JPH0240974B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an ultrasonic probe for nondestructive testing equipment that uses a pulse reflection method to emit ultrasonic waves into a subject and detect reflected ultrasonic waves from within the subject. The object of the present invention is to perform the detection and detection using different piezoelectric vibrators, respectively, and to improve the radiation efficiency and detection efficiency of ultrasonic waves. In general, a parameter TP related to the ultrasonic radiation efficiency of an ultrasonic probe and a parameter RP related to the ultrasonic detection efficiency are expressed by the following equations. Here, k, ε ^S , C ^D and t are the electromechanical coupling coefficient, constrained dielectric constant, elastic stiffness and thickness of the piezoelectric vibrator, respectively. From equation (1), it is clear that a piezoelectric vibrator that emits ultrasonic waves is suitable for a piezoelectric vibrator that has both a large electromechanical coupling coefficient and a constrained dielectric constant, and has a small elastic stiffness. Furthermore, from equation (2), it is clear that a piezoelectric vibrator for detecting ultrasonic waves has a large electromechanical coupling coefficient and a small constraint dielectric constant and elastic stiffness. This suggests that a piezoelectric vibrator suitable for emitting ultrasonic waves is not necessarily suitable for detecting ultrasonic waves. FIG. 1 shows the overall configuration of a conventional single-type ultrasonic probe. This ultrasonic probe consists of a piezoelectric vibrator 1 with electrodes 4 attached to both sides, acoustic matching layers 2 and 3 attached to the surface facing the object 10, lead wires 5, and a case 7. , the piezoelectric vibrator 1 is excited by an external transmitter 8, and the ultrasonic wave is transmitted to the subject 1 via the acoustic matching layers 2 and 3.
The ultrasonic waves reflected from inside the subject 10 are detected by the piezoelectric vibrator 1 via the acoustic matching layers 2 and 3, and the signal is sent to an external receiver 9. . In this way, in conventional ultrasonic probes of this type, the piezoelectric vibrator is used to emit and detect ultrasonic waves, and therefore the piezoelectric vibrator is used depending on whether emphasis is placed on ultrasonic emission or detection. was selected at the expense of one or the other, and it was impossible to improve the ultrasonic radiation efficiency and detection efficiency. The present invention eliminates the above-mentioned disadvantages and makes it possible to independently improve the radiation efficiency and detection efficiency of an ultrasonic probe by selecting suitable piezoelectric vibrators for each. The present invention is characterized in that, in order to achieve the above object, the following structure is adopted, and ultrasonic waves are emitted and detected using different piezoelectric vibrators.
In ultrasound probes, an acoustic matching layer is often attached to the subject side of the piezoelectric vibrator in order to efficiently radiate ultrasound waves into the subject and efficiently detect reflections from within the subject. Are known. The characteristics of the ultrasonic probe vary greatly depending on whether the acoustic matching layer is one layer or two or more layers. That is, in the case of one layer, the frequency band characteristic becomes unimodal, resulting in a narrow band ultrasonic probe. On the other hand, if two or more acoustic matching layers are made of a material with appropriate acoustic impedance, the frequency band characteristics will become bimodal, making it possible to realize an ultrasonic probe with an extremely wide band. The present invention shows a specific configuration for further increasing detection efficiency while taking advantage of the characteristics of an ultrasonic probe having two or more acoustic matching layers. That is, two to three acoustic matching layers are attached to the subject side surface of a piezoelectric vibrator that emits ultrasonic waves, and at least one of the layers is made of a piezoelectric material with electrodes attached on both sides, such as PVF ₂ , CdS, or In addition to having a structure made of CdSe or the like, in an ultrasonic probe having this structure, a piezoelectric vibrator is excited to emit ultrasonic waves into the subject, and a piezoelectric acoustic matching layer is used as a receiving section to radiate ultrasonic waves into the subject. The purpose is to detect reflected ultrasound waves from inside the subject. Next, regarding the ultrasonic probe according to the present invention,
One embodiment will be described in detail with reference to FIG. That is, this ultrasonic probe includes a piezoelectric vibrator 11 for ultrasonic emission, acoustic matching layers 12 and 14, a piezoelectric acoustic matching layer 13 for ultrasonic detection, electrodes 15 and 16, lead wires 17 and 18, It consists of a case 19, and a piezoelectric vibrator 11 is transmitted by an external transmitter 20.
