an electroacoustic transducing material that performs the transduction;
and a second layer that is provided on the side of the first layer facing the object to be measured and includes a medium having a sound speed higher than the average sound speed of the object to be measured.

The second layer has a thickness such that it includes at least the near field of ultrasound transmitted from the ultrasound probe.

The acoustic impedance of the medium is an intermediate value between the acoustic impedance of the first layer and the acoustic impedance of the object to be measured.

The second layer has quarter wavelength acoustic matching layers on the side of the object to be measured and on the opposite side.

The second layer has an acoustic matching layer on the side of the object to be measured and on the side opposite thereto, the acoustic impedance of which changes substantially continuously.

The medium includes at least one selected from the group consisting of aluminum, glass, and noulammin.

■1 Detailed explanation and operation of the invention The present invention will be explained in detail below using examples.

FIG. 2 is a diagram illustrating the principle of the present invention. As is well known, the near-field sound field limit distance is
If the aperture (diameter) is D1 and the wavelength of the object to be measured is λ, then
It can be approximated by D2/4λ. The situation is shown in Figure 2 (4
). -For example, if D=10m, the resonance frequency f
Consider the case where the human body l is the object to be measured using a = 3 MHz probe. The average sound speed of the human body is C8-1500rv/8
Then, the wavelength is λo=co/f='0.5 wa
n and nashi, follow D2/4λ. -50+++m.

The distance from the surface of the human body 10 to about 50 degrees is a near-field sound field, and the distance after 50 degrees is a far-field sound field. The larger the aperture, the wider the range of the near field. Furthermore, the higher the frequency f, that is, the shorter the wavelength λ, the wider the range of the near-field sound field, and the near-field sound field is formed to a deeper distance X of the human body 1. The graph in FIG. 3 is an analysis of this relationship.

Currently, the aperture of the probe of an ultrasonic diagnostic apparatus used for the human body is 10 to 20 mm, and the frequency is mainly 2 to 5 MHz. When considering the abdomen as a target, the diagnostic distance from the surface of the living body is required to be as deep as 160 to 200 cm. Therefore, it is clear from FIG. 3 that under this condition, almost only the near-field sound field is used.

In order to reduce the range of a near-field sound field with a complex sound field pattern within a living body, it is necessary to reduce D2/4λ. For example, in order to set D2/4λ to 20 or less, the aperture may be set to 6 or less as shown in FIG. However, as shown in Figure 4, when the aperture becomes smaller, the approximate beam width 2xλ/D in the far sound field becomes
A problem arises in that the azimuth resolution deteriorates.

In order to reduce the near-field sound field limit distance D2/4λ, the aperture can be made smaller and λ longer. However, on the other hand, the beam width in the far-field sound field is 2・(λ/b)・X It can be understood that in order to reduce λ, the aperture should be made larger and λ should be made shorter. Ultimately, these two requirements are in a mutually contradictory relationship.

A method for resolving this contradiction will be explained with reference to FIG. 2(B). A medium 20 having a sound speed C higher than the average sound speed C8 of the living body is set between the living body 1 and the probe 100. Assume that there is a relationship of C=m-Co(m)1'). Now m
= 2 as an example, the near sound field limit distance D2
/4λ is λ-C/f = 2·λ0, so this distance is strongly shortened compared to living body 1. 50 in the above example
This means that Won will have 25 cabinets. Therefore, medium 2
If the thickness of the 0 is made slightly longer than this distance, a long-distance sound field can be formed from near the surface of the living body 1.

Therefore, by acoustically connecting a medium zO having a sound speed C higher than the average sound speed C6 of the living body between the probe 10 and the living body 1, it is possible to shorten the critical distance for short-range sound iyj. A living body that is the object to be measured in a far-field sound field
□ Next, the embodiment shown in FIG. 5 will be described in detail. FIG. 5 is a sectional view showing an ultrasonic probe 30 constructed based on the principle already explained.

The ultrasonic transducer 10 is made of well-known ceramic PZ.
6D made of piezoelectric material such as T, acoustic backing material 2 behind it
is fixed. Further, in front of it, there is provided a medium 20 for short-range sound field compression where the speed of sound is higher than that of the living body, and acoustic matching layers 4 and 6 for each of the lamina 20 and the living body are formed on the amphibious surface of the medium 20. There is.

The conditions necessary for the intervening medium 20 are: (1) the sound, tc, is sufficiently faster than the average sound speed C6 of the object to be measured 1;

(2) the attenuation of ultrasonic waves is small; (3) the acoustic impedance is approximately midway between that of the transducer IO and the object to be measured 1; and (4) the density is as small as possible. Examples of materials that satisfy these conditions include aluminum, glass, and duralumin, and their properties are shown in the table below.

Among these materials, aluminum is a preferred example.

