JPH0448039B2 - - Google Patents

Description

[Detailed Description of the Invention] Background of the Invention A Technical Field The present invention relates to an ultrasonic probe used in an ultrasonic diagnostic device, etc., and in particular, a three-layer acoustic matching layer is provided on the ultrasonic output surface of a piezoelectric body. Regarding the equipped ultrasonic probe. B. Prior art and its problems For example, ultrasonic diagnostic equipment that utilizes the transmission and reflection characteristics of ultrasound waves is economical and non-invasive as a device that can observe tomographic images in real time without dissecting the living body. Due to its nature, it has been widely used in the medical field in recent years. This ultrasonic diagnostic device uses the so-called pulse echo method, which emits an ultrasonic pulse to the subject, detects the echo of this ultrasonic pulse reflected at a discontinuous point in the acoustic impedance inside the subject, and detects the echo. A two-dimensional image of the living body in B mode is displayed based on the arrival time and intensity of the echo. In this ultrasound diagnostic device, the time resolution (distance resolution), which is the resolution in the ultrasound propagation direction, is
This is one of the important factors along with the azimuth resolution in the direction perpendicular to the propagation direction in obtaining detailed and accurate tomographic images.The shorter the pulse width of the ultrasound pulse transmitted from the ultrasound probe to the living body, the more , the faster the response of the ultrasound probe when receiving an echo, the better the time resolution. In a probe that uses a single transducer (electro-acoustic conversion element) for both transmission and reception, it is possible to improve the time resolution by improving the impulse response. As shown in FIG. 1, one of the methods conventionally used to improve the impulse response of a probe is to
A backside load material (backing material) close to the acoustic impedance of the transducer 10 is placed on the back surface 14 on the opposite side from the load (living body) 16 that transmits ultrasonic waves.
There is a method in which the ultrasonic waves radiated to the back are absorbed by placing the 18 in close contact with each other. This method uses transducer 1
This has the disadvantage that the zero back reflection cannot be effectively used for transmitting and receiving ultrasonic waves, resulting in a decrease in the sensitivity of the entire probe. Another method for improving the impulse response is to form an acoustic matching layer 20 between the transducer 10 and the load 16 with a thickness of λ/4 (λ being the wavelength of the ultrasound in the material) as shown in FIG. It is. In this method, the acoustic impedance of lead zirconium titanate ceramic piezoelectric material (so-called PZT), which is generally used as a transducer, is approximately 30×10 ⁶ (Kg/
^m2・s), whereas that of biological skin is approximately
There is a large difference between 1.5 and 1.6×10 ⁶ (Kg/m ²・s).
Transducer 10 when the two are brought into direct contact
The ultrasonic waves are reflected many times between the main surface 12 of the probe and the living body surface 22, and as a result, the ultrasonic waves transmitted to the living body are lost, leading to a decrease in the sensitivity and response of the probe. The purpose is to form an acoustic matching layer to alleviate the multiple reflections. Devices with improved sensitivity and response using one or two acoustic matching layers (see FIG. 3) have already been put into practical use. On the other hand, 3
Regarding the layer structure, see (1) “Multilayer Impedance Matching
Schemes for Broadbanding of Water
Loaded Piezoelectic Transducers and High
Q Electric Resonators” (JHGOLL & B.A.
Auld，IEEE TRANSACTION ON SONICS
AND ULTRASONICS 1975 vol.SU−22.No.
1 pp.52-53) (2) “The Design of Broad−Band Fluid−
"Loaded Ultrasonic Transducers" (JH
GOLL, IEEE TRANSACTION ON
SONICS AND ULTRASONICS 1979.vol.
SU-26.No.6 pp.385-393) (3) Although there have been many reports such as Japanese Patent Application Laid-open No. 52-61987, it is still difficult to find an optimal structure that exceeds the characteristics of a one-layer or two-layer structure. There has been no proposal for a three-layer acoustic matching layer, and the current situation is that it has not yet been put into practical use. Separately, the acoustic impedance of the transducer 10 as shown in FIG.
A proposal has been made to provide a mediating layer 24 between the living body 16 of the transducer 10 whose acoustic impedance changes continuously up to 2.
