Abstract:
An ultrasonic transducer includes a backing element, an active element overlying the backing layer, and a matching element overlying the active element, the matching element having an inner surface that contacts the active element and an outer surface with a non-homogeneous texture and/or material composition. The matching element may be formed by subtractive or deposition techniques.

Description:
PRIORITY CLAIM 
     The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/561,534, filed Nov. 18, 2011, which application is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     This application generally relates to ultrasonic transducers. The application further relates to ultrasonic transducers used for medical imaging. 
     A trade-off in medical ultrasound imaging is depth of penetration and spatial resolution. Higher ultrasound imaging frequencies enable higher spatial resolution at the expense of depth of penetration. Lower ultrasound imaging frequencies enable deeper penetration at the expense of spatial resolution. It would be useful if a single ultrasound imaging device was able to image across a broad range of frequencies in order to operate at a higher frequency for better spatial resolution and at a lower frequency for deeper penetration. 
     Broad bandwidth ultrasound imaging devices may include use of high sensitivity materials (e.g., single crystal piezoelectric composites), use of multiple matching layers, use of multiple transducers, and use of multiple devices. These approaches can be expensive and be difficult to implement from a manufacturing perspective, particularly for small, single-use, high-frequency ultrasound devices that are used in relatively high volumes (e.g., intravascular ultrasound catheters). 
     It would be advantageous to have an ultrasound transducer structure and corresponding manufacturing process that enables broadband imaging performance for small, single-use, high-frequency ultrasound devices. It would be further advantageous if the transducer is cost effective and easy to manufacture. 
     SUMMARY 
     In one embodiment, an ultrasonic transducer includes a backing element, an active element overlying the backing element, and a matching element overlying the active element. The matching element having an inner surface that contacts the active element and an outer surface with a non-homogenous texture and/or material composition. 
     The matching element may be a single matching layer where the outer surface has a first region with a first texture and a first material composition and a second region with a second texture and a second material composition. The first texture differs from the second texture and/or the first material composition differs from the second material composition. 
     The first and second textures of the matching layer may be coarse or rough. The first and second regions may have a reduced thickness in the matching layer. The first and second textures may be formed by ablation. The first and second textures may be formed by abrasion. 
     Alternatively, the matching element may include a plurality of matching regions having different thicknesses. The matching regions may be arranged side-by-side on the active element. At least two of the matching regions may be overlapping. 
     Furthermore, the matching layer may include a plurality of discrete matching regions of a first material over the active element. The matching element may further include a fill-in matching region of a second material with a different composition from the first material deposited between the discrete matching regions over the active element. The discrete matching regions of the first material and the discrete matching regions of the second material may be of the same thickness thereby forming a matching layer formed from two materials with different compositions. 
     In a further embodiment, a method of making an ultrasonic transducer includes the steps of providing a backing element, providing an active element overlying the backing layer, and forming a matching element over the active element, the matching element having an inner surface that contacts the active element and an outer surface with a non-homogeneous texture and/or material composition. 
     The matching element may be a single matching layer and the forming step may include providing the outer surface with a first region having a first texture and a first material composition and a second region having a second texture and a second material composition. The first texture differs from the second texture and/or the first material composition differs from the second material composition. 
     The matching layer has a thickness, and the step of providing the outers surface with first and second regions includes a step of reducing the thickness of the matching layer. The reducing step may include ablation. The reducing step may include abrasion. 
     The forming step may include providing the matching layer with a plurality of matching regions. The step of providing the matching layer with a plurality of matching regions may include arranging the matching regions side-by-side on the active element. The step of providing the matching layer with a plurality of matching regions may include overlapping at least two of the matching regions. 
     The forming step may include depositing a plurality of discrete matching regions of a first material on the active element. The forming step may further include forming a fill-in matching region of a second element between the discrete matching regions of the first material deposited on the active element. The method may further include the further step of causing the discrete matching regions of the first material and the discrete matching regions of the second material to have the same thickness. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings illustrate some particular embodiments of the present invention and therefore do not limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description. Some embodiments will hereinafter be described in conjunction with the appended drawings, wherein like numerals denote like elements. 
