JP2007289283A - Ultrasonic probe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultrasonic probe having a piezoelectric element a backing layer, and two layers of acoustic alignment layers which turns a frequency characteristic into a broadband. <P>SOLUTION: The ultrasonic probe includes a piezoelectric substance formed of a material having an acoustic impedance Z<SB>0</SB>and electrodes formed on both the face of the piezoelectric substance, a wave transducer having a mechanical resonance frequency f<SB>0</SB>, the backing layer formed of a material having an acoustic impedance Z<SB>B</SB>, a first acoustic alignment layer having an acoustic impedance Z<SB>1</SB>and a thickness M<SB>1</SB>times thicker than an alignment thickness λ<SB>1</SB>/4, and a second acoustic alignment layer having an acoustic impedance Z<SB>2</SB>and a thickness M<SB>2</SB>times thicker than an alignment thickness λ<SB>2</SB>/4, and Z<SB>B</SB>, Z<SB>1</SB>, Z<SB>2</SB>, M<SB>1</SB>, and M<SB>2</SB>satisfy 2<M<SB>1</SB>+M<SB>2</SB>≤2+0.68(Z<SB>B</SB>/Z<SB>0</SB>), 4≤Z<SB>1</SB>≤22, 2≤Z<SB>2</SB>≤8, and 5<Z<SB>B</SB>. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention includes a piezoelectric vibrator that transmits and receives ultrasonic waves, a backing layer for damping unnecessary vibrations, and an acoustic matching layer for matching acoustic impedance, and includes a medical ultrasonic diagnostic apparatus and the like The present invention relates to an ultrasonic probe used in the above.

  In the medical field, various imaging techniques have been developed in order to observe and diagnose the inside of a subject. In particular, ultrasonic imaging that acquires internal information of a subject by transmitting and receiving ultrasonic waves enables real-time image observation, and other medical uses such as X-ray photographs and RI (radio isotope) scintillation cameras. Unlike imaging technology, there is no radiation exposure. Therefore, ultrasonic imaging is used as a highly safe imaging technique in a wide range of areas including gynecological system, circulatory system, digestive system, etc. in addition to fetal diagnosis in the obstetrics field. Ultrasound imaging is an image generation technology that uses the property that ultrasound is reflected at the boundary between regions with different acoustic impedances (for example, the boundary between structures), and transmits an ultrasonic beam into a subject such as a human body. Then, by receiving the ultrasonic echo generated in the subject and obtaining the reflection point and reflection intensity at which the ultrasonic echo is generated, the outline of the structure (eg, viscera or lesion tissue) existing in the subject is obtained. To extract.

  In an apparatus for performing such ultrasonic imaging (referred to as an ultrasonic diagnostic apparatus, an ultrasonic imaging apparatus, or the like), PZT (lead zirconate titanate: Pb (lead)) is used as an ultrasonic transducer for transmitting and receiving ultrasonic waves. Generally, vibrators (piezoelectric vibrators) with electrodes formed on both sides of piezoelectric ceramics such as piezoelectric ceramics represented by zirconate titanate and polymer piezoelectric materials represented by PVDF (polyvinyliden difluoride) are common. It is used for.

  When a voltage is applied to the electrodes of the vibrator, the piezoelectric body expands and contracts due to the piezoelectric effect, and ultrasonic waves are generated. Therefore, an ultrasonic beam transmitted in a desired direction can be formed by arranging a plurality of transducers in a one-dimensional or two-dimensional manner and sequentially driving the transducers. The vibrator expands and contracts by receiving propagating ultrasonic waves to generate an electrical signal. This electric signal is used as an ultrasonic reception signal.

By the way, the propagation efficiency of the ultrasonic wave in the boundary surface where different substances contact changes according to the acoustic impedance of those substances. Specifically, since ultrasonic waves are easily reflected at the boundary surface where the difference in acoustic impedance is large, the propagation loss of ultrasonic waves increases. Here, the acoustic impedance is a material-specific constant represented by the product of the acoustic medium density and the sound velocity, and MRayl (megarail) is generally used as the unit (1 MRayl = 1 × 10 ⁶ kg · m- ² · s- ¹ ). For example, the acoustic impedance of a piezoelectric ceramic such as PZT is about 23 MRayl to about 35 MRayl, and the human body has an acoustic impedance of about 1.5 MRayl.