(No. 21082), the disadvantage is that it is extremely difficult to select and manufacture appropriate materials. Purpose of the Invention Therefore, the present invention has an optimal three-layer acoustic matching layer structure that can exhibit better response than a single-layer or two-layer type acoustic matching layer without significantly losing sensitivity. The purpose is to provide an ultrasonic probe. According to the invention, this object is achieved by an ultrasound probe as follows. That is, this ultrasonic probe uses a ceramic piezoelectric material based on zirconate lead titanate as a piezoelectric body, and has an acoustic impedance of 7 to 11×10 ⁶ kg/cm as an acoustic matching layer.
^m2・s first matching material, 2.0~4.0×10 ⁶ Kg/
^m2・s second matching material, 1.6~2.0×10 ⁶ Kg/
This is a three-layer structure in which a third matching material having a diameter of m ² ·s is sequentially laminated. DETAILED DESCRIPTION OF THE INVENTION Next, the ultrasonic probe of the present invention will be described in detail with reference to the accompanying drawings. The present invention theoretically analyzes the pressure waves propagating in each medium that makes up the ultrasonic probe, directly obtains the received waveform, and calculates the sensitivity and responsiveness of the probe, thereby creating an optimal matching layer. This was discovered, and the principles of its analysis are shown in Figures 5 to 11. First, in FIG. 5, the thickness of the transducer 10 is λ/2 (λ is the wavelength in the piezoelectric material), and the thickness between the transducer 10 and the load 16 is λ'/4 (λ' is the wavelength in the piezoelectric material). In an ultrasonic probe 30 having an acoustic matching layer 20 (wavelength within the matching layer) and having a back load material 18 in close contact with the back side, the electrodes 24 on both sides of the transducer 10,
8 is an explanatory diagram schematically showing a temporal change in a pressure wave when the single pulse voltage V _io shown in FIG. 7 is applied to 26. FIG. In the figure, the vertical axis represents the passage of time. Four pressure waves A1 are generated from both sides of the transducer 10.
~A4 is released, but the pressure wave that occurs at time t=0 when the single pulse voltage _Vio is applied (A1) ₀ ~ (A1) ₄
The amplitude of each is shown by the following equation. (A1) ₀ = -hC・V _io・Z _B / (Z _B + Z _T ) (A2) ₀ = hC・V _io・Z _T / (Z _B + Z _T ) (A3) ₀ = hC・V _io・Z _T / (Z _T + Z _M ) (A4) ₀ = -hC・V _io・Z _M / (Z _T + Z _M ) However, h is the piezoelectric coefficient in the thickness direction of the transducer 10, C is the capacitance, Z _B , Z _T , and Z _M (=Z ₁ ) are the acoustic impedances of the back load material 18, the transducer 10, and the matching layer 20, respectively. These four waves emit or transmit pressure waves AF into the load 16 while repeating reflection and transmission at the boundaries of each medium. Here, it is assumed that the transducer 10 is performing piston motion at a resonant frequency due to thickness vibration, and echoes from the back surface of the back loading material 18 are absorbed by the back loading material 18 and are almost ignored. Furthermore, it is assumed that there is almost no influence due to the attenuation within the transducer 10, the matching layer 20, the electrodes 24, 26, and the thickness of the adhesive layer between each medium. Since the peaks of pressure waves that interfere with each other at the boundaries of each medium coincide, A2 to A4 and load 16
The pressure wave AF transmitted to can be generalized and expressed by the following recurrence formula. (A2) _o =R _TB・(A3) _o-1 (A3) _o =R _TM・(A2) _o-1 +T _TM・R _MF・(A4) _o-1 (A4) _o =T _MT・(A2 ) _o-1 +R _MT・R _MF・(A4) _o-1 (AF) _o ′=T _FM・(A4) _oHere , the subscript n indicates the time of occurrence of the pressure wave when t _o =
This means that n·t ₁ , and n' means that t ₁ /2 is behind t _o . t ₁ is the time for the sound wave to propagate one way through the transducer 10 . Furthermore, R _TB , R _TM , R _MF , T _TM , T _MT , and T _FM are reflection and transmission coefficients at the medium boundary, which are given by the following equations. R _TB = (Z _B - Z _T ) / (Z _T + Z _B ), R _TM = Z _M - Z _T ) / (Z _T + Z _T ) R _MF = (Z _F - Z _M ) / (Z _M + Z _F ) T _TM = 2Z _T / (Z _T + Z _M ), T _MT = 2Z _M / (Z _M + Z _T ) T _FM = 2Z _F / (Z _F + Z _M ) On the other hand, the transmitted wave AF is transmitted from the same probe 30. When the pressure waves A1 to A4 _are received at
'' (the subscript n'' means a time t ₁ /2 before t _o , i.e., n'' = n'-1), when t _o is incident on the transducer 10, The force acting towards the main surface 14 is (A2) _o − (A3) _o-1 ,
On the 12 side, (A3) _o - (A2) _o-1 , so the received voltage generated at the time t _o is the transducer 10
_{Considering the} ^piston _{motion of _} ^_ _{_ _} }]=K{(A2) _o +(A3) _o
} However, K is a proportionality constant. (A2) _o and (A3) _o are given by the following recurrence formula. (A2) _o =R _TB・(A3) _o-1 (A3) _o =R _TM・(A2) _o-1 +T _TM・R _MF・(A4) _o-1 +T _MF・T _TM・(AF) _o ″ (A4) _o =T _MF・(A2) _o-1 +R _MT・R _MF・(A4) _o-1 +T _MF・R _MT・(AF) _o ″ By performing the above calculation, the single pulse voltage V _io The received waveform for can be obtained. Based on the same idea, in FIGS. 8 and 9, a thickness λ ₁ /
4 and λ ₂ /4 (λ ₁ and λ ₂ are the wavelengths within the matching layer, respectively), the matching layer has a two-layer structure in which the first matching layer 32 and the second matching layer 34 with acoustic impedances Z ₁ and _Z 2 are formed. 3 shows temporal changes in pressure waveforms during transmission and reception with respect to the probe 40. In the same way as above, when sending
The pressure waves of A2 to A5 and AF are expressed by the following recurrence formula. (A2) _o = R _TB・(A3) _o-1 (A3) _o =R _T1・(A2) _o-1 +R ₁₂・T _T1・(A4) _o-1 +T ₁₂・T _T1・(A5) _{o -1} (A4) _o = T _1T・(A2) _o-1 +R ₁₂・R _1T・(A4) _o-1 +T ₁₂・R _1T・(A5) _o-1 (A5) _o = T ₂₁・R _2F・(A4) _o-1 +R ₂₁・R _2F (A5) _o-1 (AF) _o = T ₂₁・T _F2・(A4) _o-1 +R ₂₁・T _F2・(A5) _o-1 When receiving again (A2) _o and (A3) _o are given by the following equation. (A2) _o = R _TB・(A3) _o-1 (A3) _o =R _T1・(A2) _o-1 +R ₁₂・T _T1・(A4) _o-1 +T ₁₂・T _T1・(A5) _{o -1} (A4) _o = T _1T・(A2) _o-1 +R ₁₂・R _1T・(A4) _o-1 +T ₁₂・R _1T・(A5) _o-1 (A5) _o = T ₂₁・R _2F・(A4) _o-1 +R ₂₁・R _2F・(A5) _o-1 +T _2F・(AF ₎ λ ₁ /4, λ ₂ /
4, λ ₃ /4 (λ ₁ , λ ₂ , λ ₃ are each wavelength within the matching layer)
The time change of the pressure waveform during transmission and reception for the probe 50 formed with the first to third acoustic matching layers 42, 44, and 46 of the three-layer structure with acoustic impedances Z ₁ , Z ₂ , and Z ₃ is shown below. Similarly to the above, the pressure waves of A2 to A6 and AF during transmission are (A2) _o = R _TB・(A3) _o-1 (A3) _o = R _T1・(A2) _o-1 +R ₁₂・T _T1・(A4) _o-1 +T ₁₂・T _T1・(A5) _o-1 (A4) _o =T _1T・(A2) _o-1 +R ₁₂・R _1T・(A4) _o-1 +T ₁₂・R _1T・(A5) _o-1 (A5) _o =T ₂₁・R ₂₃・(A4) _o-1 +R ₂₁・R ₂₃・(A5) _o-1 +R _3F・T ₂₃・(A6) _{o -1} (A6) _o =T ₂₁・T ₃₂・(A4) _o-1 +R ₂₁・T ₃₂・(A5) _o-1 +R _3F・R ₃₂・(A6) _o-1 (AF) _o ′=T _F3・(A6) _o Also, for reception, it is given by the following recurrence formula. (A2) _o ~ (A4) ₄ is the same as when sending, (A5) _o = T ₂₁・R ₂₃・(A4) _o-1 +R ₂₁・R ₂₃・(A5) _o-1 +R _3F・T ₂₃・(A6) _o-1 +T _3F・T ₂₃・(AF) _o ″ (A6) _o =T ₂₁・T ₃₂・(A4) _o-1 +R ₂₁・T ₃₂ (A5) _o-1 +R _3F・R ₃₂・(A6) _o-1 +T _3F・R ₃₂・(AF) _o ″ However, n″=n′-1 Based on the above analysis method, the back load 18 is air (Z _B ≒ 0) and the transducer 10 and load 16