         FIG. 1  is a perspective view of a prior art transducer stack in accordance with an embodiment. 
         FIG. 2  is a section view of a transducer stack with a matching element having two matching regions in accordance with an embodiment. 
         FIG. 3  is a section view of a transducer stack with a matching element having more than two matching regions in accordance with an embodiment. 
         FIG. 4  illustrates laser ablation of a matching element of a transducer stack in accordance with an embodiment. 
         FIG. 5  is a perspective view of a transducer stack with a laser-ablated matching element in accordance with an embodiment. 
         FIG. 5A  is a section view of the transducer stack shown in  FIG. 5 . 
         FIG. 6  illustrates micro-abrasive blasting of a matching element of a transducer stack in accordance with an embodiment. 
         FIG. 7  is a perspective view of a transducer stack with a laser-ablated and micro-abrasive blasted matching element in accordance with an embodiment. 
         FIG. 7A  is a section view of the transducer stack shown in  FIG. 7 . 
         FIG. 8  illustrates a time-domain response of an ultrasonic transducer stack before ablation in accordance with an embodiment. 
         FIG. 9  illustrates a frequency-domain response of a transducer stack before ablation in accordance with an embodiment. 
         FIG. 10  illustrates a time-domain response of a transducer stack after ablation in accordance with an embodiment. 
         FIG. 11  illustrates a frequency-domain response of a transducer stack after ablation in accordance with an embodiment. 
         FIG. 12  illustrates a time-domain response of a transducer stack after ablation that is excited at a first frequency in accordance with an embodiment. 
         FIG. 13  illustrates a frequency-domain response of a transducer stack after ablation that is excited at a first frequency in accordance with an embodiment. 
         FIG. 14  illustrates a time-domain response of a transducer stack after ablation that is excited at a second frequency in accordance with an embodiment. 
         FIG. 15  illustrates a frequency-domain response of a transducer stack after ablation that is excited at a second frequency in accordance with an embodiment. 
         FIG. 16  is a top view of a matching element stencil in accordance with an embodiment. 
         FIG. 17  is a section view of a transducer stack having matching regions formed from a first material based on the stencil shown in  FIG. 16 . 
         FIG. 18  is a section view of a transducer stack having a second material formed over matching regions formed from a first material based on the stencil shown in  FIG. 16 . 
         FIG. 19  is a section view of a transducer stack having matching regions formed from a first material and a matching region formed from a second material based on the stencil shown in  FIG. 16 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following detailed description is exemplary in nature and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides some practical illustrations for implementing some embodiments of the present invention. Examples of constructions, materials, dimensions, and manufacturing processes are provided for selected elements, and all other elements employ that which is known to those of ordinary skill in the field of the invention. Those skilled in the art will recognize that many of the noted examples have a variety of suitable alternatives. 
     For example, this application provide certain examples of a transducer stack appropriate for use in an intravascular ultrasound (IVUS) catheter having an ultrasound transducer disposed within the catheter sheath. These examples are given for illustrative purposes only and do not limit the application of the invention to only IVUS catheters. 
       FIG. 1  illustrates a prior art ultrasound transducer stack  100  having a backing layer  104 , an active element  101  that includes a single active layer  102 , and a matching element  105  that includes a single matching layer  106 . Transducer stack  100  is illustrated as having rectangular shape. In other examples, the transducer stack  100  may have other shapes, including square, circle, and oval. Transducer stack  100  may also include at least one electrode layer (not shown), which may be formed from metal, including gold and chrome. In one example, the transducer stack  100  may include two electrode layers located on the top and bottom surfaces of active element  101 , respectively. The electrode layer generally facilitates electrical excitation of the active layer. Transducer stack  100  may be electrically connected to a signal generator (not shown) to electrically excite the transducer stack. Transducer stack  100  may also be electrically connected to a receiver (not shown) to detect pressure fields that are converted to electrical signals by the transducer stack. 
