DESCRIPTION JPH07107595

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DESCRIPTION JPH07107595
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to
transducers and, more particularly, to wideband phased array transducers used in the medical
diagnostic field.
[0002]
BACKGROUND OF THE INVENTION Ultrasound devices are often used to view the organs of the
human body. Typically, these devices include a transducer array that converts electrical signals
into pressure waves. The transducer array is generally in the form of a hand-held probe that can
be positioned to direct the ultrasound beam to the area of interest. The transducer array can
have, for example, 128 transducer elements to generate an ultrasound beam. Electrodes are
disposed at the front and back portions of the transducer elements to excite each element
individually to generate pressure waves. The pressure waves generated by the transducer
elements are directed towards an object to be observed, such as the patient's heart to be
examined. Each time the pressure wave strikes a tissue with different acoustic properties, the
wave is reflected back. An array of transducers converts these reflected pressure waves into
corresponding electrical signals. An example of a conventional phased array acoustic imager is
disclosed in US Pat. No. 4,550,607 to Maslak et al., Dated November 5, 1985. The circuit shown
in this patent combines the input signals received by the transducer array to produce a focused
image on the display screen. A broadband converter is a converter that can operate over a wide
range of frequencies without loss of sensitivity. As a result of the wide band operation of the
broadband converter, the resolution along the range axis is improved and the image quality is
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better.
[0003]
One possible application of a broadband converter is the contrast harmonic imaging. Harmonic
contrast imaging safely injects a contrast material such as a protein ball microballoon into the
body to indicate how active a tissue such as the heart is. The diameter of these microballoons is
typically 1 to 5 μm, and when injected into the body, observed by ultrasound imaging to
determine how well the tissue under test works Can. Harmonic contrast imaging is an alternative
to the thallium test that injects radioactive material into the body and observes computer
generated tomographic images. The thallium test methods are undesirable because they use
potentially harmful radioactive materials and typically require at least one hour to produce a
computer image. This is different from harmonic contrast imaging that can use real-time
ultrasound techniques in addition to using safe microballoons. Ultrasonic Imaging, Vol. 14
(1992)In the article "Simulated Capillary Blood Flow Measurement Using a Nonlinear Ultrasonic
Contrast Agent", page 134-158, B. Schrope et al. Disclose that contrast materials can be clearly
observed at the second harmonic. That is, in fundamental harmonics, heart and muscle tissue can
be clearly observed by ultrasonic techniques. However, at the second harmonic, the contrast
material itself can be clearly observed, so it can be determined how well each tissue works.
[0004]
Harmonic contrast imaging requires that the converter be able to operate over a wide range of
frequencies (ie both the fundamental harmonic and the second harmonic), but typically the
existing converter works in such a wide range You can not do it. For example, the bandwidth of
the converter with a center frequency of 5 MHz and a bandwidth ratio to the center frequency of
70% is 3.25 MHz to 6.75 MHz. If the fundamental harmonic is 3.5 MHz, then the second
harmonic is 7.0 MHz. Thus, a converter having a center frequency of 5 MHz can not operate well
at both the fundamental and second harmonics. In addition to the desire for transducers that can
operate over a wide range of frequencies, two dimensional transducer arrays are also desired to
enhance the resolution of the generated image. An example of a two-dimensional transducer
array is disclosed in Haan U.S. Pat. No. 3,833,825 dated October 3, 1974. A two dimensional
array can improve control of the excitation of the ultrasound beam along a height (elevation) axis
that is not used in conventional one dimensional arrays. However, typically a two-dimensional
array is difficult to manufacture because it requires cutting each element into several segments
along the height axis and connecting leads to excite each segment. For example, a twodimensional array with 128 elements in the azimuth (azimuthal) axis would require at least a
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total of 256 segments of two segments in height and leads to interconnect these segments .
Furthermore, as there are at least twice as many segments as there are in a one-dimensional
array, which must be excited individually, in order to excite several of each segment at an
appropriate time in the ultrasound scan You need fairly complex software.
[0005]
Furthermore, typical prior art transducers having a plane parallel to the object to be inspected
produce undesirable reflections called "ghost echoes" at the interface between the transducer and
the object to be inspected. These undesirable reflections reduce the clarity of the resulting image.
[0006]
SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to
provide a broadband transducer array for use in acoustic imaging devices that is easy and
inexpensive to manufacture. Another object of the invention is to provide a broadband
transducer array that can be used for harmonic contrast imaging. Another object of the present
invention is to provide a transducer element and matching layer which together have a negative
curvature to provide additional focusing to the part of interest. Another object of the present
invention is to provide a transducer array that can be used in an acoustic imaging device that can
be brought close to (simulate a two-dimensional transducer array) at least at lower frequencies. is
there. Yet another object of the invention is to better suppress the generation of unwanted
reflections on the surface of the object to be examined. Another object of the invention is to
further increase the sensitivity and bandwidth of the transducer by placing one or more
matching layers on the front portion of the piezoelectric layer facing the area to be inspected.
[0007]
In order to achieve the above objects, several preferred embodiments of the present invention
are provided. An array ultrasonic transducer according to a first embodiment of the invention
comprises a plurality of transducer elements arranged in contact with one another. Each element
has a front portion facing the area to be inspected, a rear portion, two side portions, and a
transducer thickness between the front and rear portions. The transducer thickness is the largest
thickness at the side portions and the smallest thickness between the two side portions.
