Patent Publication Number: US-2004054287-A1

Title: Ultrasonic imaging devices and methods of fabrication

Description:
BACKGROUND OF THE INVENTION  
       [0001] This application relates to ultrasonic imaging devices such as ultrasonic imaging catheters and sensors and to methods for fabricating these devices.  
       [0002] Ultrasonic imaging techniques are often used to gather images during the diagnosis and treatment of medical conditions. An ultrasonic imaging catheter may be used to gather images from within a patient&#39;s body. During percutaneous transluminal coronary angioplasty procedures, for example, images may be acquired from within the blood vessels of a cardiac patient to help a physician to accurately place an expandable balloon.  
       [0003] In a typical ultrasound imaging catheter configuration, a piezoelectric ultrasound transducer array at the distal end of the catheter may be used to generate high-frequency acoustic signals that radiate towards the image target (e.g., a patient&#39;s blood vessel). The transducer array gathers corresponding reflected acoustic signals. Image processing techniques are used to convert the reflected acoustic signals into images for the physician.  
       [0004] It is an object of the present invention to provide improved ultrasonic imaging catheters and sensors and methods for fabricating such devices.  
       SUMMARY OF THE INVENTION  
       [0005] This and other objects of the invention are accomplished in accordance with the principles of the invention by providing ultrasonic imaging catheters and sensors and methods for their fabrication.  
       [0006] An imaging catheter constructed in accordance with the invention may have a sensor at its distal tip. The sensor may have a transducer array formed from piezoelectric elements. During the manufacturing process, a uniform acoustic matching layer may be formed over the surface of all of the elements in the transducer array in parallel. These simultaneously-formed acoustic matching layer portions on the transducer array elements improve the performance of the transducer array when the imaging catheter is used to gather images of a suitable image target (e.g., the blood vessels or other body lumens of a patient). Specifically, the matching layer matches the acoustic impedance of the transducer elements to the surrounding medium (e.g., blood, tissue, etc.) by serving as an intermediate layer of intermediate impedance and suitable thickness. Thus, the matching layer may be formed using a material that has an acoustic impedance between that of the transducer elements and the body fluid or other substance in which the sensor is immersed during operation.  
       [0007] The sensor may have a flexible circuit (“flex circuit”) on which the transducer array elements are formed. The flex circuit substrate may help to match the acoustic impedance of the piezoelectric transducer elements to the acoustic impedance of the medium in which the sensor is immersed. To enhance the acoustic matching capabilities of the flex circuit, the flex circuit substrate may be formed using a flexible substrate material with a relatively high acoustic impedance for flexible polymeric materials. As an example, within the polyimide group of materials, Upilex has a higher acoustic impedance than Kapton. The flex circuit may have conductors to which the transducer array elements are electrically connected.  
       [0008] The transducer array elements may be mounted to the flex circuit in the form of a block of piezoelectric material that is subsequently divided to form the individual transducer array elements. During the process of dividing the transducer array into individual elements, the flex circuit that underlies the transducer array may be divided as well, so as to acoustically decouple the transducer array elements from adjacent elements. This is expected to improve imaging performance by reducing cross-talk.  
       [0009] Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0010]FIG. 1 is a perspective view of an illustrative ultrasonic imaging catheter and accompanying image processing equipment in accordance with the present invention.  
     [0011]FIG. 2 is a perspective view of the distal tip of an illustrative support lumen that may be used in the core of an ultrasonic imaging catheter device in accordance with the present invention.  
     [0012]FIG. 3 is a perspective view of the distal tip of another illustrative support lumen that may be used in the core of an ultrasonic imaging catheter device in accordance with the present invention.  
     [0013]FIG. 4 is a perspective view of a transducer array and integrated circuits that have been formed on a flex circuit in accordance with the present invention.  
     [0014]FIG. 5 is a cross-sectional side view of a portion of an illustrative ultrasonic imaging catheter in the vicinity of the transducer array and accompanying integrated circuits in accordance with the present invention.  
     [0015]FIG. 6 is a cross-sectional view of the illustrative catheter of FIG. 5 taken along the line  6 - 6  in accordance with the present invention.  
     [0016]FIG. 7 is a perspective view of an illustrative transducer array element showing the placement of the transducer electrodes in accordance with the present invention.  
     [0017]FIG. 8 a  is a cross-sectional end view of the transducer array element of FIG. 7 mounted in an illustrative fashion to a flex circuit in accordance with the present invention.  
     [0018]FIG. 8 b  is a perspective view of a portion of a flex circuit and piezoelectric transducer prior to using a conductive fillet to connect the transducer electrodes to electrical conductors on the flex circuit in accordance with the present invention.  
     [0019]FIG. 8 c  is a perspective view of a portion of a flex circuit and piezoelectric transducer after using a conductive fillet to connect the transducer electrodes to electrical conductors on the flex circuit in accordance with the present invention.  
     [0020]FIG. 9 is a cross-sectional view (in perspective) of an ultrasonic transducer array having multiple elements of the types shown in FIG. 7 and  8   a  after mounting to a flex circuit in accordance with the present invention.  
     [0021]FIGS. 10 and 11 are schematic views of illustrative template structures that may be used when fabricating transducer arrays in accordance with the present invention.  