Reflectance R _P of ultrasonic wave in the boundary surface between the substance on the acoustic impedance Z _M of the acoustic impedance Z ₀ is expressed by the following equation (1).
R _P = (Z ₀ −Z _M ) / (Z ₀ + Z _M ) (1)
Therefore, when the acoustic impedance Z ₀ = 35 MRayl of a general piezoelectric ceramic and the acoustic impedance Z _M = 1.5 MRayl of a living body are substituted into the equation (1), the reflectance becomes R _P = 0.92. That is, when using a piezoelectric ceramic as the material of the vibrator, it can be seen that even if the vibrator is brought into direct contact with the human body, the ultrasonic wave does not propagate so much (less than 10%).

  For this reason, acoustic impedance matching is usually achieved by inserting an acoustic matching layer between the transducer and the subject. That is, by changing the acoustic impedance stepwise from the transducer toward the subject, the reflectance of the ultrasonic wave at each boundary surface is lowered. The acoustic matching layer theoretically has a multilayer structure, which can further improve the propagation efficiency. However, generally, about one to three layers are provided for reasons of design.

  In order to increase the propagation efficiency of ultrasonic waves, it is necessary to set an optimum acoustic impedance and thickness for each acoustic matching layer. For example, Non-Patent Document 1 discloses an optimal acoustic impedance obtained based on energy transmission theory.

  In Non-Patent Document 2, in consideration of reflection of ultrasonic waves at the interface of the acoustic matching layers, the thickness of the acoustic matching layers is set to ¼ of the wavelength of the ultrasonic waves propagating through the respective acoustic matching layers. Is disclosed. The reason is as follows. Even if the vibrator is excited by a drive signal having an ideal impulse waveform, due to the influence of the frequency band characteristic of the vibrator, the transmitted ultrasonic wave shows a waveform that continues while the vibration is attenuated. Therefore, the resolution in the propagation direction of ultrasonic waves (that is, distance resolution) is not very good. Here, since the ultrasonic waves are reflected at the interface of the acoustic matching layer, the phase of the ultrasonic waves reciprocating in the acoustic matching layer is compared with that of the direct wave (the ultrasonic wave propagated from the transducer or the lower acoustic matching layer). Therefore, the wavelength is shifted by 1/2 (half wavelength). Therefore, when the thickness of the acoustic matching layer is set to ¼ of the wavelength, the waveform after the half wavelength of the direct wave is suppressed by the reflected wave due to the superposition of the reflected wave and the direct wave. Thereby, the waveform of the direct wave can be brought close to an ideal impulse waveform.

As a related technique, Patent Document 1 discloses that in an ultrasonic probe in which two acoustic matching layers are provided on a piezoelectric body, the thickness of the first acoustic matching layer formed on the piezoelectric body is t ₁ , the thickness of the second acoustic matching layer formed on the first acoustic matching layer is t _2, also the longitudinal wave speed of sound in the material of first acoustic matching layer and v _1, the material of the second acoustic matching layer When the velocity of the longitudinal wave is v ₂ and the mechanical resonance frequency of the piezoelectric body used in the ultrasonic probe is f ₀ , the thicknesses of the first and second acoustic matching layers are (v ₁ / 4f ₀ ). × 1.15 ≦ t ₁ ≦ (v ₁ / 4f ₀ ) × 1.20 and (v ₂ / 4f ₀ ) × 1.15 ≦ t ₂ ≦ (v ₂ / 4f ₀ ) × 1.20 Is disclosed. That is, in Patent Document 1, in order to reduce loss and loss fluctuation near the center frequency and realize high sensitivity and high resolution, each acoustic matching is compared with the theoretical value described in Non-Patent Document 2. The layer thickness is increased (eg 1.15 to 1.20 times relative to λ / 4).

  By the way, ultrasonic waves have characteristics such as deep reachability in a low frequency band and high resolution in a high frequency band. For this reason, usually, multiple types of ultrasonic probes with different center frequency bands are prepared, and these are used in accordance with the diagnosis target, so that image generation utilizing such ultrasonic characteristics is possible. Has been done. However, when using an endoscope-type probe inserted into the digestive organ or a catheter-type probe inserted into a blood vessel, the probe is inserted into or extracted from the subject (human body). Therefore, it is a great physical burden on the subject to perform such work many times. Therefore, in particular, these probes are required to have a performance capable of covering a wide frequency band from a low frequency to a high frequency so that a lot of ultrasonic image information can be obtained without exchanging the probes. Also in a general ultrasonic probe, the image quality of the obtained ultrasonic image can be improved by widening the frequency characteristics.

