CA2268415A1 - Single element ultrasonic collimating transducers and a method and apparatus utilizing ultrasonic transducers in 3d tomography - Google Patents
Single element ultrasonic collimating transducers and a method and apparatus utilizing ultrasonic transducers in 3d tomography Download PDFInfo
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Classifications
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B06—GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS IN GENERAL
- B06B—METHODS OR APPARATUS FOR GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS OF INFRASONIC, SONIC, OR ULTRASONIC FREQUENCY, e.g. FOR PERFORMING MECHANICAL WORK IN GENERAL
- B06B1/00—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency
- B06B1/02—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy
- B06B1/06—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction
- B06B1/0688—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction with foil-type piezoelectric elements, e.g. PVDF
- B06B1/0692—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction with foil-type piezoelectric elements, e.g. PVDF with a continuous electrode on one side and a plurality of electrodes on the other side
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B06—GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS IN GENERAL
- B06B—METHODS OR APPARATUS FOR GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS OF INFRASONIC, SONIC, OR ULTRASONIC FREQUENCY, e.g. FOR PERFORMING MECHANICAL WORK IN GENERAL
- B06B1/00—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency
- B06B1/02—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy
- B06B1/06—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction
- B06B1/0644—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction using a single piezoelectric element
- B06B1/0662—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction using a single piezoelectric element with an electrode on the sensitive surface
-
- G—PHYSICS
- G10—MUSICAL INSTRUMENTS; ACOUSTICS
- G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
- G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
- G10K11/18—Methods or devices for transmitting, conducting or directing sound
- G10K11/26—Sound-focusing or directing, e.g. scanning
- G10K11/32—Sound-focusing or directing, e.g. scanning characterised by the shape of the source
-
- G—PHYSICS
- G10—MUSICAL INSTRUMENTS; ACOUSTICS
- G10K—SOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
- G10K11/00—Methods or devices for transmitting, conducting or directing sound in general; Methods or devices for protecting against, or for damping, noise or other acoustic waves in general
- G10K11/18—Methods or devices for transmitting, conducting or directing sound
- G10K11/26—Sound-focusing or directing, e.g. scanning
- G10K11/34—Sound-focusing or directing, e.g. scanning using electrical steering of transducer arrays, e.g. beam steering
Abstract
A single element resonant or wideband collimating ultrasound piezotransducer with and without electronic scanning capabilities, which can be electronically optimized for far or near field imaging. In a resonant implementation, a plurality of concentric ring electrodes (further subdivided into segments to provide electronic scanning capability) are provided on both surfaces of the piezoelement. In the alternative implementation a plurality of sector shaped electrodes is placed on the front and back surface of non-homogeneously polarized piezoelement. The geometry of the electrodes is determined by calculation of pressure distribution on the working surface of the piezoelement which maximizes specific optimization criterion. Similarly, polarization vector in non-uniformly polarized element is calculated to maximize specific optimization criterion. In a wide band implementation the body of the piezoelement forms an acoustic trap. The plurality of electrodes of various geometries on working and side surfaces of the piezoelement provide collimating and electronic scanning capabilities.
Description
SINGLE ELEMENT ULTRASONIC COLLIMATING
TRANSDUCERS AND A METHOD AND APPARATUS UTILISING
Field of the Invention This invention relates to resonant and wideband single element collimating piezoelectric transducers for use in biomedical and industrial applications requiring high resolution in lateral and axial directions and specifically for the use of such piezoelectric transducers in non-destructive testing, in acoustic microscopy, and in multi-parametric ultrasonic tomography for 3D image reconstruction in industrial and medical applications (such as breast or ovarian cancer detection).
Background of the Invention One of the major limitations of current ultrasonic methods is related to the performance of existing piezoelectric transducers. Ideally, ultrasonic transducers for general use and particularly for use in tomography or high resolution imaging should be wide band and should exhibit a very narrow directivity pattern to achieve the necessary frontal (lateral) and depth (axial or azimuthal) resolutions (for medical applications this is in the order of 0.1 mm to 0.5 mm). The transducers should emit a very tightly collimated beam and be fairly small so that sufficient numbers of independent transmitters and receivers can be used to transmit and receive ultrasonic signals from different points and at various angles. It is also highly desirable that these piezoelectric transducers be simple and relatively inexpensive.
There are several problems relating to the design of currently available single element ultrasonic transducers that form narrow weakly divergent ultrasonic beams. The most commonly used type is the focusing transducer, which generates a very narrow ultrasonic beam in its focal zone. The disadvantage of this type of the transducer is that the beam widens before and after the focal zone. Such a beam presents a problem in terms of resolution (when a feature of interest is outside the focal plane) and requires precise placement of transducers.
Focusing is usually achieved by shaping the piezoelectric element or with the help of an acoustic lens. It is possible to stretch the focal zone by using a toroidal lens. A similar result may be obtained by using a zone plate as well as annular or spherically concave transducers. Currently the best results in focusing an acoustic field to achieve a narrow main lobe in the directivity pattern and low level for the lateral lobes are provided by multi-element transducers (phased arrays). In such arrays, each piezoelectric element is excited by the uniform electric field independently of all other elements; the electric fields applied to different elements are different in amplitude and phase. With the proper choice of exciting electric signals, the interference of the emitted ultrasonic beams will generate a narrow main lobe and low level lateral lobes in the directivity pattern. However, such transducers are relatively bulky, expensive and complex.
In Soviet patents SU 1380803, SU 1404926, and a later US patent US
5081995, small single element simple and inexpensive piezoelectric transducers that can form a narrow weakly divergent ultrasonic beams are described. The transducer comprises a disk piezoelectric element having on its surfaces several pairs of concentric ring electrodes of a predetermined size.
Members of an electrode pair are arranged on opposite faces of the piezoelement. Each of the pairs of electrodes is excited in opposite phase and with specific amplitude (representing Bessel function Jo(r/R), where r is the radial position of a given electrode and R is the radius of the piezoelement).
These single element transducers exhibit a smooth pressure distribution within the piezoelectric element unlike the discrete distribution in a multi-element phased array. A major disadvantage of this type of transducer, however, is that its resolution decreases significantly with distance from the face of the piezoelement (the angle of beam divergence is relatively large), size and location of the side lobes cannot be controlled and selective beam optimization for either far or near field is not possible. This transducer is also not wide band;
TRANSDUCERS AND A METHOD AND APPARATUS UTILISING
Field of the Invention This invention relates to resonant and wideband single element collimating piezoelectric transducers for use in biomedical and industrial applications requiring high resolution in lateral and axial directions and specifically for the use of such piezoelectric transducers in non-destructive testing, in acoustic microscopy, and in multi-parametric ultrasonic tomography for 3D image reconstruction in industrial and medical applications (such as breast or ovarian cancer detection).
Background of the Invention One of the major limitations of current ultrasonic methods is related to the performance of existing piezoelectric transducers. Ideally, ultrasonic transducers for general use and particularly for use in tomography or high resolution imaging should be wide band and should exhibit a very narrow directivity pattern to achieve the necessary frontal (lateral) and depth (axial or azimuthal) resolutions (for medical applications this is in the order of 0.1 mm to 0.5 mm). The transducers should emit a very tightly collimated beam and be fairly small so that sufficient numbers of independent transmitters and receivers can be used to transmit and receive ultrasonic signals from different points and at various angles. It is also highly desirable that these piezoelectric transducers be simple and relatively inexpensive.
There are several problems relating to the design of currently available single element ultrasonic transducers that form narrow weakly divergent ultrasonic beams. The most commonly used type is the focusing transducer, which generates a very narrow ultrasonic beam in its focal zone. The disadvantage of this type of the transducer is that the beam widens before and after the focal zone. Such a beam presents a problem in terms of resolution (when a feature of interest is outside the focal plane) and requires precise placement of transducers.
Focusing is usually achieved by shaping the piezoelectric element or with the help of an acoustic lens. It is possible to stretch the focal zone by using a toroidal lens. A similar result may be obtained by using a zone plate as well as annular or spherically concave transducers. Currently the best results in focusing an acoustic field to achieve a narrow main lobe in the directivity pattern and low level for the lateral lobes are provided by multi-element transducers (phased arrays). In such arrays, each piezoelectric element is excited by the uniform electric field independently of all other elements; the electric fields applied to different elements are different in amplitude and phase. With the proper choice of exciting electric signals, the interference of the emitted ultrasonic beams will generate a narrow main lobe and low level lateral lobes in the directivity pattern. However, such transducers are relatively bulky, expensive and complex.
In Soviet patents SU 1380803, SU 1404926, and a later US patent US
5081995, small single element simple and inexpensive piezoelectric transducers that can form a narrow weakly divergent ultrasonic beams are described. The transducer comprises a disk piezoelectric element having on its surfaces several pairs of concentric ring electrodes of a predetermined size.
