Abstract:
A piezoelectric transducer formed of a body of piezoelectric material having first and second opposed sides and first and second electrically conductive layers on the first and second sides respectively of the piezoelectric body, wherein the piezoelectric body and the electrically conductive layers are so constructed that they form a plurality of separate adjacent series-connected transducer elements. A method of manufacturing such a transducer is also disclosed. The piezoelectric body may have a substantially uniform direction of polarization, or alternating zones of opposite polarization. The conductive layers may be continuous or discontinuous, together forming isolated electrode pairs to define the individual transducer elements. The elements can be hard wired or connected through a switching circuit to display either circumferential or axial or other ultrasonic focal patterns, and may be connected in a parallel, rather than a series configuration. When connected in series, advantageously higher impedance can be obtained, compared to the parallel configuration.

Description:
FIELD 
   The present invention relates to multi-element piezoelectric transducers. 
   BACKGROUND 
   Piezoelectric transducers capable of generating pulses of ultrasonic energy, e.g., sound waves, in response to an electrical excitation are formed of one or more bodies of piezoelectric material, usually a ceramic such as Lead Zirconate Titanate (PZT), with electrodes on opposite sides for coupling to an electric power supply. Such transducers have many applications, including in the medical field. Among the medical applications are ultrasound imaging, lithotripsy, i.e., using ultrasound pulses to break up kidney stones or the like, and lysing or destroying the cell walls of adipose/cellulite tissue for cosmetic procedures, generally by causing cavitation in the tissue. 
   For such medical and cosmetic purposes, it is often desirable to be able to focus the ultrasonic output of the transducer. To achieve this, the transducers are often comprised of a cup-shaped piezoelectric ceramic shell and conductive layers forming electrodes covering the convex outside and concave inside of the piezoelectric shell. Typically, the transducers are hemispherical, with the “open end”, i.e., the equatorial plane positioned toward the subject being treated. 
   The transducer is excited to vibrate and generate ultrasound by pulsing it using an AC power supply generally operating at a resonant frequency of vibration of the piezoelectric material. 
   A hemispherical transducer in which the conductive surfaces define a single electrode pair exhibits an “axial focal pattern”. This is an ellipsoidal pattern having a relatively small cross section and relatively long axis coincident with a “longitudinal” axis of the transducer, i.e., a line through the center of rotation of the transducer perpendicular to the equatorial plane. 
   SUMMARY OF THE INVENTION 
   According to some embodiments of the invention, there is provided a transducer divided into a plurality of transducer elements, adapted to be simultaneously excited with AC voltages having different phases.  FIG. 1  is a perspective drawing, with a portion cut away, which shows the structure of a multi-element, cup-shaped focusing transducer  100  in schematic form. Transducer  100  is comprised of a shaped ceramic body  110 , and bottom and top layers forming electrically conductive surfaces  114  and  116 , respectively, on the concave inner and convex outer sides  106  and  108  of body  110 . Surfaces  114  and  116  may comprise conductive metal layers painted onto or otherwise applied to ceramic body  110 , e.g., by spraying or by dripping conductive paint onto the piezoelectric body  110  while spinning it. A longitudinal axis of the transducer is indicated at  130 . The equatorial plane is indicated at  132 . 
   For simplicity, transducer  100  will be described as hemispherical, and its features will be described in that context. It should be understood, however, that the transducer can be configured as an essentially flat panel, a cylinder, a spherical cap, i.e., less than a hemisphere, (sometimes also referred to as a spherical segment), and that other non-spherical configurations are also possible. 
   The separate transducer elements are optionally created by scoring through the top and bottom conductive surfaces  116  and  114 , for example, along meridians of the hemisphere, or in any other desired pattern, to create electrically isolated electrode pairs. In the example shown, transducer  100  is comprised of four transducer elements  118   a - 118   d . Score lines  120   a - 120   d  extend completely through conductive layer  116  to form spherical triangles that define outer electrodes  116   a - 116   d  respectively. Similar score lines (not visible in  FIG. 1 ) extending completely through inner conductive layer  114 , and aligned with score lines  120   a - 120   d , define the inner electrodes. An axial opening  124  at the top pole of the transducer body is ordinarily also provided to facilitate manufacturing, and to allow insertion of other medical instruments or sensors during use. 