     [0022]FIG. 12 is a side view of an illustrative template structure prior to insertion of a block of piezoelectric material for processing to form a transducer array in accordance with the present invention.  
     [0023]FIG. 13 is a side view similar to that of FIG. 12 showing how a piezoelectric block may be inserted into the template during the process of fabricating the transducer array in accordance with the present invention.  
     [0024]FIG. 14 is a side view of the template and piezoelectric block of FIG. 13 after the block and template have been covered by a flexible cover in accordance with the present invention.  
     [0025]FIG. 15 is a side view of the template structure after the flexible sheet of FIG. 14 has been removed in accordance with the present invention.  
     [0026]FIG. 16 is a side view of the template structure of FIG. 15 after a layer of acoustically matching material has been deposited on the piezoelectric block and planarized in accordance with the present invention.  
     [0027]FIG. 17 is a side view of the piezoelectric block after removal from the template structure of FIG. 16 in accordance with the present invention.  
     [0028]FIG. 18 is a flow chart of steps involved in fabricating an ultrasonic imaging sensor and catheter in accordance with the present invention.  
     [0029]FIG. 19 is a perspective view showing the end of a portion of an illustrative sensor in which transducer array elements have been acoustically decoupled from each other by forming cuts through flex circuit portions between adjacent elements in accordance with the present invention.  
     [0030]FIG. 20 is a perspective view of the distal portion of an illustrative ultrasonic imaging catheter showing (from an exterior perspective) the location of cuts made to acoustically isolate individual transducer array elements from each other in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0031] An illustrative catheter-based ultrasonic imaging system  10  in accordance with the invention is shown in FIG. 1. A catheter  12  with a ultrasonic sensor  16  may be used to gather images from locations inside a patient&#39;s body during surgical and diagnostic procedures. For example, catheter  12  may be used during percutaneous procedures such as cardiac surgery to gather images from inside a patient&#39;s blood vessels. The catheter may be used for other ultrasound applications if desired.  
     [0032] The catheter may have any suitable dimensions. For example, the catheter may have an overall length of about 100-200 cm. The distal portion of the catheter may have a length of about 35 cm and the proximal portion of the catheter may have a length of about 125 cm.  
     [0033] Image data from sensor  16  may be provided to image processing and display equipment  14  using cables within catheter  12 . Equipment  14  may use ultrasound image processing techniques to process the image data and to display corresponding images on a display screen for a physician or other suitable user. Equipment  14  provides signals to sensor  16  that control the operation of sensor  16 . For example, equipment  14  may provide drive signals for the transducer elements in sensor  16  that cause those elements to emit appropriate acoustic signals into the surrounding area of the patient&#39;s body. Appropriate acoustic signals may, for example, be acoustic pulses of approximately 25 ns in duration.  
     [0034] The peak drive voltage that is impressed upon the piezoelectric transducer elements to generate these pulses may be about 10 V. The acoustic echoes from the patient&#39;s body may induce voltages in the transducer elements on the order of 10-30 μV. Because the induced signals from reflections are typically several orders of magnitude lower than the drive signal, the acoustic impedance of the sensor is preferably matched to the surrounding environment. Individual transducer elements may also be acoustically isolated from each other to improve imaging performance.  
     [0035] Sensor  16  may be mounted on any suitable medical device such as a catheter of the type shown in FIG. 1 or a probe. For example, sensor  16  may be mounted on a catheter that contains a hollow tube. A guide wire that runs through the hollow tube may be used to assist in the placement of the catheter at a desired location within the patient&#39;s body. In another suitable arrangement, sensor  16  may be affixed to the end of a probe without a hollow tube that is manipulated at its proximal end by a physician or other suitable user. For clarity, the present invention will be describe primarily in the context of sensor arrangements involving catheters. This is, however, merely illustrative. Sensor  16  may be used with any suitable intraluminal and/or medical instrument or equipment.  
     [0036] Sensor  16  may be permanently affixed to a medical instrument or may be provided as part of a separate structural element that is removable from the instrument. For example, sensor  16  may be provided as part of an interchangeable catheter tip.  
     [0037] Additional components may be provided on catheter  12  if desired. For example, fiber-optic cables may be used to provide video imaging capabilities, expandable balloons may be used to deploy stents, atherectomy tools may be used to clear blockages, snares or probe tips may be provided for use during surgery, irrigation ports or guide wire passages may be provided, and temperature sensors, flow sensors (e.g., Doppler flow sensors), pressure sensors, and combinations of such sensors may be provided, etc. These are merely illustrative examples. Any suitable components may be included in catheter  12  if desired.  
     [0038] Sensor  16  may include a transducer array based on a number of individual transducer elements. Any suitable number of elements may be used (e.g., 16, 32, 64, 80, 128, 256, more than 256, less than 16, 32-256, etc.). The transducer elements may be formed of a piezoelectric material (e.g., a ceramic piezoelectric material, a polymeric piezoelectric material, a single crystal piezoelectric material, a composite piezoelectric material having ceramic particles in a matrix, etc.). When driven by suitable AC electrical signals, the transducer elements produce acoustic waves that radiate into the patient&#39;s body. These transducer elements (or a separate set of receiver transducer elements) are used to gather reflected acoustic waves and to convert those signals into acoustic energy.  
