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DESCRIPTION JP2007288397

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DESCRIPTION JP2007288397
In an ultrasonic probe having an acoustic matching layer having a two-layer structure, the
frequency characteristics are made wider than before by reviewing the ideal value of the acoustic
impedance in each acoustic matching layer. An ultrasonic probe includes a piezoelectric body 11
formed of a piezoelectric ceramic and electrodes 12 and 13 formed at both ends of the
piezoelectric body, and transmits an ultrasonic wave according to an applied voltage. A first
acoustic matching device having an acoustic impedance that is formed on a main surface of the
vibrator and receives an ultrasonic wave to generate a voltage, and the ratio of the vibrator to the
acoustic impedance is 0.265 to 0.294 A layer 14 and a second acoustic matching layer 15
formed on the first acoustic matching layer and having an acoustic impedance whose ratio to the
acoustic impedance of the vibrator is 0.069 to 0.079. [Selected figure] Figure 1
Ultrasonic transducer
[0001]
The present invention relates to an ultrasonic probe having a piezoelectric vibrator for
transmitting and receiving ultrasonic waves and an acoustic matching layer for matching
acoustic impedance, and used in a medical ultrasonic diagnostic apparatus or the like.
[0002]
Conventionally, piezoelectric transducers such as PZT (lead zirconate titanate: Pb (lead) zirconate
titanate) or PVDF (polyvinylidene fluoride: polyvinylidene difluoride) have been used as
ultrasonic transducers for transmitting and / or receiving ultrasonic waves. A vibrator
(piezoelectric vibrator) in which electrodes are formed at both ends of a piezoelectric material
such as a polymeric piezoelectric material represented by the above is generally used.
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[0003]
When a voltage is applied to the electrodes of such a vibrator, the piezoelectric element expands
and contracts due to the piezoelectric effect to generate an ultrasonic wave.
Furthermore, ultrasonic beams can be formed in a desired direction by arranging a plurality of
transducers in a one-dimensional or two-dimensional manner and driving by a plurality of drive
signals given a predetermined delay.
On the other hand, the transducer expands and contracts by receiving the propagating ultrasonic
wave to generate an electrical signal. This electrical signal is used as a reception signal of
ultrasonic waves.
[0004]
The ultrasound diagnostic apparatus transmits an ultrasonic wave to a subject, receives a
reflected wave from the subject, and displays an image based on the received signal, thereby
examining organs and blood vessels in the body. . However, in the case of using a piezoelectric
ceramic for a vibrator, it is large between the acoustic impedance of the vibrator (represented by
the product of the acoustic medium density and the speed of sound) and the acoustic impedance
of the subject (human body etc.) At the interface where there is a difference and such an acoustic
impedance difference, the reflection of the ultrasonic wave occurs, resulting in the propagation
loss of the ultrasonic wave.
[0005]
The acoustic impedance is a constant specific to a substance expressed by the product of the
acoustic medium density and the speed of sound, and the unit thereof is generally MRayl (Mega
Rail), and 1 MRayl = 1 О 10 <6> kg иии It is m <-2> * s <-1>. The acoustic impedance of a common
piezoelectric ceramic is about 23 MRayl to about 35 MRayl, and the acoustic impedance of the
human body is about 1.5 MRayl.
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[0006]
When the piezoelectric vibrators are brought into direct contact with the human body, most of
the ultrasonic waves are reflected at the contact interface due to the difference in their acoustic
impedances. Assuming that the acoustic impedance of the piezoelectric vibrator is Z0 and the
acoustic impedance of the human body is ZM, the reflectance RP of the ultrasonic wave at the
contact interface is given by the following equation (1). RP = (Z0-ZM) / (Z0 + ZM) (1) If Z0 = 35
MRayl and ZM = 1.5 MRayl in the formula (1), then RP = 0.92, and ultrasonic waves do not
propagate even by 10%. I understand that.
[0007]
In order to solve this problem, an acoustic matching layer is inserted between the transducer and
the object to achieve matching of the acoustic impedance. Furthermore, by making the acoustic
matching layer into a multilayer structure, the propagation efficiency of ultrasonic waves is
improved. Here, it is possible to theoretically obtain the ideal value of the acoustic impedance in
each acoustic matching layer in order to maximize the propagation efficiency of ultrasonic waves.