is excited, emits ultrasonic waves into the interior of the subject 22 via the acoustic matching layers 12 and 14 and the piezoelectric acoustic matching layer 13, and emits the reflected ultrasound waves from inside the subject 22 to the acoustic matching layer 14. The piezoelectric acoustic matching layer 13 detects the signal via the piezoelectric acoustic matching layer 13, and sends the signal to the external receiver 21. In order to maintain the sensitivity and distance resolution of this ultrasound probe, the acoustic impedance of the piezoelectric acoustic matching layer material must be between the acoustic impedance of the piezoelectric transducer and the acoustic impedance of the object. The acoustic mismatch can be eliminated by forming a multilayer structure using any material within this range. Figure 3 shows the sensitivity and pulse decay time of an ultrasound probe using PZT-5A as the piezoelectric vibrator, PVF ₂ as the piezoelectric acoustic matching layer, and fused silica and epoxy resin as the acoustic matching layer, and the piezoelectric This figure shows the relationship between the thickness of the acoustic matching layer and the wavelength ratio of ultrasonic waves. That is, FIG. 3 is shown to confirm the influence of the piezoelectric acoustic matching layer as a matching layer in the configuration of the present invention, where the solid line shows the sensitivity and the dotted line shows the pulse decay time. t/λ=0 is the characteristic of a conventional probe without a piezoelectric acoustic matching layer. In general, where the sensitivity changes greatly, the performance of the probe tends to become unstable, so to prevent this, it is necessary to precisely adjust the thickness of the matching layer, which significantly increases manufacturing costs. There are drawbacks. Therefore, 0<t/λ<0.2 and 0.35<t/
The range of λ<0.5 cannot be adopted because the sensitivity changes greatly. On the other hand, in the range of 0.2<t/λ<0.35, the change in sensitivity is small and the pulse decay time is in a good range of −0.3 to 0 μsec. Therefore, it is preferable that the thickness of the piezoelectric acoustic matching layer be 0.2 to 0.35 times the wavelength of the ultrasonic wave. Next, the detection efficiency of the ultrasonic probe having the structure based on the present invention will be explained in comparison with a conventional example. Since the vibration mode of the piezoelectric vibrator shown in FIGS. 1 and 2 is thickness longitudinal vibration, Equation 2 can be expressed as follows. In the ultrasonic probe structured according to the present invention, if the parameter related to the detection efficiency of ultrasonic waves in the piezoelectric acoustic matching layer is RP ₁ , then k _t = h ₃₃ √ ^S ₃₃
C ^D ₃₃ , t=Vt ^D /a・f (Vt ^D is the sound velocity of longitudinal waves, is the ultrasonic frequency, a is the proportionality constant), V ^D _t = √ ^D ₃₃
Since ρ (ρ is the density) and Z ₀ =ρ·v ^D _t (Z ₀ is the acoustic impedance), Equation 3 is transformed as follows. RP ₁ =h ₃₃ /a・f・Z ₀ (4) Here, since the thickness of the piezoelectric acoustic matching layer is 0.2 to 0.35 times the wavelength of the ultrasonic wave, λ=V ^D _t /f,
From t=V ^D _t /a·, (0.2≦1/a≦0.35), the proportionality constant a becomes 2.86≦a≦5 (5). Furthermore, if RP ₂ is the parameter related to the ultrasonic detection efficiency when emitting and detecting ultrasonic waves using the same piezoelectric vibrator as in the conventional example, then k′ _t = h′ ₃₃ √ ^S ′ ₃₃ ^D ′ ₃₃ , t=v ^D ′ _t /a′・f
, V ^D ′ _t =√
Since C ^D ′ ₃₃ /ρ′ and Z′ ₀ =ρ′・v ^D ′ _t , Equation (3)
is transformed as follows. RP ₂ = h′ ₃₃ /a′・f・Z′ ₀ (6) Here, the thickness of the piezoelectric vibrator is equal to the wavelength of the ultrasonic wave.