   Appropriate wiring (not shown) connects the respective electrode-pairs to a suitable power supply or power supplies. When so configured and connected, the portions of the piezoelectric material between the respective electrode-pairs effectively function as separate transducers. If the exciting voltages for the adjacent transducer elements are of opposite phase, the resulting composite focal pattern is “circumferential”, i.e., it exhibits substantially zero ultrasound pressure along transducer axis  130  and peaks in ultrasound pressure for each element symmetrically located along the circumference of a circle having its center along axis  130 . 
   It is often desirable for tissue treatment to generate ultrasound at 1 MHz. However, the resonant frequency of vibration of a transducer as described above is inversely proportional to the thickness of the piezoelectric body. For example, a spherical focusing PZT transducer about 10 cm in diameter having a thickness of about 10 mm typically has a resonant frequency of about 200 KHz. To achieve a resonant frequency of vibration of about 1 MHz, the PZT body should have a thickness of about 2 mm. 
   The decreased thickness at higher frequencies can present a problem. For a multi-element transducer constructed in the manner described above, each transducer element behaves like a capacitor having electrical impedance inversely proportional to the thickness of the PZT material. The electrical impedance of each transducer element due to such capacitance is relatively low, and is typically about 10 Ohms at a frequency of about 200 KHz. However, for transducers operating at a frequency of about 1 MHz, the impedance of each transducer element is typically about 2 Ohms. 
   Matching a low impedance load to a power supply designed to drive a relatively high impedance load is generally accomplished by coupling the load to the power supply using a matching circuit. However, matching circuits become increasingly inefficient and wasteful of energy as the difference between the impedance of a load and the impedance for which the power supply is designed increases. 
   According to some embodiments of the invention, there is provided a piezoelectric transducer comprising a plurality of transducer elements that exhibits relatively high electrical impedance. 
   According to some embodiments of the invention, there is provided a multi-element piezoelectric transducer in which the transducer elements are electrically connected in series selectively in a first or a second configuration. 
   In the first configuration, and according to some aspects of the method of the invention, the transducer elements are so connected that a voltage applied to the transducer induces electric fields in the same direction relative to a direction of polarization of the piezoelectric material in all of the transducer elements. These embodiments are sometimes referred to below as “matching field” configurations. Such configurations yield axial focal patterns as described above. 
   In the second configuration, and according to some embodiments of the method of the invention, the transducer elements are so connected that a voltage applied to the transducer induces electric fields in opposite directions in adjacent transducer elements relative to a direction of polarization of the piezoelectric material in the transducer. These embodiments are sometimes referred to below as “alternating field” configurations. Such configurations yield circumferential focal patterns as described above. 
   According to some embodiments of the invention, a switching circuit is provided whereby a series-connected multi-element transducer can be switched between alternating and matching field configurations, thereby selectably providing circumferential or axial focal patterns. 
   As a consequence of the series connection of the transducer elements in a multi-element transducer, in accordance with an embodiment of the invention, relatively high impedance is obtained for the multi-element transducer compared to that of conventional transducers. 
   Thus, according to an aspect of the invention, a multi-element piezoelectric transducer is provided which is formed of a body of piezoelectric material having first and second opposed sides, and first and second electrically conductive layers on the first and second sides respectively of the piezoelectric body, wherein the body of piezoelectric material and the electrically conductive layers are so constructed that they form a plurality of separate adjacent series-connected transducer elements. 
   Optionally, in such a transducer, the transducer elements are so arranged that a potential difference applied thereto generates an electric field in opposite directions in immediately adjacent transducer elements relative to a polarization direction of the piezoelectric material. 
   Optionally, in such a transducer, the first and second electrically conductive layers are comprised of respective pluralities of first and second electrically isolated portions, respective ones of said first and second electrically isolated portions being aligned to form electrode pairs which cooperate with intervening portions of the piezoelectric body to define the plurality of spaced piezoelectric transducer elements, and the electrodes are so connected together that the transducer elements are connected in a first series configuration in which an applied potential difference generates an electric field in the same direction in each transducer element relative to a polarization direction of the piezoelectric material. 