Non-Patent Document 1 lists the optimal acoustic impedance obtained based on the energy
transfer theory (see FIG. 7).
[0008]
By providing the acoustic matching layer, the propagation efficiency of ultrasonic waves is
improved and the sensitivity is increased. However, there is also an adverse effect that the
oscillation frequency band is narrowed. Therefore, in accordance with Non-Patent Document 1,
the frequency bandwidth in an ultrasound probe having two acoustic matching layers is
calculated. Here, a general piezoelectric ceramic (about Z0 = 35 MRayl, an electromechanical
coupling constant K33 = about 70) and a general backing material (acoustic impedance is about
1.75 MRayl to about 20.0 MRayl) are used. Do.
[0009]
Generally, high acoustic impedance backing material is made by mixing high density powder with
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damping resin, but 20 MRayl is the upper limit of acoustic impedance in consideration of balance
with acoustic attenuation ability. It is considered a value. Further, from the viewpoint of holding
the piezoelectric vibrator, 1.75 MRayl when manufactured by using a resin material such as
epoxy-polystyrene resin is taken as the lower limit value of the acoustic impedance.
[0010]
Further, Non-Patent Document 2 discloses that the thickness of each acoustic matching layer is
set to 1?4 of the oscillation wavelength in view of the reflection of ultrasonic waves at the
interface of the acoustic matching layer. The same conditions apply.
[0011]
As a result, the frequency bandwidth fBW (%) is approximately 60% to 65%.
The frequency bandwidth fBW (%) is expressed by the following equation (2). fBW (%) = 100 О
(fH?fL) / fC (2) Here, the frequencies fH and fL are two frequencies at which the sound pressure
is attenuated by 6 dB from the peak value (fL <fH) and the frequency fC Is the center frequency
of the frequency fH and the frequency fL.
[0012]
Generally, ultrasound in the low frequency region has a low attenuation factor, so the image
obtained thereby is a high-intensity but coarse image, and ultrasound in the high frequency
domain has a high attenuation factor. Is a low but fine image. In order to make use of those
characteristics, an ultrasonic probe is selectively used depending on the subject of diagnosis.
[0013]
However, endoscope-type probes inserted into digestive organs and catheter-type probes
inserted into blood vessels are required to have the ability to cover a wide frequency range from
low frequency range to high frequency range. ing. Thereby, the number of times of inserting the
probe into the human body can be reduced. Thus, in the probe inserted into the human body, the
frequency bandwidth fBW (%) is desirably about 65% or more. Furthermore, also in a general
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ultrasonic probe, the image quality of the obtained ultrasonic image can be enhanced by realizing
the wide-band frequency characteristics.
[0014]
Under such circumstances, when setting the acoustic impedance of the acoustic matching layer,
the resulting frequency characteristics should also be considered. In the case where the acoustic
matching layer has a multilayer structure, the ideal value of the acoustic impedance in each
acoustic matching layer is set to maximize the propagation efficiency of ultrasonic waves.
However, the resulting frequency band is not necessarily suitable for achieving wide-band
frequency characteristics.
[0015]
As a related technology, Patent Document 1 below discloses an ultrasonic probe having a
plurality of passbands and the sum of the plurality of passbands effectively realizes broadband
characteristics and high sensitivity. ing. In this ultrasonic probe, two pass bands are realized by
making the resonance frequencies of the two transducers differ significantly.
[0016]
Further, Patent Documents 2 to 4 below disclose an ultrasonic probe having an acoustic
matching layer having a three-layer structure, and having an acoustic impedance in the acoustic
matching layer in a specific range. According to this ultrasonic probe, it has low-loss and lowripple, broader frequency band characteristics than ultrasonic probes having a two-layer acoustic
matching layer, and excellent ultrasonic pulse response characteristics. It can be realized.
[0017]
Furthermore, in Patent Document 5 below, the combination of the acoustic impedance of the
backing layer, the thickness of the acoustic matching layer, and the acoustic impedance can be
set appropriately to easily emit ultrasonic pulses of appropriate pulse width and amplitude. An
ultrasound probe is disclosed.
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[0018]
As described above, various techniques have been developed to realize wide-band frequency
characteristics in an ultrasonic probe.