     [0039] In one suitable arrangement, sensor  16  includes a number of transducer elements that are formed on a flexible substrate that includes conductive electrodes for carrying electrical signals to and from the transducer elements. This type of substrate, which is referred to as a “flex circuit,” is relatively thin (e.g., 5 μm to 200 μm thick or any other suitable thickness) and may be rolled up or otherwise manipulated to form a cylinder that is affixed to the cylindrical core of the catheter  12  at an appropriate distal location as shown in FIG. 1. The flex circuit portion may be on the outermost portion of the sensor, so that during operation acoustic energy from the transducer array elements radiates radially outward through the flex circuit into the patient&#39;s body. The outer diameter of the flex circuit after it has been attached to the catheter may be on the order of 0.5 mm to 3 mm or any other suitable diameter.  
     [0040] It may be desirable to place an acoustic backing layer between the catheter core and the transducer array elements to reduce acoustic interference in the form of spurious “ringing” signals reflected from the core into the transducer elements. The backing layer and rolled-up cylindrical transducer array may be mounted in a groove in the catheter core. Suitable catheter arrangements having a groove for accommodating the backing layer material and cylindrical sensor elements are shown in FIGS. 2 and 3.  
     [0041] In the arrangement of FIG. 2, catheter  12  has a main cylindrical core or body portion formed from a hollow or solid tube or a series of tubes or tube sections or other individual parts. The catheter core may be formed using metal, plastic, a combination of metal and plastic parts, or any other suitable material or combination of materials. All or a portion of the catheter (e.g., in the vicinity of the catheter tip) may be formed using a radiopaque material, e.g., a material that is visible during x-ray fluoroscopy, such as a metal alloy containing platinum or iridium. A typical hollow tube may have a diameter of approximately 0.024 inches and a wall thickness of 25 μm. At the distal tip of catheter  12 , the elements of core  18  may be shaped to form a circumferential groove  20 . Groove  20  is preferably of sufficient depth to accommodate all or at least some of the thickness of the backing layer material and the transducer array elements when the catheter is fully assembled. In the illustrative arrangement of FIG. 2, groove  20  is formed by forming an indentation with the components of catheter core  18  between a distal portion attached to the main length of the catheter and a tip portion  22 . Tip  22  may be formed from the same material or materials as catheter core  18  or may be formed from a soft material that, in combination with the tapered shape of tip  22 , facilitates insertion of catheter  12  into narrow lumens such as blood vessels. A guide wire lumen  24  may be provided in catheter  12  so that catheter  12  may be advanced over guide wire  26 .  
     [0042] In the illustrative arrangement of FIG. 3, annular rings  22  have been used to form spacers that define a groove  20 . Rings  22  may be formed of any suitable material. For example, rings  22  may be formed from rubies, which advantageously may be machined to tight tolerances. Other suitable materials that may be used include plastic, ceramic, metal, epoxy, composites of such materials, etc. Rings  22  may be affixed to core  18  using an adhesive such as a cyanoacrylate adhesive. Cyanoacrylate adhesive may also be used to affix other portions of core  18  to each other. For example, when core  18  is formed from nested or overlapping tubes, cyranoacrylate adhesive may be used to secure the tubes to each other.  
     [0043] As shown in FIGS. 2 and 3, a guide wire lumen  24  that runs longitudinally through the center of catheter  12  may be provided. This allows catheter  12  to be guided over a guide wire  26 , which facilitates placement of catheter  12  during use in a patient.  
     [0044] The arrangements for defining the groove  20  on catheter core  18  that are shown in FIGS. 2 and 3 are merely illustrative. Any suitable arrangement may be used to define groove  20  if desired. The groove may be defined by forming an indentation in the tube or tubes or cylinder of material that is used to form the main body of catheter  12 , may be formed by adding additional spacers, rings or other structures over the outer circumference of the core  18 , may be formed by attaching a specially formed tip having an integral groove portion, or may be formed using other suitable approaches or combinations of such approaches.  
     [0045] As shown in FIGS. 2 and 3, groove  20  may have a length L (as measured along the longitudinal axis of catheter  12 ). The length L may be sufficient to accommodate the transducer array when the transducer array and other components on the flex circuit are rolled up to form a cylinder around the distal end of catheter  12 . For example, if the corresponding dimension of the transducer array is 1 mm, then length L should be 1 mm plus a small clearance to allow the transducer array to be mounted in groove  20 .  
     [0046] An illustrative sensor  16  having a transducer array  28  with a dimension L for mounting in a groove  20  of the type shown in FIGS. 2 and 3 is shown in FIG. 4. Transducer array  28  may have a number of individual elements, each of which is aligned in parallel with the illustrative element  30  shown in FIG. 4. The transducer array  28  is mounted on a flex circuit  32 . The flex circuit may be formed from a flexible substrate material such as polyimide, which is electrically insulating. If desired, the flex circuit may be formed from a substance having a relatively high acoustical impedance for flexible polymeric materials. Within the polyimide group of materials, Upilex has a higher acoustic impedance than Kapton. With one suitable arrangement, the flex circuit may be formed form a substance that is flexible and that has an acoustic impedance of at least 3.5 MRayls or an acoustic impedance in the range of 3.5 to 4.5 MRayls. A suitable flex circuit substrate material that has an acoustic impedance of about 4 MRayls is available under the trade name Upilex-S, from Ube Industries, Inc. of Yamaguchi, Japan. Using a flex circuit with an acoustic impedance of 3.5 to 4.5 MRayls may significantly improve the acoustic impedance matching between sensor  16  and the medium in which catheter  12  is immersed (typically a patient&#39;s blood). Such acoustic impedance matching is expected to improve the performance of the sensor  16 .  
