JPH0417658B2 - - Google Patents

Description

[Detailed Description of the Invention] (A) Technical Field of the Invention The present invention is directed to an ultrasonic transducer, particularly an ultrasonic transducer used for ultrasonic diagnosis of the human body, in which a transducer that is reflected back from a subject is used. The sound pressure ratio between the sound wave and the sound wave emitted after being re-reflected inside the transducer is, for example, -15 dB or less, and the acoustic impedance of the surface of the test object and the sound seen from the test object side are The present invention relates to an ultrasonic transducer in which the impedance is substantially equal and the re-reflection is substantially eliminated.

(B) Technical background and problems Conventionally, in ultrasonic transducers equipped with ultrasonic transducers made of piezoelectric elements, acoustic matching with a thickness of λ/4 is used on the radiation side of the transducer. It is known to arrange layers to efficiently insert emitted ultrasonic waves into a subject and to adjust the duration of emitted ultrasonic pulses (Japanese Patent Publication No. 55-33020). .

However, the sound waves that are emitted toward the subject, reflected, and returned may be re-reflected inside the transducer and re-radiated toward the subject. The re-emitted sound is received by the transducer again from the tissue of the specimen and creates a false image. Such a phenomenon will be referred to as multiple reflection, and a method for improving the multiple reflection has been proposed, for example, in Japanese Patent Laid-Open No. 54-21082.

This has the following structure. In other words, a continuously variable layer in which the acoustic impedance changes continuously is provided on the radiation side of the transducer, that is, between the transducer and the subject, and the acoustic impedance of the subject side surface of the continuous variable layer is determined by the acoustic impedance of the subject. The acoustic impedance of the side surface of the transducer in the continuously variable layer is selected to be equal to the acoustic impedance of the transducer.

Although it is possible to reduce the above-mentioned multiple reflections to some extent with this configuration, since an acoustic damper material is generally placed on the back surface of the transducer, the difference in acoustic impedance between the transducer and the acoustic damper material may be affected. Due to the reflection caused by this, it cannot achieve sufficient effect.

(C) Object and Structure of the Invention The present invention aims to solve the above points, and the ultrasonic transducer of the present invention includes an ultrasonic transducer made of a piezoelectric element, and A radiation-side acoustic matching layer disposed on the radiation side of the sonic transducer and/or a back-side acoustic matching layer disposed on the back side of the ultrasonic transducer, and the outermost layer on the back side. In an ultrasonic transducer having an acoustic damper, the acoustic impedance seen from the radiation side surface of the transducer to the outermost acoustic damper on the transducer side is in contact with the radiation side surface in a desired frequency band. The acoustic impedance of the acoustic medium is substantially equal to the acoustic impedance of the acoustic medium. This will be explained below with reference to the drawings.

(D) Embodiments of the Invention Fig. 1 is an explanatory diagram illustrating a mode of detecting echoes from a subject using an ultrasonic transducer, and Figs. 2 and 3 illustrate the principle to which the present invention is applied. The explanatory drawings and FIGS. 4 to 8 each show an embodiment of the present invention.

In Fig. 1, 1 is a converter, 2 is a radiation side λ/
4 is an acoustic matching layer with a thickness, 5 is an acoustic damper, 6 is a subject (acoustic medium), 7 is a reflector inside the subject, 9
represents an ultrasonic transducer.

The ultrasonic waves from the transducer 1 are emitted toward the inside of the object 6 via the matching layer 2, and the acoustic waves reflected by the reflector 7 are received by the transducer 1 via the matching layer 2, thereby reducing the time difference. The presence of the reflector 1 is detected. On the other hand, ultrasonic waves directed toward the back side from the transducer 1 are preferably absorbed by the acoustic damper 5.

As mentioned above, in the device disclosed in Japanese Patent Application Laid-Open No. 54-21082, the radiation-side acoustic matching layer 2 shown in FIG. 1 can be considered to be composed of the above-mentioned continuously variable layer. In this case, in the disclosed method, it is possible to prevent the reception wave from the specimen 6 from being reflected from the radiation side surface of the transducer 1 (boundary with the acoustic damper 5). It is not possible to prevent reflection from the boundary with Child 1.

The reflection of sound waves will be discussed with reference to FIG. Now, as shown in Figure 2, the acoustic impedance Z ₁
Consider a situation in which a flat plate with an acoustic impedance Z 2 and a thickness 1/4 of the wavelength λ of a sound wave is placed between a medium with an acoustic impedance Z ₃ and a medium with an acoustic impedance Z ₃ .
When considering that sound waves pass through a flat plate from the side of the medium with acoustic impedance Z ₃ in this state, the plane where the flat plate is in contact with the medium with acoustic impedance Z ₃ (indicated by the chain line in the figure) is viewed from the arrow side. The input acoustic impedance Z _io can be expressed as Z _io = Z ₂ ² /Z ₁ ...(1), and the sound pressure of the sound wave reflected to the medium side with acoustic impedance Z ₃ is Z _io = Z ₃ Minimum under the condition that This may be because the reflected component from the boundary between impedances Z ₂ and Z ₃ and the reflected component from the boundary between impedances Z ₂ and Z ₁ cancel each other out.