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Furthermore, the maximum thickness is less than or equal to 140% of the minimum thickness. In
this embodiment, the thickness of the element along the distance (Z) axis is changed by about 20
to 40% (ie, the maximum thickness is 120 to 140 of the minimum thickness) in order to widen
the bandwidth and shorten the pulse width. %) Is preferable. This improves the resolution along
the distance axis. According to a second embodiment of the invention, the transducer which
generates an ultrasound beam when excited comprises a plurality of piezoelectric elements. Each
element has a thickness at at least a first point on the surface facing the area to be inspected that
is smaller than that at at least a second point on the surface, and the surface is non-planar.
Furthermore, the aperture of the ultrasound beam generated according to the invention varies
inversely with the excitation frequency of the element. In general, if the maximum thickness of
the piezoelectric element is greater than 140% of the minimum thickness of the piezoelectric
element, the transducer can generate a beam close to that generated by the two dimensional
array at lower frequencies. This is based on the fact that at lower frequencies the output pressure
wave generated by the transducer has at least two peaks. At lower frequencies, typically the
entire aperture is activated. Thus, this second embodiment approaches the excitation of a wide
aperture two dimensional transducer array.
[0008]
In a third embodiment, a two crystal converter component design is provided. The design
comprises a first piezoelectric portion, the thickness at at least one point on the first surface
facing the test area of the first piezoelectric portion being at least one other point on the first
surface And the first surface is non-planar. An interconnection circuit may be disposed between
the first and second piezoelectric portions. A matching layer can be disposed on the first
piezoelectric portion. In a fourth embodiment, a composite structural transducer is provided
comprising a plurality of vertical columns of piezoelectric material, the distance between the
columns of the transducer and the polymer layer being varied. This structure can be modified to
obtain the desired transducer configuration. Also, performance can be further enhanced by
placing a matching layer on this composite transducer structure. The transducers of all the above
embodiments are operable over a wide frequency range to allow correct apodization. Generally,
they are easier to assemble than prior art devices, as these embodiments do not require the back
acoustic ports of the elements to be aligned. A first preferred method of the invention for
manufacturing a transducer comprises forming a plurality of transducer elements in contact with
one another. Each element consists of a front portion facing the area to be inspected, a rear
portion, two side portions and a transducer thickness between the front and rear portions.
Furthermore, the transducer thickness is the largest thickness in the side portions and the
smallest thickness between the two side portions, the largest thickness being less than or equal
to 140% of the smallest thickness. An electric field is established through at least one portion of
each element.
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[0009]
A second preferred method of the invention for manufacturing a transducer comprises forming a
plurality of piezoelectric elements. The thickness at at least one point on the front surface facing
the inspected area of each element is less than the thickness at at least one other point on the
surface, the surface being non-planar. An electric field is established through at least one portion
of each element. For example, electrodes may be provided on the front surface and the rear
portion of each piezoelectric element to apply an electric field. Typically, when the maximum
thickness of the piezoelectric element is greater than 140% of the minimum thickness of the
piezoelectric element, the excitation pulse generated by the transducer when the excitation pulse
is applied to the electrode causes the aperture of the ultrasonic beam generated by the
transducer to And vice versa. A third preferred method of the invention for manufacturing a
transducer comprises forming a piezoelectric element of composite material having a front
portion facing the area to be inspected. The thickness of at least one point on the front portion is
less than the thickness of at least one other point on the front portion. First and second
electrodes can be disposed on the piezoelectric element. The element can be deformed into the
desired shape. The transducer of all embodiments as well as the transducer produced by the
above method is in the form of a hand-held probe and can be adjusted in position during
excitation to direct the ultrasound beam to the area of interest. Furthermore, the transducers of
all embodiments as well as the transducers produced by the above method can be arranged in a
housing for positioning in a hand-held probe. Other types of probe and beam directing
techniques are also conceivable. The ultrasound apparatus for generating an image comprises a
transmitting circuit for transmitting an electrical signal to the transducer probe, a receiving
circuit for processing the signal received by the transducer probe, and a display for generating
an image of the object to be observed. The converter converts the electric signal supplied from
the transmission circuit into a pressure wave, and converts the pressure wave reflected from the
object to be observed into a corresponding electric signal. These electrical signals are processed
in the receiving circuit and finally displayed.
[0010]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a schematic view of an
ultrasonic apparatus 1 for generating an image of an object to be observed or a body 5. As shown
in FIG. The ultrasound apparatus 1 has a transmitter circuit 2 for transmitting an electrical signal
to the transducer probe 4, a receiver circuit 6 for processing the signal received by the
transducer probe, and a display device 8 for generating an image of the object 5 to be observed.
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doing. See also FIG. The probe 4 comprises an array 10 of transducer elements 11. Typically,
128 elements 11 are present in the azimuth (Y) axis to form a broadband transducer array 10.
However, the transducer elements 11 in the array can consist of any number of transducer
elements 11, each arranged in any desired geometric form. Transducer array 10 is supported by
backing block 13. The probe 4 may be hand-held and may be adjusted in position to direct the
ultrasound beam to the area of interest. The transducer element 11 converts the electrical signal
supplied from the transmission circuit 2 into a pressure wave. The transducer elements 11 also
convert pressure waves reflected from the object to be observed into corresponding electrical
signals. These electrical signals are processed in the receiving circuit 6 and finally displayed on
the display device 8.
