BACKGROUND OF THE INVENTION
[0001] This invention relates to transducers and more particularly to broadband phased array
transducers for use in the medical diagnostic field.
[0002] 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 received by the
transducer array to produce a focused image on the display screen.
[0003] 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.
[0004] 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.
[0005] 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 of clearly viewing the contrast agent
itself and thus may determine how well the respective tissue is performing.
[0006] 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.
[0007] 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. 3,833,825 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 the amount of segments which would have to be individually
excited as compared with a one-dimensional array.
[0008] 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.
[0009] A known ultrasonic wave transducer is set forth in US-A-4,350,917 wherein an ultrasonic
wave transducer is formed by a spherical body of piezoelectric material having non-uniform
thickness which is narrow at one end of the sphere and thick at an opposite end of
the sphere. Each location on the transducer is resonant at a different frequency according
to the thickness at that point. By changing the frequency of the applied excitation
signal, the origin and direction of the radiation can be altered.
[0010] US-A-3,666,979 discloses a transducer element having a non-planar front surface which
faces the range direction, i.e. the region of examination when the transducer array
is in use, and which has a uniform thickness.
[0011] EP-A-0 397 961 discloses a transducer element having a minimum thickness near its
centre and a maximum thickness near each end but the transition between the minimum
and maximum of thicknesses is in a stair step fashion.
SUMMARY OF THE INVENTION
[0012] According to this invention there is provided a transducer array as claimed in claim
1 herein.
[0013] In a feature of this invention there is provided an array in combination with an
ultrasound system as claimed in claim 16 herein.
[0014] Other preferred features are defined in the dependent claims herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic view of an ultrasound system for generating an image.
[0016] FIG. 2 is a cross-sectional view of a transducer element in accordance with the first
preferred embodiment.
[0017] FIG. 3 is a cross-sectional view of a transducer element in accordance with the second
preferred embodiment.
[0018] 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.
[0019] 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.
[0020] FIG. 6 is an enlarged view of a single broadband transducer element of the transducer
array constructed in accordance with the present invention.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] FIG. 11 is an example of a typical acoustic impedance frequency response plot resulting
from operation of a prior art transducer.
[0026] 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.
[0027] FIG. 13 is a cross-sectional view of an alternate two crystal design.
[0028] FIG. 14 is a cross-sectional view of a composite transducer element in accordance
with a fourth preferred embodiment.
[0029] FIG. 15 is a cross-sectional view of the composite transducer element of FIG. 14
which is deformed.
[0030] 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.
[0031] 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.
[0032] FIG. 18 shows a partial perspective view of a linear transducer array in accordance
with the present invention.
[0033] 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.
[0034] FIG. 20 shows an impulse response and the corresponding frequency spectrum for the
transducer element of FIG. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] 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.
[0036] 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 geometrical configuration.
The transducer array 10 is supported by backing block 13.
[0037] 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.
[0038] 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 is 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).
[0039] The front portion 12 has a continuously curved surface, front portion 12 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 preferably a planar surface
may also be, for example, a concave or convex surface.
[0040] 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.
[0041] 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.
[0042] 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).
[0043] 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 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.
[0044] The slight variation in thickness of the element, i e the distance 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.
[0045] 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 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.
[0046] 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.
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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] 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.
[0052] 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
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).
[0053] 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 0.381mm
(.015 INCHES) and a center thickness LMIN of 0.109mm (.00428 INCHES). 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 Δf, which is the difference between f
a, the anti-resonant frequency (i.e., maximum impedance), and f
r, 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.
[0054] 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.
[0055] 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:
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.
[0056] By the addition of matching layer 24, the fractional bandwidth can be improved. Further,
the 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.
[0057] 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 = (½) (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 x 10
6 (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
x 10
6 (3 Mega Rayls).
[0058] 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 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.
[0059] 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.
[0060] 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 + (w
2/8h), 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.
[0061] 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.
[0062] 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.
[0063] 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.
[0064] 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 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.
[0065] 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.
[0066] 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.
[0067] Typically, the radius of curvature of the first backing block 13' is approximately
44mm but may vary as 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.
[0068] 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.