The sound velocity of aluminum is 6.4.20 nV"s, which is about 4.3 times the average sound velocity of a living body, which is 1.500%'8. If the transducer IO is a circular plate as a whole, its diameter ( aperture) D = 13 ttrm, frequency f =
Question 3 (Assuming 2, in the conventional probe, the near field limit distance is 84.5'am, but in the case of the probe 30 constructed according to the principles of the present invention, the thickness of the medium 20 is 84.5'am. value 4
．． If it is one-third or more, that is, about 20 degrees or more, a long-distance sound field will be substantially formed deeper than the surface of the living body 1.

Now, the acoustic matching layers 4 and 6 are so-called 174 wavelength matching layers used to improve the sensitivity and response of the probe 30 as a whole.

The acoustic impedance of the vibrator 10 is approximately 30×105 (9
/cm2. s), when the medium 20 is aluminum, its acoustic impedance is 17.3X10, so the acoustic impedance of the acoustic matching layer 4 is 19~2
2 is preferred. Furthermore, the acoustic impedance of the acoustic matching layer 6 is preferably 4 to 6, assuming that of the living body 1 to be 1.505.

These acoustic impedance values are given in 4l-PC-91'-2 on page 89 of the 41st lecture proceedings of the Japanese Society of Ultrasonics in Medicine.
It can be easily calculated using the method described in ``Study of characteristics of multilayered ultrasonic probe''. Furthermore, acoustic matching layers 4 and 6 are preferably provided in order to minimize multiple reflections at the two boundaries 45 and 56 of the medium 200.
The acoustic impedance is 30 to 17.3 and 17, respectively.
．． It is formed so that it changes almost continuously in the X direction from 3 to 1.5. Matching layers 4 and 6 are described in Japanese Patent Application Laid-Open No. 1985/1. -21
It is possible to manufacture it by the method described in 082, that is, by mixing tungsten powder into epoxy.

Even when the resonator has a low acoustic impedance, such as a polymer system (PVDF-, FF vinyldene litz, etc.) or a composite of a polymer system and an inorganic material, a matching layer with continuously changing acoustic impedance can be used. It is valid.

Next, the embodiment shown in FIGS. 6A and 6B will be described in detail. Figure 6B shows the ultrasonic probe 30 shown in Figure 5.
An example of an ultrasonic measuring device in the case of using (history) is shown.The configuration is the same as that of a B-mode device using the conventional pulse echo method except for the part 200 (data correction section) within the dotted line frame.
Figure A shows the configuration of a B-mode device using the conventional pulse echo method, and its operation is generally as follows.

The probe 30 set on the subject 1 is scanned by the scanning unit 40 on its surface perpendicularly to the azimuth direction. Probe 30
A transmitter circuit 22 and a receiver circuit 50 are connected as shown in the figure, the former transmits ultrasonic waves through the probe 30 under the control of the control circuit 140, and the latter transmits ultrasonic waves reflected from inside the subject l. receive echoes.

The received echo signal is logarithmically amplified by a logarithmic amplifier 60, and is converted into a so-called STC signal by an STC circuit 70.
Receive correction. This is detected by the detection circuit 80, converted into a digital signal, and stored in the memory 90. The echo data stored in the memory 90 is displayed as a so-called B-mode echo image on the display electrician 30 having, for example, a CI cape.

In FIG. 6B, the data correction unit 200 includes a correction circuit ioo that performs correction calculations, a correction data memory 110 that stores correction data in advance, and an arithmetic circuit 120 that performs calculations to calculate the acoustic characteristics of the subject 1. Consisting of The data correction unit 200 performs correction to remove the influence on the received signal due to multiple reflections of the ultrasonic waves by the boundaries 45 and 56 of the intervening medium 20. This will be explained in detail below.

As already explained in the embodiment of the probe 30 in FIG.
In order to minimize the reflected waves at the boundaries 45 and 56 of the medium 20 for short-range sound field compression, the sound #matching layers 4 and 6 are
has been established. However, if even a small amount of reflected waves from the boundaries 45 and 56 remain, there is a risk that they will be mixed into the echo signal to be measured from the object 1 as unnecessary multiple reflected waves. FIG. 7 conceptually shows the state of multiple reflections at this boundary. Reference/Ir1-+;.

1000.2000.3000 and 4000 are examples of these multiple reflected waves. The relationship between these echoes (number n) and multiple reflected waves is shown in FIG.

FIG. 8 (N 500 is a driving waveform applied from the transmitting circuit 22 to the probe 30. FIG. 8(B) shows a multiple reflected wave 1000 generated for the above-mentioned reason.

2000.3000 and 4000 are shown.

Due to the nature of multiple reflections, the intervals between each wave are all equal. FIG. 8(C) shows an echo signal train from the object to be measured 1, which propagates back and forth through the gold layers 4 and 6 in the medium 20 and sound v' between the vibrator 1° and the object to be measured l. The Sheco signal train begins to be received by the vibrator 10 at time 520, which is delayed from the driving time 510 by a certain amount of time. Therefore 6H
+) What is mixed into the target echo signal as unnecessary multiple reflected waves becomes the second reflected wave 2000 and subsequent reflected waves.