       FIG. 1  shows active element  101  which includes active layer  102 . Active layer  102  may also be referred to as a piezoelectric layer. In other examples, active element  101  may include multiple active layers. Active layer  102  may be composed of a ceramic material such as lead zirconate titanate, commonly known as PZT. The thickness of active layer  102  determines the thickness resonance of the layer. For example, a 36 μm Motorola 3203HD material has a thickness resonance of approximately 63 MHz. Alternatively, active layer  102  may be composed of a composite material such as lead magnesium niobate-lead titanate single crystal, commonly known as PMN-PT and polymer, wherein a resonance is determined by a longitudinal length mode rather than a thickness mode. 
     Backing layer  104  may be composed of an electrically conductive epoxy, such as a tungsten-loaded epoxy. In the example of a transducer stack for use in an IVUS catheter, the thickness of backing layer  104  may be 200 μm or greater. In other examples, the appropriate thickness of backing layer  104  should be sufficiently thick to attenuate ultrasonic vibrations from active element  101  in the backwards direction (toward backing layer). 
       FIG. 1  also illustrates matching element  105  which includes matching layer  106 . Matching layer  106  may be composed of an electrically conductive epoxy, such as a silver-loaded epoxy. Matching layer  106  provides a better acoustic impedance match between active element  101  and the medium in which transducer stack  100  is located. Matching layer  106  may have a uniform thickness that is equal to one quarter of the wavelength at the nominal center frequency of transducer stack  100  and is commonly referred to as a quarter-wave matching layer. Matching element  105  improves the efficiency of the transducer stack to transmit ultrasound vibrations into the surrounding medium and to receive ultrasound vibrations from the surrounding medium. While transducer stack  100  is shown for illustrative purposes in  FIG. 1  to have matching element  105  having only one matching layer  106 , in other examples, matching element  105  may have more than one matching layer to further improve efficiency. 
       FIGS. 2 and 3  are section views of transducer stacks that illustrate matching elements having side-by-side matching regions.  FIG. 2  shows transducer stack  120  which includes backing layer  104 , active element  101  including active layer  102 , and matching element  122 . Matching element  122  includes a quarter-wave matching region  124  tuned to a first wavelength λ 1  with a thickness equal to λ 1 /4. Matching element  122  also includes a quarter-wave matching region  126  tuned to a second wavelength λ 2  with a thickness equal to λ 2 /4. Matching regions  124 ,  126  may be formed from the same material, such as a silver-loaded epoxy. In other examples, matching regions  124 ,  126  may be formed from materials with different composition. For example, matching region  124  may be formed from a silver-loaded epoxy having a first volume concentration of silver, while matching region  126  may be formed from a silver-loaded epoxy having a second volume concentration of silver. The volume concentration of silver may affect mass density and speed of sound of matching element  122  which in turn affect the corresponding wavelength at a given ultrasound frequency. The first and second regions of matching regions  124 ,  126  may then exhibit different corresponding quarter wavelengths. The volume concentration of silver may also affect acoustic impedance of matching element  122 . Matching regions  124 ,  126  may exhibit different efficiencies at different ultrasound frequencies to transmit ultrasound vibrations into the surrounding medium and to receive ultrasound vibrations from the surrounding medium. 
       FIG. 3  illustrates transducer stack  130  having matching element  132  which includes a plurality of matching regions  134 - 138 . As can be appreciated, matching regions  134 - 138  of matching element  132  may be formed from the same material or different materials. Matching regions  134 - 138  of the matching element  132  may then exhibit different corresponding quarter wavelengths. The portions of transducer stack  130  having matching regions  134 - 138  of matching element  132  may exhibit different efficiencies at different ultrasound frequencies to transmit ultrasound vibrations into the surrounding medium and to receive ultrasound vibrations from the surrounding medium. 