[0069] 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
Θ
g, the electronic steering angle in sector format Θ
s, the wavelength of the object being examined at the highest operating frequency of
the transducer λ, and the spacing between the elements S is given by the equation:
Therefore, for a given grating lobe angle, the design of the transducer aperture
is restricted by the upper operating frequency of the transducer.
[0070] 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.
[0071] 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 + (w
2/8h), 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 as being plano-concave in structure. The thickness of each
of the portions 42 and 44 is 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.
[0072] 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.
[0073] 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 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.
[0074] 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.
[0075] 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.
[0076] 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 + (w
2/8h), 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,
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 + (w
2/8h'), 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.
[0077] 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.
[0078] 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 with polymer material to ensure electrical isolation between
elements.
[0079] The front portion 66 is curved surface, wherein the thickness of the structure 60
is greater at each of the side portions 70, 72 and decreases in thickness at the center.
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.
[0080] 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.
[0081] 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.
[0082] 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
transducer array may be capable of transmitting and receiving at the second harmonic
frequency.
[0083] 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 0.109mm, 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 1MHZ 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.
[0084] 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.
[0085] 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 approximately
0.40 usec. This helps produce a clearer image when viewing the far field.
[0086] 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.
[0087] 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 20mm in the
range direction.
[0088] 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
Corporation's 128 XP System having acoustic response technology (ART) capability.
[0089] 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.
[0090] 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 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.
[0091] 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.
[0092] 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.
[0093] 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 2 x 10
-7 m - 3 x 10
-7 m (2000-3000 Angstroms) and may be disposed on the transducer structure by use of
sputtering techniques.
[0094] 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 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.
1. Meßwandlergruppe zur Erzeugung eines Ultraschallstrahls bei Erregung, wobei der Meßwandler
folgendes umfaßt:
eine Mehrzahl von Meßwandlerelementen (11), die aufeinanderfolgend entlang einer Azimutalachse
(Y) angeordnet sind, wobei jedes der genannten Meßwandlerelemente eine nicht ebene
vordere Oberfläche (12) aufweist, die in eine Bereichsrichtung in Richtung eines Untersuchungsbereichs
zeigt, wenn sich die Meßwandlergruppe im Einsatz befindet, mit einer hinteren Oberfläche
(14) und zwei seitlichen Oberflächen (16, 18), die im wesentlichen parallel zueinander
sind, welche die vordere und die hintere Oberfläche an jedem Ende des Elements verbinden,
so daß jedes der Meßwandlerelemente (11) eine Dicke (L) aufweist, die in der Bereichsrichtung
zwischen der vorderen und der hinteren Oberfläche (12, 14) gemessen wird, wobei sich
die kleinste Abmessung der Dicke ungefähr in der Mitte zwischen den genannten Enden
befindet, und wobei sich die größte Abmessung an jedem Ende befindet, wobei die genannte
Dicke von der geringsten Dickenabmessung zu der größten Dickenabmessung stetig zunimmt.
2. Meßwandler nach Anspruch 1, wobei die vordere Oberfläche konkav ist.
3. Meßwandler nach Anspruch 1, wobei die vordere Oberfläche einen Krümmungsradius entlang
einer Elevationsachse (X) aufweist, der sich von einem Krümmungsradius entlang der
Azimutalachse (Y) unterscheidet.
4. Meßwandler nach Anspruch 1, wobei die hintere Oberfläche (14) allgemein planar ist.
5. Meßwandler nach Anspruch 1, wobei die hintere Oberfläche (14) eine konkave Form aufweist.
6. Meßwandler nach Anspruch 1, wobei die hintere Oberfläche (14) eine konvexe Form aufweist.
7. Meßwandler nach Anspruch 1, wobei jedes der genannten Elemente (11) plankonkav ist.
8. Meßwandler nach einem der vorstehenden Ansprüche, wobei der Meßwandler ferner mindestens
eine akustische Anpaßschicht (24) umfaßt, die sich an der vorderen Oberfläche (12)
jedes Meßwandlerelements befindet.