   Optionally, in such a transducer, the first and second electrically conductive layers are comprised of respective pluralities of first and second electrically isolated portions, respective ones of said first and second electrically isolated portions are aligned to form electrode pairs which cooperate with intervening portions of the piezoelectric body to define the plurality of spaced piezoelectric transducer elements; and the electrodes are so connected together that the transducer elements are connected in a second series configuration in which an applied potential difference generates electric fields in opposite directions in immediately adjacent transducer elements relative to a polarization direction of the piezoelectric material. 
   Optionally, in such a transducer, the first and second electrically conductive layers extend substantially continuously over a portion of the body of piezoelectric material, and the adjacent series-connected transducer elements are defined by adjacent zones of alternating polarization direction in the piezoelectric material between the conductive layers. 
   Optionally, in such a transducer, the piezoelectric body is hemispherical in shape, and the adjacent zones of the piezoelectric body are in the shape of hemispherical triangles separated by spaced meridians of the hemispherical body. Alternatively, the zones are crescent-shaped. 
   Optionally, in such a transducer, the piezoelectric body is in the form of a spherical cap. 
   Optionally, in such a transducer, the piezoelectric body is cup-shaped, and the adjacent zones of the piezoelectric body are crescent-shaped. Alternatively, a first zone is in the form of a cap, and other zones are annular. 
   Optionally, in such a transducer, there is provided a switching arrangement operative to connect the transducer elements together in a matched field configuration or in an alternating field configuration. Optionally, in such a transducer, in the matched field configuration, first and second electrodes of adjacent transducer elements are connected together. Optionally, in such a transducer, in the alternating field configuration, the first electrodes of at least one pair of adjacent transducer elements are connected together. Optionally, in such a transducer, in the alternating field configuration, the electrical connections connect second electrodes of at least one pair of adjacent transducer elements together. Optionally, in such a transducer, in the alternating field configuration, transducer element pairs having their first electrodes connected together are interleaved with transducer element pairs having their second electrodes connected together. 
   Optionally, in such a transducer, the electrically isolated portions are defined by a plurality of aligned discontinuities between adjacent portions of the conductive layers. 
   Optionally, in such a transducer, the piezoelectric body is hemispherical in shape, and the aligned discontinuities extend along spaced meridians of the hemispherical body. 
   According to an aspect of the invention, a piezoelectric transducer is provided which is formed of a shaped body of piezoelectric material, a first electrically conductive layer on a first side of the piezoelectric body, a second electrically conductive layer on a second side of the piezoelectric body which is opposite the first side, the first and second electrically conductive layers being comprised of a plurality of respectively aligned electrically isolated first and second electrode portions forming electrode pairs which cooperate with intervening portions of the piezoelectric body to define a plurality of spaced piezoelectric transducer elements, a plurality of signal paths which connect the transducer elements together, and a switching circuit operable to connect signal paths so the individual transducer elements are selectably connected together in series or in parallel. 
   Optionally, in such a transducer, the transducer body is cup-shaped, the first electrically conductive layer is on a concave side of the transducer body, and the second electrically conductive layer is on a convex side of the transducer body. 
   Optionally, in such a transducer, the piezoelectric body is characterized by a direction of polarization, and the switching circuit is operable to connect adjacent transducer elements in an alternating field configuration, or to connect the transducer elements in a matched field configuration, and either in series or parallel for each polarization configuration. 
   Optionally, in such a transducer, the electrically isolated portions are defined by a plurality of aligned discontinuities between adjacent portions of the conductive layers. 
   Optionally, in such a transducer, the piezoelectric body is hemispherical in shape, and the aligned discontinuities extend along spaced meridians of the hemispherical body. 
   Optionally, in such a transducer, the piezoelectric body is in the form of a spherical cap. 
   Optionally, in such a transducer, the switching circuit is operable to connect the signal paths so the individual transducer elements are selectably connected together in series or in parallel. 