     [0047] Electrical conductors  34  are formed on the surface of the flex circuit substrate. The electrical conductors may be formed, for example, from a malleable metal such as gold. A suitable adhesion layer such as a thin layer of chromium may be used to facilitate adhesion of the gold or other conductor material to the substrate. Metal layers may be deposited by sputtering, evaporation, or any other suitable technique. Wet or dry etching or other suitable patterning techniques may be used to pattern the deposited metal to form electrical conductors  34 .  
     [0048] Each transducer element  30  may have two opposing electrodes. The main portion of the electrodes is located on the upper and lower surfaces of the transducer array when the array is oriented as shown in FIG. 4. Smaller portions of the electrodes extend over the ends  35  and  36  of the elements  30  in transducer array  28 . Electrical signals may be conducted between the conductors  34  and the main portions of the electrodes by forming electrical contacts between the conductors  34  and the end portions  35  and  36 .  
     [0049] By connecting the electrodes on each transducer element  30  to corresponding conductors  34 , drive signals for the transducer elements  30  may be conveyed to the elements  30 . Similarly, electrical signals that are produced by the elements  30  when reflected acoustic waves are detected by elements  30  may be conveyed from the elements.  
     [0050] In some transducer arrays (e.g., arrays with 64 elements or more), there may be so many conductors  34  that it is cumbersome to route all of these conductor lines to equipment  14  in a single cable along the length of the catheter  12 . Accordingly, electrical multiplexer circuits  38  (e.g., time-division multiplexing circuits or other suitable multiplexing circuits) may be used to reduce the relatively large number of conductors  34  that are directly connected to transducer array  28  into a smaller number of conductors  34  at the input/output  40 . The conductors at input/output  40  may be soldered, welded, or otherwise electrically connected to wires in a suitable cable that runs along the length of catheter  12  to equipment  14 . If desired, circuits  38  may include drive circuitry for generating drive signals and/or preprocessing circuitry for at least partially processing the electrical signals that are produced when the transducer elements  30  in array  28  are used to detect acoustical information.  
     [0051] After circuits  38  and transducer array  28  have been mounted on flex circuit  32 , as shown in FIG. 4, the flex circuit and mounted components may be formed into a cylindrical shape and attached to the distal section of catheter  12 , so that array  28  protrudes into groove  20  (see FIGS. 2 and 3). A cross-sectional side view of an illustrative catheter  12  after the flex circuit and components of FIG. 4 have been attached to the catheter is shown in FIG. 5.  
     [0052] As shown in FIG. 5, the ends  35  and  36  of array  28  are held in place between the inner ends of groove  20  (defined by the circumferential inner end faces of annular spacer rings  22  in the embodiment of FIG. 5). Integrated circuits  38  may surround the catheter core as shown in FIG. 5.  
     [0053] The body of catheter  12  may have a guide wire tube  106  (e.g., a high-density polyethelyne tube) surrounded by an optional outer tube  108  (e.g., a medium-density polyethylene tube) and a corresponding tube extension  110 . Circuits  38  and array  28  may be wrapped around marker tube  112  and backing layer  46 . If five circuits  38  are involved, the cross-section of the circuits will form a pentagon. If four circuits  38  are involved, the cross-sectional shape will be square. Other numbers of circuits  38  may be used if desired.  
     [0054] At the input/output  40  of the flex circuit, cable wires  42  may be soldered, welded, or otherwise electrically connected to the conductors  34  on the flex circuit. The catheter  12  may have a longitudinal lumen through which the cable  42  passes to equipment  14  (FIG. 1). Plastic tube  118  may be affixed to tube extension  110  using cyanoacrylate adhesive  120 . Cyanoacrylate adhesive may also be used as the adhesive  122  for affixing outer tube  108  and tube extension  110  to marker tube  112 . An ultraviolet-curable adhesive  124  may be used to seal and attach the sensor  16  to the rest of catheter  12 .  
     [0055] Stiffening member  126  may be used to stiffen the proximal portion of catheter  12 .  
     [0056] This is merely one suitable arrangement for mounting the flex circuit and components such as circuits  38  and transducer array  28  to the catheter  12 . Any suitable arrangement may be used if desired. For example, separate tubes may be provided as unitary structures. Single tubes or structures may be provided in the form of individual parts that are affixed using adhesives or other suitable arrangements. Different types of adhesives and tubing may be used, etc.  