     This application discloses a transducer stack having a matching layer that is matched at more than one ultrasound frequency to improve the transmit and receive efficiency of the transducer stack for a broader range of ultrasound frequencies. There are a number of techniques that may be used to form a matching element that is matched at more than one ultrasound frequency. Subtractive techniques like machining, grinding or etching may be used to modify the thickness profile of a matching layer in a matching element. Other subtractive techniques such as laser ablation or micro-abrasive blasting modify the thickness of the matching element and may also modify the composition profile of the matching element. For example, when a matching layer formed from silver-loaded epoxy is laser ablated or abrasively blasted, more of the softer epoxy may be removed compared to the silver. This would change the mass density of the ablated/blasted regions which may have an effect on the ultrasonic properties of the matching element. Generally, subtractive techniques will also increase the effective surface area of the matching element which can impact on the ultrasonic properties of the matching element. 
     These subtractive techniques may be used individually, or in combination to form a matching element with a coarse or roughened surface. The coarse or roughened surface of the matching element creates a varying and non-uniform thickness allowing the matching element to match to more than one ultrasound frequency. Furthermore, the coarse or roughened surface of the matching element results in an increased effective surface area of the matching element, can impact the ultrasonic properties of the matching element. Precise control of the matching element modification process will provide further improvements. An ultrasound transducer stack having a matching element with finely controlled, coarseness or roughness enables balancing the amount of transducer area matched to different ultrasound frequencies. 
     One example of a subtractive technique that may be used to form a matching element able to match at more than one ultrasound frequency is laser ablation.  FIG. 4  illustrates a laser system  200  ablating a surface of matching element  105  of transducer stack  300 . Laser system  200  includes a light source (not shown) that may operate in the near-infrared spectrum wherein the optical wavelength can vary between 800 nm and 2500 nm. Exemplary laser sources that operate in the near-infrared spectrum include neodymium-doped yttrium aluminum garnet (or ND:YAG) lasers, laser diodes, and fiber lasers. The light source generates laser beam  202  that may be directed through lens  204 . Focused laser beam  206  ablates a surface of matching element  105  to form an ablated region (see  FIG. 5 ). Laser system  200  may be repeatedly translated to ablate multiple regions of a surface of matching element  105 . Alternatively, transducer stack  300  may be translated relative to the laser system  200 . The ablated region size and depth for a given laser system may be controlled by the laser system pulse energy, pulse duration and laser beam diameter. 
       FIG. 5  illustrates transducer stack  300  having matching element  105  including matching layer  306 . Matching element  105  is shown to have five (5) ablated regions  310 - 318 .  FIG. 5A  shows a section view of transducer stack  300  including ablated regions  310 ,  312 . In the example of a transducer stack for use in an IVUS catheter, the number of ablated regions may range from one (1) to 40, wherein the diameter of the ablated regions may range from 50 μm to 500 μm. The ablated regions may be distributed uniformly or unevenly across the face of the matching layer. In other examples, the appropriate size, number and location of laser-ablated regions on the matching element may vary depending on the specific application of the transducer stack. 
     Another example of a subtractive technique that may be used to form a matching element able to match at more than one ultrasound frequency is micro-abrasive blasting.  FIG. 6  illustrates micro-abrasive blasting system  400  that is ablating matching element  305 . Micro-abrasive blasting system  400  includes abrasive nozzle  402 . Micro-abrasive blasting system  400  delivers a stream of abrasive particles  404  to matching element  305  of transducer stack  500  typically using a pressurized gas such as nitrogen or dry air. In the example of a transducer stack for use in an IVUS catheter, the size of the abrasive particles may range from 10 μm to 200 μm and include soft abrasives such as wheat starch or sodium bicarbonate; the depths of the ablated regions generally range between 0.1 μm and 10 μm; the pressure of the pressurized gas may range between 40 PSI and 140 PSI; and the area of the abrasive-blasted region is generally the entire surface area of the matching element. In other examples, the appropriate size and hardness of the abrasive particles, depth of ablated regions, pressure of the pressurized gas, and area of abrasive blasting may vary depending on the specific application of the transducer stack. 
