Abstract:
An ablation catheter apparatus with a monopole antenna that is arranged to provide a relatively uniform electric field and a method for using such an ablation catheter apparatus are disclosed. According to one aspect of the present invention, an ablation catheter includes an elongated flexible tubular member that is adapted to be inserted into the body of a patient, and a transmission line that is disposed within the tubular member. The transmission line has a distal end and a proximal end which is arranged to be connected to an electromagnetic energy source. The catheter also includes a monopole antenna with tip section and a body section that includes a distal end and a proximal end. The tip section and the body section are arranged to produce a relatively uniform electric field around the monopole antenna which is sufficiently strong to cause tissue ablation. The proximal end of the body-section of the monopole antenna is arranged to be electrically coupled to the transmission line.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 10/988,028, filed on Nov. 12, 2004, which is a continuation of application Ser. No. 09/321,666, filed May 28, 1999, now issued as U.S. Pat. No. 6,277,113, which applications are incorporated herein in their entireties by this reference. 
     
    
     FIELD OF INVENTION 
       [0002]    The present invention relates generally to ablation catheter systems that use electromagnetic energy in the microwave frequency range to ablate internal bodily tissues. More particularly, the present invention relates to a monopole tip for a catheter that enables distal fire capabilities while enabling a relatively even electromagnetic field to be created at the sides of the monopole tip to facilitate the ablation of cardiac tissue. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    Catheter ablation is a therapy that is becoming more widely used for the treatment of medical problems such as cardiac arrhythmias, cardiac dysrhythmias, and tachycardia. Most presently approved ablation catheter systems utilize radio frequency (RF) energy as the ablating energy source. However, RF energy has several limitations which include the rapid dissipation of energy in surface tissues. This rapid dissipation of energy often results in shallow “burns,” as well as a failure to access deeper arrhythmic tissues. As such, catheters which utilize electromagnetic energy in the microwave frequency range as the ablation energy source are currently being developed. Microwave frequency energy has long been recognized as an effective energy source for heating biological tissues and has seen use in such hyperthermia applications as cancer treatment and the preheating of blood prior to infusions. Catheters which utilize microwave energy have been observed to be capable of generating substantially larger lesions than those generated by RF catheters, which greatly simplifies the actual ablation procedures. Some catheter systems which utilize microwave energy are described in the U.S. Pat. No. 4,641,649 to Walinsky; U.S. Pat. No. 5,246,438 to Langberg; U.S. Pat. No. 5,405,346 to Grundy, et al.; and U.S. Pat. No. 5,314,466 to Stern, et al., each of which is incorporated herein by reference in its entirety. 
         [0004]    Cardiac arrhythmias, which may be treated using catheter ablation, are generally circuits, known as “reentry circuits,” which form within the chambers of the heart. As is known to those skilled in the art, reentry circuits are abnormal electrical pathways that may form in various areas of the heart. For example, reentry circuits may form around veins and/or arteries which lead away from and to the heart. Cardiac arrhythmias may occur in any area of the heart where reentry circuits are formed. 
         [0005]    The catheters used for treatment of cardiac arrhythmias, dysrhythmias, and tachycardia may have a variety of different antenna configurations to create electromagnetic fields used in ablation. Some catheters have antennas that essentially protrude from the distal ends of the catheters. In other words, some catheters have antennas which form the distal tips of the catheters. A monopole antenna is typically configured to form the distal tip of a catheter. 
         [0006]      FIG. 1   a  is a diagrammatic representation of a distal end of a catheter with a monopole antenna at its tip. A distal end  102  of a catheter has a monopole antenna  108  at its tip. As shown, monopole antenna  108  has a rounded shape, and is coupled to a center conductor  112  of a co-axial transmission line  116 . Typically, monopole antenna  108  is formed from a metallic material. Distal end  102  of the catheter may also include electrodes  120 , which may be used for mapping processes, that may be coupled to processing equipment (not shown) using ECG wires  122 . 
         [0007]    Monopole antenna  108  is often arranged to be used in ablating tissue. Center conductor  112  transmits energy, e.g., electromagnetic energy, to monopole antenna  108  to allow an electromagnetic field to be formed with respect to monopole antenna.  FIG. 1   b  is a diagrammatic representation of a monopole antenna, i.e., monopole antenna  108  of  FIG. 1   a , shown with electromagnetic field lines. Electromagnetic field lines  130  generally radiate from monopole antenna  108  in a substantially ellipsoidal pattern. Hence, near sides  134 , “hot spots”  138  of electromagnetic energy are typically formed. Hot spots  138  are generally associated with the highest amounts of electromagnetic energy radiated by monopole antenna  108 . The existence of hot spots  138  causes certain portions of a myocardium of heart, for example, such as those that are substantially contacted by a hot spot to be ablated more than other portions. 
