Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
-- 21 29946
BROADBAND PHASED ARRAY TRANSDUCER DESIGN
WITH FREQUENCY CONTROLLED TWO DIMENSION CAPABILITY
AND METHODS FOR MANUFACTURE THEREOF
BACKGROUND OF THE INVENTION
This invention relates to transducers and more
particularly to broadband phased array transducers for
use in the medical diagnostic field.
Ultrasound machines are often used for
observing organs in the human body. Typically, these
machines contain transducer arrays for converting
electrical signals into pressure waves. Generally, the
transducer array is in the form of a hand-held probe
which may be adjusted in position to direct the
ultrasound beam to the region of interest. Transducer
arrays may have, for example, 128 transducer elements for
generating an ultrasound beam. An electrode is placed at
the front and bottom portion of the transducer elements
for individually exciting each element, generating
pressure waves. The pressure waves generated by the
transducer elements are directed toward the object to be
observed, such as the heart of a patient being examined.
Each time the pressure wave confronts tissue having
different acoustic characteristics, a wave is reflected
backward. The array of transducers may then convert the
reflected pressure waves into corresponding electrical
signals. An example of a previous phased array acoustic
imaging system is described in U.S. Patent No. 4,550,607
granted November 5, 1985 to Maslak et al.
That patent
illustrates circuitry for combining the incoming signals
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received by the transducer array to produce a focused
image on the display screen.
Broadband transducers are transducers capable
of operating at a wide range of frequencies without a
loss in sensitivity. As a result of the increased
bandwidth provided by broadband transducers, the
resolution along the range axis may improve, resulting in
better image quality.
one possible application for a broadband
transducer is contrast harmonic imaging. In contrast
harmonic imaging, contrast agents, such as micro-balloons
of protein spheres, are safely injected into the body to
illustrate how much of a certain tissue, such as the
heart, is active. These micro-balloons are typically one
to five micrometers in diameter and, once injected into
the body, may be observed via ultrasound imaging to
determine how well the tissue being examined is
operating. Contrast harmonic imaging is an alternative
to Thallium testing where radioactive material is
injected into the body and observed by computer generated
tomography. Thallium tests are undesirable because they
employ potentially harmful radioactive material and
typically require at least an hour to generate the
computer image. This differs from contrast harmonic
imaging in that real-time ultrasound techniques may be
used in addition to the fact that safe micro-balloons are
employed.
In B. Schrope et al., "Simulated Capillary
Blood Flow Measurement Using a Nonlinear Ultrasonic
Contrast Agent," Ultrasonic Imaging, Vol. 14 at 134-58
(1992),
Schrope discloses that an observer may clearly see the
contrast agents at the second operating harmonic. That
is, at the fundamental harmonic, the heart and muscle
tissue is clearly visible via ultrasound techniques.
However, at the second harmonic, the observer is capable
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2 1 2q~46
of clearly viewing the contrast agent itself and thus may
determine how well the respective tissue is performing.
Because contrast harmonic imaging requires that
the transducer be capable of operating at a broad range
of frequencies (i.e. at both the fundamental and second
harmonic), existing transducers typically cannot function
at such a broad range. For example, a transducer having
a center frequency of 5 Megahertz and having a 70% ratio
of bandwidth to center frequency has a bandwidth of 3.25
Megahertz to 6.75 Megahertz. If the fundamental harmonic
is 3.5 Megahertz, then the second harmonic is 7.0
Megahertz. Thus, a transducer having a center frequency
of 5 Megahertz would not be able to adequately operate at
both the fundamental and second harmonic.
In addition to having a transducer which is
capable of operating at a broad range of frequencies,
two-dimensional transducer arrays are also desirable to
increase the resolution of the images produced. An
example of a two-dimensional transducer array is
illustrated in U.S. Patent No. 3,833,$25 to Haan issued
September 3, 1974~
Two-dimensional arrays allow for increased control of the
excitation of ultrasound beams along the elevation axis,
which is otherwise absent from conventional single-
dimensional arrays. However, two-dimensional arrays are
also difficult to fabricate because they typically
require that each element be cut into several segments
along the elevation axis, connecting leads for exciting
each of the respective segments. A two-dimensional array
having 128 elements in the azimuthal axis, for example,
would require at least 256 segments, two segments in the
elevation direction, as well as interconnecting leads for
the segments. In addition, they require rather
complicated software in order to excite each of the
several segments at appropriate times during the
ultrasound scan because there would be at least double
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the amount of segments which would have to be
individually excited as compared with a one-dimensional
array.
Further, typical prior art transducers having
parallel faces relative to the object being examined tend
to produce undesirable reflections at the interface
between the transducer and object being examined,
producing what is called a "ghost echo." These
undesirable reflections may result in a less clear image
being produced.
SUMMARY OF THE INVENTION
Consequently, it is a primary objective of this
invention to provide a broadband transducer array for use
in an acoustic imaging system that is easier and less
expensive to manufacture.
It is also an objective of this invention to
provide a broadband transducer array capable for use in
contrast harmonic imaging.
It is another objective of the present
invention to provide a transducer element and a matching
layer both having a negative curvature to allow for
additive focusing in the field of interest.
It is also an objective of the present
invention to provide a transducer array for use in an
acoustic imaging system that is capable of simulating a
two-dimensional transducer array at least at lower
frequencies.
It is a further objective of the present
invention to better suppress the generation of
undesirable reflections at the surface of the object
being examined.
It is another objective of the present
invention to further increase the sensitivity and
bandwidth of the transducer by disposing one or more
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matching layers on the front portion of a piezoelectric
layer that is facing a region of examination.
To achieve the above objectives, there are
provided several preferred embodiments of the present
invention. In a first embodiment of this invention, an
array-type ultrasonic transducer comprises a plurality of
transducer elements disposed adjacent to one another.