     Subtractive techniques may be used in combination to further increase the transmit and receive efficiency of a transducer stack over a broader range of frequencies.  FIG. 7  illustrates transducer stack  500  having an abrasive-blasted and a laser-ablated matching element  505 . The surface of matching element  505  is shown to have laser-ablated regions  510 - 518 .  FIG. 7A  shows a section view of transducer stack  500  that includes ablated regions  510 ,  512  that have been laser-ablated and abrasive-blasted. 
       FIGS. 8 to 11  illustrate the effect of matching layer ablation on pulse-echo time-domain and frequency-domain responses of an ultrasonic transducer stack to a short-time electrical excitation. Measurement of the pulse-echo time-domain and frequency-domain responses of an ultrasonic transducer stack are known to those skilled in the art of ultrasound imaging.  FIG. 8  shows a time-domain pulse-echo response  402  of the transducer stack  300  before ablation of the matching layer  106 , as illustrated in  FIG. 4 .  FIG. 9  shows a pulse-echo (frequency-domain) power spectrum  404  that corresponds to the time-domain pulse-echo response  402  of the transducer stack  300  before ablation of a matching layer  106 .  FIG. 10  shows a time-domain pulse-echo response  412  of the transducer stack  500  after laser ablation and abrasive blasting of the matching layer  506 , as illustrated in  FIG. 7A .  FIG. 11  shows a pulse-echo (frequency-domain) power spectrum  414  that corresponds to the time-domain pulse-echo response  412  of the transducer stack  500  after laser ablation and abrasive blasting of the matching layer  506 . The effects of matching element ablation on pulse-echo time-domain and frequency-domain responses of the ultrasonic transducer stack are decreased time-domain pulse length, increased center frequency, and increased bandwidth. These effects generally provide improved image quality of ultrasound devices. 
     The increased bandwidth further enables imaging at more than one frequency.  FIGS. 12 to 15  show the pulse-echo time-domain and frequency-domain responses of transducer stack  500  having an ablated matching element  505 , as illustrated in  FIG. 7 .  FIGS. 12 and 13  respectively show a pulse-echo time-domain response  422  and a pulse-echo (frequency-domain) power spectrum  424  of a short-time electrical excitation having a first frequency.  FIGS. 14 and 15  respectively show a pulse-echo time-domain response  432  and a pulse-echo (frequency-domain) power spectrum  434  of a short-time electrical excitation having a second frequency, wherein the second frequency is lower than the first frequency. The pulse-echo time-domain response  422  of the transducer to the first-frequency, short-time, electrical excitation is shorter than the pulse-echo time-domain response  432  of the transducer to the second-frequency, short-time, electrical excitation. The pulse-echo power spectrum  424  of the transducer to the first-frequency, short-time, electrical excitation has a higher center frequency than that of the pulse-echo power spectrum  434  of the transducer to the second-frequency, short-time, electrical excitation. A transducer operating with a shorter time-domain pulse and higher center frequency will generally enable imaging with better spatial resolution and a smaller depth of penetration. Conversely, a transducer operating with a longer time-domain pulse and lower center frequency will generally enable imaging with a larger depth of penetration and lower spatial resolution. 
     Deposition techniques may also be used to increase the transmit and receive efficiency of a transducer stack over a broader range of frequencies. In one technique, one or more stencils may be used to form a matching layer of a matching element, the matching layer having multiple matching regions formed from materials with different compositions. Stencils can be developed from metals, such as stainless steel. Stencil patterns can be fabricated using known processes, such as photochemical machining. A stencil includes at least one cut-out hole that may be of a variety of shapes, including circle, rectangle, or triangle. In the example of a transducer stack for use in an IVUS catheter having a width of approximately 0.5 mm and a length of approximately 0.75 mm, the thickness of the stencil may range from 0.05 mm to 1 mm, and the cut-out holes may vary in size from approximately 0.025 mm to 0.5 mm. In other examples, the dimensions of the stencil and the size and shape of the cut-out-holes may vary depending on the specific application of the transducer stack. 