Members of an electrode pair are arranged on opposite faces of the piezoelement. Each of the pairs of electrodes is excited in opposite phase and with specific amplitude (representing Bessel function Jo(r/R), where r is the radial position of a given electrode and R is the radius of the piezoelement).
These single element transducers exhibit a smooth pressure distribution within the piezoelectric element unlike the discrete distribution in a multi-element phased array. A major disadvantage of this type of transducer, however, is that its resolution decreases significantly with distance from the face of the piezoelement (the angle of beam divergence is relatively large), size and location of the side lobes cannot be controlled and selective beam optimization for either far or near field is not possible. This transducer is also not wide band;
i.e. it cannot emit very short ultrasonic pulses. Consequently for applications that require very good depth resolution, these transducers are not suitable.
Various techniques have been used in order to create a wide band transducer that could emit and receive short ultrasonic pulses without distortions and thus exhibit high depth resolution. The most common of these techniques utilizes mechanical damping of the piezoelectric element. This is a very simple method and gives reasonably good results. But the noise level is relatively high because it is impossible to damp all the oscillations of the piezoelectric element. Several other methods, such as compensating electronic circuits (RLC correcting circuits) shaped piezoelectric elements, piezoelements with side electrodes, specialized piezoelectric materials with high attenuation indices were implemented and provide reasonably good results. However, all of these implementations have significant disadvantages in that it is necessary to tune each transducer, its electronic components, the shape of a radiated acoustic field, etc. for each specific application and system design.
In Soviet Patent Nos. SU 652808, SU 700916, and SU 847524, several wide band, simple and inexpensive piezoelectric transducers are described.
These transducers are of the so-called "surface excited thick piezoelectric"
type.
The piezoelectric element in these transducers has two electrodes located on one surface only. Thus, the only source of acoustic waves (for radiation) and electric charges (for reception) lies in a small piezoelectric element area between the electrodes. The transducer has a conical shape and eventually absorbs all ultrasonic pulses propagated within it; i.e. the body of the transducer acts as an acoustic trap. Transducers of this type are extremely wide band, simple and inexpensive. On the other hand, this type of transducer cannot form collimated beams. In other words, this transducer provides high depth resolution but has poor frontal resolution.
Thus, it may be seen that currently available single element transducers have either relatively low frontal (lateral) or azimuthal (depth) resolutions.
Linear phased arrays made up of multiple piezoelectric elements, on the other hand, provide very good frontal resolution (albeit only in elevation plane) but they are not wide band, have relatively large dimensions, are complicated and expensive to manufacture. Furthermore, attempting to use currently available phased arrays in ultrasonic tomography would be impractical. At least 20 independent transducers would have to be located around the object of interest (e.g., breast in medical ultrasound tomography), requiring very small transducers that are only a few millimeters in length. This would significantly downgrade the frontal resolution and appreciably complicate the manufacturing procedure, making these devices prohibitively expensive.
Description of the Invention and Outline of the Preferred Embodiment The present invention describes a new method for designing and constructing resonant or wide band single element ultrasonic transducers that are capable of producing a weakly divergent (collimated) ultrasonic beam, when driven by a suitable electronic circuit. The transducers described herein provide significant improvements in performance compared to prior art.
A transducer according to the prior art, which was intermitted by the applicant of the subject invention for generating a narrow weakly divergent ultrasonic beam comprises an axially polarized generally circular flat disk piezoelectric element to which a series of ring electrodes are attached. One surface of the piezoelectric element is termed the working surface and is generally the surface from which an ultrasonic beam is emitted or by which an ultrasonic signal is received. In order to produce a narrow weakly divergent ultrasonic beam from this type of transducer, the ring electrodes are arranged in concentric pairs with one member of the pair being wider than the other. The wider electrode of a pair is usually located on the working surface of the piezoelectric element and the narrower electrode of the pair being on the opposite surface of the element. An alternating arrangement, where a wider electrode of the pair is placed next to a narrow electrode of the adjacent pair is also possible and is easier to implement with certain types of fabrication technology. The pairs of electrodes are excited by an electric field applied thereto in order to provide the desired pressure distribution on the working s surface of the piezoelement. The pressure distribution in this prior art design is the Bessel function Jo(r/R).
Present invention provides further significant performance improvement for this type of a resonant transducer by modifying the electrode geometry and amplitudes and phases of excitation voltages in such a way as to obtain pressure distribution on transducer working surface maximizing the specific optimization criterion, as described below. Present invention introduces a method for determining optimal pressure distribution on the surface of the piezoelement. This method involves the selection of optimization criterion that is used in the transducer design model to obtain optimal trade off between the width and amplitude of the main lobe and size and position of the side lobes in the directivity pattern. The optimization criterion should be selected in such a way as to provide the best representation of the desired characteristics of the generated ultrasonic beam. The method for the transducer design is as follows:
The desired radial pressure distribution P(r) on the piezoelement's working surface is represented by the sum of a series of Bessel functions of the first kind and zero order Jo as follows:
__ ~ 'IO~an R
+ ~ An ',1 \
an where coefficients A" are defined as numerical constants providing the solution of the synthesis problem for beam profile, based on the assumption that the emitted ultrasonic beam is very narrow and weakly divergent; R is the piezoelement radius (in the case of resonant transducer) or face electrode disk radius (in the case of wide band transducer, as described below); J~ is the Bessel function of the first kind and first order, an is the nth root of the equation Jo(a)=0. Thus, a~=2.4, a2=5.5, a3=8.6, etc. Coefficients An are determined using a specific algorithm to obtain the optimal directivity pattern in terms of the relationship between beam width and position and size of the side lobes. Below is the description of the algorithm:
Directivity pattern for a given transducer is written as:
a J (k R sin 9) D(8)=KRz A° Jl (kRsin9)+KRz ~A" an -kz Rz sinz B ' an #kRsin9 kRsinB n=~ O.SJ,(a") , an =kRsin9 where K is the normalization coefficient.
A realistic or practical number of terms n in equation for D(9) is selected.
Usually n is selected empirically as a compromise between the desired directivity function and complexity of the transducer design. Generally speaking, the larger n would allow to obtain the directivity function with narrower main lobe and smaller side lobes but at a cost of a more complex design, lower signal intensity, and a larger number of electrode pairs. With n~oo and appropriate selection of coefficients Ar,, D(8) will approximate s-function, representing the phenomenon of superdirectivity. The practical value of n is usually between 5 and 20. The algorithm is implemented numerically as follows:
First, the optimization criterion Q for the desired directivity pattern (criterion of directivity narrowness) is defined. An example of such a criterion is given here as:
D ( 9)~ B ° ~3) j D ( a)da °
Other suitable optimization criteria that represent the desired characteristics of the directivity pattern, of cause, can be used. The above was chosen because it is relatively simple and has the following properties:
the smaller are the side lobes and narrower the main lobe, the smaller is the area under the integral;
the larger is the amplitude of the main lobe (numerator in the expression), the more energy is concentrated within it. Thus, the larger the criterion Q, the narrower and less divergent is the ultrasonic beam defined by D(8).
We then computed a set of N coefficients A" (n=1,2,3...N), based on the requirement to maximize the value of criterion Q . Each set consisted of a specific number of coefficients, e.g., 5, 8, 12, or 20. The more coefficients we used, the greater was the value of the criterion Q, narrower the main lobe in the directivity pattern, and smaller the side lobes.
Finally, we calculated the pressure distribution P(r) by substituting the values of coefficients A" obtained in the previous step. The peaks (maxima and minima) of the function P(r) define the positions of the ring electrodes on the surface of the piezoelement, i.e. each ring electrode (from 1 to N) is placed at a distance r(n) from the center of the piezoelement, where r(n) is the location of the nth peak of the function P (r) . The width of the nth electrode ring is approximately equal to the distance between the adjacent zero values of the function P(r) (for the wider electrode in the pair). In practical implementation, the width of the nth electrode ring is equal to the distance between the adjacent zero values of the function P(r) minus the width of the gap between the adjacent ring electrodes necessary to provide electrical separation between the rings. This gap would usually be in the range of 50 to 500 Vim. Width of the narrower electrode in the pair is between 30% and 50% of the width of the wider electrode (usually 40%). To achieve better transducer efficiency, wider electrodes would normally be placed on the working surface of the transducer and narrower ones on the opposite surface. An alternative implementation of the above design involves placement of a single solid disk electrode equal to the radius of the piezoelement on its working surface and a plurality of narrow (as described above) ring electrodes on the back surface of the piezoelement.
