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DESCRIPTION JP2001016677

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DESCRIPTION JP2001016677
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
ultrasonic probe and a transmitter for an ultrasonic therapeutic apparatus.
[0002]
2. Description of the Related Art An ultrasonic probe comprises an ultrasonic transmitting and
receiving element having a piezoelectric element. The ultrasonic probe is used to image the
internal state of the object by directing ultrasonic waves toward the object and receiving
reflection echoes from interfaces of different acoustic impedances in the object. An ultrasonic
imaging apparatus incorporating such an ultrasonic probe is applied to, for example, a medical
diagnostic apparatus for inspecting the inside of a human body, and an inspection apparatus for
detecting flaws in metal welding.
[0003]
Recently, as one of the medical diagnostic devices, in addition to a tomogram (B-mode image) of
the human body, blood flow using Doppler shift due to the blood flow of ultrasonic waves for
heart, liver, carotid artery etc. The "color flow mapping (CFM) method" capable of colordisplaying the speed of the image in two dimensions has been developed, and its diagnostic
capability has been dramatically improved by the medical diagnostic device. The medical
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diagnostic apparatus adopting the CFM method is used to diagnose all organs and organs of the
human body such as the uterus, liver, and spleen, and in the future, research is being conducted
for an apparatus capable of diagnosing coronary thrombus.
[0004]
In the case of the former B-mode image, it is required that a high resolution image be obtained
with high sensitivity in order to clearly show small lesions and gaps due to physical changes to a
deep part. In the case of the Doppler mode capable of obtaining the CFM image of the latter, the
signal level obtained is smaller than that in the case of the B mode, since reflection echoes from
minute blood cells having a diameter of about several μm are used. High sensitivity is required.
[0005]
Also, in recent years, a harmonic imaging (HI) method that can obtain a high resolution image by
using the non-linear effect of ultrasound has attracted attention. In this method, it is used that
ultrasonic waves propagating in a living body cause waveform distortion due to acoustic nonlinearity of living body tissue and cumulatively generate high-order harmonics. While the
conventional B-mode image mainly images the fundamental wave component in the echo signal,
the HI method mainly images the second harmonic component in the echo. In the HI method, a
high-resolution and artifact-free image can be obtained because the beam width of the second
harmonic is about (1/2) 1/2 of the fundamental beam width and the occurrence of grating lobes
is small. There is a feature.
[0006]
The present inventors replace at least lead titanate having an extremely large electromechanical
coupling coefficient and an acoustic impedance close to that of a living body in place of PZT (lead
zirconate titanate) ceramic currently widely used as an ultrasonic transmitting / receiving
element. The application of the solid solution type piezoelectric single crystal that has been
contained has been proposed, for example, in JP-A-6-38963 or JP-A-7-99348. Such an ultrasonic
transmitting / receiving element has high sensitivity and a wide band compared to the
conventional PZT ceramic probe, so the transmission is based on the fundamental wave and the
reception is based on the second harmonic, in addition to the image quality improvement of the
conventional B mode image and CFM image. It is extremely effective for the HI method that uses
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waves. The point in the HI method is to prevent the transmission of the second harmonic
component during transmission. For this purpose, it is effective to set the drive signal voltage to,
for example, a sine burst and reduce the second harmonic component. However, in an ultrasonic
probe using a piezoelectric single crystal, the piezoelectric single crystal is gradually depolarized
depending on the amplitude of the drive signal due to the polarization direction and the reverse
direction component of the bipolar drive signal voltage represented by this sine burst. There was
a problem of deterioration.
[0007]
As described above, the conventional ultrasonic probe using a piezoelectric single crystal has a
problem of depolarization due to a drive signal.
[0008]
It is an object of the present invention to solve such problems and to provide an ultrasonic
oscillator which does not depolarize at the time of driving and a method of driving the same.
[0009]
SUMMARY OF THE INVENTION According to the present invention, there is provided a
piezoelectric body consisting of a solid solution piezoelectric single crystal containing at least
lead titanate, and an ultrasonic oscillation surface of the piezoelectric body and an opposing
surface thereof. It is an ultrasonic oscillation device characterized by having a pair of electrodes
and voltage supply means which applies a drive signal voltage and a direct current voltage
between the pair of electrodes.