   According to an aspect of the invention, a method of manufacturing a piezoelectric transducer is provided comprising the steps of forming a body of piezoelectric material having first and second opposed sides, polarizing the material according to a desired pattern, and forming first and second electrically conductive layers on the first and second sides, respectively, of the piezoelectric body, wherein the body of piezoelectric material and the electrically conductive layers are so formed that they define a plurality of separate adjacent series-connected transducer elements. 
   Optionally, according to such a method, the transducer elements and conductive layers are so formed that a potential difference applied to the conductive layers generates an electric field in opposite directions in immediately adjacent transducer elements relative to a polarization direction of the piezoelectric material. 
   Optionally according to such a method, the first and second electrically conductive layers are formed by dividing the conductive layers into respective pluralities of first and second electrically isolated portions, the conductive layers are so divided that respective ones of said first and second electrically isolated portions are aligned to form electrode pairs which cooperate with intervening portions of the piezoelectric body to define the plurality of spaced piezoelectric transducer elements, and by connecting the electrodes together so that the transducer elements are connected in a first series configuration in which an applied potential difference generates an electric field in the same direction in each transducer element relative to a polarization direction of the piezoelectric material. 
   Optionally according to such a method, the first and second electrically conductive layers extend substantially continuously over a portion of the body of piezoelectric material and the adjacent series-connected transducer elements are defined by forming adjacent zones of alternating polarization direction in the piezoelectric material between isolated aligned portions of the conductive layers. 
   Optionally, according to such a method, the body of piezoelectric material is hemispherical in shape, and the adjacent zones of alternating polarization are in the shape of spherical triangles. 
   Optionally, according to such a method, the body of piezoelectric material is cup-shaped, and the adjacent zones of alternating polarization are in the shape of triangles, or the adjacent zones of alternating polarization are crescent-shaped, or a first zone of alternating polarization is formed as a cap at a polar region of the cup-shaped body, and other zones of alternating polarization are formed as annuli. 
   According to an aspect of the invention, a piezoelectric transducer is provided in the form of a body of piezoelectric material having first and second opposed sides, and first and second substantially continuous electrically conductive layers on the first and second sides respectively of the piezoelectric body, wherein the body of piezoelectric material and the electrically conductive layers are so constructed that they form a plurality of separate adjacent connected transducer elements. In a transducer according to this aspect of the invention, the separate transducer elements cooperate to form a circumferential acoustical focal pattern. 
   According to an aspect of the invention, a method of manufacturing a piezoelectric transducer is provided which comprises forming a body of piezoelectric material having first and second opposed sides, polarizing the material according to a desired pattern, and forming first and second substantially continuous electrically conductive layers on the first and second sides respectively of the piezoelectric body wherein the body of piezoelectric material and the electrically conductive layers are so formed that they define a plurality of separate adjacent connected transducer elements. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Non-limiting examples of embodiments of the invention are described below with reference to figures attached hereto that are listed following this paragraph. Identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale. 
