Abstract:
A method of mounting a transducer to a driveshaft which eliminates the need for a transducer housing, the improved method directly attaches the transducer to a rigid distal tip of a driveshaft which is part of a rotatable imaging core of a catheter assembly. The method contemplates heat treating the distal tip of the driveshaft to make it rigid, machining the distal tip to be dimensioned to hold the transducer, and attaching the transducer to the distal tip by clamping, crimping, or an adhesive.

Description:
PRIOR APPLICATIONS  
       [0001]    This application is a divisional of co-pending U.S. patent application Ser. No. 09/755,873, filed on Jan. 4, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates, in general, to transducer mountings for ultrasound catheter assemblies used in diagnostic or therapeutic applications.  
         DISCUSSION OF RELATED ART  
         [0003]    Mechanically scanned ultrasound catheter assemblies employ a single transducer mounted inside a rotating housing. In particular, the transducer transmits and receives ultrasonic waves while the transducer housing rotates about a fixed axis in an acoustic window located at a distal tip of the catheter. The rotational motion of the transducer housing is accomplished by a flexible driveshaft that extends through an axially disposed lumen of the catheter, wherein the driveshaft has one end connected to the transducer housing. Once the distal end of the catheter is positioned, for example, in a patient&#39;s vascular system, a cross-sectional image of the tissue surrounding the distal catheter tip is produced by using imaging and control circuitry that are electrically coupled to the transducer via an electrical conductor extending through the drive shaft.  
           [0004]    With respect to FIG. 1, a conventional prior art ultrasound catheter assembly  100 , which appears in U.S. Pat. No. 5,842,994, is depicted. U.S. Pat. No. 5,842,994 is hereby incorporated by reference. The catheter assembly  100  comprises a first elongate tubular element  120 , which forms an axially disposed lumen  130 . An acoustic imaging window  140  is attached to a distal end of the first tubular element  120 , thereby forming an enclosed tip of the catheter assembly  100 . A flexible driveshaft  150  extends through the lumen  130  and is connected at a distal end to a transducer housing  60  disposed in the acoustic imaging window  140 . The transducer housing  60  has a generally cylindrical transducer  170  mounted therein, exposing a circular active surface area, or aperture. A second elongate tubular element  180  forms an additional lumen  190  used for other catheter functions such as housing pullwires or delivering liquid to a distally disposed balloon during angioplasty.  
           [0005]    With further reference to FIG. 1, the transducer housing  60  is an expensive, high-precision part that requires a particular joining operation to attach it to the driveshaft  150 . In addition, the joint created between the driveshaft  150  and the transducer housing  60  may potentially fail. Therefore, it would be advantageous to eliminate the transducer housing  60  from catheter assembly  100 . FIG. 1A depicts an alternative conventional prior art ultrasound catheter, which includes opening  65  adapted to permit flushing from the proximal end of catheter assembly  100  and, if a separate lumen is provided, dimensioned for the passage of guide wires. This alternative catheter does not include first elongate tubular element  120  or second elongate tubular element  180 .  
         SUMMARY OF THE INVENTION  
         [0006]    An aspect of the invention involves a method of making a transducer mounting to a driveshaft by providing an elongate tubular element including a lumen, providing a rotatable imaging core adapted to pass through the lumen, the imaging core including a flexible driveshaft and a transducer. The method optionally contemplates treating and machining the distal tip of the driveshaft, and then attaching the transducer to the distal tip.  
           [0007]    Another separate aspect of the invention involves a method of making a transducer mounting to a driveshaft comprising a step of treating the distal tip of the driveshaft by hardening the distal tip with a welding process, wherein the driveshaft is initially made of flexible wound wires and the welding process joins the wound wires of the distal tip together such that a rigid distal tip is formed. This method optionally contemplates the use of electrical welding by applying electrodes at two locations on the drive shaft for electrical conductivity therebetween.  
           [0008]    A further separate aspect of the invention involves a method of making a transducer mounting to a driveshaft comprising a step of treating the distal tip of the driveshaft by hardening the distal tip with a soldering process, wherein the driveshaft is made of flexible wound wires having interstices therebetween and the soldering process fills the interstitial spaces such that a rigid distal tip is formed. This method optionally contemplates the use of a cold clamp having a high specific heat to dissipate excess heat during soldering. In addition, this method may incorporate the use of a ceramic or fiber-optic plug for keeping the central lumen open during soldering.  