         [0008]    When an ablation procedure is performed using monopole antenna  108 , the depth of cuts formed may not be uniform, since electromagnetic field lines  130  are not uniform. That is the shape, or profile, of electromagnetic field lines  130  are such that when ablation is performed, the depth associated with the ablation may not be even. The lack of even depth in an ablation procedure may cause the ablation, e.g., an ablation in the myocardium of a heart, to be unsuccessful, as all of the cardiac tissue may not be effectively ablated. Hence, the ablation procedure may have to be repeated, which is both time-consuming and inefficient. 
         [0009]    Therefore, what is needed is a monopole antenna structure for use with an ablation catheter that efficiently allows tissue to be ablated. More specifically, what is desired is a monopole antenna structure that is capable of producing a relatively field, e.g., electromagnetic field, a deep lesion, and a microwave power deposition at the tip of a catheter, i.e., a tip-firing catheter. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention relates generally to an ablation catheter with a monopole antenna that is arranged to provide an electric field that is able to produce a deep lesion, e.g., in the myocardium or a heart, and has a tip-firing capability. According to one aspect of the present invention, an ablation catheter includes an elongated flexible tubular member that is adapted to be inserted into the body of a patient, and a transmission line that is disposed within the tubular member. The transmission line has a distal end and a proximal end which is arranged to be connected to an electromagnetic energy source. The catheter also includes a monopole antenna with tip section and a body section that includes a distal end and a proximal end. The tip section and the body section are arranged to produce a relatively uniform electric field around the monopole antenna which is efficiently strong to cause deep tissue ablation. The proximal end of the body section of the monopole antenna is arranged to be electrically coupled to the transmission line. 
         [0011]    In one embodiment, the transmission line is a coaxial cable, which has a center conductor and an outer conductor. In such an embodiment, the proximal end of the monopole antenna is arranged to be electrically coupled to the center conductor. In another embodiment, the body section of the monopole antenna is tapered such that the diameter at the proximal end of the body section of the monopole antenna is smaller than the diameter at the distal end of the body section of the monopole antenna. 
         [0012]    According to another aspect of the present invention, an antenna structure arranged to be used in an ablation catheter has a longitudinal axis, and includes a body section with a first end and a second end, a tip section, and a transition section. The body section is sized such that the axial cross-sectional area about the longitudinal axis of the second end is smaller than the axial cross-sectional area about the longitudinal axis of the first end. The second end is arranged to be electrically coupled to a transmission line, and the body section is shaped to allow a relatively uniform electric field to be formed with respect to the antenna structure. The tip section has a proximal portion that has an axial cross-sectional area about the longitudinal axis which is greater than or approximately equal to the axial cross-sectional area of the first end, and the transition section is disposed between the proximal portion and the first end. 
         [0013]    In one embodiment, the first end has a diameter that is greater than the diameter of the second end, and the proximal portion has a diameter that is greater than or equal to the diameter of the first end. In such an embodiment, the tip section may have a diameter that is less than the diameter of the first end. 
         [0014]    In accordance with still another aspect of other present invention, a microwave ablation catheter includes an elongated flexible tubular member, which has a distal portion, a proximal portion, and a longitudinal catheter axis, and is adapted to be inserted into a vessel in the body of a patient. The microwave ablation catheter also includes a transmission line with a proximal end and a distal end. The transmission line is disposed within the tubular member, and the proximal end of the transmission line is suitable for connection to an electromagnetic energy source. A monopole antenna which is part of the microwave ablation catheter is coupled to the transmission line for generating an electric field sufficiently strong to cause tissue ablation, and includes a frusto-conically shaped emitting surface with an axis that is substantially parallel to the longitudinal catheter axis. In one embodiment, the monopole antenna further includes a rounded distal emitter surface. In such an embodiment, the antenna may also include a trough region between the frusto-conically shaped emitting surface and the distal emitter surface, as well as an encapsulating material that encapsulates the trough and frusto-conically shaped emitting surface such that the trough forms an anchor for the encapsulating material. 