       FIG. 1  schematically shows a perspective drawing of a multi-element piezoelectric transducer having a portion cut away to show structural details; 
       FIG. 2  is vertical cross-sectional view of  FIG. 1  showing the polarization of a piezoelectric material in a transducer such as that of  FIG. 1 ; 
       FIG. 3A  is an electrical schematic diagram of a first embodiment of the invention; 
       FIG. 3B  is a bottom plan view of a transducer showing a schematic wiring layout for the embodiment of  FIG. 3A ; 
       FIG. 4A  is an electrical schematic diagram of a second embodiment of the invention; 
       FIG. 4B  is a top plan view of a transducer showing a schematic wiring layout for the embodiment of  FIG. 4A ; 
       FIG. 5A  is an electrical schematic diagram showing how a transducer according to the first embodiment of the invention can be connected in a parallel arrangement; 
       FIG. 5B  is a top plan view of a transducer showing a schematic wiring layout for the arrangement of  FIG. 5A ; 
       FIG. 6A  is an electrical schematic diagram showing how a transducer according to the second embodiment of the invention can be connected in a parallel arrangement; 
       FIG. 6B  is a top plan view of a transducer showing a schematic wiring layout for the arrangement of  FIG. 6A ; 
       FIG. 7  is a schematic diagram showing connection of a multi-element transducer to a switching circuit; 
       FIG. 8  is a perspective drawing showing an alternative way of achieving the focal patterns of multi-element piezoelectric transducers according to the first embodiment of the invention; 
       FIG. 9  is a perspective drawing similar to  FIG. 8  illustrating how some of the concepts of the invention can be applied to a multi-element transducer configuration that is different from that of the first and second embodiments; and 
       FIG. 10  is a perspective drawing similar to  FIGS. 8 and 9  illustrating application of some of the concepts of the invention to a multi-element transducer having another configuration that is different from that of the first and second embodiments. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 2 , there is shown an enlarged vertical cross-sectional view of transducer  100  illustrated in  FIG. 1 . Again, for convenient description, it is assumed that transducer  100  is hemispherical (with the longitudinal axis indicated at  130 , and the equatorial plane indicated at  132 ), but it should be understood that spherical cap or other curvatures are also encompassed within the scope of the invention. 
   Transducer elements  118   a  and  118   d  shown in  FIG. 1  are illustrated in  FIG. 2 . As will therefore be appreciated, the drawing is sectioned along score lines  120   a  and  120   b  (see  FIG. 1 ). As illustrated, terminals  126  and  127  are connected respectively to the outer electrodes  116   a ,  116   b , . . . and  114   a ,  114   b , . . . by which the transducer elements  118   a - 118   d  are energized. 
   It should also be appreciated that the outer side  134  of transducer  100  (i.e., the convex side) is conventionally anchored to a suitable mass so that the ultrasound energy emitted by the transducer is mainly directed from the inner, i.e., concave side  136 , toward the subject under treatment. As mentioned above, and as known by those skilled in the art, if materials such as PZT are exposed during manufacture to a high-strength electric (polling) field under appropriate conditions, the material will become polarized, i.e., it will exhibit an overall orientation of positive and negative electric charge pairs in the crystal structure of the material which orientation is retained after manufacture. Then, if exposed to an electric field, the material will expand or contract, depending on the direction of the field relative to the direction of polarization. The diametrically extending arrows  128  in  FIG. 2  within piezoelectric material  110  schematically indicate polarization direction. For an electric field generated in material  110  parallel or anti-parallel to the polarization direction, the material respectively expands or contracts along the polarization direction. A first embodiment of the invention is illustrated in  FIGS. 3A and 3B .  FIG. 3A  shows an electrical schematic diagram of a transducer  300  having four transducer elements  118   a - 118   d . Element  118   a , for example, is comprised of inner electrode  114   a , outer electrode  116   a , and an intervening portion  324   a  of shaped piezoelectric body  110  (see  FIGS. 1 and 2 ). Respective transducer elements  118   b - 118   d  are comprised of inner electrodes  114   b - 114   d , outer electrodes  116   b - 116   d , and the intervening portions  324   b - 424   d  of piezoelectric body  110 . 
   In the embodiment of  FIG. 3A , the transducer elements are connected in series in an alternating field configuration relative to the direction of polarization of the piezoelectric material. To illustrate this conveniently, arrows  328  indicate the direction of polarization, and double arrows  330   a - 330   d  indicate the field direction relative to the direction of polarization. Plus (+) and minus (−) signs at the electrodes of the transducer elements indicate instantaneous voltage drop directions for a voltage having the polarity indicated at input terminals  310  and  312 , by which transducer  300  is connected to a power supply (not shown). 