These reflected waves can be measured in advance by the method shown in FIG. That is, the probe 30 of the apparatus shown in FIG. 6A or 6B is acoustically connected to an anechoic chamber 400 as shown in FIG. 9(4). In this case, the medium aOO is suitably a substance having substantially the same acoustic impedance as the object to be measured 1, for example, if the object d111 to be measured l is a human body, degassed water or physiological saline is suitable. In order to prevent the ultrasonic waves transmitted to this tank 400 from returning to the probe 30 again, the entire inner surface of the tank 400 is provided with an absorber 402 having irregularities.

In addition, as shown in FIG. 9(B), the probe 30 is acoustically connected to an attenuation medium 600 such as silicone rubber that has the same acoustic impedance as the human body, and the reflection from the bottom surface 610 of the silicone COMRO 00 is measured. To prevent the waves from returning to the probe 30, make the thickness of the silicone COMRO 00 sufficiently thick to sufficiently attenuate the waves reflected from the bottom surface, or alternatively, make the bottom surface 610 as shown in FIG. It may be shaped like an absorber 402.The reflected waves are measured under these conditions, and all these signals are stored in the correction data memory 110 shown in FIG. 6B.

In the correction circuit ioo of FIG. 6B, by subtracting unnecessary reflected wave signals stored in advance in the memory 110 from the output signal of the receiving circuit 50 and inputting them to the logarithmic amplification circuit 60, multiple reflected waves such as 1000 are mixed. can be removed.

As described above, by using the probe 30 that compresses the near-field sound field, it is possible to substantially form a far-field sound field in the object to be measured 1. As is well known, the far sound field forms spherical waves, and unlike the near field, it does not form a sound field due to complex interference. Therefore, JP-A-58-55
Even when a method of correcting (calibrating) the transmitted sound field using a reference medium as reported in 850 is used, it is presumed that the accuracy of calibration will be improved compared to the conventional method. This calibration is performed in correction circuit 100 with input from calibration data memory 110 of FIG. 6B. Arithmetic circuit 12
When the value is 0, calculations for measuring the acoustic characteristics (for example, the attenuation coefficient) of the object 1 to be measured are performed. It is also possible that the echo signal undergoes multiple reflections at the boundary 45''! or 56, but
I think this is so small that it can be ignored.

■1 Specific Effects of the Invention As described above, according to the present invention, the near-field sound field transmitted from the ultrasonic probe can be compressed, and the near-field sound field transmitted from the ultrasonic probe can be compressed into the object to be measured without deteriorating the azimuth resolution. A far-field sound field can be created. Therefore, it is possible to eliminate the adverse effects on the received waves caused by the transmitting sound field of the probe and the near-field sound field having complicated sound field characteristics.

[Brief explanation of the drawing]

1 to 4 are explanatory diagrams for explaining the present invention in detail; FIG. 5 is a cross-sectional view showing an embodiment of an ultrasonic probe realized according to Akira Honzuko's principle; and FIG. 6A. 6B is a block diagram showing an embodiment of an ultrasonic measuring device using the ultrasonic probe shown in FIG. 5, and FIGS. FIG. 3 is an explanatory diagram illustrating the removal of the influence of multiple reflections that occur in FIG. Explanation of symbols for main parts 4.6 Acoustic matching layer 10 Ultrasonic transducer 20 Intervening medium 30 Ultrasonic probe 90 Memory 100 Correction circuit 110 . . . Memory for correction data 120 . F Shishima Co., Ltd. Agent Takao Katori-9-11 Tokukai Sho GO-188842 (7) n Kaiji GO-188842 (8) 1 Toki Sho Go-188842 (9) -265-

Claims (1)

[Claims] 1. An ultrasonic probe that transmits an ultrasonic pulse to an object to be measured and receives an ultrasonic echo signal reflected from the object, the ultrasonic probe comprising: a first layer including an electroacoustic transducer for mutually converting a signal and an ultrasonic signal; and a first layer provided on the side of the first layer facing the object to be measured, the average sound velocity of the object to be measured. An ultrasonic probe comprising: a second layer containing a medium having a high sound speed; 2. The second layer has a thickness such that it includes at least a small near-field sound field of the ultrasound transmitted from the ultrasound probe, as set forth in claim 1. Ultrasonic probe. 3. The ultrasound probe according to claim 1, wherein the acoustic impedance of the medium is an intermediate value between the acoustic impedance of the first layer and the acoustic impedance of the object to be measured. 4. The ultrasonic wave according to claim 1, wherein the second layer has a 1/4 wave sound # matching layer on the side of the object to be measured and on the opposite side, respectively. probe. 5. The second layer has an acoustic matching layer on the side of the object to be measured and on the opposite side thereof, the acoustic impedance of which changes almost continuously. Ultrasonic probe. 6. The ultrasonic probe according to claim 2 or 3, wherein the medium includes at least one selected from the group consisting of aluminum, glass, and thulamine.