However, even if the propagation efficiency of ultrasonic waves is improved by providing an acoustic matching layer, there is a problem that the frequency band is narrowed. Here, the frequency bandwidth f _BW (%) is expressed by the following equation (2). In Expression (2), the frequencies f _H and f _L are two frequencies at which the sound pressure is attenuated by 6 dB from the peak value (f _L <f _H ), and the frequency f _C is the frequency f _H and the frequency f _L. Center frequency.
f _BW (%) = 100 × (f _H −f _L ) / f _C (2)

For example, a piezoelectric ceramic (acoustic impedance of about 35 Mrayl, the electromechanical coupling constant of about _K 33 = 0.7) and the backing layer to the ultrasonic probe and a (acoustic impedance of about 3.5MRayl～6.0MRayl) When the first acoustic matching layer (acoustic impedance is about 8.92 MRayl) and the second acoustic matching layer (acoustic impedance is about 2.34 MRayl) are provided, as described in Non-Patent Document 2. When the thickness of each acoustic matching layer is designed to be ¼ of the wavelength, the frequency bandwidth f _BW (%) is about 60% to 65%.
As described above, the condition of the acoustic matching layer that maximizes the propagation efficiency of ultrasonic waves is not necessarily suitable for realizing a broadband frequency characteristic.
JP 58-177640 A (first page) “Ultrasonic Handbook”, Maruzen, p. 116 Masayasu Ito, Tsuyoshi Mochizuki “Ultrasound Diagnostic Device”, Corona, p. 30

  Therefore, in view of the above points, the present invention includes a piezoelectric vibrator that transmits and receives ultrasonic waves, a backing layer for damping unnecessary vibrations, and two acoustic matching layers for matching acoustic impedance. The purpose of the ultrasonic probe is to broaden the frequency characteristics.

To solve the above problems, an ultrasonic probe according to one aspect of the present invention includes electrodes formed on both surfaces of the piezoelectric body and the piezoelectric body is formed of a material having an acoustic impedance Z ₀ A vibrator that generates a voltage by transmitting an ultrasonic wave according to an applied voltage and receiving the ultrasonic wave, the vibrator having a mechanical resonance frequency f _0, and a first of the vibrator on the main surface, and a backing layer formed of a material having an acoustic impedance Z _B, on the second major surface of the vibrator, the first being formed by a first material having an acoustic impedance Z ₁ an acoustic matching layer, a first acoustic matching layer having a first material and the mechanical resonance frequency f ₀ thickness of M _1/4 times the ultrasonic wavelength lambda ₁ determined based on of, said On one acoustic matching layer A second acoustic matching layer formed of a second material having impedance Z ₂ , wherein M ₂ / with respect to the wavelength λ _{2 of the} ultrasonic wave determined based on the second material and the mechanical resonance frequency f ₀ A second acoustic matching layer having a thickness of 4 times, and the above Z _B , Z ₁ , Z ₂ , M ₁ , and M ₂ are 2 <M ₁ + M ₂ ≦ 2 + 0.68 (Z _B / Z ₀ ), and 4 ≦ Z ₁ ≦ 22, ₂ ≦ Z ₂ ≦ 8, and 5 <Z _B.

  According to the present invention, the acoustic impedance of the backing layer and the thickness and acoustic impedance of the two acoustic matching layers are set in consideration of the frequency characteristics of the ultrasonic wave, so that an ultrasonic probe capable of transmitting and receiving broadband ultrasonic waves is set. It becomes possible to obtain a tentacle.

Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings. The same constituent elements are denoted by the same reference numerals, and the description thereof is omitted.
FIG. 1 is a partial cross-sectional perspective view showing the structure of an ultrasonic probe according to an embodiment of the present invention. As shown in FIG. 1, the ultrasonic probe 1 includes a piezoelectric body 11 and electrodes 12 and 13 formed on both surfaces thereof, a first acoustic matching layer 14, a second acoustic matching layer 15, and the like. , A backing layer 16. Further, the ultrasonic probe 1 may include a filler 17 and an acoustic lens 18 in addition to these components.