[0010]
According to the present invention, depolarization of the piezoelectric body can be prevented
even when a sine burst is used as the drive signal voltage.
[0011]
Further, by using the ultrasonic wave emitting surface as an ultrasonic wave receiving surface, it
can be used as an ultrasonic probe.
Furthermore, the characteristics of the piezoelectric single crystal can be utilized by using a sine
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burst as a drive signal voltage and receiving the second harmonic of the ultrasonic wave.
[0012]
Also, as the piezoelectric single crystal, Pb [(B1, B2) 1-xTix)] O3 (where x is 0.05 ≦ x ≦ 0.55, B1
is Zn, Mg, Ni, Sc, In). It is desirable to use a composition represented by at least one group
selected from the group of and Yb, and at least one group of B2 selected from the group of Nb
and Ta.
[0013]
The composition is preferably such that 0.98 ≦ A / B <1 where A / B is a stoichiometric ratio of
Pb to [(B1, B2) 1-xTix].
[0014]
Preferably, the magnitude of the DC electric field defined by the magnitude of the DC voltage and
the distance between the pair of electrodes is 12 [V / mm] or more.
[0015]
Another invention is an ultrasonic oscillation that applies a drive signal voltage to an ultrasonic
oscillation surface of a piezoelectric body made of a solid solution type piezoelectric single
crystal containing at least lead titanate and a pair of electrodes respectively provided on the
opposite surface. A method of driving an apparatus, comprising the step of: superimposing a DC
voltage on the drive signal voltage.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have
intensively studied an ultrasonic oscillator using a piezoelectric single crystal containing lead
titanate which is excellent in the reception of the second harmonic due to the width of the band.
Focusing on the fact that the piezoelectric single crystal is easily depolarized compared to other
piezoelectrics when using an AC power supply as a drive signal voltage, the drive signal voltage
is applied while applying a DC voltage in the polarization direction. It was confirmed that the
depolarization of the piezoelectric single crystal can be prevented, and the present invention has
been made.
[0017]
This will be described in detail with reference to FIG.
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[0018]
A plurality of piezoelectric bodies 1 made of single crystal are bonded to a backing material 2
separately from each other.
Each piezoelectric body 1 vibrates in the direction of arrow A in the figure.
The first electrode 3 is formed from the ultrasonic wave transmitting / receiving surface of each
of the piezoelectric bodies 1 to the side surface thereof and part of the surface opposite to the
transmitting / receiving surface.
The second electrode 4 is formed on the surface of each of the piezoelectric bodies 1 opposite to
the transmission / reception surface so as to be separated from the first electrode 3 by a desired
distance.
An ultrasonic transmitting and receiving element is constituted by the piezoelectric body 1 and
the first and second electrodes 3 and 4.
[0019]
The acoustic matching layer 5 is formed on the ultrasonic wave transmitting / receiving surface
of each of the piezoelectric bodies 1 including the respective first electrodes 3.
The acoustic lens 6 is formed over the entire acoustic matching layer 5 to focus the ultrasonic
waves generated by the vibration of the piezoelectric body 1.
The ground electrode plate 7 is connected to each of the first electrodes 3.
A flexible printed circuit board 8 having a plurality of conductors (cables) is connected to the
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second electrodes 4 by, for example, soldering.
[0020]
The ultrasonic probe 9 having the structure shown in FIG. 1 is manufactured, for example, by the
following method.
[0021]
First, a conductive film is deposited on a block-shaped single crystal piece by sputtering, and the
conductive film is left on the ultrasonic transmitting / receiving surface and the surface opposite
to the transmitting / receiving surface by selective etching technology.
Subsequently, after connecting the ground electrode 7 by soldering, for example, on the
conductive film end of the single crystal piece on the ultrasonic wave transmitting / receiving
surface side, an acoustic matching layer is formed on the ultrasonic wave transmitting / receiving
surface of the single crystal piece. Form.