In such implementation the disk electrode is attached to ground and ring electrodes are driven by voltages of alternating polarity A further aspect of the invention provides for the procedure of electronic optimization (by changing excitation voltages and phases applied to ring electrodes) of the performance of the above described piezoelements in the near field, by maximizing the "near field beam optimization criterion" (which is selected to optimize the transducer performance for a given application), for either far or near zone. The following procedure is used:
The piezoelement is represented as a set of ring radiators driven by voltages with alternating polarities and various amplitudes. The optimization procedure is applied to a given set of electrodes placed on the surface of the piezoelement following the above described procedure.
As a criterion of optimal near zone collimation we use the product M
P (r ~ z ) I z=z, , r=0 M =~Qr~ Qr=
P ~r, Z~ I = dr z z, where each factor Q; is a modified criterion (3), applied this time not to directivity pattern, but to radial pressure profile at various z values (where z is the distance from the face of the piezoelement). Near zone is divided into m segments along the z axis, and z; is the distance to the ith segment. P(r,z) is the acoustic pressure at the coordinates r,z, generated by all ring electrodes positioned to optimize D(e) as previously described.
Further aspect of the invention enables electronic beam steering in a single element transducer design. In accordance with this aspect of the invention there is provided an ultrasonic transducer comprising a resonant piezoelectric element, a plurality of ring electrodes arranged on the surfaces of the said element according to the method described above. Each of the ring electrodes is further subdivided into 5 to 20 segments. Each ring electrode segment is adapted to be powered by an electric voltage of specified phase and amplitude in order to generate a collimated ultrasonic beam, and means associated with the electrodes, for electronically steering the beam by varying the voltage amplitude and phase applied to the electrodes. With the segmental electrode geometry, changing amplitude and phase of the excitation voltage on individual electrodes will modify the directivity pattern of the transducer.
Furthermore, this design enables annular focusing and sector scanning in both azimuthal and elevation planes. In reception mode, selective directional sensitivity is achieved by monitoring signals from specific sets of electrodes.
Further aspect of the invention provides an alternative method of implementation of the transducer design described above. Once an optimal (desired) pressure distribution on the surface of the piezoelement has been determined, the piezoelement is then nonuniformly polarized. The polarization vector is such that it approximates the desired radial pressure distribution on the surface of the piezoelement. Then both sides of such piezoelement are covered with solid disk electrodes (as in conventional piston implementation). Such element will emit a collimated beam when excitation signal is provided.
Dividing the surface disk electrodes into sectors allows to steer the beam by controlling the phase and amplitude of the excitation signal to individual sectors. This design offers significant advantage compared to one with ring electrodes, namely, it significantly reduces the number of electrodes and requires much less complex electronic circuitry to drive the transducer. However the process of polarizing such piezoelement is significantly more complex than for conventional piezoelements.
A still further aspect of the invention provides for a wide band collimating ultrasonic transducer. The transducer includes a solid body of piezoelectric material that acts as an acoustic trap (e.g. a solid cone, a long cylinder, a frustum of a cylinder, or other suitable shape). A series of ring electrodes are placed on the working surface of the said piezoelement according to the method described above, while a set of corresponding narrow ring electrodes are placed inside the body of the piezoelement during its fabrication. These internal electrodes being composed of a material of similar acoustic impedance to that of the piezoelectric element. An electrical circuit is connected to drive each of the electrodes. Such piezoelectric transducer is wide band and can emit and receive very short ultrasonic pulses without significant distortions, thus providing high depth resolution as well as beam collimation.
Further aspect of the invention provides an alternative method of implementation of the wide band transducer design. This design comprises a single conically shaped piezoelectric element with a single disk electrode on its working surface and a plurality of ring electrodes on the side surface of the cone adapted to be connected to an electric voltage of predetermined amplitude and phase in order to generate an optimal (desired) pressure distribution on the surface of the piezoelement that will result in a generation of a collimated beam.
Further aspect of the invention provides for yet another method of implementation of the wide band transducer design. This design comprises a single piezoelectric element shaped as a cone, cylinder, or other suitable shape, with a plurality of ring electrodes on its working surface and a single ring electrode on the side surface. These electrodes are adapted to be connected to an electric voltage of predetermined amplitude and phase in order to generate an optimal (desired) pressure distribution on the surface of the piezoelement that will result in a generation of a collimated beam.
Description of the drawings illustrating embodiments of the invention.
In drawings which illustrate embodiments of the invention, Fig 1 is the schematic diagram of the resonant piezoelement with 5 ring electrodes, the beam profile emitted by such piezoelement, and the pressure distribution on the surface of the piezodisk; Fig. 2 is the front and back view of the linear resonant piezoelement with 9 pairs of linear electrodes; Fig. 3 is the embodiment of a wide band piezoelement with a single disk electrode on the working surface and a series of ring electrodes on the side surface; Fig. 4 illustrates an alternative embodiment of a wide band transducer which incorporates a series of internal and surface ring electrodes; Fig. 5 is the embodiment of a wide band piezoelement with a plurality of ring electrodes on the working surface and a single ring electrode on the side surface; Fig. 6 is the embodiment of a collimating piezoelement with the capability of electronic beam steering; Fig.
is a schematic diagram of a preferred embodiment of a multi-parametric ultrasonic tomography apparatus; and Fig. 8 is a schematic diagram of a preferred embodiment of a multi-parametric ultrasonic tomography apparatus illustrating the image reconstruction procedure.
The embodiment of a resonant piezoelement is shown in Fig. 1. This particular implementation utilizes 5 pairs of ring electrodes - 5 ring electrodes 102 on the working surface and 5 corresponding ring electrodes 104 on the back surface. Transducers with greater number of electrode pairs, e.g. 8 or 12 can be easily designed and implemented using the same approach. The implementation shown in Fig. 1 provides for a significantly improved transducer performance compared to the prior art, by generating a more tightly collimated beam (ultrasonic beam is narrower and less divergent) and the side lobes in the directivity pattern are much smaller. The geometry (position and width) of electrodes and excitation voltages, were calculated using Eq. 4 to produce radial pressure distribution on the piezoelement working surface having profile 108. This pressure profile generates tightly collimated ultrasonic beam 106.
The embodiment of a linear (rectangular) collimating piezoelement is shown in Fig. 2. The working surface of the linear piezoelement 203 is composed of several rectangular electrodes 202, while matching narrow rectangular electrodes 204 are placed on the back surface of the piezoelement.
This type of linear electrode geometry provides a narrow weakly divergent knifelike ultrasonic beam in one plane. With this electrode geometry, changing amplitude and phase of the excitation voltage on individual electrodes can modify the directivity pattern of the element and thus allow to steer the beam in one plane.
A still further aspect of the invention provides for an ultrasonic transducer comprising a single sonically shaped piezoelectric element with a single disk electrode on its working surface and a plurality of electrodes adapted to be connected to an electric voltage of predetermined amplitude and phase on its side surface in order to generate a collimated ultrasonic beam.
Referring to Figure 3, a wide band collimating piezoelement according to the present invention is shown. The piezoelement includes a conical body of piezoelectric material 306, on one end of the body (working surface) is located a circular disk electrode 301, and a series of electrodes 303 attached to the side of the cone, which is axially polarized. An electrical circuit, not shown here, is connected to drive each of the electrodes. The position of the side ring electrodes and driving voltages on them are selected such that electric field distribution on the surface of the disk electrode 301 is the sum of the series of Bessel functions as defined in the specifications. The transducer body 306, in this case a conical body, acts as an acoustic trap. Consequently, such piezoelectric transducer is wide band and can emit and receive very short ultrasonic pulses without significant distortions and provides high depth resolution as well as beam collimation.
The embodiment of a wide band collimating piezoelement according to the present invention is shown in Fig. 4. The piezoelement includes a conical body of axially polarized piezoelectric material 403, on one end of the body (working surface) are located concentric ring electrodes 102 and a series of internal ring electrodes 404 are located inside the body of the element. The surface ring electrodes 102 have varying dimensions as shown in the plan view 401. The piezoelement body as shown in Figure 4, has a conical shape which acts as an acoustic trap (other shapes e.g., cylinder, a frustum of a cylinder, etc. can also be utilized). The internal electrodes 404 should be made from materials such as cast iron, zinc, an alloy of tungsten and tin, vanadium or other suitable conductive material whose acoustic impedance z matches closely the acoustic impedance of the piezoelectric element material. The acoustic impedance of the metals listed above is generally in the range of 31-36 x 106 kilograms per meter squared per second; for piezoelectric ceramics PZT-5A or PZT-5H (the most commonly used piezoelectric materials) z = 33-34 x 106 kilograms per meter squared per second. Thus, it is possible to match the acoustic impedances of the internal electrodes and the PZT material to insure that the internal electrodes will not cause significant reflection of acoustic waves. The piezoelements of the subject invention should be made from one-piece piezoelectric material. If the piezoelements are manufactured using the commonly used scintering method, the internal electrodes with the connectors should be placed inside the ceramic material when it is being formed from powdered state. If the piezoelement is manufactured using the epitaxial method, then the internal electrodes with their connectors may be drawn from the gas phase. The voltage dividers for providing the voltage phase and amplitude difference between adjacent electrodes (not shown in the Fig.) can be placed inside the transducer housing or alternatively these voltage dividers may be external to the housing.