     [0057] The backing layer  46  that is used to support the transducer array  28  preferably has a relatively high acoustic attenuation, so that acoustic signals propagating radially inward to the center of the catheter core from transducer array  28  are absorbed. The impedance of layer  46  is preferably above 3 MRayls, and even more preferably above 4 MRayls (e.g., 4.2 MRayls). Suppressing spurious acoustic signals in this way helps to reduce ringing and improves the signal to noise ratio of the sensor  16 . Any suitable material may be used for backing layer  46 . For example, backing layer  46  may be formed from a mixture of epoxy and hollow microspheres or any other suitable material that has a high acoustic absorbance. It may also be advantageous for the backing layer to be formed from an adhesive so that it may reduce “ringing,” as well as attach/secure the transducer array to the catheter body.  
     [0058] During the process of forming transducer array  28 , an acoustic matching layer may be formed on each of the array elements  30 . The matching layer may be formed, for example, from a 20-30 μm thick layer of material having an acoustic impedance that is preferably in the range of 5-12 MRayls or more preferably in the range of 8-10 MRayls. An illustrative matching layer material that may be used is Eccosorb™, available from Emerson &amp; Cuming Microwave Products, Inc. of Randolph, Mass.  
     [0059] The matching layer may be disposed between transducer array elements  30  and flex circuit substrate  32 . During operation, acoustical signals are transmitted from the transducer array elements through the matching layer and the flex circuit substrate. The thicknesses (totaling about a quarter-wavelength of the acoustical signal wavelength) and acoustical impedances of the matching layer and flex circuit substrate may be selected to provide good acoustical impedance matching between transducer array  28  and the surrounding tissue or other substances in the patient&#39;s body that are being imaged by the ultrasound sensor  16 .  
     [0060] The quality of sensor  16  may be characterized in terms of performance metrics such as sensitivity (efficiency), ringdown (the ability of the transducer to stop vibrating immediately after an acoustic pulse has been produced by the transducer array), bandwidth (the ability of the transducer to launch and receive a wide frequency range of acoustic signals), and cross-talk (the relative electrical/acoustic isolation of individual elements of the transducer array from adjacent elements).  
     [0061] The use of an acoustic matching layer on transducer array elements  30  improves the ringdown performance of the sensor  16  significantly as compared to configurations in which acoustic matching capabilities are only provided using the flex circuit substrate material itself. This is due primarily to the higher acoustic impedance properties of the matching layer compared to those available from suitable flexible circuit substrates.  
     [0062] The acoustic matching layer is used to form an acoustical antireflection coating that helps couple acoustical signals into and out of the transducer array  28 . Ideally, the acoustic matching layer would have an acoustic impedance roughly equal to the geometric mean of the acoustic impedance of the piezoelectric material of the transducer array  28  (about 31 MRayls) and the medium in which the catheter is immersed (typically a patient&#39;s blood or other body fluid, which has an acoustic impedance of about 1.5 MRayls). The geometric mean of these two values (given by the square root of their product) is about 6.8 MRayls.  
     [0063] The materials that are most suitable for the substrate of flex circuit  32  are drawn polymer films such as polyimide (Upilex or Kapton). Such films are flexible enough to roll up into the cylindrical shape needed for sensor  16  after the electrical conductors  34  have been formed and the components such as circuits  38  and array elements  30  have been mounted on the flex circuit  32 . However, such polyimide-like films typically have acoustical impedances of about 3.2 MRayls or less, which is significantly below the optimum value of about 6.8 MRayls. By using a matching layer with an acoustic impedance close to 6.8 MRayls (e.g., 5-12 or 6-8 MRayls), the acoustic matching between the transducer array and the medium in which the catheter is operating (e.g., blood) is improved, and ringdown (due to reflected energy at the interface between the device and the medium in which it is operating) is significantly improved.  
     [0064] To ensure a high level of uniformity in the matching layer thickness and to improve the efficiency of the manufacturing process, the matching layer for all of the elements  30  in array  28  may be deposited and planarized in parallel. The acoustic matching layer and the other portions of the catheter tip are shown in cross section in FIG. 6 (taken along the line  6 - 6  in FIG. 5).  
     [0065] As shown in FIG. 6, catheter core  18  may have a lumen  24  in which a guide wire may be disposed during use of the catheter. Core  18  may be solid or may be formed using a hollow tube (e.g., a hollow plastic tube). If core  18  is a hollow tube, lumen  24  may be the same size as the inner bore of the tube or may be provided by nesting a separate hollow tube within the catheter tube. These are merely illustrative configurations. Any suitable configuration may be used if desired.  
     [0066] A backing layer  46  that is highly absorbing to acoustic waves may be provided on the outer surface of the hollow tube or cylinder that forms core  18 . The transducer array  28  may be attached to core  18  after backing layer  46 . Matching layer  48  is disposed between the flex circuit  32  and the array  28 . The array  28  is shown as forming an annular ring in the drawing of FIG. 6, but is actually composed of many individual transducer array elements  30  (three of which are shown in FIG. 6). Similarly, the matching layer  48  is shown as forming a continuous layer of material in the drawing, but actually lies on top of each of the transducer array elements  30 , as indicated by the three illustrative matching layer portions  50 . The drawing of FIG. 6 is not to scale.  