The acoustic impedance of Z _T = 30, Z _F = 1.5×
10 ⁶ Kg/m ² s, the received waveforms are calculated for each of the probes 30, 40, and 50 with a one-layer to three-layer structure, and the relative sensitivity S is calculated from the maximum pulse height, around the maximum pulse on the time axis. Responsiveness D was calculated from the variance of . The results are shown in FIGS. 12 to 16.
Note that the received waveform obtained by analyzing the two-layered probe 40 as an example generally matches the waveform obtained by actually manufacturing and measuring an actual probe, which indicates that the principles of the present invention are valid. I understand that. This is shown in FIGS. 17 to 20. In these figures, Figure A shows an actually measured waveform, and Figure B shows an analyzed waveform. 12 and 13 relate to a probe 30 having one matching layer, and the acoustic impedance _Z1 of the matching layer 20 is varied in various ways. According to this,
Both sensitivity and responsiveness are at their highest levels around Z ₁ =4×10 ⁶ Kg/m ² ·s. FIG. 14 shows the characteristics of the two-layer type probe 40, where the acoustic impedance Z of the first acoustic matching layer 32 is set as ₁ parameter, and each
It shows how Z ₂ of the second layer is changed variously for each value of Z ₁ . From the figure, improvement in responsiveness is achieved by setting Z ₁ to 6 to 7 and Z ₂ to
It is optimal to set the relative sensitivity to about 2.0 to 2.2×10 ⁶ Kg/m ²・s, and in this case, the relative sensitivity is the optimal combination (Z ₁ = 5 to
6, Z ₂ = 1.7 to 1.9×10 ⁶ Kg/m ²・s)
It has only decreased to a certain extent. × in Figures 15 and 16
The data indicated by the mark is for the two-layer type probe 4 mentioned above.
From the characteristics of 0, we found the optimal combination of Z ₂ for each value of Z ₁ and plotted the relative sensitivity S and D. The numbers on the graph are the value of Z ₂ × 10 ⁶ Kg/m ²・It is represented by s. The data indicated by black circles in FIGS. 15 and 16 shows the characteristics of the three-layer type probe 50, and the optimum combination of Z ₂ and Z ₃ is determined for each value of Z ₁ . The heading is a plot of S and D.
The upper row of numbers in the graph represents the value of Z ₂ and the lower row represents the value of Z ₃ in ×10 ⁶ Kg/m ² ·s. As a result,
The range in which the response is further improved than that of the two-layer type is roughly as follows. First matching layer Z ₁ : 6 to 12 (×10 ⁶ Kg/m ² ·s) Second matching layer Z ₂ : 2 to 4 Third matching layer Z ₃ : 1.6 to 2.0 The response in this range is approximately It has a value of 1.7 to 2,
The decrease from the optimal combination of sensitivities is less than a few percent.
Examples of analyzed received waveforms are shown in FIGS. 21 and 22. 23 and 24 show the transducer 10
Acoustic impedance Z _T of 20 x 10 ⁶ and 40 x 10 ⁶
(Kg/ ^m2・s), each probe 30, 40,
The 50 characteristics were calculated in the same manner as above, and each
The range of the three-layer type that exhibits improved responsiveness than the two-layer type is as follows.