Referring to Figure 5, an alternative embodiment of a wide band transducer is illustrated.
The piezoelement 505 is shaped as a frustum of a cylinder (conical or other shapes can also be utilized) comprising a plurality of ring shaped piezoelectric elements 502 on its working surface and a single ring electrode 503 on its side surface. The electrodes are adapted to be connected to an electric voltage of predetermined amplitude and phase in order to generate a collimated ultrasonic beam as described in specifications.
Fig. 6 is a schematic diagram of the working surface of a resonant or wide band transducer 603 comprising a plurality of ring electrodes subdivided into sectors 601. The electrodes 601 being adapted to be powered by an electric voltage of specified phase and amplitude in order to generate a collimated ultrasonic beam and to steer the beam by varying the voltage amplitude and phase applied to individual electrodes.
Referring to Figure 7, a schematic diagram of a preferred embodiment of a multi-parametric ultrasonic tomography apparatus which utilizes an embodiment of the transducers of the subject invention is shown generally by numeral 700.
The transducers 703 generate collimated beams 706 when excited by appropriate signals. Emitting and receiving transducers 703 are arranged to surround the object 715, e.g. breast. Acoustic coupling between the object 715 and transducers 703 is provided by the layer of coupling material 711.
Generally, for tomographic applications, such as breast tomography, or for tomographic imaging of solid materials, object is insonified from many points and at different angles. The transducers are controlled by suitable circuitry that activate each transducer in turn while other transducers including the radiating transducer receive the reflected, refracted, transmitted and scattered ultrasonic signals. Consequently, each area of the object will be insonified at various angles and from different points and signals will be received from various transducers and at different angles. All these signals are stored in computer memory or suitable storage means. A suitable algorithm, as illustrated in Fig.
8, is used to reconstruct a set of 2D images of the cross-section of the object.
The 2D images are constructed by measuring the following reconstruction parameters: acoustic impedance z, ultrasonic velocity c, bulk modules of elasticity k, density p and absorption index a. Thus five different images related to different material (tissue) properties can be reconstructed. By changing the position of the transducers relative to the object, it is possible to generate images of sequential cross-sections. Five 3D images of the entire object (e.g., breast) may then be reconstructed from a series of these 2D sections.
Figure 8 illustrates the process of multiparametric image reconstruction using the embodiment of a multi-parametric ultrasonic tomography apparatus 700. The said apparatus includes a set of two linear arrays 704 and 705 of small single element focusing wide band or resonant transducers numbered from 801 to 814. In the apparatus shown, each array 704 and 705 comprises seven transducers, each arranged in a suitable housing. The transducers as shown in Figure 8 are numbered 801 to 807 in the first array 704 and 808 to 814 in the second array 705. As may be seen from Figure 8, the object to be insonified 715 is placed between the transducer arrays 704 and 705. A
lubricating/coupling layer 711 is introduced between the surface of the object 715 and the transducer arrays 704 and 705. The transducers are driven by a suitable electronic circuit (not shown). The implementation of the 2D imaging technique using the above-described arrangement is now explained with specific reference to Figure 8, as follows. First transducer radiates a short ultrasonic pulse; all other transducers (including the emitting one) receive signals corresponding to the reflected, transmitted, refracted and scattered ultrasonic pulse 815. The amplitude, phase and time of arrival of each received signal are determined. Depending upon the internal structure of the object's cross-section, some transducers may receive several pulses (e.g., reflections from different interfaces) while others may receive one pulse only or none at all. All the data regarding the parameters of the received pulses are stored in a computer (not shown). In order to enhance the signal-to-noise ratio, this process is repeated several times and the average of the data received is taken. All the receiving transducers are rotated (either electronically or mechanically) to detect the signals arriving at different angles. The emitting transducer also rotates, performing the sector scanning so that the entire cross-section of the object 715 is insonified from various angles. This series of steps is repeated for each of the transducers. As a result, object's cross-section is insonified by ultrasonic waves from different points on the surface and at various angles. This process provides detailed information about the acoustic properties of the internal structures of a selected cross-section.
Generally speaking, five 2D images of the particular cross-section are determined. Each image corresponds to the distribution of one of the following physical parameters - namely, acoustic impedance, sound velocity, attenuation index, bulk modulus of elasticity and density. Such process greatly enhances the probability of detecting a small fault or lesion compared to other conventional techniques. For illustration purposes, the cross-section of the object 715 shown in Figure 8, is indicated as having several features, namely an outer region shown as numeral 712 and an inner region indicated by numeral 714. The region 714 also has an inner region shown as numeral 716 that differs with respect to at least one parameter. For example region 716 may exhibit a different bulk modulus of elasticity than region 714.
To begin, we first determine the acoustic parameters of the region adjacent to the transducers, in this case region 712. If any of the transducers, for example transducer 809, emits an ultrasonic pulse normal to the object's surface, then the first signal received by it would correspond to the pulse reflected by the interface of the lubricant 711 and object 715. If the phase of this pulse coincides with the phase of the exciting pulse, it means that the acoustic impedance z~ of region 712 is greater than the acoustic impedance of z~ of the lubricant 711. If the amplitude of the exciting pulse is Ao, then the amplitude A~
of the received signal can be written as follows:
Ai = Ao z~ zL (5) Zl + ZG
Since we always know the amplitude Ao of the emitted pulse and the impedance z~ of the lubricant 711, we can calculate the acoustic impedance z, of region 712 by measuring the amplitude A~ and using equation 5 above. We can then use any pair of neighboring transducers inclined at critical angles to the surface, one of them acting as an emitter and the other as a receiver. For example, we can select transducers 813 and 814 as shown in Figure 8. The, so-called, critical angle can be determined by finding the angle at which the receiving transducer's signal is at maximum. In the case illustrated, the ultrasonic pulse will propagate inside region 712 from transducer 813 to transducer 814. Knowing the distance d between these transducers and measuring the pulse time-of flight dt, we may calculate the sound speed c~ in region 712 using the following equation:
c~ - Ot (6) Using values of z~ and c~ for region 712, we may then compute its density p~ and bulk modulus of elasticity k~ from the following equations:
zi - P~c~ (7) and c 2 _ k~ (8) P~
Next, we may use, for example, transducer 814 as an emitter and transducer 808 as a receiver, both of which are electronically inclined to have their beam directions at critical angles. We will then measure the value of the amplitude 8 of the received pulse and then, using the distance r between these transducers, we may calculate the absorption index a~ of region 712 from the following equation:
B = Ao exp~-a,r~ (g).
Now having determined all five acoustic parameters z~, c~, p~, k~ and a~, for region 712, we can proceed further and determine these parameters for region 714.
We may consider the ultrasonic pulses emitted for example, by transducer 803. Assume that transducer 803 is inclined at an angle 6~ with respect to the object's surface. The pulse emitted from this transducer is reflected by the interface between region 712 and region 714 in several directions as shown in Figure 8. However, these reflected signals received by transducers 804, 805 and 806, respectively will have different amplitudes.
Signals with a maximal amplitude will be received by transducer 804 at an inclination angle 82 as shown in Figure 8. Knowing this angle 62, the inclination angle 9~ of the emitting transducer 803, time of pulse arrival and ultrasonic velocity in region 712 (which was calculated earlier), the location of the boundary of the interface between region 712 and region 714 may be determined. Then, by measuring the phase and the amplitude A2 of the pulse received by transducer 804 and substituting the known values of length s of the distance this ultrasonic pulse had traveled, impedance z~, absorption index a~
of region 712 and inclination angles 0~, and 0z, of transducers 803 and 804 respectively, the acoustic impedance of region 714 is calculated, using the following formula:
Az = A, exp~-a,s) ZZ Z' cos,Q (10).
Z2 ~- Zt In this equation, the angle beta is the inclination angle of the receiving transducer (transducer 804) and determines the direction of specular reflection for the corresponding section of the interface between region 712 and region 714.
This procedure is then applied to the analysis of the signals received by all other transducers. Taking into account their positions and angles of inclination, it is possible to determine the entire position and shape of first interface and all acoustic parameters of the tissue in region 714.