[0011]
FIGS. 2, 4 and 6 show a first embodiment of the present invention. The transducer element 11
comprises a front part 12, a rear part 14, a central part 19 and two side parts 16 and 18. The
front portion 12 is a surface located toward the inspection area. The rear portion 14 may be
shaped as desired, but is generally planar. The front portion 12 is non-planar. The thickness
along the distance axis of element 11 is such that the thickness at each side portion 16 and 18 is
large and the thickness between the two sides is small. The side parts 16 and 18 referred to here
are not only the side 15 of the respective element 11 but also if the thickness of the element is
greater than the thickness on the inside of the element (for example, the thickness on each side
of the element is tapered In the case of) also includes the internal area of the element. Although
the front portion 12 is illustrated as a continuously curved surface, the element has a large
thickness on each side portion 16 and 18 and a negative “curved” front portion that decreases
in thickness towards the central portion 19 If it is 12, the front portion 12 can be a stepped
configuration, a series of linear segments, or any other configuration. The rear portion 14, which
is preferably planar, can also be concave or convex, for example.
[0012]
The element 11 has a maximum thickness LMAX and a minimum thickness LMIN, measured
along the distance (Z) axis. Preferably, the thicknesses of the side portions 16 and 18 are both
equal to LMAX, and the thickness is LMIN at or near the center of the element 11. However, it is
not necessary for each side portion 16, 18 to have the same thickness in order to realize the
invention, nor for LMIN to be at the exact center of the transducer element. In a first preferred
embodiment, the value of LMAX is less than or equal to 140% of the value of LMIN. This allows
the bandwidth activation energy to be increased, generally without the need to reprogram the
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ultrasound system to produce the ultrasound beam. Furthermore, if the value of LMAX is less
than or equal to 140% of the value of LMIN, then the output beam width will be the same for
different excitation frequencies. The increase in bandwidth activation energy of the transducer
arrangement of the present invention is such that the transducer is free resonant (i.e. without
matching layer) or is an optically matched transducer as described below In the case of (i.e.
having at least two matching layers) it is approximated by LMAX / LMIN. In the first preferred
embodiment shown in FIGS. 2, 4 and 6, the bandwidth is increased by 40% by increasing the
thickness of LMAX to LMIN by 40% (e.g. making LMAX 140% of LMIN). be able to.
[0013]
For example, if the transducer has an LMAX of 0.3048 mm and an LMIN of 0.254 mm, the
bandwidth is increased by 20% as compared to a transducer having a uniform thickness of 0.254
mm. Similarly, if the transducer has an LMAX of 0.3556 mm and an LMIN of 0.254 mm, the
bandwidth is increased by 40% as compared to a transducer having a uniform thickness of 0.254
mm. In this embodiment, the change in thickness of the element along the distance axis is on the
order of 20-40% (i.e. making the maximum thickness greater than or equal to 120% of the
minimum thickness, or 140 of the minimum thickness). Less than or equal to), which broadens
the bandwidth and shortens the pulse width. This increases the maximum bandwidth by about
20 to 40%, respectively. Furthermore, this improves the resolution along the distance axis. By
slightly changing the thickness of the front portion 12 to the rear portion 14 of the first
embodiment, for example, the converter was activated at three different frequencies (eg 2 MHz,
2.5 MHz and 3 MHz) known as three frequency operating modes In the case, better converter
performance can be obtained. Such a three frequency mode of operation can be used for cardiac
applications. Furthermore, with slight variations in transducer thickness, it is possible to improve
transducer performance even in other three frequency operating modes such as 2.5 MHz, 3.5
MHz and 5 MHz.
[0014]
Preferably, the element 11 has a plano-concave structure and is made of lead zirconate titanate
piezoelectric material (PZT). However, as described below, element 11 can also be formed of a
composite material such as polyvinylidene fluoride (PVDF) or other suitable material. See also
FIG. Electrodes 23 and 25 can be appropriately positioned on the front 12 and rear 14 portions
of the element 11, as is known, to excite the element 11 to generate the desired beam. The
electrodes 25 may be disposed directly on the piezoelectric element 11 or alternatively may be
disposed on the matching layer 24. In the latter case, the matching layer 24 will be disposed
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directly on the piezoelectric element 11. Electrodes 23 and 25 establish an electric field through
element 11 to generate the desired ultrasound beam. An example arrangement of electrodes for
piezoelectric material is disclosed in US Pat. No. 4,611,141 to Hamada et al., Dated October 9,
1986. The first electrode 23 provides a signal for exciting the respective transducer element, and
the second electrode 25 is grounded. Leads 17 (FIG. 4) can be used to excite each first electrode
23 on each transducer element 11, and all the second electrodes 25 can be connected to
electrical ground. As known in the art, sputtering techniques can be used to place the electrodes
on the piezoelectric layer. Alternatively, the interconnection circuits described below can be used
to provide electrical excitation of each transducer element.
