Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 60/686,291 filed Jun. 1, 2005, the disclosure of which is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to ultrasonic transducers.  
         [0003]     Ultrasonic transducers find wide application in industry and in medicine. For example, in certain medical procedures, they are used to produce ultrasonic energy which heats tissue within the body of a living subject. In one such type of procedure, known as ablation, sufficient heat is applied to kill undesired tissue. Typically, this requires heating tissue to a temperature of 60-80° C. Furthermore, it is desirable that tissue be heated rapidly in order to avoid incidental damage to surrounding tissue.  
         [0004]     In one common medical application, ultrasonic transducers are utilized in catheters which must pass through small spaces within the body. For example, catheters are often passed through the circulatory system in order to be placed into the heart. It will be appreciated that the ultrasonic transducers on such catheters must be quite small, yet they must be capable of emitting a substantial amount of ultrasonic power in order to be useful.  
         [0005]     A typical ultrasonic transducer utilized in a catheter includes an active element in the form of a piezoelectric sleeve. Piezoelectric elements deform physically when subjected to an electric field. Hence, when a sufficiently rapidly varying electrical signal is applied, the piezoelectric sleeve vibrates at ultrasonic frequencies, and ultrasonic energy is radiated. Typically, the piezoelectric sleeve is mounted on a supporting tube, typically made of surgical steel, which provides a lumen for the catheter. The supporting tube must be electrically isolated from the piezoelectric sleeve. When the piezoelectric sleeve vibrates, it produces substantial heat, and excessive heat must not be transmitted to the supporting tube.  
         [0006]     In order to provide efficient ultrasonic radiation, a backing medium is usually provided at the inner face of the piezoelectric sleeve. This backing is made of a material which has a substantially different ultrasonic impedance than the piezoelectric material, so that ultrasonic energy impinging upon the interface between the piezoelectric sleeve and the backing medium is reflected outwardly, increasing the total ultrasonic radiation away from the sleeve.  
         [0007]     One known backing medium is air. Air backing is achieved by constructing the transducer so that air is in contact with substantially entire inner surface of the active element, providing the necessary reflection of ultrasonic energy. U.S. Pat. Nos. 6,599,288 and 6,607,502, both to Maguire et al., disclose a catheter wherein the ultrasound transducer is mounted onto a catheter shaft without any support structure between the two, i.e., the transducer is suspended about the catheter shaft. This may isolate the transducer from the shaft by providing a layer of air between the two, however, a catheter with such complicated structure is difficult to manufacture and assemble.  
         [0008]     Ultrasonic transducers have also been provided with solid backing. For example, the backing could be a brass sleeve inwardly of the piezoelectric sleeve.  FIG. 1  is a schematic representation of the internal construction of a known water backed transducer  10 . An active element  12  of transducer  10  is a cylindrical sleeve made of a piezoelectric material. Cables  13  provide an electrical actuating signal for transducer  10 . For this purpose, conductive regions (not shown) are provided on the outer and inner surfaces of sleeve  12 . One conductor of each of cables  13  is connected to the conductive region on the outer surface of sleeve  12 , and the other conductor is connected to a backing element  14 . Since backing element  14  is made of an electrically conductive material (see below), it will make the necessary electrical contact with the conductive region on the inner surface of sleeve  12 .  
         [0009]     Backing element  14  is generally cylindrical, made of brass, and is provided inside piezoelectric sleeve  12 . Backing element  14 , at each axial end, includes a plurality of radially extending protrusions  14   a.  The diameter of the rear surface of sleeve  12  is greater than the diameter of the opposed surface of backing element  14 , so that a cylindrical space  16  is formed there between. Space  16  is filled with water and absorbs heat generated by active element  12 . Backing element  14  has an axial bore which receives a supporting tube  18 , made of stainless steel. Between tube  18  and backing element  14 , there is provided a layer  20  of polyimide insulation. The water in this water backed transducer  10  provides a coolant which reduces heat transmission to the tube. The water back of transducer  10  therefore serves dual functions of providing ultrasonic reflection and limiting heat transfer to supporting tube  18 .  