The piezoelectric body 11 and the electrodes 12 and 13 constitute a vibrator (piezoelectric vibrator) that expands and contracts due to a piezoelectric effect when a voltage is applied to generate ultrasonic waves. The piezoelectric body 11 is made of piezoelectric ceramic. This is because piezoelectric ceramics have a high electrical / mechanical energy conversion capability, and therefore can generate powerful ultrasonic waves that can reach deep inside the body and have high reception sensitivity. Specific materials include PZT (lead zirconate titanate: Pb (Ti, Zr) O ₃ ), a metamorphic material having a similar perovskite crystal structure, PMN-PT (Pb (Mg _1/3). Nb _2/3 ) O ₃ —PbTiO ₃ solid solution), relaxer-based materials including PNN-PT (Pb (Ni _1/3 Nb _2/3 ) O ₃ —PbTiO solid solution), and the like are used.

In the following, the acoustic impedance of the piezoelectric layer 11 is Z _0, and the mechanical resonance frequency of the vibrator including the piezoelectric layer 11 and the electrode layers 12 and 13 is f ₀ . In the present embodiment, a piezoelectric material having an acoustic impedance Z ₀ of, for example, about 23 to 35 MRayl is used as the piezoelectric layer 11. Further, the vibrators 11 to 13 are designed so that the mechanical resonance frequency f ₀ is, for example, about 3.5 MHz to 12 MHz.

The first and second acoustic matching layers 14 and 15 are disposed on the top surfaces of the transducers 11 to 13, and by matching the acoustic impedance between the transducer and a subject such as a human body, Increase propagation efficiency.
The first acoustic matching layer 14 is made of, for example, quartz glass or organic material (epoxy resin, urethane resin, silicon resin, acrylic resin, etc.) mixed with a material powder (tungsten, ferrite powder, etc.) having high acoustic impedance. It is made of material. In the following, the acoustic impedance of the acoustic matching layer 14 and Z _1.

The thickness t ₁ of the acoustic matching layer 14, as _one wavelength of the ultrasonic wave lambda propagating the acoustic matching layer 14 is designed such that the M ₁ times the matching thickness lambda _1/4 (i.e., _{t 1 = M 1 · λ 1} /4). Here, the matching thickness is a thickness of ¼ of the wavelength of the ultrasonic wave propagating through the acoustic matching layer. The reason why the matching thickness is used as a reference in the present application is that, in a general ultrasonic probe, the acoustic matching layer is designed to have this thickness (see Non-Patent Document 2). In the following, the ratio M ₁ of the thickness t ₁ for matching thickness lambda _1/4, also referred to as matching ratio.

The wavelength λ _i of the ultrasonic wave propagating through the acoustic matching layer is expressed by the following equation (3) using the sound velocity v and the frequency f propagating through the medium (material of the acoustic matching layer).
λ _i = v / f (3)
The sound velocity v is expressed by the following equation (4), where K is the bulk modulus of the medium and ρ is the density.
v = (K / ρ) ^1/2 (4)
After all, from the equations (3) and (4), the wavelength λ _i is expressed by the following equation (5).
λ _i = (K / ρ) ^1/2 / f (5)

Therefore, the wavelength λ ₁ of the ultrasonic wave propagating through the acoustic matching layer 14 is expressed by the equation (5) and the vibrators 11 to 13, where K ₁ is the bulk modulus of the material constituting the acoustic matching layer 14 and ρ ₁ is the density. using the mechanical resonance frequency f _0, it is represented by the following equation.
λ ₁ = (K ₁ / ρ ₁ ) ^1/2 / f ₀

The second acoustic matching layer 15 is formed of a material having an acoustic impedance lower than that of the first acoustic matching layer 14, such as an organic material (epoxy resin, urethane resin, silicon resin, acrylic resin, etc.). . In the following, the acoustic impedance of the acoustic matching layer 15 and Z _2.

The thickness t ₂ of the acoustic matching layer 15, a ₂ wavelength of the ultrasonic wave lambda propagating the acoustic matching layer 15 is designed to be _twice M matching thickness lambda _2/4 (i.e., _{t 2 = M 2 · λ 2} /4). The wavelength λ ₂ is expressed by the following equation using the equation (5) and the mechanical resonance frequency f ₀ of the vibrators 11 to 13, where K ₂ is the bulk modulus of the material constituting the acoustic matching layer 15 and ρ ₂ is the density. It is represented by
λ ₂ = (K ₂ / ρ ₂ ) ^1/2 / f ₀
The conditions of the acoustic impedances Z ₁ and Z ₂ and the matching ratios M ₁ and M ₂ will be described in detail later.