This process is then repeated for region 716 in order to determine the position and shape of the interface between this region and its surrounding region 714. By continuing this process, one will determine the locations and acoustic parameters of all features within the analyzed cross-section. As a result, the images of the cross-section depicting five different acoustic parameters are constructed.
These steps can be continued by changing the relative position of the apparatus and the object until the entire object is scanned. Utilizing the 2D
images for different cross-sections, a 3D image of the entire object is then reconstructed using any one of the five different parameters.
Thus, with the use of the algorithm described above, and the single element collimating transducers of the subject invention, a reliable 3D image of the object of interest is constructed. The transducers and apparatus described herein can be used in many forms of non-destructive testing, materials evaluation, imaging of babies' heads, breast and testicles imaging, and the like.
Various techniques have been used in order to create a wide band transducer that could emit and receive short ultrasonic pulses without distortions and thus exhibit high depth resolution. The most common of these techniques utilizes mechanical damping of the piezoelectric element. This is a very simple method and gives reasonably good results. But the noise level is relatively high because it is impossible to damp all the oscillations of the piezoelectric element. Several other methods, such as compensating electronic circuits (RLC correcting circuits) shaped piezoelectric elements, piezoelements with side electrodes, specialized piezoelectric materials with high attenuation indices were implemented and provide reasonably good results. However, all of these implementations have significant disadvantages in that it is necessary to tune each transducer, its electronic components, the shape of a radiated acoustic field, etc. for each specific application and system design.
In Soviet Patent Nos. SU 652808, SU 700916, and SU 847524, several wide band, simple and inexpensive piezoelectric transducers are described.
These transducers are of the so-called "surface excited thick piezoelectric"
type.
The piezoelectric element in these transducers has two electrodes located on one surface only. Thus, the only source of acoustic waves (for radiation) and electric charges (for reception) lies in a small piezoelectric element area between the electrodes. The transducer has a conical shape and eventually absorbs all ultrasonic pulses propagated within it; i.e. the body of the transducer acts as an acoustic trap. Transducers of this type are extremely wide band, simple and inexpensive. On the other hand, this type of transducer cannot form collimated beams. In other words, this transducer provides high depth resolution but has poor frontal resolution.
Thus, it may be seen that currently available single element transducers have either relatively low frontal (lateral) or azimuthal (depth) resolutions.
Linear phased arrays made up of multiple piezoelectric elements, on the other hand, provide very good frontal resolution (albeit only in elevation plane) but they are not wide band, have relatively large dimensions, are complicated and expensive to manufacture. Furthermore, attempting to use currently available phased arrays in ultrasonic tomography would be impractical. At least 20 independent transducers would have to be located around the object of interest (e.g., breast in medical ultrasound tomography), requiring very small transducers that are only a few millimeters in length. This would significantly downgrade the frontal resolution and appreciably complicate the manufacturing procedure, making these devices prohibitively expensive.
Description of the Invention and Outline of the Preferred Embodiment The present invention describes a new method for designing and constructing resonant or wide band single element ultrasonic transducers that are capable of producing a weakly divergent (collimated) ultrasonic beam, when driven by a suitable electronic circuit. The transducers described herein provide significant improvements in performance compared to prior art.
A transducer according to the prior art, which was intermitted by the applicant of the subject invention for generating a narrow weakly divergent ultrasonic beam comprises an axially polarized generally circular flat disk piezoelectric element to which a series of ring electrodes are attached. One surface of the piezoelectric element is termed the working surface and is generally the surface from which an ultrasonic beam is emitted or by which an ultrasonic signal is received. In order to produce a narrow weakly divergent ultrasonic beam from this type of transducer, the ring electrodes are arranged in concentric pairs with one member of the pair being wider than the other. The wider electrode of a pair is usually located on the working surface of the piezoelectric element and the narrower electrode of the pair being on the opposite surface of the element. An alternating arrangement, where a wider electrode of the pair is placed next to a narrow electrode of the adjacent pair is also possible and is easier to implement with certain types of fabrication technology. The pairs of electrodes are excited by an electric field applied thereto in order to provide the desired pressure distribution on the working s surface of the piezoelement. The pressure distribution in this prior art design is the Bessel function Jo(r/R).
Present invention provides further significant performance improvement for this type of a resonant transducer by modifying the electrode geometry and amplitudes and phases of excitation voltages in such a way as to obtain pressure distribution on transducer working surface maximizing the specific optimization criterion, as described below. Present invention introduces a method for determining optimal pressure distribution on the surface of the piezoelement. This method involves the selection of optimization criterion that is used in the transducer design model to obtain optimal trade off between the width and amplitude of the main lobe and size and position of the side lobes in the directivity pattern. The optimization criterion should be selected in such a way as to provide the best representation of the desired characteristics of the generated ultrasonic beam. The method for the transducer design is as follows:
The desired radial pressure distribution P(r) on the piezoelement's working surface is represented by the sum of a series of Bessel functions of the first kind and zero order Jo as follows:
__ ~ 'IO~an R
+ ~ An ',1 \
an where coefficients A" are defined as numerical constants providing the solution of the synthesis problem for beam profile, based on the assumption that the emitted ultrasonic beam is very narrow and weakly divergent; R is the piezoelement radius (in the case of resonant transducer) or face electrode disk radius (in the case of wide band transducer, as described below); J~ is the Bessel function of the first kind and first order, an is the nth root of the equation Jo(a)=0. Thus, a~=2.4, a2=5.5, a3=8.6, etc. Coefficients An are determined using a specific algorithm to obtain the optimal directivity pattern in terms of the relationship between beam width and position and size of the side lobes. Below is the description of the algorithm:
Directivity pattern for a given transducer is written as:
a J (k R sin 9) D(8)=KRz A° Jl (kRsin9)+KRz ~A" an -kz Rz sinz B ' an #kRsin9 kRsinB n=~ O.SJ,(a") , an =kRsin9 where K is the normalization coefficient.
A realistic or practical number of terms n in equation for D(9) is selected.
Usually n is selected empirically as a compromise between the desired directivity function and complexity of the transducer design. Generally speaking, the larger n would allow to obtain the directivity function with narrower main lobe and smaller side lobes but at a cost of a more complex design, lower signal intensity, and a larger number of electrode pairs. With n~oo and appropriate selection of coefficients Ar,, D(8) will approximate s-function, representing the phenomenon of superdirectivity. The practical value of n is usually between 5 and 20. The algorithm is implemented numerically as follows:
First, the optimization criterion Q for the desired directivity pattern (criterion of directivity narrowness) is defined. An example of such a criterion is given here as:
D ( 9)~ B ° ~3) j D ( a)da °
Other suitable optimization criteria that represent the desired characteristics of the directivity pattern, of cause, can be used. The above was chosen because it is relatively simple and has the following properties:
the smaller are the side lobes and narrower the main lobe, the smaller is the area under the integral;
the larger is the amplitude of the main lobe (numerator in the expression), the more energy is concentrated within it. Thus, the larger the criterion Q, the narrower and less divergent is the ultrasonic beam defined by D(8).
We then computed a set of N coefficients A" (n=1,2,3...N), based on the requirement to maximize the value of criterion Q . Each set consisted of a specific number of coefficients, e.g., 5, 8, 12, or 20. The more coefficients we used, the greater was the value of the criterion Q, narrower the main lobe in the directivity pattern, and smaller the side lobes.
Finally, we calculated the pressure distribution P(r) by substituting the values of coefficients A" obtained in the previous step. The peaks (maxima and minima) of the function P(r) define the positions of the ring electrodes on the surface of the piezoelement, i.e. each ring electrode (from 1 to N) is placed at a distance r(n) from the center of the piezoelement, where r(n) is the location of the nth peak of the function P (r) . The width of the nth electrode ring is approximately equal to the distance between the adjacent zero values of the function P(r) (for the wider electrode in the pair). In practical implementation, the width of the nth electrode ring is equal to the distance between the adjacent zero values of the function P(r) minus the width of the gap between the adjacent ring electrodes necessary to provide electrical separation between the rings. This gap would usually be in the range of 50 to 500 Vim. Width of the narrower electrode in the pair is between 30% and 50% of the width of the wider electrode (usually 40%). To achieve better transducer efficiency, wider electrodes would normally be placed on the working surface of the transducer and narrower ones on the opposite surface. An alternative implementation of the above design involves placement of a single solid disk electrode equal to the radius of the piezoelement on its working surface and a plurality of narrow (as described above) ring electrodes on the back surface of the piezoelement.