[Table] Therefore, when the piezoelectric material used for the transducer 10 is, for example, PZT and Z _T =20 to 40×10 ⁶ Kg/m ² s, the configuration range of the three-layer type is improved over the two-layer type. First matching layer 5 to 15 (×10 ⁶ Kg/m ²・s) Second matching layer 1.9 to 4.4 (×10 ⁶ Kg/m ²・s) Third matching layer 1.6 to 2.0 (×10 ⁶ Kg / ^m2・s). The following table shows the previously reported three-layer type probe and the probe optimized based on the principles of the present invention using a transducer 10 with Z _T = 33.7 x 10 ⁶ Kg/m ² s. A comparison of the characteristics with that of children shows that responsiveness has been significantly improved. 25th
Comparing the analytical received waveform for the conventional three-layer type shown in Figure 26 and the analytical received waveform for the three-layer type according to the present invention shown in Figure 27, the trailing portion of the received waveform almost disappears. I can see that.

[Table] In order to realize a probe with a three-layer structure according to the present invention, the following materials may be used, for example, according to the acoustic impedance range of each matching layer. First matching layer: GeSe, As ₂ S ₃ glass, a mixture of epoxy resin and W powder, a mixture of a thermoplastic polymer or a polymer material consisting of a thermoplastic polymer and a rubber-like elastic polymer and an inorganic powder, etc. 2 matching layers: polystyrene, polyethylene, polyester, ABS polycarbonate, acrylic, epoxy resin,
etc. 3rd matching layer: silicone/rubber, neoprene/
Rubber, etc. Specific effects of the invention As described above, according to the ultrasonic probe of the present invention,
By forming a three-layer acoustic matching layer between the transducer and the load and optimizing the acoustic impedance of each matching layer, sensitivity is improved compared to probes with one or two acoustic matching layers. It is possible to improve responsiveness without losing much. Therefore, since it is possible to transmit and receive ultrasonic signals in a wider band, the time resolution is improved and the performance of the apparatus is improved.

[Brief explanation of the drawing]

FIGS. 1 to 4 are schematic diagrams showing conventional probes, and FIGS. 5 and 6 are explanatory diagrams showing the propagation of pressure waves during transmission and reception of a single-layer probe. Figure 7 is a diagram showing the signal waves applied to both ends of the transducer, Figures 8 and 9 are explanatory diagrams showing the propagation of pressure waves during transmission and reception of a two-layer type probe, Figures 10 and 11 are explanatory diagrams showing the propagation of pressure waves during transmission and reception with a three-layer type probe, and Figures 12 and 13 are the sensitivity and responsiveness of a single-layer type probe, respectively. Line diagram showing, 1st
Figure 4 is a diagram showing the sensitivity and response characteristics of a two-layer type probe, and Figures 15 and 16 are diagrams showing the sensitivity and response characteristics of a three-layer type probe according to the present invention. Figures 17 to 20 are diagrams comparing received waveforms and measured waveforms obtained by analyzing two-layer type probes.
Figures 1 and 22 are diagrams showing received waveforms obtained by analyzing the optimized three-layer type probe according to the present invention, and Figures 23 and 24 are diagrams showing the acoustic impedance of the transducer. Figures 25 and 26 are lines showing the received waveforms obtained by analyzing the conventional three-layer type probe. figure,
FIG. 27 is a diagram showing received waveforms obtained by analyzing the three-layer type probe of the present invention. Explanation of symbols of main parts: 10...Transducer as a piezoelectric body, 24, 26...Electrode, 42...First acoustic matching layer, 44...Second acoustic matching layer, 46...Third acoustic matching layer.

Claims (1)

[Scope of Claims] 1. An ultrasonic probe having a pair of electrodes formed on both sides of a piezoelectric body and an acoustic matching layer provided on one of the pair of electrodes, comprising: The body is a ceramic piezoelectric material based on lead zirconium titanate, and the acoustic matching layer has an acoustic impedance of 7 to 7.
11X10 ⁶ Kg/m ²・s first matching material, 2.0~
4.0X10 ⁶ Kg/m ²・s second matching material, 1.6~
An ultrasonic probe characterized in that it has a three-layer structure in which a third matching material having a yield of 2.0X10 ⁶ Kg/m ² ·s is sequentially laminated.