[0022]
Subsequently, a flexible printed wiring board 8 having a plurality of conductors (cables) on the
end of the conductive film located on the side opposite to the ultrasonic wave transmitting /
receiving surface of the single crystal piece is connected, for example, by soldering. Thereafter,
these are adhered onto the backing material 2.
Thereafter, the blade is used to cut a plurality of times from the acoustic matching layer to the
conductive film located on the surface of the single crystal piece opposite to the ultrasonic wave
transmitting / receiving surface, thereby forming the first on the backing material 2, A plurality
of piezoelectric bodies 1 having second electrodes 3 and 4 arranged in an array and separated
from each other and a plurality of acoustic matching layers 5 arranged on the respective
piezoelectric bodies 1 are formed.
[0023]
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Then, an ultrasonic lens is fabricated by forming an acoustic lens 6 in the acoustic matching
layer 4.
The piezoelectric body 1 is formed of a solid solution single crystal of lead zinc niobate-lead
titanate.
[0024]
Such a single crystal can be produced, for example, by the following method.
[0025]
First, using PbO, ZnO, Nb 2 O 5, and TiO 2 chemically as high purity starting materials, after
correcting their purity, the desired molar ratio of zinc niobate (PZN) and lead titanate (PT) Weigh
so as to be and further add PbO as flux.
Pure water is added to this powder, and mixing is carried out for a desired time by, for example, a
ball mill containing ZrO 2 balls. After removing the water content of the obtained mixture, the
mixture is sufficiently pulverized by, for example, a pulverizer such as a lycai machine, and then
placed in a rubber-type container, and a rubber press is performed at a desired pressure.
[0026]
The solid removed from the rubber mold is placed in a container of the desired volume, for
example of platinum, and melted at the desired temperature. After cooling, the solid is placed in
the container, sealed with a lid made of platinum, for example, and the container is placed at the
center of the electric furnace. The temperature is raised to a temperature higher than the melting
temperature, gradually cooled to near the melting temperature at a desired temperature lowering
rate, and then cooled to room temperature.
[0027]
Thereafter, nitric acid having a desired concentration is added to the vessel, and the vessel is
boiled to take out a solid solution single crystal.
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[0028]
The solid solution single crystal of zinc niobate-lead titanate can be produced similarly by, for
example, the Bridgman method, Kilopohrs method, hydrothermal growth method, etc. in addition
to the above-mentioned flux method. .
[0029]
As the solid solution type single crystal of lead zinc niobate-lead titanate, it is preferable to use
one having a composition in which the mole fraction of lead titanate is 20% or less.
By using a piezoelectric body made of such solid solution type single crystal, the speed of sound
can be made 20% or more slower than that of a piezoelectric body made of PZT ceramic, so an
ultrasonic probe with high sensitivity can be obtained. Becomes possible.
[0030]
Here, although lead zinc niobate-lead titanate is mentioned as an example, a solid solution type
piezoelectric single crystal containing lead titanate obtained by replacing the starting materials
ZnO and Nb 2 O 5 with other elements is manufactured. It can also be done.
[0031]
Pb [(B1, B2) 1-xTix)] BO3 (where x is 0.05 ≦ x ≦ 0.55, B1 is selected from the group of Zn, Mg,
Ni, Sc, In and Yb) It is desirable to use a solid solution piezoelectric single crystal containing lead
titanate represented by at least one selected from the group consisting of Nb and Ta as B2.
[0032]
The reason why x in the above general formula is defined is as follows.
If the value of x is less than 0.05, the Curie temperature of the solid solution single crystal is low,
which may cause depolarization when the flexible printed wiring board 7 and the ground
electrode plate 8 are soldered or when the solid solution single crystal is cut. There is.
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On the other hand, when x exceeds 0.20, not only a large electromechanical coupling coefficient
can not be obtained, but also the dielectric constant is lowered, which may make it difficult to
match the electrical impedance of the transmission / reception circuit unit.
More preferably x is 0.06 to 1.2.
[0033]
Furthermore, when the stoichiometric ratio of Pb to (Zn1 / 3Nb2 / 3) 1-xTix of the composition
represented by the above general formula is A / B, 0.98 ≦ A / B <1.00. It is desirable to
[0034]
If the A / B in the general formula deviates from the above range, the reliability of the obtained
ultrasonic probe in actual operation may be reduced.