Since it is generally 0.4 to 0.45 times, the proportionality constant a′ is 2.2≦a′≦2.5 (7). In order for the detection efficiency of the ultrasonic probe having the structure based on the present invention to be superior to the conventional one, RP ₁ <RP ₂ (8) must be satisfied. Therefore, from equations (4) to (8), h ₃₃ /Z ₀ ≧a/a' h ₃₃ '/Z ₀ '. Here, in order for a/a' to be maximum, a=5 and a'=2.2, so h ₃₃ /Z ₀ ≧2.3h' ₃₃ /Z' ₀ (9), and this condition is satisfied. At this time, the ultrasonic probe structured according to the present invention has improved detection efficiency compared to the conventional one. As a piezoelectric vibrator, Pb (Mg _1/3 Nb _2/3 )
O ₃ ―PbTiO ₃ ―PbZrO ₃ -based ceramics, PbZrO ₃
-Using PbTiO ₃ -based ceramics or PbTiO ₃ -based ceramics as a piezoelectric acoustic matching layer,
According to the characteristics of each material shown in Table 1, an ultrasonic probe using PVF ₂ , CdS, or CdSe satisfies the above conditions in the following combinations. That is, Pb(Mg _1/3 Nb _2/3 )O ₃ -PbTiO ₃ -PbZrO ₃ -based ceramics and PVF ₂ or CdS PbZrO ₃ -PbTiO _3- based ceramics and PVF ₂ ,
It is a combination of CdS or CdSe PbTiO ₃ ceramics and PVF ₂ .

[Table] Table 2 shows various constants and their calculations required to calculate the parameter RP related to the ultrasonic detection efficiency of the conventional example shown in Fig. 1 and the structure of the ultrasonic probe according to the present invention shown in Fig. 2. This shows the results. In the conventional example, PZT-5A is used as the piezoelectric vibrator, and fused silica and epoxy resin are used as the acoustic matching layer.
PZT-5A, fused silica and epoxy resin as acoustic matching layer, PVF ₂ as piezoelectric acoustic matching layer
was used. The detection efficiency is expressed by equation (3), i.e. In the ultrasonic probe having the structure according to the present invention, from the constants in Table 2, k _t =0.2, ε ^S ₃₃ =8×ε ₀ , C ^D ₃₃ =0.95×10 ¹⁰ , t=
0.11×10 ⁻³ Here, ε ₀ is the dielectric constant in vacuum, and ε ₀ =8.854×10 ⁻¹² . Therefore, the detection efficiency RP ₁ is RP ₁ =26.8×10 ⁻⁶ . Here, the sound speed V of PVF ₂ is generally V=
2260m/sec, and the frequency of ultrasound =
If it is 5MHz, the wavelength λ is λ=V/=0.452mm. Furthermore, since t=0.11 mm, t/λ≈0.24, so the condition of 0.2≦t/λ≦0.35 is satisfied. On the other hand, the ultrasonic detection efficiency RP ₂ in the conventional example is
Similarly, from the constants in Table 2, k _t =0.5, ε ^S ₃₃ =830×ε ₀ , C ^D ₃₃ =14.7×10 ¹⁰ t=0.35×10 ⁻³ Therefore, detection efficiency RP ₂ =5.3×10 ⁻ It becomes ⁶ . As described above, the detection efficiency of the ultrasonic probe having the structure based on the present invention is approximately five times higher than that of the conventional probe, and this tendency was also confirmed through experiments.

[Table] Set. Further, ε ₀ is the dielectric constant in vacuum.