The backing layer 16 is disposed on the lower surface of the transducers 11 to 13 (that is, the surface opposite to the acoustic matching layers 14 and 15), and attenuates unnecessary ultrasonic waves generated from the transducers 11 to 13. The backing layer 16 is formed of a material having a large acoustic attenuation, such as an epoxy resin or rubber. In the following, the acoustic impedance of the backing layer 16 and _{Z B.}

  The filler 17 reduces the interference between the plurality of piezoelectric vibrators and suppresses the lateral vibrations of the vibrators 11 to 13 so that the vibrators 11 to 13 vibrate in the vertical direction. The acoustic lens 18 converges the ultrasonic wave so as to form a focal point at a desired depth.

  In the present embodiment, in the ultrasonic probe having such a configuration, the backing layer 16 and the two acoustic matching layers 14 and 15 are provided as follows in order to realize a wide frequency characteristic of ultrasonic waves. Designed using methods. In the following, a case where the acoustic lens 18 is not arranged will be described.

  First, a method for calculating the frequency characteristics of the ultrasonic probe will be described. By replacing each component included in the ultrasonic probe with an equivalent four-terminal circuit, the ultrasonic transmission system can be divided into four layers in which a backing layer, a piezoelectric vibrator, and an acoustic matching layer are connected in series. Think of it as a terminal network. Each component has a specific acoustic impedance, and a specific frequency characteristic is generated when transmitting or receiving an ultrasonic wave via an ultrasonic transmission system. Therefore, by inputting a voltage to one end of the four-terminal network and terminating the other end by an equivalent circuit of the subject, the oscillation performance of the ultrasonic probe is determined by a transmission / reception characteristic VTG (Voltage Transfer Gain). Can be calculated as

ここで、

である。また、Ｋは電気機械結合定数を表し、Ｚ_０は圧電振動子の音響インピーダンスを表し、Ｚ_Ａは音響整合層の音響インピーダンスを表し、Ｚ_Ｂはバッキング層の音響インピーダンスを表し、ｆ_０は圧電振動子の機械共振周波数を表し、ω_０は２πｆ_０を表し、Ｃ_０は容量値を表している。さらに、ｋ_Ａ、ｋ_Ｂは、圧電振動子の厚さに対する音響整合層の厚さ及びバッキング層の厚さからそれぞれ決定されるパラメータである。 The input impedance Z of the transmission system is expressed by the following equation (6).

here,

It is. K represents the electromechanical coupling constant, Z ₀ represents the acoustic impedance of the piezoelectric vibrator, Z _A represents the acoustic impedance of the acoustic matching layer, Z _B represents the acoustic impedance of the backing layer, and f ₀ represents the piezoelectric impedance. The mechanical resonance frequency of the vibrator is represented, ω ₀ represents 2πf ₀ , and C ₀ represents a capacitance value. Further, k _A and k _B are parameters respectively determined from the thickness of the acoustic matching layer and the thickness of the backing layer with respect to the thickness of the piezoelectric vibrator.

  In order to perform a simulation in which this equivalent circuit is applied to an acoustic matching layer having a two-layer structure, an ultrasonic transmission system includes a backing layer, a piezoelectric vibrator, a first acoustic matching layer, and a second acoustic matching layer in series. Think of it as a connected four-terminal network. In this transmission model, assuming that the transmission sound pressure is equal to the reception sound pressure, the signal transfer function T is obtained based on the input impedance of the transmission system and the constants of the four-terminal network. By calculating 20 · log (T) at each frequency with respect to this signal transfer function T, the transmission / reception characteristic VTG is obtained.

Using such a frequency characteristic calculation method, the following five parameters are changed to obtain the frequency characteristic of the ultrasonic wave generated from the ultrasonic probe by simulation.
(i) Acoustic impedance Z _B of the backing layer 16 (for example, Z _B = 5 MRayl, 10 MRayl, 15 MRayl, 20 MRayl)
(ii) the acoustic impedance _Z 1 of the acoustic matching layer _{14 (4MRayl ≦ Z 1 ≦ 30MRayl} )
(ii) Acoustic impedance Z ₂ of the acoustic matching layer 15 (1 MRayl ≦ Z ₂ ≦ 8 MRayl)
(iv) Thickness t ₁ of the acoustic matching layer 14 (or matching ratio M ₁ , 0.4 ≦ M ₁ ≦ 1.6)
(v) Thickness t ₂ of the acoustic matching layer 15 (or matching ratio M ₂ , 0.4 ≦ M ₂ ≦ 1.6)