Each of the elements comprises a front portion facing a
region of examination, a back portion, two side portions,
and a transducer thickness between the front and back
portions. The transducer thickness is a maximum
thickness at the side portions and a minimum thickness
between the side portions. Further, the maximum
thickness is less than or equal to 140 percent of the
minimum thickness. Variation in thickness of the element
along the range axis as much as 20 to 40 percent is
preferred in this embodiment resulting in increased
bandwidth and shorter pulse width (i.e., the maximum
thickness is between 120 and 140 percent the value of the
minimum thickness). This provides improved resolution
along the range axis.
In a second embodiment of this invention, a
transducer for producing an ultrasonic beam upon
excitation comprises a plurality of piezoelectric
elements. Each of the elements comprises a thickness at
at least a first point on a surface facing a region of
examination being less than a thickness at at least a
second point on the surface, the surface being generally
non-planar. In addition, the aperture of an ultrasound
beam produced by the present invention varies inversely
as to a frequency of excitation of the element.
Generally, where the maximum thickness of the
piezoelectric element is greater than 140 percent of the
minimum thickness of the piezoelectric element, the
transducer may simulate the beam produced by a two-
dimensional array at lower frequencies. This is due to
21299q~
the fact that at lower frequencies, the exiting pressure
wave generated by the transducer has at least two peaks.
Further, the full aperture is typically activated at
lower frequencies. Consequently, the second embodiment
simulates the excitation of a wider aperture two-
dimensional transducer array.
In a third preferred embodiment, a two crystal
transducer element design is provided comprising a first
piezoelectric portion with a thickness at at least one
point on a first surface facing a region of examination
being less than a thickness at at least one other point
on the first surface, the first surface being generally
non-planar. An interconnect circuit may be disposed
between the first piezoelectric portion and a second
piezoelectric portion. A matching layer may be disposed
on the first piezoelectric portion.
In a fourth preferred embodiment, a composite
structure transducer is provided comprising a plurality
of vertical posts of piezoelectric material comprising
varying thickness and polymer layers in between the
posts. This structure may be deformed to produce the
desired transducer configuration. In addition, a
matching layer may be disposed on the composite
transducer structure to further increase performance.
The transducer of all embodiments allows for
the transducer to operate at a broader range of
frequencies and allows for correct apodization. Because
the embodiments do not require matching the back acoustic
port of the element, they generally are easier to
fabricate than prior art devices.
A first preferred method of the invention for
making a transducer is disclosed by forming a plurality
of transducer elements disposed adjacent to one another.
Each of the elements comprises a front portion facing a
region of examination, a back portion, two side portions,
and a transducer thickness between the front and back
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portions. Further, the transducer thickness is a maximum
thickness at the side portions and a minimum thickness
between the side portions, the maximum thickness being
less than or equal to 140 percent of the minimum
thickness. An electric field is established through at
least one portion of each of the elements.
A second preferred method of the invention for
making a transducer is disclosed by forming a plurality
of piezoelectric elements, each of the elements
comprising a thickness at at least one point on a front
surface facing a region of examination being less than a
thickness at at least one other point on the surface, the
surface being generally non-planar. An electric field is
established at least through one portion of each of the
elements. For example, electrodes may be placed on the
front surface and back portion of each of the
piezoelectric elements to provide the electric field.
Upon application of an excitation pulse to the
electrodes, the aperture of an ultrasound beam produced
by the transducer varies inversely as to the frequency of
the excitation pulse, where the maximum thickness of the
piezoelectric element is typically greater than 140
percent of the minimum thickness of the piezoelectric
element.
A third preferred method of the invention for
making a transducer is disclosed by forming a
piezoelectric element comprising composite material
comprising a front portion facing a region of
examination, the thickness of at least one point on the
front portion being less than the thickness on at least
one other point on the front portion. First and second
electrodes may also be placed on the piezoelectric
element. The element may be deformed to the desired
shape.
The transducer of all embodiments as well as
those made by the disclosed methods may be in the form of
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a hand-held probe which may be adjusted in position
during excitation to direct the ultrasound beam to the
region of interest. Further, the transducer of all
embodiments as well as those made by the disclosed
methods may be placed in a housing for placement in a
hand-held probe. Other types of probes and manners of
directing the beam are possible. The ultrasound system
for generating an image comprises transmit circuitry for
transmitting electrical signals to the transducer probe,
receive circuitry for processing the signals received by
the transducer probe, and a display for providing the
image of the object being observed. The transducers
convert the electrical signals provided by the transmit
circuitry to pressure waves and convert the pressure
waves reflected from the object being observed into
corresponding electrical signals which are then processed
in the receive circuitry and ultimately displayed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of an ultrasound
system for generating an image.
FIG. 2 is a cross-sectional view of a
transducer element in accordance with the first preferred
embodiment.
FIG. 3 is a cross-sectional view of a
transducer element in accordance with the second
preferred embodiment.
FIG. 4 is a perspective view of a broadband
transducer array further illustrating the probe of FIG.
1 in accordance with the first preferred embodiment.
FIG. 5 is a perspective view of a broadband
transducer array further illustrating the probe of FIG.
1 and the beam widths produced for low and high
frequencies in accordance with the second preferred
embodiment.
212994~
g
FIG. 6 is an enlarged view of a single
broadband transducer element of the transducer array
constructed in accordance with the present invention.
FIG. 7 is a perspective view of a broadband
transducer array in accordance with the present invention
further illustrating the probe of FIG. 1 and having a
curved matching layer disposed on a front portion of the
transducer elements.
FIG. 8 is a cross-sectional view of a single
broadband transducer element in accordance with the
present invention having a curved matching layer and
further having a coupling element thereon.
FIG. 9 is a view of the exiting beam width
produced by the broadband transducer elements from low to
high frequencies as compared to the width of the
transducer element in accordance with the second
preferred embodiment.
FIG. 10 is an example of a typical acoustic
impedance frequency response plot resulting from
operation of the transducer constructed in accordance
with the second preferred embodiment.
FIG. 11 is an example of a typical acoustic
impedance frequency response plot resulting from
operation of a prior art transducer.