         [0015]    These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0016]    The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
           [0017]      FIG. 1   a  is a diagrammatic representation of a distal end of a catheter with a monopole tip. 
           [0018]      FIG. 1   b  is a diagrammatic representation of a monopole antenna, i.e., monopole antenna  108  of  FIG. 1   a , shown with electromagnetic field lines. 
           [0019]      FIG. 2   a  in a diagrammatic representation of an ablation catheter in accordance with an embodiment of the present invention. 
           [0020]      FIG. 2   b  is a perspective representation of a monopole antenna with a tapered configuration, i.e., monopole antenna  202  of  FIG. 2   a , in accordance with an embodiment of the present invention. 
           [0021]      FIG. 3   a  is a diagrammatic side view representation of a monopole antenna, shown with a contour plot of the magnitude of electric field lines, in accordance with an embodiment of the present invention. 
           [0022]      FIG. 3   b  is a diagrammatic side view representation of a monopole antenna, i.e., monopole antenna  302  of  FIG. 3   a , shown with relative specific absorption rates, in accordance with an embodiment of the present invention. 
           [0023]      FIG. 4  is a diagrammatic cross-sectional representation of a distal end of a catheter which includes a monopole antenna in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0024]    When the electromagnetic field associated with an antenna in an ablation catheter is not uniform, the depth of an ablation formed in cardiac tissue using the catheter is often uneven. Ablation catheters with conventional monopole antennas generally do not emit uniform electric fields. Instead, the contour of electric field lines, as well as hot spots in the electric field around a monopole antenna, are such that ablation of cardiac tissue, as for example in a myocardium of a heart, are often uneven. As a result, the ablation of the tissue may not be successful. 
         [0025]    An ablation catheter that has a monopole antenna which is shaped to enable a substantially uniform field, e.g., electromagnetic or electric field, to be formed around the monopole antenna allows the depth of an ablation of tissue to occur substantially uniformly. In addition, such a monopole antenna allows the catheter to have forward firing, or tip-firing, capabilities. That is, the distal tip of the monopole antenna may also be used to ablate tissue. 
         [0026]    When the depth of an ablation is relatively uniform, i.e., has a substantially uniform depth, an overall ablation process may be more efficiently performed, as it may be unnecessary to repeatedly ablate the same area of tissue to obtain an even depth of ablation. When an overall ablation process is more efficient, in that the time spent performing ablation may be reduced. 
         [0027]    A monopole antenna which includes a tip section and a tapered body section enables hot spots in the electromagnetic field formed around the body section to be substantially eliminated.  FIG. 2   a  is a diagrammatic representation of an ablation catheter with a monopole antenna, which includes a tip section and a tapered body section, in accordance with an embodiment of the present invention. An ablation catheter  180 , which is suitable for use as a microwave ablation catheter, is generally arranged to be introduced into the body of a patient through a blood vessel, e.g., the femoral vein. Catheter  180  may be considered to be an overall elongated, flexible, tube. It should be appreciated that for ease of illustration, catheter  180  has not been drawn to scale. 
         [0028]    Since catheter  180  is arranged to be used within the body of a patient, materials used to form catheter  180  are typically biocompatible materials. Suitable biocompatible materials used to form catheter  180  include, but are not limited to medical grade polyolefins, fluoropolymers, polyurethane, polyethylene, or polyvinylidene fluoride. In one embodiment, a PEBAX resin, which is available commercially from Elf Atochem of Germany, may be used in the formation of catheter  180 . 
         [0029]    Catheter  180  includes a monopole antenna  202  from which an electric field may be emitted to cause ablation. As shown, monopole antenna  202  is located at the distal end of catheter  180 . Monopole antenna  202 , which may be machined from a material such as stainless steel using a mill or a lathe, will be discussed below with reference to  FIG. 2   b . Typically, once catheter  180  is introduced into the body of a patient, catheter  180  is manipulated through a blood vessel and into the heart such that monopole antenna  202  may be positioned within a cardiac chamber in which an ablation procedure is to be performed. 
         [0030]    Catheter  180  also includes electrodes  204  which are positioned on catheter  180  such that they are located proximally with respect to monopole antenna  202 . Electrodes  204  are generally arranged to detect electro-physiological signals from cardiac tissue. Hence, electrodes  204 , which are generally electrode bands, may be used to map the relevant region of the heart, i.e., the portion of the heart with which an ablation procedure is associated, prior to or after an ablation procedure. Electrodes  204  may also be used to aid in positioning catheter  180  during, an ablation procedure. In general, although electrodes  204  may be formed from any suitable material which has biocompatible characteristics, electrodes  204  are typically formed from materials which include, but are not limited to, stainless steel and iridium platinum. 