[0015]
FIGS. 3 and 5 show a second preferred embodiment of the present invention, in which parts
identical to those of the first embodiment are given the same reference numerals. With reference
to FIGS. 6 and 8 for the first embodiment, the figures are also used in the second preferred
embodiment as the two embodiments are similar. Furthermore, the thickness at at least a first
point on the front portion 12 is less than the thickness at at least a second point on the front
portion. The front part is nonplanar. In a second preferred embodiment, the value of LMAX is
greater than 140% of the value of LMIN. When the value of LMAX is greater than 140% of the
value of LMIN, the width of the generated output beam typically varies with frequency.
Furthermore, the lower the frequency, the wider the output beam. FIG. 9 shows a typical change
from low frequency to high frequency of the output beam width or aperture along the height (X)
direction generated by the broadband converter according to the second embodiment. At high
frequencies, such as 7 MHz, the beam has a narrow aperture. As the frequency is reduced, the
beam aperture will expand. Furthermore, at a sufficiently low frequency, such as 2 MHz, the
beam is effectively generated from the entire aperture of the transducer element 11. As shown in
FIG. 9, the output pressure wave at low frequency has two peaks and approaches the excitation
of a wide aperture two dimensional transducer array.
[0016]
FIG. 5 also shows the change in beam width of the entire transducer array as a function of
frequency for the second preferred embodiment. At higher excitation frequencies the output
beam width has a narrow aperture and the beam is generated from the center of element 11. On
the other hand, at lower excitation frequencies the output beam width has a wide aperture and
the beam is generated from the entire aperture of element 11. By controlling the excitation
frequency, it is possible to control which section of the transducer element 11 is to generate an
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ultrasound beam. That is, at higher excitation frequencies the beam is mainly generated from the
center of the transducer element 11 and at lower excitation frequencies the beam is mainly
generated from the entire aperture of the transducer element 11. Furthermore, the greater the
curvature of the front portion 12, the closer the element 11 is to the wide aperture two
dimensional transducer array. In order to pursue the purpose of the second preferred
embodiment, i.e. to increase the bandwidth by more than 40%, it may be necessary to reprogram
the ultrasound system to excite the transducer over such a wide range of frequencies. Absent. As
can be seen from the equation LMAX / LMIN, the greater the change in thickness, the greater the
spread of the bandwidth. In accordance with the principles of the present invention, a bandwidth
increase of 300% or more can be achieved for a given design. That is, the thickness LMAX is
approximately three times larger than the thickness LMIN. For example, the bandwidth of a
single transducer element can be extended to the range of 2 MHz to 11 MHz, but even wider
ranges can be achieved according to the principles of the present invention. The transducer array
manufactured according to the invention can be operated at such a wide range of frequencies (i.e.
the converter is operable at the main fundamental harmonic frequency and also at the main
second harmonic frequency A harmonic contrast imaging method, which is operable at (1),
observing both the fundamental harmonic and the second harmonic, can be achieved using a
single transducer array according to the invention.
[0017]
As shown in FIGS. 10 and 11, varying the thickness of the transducer element 11 significantly
increases the bandwidth. 10 and 11 show an example of the effect on bandwidth when using a
plano-concave transducer element 11, the results may vary depending on the particular form
used. FIG. 10 shows the impedance plotted for the transducer element 11 made in accordance
with the second preferred embodiment of the present invention. The thickness of the side portion
of this transducer element 11 is 0.015 inch (0.381 mm) and the center thickness is 0.00428 inch
(0.109 mm). As is apparent from the figure, the elements have a bandwidth of about 3.5 MHz to
10.7 MHz. By contrast, as shown in FIG. 11, a common element with a uniform thickness of
0.381 mm is typically a bandwidth of about 4.5 MHz to 6.6 MHz. That is, by comparing .DELTA.f,
which is the difference between the antiresonance frequency (i.e. maximum impedance) fa and
the resonance frequency (i.e. minimum impedance) fr, the fractional bandwidth of the prior art
design is about 38%. Whereas it can be seen that the transducer element produced according to
the invention is provided with 100% partial bandwidth. Thus, the frequency of the energy
emitted by controlling the shape of the curvature of the transducer element (i.e. it may be
cylindrical, parabolic, gaussian or even stepped and may even be triangular) The content can be
controlled effectively. It is to be understood that the use of each of these shapes, as well as
others, is within the scope of the present invention.
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[0018]
The transducer structure produced according to the invention shown in FIGS. 7 and 8 with the
same reference numbers for identical components has a curved matching layer 24 arranged on
the front portion 12 of the transducer element 11. Have. The matching layer 24 is preferably
made of filled polymer. Furthermore, the thickness of matching layer 24 is such that LML at a
given point on the transducer surface is the thickness of the matching layer, LE is the thickness
of the transducer element, CML is the speed of sound in the matching layer, and CE is the
element It is preferable to approximate by the equation LML = (1/2) (LE) (CML / CE) as the sound
velocity of The curvature of the front portion 12 may be different from the curvature of the top
portion 26 of the matching layer 24 since the thickness of the matching layer 24 depends on the
thickness of the element at a given point on the transducer surface. While it is preferred to use
the above equation to form one or more matching layers, the matching layers can have a
constant thickness for ease of manufacture. By adding the matching layer 24, partial bandwidth
can be improved. Furthermore, the sensitivity of the transducer can be increased. However, the
difference in thickness between the edge and the center of the assembled substrate controls the
desired increase in bandwidth and the shape of the curve controls the shape of the baseband in
the frequency domain. Furthermore, since both the transducer element 11 and the matching
layer 24 have a negative curvature, an additional focusing on the part of interest is obtained.