9. Meßwandler nach Anspruch 8, wobei die Dicke LML der genannten Anpaßschicht näherungsweise
durch (1/2) (LE) (CML/CE) gegeben ist, wobei LML für einen gegebenen Punkt auf der
vorderen Oberfläche jedes Elements die Dicke der Anpaßschicht bezeichnet, wobei LE
die Dicke des Meßwandlerelements darstellt, wobei CML die Schallgeschwindigkeit der
Anpaßschicht bezeichnet, und wobei CE für die Schallgeschwindigkeit des Elements steht.
10. Meßwandler nach Anspruch 8 oder 9, wobei an der genannten Anpaßschicht (24) ein Kopplungselement
(27) angeordnet ist.
11. Meßwandler nach Anspruch 10, wobei eine Oberfläche des genannten Kopplungselements
(27), die in die Bereichsrichtung ausgerichtet ist, eine konkave Form aufweist.
12. Meßwandler nach einem der vorstehenden Ansprüche, wobei das genannte Element (11)
aus einem der folgenden Stoffe hergestellt wird, die Blei(II)-tinatatzirkonat, Verbundwerkstoffe
und Polyvinylidenfluorid umfassen.
13. Meßwandler nach einem der vorstehenden Ansprüche, wobei die genannte vordere Oberfläche
(12) einen Krümmungsradius aufweist, der näherungsweise durch die Gleichung h/2+(w2/8h) gegeben ist, wobei h die Differenz zwischen der minimalen und der maximalen Dicke
des genannten Meßwandlerelements bezeichnet, und wobei w für die Breite des genannten
Meßwandlerelements steht, die zwischen den genannten seitlichen Oberflächen (16, 18)
gemessen wird.
14. Meßwandler nach einem der vorstehenden Ansprüche, wobei die genannte maximale Dicke
kleiner oder gleich 140% der genannten minimalen Dicke ist.
15. Meßwandler nach einem der Ansprüche 1 bis 13, wobei die genannte maximale Dicke kleiner
oder gleich 140% der genannten minimalen Dicke und größer oder gleich 120% der genannten
minimalen Dicke ist.
16. Meßwandler nach einem der vorstehenden Ansprüche in Kombination mit einem Ultraschallsystem
zur Erzeugung eines Bildes, wobei das System einen Übermittlungsschaltkreis (2) zur
Übermittlung elektrischer Signale an die Meßwandlergruppe (4, 10) aufweist, wobei
die Meßwandlergruppe im Einsatz einen Ultraschallstrahl übermittelt, der durch eine
bestimmte Frequenzerregung erzeugt wird, und Druckwellen empfängt, die von einem zu
untersuchenden Körper zurückgestrahlt werden, wobei ein Empfangsschaltkreis (6) zur
Verarbeitung der von der genannten Meßwandlergruppe empfangenen Signale vorgesehen
ist, und mit einer Anzeige (8), die dazu dient, ein Bild eines überwachten Objekts
vorzusehen.
17. Meßwandler nach einem der vorstehenden Ansprüche, wobei der Meßwandler ferner folgendes
umfaßt:
eine Mehrzahl erster Elektroden (74), wobei sich jede der genannten ersten Elektroden
auf einem hinteren Teilstück eines entsprechenden der genannten Elemente befindet;
und
eine Mehrzahl zweiter Elektroden (76), wobei sich jede der genannten zweiten Elektroden
auf einer vorderen Oberfläche eines entsprechenden der genannten Elemente befindet;
wobei ein elektrisches Feld zwischen den genannten ersten und zweiten Elektroden an
dem genannten mittleren Teilstück größer ist als an den genannten seitlichen Teilstücken.
1. Ensemble de transducteurs pour produire un faisceau d'ultrasons par excitation, le
transducteur comprenant :
une pluralité d'éléments transducteurs (11) séquentiellement agencés le long d'un
axe d'azimut (Y), chacun desdits éléments transducteurs présentant une surface avant
non plane (12) qui est tournée dans une direction de visée vers une région à examiner
lorsque l'ensemble de transducteurs est en utilisation, une surface arrière (14) et
deux surfaces latérales (16,18) qui sont sensiblement parallèles l'une à l'autre et
relient les surfaces avant et arrière à chaque extrémité de l'élément de sorte que
chaque élément transducteur 11 a une épaisseur (L) mesurée dans la direction de visée
entre les surfaces avant et arrière (12,14) qui est minimale sensiblement à mi-distance
entre lesdites extrémités et maximale à chaque extrémité, ladite épaisseur augmentant
de façon continue de l'épaisseur minimale à l'épaisseur maximale.