         [0031]    A handle  205  is often located near a proximal end of catheter  180 , although it should be appreciated that handle  205  is not necessarily included as a part of catheter  180 . Handle  205  is arranged to enable a user, i.e., an individual who is performing an ablation procedure on a patient, to grip and to manipulate catheter  180 . In the described embodiment, a connector  206  is located on catheter  180  such that connector  206  is proximal to handle  205 . Connector  206  is arranged to couple a transmission line (not shown), which is located within catheter  180 , to a power supply, or similar device, that is designed to generate controlled electromagnetic energy. 
         [0032]    As mentioned above, monopole antenna  202  is arranged to provide an electric field, e.g., an electromagnetic field, to allow tissue to be ablated. In the described embodiment, monopole antenna  202  is shaped such that the electric field which is generated is effectively confined to the monopole region associated with monopole antenna  202 . With reference to  FIG. 2   b , a monopole antenna with a tapered body section will be described in accordance with an embodiment of the present invention.  FIG. 2   b  is a perspective representation of monopole antenna  202  of  FIG. 2   a . Monopole antenna  202  includes a body section  208 , an intermediate section  210 , and a tip section  214 . In the described embodiment, body section  208  has a tapered shape, e.g., body section  208  is shaped substantially as a conical structure with no single apex point. That is, body section  208 , which includes an emitting surface, may have a frusto-conical shape. A proximal end  218  of body section  208  generally has the smallest axial cross-sectional area, about a longitudinal axis of monopole antenna  202 , associated with body section  208 . By way of example, the diameter of proximal end  218 , about the longitudinal axis of monopole antenna  202 , is typically smaller than any other diameter, along the same axis, that is associated with body section  208 . 
         [0033]    Intermediate section  210  effectively separates body section  208  from tip section  214 . One purpose of intermediate, or “trough,” section  210  is to allow a material which is used to encase body section  208  to be anchored with respect to monopole antenna  202 . In other words, intermediate section  210  is shaped such that a material which effectively encapsulates body section  208  and, further, at least part of intermediate section  210 , is generally prevented from “peeling away” from intermediate section  210  and body section  208 . The encapsulating material serves as a plug that holds monopole antenna  202  against a catheter, e.g., catheter  180  of  FIG. 2   a . In general, any suitable material may be used to form a plug that essentially encases body section  208 . Such materials include, but are not limited to, Teflon, such as PolyTetraFluoroEthylene (PTFE), and Polyethylene (PE). 
         [0034]    As shown, intermediate section  210  has an axial cross-sectional area that is less than the largest axial cross-sectional area associated with body section  208 , i.e., the axial cross-sectional area associated with a distal end  222  of body section  208 . In one embodiment, since intermediate section  210  and body section  208  have substantially circular cross-sectional areas, the diameter of intermediate section  210  is less than the diameter of distal end  222  of body section  208 . 
         [0035]    Tip section  214  typically includes a distal portion  214   a  and a proximal portion  214   b . Distal portion  214   a  generally has a rounded shape. In the described embodiment, distal portion  214   a  has an approximately hemispherical shape. Proximal portion  214   b  has a substantially cylindrical shape, although it should be appreciated that the shape of proximal portion  214   b  may vary widely. In some embodiments, tip section  214  may include only distal portion  214   a.    
         [0036]    Generally, the dimensions associated with monopole antenna  202  may vary, depending upon the overall configuration of a catheter in which monopole antenna  202  is used. By way of example, the dimensions may vary in order to achieve electric field lines of a particular shape. Typically, body section  208  has a longitudinal length in the range of approximately 0.25 inches to approximately 0.4 inches, e.g., approximately 0.3 inches. The longitudinal length of intermediate section  210  may range from approximately 0.07 inches to approximately 0.10 inches, e.g., the longitudinal length of intermediate section  210  may be approximately 0.09 inches. Finally, the longitudinal length of tip section  214  may range from total length of approximately 0.08 inches to approximately 0.1 inches. In one embodiment, distal portion  214   a  of tip section  214  may have a longitudinal length of approximately 0.06 inches. 