FIG. 12 is a cross-sectional view of a two
crystal design having interconnect circuitry between the
two crystal elements in accordance with the third
preferred embodiment.
FIG. 13 is a cross-sectional view of an
alternate two crystal design.
FIG. 14 is a cross-sectional view of a
composite transducer element in accordance with a fourth
preferred embodiment.
FIG. 15 is a cross-sectional view of the
composite transducer element of FIG. 14 which is
deformed.
2129946
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FIG. 16 is a cross-sectional view of a
piezoelectric layer and surface grinder wheel
illustrating a preferred method for machining the surface
of the piezoelectric layer.
FIG. 17 is a cross-sectional view of a
piezoelectric layer and surface grinder wheel
illustrating another preferred method for machining the
surface of the piezoelectric layer.
FIG. 18 shows a partial perspective view of a
linear transducer array in accordance with the present
invention.
FIG. 19 shows a partial perspective view of a
curvilinear transducer array in accordance with the
present invention with a portion of the flex circuit
removed at one end for purposes of illustration.
FIG. 20 shows an impulse response and the
corresponding frequency spectrum for the transducer
element of FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the accompanying drawing FIG.
1, there is provided a schematic view of an ultrasound
system 1 for generating an image of an object or body 5
being observed. The ultrasound system 1 has transmit
circuitry 2 for transmitting electrical signals to the
transducer probe 4, receive circuitry 6 for processing
the signals received by the transducer probe, and a
display 8 for providing the image of the object 5 being
observed.
Referring also to FIG. 4, the probe 4 contains
an array 10 of transducer elements 11. Typically, there
are one hundred twenty eight elements 11 in the y -
azimuthal axis forming the broadband transducer array 10.
However, the array can consist of any number of
transducer elements 11 each arranged in any desired
212994~
--11--
geometrical configuration. The transducer array 10 is
supported by backing block 13.
The probe 4 may be hand-held and can be
adjusted in position to direct the ultrasound beam to the
region of interest. The transducer elements 11 convert
the electrical signals provided by the transmit circuitry
2 to pressure waves. The transducer elements 11 also
convert the pressure waves reflected from the object 5
being observed into corresponding electrical signals
which are then processed in the receive circuitry 6 and
ultimately displayed 8.
Referring to FIGS. 2, 4, and 6, there is
provided the first embodiment of the present invention.
Transducer element 11 has a front portion 12, a back
portion 14, a center portion 19, and two side portions 16
and 18. The front portion 12 is the surface which is
positioned toward the region of examination. The back
portion 14 may be shaped as desired, but is generally a
planar surface. The front portion 12 is generally a non-
planar surface, the thickness along the z-axis of element
11 may be greater at each of the side portions 16 and 18
and smaller between the side portions. The term side
portion 16, 18 refers not only to the sides 15 of the
respective element 11, but may also include a region
interior to the element 11 where the thickness of the
element is greater than a thickness toward the interior
of the element (e.g., where the thickness of each of the
sides of the element are tapered).
Although the front portion 12 is illustrated
having a continuously curved surface, front portion 12
may include a stepped configuration, a series of linear
segments, or any other configuration wherein the
thickness of element 11 is greater at each of the side
portions 16 and 18 and decreases in thickness at the
center portion 19, resulting in a negatively "curved"
front portion 12. The back portion 14 which is generally
-12- 2129~6
preferably a planar surface may also be, for example, a
concave or convex surface.
Element 11 has a maximum thickness LMAX and a
minimum or smallest thickness LMIN, measured along the
range axis. Preferably the side portions 16 and 18 both
are equal to the thickness LMAX and the center of element
11, or substantially near the center of element 11, is at
the thickness of LMIN. However, each of the side
portions 16, 18 do not have to be the same thickness and
LMIN does not have to be in the exact center of the
transducer element to practice the invention.
In the first preferred embodiment, the value of
LMAX is less than or equal to 140 percent the value of
LMIN. This allows for an increase in bandwidth
activation energy generally without the need to reprogram
the ultrasound machine for generating the ultrasound
beam. Further, when the value of LMAX is less than or
equal to 140 percent the value of LMIN, the exiting beam
width is generally the same for different exciting
frequencies.
The increase in bandwidth activation energy for
the transducer configuration of the present invention is
approximated by LMAX/LMIN where the transducer is of the
free resonator type (i.e., does not comprise a matching
layer) or is an optimally matched transducer (i.e., has
at least two matching layers), to be discussed later. In
the first preferred embodiment shown in FIGS. 2, 4, and
6, the bandwidth may be increased by 40 percent by
increasing the thickness of LMAX relative to LMIN by 40
percent, respectively (e.g., LMAX is 140 percent of the
value of LMIN).
If, for example, a transducer has an LMAX of
0.3048mm and an LMIN of 0.254mm, the bandwidth is
increased by 20 percent as compared to a transducer
having a uniform thickness of 0.254mm. Similarly, if a
transducer has an LMAX of 0.3556mm and an LMIN of
21~9!31~
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0.254mm, the bandwidth is increased by 40 percent as
compared to a transducer having a uniform thickness of
0.254mm. Variation in thickness of the element along the
range axis as much as 20 to 40 percent is preferred in
this embodiment resulting in increased bandwidth and
shorter pulse width (i.e., the maximum thickness is
greater than or equal to 120 percent of the minimum
thickness or less than or equal to 140 percent of the
minimum thickness). This results in the maximum
bandwidth increase, approximately 20 to 40 percent,
respectively. Further, this provides improved resolution
along the range axis.
The slight variation in thickness of the front
portion 12 relative to the back portion 14 of the first
embodiment allows for better transducer performance
where, for example, the transducer is activated at three
different frequencies, such a 2MHz, 2.5MHz, and 3MHz,
known as a tri-frequency mode of operation. Such a tri-
frequency mode of operation may be used in cardiac
applications. Moreover, the slight variation in
transducer thickness may also improve transducer
performance for other tri-frequency modes of operation,
such as operation at the frequencies of 2.5MHz, 3.5MHz,
and 5MHz.
