Abstract:
The present invention generally relates to medical devices, and more particularly to an improved medical imaging device. In one embodiment, an imaging device includes a drive shaft having proximal and distal ends received within the lumen; and an imaging transducer assembly coupled to the distal end of the drive shaft and positioned at the distal portion of the elongate member. The imaging transducer assembly includes one or more imaging transducers formed with a piezoelectric composite plate using photolithography based micromachining.

Description:
FIELD OF THE INVENTION 
       [0001]    The field of the invention relates to imaging devices, and more particularly to micromachined imaging transducers. 
       BACKGROUND OF THE INVENTION 
       [0002]    Intraluminal, intracavity, intravascular, and intracardiac treatments and diagnosis of medical conditions utilizing minimally invasive procedures are effective tools in many areas of medical practice. These procedures are typically performed using imaging and treatment catheters that are inserted percutaneously into the body and into an accessible vessel of the vascular system at a site remote from the vessel or organ to be diagnosed and/or treated, such as the femoral artery. The catheter is then advanced through the vessels of the vascular system to the region of the body to be treated. The catheter may be equipped with an imaging device, typically an ultrasound imaging device, which is used to locate and diagnose a diseased portion of the body, such as a stenosed region of an artery. For example, U.S. Pat. No. 5,368,035, issued to Hamm et al., the disclosure of which is incorporated herein by reference, describes a catheter having an intravascular ultrasound imaging transducer. These are generally known in the art as Intravascular Ultrasound (“IVUS”) devices. 
         [0003]      FIG. 1  shows an example of an imaging transducer assembly  1  known in the art. The imaging transducer  1  is typically within the lumen  10  of a guidewire or catheter (partially shown), having an outer tubular wall member  5 . To obtain an image of a blood vessel the imaging transducer assembly  1  may be inserted into the vessel. The transducer assembly  1  may then interrogate the cross-sectional-plain of the vessel from the inside by rotating while simultaneously emitting energy pulses, e.g., ultrasound pulses, and receiving echo signals. 
         [0004]    On the distal end of the assembly  1  is an imaging element  15 , specifically, an imaging transducer  15  that includes a layer of piezoelectric ceramic (“PZT”)  80 , “sandwiched” between a conductive acoustic lens  70  and a conductive backing material  90 , formed from an acoustically absorbent material (e.g., an epoxy substrate having tungsten particles). During operation, the PZT layer  80  is electrically excited by both the backing material  90  and the acoustic lens  70  to cause the emission of energy pulses. 
         [0005]    The transducer assembly  1  of  FIG. 1  shows a single imaging element  15 . Also known in the art is the utilization of an array of imaging elements, e.g., an array of imaging transducers, instead of just one imaging element  15 . An array of imaging transducers provides the ability to focus and steer the energy pulses without moving the assembly  1 . An example of such an array  100  is shown in  FIG. 2 , which also illustrates a known process  200  for creating the array  100 , commonly referred to as “dice and fill.” In the process  200 , a plate of poled PZT ceramic  210  is obtained. A saw  220  is then used on the ceramic  210 , forming a plurality of kerfs  230  and an array of posts  240 , which serve as the PZT layer for the array of transducers  100 . The kerfs  230  are then backfilled with polymer materials, such as epoxy  250 , to form composite structures. Transducers based on this architecture can exhibit high bandwidth, high sensitivity, good acoustic impedance matching to tissue, and desirable array properties such as low inter-element cross-talk and low side-lobe levels. However, transducers based on this architecture generally do not operate at frequencies much above 20 Megahertz (“MHz”). Accordingly, an improved imaging device would be desirable. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention generally relates to medical devices, and more particularly to an improved medical imaging device. In one embodiment, an imaging device includes a drive shaft having proximal and distal ends received within the lumen; and an imaging transducer assembly coupled to the distal end of the drive shaft and positioned at the distal portion of the elongate member. The imaging transducer assembly includes one or more imaging transducers formed with a piezoelectric composite plate using photolithography based micromachining. 
         [0007]    Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0008]    In order to better appreciate how the above-recited and other advantages and objects of the present inventions are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated in the accompanying drawings. It should be noted that the components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views. However, like parts do not always have like reference numerals. Moreover, all illustrations are intended to convey concepts, where relative sizes, shapes and other detailed attributes may be illustrated schematically rather than literally or precisely. 
           [0009]      FIG. 1  is a cross-sectional side view of an imaging transducer assembly known in the art. 
           [0010]      FIG. 2  is an illustration of a technique for manufacturing an array of transducers known in the art. 
           [0011]      FIG. 3  is an illustration of a photolithography based micromachining process in accordance with a preferred embodiment of the present invention. 
           [0012]      FIG. 4   a  is an imaging transducer having a 2-2 configuration in accordance with a preferred embodiment of the present invention. 
           [0013]      FIG. 4   b  is an imaging transducer having a 1-3 configuration in accordance with a preferred embodiment of the present invention. 