[0019]
One or more matching layers can be added to the front portion 12 to focus the ultrasound beam
onto the portion of interest and improve the sensitivity of the transducer. Preferably, two
matching layers are disposed on the piezoelectric or transducer element 11 to form an optically
matched transducer. Each matching layer is calculated by the equation LML = (1/2) (LE) (CML /
CE). Specifically, when calculating the thickness LML of the first matching layer, the value of the
speed of sound CML of the first material is used. When calculating the thickness LML of the
second matching layer, the value of the speed of sound CML of the second material is used.
Preferably, the value of the acoustic impedance of the first matching layer (that is, the matching
layer closer to the piezoelectric element) is about 10 megarails (Mega Rayls), and the second
matching layer (that is, the one closer to the object to be observed). The value of the acoustic
impedance of the matching layer is approximately 3 megarails. A coupling element 27 with
acoustic properties to be examined can be arranged directly on the matching layer or, for
example, on the second electrode 25 if no matching layer is used. The coupling element 27 can
provide comfort to the patient as it can relieve any sharp surface of the transducer structure in
contact with the body under examination. The coupling element 27 can be used, for example, in
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applications where the bending of the front part 12 or the top part 26 is large.
[0020]
The binding element 27 can be formed of unfilled polyurethane. The bonding element can have a
generally flat, slightly concave or slightly convex surface 29. Preferably, the curvature of the
surface 29 is slightly concave so as to be able to hold an ultrasonic gel 28 such as Aquasonic®, a
product of Parker Labs, Orange, NJ, between the probe 4 and the object to be examined. This
provides a strong acoustic contact between the probe 4 and the object to be examined. The
matching layer and the bonding layer described above can be arranged in all the embodiments
described above. The thickness of the transducer element can be varied using a machine such as
a numerical control machine tool widely used in the ultrasound industry. The machine tool can
process the initial piezoelectric layer to give LMAX and LMIN the desired thickness change. FIG.
16 shows a first method of processing the piezoelectric layer 80 when it is desired to bend the
front portion. Coordinates defining the radius of curvature R approximated by the formula h / 2 +
(w2 / 8h), where h is the thickness difference between LMAX and LMIN and w is the width of the
transducer element along the height (elevation) axis Is input to the numerical control machine.
The numerical control machine processes the piezoelectric layer with a grinding wheel 84 having
the same dimension width as the piezoelectric layer 80. The grinding wheel 84 rotates about an
axis 86 parallel to the height axis. The grinding wheel 84 includes grinding material such as
aluminum oxide. The grinding wheel 84 preferably starts processing from one end of the
piezoelectric layer 80 and proceeds along the azimuth direction until reaching the other end of
the piezoelectric layer.
[0021]
FIG. 17 shows an alternative method of processing the piezoelectric layer 80. In this method, the
grinding wheel 84 is angled such that one corner 88 thereof contacts the surface of the
piezoelectric layer 80. In a given orientation area, the grinding wheel 84 starts grinding from one
side along the height axis of the piezoelectric layer 80 and continues grinding until it reaches the
other side along the height axis of the piezoelectric layer 80 ( For example, the grinding wheel
performs the desired grinding along the height axis at the point where the index is provided in
the azimuth axis). The grinding wheel 84 rotates about an axis 90. The grinding wheel 84 then
moves to the next region or indicator along the azimuthal axis and repeats machining along the
height axis from one side of the piezoelectric layer to the other. This process is repeated until the
entire piezoelectric layer 80 is processed to obtain the desired curvature 82. The processed
surface can be ground or polished to obtain a smooth surface. This is particularly desirable when
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using the converter at very high frequencies, such as 20 MHz. See also FIGS. 7 and 18. As is
known, the piezoelectric material is incised in the form of pleats and their incisions 94 form a
plurality of electrically independent piezoelectric elements 11. The incision 94 provides a
plurality of matching layers 24, piezoelectric elements 11 and electrodes 23. The incision 94 can
also extend slightly into the backing block 13 to ensure electrical isolation between the
transducer elements.
[0022]
Please refer to FIG. The metallization layer can also be deposited directly on top of the
piezoelectric layer to form the second electrode 25 before making the cuts. Preferably, the
second electrode 25 is disposed on the top portion 26 of the matching layer 24, if the matching
layer 24 is also used. However, the top portion 26 of the matching layer 24 is shorted to the
second electrode by metallization across the edge of the matching layer or by using a conductive
material such as magnesium or conductive epoxy Is preferred. Furthermore, if a matching layer is
used, the cuts are made after the matching layer is placed on top of the piezoelectric layer. In the
preferred embodiment, the second electrode is held at ground potential. If a flex circuit 96,
described below, is used, cuts are made through the flex circuit to form the individual electrodes
23. If the converter is designed to operate in a sector format, the length S, which is the distance
between elements along the azimuthal direction, should approximate the half wavelength in the
object under test at the highest operating frequency of the converter preferable. This
approximation is also applied to the two crystal design described later. If the transducer is
designed for linear operation, or if the geometry of the transducer array is curvilinear, then the
value S is 1 and 2 wavelengths within the object under test at the highest operating frequency of
the transducer. Can vary between
[0023]
FIG. 19 illustrates a curvilinear transducer array fabricated in accordance with the principles of
the present invention. More specifically, a curvilinear array is fabricated similar to the linear
transducer array of FIG. However, the piezoelectric element 11 is not directly mounted on the
large backing block 13 as shown in FIG. 18, but the piezoelectric element 11, the flexible circuit
96 and the corresponding electrode 23 have a first backing thickness of about 1 mm. It is
arranged directly on the block 13 '. This allows the array to be easily bent by the desired amount
to widen the field of view. Typically, the radius of curvature of the first backing block 13 'is about
44 mm, but can be varied as required. The first backing block 13 'can be adhered using epoxy
glue to a second backing block 13 "having a thickness of about 2 cm in the distance direction.