   Thus, for the illustrated embodiment, terminals  310  and  312  are connected to terminals  114   a  and  114   d  respectively of transducer elements  118   a  and  118   d . Terminals  116   a  and  116   b  of transducer elements  118   a  and  118   b  are connected together by a signal path  314 , and the terminals  114   b  and  114   c  of transducer elements  118   b  and  118   c  are connected together by a signal path  318 . Terminals  116   c  and  116   d  of transducer elements  118   c  and  118   d  are connected together by a signal path  316 . As a consequence, the induced electric fields in adjacent transducer elements are in opposite (alternating) directions, and the mechanical vibrations generated by adjacent sectors are 180° are out of phase relative to each other.  FIG. 3B  shows a schematic bottom plan view of transducer  300  and an exemplary wiring layout by which the electrical configuration of  FIG. 3A  may be achieved. In the figure, electrodes  114   a - 114   d  on the concave, bottom side of the transducer elements  118   a - 118   d , respectively, are shown. 
   The embodiment illustrated in  FIGS. 3A and 3B  exhibits a circumferential focal pattern with one peak for each transducer element. In addition, since the impedance of N like circuit elements connected in series is related to N times the impedance of a single element while the impedance of N such elements connected in parallel is related to 1/N times the impedance of a single element, the four-element series-connected transducer illustrated in  FIG. 3  exhibits electrical impedance which can be 16 times that of conventional transducers having the same elements connected in parallel. 
   A second embodiment of the invention is illustrated in  FIGS. 4A and 4B . Here, a four-element transducer  400  is arranged with its elements  118   a - 118   d  connected in series in matched field configuration. Thus, input leads  410  and  412  are connected respectively to the “−” side terminal  116   a  of element  118   a , and the “+” side terminal  116   d  of element  118   d . Likewise, the “+” side terminal  116   a  of element  118   a  is connected to the “−” side terminal  11   4   b  of element  118   b  by signal path  414 , the “+” side terminal  116   b  of element  118   b  is connected to the “−” side terminal  114   c  of element  118   c  by a signal path  416 , and the “+” side terminal  116   c  of element  118   c  is connected to the “−” side terminal  114   d  of element  118   d  by a signal path  418 . As a consequence, the electric fields (indicated by double arrows  430   a - 430   d ) are in the same direction relative to the polarization of the piezoelectric material (indicated by single arrows  428 ) in all of the transducer elements, and the mechanical vibrations generated by all the elements are in phase relative to each other.  FIG. 4B  is a schematic top plan view of transducer  400  which shows electrodes  116   a - 116   d , and an exemplary wiring layout by which the electrical configuration of  FIG. 4A  may be achieved. 
   The embodiment illustrated in  FIGS. 4A and 4B  exhibits an axial focal pattern, i.e., having one peak along the transducer axis. In addition, like the embodiment of  FIGS. 3A and 3B , the impedance can be  16  times that of prior art transducers in which the elements are connected in parallel. In some instances, it is desirable to be able to switch a transducer between the alternating field configuration of  FIGS. 3A and 3B  and the matched field configuration of  FIGS. 4A and 4B . This can be achieved by connecting the input terminals (designated as  310  and  312  in  FIGS. 3A and 3B  and as  410  and  412  in  FIGS. 4A and 4B ) and the signal paths between the transducer elements through an appropriate switching circuit as illustrated schematically in  FIG. 7 . 
   Here, a four-element transducer such as transducer  300  illustrated in  FIG. 3A  (or transducer  400  illustrated in  FIG. 4A ) has its elements  118   a - 118   d  connected to a switching circuit  702 . Terminals PS 1  and PS 2  by which a power supply (not shown) is connected to energize the transducer are provided on switching circuit  702 , and also a set of control terminals C 1  . . . Cn. As will be understood by those skilled in the art, there are numerous suitable internal configurations for switching circuit  702 , and details of such configurations are omitted in the interest of brevity. 
   Using a switching circuit as illustrated in  FIG. 7 , it is possible to switch between series-connected alternating and matched field configurations as shown in  FIGS. 3A and 4A  to selectably obtain an axial or circumferential focal pattern. 
   Using such a switching circuit with appropriate internal connections, it is also possible to obtain alternating and matched field configurations in which the transducer elements are connected in parallel. An alternating field configuration, with the transducer elements connected in parallel, is illustrated in  FIGS. 5A and 5B . 