In such implementation the disk electrode is attached to ground and ring electrodes are driven by voltages of alternating polarity A further aspect of the invention provides for the procedure of electronic optimization (by changing excitation voltages and phases applied to ring electrodes) of the performance of the above described piezoelements in the near field, by maximizing the "near field beam optimization criterion" (which is selected to optimize the transducer performance for a given application), for either far or near zone. The following procedure is used:
The piezoelement is represented as a set of ring radiators driven by voltages with alternating polarities and various amplitudes. The optimization procedure is applied to a given set of electrodes placed on the surface of the piezoelement following the above described procedure.
As a criterion of optimal near zone collimation we use the product M
P (r ~ z ) I z=z, , r=0 M =~Qr~ Qr=
P ~r, Z~ I = dr z z, where each factor Q; is a modified criterion (3), applied this time not to directivity pattern, but to radial pressure profile at various z values (where z is the distance from the face of the piezoelement). Near zone is divided into m segments along the z axis, and z; is the distance to the ith segment. P(r,z) is the acoustic pressure at the coordinates r,z, generated by all ring electrodes positioned to optimize D(e) as previously described.
Further aspect of the invention enables electronic beam steering in a single element transducer design. In accordance with this aspect of the invention there is provided an ultrasonic transducer comprising a resonant piezoelectric element, a plurality of ring electrodes arranged on the surfaces of the said element according to the method described above. Each of the ring electrodes is further subdivided into 5 to 20 segments. Each ring electrode segment is adapted to be powered by an electric voltage of specified phase and amplitude in order to generate a collimated ultrasonic beam, and means associated with the electrodes, for electronically steering the beam by varying the voltage amplitude and phase applied to the electrodes. With the segmental electrode geometry, changing amplitude and phase of the excitation voltage on individual electrodes will modify the directivity pattern of the transducer.
Furthermore, this design enables annular focusing and sector scanning in both azimuthal and elevation planes. In reception mode, selective directional sensitivity is achieved by monitoring signals from specific sets of electrodes.
Further aspect of the invention provides an alternative method of implementation of the transducer design described above. Once an optimal (desired) pressure distribution on the surface of the piezoelement has been determined, the piezoelement is then nonuniformly polarized. The polarization vector is such that it approximates the desired radial pressure distribution on the surface of the piezoelement. Then both sides of such piezoelement are covered with solid disk electrodes (as in conventional piston implementation). Such element will emit a collimated beam when excitation signal is provided.
Dividing the surface disk electrodes into sectors allows to steer the beam by controlling the phase and amplitude of the excitation signal to individual sectors. This design offers significant advantage compared to one with ring electrodes, namely, it significantly reduces the number of electrodes and requires much less complex electronic circuitry to drive the transducer. However the process of polarizing such piezoelement is significantly more complex than for conventional piezoelements.
A still further aspect of the invention provides for a wide band collimating ultrasonic transducer. The transducer includes a solid body of piezoelectric material that acts as an acoustic trap (e.g. a solid cone, a long cylinder, a frustum of a cylinder, or other suitable shape). A series of ring electrodes are placed on the working surface of the said piezoelement according to the method described above, while a set of corresponding narrow ring electrodes are placed inside the body of the piezoelement during its fabrication. These internal electrodes being composed of a material of similar acoustic impedance to that of the piezoelectric element. An electrical circuit is connected to drive each of the electrodes. Such piezoelectric transducer is wide band and can emit and receive very short ultrasonic pulses without significant distortions, thus providing high depth resolution as well as beam collimation.
Further aspect of the invention provides an alternative method of implementation of the wide band transducer design. This design comprises a single conically shaped piezoelectric element with a single disk electrode on its working surface and a plurality of ring electrodes on the side surface of the cone adapted to be connected to an electric voltage of predetermined amplitude and phase in order to generate an optimal (desired) pressure distribution on the surface of the piezoelement that will result in a generation of a collimated beam.
Further aspect of the invention provides for yet another method of implementation of the wide band transducer design. This design comprises a single piezoelectric element shaped as a cone, cylinder, or other suitable shape, with a plurality of ring electrodes on its working surface and a single ring electrode on the side surface. These electrodes are adapted to be connected to an electric voltage of predetermined amplitude and phase in order to generate an optimal (desired) pressure distribution on the surface of the piezoelement that will result in a generation of a collimated beam.
Description of the drawings illustrating embodiments of the invention.
In drawings which illustrate embodiments of the invention, Fig 1 is the schematic diagram of the resonant piezoelement with 5 ring electrodes, the beam profile emitted by such piezoelement, and the pressure distribution on the surface of the piezodisk; Fig. 2 is the front and back view of the linear resonant piezoelement with 9 pairs of linear electrodes; Fig. 3 is the embodiment of a wide band piezoelement with a single disk electrode on the working surface and a series of ring electrodes on the side surface; Fig. 4 illustrates an alternative embodiment of a wide band transducer which incorporates a series of internal and surface ring electrodes; Fig. 5 is the embodiment of a wide band piezoelement with a plurality of ring electrodes on the working surface and a single ring electrode on the side surface; Fig. 6 is the embodiment of a collimating piezoelement with the capability of electronic beam steering; Fig.
is a schematic diagram of a preferred embodiment of a multi-parametric ultrasonic tomography apparatus; and Fig. 8 is a schematic diagram of a preferred embodiment of a multi-parametric ultrasonic tomography apparatus illustrating the image reconstruction procedure.
The embodiment of a resonant piezoelement is shown in Fig. 1. This particular implementation utilizes 5 pairs of ring electrodes - 5 ring electrodes 102 on the working surface and 5 corresponding ring electrodes 104 on the back surface. Transducers with greater number of electrode pairs, e.g. 8 or 12 can be easily designed and implemented using the same approach. The implementation shown in Fig. 1 provides for a significantly improved transducer performance compared to the prior art, by generating a more tightly collimated beam (ultrasonic beam is narrower and less divergent) and the side lobes in the directivity pattern are much smaller. The geometry (position and width) of electrodes and excitation voltages, were calculated using Eq. 4 to produce radial pressure distribution on the piezoelement working surface having profile 108. This pressure profile generates tightly collimated ultrasonic beam 106.
The embodiment of a linear (rectangular) collimating piezoelement is shown in Fig. 2. The working surface of the linear piezoelement 203 is composed of several rectangular electrodes 202, while matching narrow rectangular electrodes 204 are placed on the back surface of the piezoelement.
This type of linear electrode geometry provides a narrow weakly divergent knifelike ultrasonic beam in one plane. With this electrode geometry, changing amplitude and phase of the excitation voltage on individual electrodes can modify the directivity pattern of the element and thus allow to steer the beam in one plane.
A still further aspect of the invention provides for an ultrasonic transducer comprising a single sonically shaped piezoelectric element with a single disk electrode on its working surface and a plurality of electrodes adapted to be connected to an electric voltage of predetermined amplitude and phase on its side surface in order to generate a collimated ultrasonic beam.
Referring to Figure 3, a wide band collimating piezoelement according to the present invention is shown. The piezoelement includes a conical body of piezoelectric material 306, on one end of the body (working surface) is located a circular disk electrode 301, and a series of electrodes 303 attached to the side of the cone, which is axially polarized. An electrical circuit, not shown here, is connected to drive each of the electrodes. The position of the side ring electrodes and driving voltages on them are selected such that electric field distribution on the surface of the disk electrode 301 is the sum of the series of Bessel functions as defined in the specifications. The transducer body 306, in this case a conical body, acts as an acoustic trap. Consequently, such piezoelectric transducer is wide band and can emit and receive very short ultrasonic pulses without significant distortions and provides high depth resolution as well as beam collimation.
The embodiment of a wide band collimating piezoelement according to the present invention is shown in Fig. 4. The piezoelement includes a conical body of axially polarized piezoelectric material 403, on one end of the body (working surface) are located concentric ring electrodes 102 and a series of internal ring electrodes 404 are located inside the body of the element. The surface ring electrodes 102 have varying dimensions as shown in the plan view 401. The piezoelement body as shown in Figure 4, has a conical shape which acts as an acoustic trap (other shapes e.g., cylinder, a frustum of a cylinder, etc. can also be utilized). The internal electrodes 404 should be made from materials such as cast iron, zinc, an alloy of tungsten and tin, vanadium or other suitable conductive material whose acoustic impedance z matches closely the acoustic impedance of the piezoelectric element material. The acoustic impedance of the metals listed above is generally in the range of 31-36 x 106 kilograms per meter squared per second; for piezoelectric ceramics PZT-5A or PZT-5H (the most commonly used piezoelectric materials) z = 33-34 x 106 kilograms per meter squared per second. Thus, it is possible to match the acoustic impedances of the internal electrodes and the PZT material to insure that the internal electrodes will not cause significant reflection of acoustic waves. The piezoelements of the subject invention should be made from one-piece piezoelectric material. If the piezoelements are manufactured using the commonly used scintering method, the internal electrodes with the connectors should be placed inside the ceramic material when it is being formed from powdered state. If the piezoelement is manufactured using the epitaxial method, then the internal electrodes with their connectors may be drawn from the gas phase. The voltage dividers for providing the voltage phase and amplitude difference between adjacent electrodes (not shown in the Fig.) can be placed inside the transducer housing or alternatively these voltage dividers may be external to the housing.