     [0067] As described in connection with FIG. 4, the electrodes on each transducer element are electrically connected to corresponding electrical conductors  34  on flex circuit  32  prior to installation of the transducer elements in the catheter. A perspective view of a block of piezoelectric material  52  that is to be used to form transducer array  28  is shown in FIG. 7. The piezoelectric block  52  of FIG. 7 has not yet been divided into individual elements  30 . Electrodes  54  and  56  may be formed on block  52  using sputtering, evaporation, or other suitable deposition techniques and wet or dry etching or shadow masking or other suitable pattering techniques. These are merely illustrative methods for forming electrodes  54  and  56 . Any suitable techniques may be used to form electrodes  54  and  56  if desired. Electrodes  54  and  56  may be formed from gold (e.g., with an underlying adhesion layer of chromium or the like) or any other suitable metal or conductor.  
     [0068] When piezoelectric block  52  is installed on flex circuit  32 , the end faces  35  and  36  are electrically connected to conductors  34  on the flex circuit substrate. One suitable technique for electrically connecting end faces  35  and  36  to conductors  34  is to use conductive portions such as conductive fillets  58  and  60  (e.g., silver paste fillets), as shown in FIG. 8 a . (FIGS. 7 and 8 a  and the other FIGS. are not drawn to scale. For example, matching layer  48  may be about 20-30 microns thick and piezoelectric block  52  may be about 66-77 microns thick and 500-1000 microns in length L—i.e., a  14 × aspect ratio.)  
     [0069] Conductive portion  58  electrically connects end face portion  35  of electrode  54  to portion  34   a  of conductors  34 . Conductive portion  60  electrically connects end face portion  36  of electrode  56  to portion  34   b  of conductors  34 . Conductors  34  carry signals (drive signals for the array elements or acoustic echo signals that have been converted by the array elements into electrical signals) between the piezoelectric elements and the electronics in circuits  38  and equipment  14 .  
     [0070] As shown in the lower portion of FIGS. 8 a  and  8   b , matching layer  48  lies under lower electrode  54 , between piezoelectric block  52  and the substrate of flex circuit  32 . FIG. 8 c  shows how fillet  58  is used to form electrical contact with end face  35  and conductor  34 .  
     [0071] After the electrodes on block  52  have been electrically connected to conductors  34  as shown in FIG. 8 a ,  8   b , and  8   c , the piezoelectric block  52  may be divided into individual transducer elements  30 , as shown in FIG. 9. Block  52  may be divided into elements  30  by sawing block  52  (e.g., to leave spaces such as kerfs  62  between each respective pair of elements  30 ), by scoring block  52  (e.g., using a knife edge), by laser-cutting or water-jet-cutting of block  52 , or by using any other suitable cutting or dicing scheme.  
     [0072] The matching layer portions on each transducer element  30  may be formed simultaneously using any suitable technique. One illustrative approach for forming matching layer  48  on piezoelectric block  52  involves using template structures such as template structures  64  of FIGS. 10 and 11. During the manufacturing process, rectangular blocks of piezoelectric material such as block  52  of FIG. 7 may be inserted into the recessed holes  68  defined in template layer  66  of structure  64 . The matching layer may be deposited over the piezoelectric blocks. The sides of the holes  68  help to prevent the matching layer material from coating the side walls of block  52  and ends such as ends  35  and  36 . The template structure  64  of FIG. 11 has larger holes  68 , which may be used to protect the walls of larger blocks of piezoelectric material. Such larger blocks may then be cut down to form smaller, array-sized blocks of the type handled by holes  68  of FIG. 10. In the illustrative examples of FIGS. 10 and 11, only a relatively small number of holes  68  are shown. In practice, larger number of holes  68  may be provided (e.g., 50-100 or more) to improve the throughput and economies of scale involved in processing multiple piezoelectric blocks at a time.  
     [0073] FIGS.  12 - 17  show side views of the template structure and piezoelectric block  52  during processing. As shown in FIG. 12, hole  68  may have a length L that is substantially the same as the dimension L of the piezoelectric block  52 . Template material  66  may be wax or plastic or any other suitable material for protecting the sidewalls of piezoelectric block  52 . Preferably, template material  66  flows when heated. The template material  66  may have a thickness T that is about the same as the thickness of the piezoelectric block. For example, the thickness T may be the same as the thickness of the piezoelectric block  52  or thickness T may be thinner than the piezoelectric block thickness by up to about 10 μm. Using a template  66  that is equal or less than the piezoelectric block in thickness helps to ensure that the tops of the blocks are well sealed during subsequent top-layer masking operations. Template material  66  may be supported by a substrate  70  (e.g., a stainless steel substrate or carrier).  
     [0074] As shown in FIG. 13, each hole  68  in template structure  64  may be filled with a block  52  with an upper electrode  56  and a lower electrode  54 .  
     [0075] After block  52  has been inserted into the hole defined by the template  66 , a flexible masking member  72  (e.g., a flexible silicone cover) may be pressed against the upper surface of the piezoelectric blocks  52  and template portions  66  as shown in FIG. 14. The template structure and blocks  52  may then be heated. This causes the template material to flow. As the plastic or wax or other template material of template layer  66  flows, it coats the side walls of blocks  52 . However, the flexible cover  72  seals off the top layer of block  52 , thereby preventing the wax or other template material from flowing over top electrode  56 . By protecting top electrode  56  in this way, subsequent cleaning operations may be minimized.  