   Here, a four-element transducer  500  having the same piezoelectric transducer element configuration as illustrated in  FIGS. 3A and 4A , is arranged so that a first power supply terminal  510  is connected to the “+” side terminals  116   a  and  116   c  of transducer elements  118   a  and  118   c , and to the “−” side electrodes  114   b  and  114   d  of transducer elements  118   b  and  118   d . A second power supply terminal  512  is connected to the “−” side terminals  114   a  and  114   c  of transducer elements  118   a  and  118   c , and the “+” side terminals  116   b  and  116   d  of transducer elements  118   b  and  118   d . As in the case of transducer  300  (see  FIG. 3 ), the induced electric fields (indicated by double arrows  530   a - 530   d ) are in opposite directions relative to the polarization of the piezoelectric material in adjacent transducer elements (indicated by single arrows  528 ), and the mechanical vibrations generated by adjacent transducer elements are 180° out of phase relative to each other.  FIG. 5B  shows a top plan view of transducer  500 , with electrodes  116   a - 116   d  visible, and an exemplary wiring layout by which the electrical configuration of  FIG. 5A  may be achieved. 
   The arrangement illustrated in  FIGS. 5A and 5B  exhibits a circumferential focal pattern with one peak for each transducer segment. However, its electrical impedance is lower by a factor of about  16  as explained above compared to that of the series connected configuration shown in  FIGS. 3A and 3B . 
   The configuration of  FIGS. 5A and 5B  can readily be provided for in the design of switching circuit  702 , as will be apparent to those skilled in the art in light of the description herein. 
   A parallel-connected transducer having a matched-field configuration may also be provided for in the design of switching circuit  702 . Such a transducer configuration is shown at  600  in  FIGS. 6A and 6B . Here, power supply terminals  610  and  612  are respectively connected to the “+” and “−” side terminals  116   a - 116   d  and  114   a - 114   d  of all the transducer elements  118   a -  118   d . As in the embodiment of  FIG. 4 , the electric fields (indicated by double arrows  630   a - 630   d ) are in the same direction relative to the polarization of the piezoelectric material (indicated by single arrows  628 ) in all of the transducer elements, and the mechanical vibrations generated by all the elements are in phase relative to each other.  FIG. 6B  shows a top plan view of transducer  600 , with electrodes  116   a - 116   d  visible, and an exemplary wiring layout by which the electrical configuration of  FIG. 6A  may be achieved. 
   The configuration of  FIGS. 6   a  and  6 B is characterized by an axial focal pattern and electrical impedance at 1 MHz that is lower than that of the corresponding serially connected transducer of  FIGS. 4A and 4B  by a factor of 16. 
   From the foregoing description, it will readily be appreciated that desirable electrical impedance levels can be achieved according to the invention by taking advantage of the polarization of piezoelectric ceramic material and by connecting a segmented transducer with the elements in series, either in an alternating polarization configuration or in a matched polarization configuration. 
   Quite apart from the above-stated benefit of the invention, by the use of a switching circuit of straightforward design, the same multi-element transducer construction can be used to provide both alternating and matched polarization configurations, and to provide these configurations with series-connected elements or parallel-connected elements, thereby achieving flexibility in selection of both focal patterns, and electrical impedance. 
   There are also other ways in which some of the concepts of the invention can be applied. For example, while the transducers discussed above are all constructed of four elements, any other desired even numbers of elements are also possible. As will be appreciated, as the number of elements is increased, the relative increase in impedance for series-connected arrangements compared to parallel-connected arrangements will be larger. 
   Using some of the concepts of the invention, it is also possible to obtain a multiple-element transducer having an alternating field configuration without the need for multiple isolated electrode pairs. One way to accomplish this is illustrated in  FIG. 8 , which shows an outside perspective view of a cup-shaped hemispherical piezoelectric body  800  without an outer electrode. 
   Here piezoelectric body  800 , instead of being formed with a uniform direction of polarization, is formed with four adjacent zones of alternating polarization  802   a ,  802   b ,  802   c , and  802   d  (or any other desired even number of alternating zones). This may be done, for example, by applying a suitable electric polling field with the desired polarity to each zone. Appropriate ways to accomplish this will be apparent to those skilled in the art. 