[0035]
The thickness of the piezoelectric body 1 in the vibration direction is preferably 200 to 400 μm.
[0036]
The piezoelectric body 1 preferably has an average surface roughness of 0.4 μm or less and a
maximum surface roughness of 4 μm or less on the ultrasonic wave transmitting / receiving
surface and the surface opposite to the transmitting / receiving surface. .
If the average surface roughness and the maximum surface roughness exceed 0.4 μm and 4
μm, respectively, long-term reliability such as sensitivity may be reduced.
More preferably, the average surface roughness and the maximum surface roughness are 0.3
μm or less and 3 μm or less, respectively.
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[0037]
The piezoelectric body 1 preferably has a (001) plane as the ultrasonic wave transmitting /
receiving surface.
Such a piezoelectric body 1 is produced by cutting out perpendicularly to the [001] axis (C axis)
of the solid solution type single crystal.
[0038]
The first and second electrodes 3 and 4 are formed of, for example, a two-layer conductive film
of Ti / Au, Ni / Au or Cr / Au, or silver baking including a glass frit.
[0039]
The arrangement of the electrodes 3 and 4 and the attachment of the ground electrode plate 7
and the flexible printed wiring board 8 to the electrodes 3 and 4 are not limited to those shown
in FIG.
For example, the bonding between the ground electrode plate 7 and the flexible printed wiring
board 8 and the electrodes 3 and 4 may be performed by using a conductive paste or resistance
welding other than soldering.
[0040]
The electrodes 3 and 4 are connected to the pulser circuit 11 via the coaxial cable 10. A direct
current voltage can be superimposed on the pulser circuit 11 by a direct current power supply
12 and applied to the piezoelectric body 1.
[0041]
Then, by applying a drive signal voltage in which a direct current voltage is superimposed to the
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piezoelectric body 1, the piezoelectric body 1 is vibrated and an ultrasonic wave is emitted from
the probe. Further, at the time of reception, the received ultrasonic waves are converted into
electric signals by the piezoelectric body, and the beam former 14 delays the reception signals of
the respective channels as desired, and is then phase-added by the adder. After that, when the
fundamental wave is measured, the signal is passed through the fundamental wave pass filter 15,
and when the second harmonic is measured, the signal is passed through the high pass filter 16
for removing the fundamental wave component. Ru.
[0042]
The present invention is characterized in that the depolarization of the piezoelectric body is
suppressed by applying a direct current voltage superimposed on a drive signal voltage.
[0043]
That is, when a sine burst is applied, there is a timing at which a voltage is applied in the
depolarizing direction, which causes the piezoelectric body according to the present invention to
depolarize, but a DC voltage in the polarization direction The depolarization of the piezoelectric
substance according to the present invention can be suppressed by superimposing.
[0044]
Considering that a sine burst of about 200 to 1000 V / mm is usually applied as a drive signal,
the DC voltage to be superimposed is 12 V / mm or more, more preferably 15 to 60 V in the
direction of polarization of the piezoelectric body. It is preferable to set to about / mm.
[0045]
If the value of the direct current voltage is too small, the function to reduce the depolarization is
not sufficient. If it is too large, the depolarization preventing characteristics do not change
significantly for the power consumption.
[0046]
Further, since the piezoelectric body according to the present invention has a wide band, it is
desirable to measure and detect the second harmonic component in the received wave when the
piezoelectric body is used as a transmitter / receiver of ultrasonic waves. .
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The measurement means of the second harmonic is not particularly limited, and any known
device capable of filtering the fundamental wave can be used.
[0047]
FIG. 2 is a schematic view showing an ultrasonic oscillation apparatus according to the present
invention, but the control apparatus 20 such as a drive signal, a drive system such as a direct
current power source, a measurement system of received waves such as the filter Or the like, and
it is not necessary to integrate them with the ultrasonic transmitting / receiving element or the
ultrasonic transmitting element, and as shown in the figure, the probe 9, the control device 20
and the monitor can be connected and used respectively.