       FIG. 16  illustrates a top view of an example of a stencil  600  that may be used to deposit a first material on a transducer stack that may be used in an IVUS catheter. The stencil length is approximately 0.75 mm, width is approximately 0.5 mm, and thickness is approximately 0.05 mm. The stencil  600  includes five (5) cut-out holes  610 - 618  wherein the cut-holes are circular in shape and have diameters of approximately 0.15 mm. 
       FIG. 17  shows a section view of transducer stack  700  including matching element  705  having partial matching layer  706 . Partial matching layer  706  includes matching regions  710 ,  712  formed from a first material. Matching regions  710 ,  712  are formed by aligning stencil  600 , shown in  FIG. 16 , with the top surface of transducer stack  700 . The first material, such as an epoxy containing a first volume fraction of silver, is then applied to transducer stack  700 . Excess first material may be removed by scraping the top surface of stencil  600  using a razor blade or other sharp-bladed instrument after the first material is applied. Stencil  600  may then be removed from the top surface of transducer stack  700  thereby forming matching regions  710 ,  712  formed from the first material. Matching regions  710 ,  712  may then be permitted to cure before depositing additional materials. 
       FIG. 18  shows a section view of transducer stack  700  after having a second material  714 , having a different composition from the first material, deposited on the top face of transducer stack  700  that include matching regions  710 ,  712  formed from the first material. The second material  714  may then be permitted to cure before applying a subtractive technique to reduce the thickness of matching layer  706  to a target thickness. The thickness of matching layer  706  can be reduced by a variety of techniques, for example machining.  FIG. 19  shows a section view of the transducer stack  700  having a matching element  705  with a matching layer  706 , the matching layer  706  including matching regions  710 ,  712  formed from the first material and matching region  714  formed from the second material. It can be appreciated that in other examples, a matching element may include a matching layer formed from more than two materials, each material having a different composition. 
     This application discloses a number of subtractive and deposition techniques, each method may be used individually to increase the transmit and receive efficiency of a transducer stack over a broad range of frequencies. It can be appreciated, that any of these methods may also be used in combination with each other to further increase the efficiency of a transducer stack. For example, as noted above,  FIG. 7  shows a transducer stack  500  having a matching element having been both laser-ablated and abrasive-blasted. In another example, the transducer stack  700 , as shown in  FIG. 19 , may have its matching element  705  coarsened or roughened using either laser ablation, abrasive blasting, or both. In yet another example, these techniques may be performed on transducer stack  120  and  130  as shown in  FIGS. 2 and 3 , respectively. 
     Furthermore, the subtractive and deposition techniques disclosed in this application may be used individually or in combination on varying transducer stacks. For example, these techniques may be performed on transducer stack  100  shown in  FIG. 1  which includes backing layer  104 , active element  101  having a single active layer  102 , and matching element  105  having a single matching layer  106 . In another example, these techniques may be performed on a transducer stack including a backing layer, an active element having one or more active layers, and a matching element having one or more matching layers. In yet another example, using  FIG. 19  as a reference, these techniques may be applied to a transducer stack  700  having an active element  101  with more than one active layer, and a matching element  705  with more than one matching layer, where one of those layers is similar to matching layer  706 . 
     In some embodiments, an ultrasonic transducer is provided. The transducer can include an active element having a first side and a second side. The transducer can include a backing element attached to the first side of the active element. The transducer can include a matching element attached to the second side of the active element. The matching element may have a surface that is coarse or roughened causing the matching element to have a non-uniform thickness. 
     Such an ultrasonic transducer can include a variety of characteristics. In some embodiments, the coarse or roughened surface of the matching element may include a plurality of concavities. In such embodiments, the concavities may be ablated regions. In some embodiments, the ablated regions may have diameters ranging between 50 μm and 500 μm. In some embodiments, by use of micro-abrasive blasting, the ablated regions may cover up to the entire surface of the transducer. In some embodiments, the matching element may include at least two matching layers. In some embodiments, the active element may further comprise two active layers. 