         [0037]    In addition to having a longitudinal length that may vary, monopole antenna  202  has diameters that may also be widely varied. As discussed above, body section  208  may have a tapered shape, e.g., a frusto-conical shape. Accordingly, the diameters along the longitudinal axis of body section  208  will generally vary. For example, the proximal end  218  of body section  208  may have a diameter which ranges between approximately 0.025 inches to approximately 0.04 inches, while the distal end  222  of body section  208  may have a diameter which ranges from approximately 0.06 inches to approximately 0.08 inches. It should be appreciated that the ranges of diameters may vary widely depending upon the requirements of an overall catheter system. 
         [0038]    The diameter of intermediate section  210  may also be widely varied. In general, the diameter of intermediate section  210  may be any suitable diameter that is less than or equal to the diameter of distal end  222  of body section  208 . However, the diameter of intermediate section  210  is preferably less than the diameter of distal end  222  of body section  208 , in order for a plug to be securely formed around body section  208 , as previously mentioned. By way of example, when distal end  222  of body section  208  has a diameter which ranges between approximately 0.6 inches and approximately 0.8 inches, then intermediate section  210  may have a diameter which ranges between approximately 0.04 inches to approximately 0.06 inches. 
         [0039]    Like the other diameters associated with monopole antenna  202 , the diameter associated with tip section  214  may also vary. In the described embodiment, the diameter associated with proximal portion  214   b  is substantially the same as a diameter associated with distal portion  214   a . That is, when proximal portion  214   b  is approximately cylindrical in shape, and distal portion  214   a  is substantially hemispherical in shape, the diameters of proximal portion  214   b  and distal portion  214   a  may be approximately the same. For instance, the diameters may be in the range of approximately 0.08 inches to approximately 0.1 inches, although it should be understood that the diameters may be widely varied. 
         [0040]    A monopole antenna such as monopole antenna  202  may be formed from substantially any conductive material. In general, monopole antennas are preferably formed from materials with relatively high conductivity characteristics. Since catheters which include monopole antennas are typically arranged to be inserted into human bodies, the monopole antennas are further formed from biocompatible materials, or are coated with a conductive biocompatible material, e.g., silver or platinum. 
         [0041]    Monopole antenna  202 , as mentioned above, is shaped to enable a substantially elliptical electromagnetic field to be formed around antenna  202 .  FIG. 3   a  is a diagrammatic side view representation of a monopole antenna, shown with contour lines associated with the magnitude of an associated electric field, in accordance with an embodiment of the present invention. Contour lines  304  are shown with respect to field propagation at ninety degrees of a cycle. As will be appreciated by those skilled in the art, a cycle is a phase shift of 360 degrees. The number of cycles per second will generally vary depending upon the frequency that is being used, which often varies depending upon the needs of a particular system. By way of example, in one embodiment, at a frequency of approximately 2.45 GigaHertz (GHz), the number of cycles per second is approximately 2.45×10 9 . 
         [0042]    For purposes of illustration, representative contour lines  304  of the magnitude of an electric field have been shown, although it should be appreciated that many more contour lines  304  associated with the magnitude of an electric field will generally exist. The magnitude of an electric field generally varies with the distance from monopole antenna  202 . Specifically, the magnitude of an electric field decreases as the distance from monopole antenna  202  increases. For example, the magnitude of the portion of the electric field represented by contour line  304   a  is greater than the magnitude of the portion of the electric field represented by contour line  304   c . In the described embodiment, the output power associated with monopole antenna  202  is approximately one Watt (W), and the magnitude of the electric field represented by contour line  304   a  is approximately 1000 Volts per meter (V/m). In such an embodiment, the magnitude of electric field line  304   c  may be approximately 500 V/m. 
         [0043]    Ablation procedures that are performed with monopole antenna  202  may be more efficient than those performed using a conventional monopole antenna, in that the ablation of tissue is generally more even, e.g., the depth of an ablation made in cardiac tissue may be uniform. Specifically, the tip-firing capabilities of monopole antenna  202 , as well as the deep penetration of the energy which emanates from monopole antenna  202 , may allow for a more efficient treatment of flutters and tachycardias, for example. 
         [0044]    Monopole antenna  202  has an associated specific absorption rate (SAR), as will be understood by those skilled in the art.  FIG. 3   b  is a diagrammatic side view representation of a monopole antenna, i.e., monopole antenna  302  of  FIG. 3   a , shown with a pattern specific absorption rates, in accordance with an embodiment of the present invention. The specific absorption rate associated with an antenna may be expressed as follows: 
         [0000]    
       