     [0076] After the cover  72  has been removed and the template cooled (optionally, the template may be cooled prior to removal of cover  72 ), the template layer may appear as shown in FIG. 15. Cusps  74  and other stray template material may be removed from the top electrode  56  by plasma etching (e.g., in an oxygen plasma) or any other suitable cleaning procedure.  
     [0077] As an alternative to flowing, template  66  may be provided with very tight tolerances such that the template is substantially the same width L as blocks  52 . This allows application of a matching layer to top electrode  56 , while ensuring that the matching layer does not contact the side wall portions of electrodes  54  and  56  of blocks  52 . As with the flowing approach, the height of the template may be modified and/or stray template material may be removed from the top electrode  56  by plasma etching (e.g., in an oxygen plasma) or any other suitable cleaning procedure.  
     [0078] The matching layer  48  may be formed by spreading a suitable matching layer material on top of the cleaned structures, by curing the matching layer material, and by polishing the cured matching layer (if desired). Suitable matching layer materials such as Eccosorb typically are formed from precursors that have a paste-like consistency. After curing, the matching layer becomes solid. Mechanical polishing, chemical-mechanical polishing, or any other suitable polishing or planarizing technique may be used to planarize the matching layer portions on top of each of the piezoelectric blocks  52  in the template structure  64  in parallel. A side view showing how matching layer  48  may be formed on top of block  52  and template  66  is shown in FIG. 16.  
     [0079] After the matching layer  48  has been polished and blocks  52  removed from the template and cleaned in a solvent (if desired) to remove excess template material from the side walls, each block  52  (now coated with matching layer  48  as shown in FIG. 17) is ready to be affixed to flex circuit  32  and electrically interconnected with conductors  32 . Blocks  52  may be removed from template  66  by a variety of techniques, including laser or mechanical cutting. Alternatively, the template may be thermally cooled and fractured along interfaces with blocks  52 . Further, the template material, e.g., wax, may be melted to allow easy removal of the blocks from the template. These are merely illustrative examples. Any suitable technique for removing blocks  52  having matching layers  48  from template  66  may be used if desired.  
     [0080] During the process of coating blocks  52  with matching layer  48 , the matching layer on each of multiple blocks  52  (e.g., 50-100 blocks in structure  64 ) are formed simultaneously (both during the deposition and curing phase and during the polishing phase). Preferably, matching layer  48  is segmented from a sheet covering multiple blocks  52  into individual portions that individually cover the surface of each block  52  during removal of the blocks from the template. Such segmentation may be achieved by cutting, cracking, etc.  
     [0081] Moreover, each of the transducer elements  30  that is created when a given piezoelectric block  52  is divided up has a portion of the simultaneously-formed matching layer  48 , which is also divided up during the dicing of block  52  into elements  30 . Because they are produced in parallel, the simultaneously-formed matching layer portions on the transducer elements are uniform and economical to manufacture.  
     [0082] Illustrative steps involved in fabricating an ultrasonic imaging catheter  12  having simultaneously-formed matching layer portions  48  on each of the transducer elements  30  in the array  28  are shown in FIG. 18. At step  76 , the patterned template structure  64  may be provided (e.g., by forming a patterned wax or plastic template  66  on a stainless steel carrier  70 ).  
     [0083] At step  78 , piezoelectric blocks  52  may be placed into each of the holes  68  in the patterned template structure  64 . The piezoelectric blocks  52  may be formed from any suitable piezoelectric material such as lead zirconate titonate composites. Electrodes  54  and  56  (including end-wall electrode portions  35  and  36 ) are preferably formed on the blocks  52  before the blocks are inserted into the holes  68 .  
     [0084] At step  80 , the piezoelectric blocks  52  may be covered with a suitable flexible mask (e.g., a silicone cover sheet). The cover may be pressed against the top surface of the blocks with sufficient force to seal off the tops of the blocks (and therefore electrodes  56 ).  
     [0085] At step  82 , the template material (and the blocks  52 ) may be heated. This causes the template material to flow and coat the sides of the blocks  52 .  
     [0086] At step  84 , the suitable flexible mask may be removed.  
     [0087] Excess material, e.g., wax or plastic, may be removed by cleaning the blocks  52  using a plasma etch or other suitable cleaning technique at step  86 .  
     [0088] At step  88 , the matching layer may be formed simultaneously on all of the blocks  52  in the template structure  64  and on all of the portions of each block  52  that will later become individual transducer array elements  30 . The matching layer may be deposited by applying uncured matching layer paste to the surface of the blocks  52  in template structure  64  and by applying heat and/or exposing the paste to air or any other suitable ambient environment to cure the paste.  
     [0089] At step  90 , the cured matching layer  48  may be polished or otherwise planarized.  
     [0090] At step  92 , the piezoelectric blocks  52  may be removed from the template structure  64  (e.g., by prying each block  52  from the structure  64  using a tool such as a knife, by flexing the substrate, by cooling the substrate and template structure  64  sufficiently to make the wax or plastic of the template  66  brittle enough to crack along the seams between the template  66  and the sides of the blocks  52  under pressure, by laser cutting, or using any other suitable technique). The sides of the blocks  52  will generally be clean when removed from the template, but an additional solvent cleaning step or other suitable cleaning operation may be used to further clean the template material from the blocks (and from the electrodes on the blocks) if desired.  