2 to 11 show the results of such a simulation. In each figure, each column identified by matching ratios M ₁ and M _2, 2 one acoustic matching layer 14 and 15 of the matching ratio when the M ₁ and M _2, respectively, to obtain the best bandwidth The possible combinations of acoustic impedances Z ₁ and Z ₂ and their bandwidth (%) are shown. For example, in the lower left column (M ₁ = 0.4, M ₂ = 0.4) shown in FIG. 3, the matching ratio of the acoustic matching layer 14 is M ₁ = 0.4, and the acoustic impedance is Z ₁ = 4 MRayl to 6 MRayl. When the matching ratio of the acoustic matching layer 15 is M ₂ = 0.4 and the acoustic impedance is Z ₂ = 7 MRayl to 8 MRayl, a maximum frequency bandwidth of 50% or more is obtained.

2 and 3 show the simulation results when the acoustic impedance of the backing layer 16 is Z _B = 5 MRayl.
In this case, when the matching ratio of the two acoustic matching layers 14 and 15 is (M ₁ , M ₂ ) = (1.0, 1.0) (within the thick frame shown in FIG. 2), Z ₁ = 20 MRayl to 24 MRayl, and a maximum frequency bandwidth of 80% or more was obtained in the vicinity of Z ₂ = 4 MRayl to 5 MRayl. However, with respect to the other ranges, only a bandwidth of 70% or more was obtained in a narrow range (for example, when (M ₁ , M ₂ ) = (1.2, 1.0) ( 12 ≦ Z ₁ ≦ 14, Z ₂ = 3), (M ₁ , M ₂ ) = (1.2,1.2) (12 ≦ Z ₁ ≦ 18, 3 ≦ Z ₂ ≦ 4) Near, when (M ₁ , M ₂ ) = (1.2, 1.0) (near 12 ≦ Z ₁ ≦ 14, Z ₂ = 3).

4 and 5 show the simulation results when the acoustic impedance of the backing layer 16 is Z _B = 10 MRayl.
In this case, the matching ratios are (M ₁ , M ₂ ) = (0.8, 0.8), (0.8, 1.0), (1.0, 0.6), (1.0 , 0.8), (1.2, 0.6), (1.2, 1.2), a maximum frequency bandwidth of 80% or more was obtained. From this, it was found that a relatively high frequency bandwidth can be obtained even when the matching ratio is shifted from (M ₁ , M ₂ ) = (1.0, 1.0). Further, when the matching ratio is (M ₁ , M ₂ ) = (1.2, 1.0), (1.2, 1.2), (1.0, 1.2) (shown in FIG. 4) In the thick frame), a frequency band of 70% or more or 80% or more was obtained in a relatively grouped region.

6 to 8 show simulation results when the acoustic impedance of the backing layer 16 is Z _B = 15 MRayl.
In this case, the matching ratio is (M ₁ , M ₂ ) = (0.4, 1.6), (0.6, 1.4 to 1.6), (0.8, 1.2 to 1). .4), (1.0, 1.0-1.4), (1.2, 0.8-1.0), (1.4, 0.6-0.8), (1.6 , 0.6), a frequency bandwidth of 90% or more was obtained. Among them, the matching ratio is (M ₁ , M ₂ ) = (0.6, 1.6), (0.8, 1.4), (1.0, 1.2), (1.2 , 1.0), (1.4, 0.8), (1.6, 0.6) (within the thick frame shown in FIGS. 6 to 8), a frequency bandwidth of 100% or more is obtained. I was able to get it. In these cases, a frequency band of approximately 70% or more is obtained in a wide range of combinations of acoustic impedances (Z ₁ , Z ₂ ), and 80% or more in a wide range.