Referring to Figure 5, an alternative embodiment of a wide band transducer is illustrated.
The piezoelement 505 is shaped as a frustum of a cylinder (conical or other shapes can also be utilized) comprising a plurality of ring shaped piezoelectric elements 502 on its working surface and a single ring electrode 503 on its side surface. The electrodes are adapted to be connected to an electric voltage of predetermined amplitude and phase in order to generate a collimated ultrasonic beam as described in specifications.
Fig. 6 is a schematic diagram of the working surface of a resonant or wide band transducer 603 comprising a plurality of ring electrodes subdivided into sectors 601. The electrodes 601 being adapted to be powered by an electric voltage of specified phase and amplitude in order to generate a collimated ultrasonic beam and to steer the beam by varying the voltage amplitude and phase applied to individual electrodes.
Referring to Figure 7, a schematic diagram of a preferred embodiment of a multi-parametric ultrasonic tomography apparatus which utilizes an embodiment of the transducers of the subject invention is shown generally by numeral 700.
The transducers 703 generate collimated beams 706 when excited by appropriate signals. Emitting and receiving transducers 703 are arranged to surround the object 715, e.g. breast. Acoustic coupling between the object 715 and transducers 703 is provided by the layer of coupling material 711.
Generally, for tomographic applications, such as breast tomography, or for tomographic imaging of solid materials, object is insonified from many points and at different angles. The transducers are controlled by suitable circuitry that activate each transducer in turn while other transducers including the radiating transducer receive the reflected, refracted, transmitted and scattered ultrasonic signals. Consequently, each area of the object will be insonified at various angles and from different points and signals will be received from various transducers and at different angles. All these signals are stored in computer memory or suitable storage means. A suitable algorithm, as illustrated in Fig.
8, is used to reconstruct a set of 2D images of the cross-section of the object.
The 2D images are constructed by measuring the following reconstruction parameters: acoustic impedance z, ultrasonic velocity c, bulk modules of elasticity k, density p and absorption index a. Thus five different images related to different material (tissue) properties can be reconstructed. By changing the position of the transducers relative to the object, it is possible to generate images of sequential cross-sections. Five 3D images of the entire object (e.g., breast) may then be reconstructed from a series of these 2D sections.
Figure 8 illustrates the process of multiparametric image reconstruction using the embodiment of a multi-parametric ultrasonic tomography apparatus 700. The said apparatus includes a set of two linear arrays 704 and 705 of small single element focusing wide band or resonant transducers numbered from 801 to 814. In the apparatus shown, each array 704 and 705 comprises seven transducers, each arranged in a suitable housing. The transducers as shown in Figure 8 are numbered 801 to 807 in the first array 704 and 808 to 814 in the second array 705. As may be seen from Figure 8, the object to be insonified 715 is placed between the transducer arrays 704 and 705. A
lubricating/coupling layer 711 is introduced between the surface of the object 715 and the transducer arrays 704 and 705. The transducers are driven by a suitable electronic circuit (not shown). The implementation of the 2D imaging technique using the above-described arrangement is now explained with specific reference to Figure 8, as follows. First transducer radiates a short ultrasonic pulse; all other transducers (including the emitting one) receive signals corresponding to the reflected, transmitted, refracted and scattered ultrasonic pulse 815. The amplitude, phase and time of arrival of each received signal are determined. Depending upon the internal structure of the object's cross-section, some transducers may receive several pulses (e.g., reflections from different interfaces) while others may receive one pulse only or none at all. All the data regarding the parameters of the received pulses are stored in a computer (not shown). In order to enhance the signal-to-noise ratio, this process is repeated several times and the average of the data received is taken. All the receiving transducers are rotated (either electronically or mechanically) to detect the signals arriving at different angles. The emitting transducer also rotates, performing the sector scanning so that the entire cross-section of the object 715 is insonified from various angles. This series of steps is repeated for each of the transducers. As a result, object's cross-section is insonified by ultrasonic waves from different points on the surface and at various angles. This process provides detailed information about the acoustic properties of the internal structures of a selected cross-section.
Generally speaking, five 2D images of the particular cross-section are determined. Each image corresponds to the distribution of one of the following physical parameters - namely, acoustic impedance, sound velocity, attenuation index, bulk modulus of elasticity and density. Such process greatly enhances the probability of detecting a small fault or lesion compared to other conventional techniques. For illustration purposes, the cross-section of the object 715 shown in Figure 8, is indicated as having several features, namely an outer region shown as numeral 712 and an inner region indicated by numeral 714. The region 714 also has an inner region shown as numeral 716 that differs with respect to at least one parameter. For example region 716 may exhibit a different bulk modulus of elasticity than region 714.
To begin, we first determine the acoustic parameters of the region adjacent to the transducers, in this case region 712. If any of the transducers, for example transducer 809, emits an ultrasonic pulse normal to the object's surface, then the first signal received by it would correspond to the pulse reflected by the interface of the lubricant 711 and object 715. If the phase of this pulse coincides with the phase of the exciting pulse, it means that the acoustic impedance z~ of region 712 is greater than the acoustic impedance of z~ of the lubricant 711. If the amplitude of the exciting pulse is Ao, then the amplitude A~
of the received signal can be written as follows:
Ai = Ao z~ zL (5) Zl + ZG
Since we always know the amplitude Ao of the emitted pulse and the impedance z~ of the lubricant 711, we can calculate the acoustic impedance z, of region 712 by measuring the amplitude A~ and using equation 5 above. We can then use any pair of neighboring transducers inclined at critical angles to the surface, one of them acting as an emitter and the other as a receiver. For example, we can select transducers 813 and 814 as shown in Figure 8. The, so-called, critical angle can be determined by finding the angle at which the receiving transducer's signal is at maximum. In the case illustrated, the ultrasonic pulse will propagate inside region 712 from transducer 813 to transducer 814. Knowing the distance d between these transducers and measuring the pulse time-of flight dt, we may calculate the sound speed c~ in region 712 using the following equation:
c~ - Ot (6) Using values of z~ and c~ for region 712, we may then compute its density p~ and bulk modulus of elasticity k~ from the following equations:
zi - P~c~ (7) and c 2 _ k~ (8) P~
Next, we may use, for example, transducer 814 as an emitter and transducer 808 as a receiver, both of which are electronically inclined to have their beam directions at critical angles. We will then measure the value of the amplitude 8 of the received pulse and then, using the distance r between these transducers, we may calculate the absorption index a~ of region 712 from the following equation:
B = Ao exp~-a,r~ (g).
Now having determined all five acoustic parameters z~, c~, p~, k~ and a~, for region 712, we can proceed further and determine these parameters for region 714.
We may consider the ultrasonic pulses emitted for example, by transducer 803. Assume that transducer 803 is inclined at an angle 6~ with respect to the object's surface. The pulse emitted from this transducer is reflected by the interface between region 712 and region 714 in several directions as shown in Figure 8. However, these reflected signals received by transducers 804, 805 and 806, respectively will have different amplitudes.
Signals with a maximal amplitude will be received by transducer 804 at an inclination angle 82 as shown in Figure 8. Knowing this angle 62, the inclination angle 9~ of the emitting transducer 803, time of pulse arrival and ultrasonic velocity in region 712 (which was calculated earlier), the location of the boundary of the interface between region 712 and region 714 may be determined. Then, by measuring the phase and the amplitude A2 of the pulse received by transducer 804 and substituting the known values of length s of the distance this ultrasonic pulse had traveled, impedance z~, absorption index a~
of region 712 and inclination angles 0~, and 0z, of transducers 803 and 804 respectively, the acoustic impedance of region 714 is calculated, using the following formula:
Az = A, exp~-a,s) ZZ Z' cos,Q (10).
Z2 ~- Zt In this equation, the angle beta is the inclination angle of the receiving transducer (transducer 804) and determines the direction of specular reflection for the corresponding section of the interface between region 712 and region 714.
This procedure is then applied to the analysis of the signals received by all other transducers. Taking into account their positions and angles of inclination, it is possible to determine the entire position and shape of first interface and all acoustic parameters of the tissue in region 714.
This process is then repeated for region 716 in order to determine the position and shape of the interface between this region and its surrounding region 714. By continuing this process, one will determine the locations and acoustic parameters of all features within the analyzed cross-section. As a result, the images of the cross-section depicting five different acoustic parameters are constructed.