           [0009]    An additional separate aspect of the invention involves a method of making a transducer mounting to a driveshaft comprising a step of machining the distal tip of the driveshaft by grinding or drilling the distal tip to form an arcuate recession with opposing tapered side walls. This method optionally contemplates attaching the transducer to the distal tip of the driveshaft by crimping the tapered side walls about the perimeter of the transducer so that the transducer is held in place therebetween. In addition, a clamping member or an adhesive may be used to secure the transducer to the distal tip of the drive shaft.  
           [0010]    Yet another separate aspect of the invention involves an ultrasonic imaging catheter assembly comprising an elongate tubular element including a lumen, a rotatable imaging core adapted to pass through the lumen, the rotatable imaging core including a flexible driveshaft attached to a transducer, wherein the driveshaft has a rigid distal tip adapted to be mounted to the transducer.  
           [0011]    An additional separate aspect of the invention involves an ultrasonic imaging catheter assembly having a driveshaft with a rigid distal tip which includes an arcuate recession with opposing tapered side walls, wherein the transducer is attached to the distal tip of the driveshaft by crimping the tapered side walls about the perimeter of the transducer so that the transducer is held in place therebetween. Alternatively, a clamping member or an adhesive may be used to secure the transducer to the distal tip of the drive shaft.  
           [0012]    The invention may include any one of these separate aspects individually, or any combination of these separate aspects.  
           [0013]    Other features and advantages of the invention will be evident from reading the following detailed description, which is intended to illustrate, but not limit, the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The drawings illustrate the design and utility of preferred embodiments of the present invention, in which similar elements are referred to by common reference numerals.  
         [0015]    [0015]FIG. 1 is a cut-away partial side view of a prior art ultrasound catheter assembly.  
         [0016]    [0016]FIG. 1A is a cut-away partial side view of a prior art ultrasound catheter assembly.  
         [0017]    [0017]FIG. 2 is a cut-away partial side view of an ultrasound catheter assembly according to an example embodiment of the present invention.  
         [0018]    [0018]FIG. 2A is a cross-sectional view taken along line  2 A- 2 A of FIG. 2.  
         [0019]    [0019]FIG. 3 is a cut-away partial side view of the ultrasound catheter assembly of FIG. 2.  
         [0020]    [0020]FIG. 3A is a cross-sectional view taken along line  3 A- 3 A of FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    With respect to FIGS. 2 and 3, a preferred ultrasound catheter assembly  100  includes an elongate tubular element  115  having tubular section  120 , which forms an axially disposed lumen  130 . A dome-shaped acoustic imaging window  140  is attached to a distal end of the elongate tubular element  115 , thereby forming an enclosed tip of the catheter assembly  100 . Alternatively, the shape of the acoustic imaging window  140  may be virtually any shape or combination of shapes. An imaging core  145  comprising a flexible driveshaft  150  having a rigid distal tip  160  and a generally cylindrical transducer  170  is disposed within lumen  130 . The imaging core  145  is capable of translation along its center axis  185 .  
         [0022]    As best seen in FIG. 2A, axially disposed lumen  130  has a substantially “D-shaped” cross-section wherein the inner dimensions of lumen  130  are sufficient for transducer  170  to be translated therein. With further reference to FIG. 2A, a solid section  180  of elongate tubular element  115  forms an additional lumen  190  used for other catheter functions such as, by way of non-limiting examples, housing pullwires, drug delivery, balloon angioplasty, laser ablation, or for housing a stiffening member to help prevent the collapsing of the catheter assembly.  
         [0023]    A cover tube  200  formed of a suitable material, such as a heat shrinkable nylon, urethane, polyethylene or other plastic, is disposed around tubular element  115 , wherein cover tube  200  provides both structural integrity to the catheter assembly  100 , as well as a smooth outer surface for ease in axial movement in a patient&#39;s body passage with minimal friction. Preferably, the acoustic imaging window  140  has its proximal end open and its distal end rounded and is attached to a distal outer circumferential portion of the tubular element  115  to form an enclosed catheter tip  210 , with respective ends of the cover tube  200  and acoustic imaging window  140  bonded together at a common joint  220 . The outer diameter of the proximal end of window  140  is preferably substantially equal to that of the installed cover tube  200 , so that a smooth outer surface is provided at joint  220 .  