         [0010]     Unfortunately, an ultrasonic transducer with a water back tends to produce ultrasonic energy inefficiently and permits an undesirable amount of heat to buildup in the interior of the transducer. Additionally, due to the small size of the catheter, it is difficult to provide high enough flow rate of water to obtain sufficient cooling effect. Typically, this type of transducer will convert electrical energy to ultrasound energy with an efficiency of about 45%, and the interior (center) of the transducer can be heated to a temperature in excess of 300° F. Moreover, a catheter with this complex structure is difficult to manufacture and assemble.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention overcomes the shortcomings of the prior art and discloses an ultrasonic transducer having an active element, a backing element, and a support. The backing element is located between and is in contact with the active element and the support. The backing element is hydrophobic, made of an insulating material which contains entrained air and is of sufficient thickness to provide substantial thermal insulation with respect to the active element.  
         [0012]     In another aspect the present invention discloses an ultrasonic transducer of the type including a hollow cylindrical active element made of a piezoelectric material and having inner and outer surfaces. The ultrasonic transducer also includes a backing element that is in contact with the inner surface and is mounted on a cylindrical support. The backing element is made of an insulating material which contains entrained air and is hydrophobic.  
         [0013]     Another aspect of the present invention discloses an ultrasonic transducer containing an active element having a front and a rear surface, a backing element in contact with the rear surface, and a support, the support being in contact with the backing element. The backing element is made of a soft hydrophobic material that does not significantly dampen the vibrations of the active element. The backing element may be made from a material which includes a substantial amount of entrained air. Preferably, the backing component is made of expanded polytetrafluoroethylene (EPTFE).  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The foregoing brief description and further objects, features and advantages of the present invention will be understood more completely from the following detailed description of a presently preferred, but nonetheless illustrative, embodiment in accordance with the present invention, with the reference being had to the accompanying drawings, in which:  
         [0015]      FIG. 1  is a schematic representation of the internal construction of a known water backed transducer shown in lengthwise cross-section;  
         [0016]      FIG. 2  is a schematic representation of the internal construction of an EPTFE backed transducer, also shown in lengthwise cross-section; and  
         [0017]      FIG. 3  is a schematic representation of an embodiment of a probe containing a transducer in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]      FIG. 2  is a schematic representation of the internal construction of an ultrasonic transducer  30 . In this figure, elements which are indicated by the same reference numbers as in  FIG. 1  are identical to the corresponding elements of  FIG. 1 . In  FIG. 2 , backing element  14  and the insulation layer  20  of  FIG. 1  are replaced by a cylindrical sleeve  32  (i.e., backing element) which is made of in insulating material, preferably EPTFE, which fills the entire space between the interior surface of element  14  and the opposed surface of tube  18 . EPTFE was selected because it contains entrained air, is hydrophobic and is widely accepted for medical applications inside a living body. However, it is contemplated other materials including other materials containing entrained air may be utilized, so long as they would not be harmful if placed inside a living body and would not deteriorate in that environment. The elimination of water backing element  14  and insulation layer  20  results in simpler construction and easier manufacture and assembly of transducer  30 . PTFE is a material which has long been available from DuPont under the trademark Teflon®. With an EPTFE backing in place of a water backing, the power efficiency of the transducer can be improved by about 20% or more, and the internal temperature of the transducer can be reduced from the range of 310° F. to the range of 220° F.  
         [0019]     Since EPTFE sleeve  32  is electrically nonconductive, the electrical conductors of cables  13  which were previously attached to backing element  14  are now connected directly to a conductive area on the inner surface of sleeve  12 . Also, the insulation layer  20  of  FIG. 1  may be eliminated for the same reason. Otherwise, the structure of transducer  30  is identical to that of transducer  10 .  
       EXAMPLE  
       [0020]     Two identical water backed transducers were utilized to test the efficacy of the present transducer construction. One transducer was left unchanged, and on the other, the backing element  14  and insulating layer  20  removed and replaced by sleeve  32  of EPTFE which completely filled the space between transducer  12  and tube  18 . EPTFE sleeve  32  was press fitted onto tube  18  and within sleeve  12 .  
         [0021]     Each transducer was mounted within a brass reflector, placed in a water bath and sonicated at 100 Watts for 60 seconds. During sonication, the power output of the transducer was measured, and at the conclusion of the test, the temperature inside the transducer was measured by a temperature sensor placed within tube  18 . The test was repeated several times for reach transducer.  