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The surface of the second backing block 13 '' in contact with the first backing block 13
'preferably has the same radius of curvature. As known in the art, a curvilinear array functions in
the same manner as a linear array with mechanical tolerance placed on the front of the linear
array. Because the signal at the central portion 19 of the transducer element 11 is stronger than
at the end or side portions 16 and 18, accurate apodization occurs (i.e. reduces or suppresses the
formation of side lobes). This is due to the fact that the electric field between the two electrodes
on the front part 12 and the rear part 14 is greatest on the central part 19 and reduces the
formation of side lobes. Furthermore, unwanted reflections (i.e. ghost echoes) at the interface of
the transducer and the object to be examined are well suppressed, since the front and back parts
are not flat parallel surfaces. Still further, since the transducer array manufactured according to
the present invention can operate over a wide range of frequencies, the transducer can receive
signals at center frequencies other than the transmitted center frequency. .
[0024]
With regard to the design of the distance between the elements 11 and the design of the
transducer aperture or width w, the upper operating frequency of the transducer has the greatest
influence on the grating lobes. If the distance between the elements is designed in consideration
of the maximum operating frequency of the transducer, it is possible to avoid artifacts or artifacts
of the grating lobe image (i.e. the generation of undesired multiples of the object to be observed).
More specifically, the relationship between the grating lobe angle Θg, the electronic steering
angle Θs in sector format, the wavelength λ in the object under test at the highest operating
frequency of the converter, and the distance S between elements is given by . Therefore, for a
given grating lobe angle, the transducer aperture design is limited by the upper operating
frequency of the transducer. As can be seen from the above equation, in order to sweep at a
higher frequency, it is necessary to narrow the aperture that is correlated to that frequency. For
example, at an operating frequency of 3.5 MHz, the desired distance S between elements is 220
μm and the distance S at 7 MHz is 110 μm. Since it is desirable to narrow the aperture of the
transducer element given by the above equation at higher frequencies, using that transducer
element at lower frequencies will result in a loss in resolution. This is due to the fact that
operation at lower frequencies typically requires larger element apertures. However, this results
in a two-dimensional array of transducers at lower frequencies where the value of LMAX is
greater than 140% of the value of LMIN and the resolution of the image obtained by the wider
aperture at lower frequencies is increased. It is compensated by the fact that it simulates.
[0025]
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Two crystal converter elements can be designed using the principles of the present invention.
The two-crystal transducer element 40 shown in FIG. 12 has a first piezoelectric portion 42 and a
second piezoelectric portion 44. These piezoelectric parts may be processed as two separate
pieces. Preferably, both surfaces 46 and 48 are generated by the formula h / 2 + (w2 / 8h).
Where h is the difference in thickness between LMAX and LMIN and w is the width of the
transducer element along the height axis. Although the structure of the piezoelectric portions 42
and 44 is illustrated as being plano-concave, the surfaces 46 and 48 can include a stepped
configuration, a series of linear segments, or other configurations. The thickness of each portion
42 and 44 is greater at the side portions 43, 45, 47, 49 and smaller at their respective central
portions. Furthermore, the back portions 51 and 53 of the piezoelectric portions 42 and 44
respectively are preferably planar. However, these surfaces can also be non-linear. An
interconnect circuit 50 is disposed between the first piezoelectric portion 42 and the second
piezoelectric portion 44. The interconnect circuit 50 may consist of any interconnect design used
in the field of acoustics or integrated circuits. Typically, interconnect circuit 50 is made of a layer
of copper carrying leads for exciting transducer element 40. The layer of copper can be adhered
to a piece of polyamide material, which is typically Kapton. The copper layer is preferably
coextensive with the respective piezoelectric portions 42 and 44. Additionally, the interconnect
circuitry can be gold plated to improve contact performance. Such interconnect circuitry may be
a flexible circuit manufactured by Sheldahl of Northfield, Minnesota.
[0026]
A matching layer 52 can be disposed on the piezoelectric portion 42 to further improve
performance. If the first and second piezoelectric portions are both formed of the same material,
LML at a given point on the transducer surface is the thickness of the matching layer and LE is
the first and second piezoelectric portions The thickness LML of the matching layer 52 is
approximated by (1/2) (LE) (CML / CE), where CML is the speed of sound in the matching layer,
and CE is the speed of sound in the piezoelectric portion. Electrodes or ground layers 58 and 59
can be placed directly on the matching layer 52 and on the surface 48 to connect the two
piezoelectric parts in parallel. Matching layer 52 can be coated with a conductive material such
as nickel and gold. However, if no matching layers are used, both ground layers are placed
directly on the piezoelectric portions 42 and 44. The matching layer 52 can face the inspection
area. Transducer 40 can be disposed on backing block 54 as is common in the field of
ultrasound. Furthermore, binding elements as described above can also be used. FIG. 13 shows
another two-crystal design 55 using the principles of the present invention. A first piezoelectric
portion 56 and a second piezoelectric portion 57 are provided. The shape of the piezoelectric
portion 56 is preferably plano-concave. Furthermore, the thickness of the second piezoelectric
portion 57 changes along the height direction. An interconnection circuit 50 as described above
can be used between the two piezoelectric parts to excite the two crystal converter 55. Matching
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layers and coupling elements as described above are also provided to improve performance and
patient comfort. Additionally, electrodes or ground layers 58 and 59 can be used to connect the
two piezoelectric portions in parallel.