2. Transducteur selon la revendication 1, dans lequel la surface avant est concave.
3. Transducteur selon la revendication 1, dans lequel la surface avant a un rayon de
courbure , le long d'un axe d'élévation (X), qui est différent d'un rayon de courbure
le long de l'axe d'azimut (Y).
4. Transducteur selon la revendication 1, dans lequel la surface arrière (14) est sensiblement
plane.
5. Transducteur selon la revendication 1, dans lequel la surface arrière (14) est de
forme concave.
6. Transducteur selon la revendication 1, dans lequel la surface arrière (14) est de
forme convexe.
7. Transducteur selon la revendication 1, dans lequel chacun desdits éléments (11) est
plan-concave.
8. Transducteur selon une quelconque des revendications précédentes, comprenant en outre
au moins une couche d'adaptation acoustique (24) disposée sur la surface avant (12)
de chaque élément transducteur.
9. Transducteur selon la revendication 8, dans lequel l'épaisseur LML de la couche d'adaptation
est représentée approximativement par (1/2) (LE) (CML/CE), où,pour un point donné
sur la surface avant de chaque élément, LML est l'épaisseur de la couche d'adaptation,
LE est l'épaisseur de l'élément transducteur, CML est la vitesse du son dans la couche
d'adaptation et CE est la vitesse du son dans l'élément.
10. Transducteur selon la revendication 8 ou 9, dans lequel un élément de couplage (27)
est disposé sur ladite couche d'adaptation (24).
11. Transducteur selon la revendication 10, dans lequel une surface dudit élément de couplage
(27) tournée dans la direction de visée est de forme concave.
12. Transducteur selon une quelconque des revendications précédentes, dans lequel ledit
élément (11) est constitué d'une matière choisie parmi le zirconate-titanate de plomb,
une matière composite et le fluorure de polyvinylidène.
13. Transducteur selon une quelconque des revendications précédentes, dans lequel ladite
surface avant (12) possède un rayon de courbure représenté approximativement par l'équation
h/2 + (w /8h), où h est la différence entre l'épaisseur minimale et l'épaisseur maximale
dudit élément transducteur et w est la largeur dudit élément transducteur mesurée
entre lesdites surfaces latérales (16,18).
14. Transducteur selon une quelconque des revendications précédentes, dans lequel ladite
épaisseur maximale est inférieure ou égale à 140% de ladite épaisseur minimale.
15. Transducteur selon une quelconque des revendications 1 à 13, dans lequel ladite épaisseur
maximale est inférieure ou égale à 140% de ladite épaisseur minimale, et supérieure
ou égale à 120% de ladite épaisseur minimale.
16. Transducteur selon une quelconque des revendications précédentes, en combinaison avec
un système d'ultrasons pour la génération d'une image, le système comprenant un circuit
d'émission (2) pour émettre des signaux électriques vers l'ensemble de transducteurs
(4,10), l'ensemble de transducteurs en fonctionnement émettant un faisceau d'ultrasons
produit par une excitation de fréquence donnée et recevant des ondes de pression réfléchies
par un corps en examen, un circuit de réception (6) pour traiter les signaux reçus
par le dit ensemble de transducteurs, et un affichage (8) pour fournir une image d'un
objet observé.
17. Transducteur selon une quelconque des revendications précédentes, comprenant en outre
:
une pluralité de premières électrodes (74), chacune desdites premières électrodes
étant disposée sur ladite partie arrière d'un élément correspondant parmi lesdits
éléments ; et
une pluralité de deuxièmes électrodes (76), chacune desdites deuxièmes électrodes
étant disposée sur ladite surface avant d'un élément correspondant parmi lesdits éléments
;
de sorte qu'un champ électrique entre lesdites première et deuxième électrodes est
plus grand dans la dite partie centrale que dans lesdites parties latérales.