         [0022]     The average power output for the water backed transducer was 45.4 Watts, while the average power output for the EPTFE backed transducer was 54.7 Watts. At the same time, the maximum temperature recorded inside the water backed transducer was 307° F., while the maximum temperature recorded inside the EPTFE backed transducer was 219° F.  
         [0023]     There are believed to be a number of reasons for the superior performance of the EPTFE backed transducer. Consideration of these will serve as an effective guide to the selection of alternate insulating materials for the backing element. First of all, the interface between the piezoelectric material of sleeve  12  and EPTFE sleeve (with its entrained air)  32  provides a very effective reflection of ultrasonic energy. However, there is another contribution to the more efficient energy conversion of the EPTFE backed transducer. In the water backed transducer, protrusions  14   a  effectively damped vibration of sleeve  12  wherever they touch it. EPTFE sleeve  32 , on the other hand, is very soft and has no similar deleterious effect on the vibration of sleeve  12 . This accounts, in some part, for the more efficient energy conversion of the EPTFE backed transducer.  
         [0024]     As far as the reduction in core temperature of the transducer is concerned, this is probably accounted for by the presence of the relatively thick sleeve  32  of insulating material. Additional benefits of the EPTFE backed transducer include the replacement of sleeve  20  and backing element  14  with a much simpler construction involving only a sleeve of insulating material, and the elimination of the complications introduced by the use of water inside the transducer.  
         [0025]     In a typical application, tube  18  typically has an outside diameter of approximately 1.14 mm. Transducer  12  might have outside diameter of approximately 1.5-2.5 mm, a wall thickness of approximately 0.1-0.5 mm and a length of approximately 0.5-16 mm. Sleeve  32  would fill the gap between the inside of transducer  12  and tube  18 . Sleeve  32  has a wall thickness in the range of approximately 0.25-1.25 mm. Most preferably, transducer  12  is 6 mm in length, has an outside diameter of 2.44 mm and a wall thickness of 0.116 mm. Transducer  12  may have any outside diameter which is appropriate for its application, with a progressively larger thickness for larger transducers.  
         [0026]      FIG. 3  is a schematic representation of an embodiment of a probe  40  containing a transducer in accordance with the present invention. Probe  40  includes a catheter  52  having a distal end bearing an outer, reflector balloon  54 ; an inner, structural balloon  58 ; and a transducer subassembly  50  in accordance with the present invention. U.S. Pat. No. 6,635,054 and International Publication WO 2004/073505 disclose in more detail various probe structures of this type. The disclosures of U.S. Pat. No. 6,635,054 and International Publication WO 2004/073505 are incorporated herein, in their entirety, by reference. Supporting tube  18  communicates with the interior lumen  53  of catheter  52 . Supporting tube  18  may also extend through the forward wall  59  of balloon  58 . Alternatively, tube  18  may be connected to another tubular structure  60  which extends through forward wall  59  of balloon  58 . Tube  18  may have a lumen to pass device such as a guide wire  62 , or a sensor or pass a fluid such as a contrast medium. Because the tube  18  is continuous with the lumen  53  of catheter  52 , and tube  18  or tubular structure  60  communicates with the forward wall  59 , the device provides a continuous passage. The thermal insulation provided by sleeve  32  ( FIG. 2 ) protects the devices or fluids introduced through tube  18  from the heat generated by the transducer.  
         [0027]     Prior to use, probe  40  would be in a collapsed state, in which both balloons  54  and  58  are collapsed about transducer subassembly  50 . Probe  40  could, for example, be for use in cardiac ablation, in which case it could be inserted over a guide wire, through a sheath which, in accordance with conventional practice, has previously been threaded through a patient&#39;s circulatory system and into the left atrium of the heart. However, there are other known techniques for positioning the probe, including surgical procedures.  
         [0028]     Following that, structural balloon  58  may be inflated by injecting through a lumen of catheter  52  a liquid, such as saline solution, which has an ultrasonic impedance approximating that of blood. Reflector balloon  54  is inflated by injecting through another lumen of catheter  52  a gas, such as carbon dioxide. Owing to the different ultrasound impedance of the two inflation media, the interface between balloons  54  and  58  would then reflect ultrasound waves forward, through the distal portion of balloon  58 .  
         [0029]     Although a preferred embodiment of the invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that many additions, modifications and substitutions are possible without departing from the scope and spirit of the invention.

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