Preferably, the element 11 is a plano-concave
structure and is composed of the piezoelectric material
lead zirconate titanate (PZT). However, the element 11
may also be formed of composite material as discussed
later, polyvinylidene fluoride (PVDF), or other suitable
material. Referring also to FIG. 8, electrodes 23 and 25
may appropriately be placed on the front 12 and bottom 14
portions of the element 11 in order to excite the element
to produce the desired beam, as is well known in the art.
Although electrode 25 is shown to be disposed directly on
the piezoelectric element 11, it may alternatively be
disposed on matching layer 24. As a result, the matching
- 21~9946
-14-
layer 24 may be directly disposed on piezoelectric
element 11. The electrodes 23 and 25 establish an
electric field through the element 11 in order to
produced the desired ultrasound beam.
An example of the placement of electrodes in
relation to the piezoelectric material is illustrated in
U.S. Patent No. 4,611,141 to Hamada et al. issued
September 9, 1986 and is incorporated herein by
reference. A first electrode 23 provides the signal for
exciting the respective transducer element and the second
electrode may be ground. Leads 17 may be utilized to
excite each of the first electrodes 23 on the respective
transducer elements 11 and the second electrodes 25 may
all be connected to an electrical ground. As is commonly
known in the industry, electrodes may be disposed on the
piezoelectric layer by use of sputtering techniques.
Alternatively, an interconnect circuit, described later,
may be used to provide the electrical excitation of the
respective transducer elements.
Referring now to FIGS. 3 and 5, there is shown
the second preferred embodiment of the present invention
wherein like components have been labeled similarly.
Although FIGS. 6 and 8 have been described in relation to
the first preferred embodiment, they will be used to
illustrate the second preferred embodiment in light of
the similarity of the two embodiments. Further, the
thickness at at least a first point on the front portion
12 is less than a thickness at at least a second point on
the front portion. In addition, the front portion is
generally non-planar.
In the second preferred embodiment, the value
of LMAX is greater than 140 percent the value of LMIN.
Where the value of LMAX is greater than 140 percent of
the value of LMIN, the exiting beam width produced
typically varies with frequency. In addition, the lower
the frequency, the wider the exiting beam width.
-15- 2129946
FIG. 9 illustrates the typical variation in the
exiting beam width or aperture along the elevation
direction produced by the broadband transducer from low
to high frequencies in accordance with the second
preferred embodiment. At high frequencies, such as 7
Megahertz, the beam has a narrow aperture. When the
frequency is lowered, the beam has a wider aperture.
Further, at low enough frequencies, such as 2 Megahertz,
the beam is effectively generated from the full aperture
of the transducer element 11. As shown in FIG. 9, the
exiting pressure wave has two peaks, simulating the
excitation of a wide aperture two-dimensional transducer
array at lower frequencies.
FIGS. 5 further illustrates the beam width
variation of the whole transducer array as a function of
frequency for the second preferred embodiment. At high
excitation frequencies, the exiting beam width has a
narrow aperture and is generated from the center of
elements 11. On the contrary, at low excitation
frequencies, the exiting beam width has a wider aperture
and is generated from the full aperture of elements 11.
By controlling the excitation frequency, the
operator may control which section of transducer element
11 generates the ultrasound beam. That is, at higher
excitation frequencies, the beam is primarily generated
from the center of the transducer element 11 and at lower
excitation frequencies, the beam is primarily generated
from the full aperture of the transducer element 11.
Further, the greater the curvature of the front portion
12, the more the element 11 simulates a wide aperture
two-dimensional transducer array.
In order to pursue the second preferred
embodiment, that is, increasing the bandwidth greater
than 40 percent, it may be necessary to reprogram the
ultrasound machine for exciting the transducer at such a
broad range of frequencies. As seen by the equation
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LMAX/LMIN, the greater the thickness variation, the
greater the bandwidth increase. Bandwidth increases of
300 percent, or greater, for a given design may be
achieved in accordance with the principles of the
invention. Thus, the thickness LMAX would be
approximately three times greater than the thickness
LMIN. The bandwidth of a single transducer element, for
example, may range from 2 Megahertz to 11 Megahertz,
although even greater ranges may be achieved in
accordance with the principles of this invention.
Because the transducer array constructed in accordance
with this invention is capable of operating at such a
broad range of frequencies, contrast harmonic imaging may
be achieved with a single transducer array in accordance
with this invention for observing both the fundamental
and second harmonic (i.e., the transducer is operable at
a dominant fundamental harmonic frequency and is operable
at a dominant second harmonic frequency).
The thickness variation of the transducer
element 11 greatly increases the bandwidth, as
illustrated in FIGS. 10 and 11. FIGS. 10 and 11 provide
one example of the effect of utilizing a plano-concave
transducer element 11 on bandwidth performance and
results may vary depending on the particular
configuration used. FIG. 10 illustrates an impedance
plot for a transducer element 11 produced in accordance
with the second preferred embodiment of the present
invention having an outer edge thickness LMAX of 0.015
inches (0.381mm) and a center thickness LMIN of 0.00428
inches (0.109mm). As can be seen, the element has a
bandwidth from approximately 3.5 Megahertz to 10.7
Megahertz. In contrast, a conventional element having a
uniform thickness of 0.381mm typically has a bandwidth of
approximately 4.5 Megahertz to approximately 6.6
Megahertz, as illustrated by FIG. 11. Thus, by comparing
~f, which is the difference between fal the anti-resonant
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frequency (i.e., maximum impedance), and fr, the resonant
frequency (i.e., minimum impedance), a fractional
bandwidth of 100% is provided by the transducer element
produced in accordance with the present invention versus
a fractional bandwidth of approximately 38% for the prior
art design.
Therefore, by controlling the curvature shape
of the transducer element (i.e., cylindrical, parabolic,
gaussian, stepped, or even triangular), one can
effectively control the frequency content of the radiated
energy. The use of each of these shapes, as well as
others, is considered within the scope of the present
invention.