           [0014]      FIG. 5   a  is an annular transducer array in accordance with a preferred embodiment of the present invention. 
           [0015]      FIG. 5   b  is another annular transducer array in accordance with a preferred embodiment of the present invention. 
           [0016]      FIG. 6  is a cross-sectional view of an imaging wire in accordance with a preferred embodiment of the present invention. 
           [0017]      FIG. 7  is a diagram of a medical imaging system in accordance with a preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    As mentioned above, an imaging transducer that operates at high frequencies, e.g., frequencies higher than 20 MHz, would be desirable. Such imaging transducers can provide images with higher resolution, which is desirable in applications involving dermatology, ophthalmology, laparoscopy, intracardiac and intravascular ultrasound. One approach to develop such imaging transducers is to utilize a photolithography based micromachining process. An example of such a process  300  is illustrated in  FIG. 3 . 
         [0019]    In the first step  310 , a plate or block of piezoelectric crystal material  315 , such as lead magnesium niobate lead titanate (“PMN-PT”) or lead zinc niobate-lead titanate (“PZN-PT”) is obtained. The plate  315  is preferably lapped on both sides and polished on one of the sides. The lapped and unpolished side can then be bonded to a glass carrier (not shown), which is bonded to a silicon, Si, wafer (not shown). The dimensions of the plate  315  are in the range of ten (10) millimeters (“mm”)×ten (10) mm×0.5 mm to fifteen (15) mm×fifteen (15) mm×0.5 mm; however, the dimensions could be of any size. The material of the plate  315  can be a ceramic or a single crystal. Preferably, the material of the plate  315  is a single crystal PMN-PT with electroded faces oriented along the &lt;001&gt; or &lt;011&gt; crystallographic directions. As one of ordinary skill in the art would appreciate, a single crystal structure can desirably have a high piezoelectric coefficient (e.g., d 33 &gt;1500 pC/N, k 33 &gt;0.8, k 33 &gt;0.7). The plate  315  preferably has a dielectric constant in the range of approximately 4000 to &gt;7700 and a dielectric loss of less than 0.01. 
         [0020]    In the next step  320 , a mask of photoresist  325  is applied to the plate of piezoelectric material  315 . The mask  325  defines the desired shape and/or pattern of imaging element(s) within the piezoelectric composite material  315 . In the next step  330 , electroplating is applied to the plate  315  using nickel, Ni. A hard pattern of Ni  335  is formed on the plate  315  in accordance with the mask of photoresist  325 . The pattern of Ni  335  can have a thickness of approximately 1 to 20 microns (“μm”). Other metals, such as platinum, Pt, may be used instead of, or in addition to, nickel. The use of hard and/or high molecular weight materials, such as Ni and Pt, is desirable for selectivity, to protect the covered underlying area of the plate  315  from being etched. The mask of photoresist  325  is removed after the Ni is applied. 
         [0021]    In the next step  340 , an etching process, such as reactive ion etching (“RIE”), is applied. Other etching processes can be used, such as wet-etching. In one preferred embodiment, chlorine, Cl 2  based RIE etching is used, which has an etching rate of approximately from less than 3 microns/hour to 12 microns/hour and can cause a substantially vertical etching profile (e.g., &gt;80°). In the alternative, or in addition, to Cl 2 , sulfur hexaflouride, SF 6 , based etching can be used, which has similar etching properties to that of Cl 2 . The nickel, Ni, pattern  335  protects the underlying portions of the plate  315  covered by the pattern  335  from the etching process, and thus, one or more deep posts  347  are formed in the plate  315  with one or more kerfs  345  surrounding the one or more posts  347  etched in the uncovered portions of the plate  315 . The one or more kerfs  345  can have a width in the range of approximately from less than one (&lt;1) to twelve (12) μm, and the width of the one or more posts  347  can have a width in the range of approximately from less than three (&lt;3) to thirty-six (36) μm and have a height in the range of approximately from less than five (&lt;5) to more than seventy (&gt;70) μm. In one embodiment, it is preferable to have an aspect ratio (post height/post width) of at least two (2) to one (1) to dampen the effect of lateral modes. For the dimensions of the plate  315  described above, the etching process can last approximately six (6) to eight or eighteen (8 or 18) hours. After the etching step  340 , the plate  315  is then rinsed with a solvent for cleaning. 
         [0022]    In the next step  350 , the kerfs  345  are filled with an epoxy  355  such as Epo-Tek-301. A vacuum (not shown) may be utilized to remove air bubbles and prevent any void within the kerfs  345 . In the next step  360 , the top portion of the plate  315  and epoxy  355  are lapped to a thickness of approximately forty (40) μm. An electrode pattern  375  is then applied to the plate  315  in the next step  370  to form the imaging transducer pattern. The electrode pattern  375  is preferably comprised of gold, Au, and chromium, Cr. Moreover, as one of ordinary skill in the art would appreciate, electronic circuitry, such as imaging processing circuitry, (not shown) can be bonded to the electrodes  375 . Further, the electrode pattern  375  formed on the plate  315  can define any pattern of imaging transducers, including an array, e.g., an imaging transducer at each post  347 , or a single imaging transducer. An epoxy layer  377  may be applied to the back of the plate  315 . 