As shown in Fig. 3, the acoustic impedances on the radiation side of the transducer 1 are Z _T1 , Z _T2 , ...,
N _radiation -side acoustic matching layers each having a thickness of λ/4 and having an acoustic impedance of Z _B1 , Z _B2 , ..., Z _BM on the back side of the transducer 1, each having a thickness of λ/4.
This example is adapted to an ultrasonic transducer having an M-layer rear side acoustic matching layer having a thickness of 4, and an acoustic damper 5 having an acoustic impedance _ZB . In addition,
Consider that the radiation-side surface layer 8 shown in FIG. 3 is in contact with a subject 6 having an acoustic impedance Z _T . In this case, the input impedance Z _io is: l _o Z _io =2 _N 〓 ⁱ⁼⁰ (-1) ^(Ni) l _o Z _Ti +2 _M 〓 ^j=0 (-1) ^(N+j-1) l _o Z _Bj + (-1) ^(M+N) l _o Z _B (however, it can be expressed as Z _Ti(i=0) =Z _Bj(j=0) =1 ...(3), and the acoustic impedance Z _T For received sound waves incident from the medium side with , the sound pressure of the sound waves reflected from the transducer becomes the lowest when the following relationship holds: l _o Z _io = l _o Z _T (4).

FIG. 4a shows an embodiment of the present invention, where symbols 1, 2, and 5 correspond to those in FIG. 1, 3 is an acoustic matching layer with a thickness of λ/4 on the radiation side, and 4 is a rear surface side λ/4
represents an acoustic matching layer with a thickness of . In the illustrated case, two matching layers are provided on the radiation side and one matching layer is provided on the back side. In this case, the acoustic impedance of each λ/4 matching layer 1, 2, 3, 4, and 5 is 34×10 ⁶ , 8.5×10 ⁶ , 2×10 ⁶ , 12.8
×10 ⁶ , 7.5×10 ⁶ (each unit Kg/sm ² ),
A transducer with approximately 20 dB less reflection than the conventional transducer (shown in Figure 1) in the 3.5±0.5 MHz band was obtained.

An outline of the measurement system at this time is shown in FIG. 4b, and the spectra of the reflected waves obtained are shown in FIGS. 4c and d. In the measurement system, a perfect reflector b facing a transducer a is placed in water c, the transducer is driven by a transmission circuit (not shown), and the first reflected wave and second reflected wave are received. However, the received waves are configured to pass through a receiving circuit (not shown) and a spectrum analyzer obtains the spectrum of each received wave. Using this measurement system, the first received wave and the second received wave measured for the conventional transducer are shown in FIG.
The second received wave is shown in FIG. 4d, and in the case of the one according to the present invention, the level of the second received wave is greatly reduced. FIG. 5a shows an embodiment of the present invention,
Reference numerals 1, 2, and 5 in the figure correspond to those in FIG.
In this case, the acoustic impedances of the λ/4 matching layers 2 and 5 are 3.8×10 ⁶ and 11.5×10 ⁶ (unit: Kg/s).
m ² ), the ratio of the transmission/reception spectrum to the first multiple reflection spectrum is -15 dB or less in the desired frequency band, as shown in FIG. 5b, and satisfies the desired conditions.

FIG. 6a shows one embodiment of the present invention, and the symbols 1, 2, 4, and 5 in the figure correspond to those in FIG. In this case, the acoustic impedances of matching layers 2, 4, and 5 are set to 3.8×10 ⁶ , 9.4×10 ⁶ , and 7.5×10 ⁶ (unit:
Kg/sm ² ), the ratio of the transmitting/receiving spectrum to the first multiple reflection spectrum is - in the desired frequency band, as shown in Figure 6b.
It is 15dB or less and satisfies the desired conditions.

FIG. 7a shows one embodiment of the present invention, and the symbols 1, 2, 3, and 5 in the figure correspond to those in FIG. In this case, the acoustic impedances of matching layers 2, 3, and 5 are 8.4×10 ⁶ , 2.0×10 ⁶ , and 21.8×10 ⁶ (unit:
Kg/sm ² ), the ratio of the transmitting/receiving spectrum to the first multiple reflection spectrum is - in the desired frequency band, as shown in Figure 7b.
It is 15dB or less and satisfies the desired conditions.