Referring now to FIGS. 7 and 8, wherein like
components are labeled similarly, the transducer
structure in accordance with the invention is shown
having a curved matching layer 24 disposed on the front
portion 12 of transducer element 11. The matching layer
24 is preferably made of a filled polymer. Moreover, the
thickness of the matching layer 24 is preferably
approximated by the equation:
LML = (~)(LE)(CML/CE)
where, for a given point on the transducer surface, LML
is the thickness of the matching layer, LE is the
thickness of the transducer element, CML is the speed of
sound of the matching layer, and CE is the speed of sound
of the element. The curvature of the front portion 12
may be different than the curvature of the top portion 26
of the matching layer 24 because the thickness of the
matching layer depends on the thickness of the element at
a given point of the transducer surface. Although one or
more matching layers are preferably formed using the
above equation, the matching layers may be constant in
thickness for ease of manufacturing.
By the addition of matching layer 24, the
fractional bandwidth can be improved. Further, the
21~9946
-18-
transducer may act with increased sensitivity. However,
the thickness difference between the edge and center of
the assembled substrates will control the desired
bandwidth increase, and the shape of the curvature will
control the base bandshape in the frequency domain.
Further, because both the transducer element 11 and the
matching layer 24 have a negative curvature, there is
additive focusing in the field of interest.
More than one matching layer may be added to
the front portion 12 to effect focusing in the field of
interest and to improve the sensitivity of the
transducer. Preferably, there are two matching layers
placed upon the piezoelectric element 11 resulting in an
optimally matched transducer. Each are calculated by the
equation LML = (~)(LE)(CML/CE). Specifically, for
calculating the thickness LML for the first matching
layer, the value of the speed of sound CML for that first
material is used. When calculating the thickness LML for
the second matching layer, the value of the speed of
sound CML for that second material is used. Preferably,
the value of the acoustic impedance for the first
matching layer (i.e., the matching layer closest to the
piezoelectric element) is approximately 10 Mega Rayls and
the value of the acoustic impedance for the second
matching layer (i.e., the matching layer closest to the
object being observed) is approximately 3 Mega Rayls.
A coupling element 27 having the acoustical
properties of the object being examined may be disposed
on the matching layer or directly on the second electrode
25 if, for example, the matching layer is not used. The
coupling element 27 may provide increased patient comfort
because it may alleviate any of the sharper surfaces in
the transducer structure which are in contact with the
body being examined. The coupling element 27 may be used,
for example, in applications where the curvature of the
front portion 12 or top portion 26 are large. The
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coupling element 27 may be formed of unfilled
polyurethane. The coupling element may have a surface 29
which is generally flat, slightly concave, or slightly
convex. Preferably, the curvature of surface 29 is
slightly concave so that it may hold an ultrasound gel
28, such as Aquasonic~ manufactured by Parker Labs of
Orange, New Jersey, now shown, between the probe 4 and
the object being examined. This provides strong
acoustical contact between the probe 4 and the object
being examined. The matching layer and coupling element
described may be placed on all of the embodiments
disclosed.
Machines such as a numerically controlled
machine tool which is commonly used in the ultrasound
industry may be used to provide the thickness variation
of the transducer element. The machine tool may machine
an initial piezoelectric layer in order to have the
desired thickness variation of LMAX and LMIN.
FIG. 16 shows a first method of machining the
piezoelectric layer 80 where it is desired to have a
curvature 82 on the front portion. The numerically
controlled machine is first inputted with the coordinates
for defining the radius of curvature R approximated by
the equation 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 elevation axis. Then,
a surface grinder wheel 84 on the numerically controlled
machine having a width coextensive in size with the
piezoelectric layer 80 machines the piezoelectric layer.
The surface grinder wheel rotates about an axis 86 which
is parallel to the elevation axis. The surface grinder
wheel contains an abrasive material such as Aluminum
Oxide. The surface grinder wheel preferably begins
machining at one end of the piezoelectric layer 80 along
the azimuthal direction until it reaches the other end of
the piezoelectric layer.
- 2129946
-20-
FIG. 17 shows an alternate method of machining
the piezoelectric layer 80. With this method, the
surface grinder wheel 84 is tilted such that one corner
88 of the surface grinder wheel contacts a surface of the
piezoelectric layer 80. For a given azimuthal region,
the surface grinder wheel 84 begins at one side of the
piezoelectric layer 80 along the elevation axis until it
reaches the other side of the piezoelectric layer along
the elevation axis (e.g., the surface grinder wheel makes
the desired cut along the elevation axis for a certain
index in the azimuthal axis). The surface grinder wheel
84 rotates about an axis 90. Then, the surface grinder
wheel 84 is moved to a different region or index along
the azimuthal axis and repeats the machining from one
side to the other side of the piezoelectric layer along
the elevation axis. This process is repeated until the
whole piezoelectric layer 80 is machined to have the
desired curvature 82.
The machined surface may also be ground or
polished to provide a smooth surface. This is especially
desirable where the transducer is used at very high
frequencies such as 20 MHz.
Referring also to FIGS. 7 and 18, a number of
electrically independent piezoelectric elements 11 may
then be formed by dicing kerfs 94 accomplished by dicing
the piezoelectric material, as is commonly done in the
industry. The kerfs 94 result in a plurality of matching
layers 24, piezoelectric elements 11, and electrodes 23.
The kerf may also slightly extend into the backing block
13 to ensure electrical isolation between transducer
elements.
Referring to FIG. 8, a metalization layer may
be directly deposited on top of the piezoelectric layer
prior to dicing to form the second electrodes 25. If a
matching layer 24 is also employed, the second electrode
25 is preferably disposed on the top portion 26 of
212994~
-21-
matching layer 24. However, the top portion 26 of the
matching layer 24 is preferably shorted to the second
electrode 25 via metalization across the edges of the
matching layer or by using an electrically conductive
material such as magnesium or a conductive epoxy. In
addition, where a matching layer is used, the dicing may
be done after the matching layer is disposed on top of
the piezoelectric layer. In a preferred embodiment, the
second electrode 25 is held at ground potential. If a
flex circuit 96, described later, is used, the dicing may
extend through the flex circuit, forming individual
electrodes 23.