         [0023]    Imaging transducers having an operating frequency at above 20 MHz, e.g., 30 to &gt;80 MHz, can be developed using photolithography based micromachining, such as the process  300  described above. The higher frequency of operation increases the resolution and image depth of an imaging transducer. Furthermore, the bandwidth of the imaging transducer, particularly when single crystal PMN-PT is employed as the piezoelectric, can be close to 100%, compared to only 70 to 80% for &lt;20 MHz transducers made with PZT ceramic. The greater bandwidth improves the transducer&#39;s axial resolution, which increases the imaging depth. This is desirable for high frequency transducers, which have very limited imaging depth due the strong attenuation of high frequency ultrasound in tissue. When single crystal is used, these advantages can be achieved with sensitivities equivalent to or better than ceramic transducers. These high frequency transducers can be applied to a number of medical procedures including the imaging of the anterior region of an eye for monitoring surgical procedures such as cataract treatment by lens replacement and laser in situ keratomileusis (LASIK) and tumor detection (preferably up to sixty (60) MHz for fifty (50) μm resolution); skin imaging for care of burn victims and melanoma detection (preferably twenty five (25) MHz for subcutaneous, fifty (50) MHz for dermis and one hundred plus (100+) MHz for epidermis); intra-articular imaging for detection of pre-arthritis conditions (preferably twenty five (25) to fifty (50) MHz); in-vivo mouse embryo imaging for medical research (preferably fifty (50) to sixty (60) MHz); Doppler ultrasound for determination of blood flow in vessels &lt;one hundred (100) μm in diameter (preferably twenty (20) to sixty (60) MHz); intracardiac and intravascular imaging (preferably ten (10) to fifty (50) MHz); and ultrasound guidance for the biopsy of tissue. 
         [0024]    In preferred embodiments, at least two types of imaging transducer configurations can be developed using a photolithography based micromachining process, such as the process  300  described above, the 2-2 configuration and the 1-3 configuration, which are configurations known in the art. Turning to  FIG. 4   a , an example array  400  of piezoelectric posts  410  are shown on a wafer  405  positioned in a 2-2 configuration. Polymeric material  420  is filled in between the posts  410 . The “2-2” describes the number of directions in which each section of the piezoelectric material  410  and polymeric material  420  mainly extend. The description method preferably uses an M-N labeling convention, where M is the number of directions in which the piezoelectric material  410  mainly extends and N is the number of directions in which the polymeric material  420  mainly extends. Turning to  FIG. 4   b , an example array  450  of piezoelectric posts  460  are shown on a wafer  455  positioned in a 1-3 configuration. The kerfs  470  in between the posts  460  are filled with a polymeric material. 
         [0025]    Using a photolithography based micromachining process, such as the process  300  described above, on a plate of piezoelectric material enables any pattern of imaging elements to be formed, including one dimensional and two dimensional arrays of imaging elements, which can be utilized in two dimensional and three dimensional ultrasound imaging applications, respectively. 
         [0026]    In addition, various shapes of arrays maybe formed. Turning to  FIG. 5   a , an annular array  500  of imaging transducers is shown. The array includes segmented elements  510  and a central element  515 . Turning to  FIG. 5   b , an alternative annular array  550  of imaging transducers is shown. The array  550  includes a central element  565  and annular aperture elements  560  concentrically positioned around the central element  565 . These annular array configurations may be forward-facing in an imaging catheter or guidewire and are particularly suited for blood vessels. The annular arrays are preferably formed by defining annular arrays of electrodes over a 1-3 composite structure  450 , such as that shown in  FIG. 4   b.    
         [0027]    Turning to  FIG. 6 , the imaging transducers described above may be used in a catheter and can also be placed in a distal portion  640  of a guidewire  600 . The guidewire  600  may comprise a guidewire body  620  in the form of a flexible, elongate tubular member, having an outer wall  630 . The guidewire body  620  may be formed of any material known in the art including nitinol hypertube, metal alloys, composite materials, plastics, braided polyimide, polyethylene, peek braids, stainless steel, or other superelastic materials. 
         [0028]    Turning to  FIG. 7 , a proximal portion of the guidewire  600 , such as that shown in  FIG. 6 , may be adapted to connect to circuitry  710  that processes imaging signals from the imaging transducers described above, such circuits being well known. 
         [0029]    In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. For example, the reader is to understand that the specific ordering and combination of process actions described herein is merely illustrative, and the invention can be performed using different or additional process actions, or a different combination or ordering of process actions. As a further example, each feature of one embodiment can be mixed and matched with other features shown in other embodiments. Additionally and obviously, features may be added or subtracted as desired. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.