However, in response to high demands in recent years, it is required to further broaden the
frequency characteristics of the ultrasonic probe. Japanese Patent Application Laid-Open No. 60113599 (Page 4, FIG. 7) Japanese Patent Application Laid-Open No. 60-113600 (Page 5, Page 7)
Japanese Patent Application Laid-Open No. 60-185499 (Page 7, FIG. 70) Sho 60-223 299 (page
7, FIG. 70, FIG. 71), JP-A 6-30933 (page 5, FIG. 3) "Ultrasonic Handbook", Maruzen, p. 116
Masaru Ito, Takeshi Mochizuki "Ultrasonic Diagnostic System", Corona, p. ??
[0019]
Then, in view of the above-mentioned point, in the probe for ultrasonic waves which has an
acoustic matching layer of 2 layer structure, the present invention is a frequency characteristic
than before by reviewing the ideal value of the acoustic impedance in each acoustic matching
layer Aims to broaden the bandwidth of
[0020]
In order to solve the above-mentioned subject, the probe for ultrasonic waves concerning the 1st
viewpoint of the present invention has the piezoelectric material formed of piezoelectric
ceramics, and the electrode formed in the both ends of this piezoelectric material, and is
impressed An ultrasonic wave is transmitted according to a voltage, a transducer that receives
the ultrasonic wave and generates a voltage, and an acoustic wave formed on the main surface of
the transducer and having a ratio of 0.265 to 0.294 to the acoustic impedance of the transducer
A first acoustic matching layer having an impedance, and a second acoustic matching layer
formed on the first acoustic matching layer, the second acoustic matching layer having an
acoustic impedance whose ratio to an acoustic impedance of the vibrator is 0.069 to 0.079
Equipped with
[0021]
The ultrasonic probe according to the second aspect of the present invention includes a
piezoelectric body formed of a piezoelectric ceramic and electrodes formed at both ends of the
piezoelectric body, and ultrasonic waves are generated according to an applied voltage. A
transducer that transmits and receives an ultrasonic wave and generates a voltage, and an
acoustic impedance formed on the main surface of the transducer, wherein the ratio of the
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transducer to the acoustic impedance is 0.309 to 0.338 And a second acoustic matching layer
formed on the first acoustic matching layer and having an acoustic impedance having a ratio of
0.062 to 0.088 to the acoustic impedance of the transducer.
[0022]
Furthermore, the ultrasonic probe according to the third aspect of the present invention includes
a piezoelectric body formed of a piezoelectric ceramic and electrodes formed at both ends of the
piezoelectric body, and ultrasonic waves are generated according to an applied voltage. A
transducer that transmits and receives ultrasonic waves and generates a voltage, and an acoustic
impedance that is formed on the main surface of the transducer and has a ratio of 0.353 to
0.456 to the acoustic impedance of the transducer. And a second acoustic matching layer formed
on the first acoustic matching layer and having an acoustic impedance having a ratio of 0.062 to
0.106 to the acoustic impedance of the vibrator.
[0023]
According to the present invention, by newly defining the range of the acoustic impedance of the
first and second acoustic matching layers suitable for broadening the frequency characteristics of
the ultrasonic probe, the frequency characteristics more than in the prior art Can be broadened.
[0024]
The best mode for carrying out the present invention will be described in detail below with
reference to the drawings.
The same reference numerals are given to the same components, and the description will be
omitted.
FIG. 1 is a perspective view showing an internal structure of an ultrasound probe according to an
embodiment of the present invention.
As shown in FIG. 1, the ultrasonic probe 1 has a piezoelectric body 11 that expands and contracts
by the piezoelectric effect to generate an ultrasonic wave, and an electrode 12 to which a voltage
for causing the piezoelectric body 11 to generate the piezoelectric effect is applied. And 13, the
first acoustic matching that improves the propagation efficiency of ultrasonic waves by matching
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the acoustic impedance between the piezoelectric vibrator configured by the piezoelectric body
11 and the electrodes 12 and 13 and the subject such as a human body. A layer 14 and a second
acoustic matching layer 15 and a backing layer 16 for attenuating unnecessary ultrasonic waves
generated from the piezoelectric body 11 are included.
[0025]
Furthermore, the ultrasonic probe 1 reduces interference between the plurality of piezoelectric
vibrators, suppresses lateral vibration of the piezoelectric vibrators, and causes the piezoelectric
vibrators to vibrate only in the vertical direction. And the filler 17 may be included.