9 to 11 show simulation results when the acoustic impedance of the backing layer 16 is Z _B = 20 MRayl.
In this case, the matching ratio is (M ₁ , M ₂ ) = (0.4, 1.6), (0.6, 1.0 to 1.6), (0.8, 1.2 to 1). .6), (1.0, 1.0-1.6), (1.2, 0.8-1.6), (1.4, 0.6-1.0), (1.6 , 0.4 to 0.8), a frequency bandwidth of 90% or more was obtained. Among them, the matching ratio is (M ₁ , M ₂ ) = (0.6, 1.6), (0.8, 1.4 to 1.6), (1.0, 1.2), When (1.2, 1.0), (1.4, 0.8), (1.6, 0.6 to 0.8) (in the thick frame shown in FIGS. 9 to 11), A frequency bandwidth of 100% or more could be obtained. In these cases, a frequency band of approximately 70% or more is obtained in a wide range of combinations of acoustic impedances (Z ₁ , Z ₂ ), and 80% or more in a wide range.

Next, based on the results shown in FIGS. 4 to 11, conditions necessary for obtaining a good frequency bandwidth were obtained. In the following, a case where a frequency band of approximately 80% or more (partially 70% or more) can be obtained in a relatively wide range is defined as a relatively good frequency bandwidth, and acoustic impedances Z ₁ and Z ₂ are taken. The case where the obtained ranges are ΔZ ₁ ≧ 8 (MRayl) and ΔZ ₂ ≧ 3 (MRayl) is a relatively wide range.

From FIG. 4 to FIG. 11, it is considered that the combination of the matching ratios M ₁ and M ₂ that can obtain a relatively good frequency band has a relationship in which the matching ratio of the other decreases as the matching ratio of one increases. As a result of analysis, the relationship shown in the following formula (7) was found. Equation (7) is normalized by the acoustic impedance Z ₀ of the piezoelectric layer 11.
_{2 <M 1 + M 2 ≦} 2 + 0.68 (Z B / Z 0) ... (7)

Furthermore, an analysis on the acoustic impedances Z ₁ and Z ₂ in which a relatively good frequency band is obtained within the range of the matching ratio combination (M ₁ , M ₂ ) that satisfies Expression (7) is as follows: The relationship shown in 8) was obtained. Equation (8) is normalized by the acoustic impedance Z ₀ of the piezoelectric layer 11.

Therefore, an experiment was conducted in which a parameter for satisfying conditional expressions (7) and (8) was selected to actually produce an ultrasonic probe, and frequency characteristics were measured. In this experiment, each parameter was set as follows.
Transducer: Acoustic impedance Z ₀ = 34 MRayl
Backing layer: acoustic impedance Z _B = 20 MRayl
First acoustic matching layer: acoustic impedance Z ₁ = 8 MRayl, matching ratio M ₁ = 0.8
Second acoustic matching layer: acoustic impedance Z ₂ = 4 MRayl, matching ratio: M ₂ = 1.4

  FIG. 12 shows the experimental results. In FIG. 12, the horizontal axis indicates the frequency (MHz), and the vertical axis indicates the intensity (arbitrary unit) of the transmission ultrasonic wave. As shown in FIG. 12, in this ultrasonic probe, a wide frequency band extending from about 3 MHz to about 10 MHz could be obtained. The frequency bandwidth of this ultrasonic probe was 110% of the actual measurement value with respect to the design value of 100%.

  As described above, according to this embodiment, when the acoustic matching layer has a two-layer structure, the acoustic impedance of the backing layer, the thickness of the first acoustic matching layer and the second acoustic matching layer, and the acoustic By combining the impedance appropriately, it is possible to realize a broadband ultrasonic probe.

  In the present embodiment, as shown in FIG. 1, a linear array probe in which a plurality of transducers are arranged in a line on a plane has been described. However, in the present invention, transducers are arranged on a convex surface. A convex array probe, an annular array probe in which annular transducers are arranged concentrically, a two-dimensional array probe in which a plurality of transducers are arranged in two dimensions, The present invention can be applied to various probes such as an intracavity probe that performs radial scanning in a body cavity.

  The present invention includes a piezoelectric vibrator that transmits and receives ultrasonic waves, a backing layer for damping unnecessary vibrations, and an acoustic matching layer for matching acoustic impedance, and includes a medical ultrasonic diagnostic apparatus and the like It can be used in an ultrasonic probe used in the above.