[0048]
Although FIG. 1 shows a one-dimensional array type probe 9, the ultrasonic probe of the present
invention is not particularly limited thereto.
[0049]
For example, it is possible to use a two-dimensional array type probe in which the piezoelectric
bodies 1 as shown in FIG. 3 are arranged two-dimensionally in a matrix, and a composite type in
which resin is filled between the piezoelectric bodies 1. The present invention is also applicable
to a mechanical sector probe for mechanically scanning a single element or an annular array
probe in which concentric oscillators are arranged.
[0050]
EXAMPLE An ultrasonic probe shown in FIG. 1 was produced.
[0051]
As a piezoelectric single crystal, a solid solution piezoelectric single crystal in which lead zinc
niobate and lead titanate are mixed at a molar ratio of 91: 9 was used.
[0052]
The solid solution piezoelectric single crystal was produced as follows.
[0053]
First, using PbO, ZnO, Nb 2 O 5, and TiO 2 chemically as high purity starting materials and
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correcting their purity, lead zinc niobate (PZN) and lead titanate (PT) are 91: 9. Then, PbO was
added as a flux so that the molar ratio of raw material: PbO was 45:55.
[0054]
Pure water was added to this powder and mixed for 1 hour in a ball mill containing ZrO 2 balls.
After removing the water content of the obtained mixture, it was sufficiently ground with a lycai
machine and further placed in a rubber container and subjected to a rubber press at a pressure
of 2 tons / cm 2.
In a platinum container with a diameter of 65 mm and a capacity of 250 cc, 600 g of the solid
material taken out of the rubber mold was dissolved by raising the temperature to 900 ° C. for
4 hours.
[0055]
After cooling, an additional 400 g of the solid matter was placed, sealed with a platinum lid, and
the container was placed at the center of the electric furnace.
The temperature was raised to a temperature of 1260 ° C. for 5 hours, gradually cooled to 900
° C. at a rate of 0.5 ° C./hr, and then cooled to room temperature.
During the slow cooling, oxygen gas was blown to the bottom of the crucible at 1200 cm 3 / min
using a platinum pipe to locally cool the crucible.
Thereafter, nitric acid of 30% concentration was added to the platinum container and boiled for 8
hours to take out a solid solution single crystal.
The obtained single crystal grew almost mononuclearly from the bottom of the crucible, and the
size was an irregular form having a side of about 40 mm.
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[0056]
A part of the single crystal was crushed and subjected to X-ray diffraction to examine the crystal
structure, and it was confirmed that it has a perovskite structure.
Thereafter, the single crystal was oriented using a Laue camera and the (001) plane was oriented,
and cut parallel to the plane with a cutter.
[0057]
After polishing the cut surface with a # 2000 abrasive to a thickness of 260 μm, Ti / Cu / Au
electrodes (each thickness is 0.05 / 1.0 / 0.2 μm) were formed on both sides by sputtering. .
Next, the thin plate was immersed in silicone oil and heated to 200 ° C., and then cooled to 40
° C. and polarized while applying an electric field of 0.3 kV / mm.
[0058]
As shown in FIG. 1, this single crystal vibrator was partially etched away at the opposite end of
the electrodes formed on both main surfaces.
The single crystal vibrator and the flexible wiring substrate 8 were joined by soldering, and then
adhered to the backing material 2 using an epoxy resin.
Further, the ground plate 7 was connected by soldering to form a two-layer acoustic matching
layer 5. Next, with a dicing saw, it was cut over a total of 96 ch with a 50 μm thick blade at a
pitch of 200 μm as shown in FIG. A cut was also made to the backing material, and the depth
thereof was about 0.3 mm. Subsequently, the cutting groove was filled with silicone resin, and
the acoustic lens was bonded using the same silicone resin. A coaxial cable was connected to this
probe head to complete an ultrasonic probe.