The ultrasound probe 1 may include an acoustic lens 18 for converging the ultrasound, but in
the following, the case where the acoustic lens 18 is not provided will be described.
[0026]
In the present embodiment, a piezoelectric ceramic is used as a material of the piezoelectric body
11.
Piezoelectric ceramic has high electro-mechanical energy conversion capability, so it can
generate powerful ultrasonic waves that can reach deep in the body, and also has high reception
sensitivity. Specific materials include PZT (lead zirconate titanate: Pb (Ti, Zr) O3), materials of
modified composition having a similar perovskite-based crystal structure, and materials generally
referred to as relaxor-based materials, etc. Can be used.
[0027]
When a piezoelectric ceramic is used as the material of the piezoelectric body 11, there is a large
difference between the acoustic impedance of the piezoelectric vibrator and the acoustic
impedance of the subject (such as a human body). By providing an acoustic matching layer
having an acoustic impedance between them, it is necessary to match the acoustic impedance to
increase the propagation efficiency of the ultrasonic wave. Here, the more the acoustic matching
layer has a multilayer structure, the better the ultrasonic wave propagation efficiency, but from
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the manufacturing point of view, a two-layer structure or a three-layer structure is often used. In
the present invention, the material of the piezoelectric body is not limited to the piezoelectric
ceramic. For example, the present invention is also applied to the case where a material having a
small difference between the acoustic impedance of the piezoelectric vibrator and the acoustic
impedance of a subject (such as a human body) is used.
[0028]
In the present embodiment, as a material of the first acoustic matching layer 14, for example,
quartz glass or a material powder (Tungsten, or the like) having high acoustic impedance to an
organic material (epoxy resin, urethane resin, silicon resin, acrylic resin, etc.) The material which
mixed ferrite powder etc. can be used. As a material of the second acoustic matching layer 15, for
example, an organic material (epoxy resin, urethane resin, silicon resin, acrylic resin or the like)
can be used. Moreover, as a material of the backing layer 16, an epoxy resin, rubber, or the like,
which is a material having large acoustic attenuation, can be used.
[0029]
When setting the number of acoustic matching layers and the acoustic impedance, the resulting
frequency band characteristics are also important. In order to broaden the frequency
characteristics, the following design is used in this embodiment: The method is used.
[0030]
First, a method of calculating the frequency characteristic 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 a backing layer, a piezoelectric
vibrator, and an acoustic matching layer connected in series. Think of it as a terminal network.
Each component has its own acoustic impedance, and when transmitting or receiving ultrasound
via an ultrasound transmission system, unique frequency characteristics occur. Therefore, by
inputting a voltage to one end of this four-terminal network and terminating the other end by the
equivalent circuit of the object, the oscillation performance of the ultrasonic probe is transmitted
and received characteristics VTG (Voltage Transfer Gain) It can be calculated and predicted.
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[0031]
The input impedance Z of the transmission system is expressed as (3) below, and based on the
input impedance Z, the transmission / reception characteristic VTG is calculated. ???? ???
? Further, K represents an electromechanical coupling constant, Z0 represents the acoustic
impedance of the piezoelectric vibrator, ZA represents the acoustic impedance of the acoustic
matching layer, and ZB represents the acoustic impedance of the backing layer. Also, mA
represents the ratio of ZA to Z0, and mB represents the ratio of ZB to Z0. f0 represents a
designed center frequency (mechanical resonance frequency of the piezoelectric vibrator), and
C0 represents a capacitance value. The angular frequency ?0 is expressed as 2?f0 using the
frequency f0. Further, ? represents the transmission phase, v L represents the velocity of the
longitudinal wave inside the piezoelectric vibrator, and d represents the thickness of the
piezoelectric vibrator.
[0032]
Furthermore, in order to perform a simulation in which this equivalent circuit is applied to the
acoustic matching layer having a two-layer structure, a transmission system of ultrasonic waves,
a backing layer, a piezoelectric vibrator, a first acoustic matching layer, and a second acoustic
matching layer Consider a four-terminal network connected in series. In this transmission model,
assuming that the transmission sound pressure and the reception sound pressure are equal, the
signal transfer function T is obtained based on the input impedance of the transmission system
and the constant of the four-terminal network. The transmission / reception characteristic VTG
can be obtained by calculating 20 и log (T) at each frequency with respect to the signal transfer
function T.