   For convenience, the polarization direction of zones  802   a  and  802   c  is indicated by circles, and the opposite direction polarization of zones  802   b  and  802   d  is indicated by dots surrounded by circles. It is to be understood that the change in polarization at the zone boundaries is not necessarily sharply defined, so the change may be gradual. (Lines  804 ,  806 ,  808 , and  810  in  FIG. 8  demarcating the zone boundaries are for illustration only, and are not part of the actual structure.) 
   After body  800  has been polarized, inner and outer metallic coatings are applied, as previously described, but optionally, coatings are not scored to create separate electrode pairs. In that event, there is a single inner electrode and a single outer electrode (not shown). Thus, for a given voltage polarity applied to the transducer, the field direction does not reverse from zone to zone, but because the direction of polarization of body  800  alternates between zones, and a circumferential focal pattern is achieved. 
   It should be noted that, for a configuration having single inner and outer electrodes, the transducer elements are connected in parallel, as in the arrangement of  FIGS. 5A and 5B . The transducer impedance is thus lower than in the embodiment of  FIGS. 3A and 3B , but the construction is somewhat simpler, and there is no concern about the effects of relatively high potential differences on adjacent electrode elements. To obtain the higher impedance levels associated with the embodiment of  FIGS. 3A and 3B , the inner and outer conductive coatings can be scored to create isolated electrode-pairs, and the resulting elements connected in series. In this regard, it should be recognized that with the polarization arrangement shown in  FIG. 8 , for a series-connected transducer, the individual elements should be connected as illustrated in  FIGS. 4A and 4B . 
   The transducers described above are characterized by elements shaped as hemispherical triangles, but the invention is not limited to such element shapes. For example, reference is made to copending U.S. patent application Ser. No. 11/870,445, filed Oct. 11, 2007 in the name of Andrey Rybyanets, the entire content of which is hereby incorporated herein by reference as if fully set forth. In the Rybyanets application, there is shown a transducer in which the piezoelectric body has a single direction of polarization like that of  FIG. 3A  etc. herein, but the isolated electrodes are in the shape of crescents. When connected in an alternating field configuration, a propeller or pinwheel focal pattern is obtained. 
   A similar pattern can be obtained with a multi-element transducer of the kind shown in  FIG. 9 , with the same notation used in  FIG. 8  for indicating the direction of polarization. Here, a transducer  900  may be provided in which body zones  902   a ,  902   b ,  902   c , and  902   d  are themselves crescent-shaped, rather than spherical triangles. As in  FIG. 8 , optionally continuous outer and inner electrodes (not shown) are provided, but due to the alternating polarization pattern in the adjacent zones  902   a - 902   d , a multi-element transducer is achieved. Like the transducers of the Rybyanets application, the acoustical focal pattern is in the shape of a propeller or a pinwheel. Again four zones are shown, but any even number is possible. As in  FIG. 8 , demarcation lines  904 ,  906 ,  908 , and  910  showing the zone boundaries are for illustration only, and are not part of the actual structure. 
   Yet another variation employing the concepts of  FIGS. 8 and 9  is shown in  FIG. 10  at  1000 . Again, the same notation used in  FIG. 8  is used here to indicate the direction of polarization. In the variation of  FIG. 10 , the polarization zone  1002   a  is shown as a spherical cap, or more generally, as a cup-shaped cap, and zones  1002   b ,  1002   c , and  1002   d  are shown as annuli, or more generally, as solid annuli, arranged below zone  1002   a . Demarcation lines  1004 ,  1006 , and  1008  are for illustration only and not part of the actual structure. As will be understood, line  1010  represents the outer margin at the bottom of the transducer. 
   It should also be noted that the transducers described in connection with  FIGS. 8 and 9  can be modified to incorporate isolated electrode pairs and combined with a switching circuit if desired to permit selection between non-axial and axial focal patterns. The transducer of  FIG. 10  can be similarly modified. 
   The invention has been described with reference to embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the described invention and embodiments of the invention comprising different combinations of features than those noted in the described embodiments will occur to persons of the art. The scope of the invention is limited only by the following claims.