When the transducer is designed for operation
in the sector format, the length S, which is the element
spacing along the azimuthal direction, is preferably
approximated by half a wavelength of the object being
examined at the highest operating frequency of the
transducer. This approximation also applies for the two
crystal design described later. When the transducer is
designed for linear operation, or if the transducer array
is curvilinear in form, the value S may vary between one
and two wavelengths of the object being examined at the
highest operating frequency of the transducer.
FIG. 19 shows a curvilinear transducer array
constructed in accordance with the principles of this
invention. Specifically, the curvilinear array is
constructed similarly to the linear transducer array of
FIG. 18. However, rather than directly resting on the
large backing block 13 of FIG. 18, the piezoelectric
elements 11 and flex circuit 96 with corresponding
electrodes 23 are placed directly upon a first backing
block 13' having a thickness of approximately lmm. This
allows easy bending of the array to the desired amount in
order to increase the field of view.
Typically, the radius of curvature of the first
backing block 13' is approximately 44mm but may vary as
21299 16
-22-
desired. The first backing block may be secured to a
second backing block 13" having a thickness in the range
direction of approximately 2cm by use of an epoxy glue.
Preferably, the surface of the second backing block 13"
adjacent to the first backing block 13' has a similar
radius of curvature. As is commonly know in the
industry, a curvilinear array functions similarly to a
linear array having a mechanical lens disposed in front
of the linear array.
Because the signal at the center portion 19 of
the transducer element 11 is stronger than at the end or
side portions 16 and 18, correct apodization occurs (i.e,
reduces or suppresses the generation of sidelobes). This
is due to the fact that the electric field between the
two electrodes on the front portion 12 and bottom portion
14 is greatest at the center portion 19, reducing side
lobe generation. In addition, because the front and
bottom portions are not flat parallel surfaces, the
generation of undesirable reflections at the interface of
the transducer and object being examined (i.e., ghost
echoes) are better suppressed. Further, because the
transducer array constructed in accordance with the
present invention is capable of operating at a broad
range of frequencies, the transducer is capable of
receiving signals at center frequencies other than the
transmitted center frequency.
As to the design of the spacing between the
elements 11 and the design of the transducer aperture or
width w, the upper operating frequency of a transducer
will have the greatest impact on the grating lobe. The
grating lobe image artifact (i.e., the creation of
undesirable multiple mirror images of the object being
observed) can be avoided if one designs the element
spacing to take into account the highest operating
frequency for the transducer. Specifically, the
relationship between the grating lobe angle eg, the
21299~6
-23-
electronic steering angle in sector format e5, the
wavelength of the object being examined at the highest
operating frequency of the transducer ~, and the spacing
between the elements S is given by the equation:
S S ~/(sine5 - sineg).
Therefore, for a given grating lobe angle, the design of
the transducer aperture is restricted by the upper
operating frequency of the transducer.
As illustrated by the equation, in order to
sweep at higher frequencies, it is necessary to reduce
the aperture correlating to that frequency. For example,
at an operating frequency of 3.5 Megahertz, the desired
spacing between the elements S is 220 um while at 7.0
Megahertz, the spacing S is 110 um. Because at higher
frequencies it is desirable to decrease the aperture of
the transducer element as given by the above described
equation, use of the transducer element at lower
frequencies will result in some resolution loss. This is
due to the fact that lower frequency operation typically
requires a greater element aperture. However, this is
compensated by the fact that the transducer simulates a
two-dimensional array at lower frequencies where the
value of LMAX is greater than 140 percent the value of
LMIN, which increases the resolution of the images
produced at the lower frequencies by wider aperture.
A two crystal transducer element design may be
employed using the principles of this invention.
Referring to FIG. 12, a two crystal transducer element 40
is shown having a first piezoelectric portion 42 and a
second piezoelectric portion 44. These piezoelectric
portions may be machined as two separate pieces.
Preferably, both surfaces 46 and 48 are generated by the
equation h/2 + (wZ/8h), where h is the thickness
difference between LMAX and LMIN and w is the width of
the transducer element along the elevation axis.
Although piezoelectric portions 42 and 44 are illustrated
2129946
-24-
as being plano-concave in structure, the surfaces 46 and
48 may include a stepped configuration, a series of
linear segments, or any other configuration. The
thickness of each of the portions 42 and 44 may be
greater at each of the side portions 43, 45, 47, 49 and
decrease in thickness at the respective center portions
of piezoelectric portions 42 and 44. In addition, the
back portions 51 and 53 of the piezoelectric portions 42
and 44, respectively, are preferably generally planar
surfaces. However, these surfaces may also be non-
planar.
An interconnect circuit 50 is disposed between
the first piezoelectric portion 42 and the second
piezoelectric portion 44. The interconnect circuit 50
may comprise any interconnecting design used in the
acoustic or integrated circuit fields. The interconnect
circuit 50 is typically made of a copper layer carrying
a lead for exciting the transducer element 40. The
copper layer may be bonded to a piece of polyamide
material, typically kapton. Preferably, the copper layer
is coextensive in size with each of the piezoelectric
portions 42 and 44. In addition, the interconnect
circuit may be gold plated to improve the contact
performance. Such an interconnect circuit may be a flex
circuit manufactured by Sheldahl of Northfield,
Minnesota.
To further increase performance, a matching
layer 52 may be disposed above piezoelectric portion 42.
Where both the first and second piezoelectric portions
are formed of the same material, the matching layer 52
has a matching layer thickness LML approximated by
(1/2)(LE)(CML/CE), where, for a given point on the
transducer surface, LML is the thickness of the matching
layer, LE is the thickness of the first and second
piezoelectric portions, CML is the speed of sound of the
matching layer, and CE is the speed of sound of the
21~99~6
-25-
piezoelectric portions. Ground layers 58 and 59 may be
disposed directly on the matching layer 52 and on surface
48, connecting the two piezoelectric portions in
parallel.