It is a partial cross section perspective view which shows the structure of the probe for ultrasonic waves which concerns on one Embodiment of this invention. Acoustic impedance of the backing layer is a diagram showing simulation results of transducing characteristics in the case of _{Z B = 5MRayl (M 2 =} 1.6~1.0). Acoustic impedance of the backing layer is a diagram showing simulation results of transducing characteristics in the case of _{Z B = 5MRayl (M 2 =} 0.8~0.4). Acoustic impedance of the backing layer is a diagram showing simulation results of transducing characteristics in the case of _{Z B = 10MRayl (M 2 =} 1.6~1.0). Acoustic impedance of the backing layer is a diagram showing simulation results of transducing characteristics in the case of _{Z B = 10MRayl (M 2 =} 0.8~0.4). Acoustic impedance of the backing layer is a diagram showing simulation results of transducing characteristics in the case of _{Z B = 15MRayl (M 2 =} 1.6~1.2). Acoustic impedance of the backing layer is a diagram showing simulation results of transducing characteristics in the case of _{Z B = 15MRayl (M 2 =} 1.0~0.8). Acoustic impedance of the backing layer is a diagram showing simulation results of transducing characteristics in the case of _{Z B = 15MRayl (M 2 =} 0.6~0.4). Acoustic impedance of the backing layer is a diagram showing simulation results of transducing characteristics in the case of _{Z B = 20MRayl (M 2 =} 1.6~1.4). Acoustic impedance of the backing layer is a diagram showing simulation results of transducing characteristics in the case of _{Z B = 20MRayl (M 2 =} 1.2~1.0). Acoustic impedance of the backing layer is a diagram showing simulation results of transducing characteristics in the case of _{Z B = 20MRayl (M 2 =} 0.8~0.4). It is a figure which shows the frequency characteristic of the probe for ultrasonics produced based on the parameter which satisfy | fills Formula (7) and (8).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Probe for ultrasonic waves 11 Piezoelectric body 12, 13 Electrode 14 1st acoustic matching layer 15 2nd acoustic matching layer 16 Backing layer 17 Filler 18 Acoustic lens

Claims (3)

Includes a piezoelectric body and both sides are formed the electrodes of the piezoelectric body is formed of a material having an acoustic impedance Z _0, in accordance with the applied voltage transmits ultrasonic waves, a voltage by receiving the ultrasonic waves A vibrator having a mechanical resonance frequency f ₀ ,
A backing layer formed of a material having an acoustic impedance Z _B on the first main surface of the vibrator;
On the second major surface of the vibrator, a first acoustic matching layer which is formed by a first material having an acoustic impedance Z _1, said first material and said mechanical resonant frequency f ₀ said first acoustic matching layer having a M _1/4 times the thickness with respect to the ultrasonic wavelength lambda _1, which is determined,
The first acoustic matching layer, a second acoustic matching layer formed by a second material having an acoustic impedance Z _2, on the basis of the second material and the mechanical resonance frequency f ₀ said second acoustic matching layer having a M _2/4 times the thickness of the ultrasound wavelength lambda ₂ determined,
Comprising
Z _B , Z ₁ , Z ₂ , M ₁ and M ₂ are
2 <M ₁ + M ₂ ≦ 2 + 0.68 (Z _B / Z ₀ )
and,
4 ≦ Z ₁ ≦ 22, ₂ ≦ Z ₂ ≦ 8, 5 <Z _B
Satisfies the ultrasonic probe. Includes a piezoelectric body and both sides are formed the electrodes of the piezoelectric body is formed of a material having an acoustic impedance Z _0, in accordance with the applied voltage transmits ultrasonic waves, a voltage by receiving the ultrasonic waves A vibrator having a mechanical resonance frequency f ₀ ,
A backing layer formed of a material having an acoustic impedance Z _B on the first main surface of the vibrator;
On the second major surface of the vibrator, a first acoustic matching layer which is formed by a first material having an acoustic impedance Z _1, said first material and said mechanical resonant frequency f ₀ said first acoustic matching layer having a M _1/4 times the thickness with respect to the ultrasonic wavelength lambda _1, which is determined,
The first acoustic matching layer, a second acoustic matching layer formed by a second material having an acoustic impedance Z _2, on the basis of the second material and the mechanical resonance frequency f ₀ said second acoustic matching layer having a M _2/4 times the thickness of the ultrasound wavelength lambda ₂ determined,
Comprising
Z _B , Z ₁ , Z ₂ , M ₁ and M ₂ are
2 <M ₁ + M ₂ ≦ 2 + 0.68 (Z _B / Z ₀ )
and,

Satisfies the ultrasonic probe.   The ultrasonic probe according to claim 1, wherein the piezoelectric body is made of piezoelectric ceramics.

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