FIG. 8a shows still another embodiment of the present invention,
Reference numerals 1, 4, and 5 in the figure correspond to those in FIG. 4, and 10 represents a continuous change layer as referred to in this specification. In this case, the acoustic impedance of the continuously variable layer 10 is selected to vary continuously from the acoustic impedance Z _T of the subject to the acoustic impedance of the transducer 1, and the acoustic impedance of each layer 4 and 5 is sequentially changed.
3.0×10 ⁶ and 7.5×10 ⁶ (unit: Kg/sm ² ), the ratio of the transmitting/receiving spectrum to the first multiple reflection spectrum is −15 dB or less in the desired frequency band, as shown in Figure 8b. Yes, and satisfies the desired conditions.

The medium having each of the above acoustic impedances can be selected from the following.

A Acoustic impedance is 2.0× ¹⁰⁶ or 3.2×
In the range of 10 ⁶ [Kg/sm ² ], there are synthetic resins such as polyurethane, nylon, and epoxy (trade name).

B Acoustic impedance is 10.0× ¹⁰⁶ to 13.5×
In the range of 10 ⁶ [Kg/sm ² ], there are substances that correspond to glass, crystal, quartz, etc.

C Acoustic impedance is 3.3× ¹⁰⁶ or 10.0×
In the range of 10 ⁶ [Kg/sm ² ], there are generally few single compositions that are suitable. Therefore, a material in which metal powder such as aluminum or iron is mixed into the synthetic resin such as epoxy or polyurethane is used. In this way, by increasing or decreasing the amount of metal powder, it becomes possible to adjust the acoustic impedance up to about 20×10 ⁶ [Kg/sm ² ]. In particular, the epoxy resin mixed with metal powder itself has good adhesive properties, so the adhesive required when using the glass is unnecessary, and property deterioration due to the adhesive layer can be prevented. I can do it.

In addition, we checked to what extent the reflection from the above-mentioned ultrasonic transducer itself can be tolerated. Figure 9 shows the reflection levels from each tissue of the heart. As shown in the figure, there is a tissue ○a that exhibits strong reflection up to about 20mm from the body surface, and if a conventional transducer with a lot of reflection is used, the reflected waves from this tissue will be reflected again by the ultrasonic transducer. Then, it is reflected again by the tissue ○a and received by the ultrasound transducer, and this appears as multiple reflections ○a′, and the image of the septum ○ro, located about 40 mm below the body surface, is significantly degraded due to the overlap of the multiple reflection images. It turned out that it would. Tissue ○B, which exhibits strong reflection, has a reflection level of about -25 dB, and septum ○B has a reflection level of about -60 dB. If the reflectance of the ultrasonic transducer is αdB, in order for the level of septum XX to be higher than the multiple reflection level of tissue XX, it is sufficient that (-25)×2+α<-60 holds, and the reflectance is must be lower than -10dB. In this way, it was confirmed that by using a transducer with a reflectance of -15 dB or less, a sufficiently clear image without the effects of multiple reflections could be obtained. Conventional transducers have a reflectance of about -6 to 10 dB, which means that good images cannot be obtained.

(E) Effects of the Invention As explained above, according to the present invention, the influence of multiple reflections can be substantially eliminated.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram illustrating how an echo from a subject is detected using an ultrasonic transducer; FIGS. 2 and 3 are explanatory diagrams illustrating the principle to which the present invention is applied; Figure a shows one embodiment of the present invention, and Figures 4b, 4c, and 4d are explanatory diagrams related to the embodiment shown in Figure 4a,
FIG. 5a shows another embodiment of the present invention.
FIG. 5b is an explanatory diagram related to the embodiment shown in FIG. 5a,
FIG. 6a is another embodiment of the present invention, FIG. 6b is an explanatory diagram related to the embodiment shown in FIG. 6a, and FIG. 7a is another embodiment of the present invention. Figure 7b
is an explanatory diagram related to the embodiment shown in FIG. 7a, and FIG.
Figure a shows another embodiment of the present invention, and Figure 8b
shows an explanatory diagram related to the embodiment shown in FIG. 8a. Further, FIG. 9 shows an example of the state of reflection in the heart region. In the figure, 1 is a transducer, 2 and 3 are radiation-side acoustic matching layers, 4 is a rear-side acoustic matching layer, 5 is an acoustic damper,
6 represents a subject (acoustic medium), 7 represents a reflector, and 9 represents a transducer.

Claims (1)

[Claims] 1. Equipped with an ultrasonic transducer composed of a piezoelectric element, a radiation-side acoustic matching layer disposed on the radiation side of the ultrasonic transducer, and a back-side acoustic matching layer disposed on the back side of the ultrasonic transducer. In an ultrasonic transducer that has both or one of the following layers and an acoustic damper on the outermost layer on the back side, from the radiation side surface of the transducer to the acoustic damper on the outermost layer. An ultrasonic transducer characterized in that the viewed acoustic impedance is substantially equal to the acoustic impedance of an acoustic medium that is brought into contact with the radiation side surface in a desired frequency band.