[0033]
FIG. 2 shows the calculation conditions in the simulation of transmission / reception
characteristics in comparison with the conventional example. In this embodiment, since the
acoustic matching layer having a two-layer structure is used, the acoustic impedance of the first
acoustic matching layer, the acoustic impedance of the second acoustic matching layer, the
acoustic impedance of the piezoelectric vibrator, and the electromechanical coupling constant ,
The acoustic impedance of the backing layer, and the design center frequency of the piezoelectric
vibrator.
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[0034]
In the conventional example, the first acoustic impedance is 8.92 MRayl and the second acoustic
impedance is 2.34 MRayl (same as FIG. 7). On the other hand, in the present embodiment, the
first acoustic impedance is 12 MRayl, and the second acoustic impedance is 2.7 MRayl. Other
calculation conditions are the same. That is, the acoustic impedance Z0 of the piezoelectric
ceramic is 34 MRayl, the electromechanical coupling constant K33 is 0.65, and the acoustic
impedance of the backing layer is 5.5 MRayl.
[0035]
FIG. 3 shows simulation results of the transmission / reception characteristics in the present
embodiment in comparison with the conventional example. In FIG. 3, the horizontal axis
represents frequency (MHz) and the vertical axis represents transmission / reception
characteristic VTG (dB). The solid line shows the calculation result in the present embodiment,
and the broken line shows the calculation result in the conventional example.
[0036]
In FIG. 3, when the frequency bandwidth fBW (%) is obtained using equation (2) based on two
frequencies fH and fL at which the sound pressure attenuates by 6 dB from the peak value, in the
conventional example, fH = 8. 6 MHz, fL = 4.5 MHz, fC = 6.55 MHz, and the frequency bandwidth
fBW (%) is 63%. On the other hand, in the present embodiment, since fH = 8.6 MHz, fL = 4.1 MHz
and f0 = 6.35 MHz, the frequency bandwidth fBW (%) is 71%. As described above, by changing
the acoustic impedance values of the first and second acoustic matching layers, which were
conventionally optimized, the frequency bandwidth can be improved over the conventional
design conditions.
[0037]
As ultrasonic waves have the property that the higher the frequency, the greater the attenuation
in the body and in the water, and in ultrasonic probes created by the conventional design
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guidelines, calculated values that do not take attenuation into account. In comparison, the high
frequency component is low and the frequency bandwidth is often narrow. On the other hand,
according to the present embodiment, in addition to the expansion of the frequency bandwidth,
the VTG characteristic can be designed high in the high frequency region, and the attenuation of
the high frequency component can be compensated.
[0038]
Next, an embodiment prototyped based on the above calculation results will be described. In this
embodiment, PZT is used as the piezoelectric body 11 shown in FIG. 1, quartz glass is used as the
first acoustic matching layer 14, a polyimide sheet is used as the second acoustic matching layer
15, and ferrite is contained as the backing layer 16. A rubber material is used.
[0039]
Here, the acoustic impedance of the piezoelectric body 11 is about 34 MRayl, the acoustic
impedance of the first acoustic matching layer 14 is 12 MRayl, the acoustic impedance of the
second acoustic matching layer 15 is 2.7 MRayl, and the backing layer is The acoustic impedance
of 16 is about 5.5 MRayl. Therefore, each acoustic impedance is the same condition as in the
simulation shown in FIG. Also, the design center frequency is 6.5 MHz, based on which the
ultrasonic wavelength is determined. Here, the thickness of each of the acoustic matching layers
is set to 1/4 of the ultrasonic wavelength in view of the reflection of the ultrasonic waves at the
interface of the acoustic matching layer while setting the thickness of the piezoelectric body to
1/2 of the ultrasonic wavelength. Set to
[0040]
FIG. 4 shows the measurement results of transmission / reception characteristics in the present
embodiment. In FIG. 4, the horizontal axis represents frequency (MHz), and the vertical axis
represents transmission / reception characteristic VTG (dB). The measurement result of the
transmission and reception characteristics shown in FIG. 4 is a curve close to the simulation
result of the transmission and reception characteristics shown by the solid line in FIG. Therefore,
it was proved that the above simulation is valid.