[0027]
The rear portion 61 of the first piezoelectric portion 56 is preferably planar. The curvature
radius R of the front portion 63 of the first piezoelectric portion 56 and the rear portion 65 of
the second piezoelectric portion 57 makes h be the thickness difference between LMAX and
LMIN of the piezoelectric portion 56 and w be along the height axis The width of the transducer
element is approximated by the equation h / 2 + (w2 / 8h). Preferably, the values of LMAX and
LMIN are the same for the first and second piezoelectric portions 56 and 57. The radius of
curvature R of the front portion 67 of the second piezoelectric portion 57 makes h ′ the
difference between the combined maximum thickness of both piezoelectric portions and the
minimum combined thickness, and w is the transducer element along the height axis The width is
approximated by the formula h '/ 2 + (w2 / 8h'). In order to obtain the desired radius of
curvature, the piezoelectric parts 56 and 57 can be processed by a numerical control machine as
described above. Instead of using a uniform layer of piezoelectric material, a composite structure
60 as shown in FIG. 14 can be used to form a composite material. Composite structure 60
includes vertical columns 62 or slabs of piezoelectric material of various thicknesses. Between
the pillars 62 is a polymer layer 64 which may be formed of e.g. epoxy material. For composite
materials, see, for example, Materials Research Bulletin, Vol. 13 The article "Connectivity and
Piezoelectric-Pyroelectric Composites" by R. E. Newham et al., Pp. 525-536 (1978), and Materials
Research Bulletin, Vol. 13 See "Flexible Composite Transducers" by R. E. Newham et al., P. 599607, (1978). The composite structure 60 is preferably plano-concave. An acoustic matching layer
(not shown) can be disposed on the front portion 66 to improve performance.
[0028]
Composite materials can be embedded within the polymer layer. The composite can then be
ground, processed or formed to the desired dimensions. Furthermore, the composite structure
can be scored to form individual transducer elements, as is commonly practiced in the field of
ultrasound. The gaps between each transducer element can also be filled with a polymer material
to ensure electrical isolation between the elements. Although the front portion 66 is illustrated as
a curved surface, the front portion 66 can be a stepped configuration, a series of linear segments,
or any other configuration. The thickness of structure 60 is large at each side portion 70 and 72
and small at the center. Furthermore, although the rear portion 68 is illustrated as a plane, the
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rear portion may be flat, concave or convex. Electrodes 74 and 76 similar to those described
above can be placed on the front and back portions of the composite structure. The composite
structure 60 of FIG. 14 can be deformed as shown in FIG. 15 to obtain recesses 66 'and 68'. The
deformed structure of FIG. 15 can be obtained by mechanically deforming the structure of FIG. In
some applications, the structure of FIG. 14 can be heated prior to deformation. If the fill material
between the vertical posts 62 is made of silicon rather than epoxy material, the structure of FIG.
14 can be easily deformed without the application of heat. If an epoxy material is used, the
structure of FIG. 14 should be exposed to heat at about 50 ° C. before deforming the structure.
Additionally, the composite structure can be deformed in opposite directions (not shown) to
obtain recessed portions 66 'and raised portions 68'. Forming the transducer structure of FIG. 14
not only provides a broadband transducer, but generally allows the ultrasound beam to be
focused on the region of interest. By deforming the structure as shown in FIG. 15, it is possible to
“fine tune” the focus of the ultrasound beam.
[0029]
To operate, the transducer array 10 is first activated at a higher frequency along a given scan
direction to focus the ultrasound beam at a near field (or field). The transducer gradually focuses
along a series of points along the scan line and reduces the excitation frequency as the beam is
gradually focused to a far field. If the value of LMAX is greater than 140% of the value of LMIN,
the output beamwidth having a narrow aperture at high frequencies will widen as the excitation
frequency decreases, as shown in FIG. Finally, at a sufficiently low frequency, such as 2 MHz, the
transducer 10 approaches a two dimensional array by effectively generating a beam using the
entire aperture of the transducer element 11. Further, the greater the curvature of the front
portion 12, the closer the transducer 10 is to a two dimensional array. A matching layer 24 can
also be disposed on the front portion 12 of the element 11 to further increase bandwidth and
sensitivity performance. Furthermore, when performing harmonic contrast imaging, the
transducer array element 11 is first excited at a main fundamental harmonic frequency such as
3.5 MHz to observe the heart or other tissue being observed. The transducer array element 11 is
then set to the main second harmonic reception mode, such as 7.0 MHz, to make the contrast
material more clearly visible to the tissue. This will allow you to see how well your organization is
working. When observing the fundamental harmonics, filters centered on the fundamental
frequency (e.g. electrical filters) can be used. When observing the second harmonic, a filter
centered on the second harmonic frequency can be used. Although the transducer array can be
set to the second harmonic reception mode as described above, the transducer array can transmit
and receive at the second harmonic frequency.