These steps can be continued by changing the relative position of the apparatus and the object until the entire object is scanned. Utilizing the 2D
images for different cross-sections, a 3D image of the entire object is then reconstructed using any one of the five different parameters.
Thus, with the use of the algorithm described above, and the single element collimating transducers of the subject invention, a reliable 3D image of the object of interest is constructed. The transducers and apparatus described herein can be used in many forms of non-destructive testing, materials evaluation, imaging of babies' heads, breast and testicles imaging, and the like.
Claims (17)
1. A collimating single element transducer, containing piezoelement with several concentric pairs of ring electrodes, each electrode pair consisting of one large size electrode usually located on the working surface of the piezoelement and one smaller size electrode usually located on the back surface of the piezoelement, these electrodes are centered one above the other.
2. A procedure for determining the position of the electrodes on the surface of the piezoelement of claim 1 and the relative amplitude and phase of the excitation voltages on these electrodes comprising the steps of:
a) defining the optimization criterion to match the application requirements in terms of main lobe amplitude and width, and position and size of the side lobes; b) calculating a set of coefficients A n in the equation for directivity function:
that provides the maximal value of optimization criterion; c) calculating optimal pressure distribution on the surface of the piezoelement using the calculated set of coefficients A n; d) placing ring electrodes on both surfaces of the piezoelement centered over the locations of the peaks (maxima and minima) of the function P(r).
The width of the nth electrode ring being equal to the distance between the adjacent zero values of the function P(r) (for the wider electrode in the pair) minus the width of the gap necessary to provide electrical separation between adjacent rings, and the narrower electrode in the pair being 30%
to 50% of the width of the wider electrode.
a) defining the optimization criterion to match the application requirements in terms of main lobe amplitude and width, and position and size of the side lobes; b) calculating a set of coefficients A n in the equation for directivity function:
that provides the maximal value of optimization criterion; c) calculating optimal pressure distribution on the surface of the piezoelement using the calculated set of coefficients A n; d) placing ring electrodes on both surfaces of the piezoelement centered over the locations of the peaks (maxima and minima) of the function P(r).
The width of the nth electrode ring being equal to the distance between the adjacent zero values of the function P(r) (for the wider electrode in the pair) minus the width of the gap necessary to provide electrical separation between adjacent rings, and the narrower electrode in the pair being 30%
to 50% of the width of the wider electrode.
3. A wide band collimating piezoelement shaped to act as an acoustic trap (e.g., cone) with one disk electrode located on the working surface of the element, and several ring electrodes located on the side surface of the piezoelement. The position and excitation voltages driving respective ring electrodes are determined by conventional calculations of electrostatic field required to obtain the desired acoustic pressure distribution of the claim 2 on the working surface of the piezoelement.
4. A single element wide band collimating piezotransducer with a plurality of ring electrodes located on the working surface of the element, and corresponding ring electrodes located inside the body of the piezoelement (internal electrodes). The position and dimensions of the electrodes are determined by the procedure in claim 2 as for the transducer in claim 1.
5. The internal electrodes in claim 4 are placed inside the piezoelement prior to the scintering step of the fabrication of the piezoelement.
6. The internal electrodes in claim 4 are drawn from the gas phase, if the piezoelement if fabricated using epitaxial method.
7. The internal electrodes in claim 4 are made from conducting material having the same or closely matched acoustic impedance as the material used to fabricate the piezoelement.
8. A single element wide band collimating piezotransducer with a plurality of ring electrodes located on the working surface of the element and one ring electrode located on the side surface of the piezoelement. The shape and dimensions of the piezoelement are selected to insure complete trapping of the acoustic waves propagating within the element (e.g., cone, long cylinder, frustum of a cylinder). The position and excitation voltages driving respective ring electrodes (with side electrode being connected to ground) are determined by conventional calculations of electrostatic field required to obtain the desired acoustic pressure distribution on the working surface of the piezoelement. This desired pressure distribution is defined in the form of sum of series of Bessel functions of claim 2.
9. Ultrasonic piezoelements of claims 1, 4, and 8 adapted with segmental electrode geometry, by subdividing individual ring electrodes into several segments, capable of providing annular focusing and sector scanning in both azimuthal and elevation planes by changing the excitation voltage and phase on individual electrodes.
10. Ultrasonic piezoelement of claim 1 adapted with sector electrode geometry on front and back surface of a non-uniformly polarized piezoelement. The said piezoelement capable of providing annular focusing and sector scanning in by changing the excitation voltage and phase on individual sector electrodes. Poling of the subject piezoelement is performed in such a way as to produce the polarization vector which approximates the radial pressure distribution on the surface of the piezoelement required to produce a collimated beam as defined in claim 2.
11. Ultrasonic piezoelement of claims 3 4, and 8 adapted with sector electrode geometry on the working surface of a non-uniformly polarized piezoelement capable of providing annular focusing and sector scanning in any direction by changing the excitation voltage and phase on individual electrodes. Poling of the subject piezoelement is performed in such a way as to produce the polarization vector on the working surface which approximates the radial pressure distribution on the surface of the piezoelement required to produce a collimated beam as defined in claim 2.
12. A linear piezoelement producing knife-like beam (collimated in one plane) with several pairs of linear electrodes on its front and back surfaces.
Surface electrode geometry is determined as in claim 2 with equations reduced to one-dimensional case.
Surface electrode geometry is determined as in claim 2 with equations reduced to one-dimensional case.
13. A method for electronically optimizing the performance of transducers of claims 1, 3, 4, and 8-12 in the near zone of the said transducers, based on the numerical calculation for a set of ring radiators driven by voltages with alternating polarities and specific amplitudes, which maximizes the product M defined as:
where Q i is the near-zone optimization criterion applied to radial pressure profiles at various z values (where z is the distance from the face of the piezoelement). Near zone is divided into m segments along the z axis, and z i is the distance to the ith segment. P(r,z) is the acoustic pressure at the coordinates r,z, generated by all ring electrodes positioned to optimize D(.theta.) as defined in claim 2.
where Q i is the near-zone optimization criterion applied to radial pressure profiles at various z values (where z is the distance from the face of the piezoelement). Near zone is divided into m segments along the z axis, and z i is the distance to the ith segment. P(r,z) is the acoustic pressure at the coordinates r,z, generated by all ring electrodes positioned to optimize D(.theta.) as defined in claim 2.
14. A procedure for enabling multiparametric imaging (i.e. simultaneous imaging of several different physical parameters) of an object. The said procedure determines the geometry of all interfaces within the object insonified by several transducers (usually between 5 and 50 individual transducers are to be used, depending on the size and complexity of internal structure of the object) of claims 1, 3, 4, 8-11, and 13. The interfaces for up to five acoustic parameters are determined using the following steps: a) Transducers are placed around the object; b) each transducer in sequence emits a series of pulses at various angles to the surface of the object; c) the amplitude, phase and time-of flight for each ultrasonic pulse received by every transducer, including the emitting one, are measured and recorded; d) the recorded signals are processed using standard formulae for amplitude and phase of the reflected and transmitted pulses, for acoustic impedance, bulk modulus of elasticity and pulse time-of flight, for Snell's law of refraction, and for the absorption index to determine spatial distribution of five different acoustic parameters of the object, namely, velocity of ultrasound propagation, material density, acoustic impedance, bulk modulus of elasticity, and absorption index within the tested object.
15. A procedure in claim 14 for calculating three independent acoustic characteristics (velocity of propagation of ultrasound, acoustic impedance and ultrasound absorption index) for each location within the object tested by using the measured parameters of received signals (amplitude, phase and instant of arrival).
16. A procedure in claim 14 for calculating two more derived physical parameters of the object for each location (density and bulk modulus of elasticity).
17. A procedure in claim 14 to reconstruct five different 2D images of the insonified cross-section of the object depicting the distribution of each of five different acoustic parameters within this cross-section.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2268415 CA2268415A1 (en) | 1999-04-09 | 1999-04-09 | Single element ultrasonic collimating transducers and a method and apparatus utilizing ultrasonic transducers in 3d tomography |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2268415 CA2268415A1 (en) | 1999-04-09 | 1999-04-09 | Single element ultrasonic collimating transducers and a method and apparatus utilizing ultrasonic transducers in 3d tomography |
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| Publication Number | Publication Date |
|---|---|
| CA2268415A1 true CA2268415A1 (en) | 2000-10-09 |
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ID=29588719
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2268415 Abandoned CA2268415A1 (en) | 1999-04-09 | 1999-04-09 | Single element ultrasonic collimating transducers and a method and apparatus utilizing ultrasonic transducers in 3d tomography |
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| CA (1) | CA2268415A1 (en) |
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