         [0024]    Referring to FIGS. 2 and 3, the transducer  170  is attached to the flexible driveshaft  150  at a cut-away portion  165  of rigid distal tip  160  such that its active surface  175  slopes at a slight angle with respect to the center axis  185  of driveshaft  150 . This tilting of transducer  170  helps to minimize internal reflections inside of catheter tip  210 . The transducer  170  can be fixedly attached in a number of ways including by an adhesive such as a UV (ultraviolet light) cure epoxy, by crimping of opposing tapered side walls  195  surrounding cut-away portion  165 , by a clamp  205 , any other known method of affixing, or any combination of these methods.  
         [0025]    As best seen in FIGS. 3 and 3A, driveshaft  150  has a central lumen  225  adapted for the passage of transducer wires or coaxial cable, which extend through relieved area  163 . The driveshaft  150  is made of wound wire such as a super alloy or stainless steel in order to be flexible inside of a patient&#39;s blood vessel, for example. However, distal tip  160  of driveshaft  150  preferably should be hardened and machined in order to mount transducer  170 . Hardening of distal tip  160  can be accomplished by a number of means including welding and soldering.  
         [0026]    Welding of the distal tip  160  is preferably accomplished electrically, by applying electrodes at two locations along distal tip  160  for conductivity therebetween. The two locations can be the end of distal tip  160  and, for example, location  230  along the driveshaft  150 . By running sufficient electricity between these two locations, the wound wires of driveshaft  150  will heat up, begin to liquefy, and bond together, eliminating some of the interstitial spaces between the wound wires. Although electrical welding of the distal tip  160  should be continued until the wires have fused together, it should be terminated before unwanted deformation of the distal tip  160  has occurred. After welding and a brief cool-down period, the distal tip  160  will be more rigid than the rest of the driveshaft  150  due to the fusing between the wound wires.  
         [0027]    Alternatively, the hardening of the distal tip  160  can be accomplished by a soldering process wherein the interstitial spaces between the wound wires are filled with softened metal. Although many different solders can be used, the solder is preferably a 5% silver solder mixed with 95% tin. During the soldering process, the silver solder should be heated to approximately 850-900 degrees Fahrenheit and melted into the interstitial spaces in the distal tip  160  of driveshaft  150 . Alternatively, a brazing process can be used, which requires greater temperatures to melt solder having a higher percentage of silver.  
         [0028]    During soldering, a cold clamp can be utilized to dissipate excess heat and to limit unnecessary fusion of the wound wires of the rest of the flexible driveshaft  150 . Ideally, the cold clamp is made of metal such as aluminum or copper having a high specific heat. Also, the cold clamp is circular so that it can encircle the perimeter of drive shaft  150  at location  230 . Before soldering, a plug should be inserted within central lumen  225  so that it is not stopped up by melted metal. Preferably, the plug is made of a material having a high melting point such as a ceramic rod or a piece of fiber optic. In addition, the plug can have an outer coating to prevent adherence to the solder.  
         [0029]    The hardening of the distal tip may also be accomplished using an adhesive such as an epoxy wherein an adhesive is used to fill the interstitial spaces of the wound wire of distal tip  160 . After the distal tip  160  has been hardened so that it is rigid, it can be machined to create an effective mount for transducer  170 .  
         [0030]    During machining, a cut-away portion  165  in the form of an arcuate recession with opposing tapered side walls  195  is formed on the rigid distal tip  160  using a milling or grinding process. In addition, relieved area  163  is formed within cut-away portion  165  using a similar milling or grinding process. Cut-away portion  165  is adapted to receive the cylindrical transducer  170 . The opposing tapered walls  195  of machined distal tip  160  can be crimped inwardly about the perimeter of the transducer  170  to hold it fixedly in place. Additionally, an epoxy or other adhesive such as a UV cure epoxy can be used to further secure transducer  170  to distal tip  160 . Alternatively, clamping member  205  can be used to secure the transducer  170  to the distal tip  160  of the driveshaft  150 . The clamping member is fixedly attached to the driveshaft  150  and removably attached to the transducer  170 .  
         [0031]    Any one or more of the features depicted in FIGS.  1 - 3 , or described in the accompanying text, may be interchanged with that of another figure to form still other embodiments.  
         [0032]    While preferred embodiments and methods have been shown and described, it will be apparent to one of ordinary skill in the art that numerous alterations may be made without departing from the spirit or scope of the invention. Therefore, the invention is not limited except in accordance with the following claims.