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[0041]
5 and 6 are diagrams showing frequency bandwidths when the acoustic impedances of the first
and second acoustic matching layers are changed in the above simulation. The acoustic
impedance ratio of the first acoustic matching layer to the vibrator is changed in the lateral
direction, and the acoustic impedance ratio of the second acoustic matching layer to the vibrator
is changed in the vertical direction.
[0042]
For example, in FIG. 5, when the acoustic impedance ratio of the first acoustic matching layer to
the vibrator is 0.262 and the acoustic impedance ratio of the second acoustic matching layer to
the vibrator is 0.069, The frequency bandwidth fBW (%) of the ultrasonic probe is 63.16%.
[0043]
Here, assuming that the acoustic impedance of the vibrator is 34 MRayl, the fact that the acoustic
impedance ratio of the first acoustic matching layer to the vibrator is 0.262 means that the
acoustic impedance of the first acoustic matching layer is 8.2. It means that it is 91 MRayl.
Further, the fact that the acoustic impedance ratio of the second acoustic matching layer to the
transducer is 0.069 means that the acoustic impedance of the second acoustic matching layer is
2.35 MRayl. That is, this condition is substantially the same as the condition of the conventional
example shown in FIG. 3, and the value of the frequency bandwidth is also substantially the
same.
[0044]
In FIG. 5 and FIG. 6, when the condition for the value of the frequency bandwidth to be relatively
large exceeding 63.16% in the conventional example is selected, the frequency bandwidth is
roughly shown in a region surrounded by a thick line. Be done. That is, in FIG. 5, the acoustic
impedance ratio of the first acoustic matching layer to the vibrator is 0.265 to 0.294, and the
acoustic impedance ratio of the second acoustic matching layer to the vibrator is 0.069. The
combination which is -0.079 corresponds.
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[0045]
Further, in FIG. 6, the acoustic impedance ratio of the first acoustic matching layer to the vibrator
is 0.309 to 0.338, and the acoustic impedance ratio of the second acoustic matching layer to the
vibrator is 0.062 The combination of ~0.088, and the acoustic impedance ratio of the first
acoustic matching layer to the vibrator is 0.353 to 0.456, and the acoustic impedance ratio of the
second acoustic matching layer to the vibrator is 0 The combination which is .062 to 0.106
corresponds. When the acoustic impedance of the backing layer is in the range of about 1.75
MRayl to 20.0 MRayl, substantially the same results are obtained. In practice, the acoustic
impedance of the backing layer is suitably in the range of 1.75 MRayl to 6.0 MRayl, and more
preferably in the range of 3.5 MRayl to 6.0 MRayl.
[0046]
Thus, according to the present invention, when the acoustic matching layer has a two-layer
structure, the combination of the acoustic impedance of the first acoustic matching layer and the
acoustic impedance of the second acoustic matching layer is selected. The frequency bandwidth
of the ultrasonic probe can be broadened. The present invention can be applied to an ultrasonic
probe of any shape such as a sector type, a linear type, a convex type, and a radial type.
[0047]
The present invention has a piezoelectric vibrator for transmitting and receiving ultrasonic
waves, and an acoustic matching layer for matching acoustic impedance, and is used in an
ultrasonic probe used in a medical ultrasonic diagnostic apparatus or the like. It is possible.
[0048]
It is a perspective view showing the internal structure of the probe for ultrasonic waves
concerning one embodiment of the present invention.
It is a figure which shows the calculation conditions in simulation of a transmission / reception
characteristic compared with a prior art example. It is a figure which shows the simulation result
of the transmission / reception characteristic in one Embodiment of this invention compared
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with a prior art example. It is a figure which shows the measurement result of the transmission /
reception characteristic in one Example of this invention. It is a figure which shows the frequency
bandwidth at the time of changing the acoustic impedance of a 1st and 2nd acoustic matching
layer. It is a figure which shows the frequency bandwidth at the time of changing the acoustic
impedance of a 1st and 2nd acoustic matching layer. It is a figure which shows the conventional
optimal acoustic impedance calculated | required based on the energy transfer theory.
Explanation of sign
[0049]
REFERENCE SIGNS LIST 1 ultrasonic probe 11 piezoelectric body 12, 13 electrode 14 first
acoustic matching layer 15 second acoustic matching layer 16 backing layer 17 filler 18 acoustic
lens
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