     In some embodiments, an ultrasonic transducer is provided. The transducer can include an active element having a first side and a second side. The transducer can include a backing element attached to the first side of the active element. The transducer can further include a matching element attached to the second side of the active element. The matching element may include at least one matching layer. At least one of the matching layers may include at least a first matching region formed from a first material and a second matching region formed from a second material. The first and the second materials can be formed from materials having different compositions. 
     Such an ultrasonic transducer can include a variety of characteristics. In some embodiments, a surface of the matching element may be coarse or roughened. In such embodiments, the matching element may have a non-uniform thickness. In some embodiments, the coarse or rough surface may include a plurality of concavities. In such embodiments, the concavities may be ablated regions. In some embodiments, the ablated regions may have diameters ranging between 50 μm and 500 μm. In some embodiments, by use of micro-abrasive blasting, the ablated regions may cover up to the entire surface of the transducer. In some embodiments, the matching element may include at least two matching layers. In some embodiments, the active layer may include at least two active layers. 
     Some embodiments provide a method of manufacturing an ultrasonic transducer. Some embodiments involve providing an active element having a first side and a second side. Some embodiments involve providing a backing element on the first side of the active element. Some embodiments involve forming a matching element on the second side of the active element. Some embodiments include forming a surface of the matching element such that the surface is coarse or roughened. In such embodiments, the matching element has a non-uniform thickness. 
     Such a method to form an ultrasonic transducer can include a variety of steps. In some embodiments, at least one subtractive technique may be used to form the matching element. In some embodiments, the at least one subtractive technique may include laser ablation. In some embodiments, the at least one subtractive technique may include micro-abrasive blasting. In some embodiments, the at least one subtractive technique may include both laser ablation and micro-abrasive blasting. In some embodiments, the at least one subtractive technique may include machining, grinding, or etching. In some embodiments, forming the active element may include forming at least two active layers. In some embodiments, forming the matching element may include forming at least two matching layers. 
     In some embodiments, a method of manufacturing an ultrasonic transducer. Some embodiments involve providing an active element having a first side and a second side. Some embodiments involve providing a backing element on the first side of the active element. Some embodiments involve forming a matching element on the second side of the active element. In such embodiments, the matching element may include a first matching layer. In such embodiments, the first matching layer may include a first matching region formed from a first material, and a second matching region formed from a second material having a different composition than the first material. 
     Such a method to form an ultrasonic transducer can include a variety of steps. In some embodiments, a first deposition technique may be used in forming the first matching layer. In such embodiments, the first deposition technique may include aligning a first stencil adjacent to the second side of the active element. In such embodiments, the stencil may have at least one cut-out-hole. In some embodiments, a first material may be applied to the first stencil. In some embodiments, the first stencil is removed and the first material is allowed to cure. In such embodiments, the cured first material forms the first matching region. In some embodiments, the first deposition technique may be repeated for a second stencil. In some embodiments, a second deposition technique may be used in forming the first matching layer. In such embodiments, the second deposition technique may include applying a second material to a surface of the matching element and allowing the second material to cure. In such embodiments, the cured second material forms the second matching region. In some embodiments, a first subtractive technique is used in forming the first matching layer. In such embodiments, the first subtractive technique may include reducing the thickness of the first matching layer until the thickness of the first and second matching regions are equal. In some embodiments, the first subtractive technique may include machining, grinding, or etching. In some embodiments, a second subtractive technique is used to form the matching element. In such embodiments, the matching element may have a surface that is coarse or rough. In such embodiments, the matching element may have a non-uniform thickness. In some embodiments, the second subtractive technique may include laser ablation. In some embodiments, the second subtractive technique may include micro-abrasive blasting. In some embodiments, the second subtractive technique may include both laser ablation and micro-abrasive blasting. 
     Thus, embodiments of the invention are disclosed. Although the present invention has been described in considerable detail with reference to certain disclosed embodiments, the disclosed embodiments are presented for purposes of illustration and not limitation and other embodiments of the invention are possible. One skilled in the art will appreciate that various changes, adaptations, and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.