[0030]
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It is known to apply pulses to obtain the desired excitation frequency. The impulse response 100
illustrated in FIG. 20 has a width of about 0.25 μs. The impulse response 100 is the response to
the impulse excitation of a transducer with LMIN 0.109 mm, LMAX 0.381 mm, and the radius of
curvature of the front portion 12 103.54 mm. The impulse response 100 provides a frequency
spectrum in the range of about 1 MHz to 9 MHz. In applications where looking at the far field or
where there is no constraint on selecting a given aperture of transducer element 11 to generate
an ultrasound beam, impulse excitation is used to excite transducer element 11 Is desirable.
Excitation of the full aperture of the transducer element 11 also helps to increase the resolution
along the distance axis. In order to select the aperture of the central portion 19 of the transducer
element when looking at the near field, the transducer element 11 can be excited using a series
of pulses, such as about 2 to 5 pulses. The frequencies of these pulses are correlated to the
central portion 19 of the element 11. Typically, the frequency of these pulses is about 7 MHz and
the width of the pulses is about 0.14 μs.
[0031]
As described above, in order to approach a two-dimensional array at low frequencies, a series of
pulses, such as about 2 to 5 pulses, can be applied to excite transducer element 11. The
frequency of these pulses is matched to a resonant frequency that correlates to the thickest or
side portions 16, 18 of the transducer element. Typically, the frequency of these pulses is about
2.5 MHz and the width of the pulses is about 0.40 μs. This helps to generate a clear image when
looking at a distant place. The elements 11 of the single crystal design shown in FIGS. 3, 5 and
18 are each 15 mm in height and 0.0836 mm in azimuth. The distance S between the elements is
0.109 mm and the cut length is 25.4 μm. The thickness LMIN is 0.109 mm, and the thickness
LMAX is 0.381 mm. The radius of curvature of the front portion 12 is 103.54 mm. The backing
block is formed of a filled epoxy treated with Dow Corning part no. DER 332 with its hardener
DEH 24 and having an aluminum oxide filler. 128 The dimensions of the backing block for the
transducer array of elements are 20 mm in the azimuthal direction, 16 mm in the height
direction and 20 mm in the distance direction. The shape and size of the matching layer 24 are
as follows: LML at a given point on the transducer surface is the thickness of the matching layer,
LE is the thickness of the transducer element, CML is the speed of sound in the matching layer
and CE is the speed of sound in the element , LML = (1/2) (LE) (CML / CE) to approximate.
Transducers can be used with commercially available units such as Acuson Corporation's 128 XP
system with acoustic response technology (ART) capabilities.
[0032]
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In the two crystal design shown in FIG. 12, the minimum thickness measured in the distance
direction of the first and second piezoelectric portions 42 and 44 is 0.127 mm, and the
maximum thickness is 0.2794 mm. The radius of curvature of surfaces 46 and 48 of piezoelectric
portions 42 and 44 is 184.62 mm. The distance S between the elements is 0.254 mm and the cut
length is 25.4 μm. For the two crystal design shown in FIG. 13, the minimum thickness of the
piezoelectric portions 56 and 57 is 0.127 mm and the maximum thickness is 0.2794 mm. The
radius of curvature of the front portion 63 of the first piezoelectric portion 56 and the rear
portion 65 of the second piezoelectric portion 57 is 184.62 mm. The radius of curvature of the
front portion 67 of the piezoelectric portion 57 is 92.426 mm. Finally, the composite structural
design shown in FIG. 14 is preferably sized identical to that of FIG. 4 or 5, thereby forming 128
transducer elements. The structure of FIG. 11 also has a flat back portion 68, which is
particularly desirable when focusing in the far field. The structure of FIG. 15 can be formed by
deforming both ends of the structure of FIG. 14 in the distance direction. When focusing to a
near field of about 2 cm in the inspected body, the side part of the structure of FIG. 14 should be
deformed about 0.25 mm with respect to the central part.
[0033]
The backing block, flexible circuit, piezoelectric layer, matching layer, and tie layer can each be
glued together using any epoxy material. Hysol® Hardener # HD3561 from Hysol Division of
Industry, Dexter Corp. of California, # 2039, a Hysol based material, used to glue various
materials together Can. Typically, the thickness of the epoxy material is about 2 μm. The
thickness of the flexible circuit forming the first electrode to provide adequate electrical
excitation is approximately 25 μm for a flexible circuit made by Sheldahl. The thickness of the
second electrode is typically 2000 to 3000 angstroms and can be deposited on the transducer
structure by using sputtering techniques. It should be noted that the transducer array produced
according to the invention can be operated at the third harmonic, in this example 10.5 MHz. This
provides the observer with more information. Furthermore, by adding matching layer 24, the
transducer array can be operated even at a wider range of frequencies. Thus, this also allows the
converter of the invention to operate at both main and second harmonic frequencies.
[0034]
The shape of the present invention described above is illustrated as a preferred example, and
various changes can be made to the shape, size, and arrangement of parts without departing
from the spirit of the present invention or the scope of the claims. I want you to understand.
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