The matching layer may be coated with
electrically conductive material, such as nickel and
gold. However, if the matching layer 52 is not employed,
then the ground layers are both disposed directly on the
piezoelectric portions 42 and 44. The matching layer 52
may face the region being examined. The transducer 40
may be placed on a backing block 54, as is commonly used
in the ultrasonic field. Further, a coupling element as
described earlier may also be used.
FIG. 13 illustrates another two crystal design
55 employing the principles of this invention. A first
piezoelectric portion 56 and a second piezoelectric
portion 57 are provided. The piezoelectric portion 56 is
preferably plano-concave in shape. In addition, the
second piezoelectric portion 57 has a thickness variation
along the elevation direction as well. An interconnect
circuit 50 as described above may be used in between the
two piezoelectric portions to excite the two crystal
transducer 55. A matching layer as well as a coupling
element as described earlier may also be provided to
improve performance as well as patient comfort. Further,
electrodes 58 and 59 may be used to connect the two
piezoelectric portions in parallel.
Preferably, the back portion 61 of the first
piezoelectric portion 56 is generally a flat surface.
The radius of curvature R for the front portion 63 and
the bottom portion 65 of the first and second
piezoelectric portions 56 and 57, respectively, is
approximated by the equation h/2 + (w2/8h), where h is
the thickness difference between LMAX and LMIN of
piezoelectric portion 56 and w is the width of the
transducer element along the elevation axis. Preferably,
2 ~ 29946
-26-
the value of LMAX and LMIN is the same for both the first
and second piezoelectric portions 56 and 57. The radius
of curvature R for the front portion 67 of the second
piezoelectric portion 57 is approximated by the equation
h'/2 + (w2/8h'), where h' is the thickness difference
between the combined maximum thickness for both
piezoelectric portions and the combined minimum thickness
for both piezoelectric portions and w is the width of the
transducer element along the elevation axis. To achieve
the desired radii of curvature, piezoelectric portions 56
and 57 may be machined by a numerically controlled
machine tool as described earlier.
Instead of using a uniform layer of
piezoelectric material, a composite structure 60 as shown
in FIG. 14 may be utilized formed of composite material.
The composite structure 60 contains a plurality of
vertical posts or slabs of piezoelectric material 62
having varying thickness. In between the posts 62 are
polymer layers 64 which may be, for example, formed of
epoxy material. The composite material may, for example,
be that described by R.E. Newnham et al. "Connectivity
and Piezoelectric-Pyroelectric Composites", Materials
Research Bulletin, Vol. 13 at 525-36 (1978) and R.E.
Newnham et al., "Flexible Composite Transducers",
Materials Research Bulletin, Vol. 13 at 599-607 (1978),
The
composite structure 60 is preferably plano-concave. An
acoustic matching layer, not shown, may be disposed on
the front portion 66 for increasing performance.
The composite material may be embedded in a
polymer layer. Then, the composite material may be
ground, machined, or formed to the desired size. In
addition, the individual transducer elements may be
formed by sawing the composite structure, as is commonly
done in the ultrasound industry. The gaps between each
of the respective transducer elements may also be filled
- 212~
-27-
with polymer material to ensure electrical isolation
between elements.
Although the front portion 66 is shown as a
curved surface, the front portion 66 may include a
stepped configuration, a series of linear segments, or
any other configuration wherein the thickness of the
structure 60 is greater at each of the side portions 70,
72 and decreases in thickness at the center. In
addition, although the back portion 68 is shown as a flat
surface, the back portion may be a generally planar
surface, a concave or a convex surface. Electrodes 74
and 76, similar to the electrodes described earlier, may
be placed on the front and back portions of the composite
structure.
The composite structure 60 of FIG. 14 may be
deformed as shown in FIG. 15 resulting in both a concave
portion 66' and a concave portion 68'. The deformed
structure of FIG. 15 may result by mechanically deforming
the structure of FIG. 14. In certain applications, the
structure of FIG. 14 may be heated prior to deforming.
If the filler material between the vertical posts 62 is
made of silicone rather than an epoxy material, the
structure of FIG. 14 may easily be deformed without the
application of heat. If epoxy material is used, then the
structure of FIG. 14 should be exposed to approximately
50~C before deforming the structure. In addition, the
composite structure may be deformed in the opposite
direction, not shown, resulting in a concave portion 66'
and a convex portion 68'. It should be noted that
forming the transducer structure of FIG. 14 not only
allows for a broadband transducer, but also generally
provides focusing of the ultrasound beam in the region of
interest. By deforming the structure as shown in FIG.
15, one is capable of "fine tuning" the focusing of the
ultrasound beam.
2129g4~
-28-
In operation, the transducer array 10 may first
be activated at a higher frequency along a given scan
direction in order to focus the ultrasound beam at a
point in the near field. The transducer may be gradually
focused along a series of points along the scan line,
decreasing the excitation frequency as the beam is
gradually focused in the far field. Where the value of
LMAX is greater than 140 percent the value of LMIN, the
exiting beam width, which has a narrow aperture at high
frequencies, may widen in aperture as the excitation
frequency is decreased, as illustrated in FIG. 9.
Eventually, at a low enough frequency, such as two
Megahertz, the transducer 10 simulates a two-dimensional
array by effectively generating a beam using the full
aperture of the transducer elements 11. Further, the
greater the curvature of front portion 12, the more the
transducer 10 simulates a two-dimensional array. A
matching layer 24 may also be disposed on the front
portion 12 of element 11 in order to further increase
bandwidth and sensitivity performance.