     [0091] If a template structure  64  with larger-sized holes such as holes  68  of FIG. 11 is being used, the larger sized piezoelectric blocks  52  may be cut into array-sized blocks at step  94 .  
     [0092] At step  96 , an individual block  52  may be attached to flex circuit  32  (e.g., using a suitable adhesive).  
     [0093] At step  98 , conductive fillets or portions  58  and  60  may be used to interconnect the ends  35  and  36  of the electrodes  54  and  56  to the conductors  34  of flex circuit  32  (e.g., by applying silver paste along the interface between the ends of block  52  and the flex circuit).  
     [0094] At step  100 , the block  52  may be diced, sawed or otherwise cut or divided into individual transducer array elements  30 . Dividing block  52  into elements  30  also divides simultaneously-formed matching layer  48 , such that each element  30  has an individual portion of matching layer  48 .  
     [0095] If desired, step  100  may also involve coating the back of the flex circuit (at least in the vicinity of array  28 ) with a stabilizing layer of photoresist (e.g., polymethylmethacrylate or PMMA) or other suitable material, such as Nitto tape. The stabilizing layer is preferably temporary and may be used as a sacrificial layer to facilitate dicing and/or formation of the flex circuit into a cylinder. The dicing process (or other suitable cutting process) may then be used to cut through both the piezoelectric block  52  and, optionally, the underlying flex circuit under kerfs  62  (FIG. 9).  
     [0096] The cutting process optionally forms cuts  104  through all or some of the flex circuit  32  between adjacent transducer elements  30 , as shown in FIG. 19. The drawing of FIG. 19 shows a sectional view (i.e., a cross-section across the middle of the transducer array) taken of the flex circuit  32  in a flattened condition, before being rolled up to form the cylindrical flex circuit shape of the sensor  16  that is used during operation of ultrasonic imaging catheter  12 . The drawing of FIG. 20 shows, from an exterior perspective, how longitudinal cuts  104  (cuts parallel to the longitudinal axes of the sensor and catheter) are located (e.g., evenly-spaced) around the circumference of the distal end of the cylindrical rolled-up flex circuit that forms sensor  16 .  
     [0097] The cuts  104  (which may be either downward extensions of kerfs  62  or separate cuts) help to isolate array elements  30  from each other. The cuts may extend from the edge of flex circuit  32  so as to leave each array element  30  mounted on its own piece of substrate material in a diving-board fashion. Alternatively, the ends of the “diving boards” may be left connected to each other by making cuts  104  through only the middle portions of the flex circuit  32  (i.e., the portion of flex circuit  32  between ends  36  and  36 ).  
     [0098] The underlying temporary stabilizing layer, which, by virtue of covering cuts  104 , may facilitate the holding of the flex circuit  32  in the vacuum chucks used during sawing, may be removed after cutting is complete.  
     [0099] When the optional flex circuit cutting process is used during the process of dividing the array  28  into transducer array elements  30 , the transducer array elements become more acoustically decoupled from each other. This decoupling may increase the width of the acoustic beam profile associated with each transducer array element  30 .  
     [0100] Imaging quality is improved when each transducer element operates relatively independently and has a fairly wide associated beam profile. When the transducer array elements produce overly-narrow beams, the beams cannot be combined properly. This may make it difficult or impossible to sweep the combined beam through as wide an angle as is desired during the imaging process. By cutting completely through the portions of the flex circuit that lie between adjacent transducer array elements  30 , the transducer elements produce wider beams and are less likely to induce undesirable vibrations in adjacent elements.  
     [0101] In FIG. 19, flex circuit  32  is shown cut into multiple pieces. However, as described above and as shown in FIG. 20, cuts  104  may be formed (during step  100  of FIG. 18) as local slits in the flex circuit in the vicinity of array elements  30 , such that flex circuit  32  remains one piece distal of the array  28 . Furthermore, cuts  104  may be filled with an epoxy or adhesive having a substantial acoustic mismatch to adjacent transducer elements  30 . The adhesive may comprise, for example, a UV-cured or heat-cured epoxy. The epoxy facilitates formation of the flex circuit into a cylinder and provides a fluid seal between elements  30  of the transducer array, while still providing acoustic decoupling between respective array elements.  
     [0102] The performance of the ultrasonic imaging catheter  12  may be improved by using the simultaneously-formed acoustic matching layer portions on the elements  30  (e.g., the matching layer portions of Eccosorb formed by parallel-processing the array elements while still in the form of a unitary piezoelectric block), by using the high-acoustic-impedance flex circuit substrate (e.g., the substrate formed of Upilex-S, Kapton, or other material having an acoustic impedance in the range of 3.5-4.5 MRayls or other suitable range), by isolating adjacent array element  30  by cutting through the underlying flex circuit that lies between adjacent elements, or by using these design approaches in any suitable combination (e.g., using only one of these approaches or using any two of these approaches). Moreover, these aspects of the invention may be used in any suitable combination with the various other embodiments of the invention described above.  
     [0103] It will be understood that the foregoing is merely illustrative of the principles of this invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention.