JPH0235935B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an ultrasonic imaging method for obtaining an acoustic image inside an object using ultrasonic waves. As a transducer for observing the inside of an object using ultrasonic waves, one that utilizes thickness vibration is conventionally known. In this method, a piezoelectric material such as lead zirconate titanate porcelain is formed into a disk shape having a polarization axis in the thickness direction, and circular electrodes are provided on both the front and back surfaces of the piezoelectric material. This piezoelectric plate is coupled to the object to be observed via or directly with a liquid such as water or silicone oil, and an electric field is applied between the front and back electrodes of the piezoelectric plate, thereby generating thickness vibration in the direction of the polarization axis. The interior of the object is observed by propagating sound waves inside the object and receiving the reflected waves. Although this method makes it possible to observe the inside of an object non-destructively, it has the following drawbacks. (1) To make the sound wave obliquely incident on the observed object,
It is necessary to place the piezoelectric plate obliquely on the surface of the object to be observed. In other words, the proportion of sound waves transmitted inside the observation object is greater with oblique incidence than with normal incidence, so oblique incidence is more advantageous; however, with the piezoelectric plate with the above configuration, )
Since thickness vibrations are excited in the object, in order to obtain oblique incidence on the object, the piezoelectric plate must be mechanically arranged obliquely to the surface of the object to be observed, which is troublesome. (2) Since the direction of sound wave propagation to the observed object must be controlled by the angle formed between the object surface and the piezoelectric plate, there is the inconvenience of having to mechanically change the arrangement of the piezoelectric plate. (3) In order to obtain an output signal from reflected waves within the observed object, it is necessary to use a single piezoelectric plate for both input and output, or to use two piezoelectric plates for input and output. However, the former requires a directional coupler to separate the input and output, and the latter requires two piezoelectric plates to be placed on the object to be observed, which is a hassle. SUMMARY OF THE INVENTION Therefore, the present invention aims to improve the above-mentioned drawbacks of the prior art, and in order to achieve this purpose, it comprises a piezoelectric substrate and interdigital electrodes for input and output provided spaced apart from each other on one side of the substrate. an ultrasonic transducer is coupled to the subject via a liquid, and by applying an alternating current electrical signal to the input electrode, exciting a sound wave having a velocity equal to or faster than the bulk wave velocity specific to the subject;
An object of the present invention is to provide an ultrasonic imaging method in which an acoustic image of the inside of the object is obtained by propagating a bulk wave into the object through the liquid and receiving the reflected wave. More preferably, an ultrasonic imaging method is provided in which the ultrasonic transducer is provided with velocity dispersion, and the direction of propagation of the bulk wave to the object is controlled by changing the frequency of an alternating current electric signal to the input electrode of the transducer. It is about providing. Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an example of an ultrasonic transducer used in the present invention, and FIG. 2 shows an embodiment of the present invention. The ultrasonic transducer 1 has a piezoelectric substrate 2 and interdigital electrodes 3 and 4 . The piezoelectric substrate 2 has a thickness that is approximately equal to or less than the wavelength λ of the acoustic wave on the substrate. Interdigital electrodes 3 and 4 are provided on one side of the piezoelectric substrate 2. As shown in FIG.
The electrode 3 functions as an input, and the electrode 4 functions as an output. It is clear that since the electrode configuration is arc-shaped, the focal point of the ultrasound beam will be on a vertical line passing through the center of the arc. As shown in FIG. 2, the transducer 1 having the above configuration has one side in contact with the liquid body 5, and is acoustically coupled to the liquid body 6 via the liquid body 5. As the liquid material, water, silicone oil, or any other suitable material as an acoustic propagation path can be used.
Since the transducer 1 having the above configuration excites Lamb waves, the open surface of the electrode may be placed in contact with the liquid 5 as shown in FIG. They may be placed adjacent to each other. However, from the point of view of electrode protection, the former has an advantage. When an AC electric signal is applied to the input electrode 3 of the transducer 1 in the above configuration, a Lamb wave is excited in the piezoelectric substrate, converted into a longitudinal sound wave L1 at the coupling surface with the liquid material 5, and generated at an incident angle θ ₁ . , propagates in the liquid. Here, the incident angle θ ₁ into the liquid body satisfies the following relationship. θ ₁ = sin ^-1 V _W /V _L (1) V _W is the longitudinal wave velocity inside the liquid body, V _L is the Lamb wave velocity on the piezoelectric substrate. It is converted into a bulk wave L2 at the interface with the wave L2 and propagates inside the object at an incident angle θ ₂ . The incident angle θ ₂ of the bulk wave L2 satisfies the following relationship. sinθ ₁ /sinθ ₂ =V _W /V _B (2) V _B is the bulk wave velocity in the object The bulk wave L2 propagating inside the object is reflected by the elastic discontinuities 6a such as scratches existing inside the object is,
While satisfying equations (2) and (1), the wave is converted into a bulk wave L2 - longitudinal sound wave L1 - Lamb wave , which reaches the output electrode 4 of the transducer 1, where it is converted into an electrical signal and output. If there is no elastic discontinuity 6a, the bulk wave L2 is reflected from the lower surface of the object 6 and output to the output electrode 4.
reach. Therefore, a difference occurs in the propagation path length depending on the presence or absence of the elastic discontinuity, and different delay outputs are obtained, making it possible to obtain an acoustic image inside the subject. More specifically, for example, by using a sample-and-hold circuit to focus on a specific delay position and observing changes in the output level of reflected waves, it is possible to acoustically observe the inside of an object. The propagation direction θ ₂ of the bulk wave in the object expressed by equation (2) is determined as follows from equation (1) and equation (2). sinθ ₂ =V _B /V _L (3) Therefore, it can be seen that θ ₂ is determined by the velocity of the bulk wave and the velocity of the Lamb wave. At the same time, from equation (3),
In order to send bulk waves inside the object, V _L ≧
It can be seen that the condition of V _B must be satisfied. The velocity of the bulk wave depends on the object being examined, and varies depending on the type of object. If the velocity of the Lamb wave is lower than the bulk wave, the sound waves will not pass through the interior of the object and will be reflected by its surface, making it impossible to observe the interior. Furthermore, from equation (3), it can be seen that if the Lamb wave velocity V _L is changed, the propagation direction θ ₂ of the bulk wave changes. The Lamb wave transducer 1 described above has a so-called velocity dispersion property in which the Lamb wave velocity changes depending on the frequency of the AC signal applied to the input electrode, so the propagation direction θ ₂ of the bulk wave can be electrically controlled. This makes it possible to eliminate the troublesome mechanical setting required in the conventional example. Experimental example Disc-shaped TDK piezoelectric ceramic 91A material (thickness 0.2 mm,
Number of electrode pairs on one side (diameter 13 mm, polarization axis thickness direction)
Create 1.5 arc-shaped interdigital electrodes (see Figure 1), and set the distance between the input and output electrodes.
The transducer was constructed with a diameter of 7.0 mm, an aperture length of 70°, and an electrode period of 350 μm. The Lamb wave velocity of this transducer is 3500 m/3 at a frequency of 4.00 MHz.
s, 3200 m/s at 4.14 MHz, and it was possible to change the sound wave beam θ ₁ into the liquid under the relationship of equation (1). An Al plate shown in Fig. 3 is used as the test object, and the transducer and the test object are liquid-coupled as shown in Fig. 2, and a bulk wave is transmitted inside the test object using a Lamb wave, and the reflected wave is detected. was received by the output electrode, and changes in the output level of the reflected output signal were observed using a sample-and-hold circuit. As a result, the desired results corresponding to FIG. 3 could be obtained. In the embodiments described above, imaging of the inside of the subject using Lamb waves has been described, but it is of course possible to use Rayleigh waves. However, Rayleigh waves (a) require consideration of electrode protection because the electrode surface must be in contact with the liquid, and (b) this does not inherently have velocity dispersion. must be given. FIG. 4 shows an example of a Rayleigh wave transducer that satisfies the above requirements, and includes a non-piezoelectric substrate 10 made of glass or sapphire, interdigital electrodes 3 and 4 provided on one surface of the substrate, and a piezoelectric thin film 1.
1. The non-piezoelectric substrate 10 has a thickness that satisfies the conditions for a semi-infinite elastic body, specifically a thickness several times, preferably five times or more, the wavelength λ of the surface wave. Interdigital electrodes 3 , 4 are provided on the surface of the non-piezoelectric substrate 10. The configuration of the electrode is the first
It is similar to the figure. A piezoelectric thin film 11 is provided on the surface of the substrate so as to cover the electrodes 3 and 4 . For example, Z _o O or AlN can be used as the piezoelectric thin film. The thickness of the piezoelectric thin film is selected depending on the purpose, but velocity dispersion becomes more noticeable when the thin film is relatively thin. According to this configuration, since the electrode is covered with the piezoelectric thin film, it is possible to provide both velocity dispersion and electrode protection. Further, depending on the thickness of the piezoelectric thin film, it is effective to provide velocity dispersion by forming a metal thin film (for example, gold, aluminum, etc.) with a thickness of about 5000 Å to 1 μm on the piezoelectric thin film. In the above structure, velocity dispersion can be provided by providing the electrodes 3 and 4 on the piezoelectric thin film, but since the electrodes are exposed, it is necessary to protect them with a photoresist or the like. Although the above example uses a non-piezoelectric substrate, a piezoelectric substrate having a thickness that excites surface waves, specifically, a piezoelectric substrate having a thickness five times or more of the wavelength λ of the surface wave on the substrate, Create an electrode in the same way as above, and apply a dielectric thin film (thickness:
It is possible to obtain a transducer that satisfies the above-mentioned requirements even by stacking layers (1μ or less). The transducer configured as described above is placed approximately parallel to the subject as shown in FIG. 2, with the electrode surface in contact with the liquid. The requirements for sending a bulk wave into the interior of the object and equations (1) to (3) can be similarly applied to Rayleigh waves, and in this case, the Lamb wave velocity is replaced by the Rayleigh wave velocity. FIGS. 5 and 6 are other specific examples of interdigital electrodes. Figure 5 shows four arc-shaped electrodes 20-1, 20-
2, 20-3, and 20-4, the four arc-shaped electrodes are arranged in a ring shape as shown in the figure, and a set of electrodes facing each other, for example, 20-1 and 20-3, is used for input. , 20-2 and 20-4 function as outputs. Note that 21a and 21
b is the ground. When in use, the terminals in of input electrodes 20-1 and 20-3 are electrically in phase, and the terminals out of output electrodes 20-2 and 20-3 are electrically in phase.
When also used in phase, good focusing of the acoustic beam can be obtained. Although the electrode configuration described in the previous embodiment and in FIG. 5 results in point focusing of the acoustic beam, certain applications may require linear focusing. FIG. 6 shows the configuration of an interdigital electrode in such a case, in which a pair of comb-shaped electrodes are interdigitally combined, and the sound waves are focused in a line. In the present invention, any of the above electrodes can be used. As explained above, according to the present invention, since imaging is performed using an ultrasonic transducer having interdigital electrodes, it is possible to easily make oblique incidence of sound waves into the object, and therefore, the inside of the object can be easily made obliquely incident. It is suitable for knowing the situation of the body, and the direction of bulk wave propagation within the subject can be electrically controlled. Furthermore, since a transducer having separate input and output electrodes on a single substrate is used, it can be easily placed on the subject, and there is no need to use a directional coupler to separate input and output.

[Brief explanation of drawings]

Fig. 1 shows an example of an ultrasonic transducer used in carrying out the present invention, Fig. 2 shows an example of the present invention, Fig. 3 shows an object to be examined used in the experiment, and Fig. 4 shows an ultrasonic transducer used in carrying out the present invention. Another embodiment of the acoustic transducer, FIGS. 5 and 6, respectively, show different embodiments of interdigital electrodes. 1; Ultrasonic transducer, 2; Piezoelectric substrate,
3, 4; interdigital electrode, 5; liquid material,
6; Subject.

Claims (1)

[Scope of Claims] 1. An ultrasonic transducer having input and output interdigital electrodes provided spaced apart from each other on one surface of a piezoelectric substrate is acoustically liquid-coupled to a subject via a liquid, and the By applying an alternating current electric signal to the input electrode, Lamb waves or Rayleigh waves are excited in the piezoelectric substrate at a speed equal to or higher than the bulk wave velocity specific to the subject, and the Lamb waves or Rayleigh waves are excited in the piezoelectric substrate. The longitudinal sound waves are converted into longitudinal sound waves at the interface between the liquid body and the liquid body, and the longitudinal sound waves propagate through the liquid body. An ultrasound imaging method characterized in that an acoustic image of the inside of the object is obtained by converting the bulk wave into a bulk wave and propagating the bulk wave inside the object and receiving the reflected wave. 2. The piezoelectric substrate of the ultrasonic transducer has a thickness of approximately λ (λ is the wavelength of the sound wave in the substrate) or less, and the uncut surface of the electrode is in contact with the liquid. Ultrasound imaging method. 3. The piezoelectric substrate of the ultrasonic transducer is made into a thin film, and is laminated on one surface of the piezoelectric substrate having a thickness that satisfies the conditions for a semi-infinite elastic body via interdigital electrodes, and the piezoelectric thin film is turned into a liquid. An ultrasonic imaging method according to claim 1, which is contiguous with the invention. 4. The ultrasonic wave according to claim 2 or 3, wherein the propagation direction of the bulk wave to the subject is controlled by changing the frequency of an AC electric signal to the input electrode of the ultrasonic transducer. Imaging method.