In addition, when performing contrast harmonic
imaging, the transducer array elements 11 may first be
excited at a dominant fundamental harmonic frequency,
such as 3.5 Megahertz, to observe the heart or other
tissue being observed. Then, the transducer array
elements 11 may be set to the receive mode at a dominant
second harmonic, such as 7.0 Megahertz, in order to make
the contrast agent more clearly visible relative to the
tissue. This will enable the observer to ascertain how
well the tissue is operating. When observing the
fundamental harmonic, filters (e.g., electrical filters)
centered around the fundamental frequency may be used.
When observing the second harmonic, filters centered
around the second harmonic frequency may be used.
Although the transducer array may be set to the receive
mode at the second harmonic as described above, the
- - 2129~46
-29-
transducer array may be capable of transmitting and
receiving at the second harmonic frequency.
The application of pulses to obtain the desired
excitation frequency is well known in the art. For
illustrative purposes, referring now to FIG. 20, an
impulse response 100 is shown having a width of
approximately 0.25usec. The impulse response 100 is the
transducer response to an impulse excitation where LMIN
is O.lO9mm, LMAX is 0.381mm, and the radius of curvature
of the front portion 12 is 103.54mm. The impulse
response 100 results in a frequency spectrum 102 ranging
from approximately lMHz to 9MHz. It is desirable to
excite the transducer element 11 with the use of an
impulse excitation when viewing the far field or in
applications where one is not limited to selecting a
given aperture of the transducer element 11 for producing
an ultrasound beam. Exciting the whole aperture of the
transducer element 11 also helps produce a finer
resolution along the range axis.
To select the aperture of the central portion
19 of transducer element when viewing the near field, a
series of pulses, approximately 2 to 5 pulses, may be
used to excite the transducer element 11. The pulses
have a frequency correlating to the central portion 19 of
the element 11. Typically, the frequency of the pulses
is approximately 7MHz and the width of the pulses is
approximately 0.14 usec.
To simulate a two-dimensional array at lower
frequencies, as discussed earlier, a series of pulses,
approximately 2 to 5 pulses, may be applied to excite the
transducer element 11. The pulses have a frequency which
matches the resonance frequency correlating to the
thickest or side portions 16, 18 of the transducer
element. Typically, the frequency of the pulses is
approximately 2.5MHz and the width of the pulses is
2129~4~
-30-
approximately 0.40 usec. This helps produce a clearer
image when viewing the far field.
The elements 11 for the single crystal design
shown in FIGS. 3, 5, and 18 each measure 15mm in the
elevation direction and 0.0836mm in the azimuthal
direction. The element spacing S is 0.109mm and the
length of the kerf is 25.4um. The thickness LMIN is
0.109mm and the thickness LMAX is 0.381mm. The radius of
curvature of the front portion 12 is 103.54mm.
The backing block is formed of a filled epoxy
comprising Dow Corning's part number DER 332 treated with
Dow Corning's curing agent DEH 24 and has an Aluminum
Oxide filler. The backing block for a transducer array
comprising 128 elements has dimensions of 20mm in the
azimuthal direction, 16mm in the elevation direction, and
2Omm in the range direction.
The shape and dimension of the matching layer
24 is approximated by the equation LML
(1/2)(LE)(CML/CE) where, for a given point on the
transducer surface, LML is the thickness of the matching
layer, LE is the thickness of the transducer element, CML
is the speed of sound of the matching layer, and CE is
the speed of sound of the element. The transducers may
be used with commercially available units such as Acuson
Corporationis 128 XP System having acoustic response
technology (ART) capability.
For the two crystal design of FIG. 12, the
first and second piezoelectric portions 42 and 44 have a
minimum thickness of 0.127mm and a maximum thickness of
0.2794mm, as measured in the range direction. The radius
of curvature for the surfaces 46 and 48 of piezoelectric
portions 42 and 44 are 184.62mm. The element spacing S
is 0.254mm and the length of the kerf is 25.4um.
For the two crystal design of FIG. 13,
piezoelectric portions 56 and 57 have a minimum thickness
of 0.127mm and maximum thickness of 0.2794mm. The radius
212~94fi
-31-
of curvature of the front portion 63 of the first
piezoelectric portion 56 and the back portion 65 of the
second piezoelectric portion is 184.62mm. The radius of
curvature of the front portion 67 of piezoelectric
portion 57 is 92.426mm.
Finally, the composite structure design shown
in FIG. 14 preferably has dimensions similar to that for
FIGS. 4 or 5, forming an array of 128 transducer
elements. The structure of FIG. 11 further possesses a
generally planar back portion 68 which is especially
desirable when focusing in the far field. The structure
of FIG. 15 may be formed by deforming the ends of the
structure of FIG. 14 in the range direction. Where
focusing in the near field at approximately 2cm into the
body being examined, the side portions of the structure
of FIG. 14 should be deformed by approximately 0.25mm
relative to the center portion.
Each of the backing block, the flex circuit,
the piezoelectric layer, the matching layer, and the
coupling element may be glued together by use of any
epoxy material. A Hysol~ base material number 2039
having a Hysol~ curing agent number HD3561, which is
manufactured by Dexter Corp., Hysol Division of Industry,
California, may be used for gluing the various materials
together. Typically, the thickness of epoxy material is
approximately 2um.
The flex circuit thickness for forming the
first electrode is approximately 25um for a flex circuit
manufactured by Sheldahl for providing the appropriate
electrical excitation. The thickness of the second
electrode is typically 2000-3000 Angstroms and may be
disposed on the transducer structure by use of sputtering
techniques.
It should be noted that the transducer array
constructed in accordance with the present invention may
be capable of operating at the third harmonic, such as
-
-32- 212994~
10.5 Megahertz in this example. This may further provide
additional information to the observer. Moreover, the
addition of the matching layer 24 will enable the
transducer array to operate at an even broader range of
frequencies. Consequently, this may further enable a
transducer of the present invention to operate at both a
certain dominant fundamental and second harmonic
frequency.
It is to be understood that the forms of the
invention described herewith are to be taken as preferred
examples and that various changes in the shape, size and
arrangement of parts may be resorted to, without
departing from the spirit of the invention or scope of
the claims.