FIG. 4 shows another embodiment of the invention. This ultrasonic probe has electrodes 27 attached to both sides, piezoelectric vibrators 23 and acoustic matching layers 24 and 26 arranged in a straight line, and an electrode 28 attached at a position opposite to the electrodes 27. matching layer 25,
This is a linear electronic scanning ultrasonic probe consisting of electronic switches 31 and 32, lead wires 29 and 30, and a case 33. Several piezoelectric vibrators are connected in succession via the electronic switch 31 by an external transmitter 34. 23 are excited simultaneously or with phase control, and emit ultrasonic waves into the interior of the object 36 via the acoustic matching layers 24 and 26 and the piezoelectric acoustic matching layer 25, and further radiate ultrasonic waves from inside the object 36. Ultrasonic waves are detected by the piezoelectric acoustic matching layer 25 via the acoustic matching layer 25, and the signal is sent to an external receiver 35 via the electronic switch 32. In addition,
The selection of the piezoelectric vibrator to be excited and the selection of the piezoelectric acoustic matching layer for detecting ultrasonic waves are performed by electronic switches 31 and 32, respectively. As is clear from the above detailed explanation, the present invention is an ultrasonic probe having two to three acoustic matching layers, and at least one of the layers has a thickness of 0.2 to 0.2 wavelength.
By using a 0.35x piezoelectric material in the receiving section, it is possible to maintain the characteristics of an ultrasonic probe using two to three acoustic matching layers, that is, the broadband frequency band characteristics, while maintaining the characteristics of the ultrasonic probe from inside the subject. This has the effect of further increasing the detection efficiency of reflected ultrasound waves. The structure based on the present invention is related to the basics of ultrasonic probes, and is applicable not only to single-type ultrasonic probes and linear electronic scanning ultrasonic probes as shown in the above embodiments. The present invention can also be applied to a fan-shaped electronic scanning type ultrasound probe that scans ultrasonic waves in a fan shape, and a composite electronic scanning type ultrasound probe that performs multiple ultrasound scans.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the overall configuration of a conventional single-type ultrasonic probe, FIG. 2 is a block diagram showing the overall configuration of an ultrasonic probe according to an embodiment of the present invention, and FIG. FIG. 4 is a diagram showing the relationship between the sensitivity and distance resolution of an ultrasound probe, the thickness of the piezoelectric acoustic matching layer, and the ratio of the wavelength of ultrasound, and FIG. 4 is an ultrasound probe according to another embodiment of the present invention. FIG. 3 is a block diagram showing the overall configuration of the child. 1, 11, 23...piezoelectric vibrator, 2, 3, 12,
14, 24, 26...Acoustic matching layer, 13, 25...
Piezoelectric acoustic matching layer, 4, 15, 16, 27, 2
8... Electrode, 5, 6, 17, 18, 29, 30... Lead wire, 7, 19, 33... Case, 8, 20, 3
4...Transmitter, 9,21,35...Receiver, 10,2
2, 36... Subject, 31, 32... Electronic switch.

Claims (1)

[Claims] 1. A piezoelectric vibrator and two to three acoustic matching layers, wherein at least one of the acoustic matching layers includes:
A piezoelectric material with electrodes attached to both sides is used, and the thickness of the piezoelectric acoustic matching layer is set to 0.2 to 0.35 times the wavelength of the ultrasound in the medium, and the piezoelectric vibrator is excited to create a sound inside the subject. An ultrasonic probe characterized in that it emits ultrasonic waves and detects reflected ultrasonic waves from inside a subject using the piezoelectric acoustic matching layer. 2 As a piezoelectric acoustic matching layer, the acoustic impedance is between the acoustic impedance of the piezoelectric vibrator and the acoustic impedance of the subject, and the ratio of the piezoelectric constant h ₃₃ of the piezoelectric acoustic matching layer to the acoustic impedance Z _O h ₃₃ /Z _O is higher than that of the piezoelectric vibrator
The ultrasonic probe according to claim 1, characterized in that the ultrasonic probe is 2.3 times or more larger.