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[0059]
Using this ultrasonic probe, first, a rectangular monopolar pulse as shown in FIG. 4 was applied
in the same direction as the polarization direction to measure pulse echo characteristics from an
acrylic block placed in water. As a result, an average center frequency of 3.45 MHz and a -6 dB
relative bandwidth of 82.3% were obtained for 96 channels. Next, the voltage of the three-wave
sine burst a having no bias superimposed as shown in FIG. 5 was gradually raised and applied,
and the echo signal was measured. The frequency of the sine burst is 3.5 MHz and the repetition
frequency is 6 kHz. As shown in FIG. 8, the amplitude increased linearly up to 120 Vpp, but
began to become dull above 120 Vpp and decreased more than 180 Vpp.
[0060]
A DC electric field of 1 kV / mm was applied to this probe at room temperature for 10 to 20
seconds to perform repolarization treatment. Thereafter, as shown in FIG. 6, a voltage of a sine
burst b superimposed with a direct current voltage was applied to measure pulse echo
characteristics. First, a bias voltage of 3 V was measured to 200 Vpp and repolarization was
performed, and then a bias voltage of 5 V was similarly measured.
[0061]
As a result, in the case of the bias of 3 V, although the improvement was made as compared with
the case of the bias of 0 V, the amplitude of the echo signal began to be blunted at about 150
Vpp. On the other hand, at a bias of 5 V, the amplitude of the echo signal increased linearly in the
measured 200 Vpp range. The bias of 3 V corresponds to about 12 V / mm when converted to an
electric field, and 19 V / mm at 5 V of bias.
[0062]
The relationship between the bias electric field and the amplitude of the echo signal showed
substantially the same tendency when the thickness of the transducer was changed, ie, when the
center frequency was changed. Furthermore, when the drive signal voltage is reversed in polarity
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as shown in the waveform c of FIG. 7, similar results are obtained when the rectangular wave
shown in FIG. 4 is made bipolar.
[0063]
In the above embodiment, the piezoelectric single crystal used is one in which PZT and lead
titanate (PT) are dissolved in a molar ratio of 91: 9 in the vicinity of the rhombohedral-tetragonal
phase boundary. When at least one of Mg, Sc, Ni, In or Yb is used instead of Zn and Zn, and also
when Nb is partially replaced by Ta, similar results can be obtained. For example, solid solution
of lead niobate and PT in a 70:30 molar ratio, solid solution of lead scandium niobate and PT in a
molar ratio of 58: 42, lead scandium niobate and magnesium niobate It is possible to use one in
which lead and PT are dissolved in a molar ratio of 29:34:37, or the like.
[0064]
As described above, in the present invention, when a solid solution type piezoelectric single
crystal containing at least lead titanate is used for a piezoelectric element for transmitting and
receiving ultrasonic waves, it is possible to use a bipolar drive pulse such as a sine burst.
Depolarization can be prevented by superimposing a DC voltage. As a result, the high
electromechanical coupling coefficient and low acoustic impedance of the piezoelectric single
crystal according to the present invention can be effectively used to obtain an ultrasonic image
with higher sensitivity and higher resolution than when using the conventional PZT ceramic. A
possible method of driving an ultrasound probe can be provided.
[0065]
Brief description of the drawings
[0066]
1 is a perspective view showing the configuration of an ultrasonic probe according to the present
invention
[0067]
Fig. 2 A schematic view of the ultrasonic oscillation apparatus of the present invention
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[0068]
Fig. 3 Modification of the piezoelectric structure according to the present invention
[0069]
Fig. 4 Rectangular unipolar drive pulse
[0070]
Fig. 5 Sign burst without bias applied
[0071]
Fig. 6 Biased sine burst according to the present invention
[0072]
Fig. 7 Sine burst in which polarity is reversed and bias is applied according to the present
invention
[0073]
Fig. 8 Graph showing relationship between amplitude of drive signal voltage and amplitude of
echo signal
[0074]
Explanation of sign
[0075]
1: Piezoelectric 2: Backing material 3: 4: Electrode 5: Acoustic matching layer 6: Acoustic lens 7:
Earth plate (common electrode plate) 8: Flexible printed board 9: Probe 10: Coaxial cable 11:
Coaxial cable 11: Pulser 12: DC power supply 13: pre-amplifier 14: beam former 15: fundamental
wave pass type filter 16: high pass type filter 17: monitor
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