         
           
             SAR 
             = 
             
               
                 σ 
                  
                 
                     
                 
                  
                 
                   E 
                   2 
                 
               
               2 
             
           
         
       
     
         [0000]    where σ a is the associated electrical conductivity at a particular frequency, e.g., approximately 2.45 GHz, and E 2  is the square of the magnitude of the electric field. As the magnitude of the electric field varies with distance from monopole antenna  202 , the specific absorption rate also varies. Since the specific absorption rate is a function of the magnitude of the electric field, the specific absorption rate decreases as the distance from monopole antenna  202  increases. 
         [0045]    In the described embodiment, specific absorption rate  354   a  is the highest rate associated with monopole antenna  202 , while specific absorption rate  354   c  is the lowest rate associated with monopole antenna  202 . The pattern of specific absorption rates have been shown as including three rates  354 , it should be appreciated that more rates generally exist although, in some embodiments, fewer rates may be in existence. 
         [0046]      FIG. 4  is a diagrammatic cross-sectional representation of a distal end of a catheter which includes a monopole antenna in accordance with an embodiment of the present invention. A distal end  400  of a catheter includes a monopole antenna  402  which has a tapered body section  408 , an intermediate section  410 , and a tip section  414 . For illustrative purposes, distal end  400  of catheter has not been drawn to scale. In the embodiment as shown, monopole antenna  402  also includes a surface finish  418 , or coating, that covers the exterior of tip section  414 . Surface finish  418  may be formed from a variety of different materials. By way of example, surface finish  418  may be a silver plating. It should be appreciated that in another embodiment, monopole antenna  402  may not include a surface finish. 
         [0047]    In the described embodiment, monopole antenna  402  is coupled to an electromagnetic wave generator that is external to the catheter (not shown) through a coaxial cable  430 . Specifically, a center conductor  432  is electrically coupled to a proximal end of body section  408 . As shown, body section  408  is bored out, e.g., includes a proximal bore  409 , that is arranged to allow center conductor  432  to be electrically coupled to monopole antenna  402 . In order to facilitate coupling of center conductor  432  to body section  408 , center conductor  432  extends past an outer conductor  436 , or a shield, of coaxial cable  430 . A variety-of different methods may be used to couple center conductor  432  to body section  408 . By way of example, center conductor  432  may be coupled to body section  408  using a crimping process. An inner dielectric  434  of coaxial cable  430  serves to separate center conductor  432 , which is arranged to carry electrical current, from shield  436  of coaxial cable  430 . As will be appreciated by those skilled in the art, outer conductor  436  is often used for grounding purposes. Although coaxial cable  430  is arranged to provide power to monopole antenna  402 , it should be appreciated that substantially any transmission line may be used in lieu of coaxial cable  430 . 
         [0048]    A flexible tubing  440 , is effectively an outer sleeve that is formed over coaxial cable  430 . Typically, flexible tubing  440  may be made from any flexible, biocompatible material including, but not limited to, Teflon, polyethylene, and polyurethane. The thickness of flexible tubing  440  may vary widely depending upon the requirements of a particular catheter. By way of example, the thickness of flexible tubing  440  may vary between approximately 0.005 inches and approximately 0.015 inches. 
         [0049]    Electrode bands  444  are often “pressed into” flexible tubing  440  such that electrode bands  444  may make contact with fluids and tissue that are external to the catheter. In general, electrode bands are electrically coupled to an external power supply (not shown) through electrode wires  448  which are located between flexible tubing  440  and co-axial cable  430 . Electrode bands  444  may be used to monitor electrocardiogram signals from a patient during an ablation procedure. As shown, electrode band  444   b , which is the electrode band which is most distally positioned with respect to distal end  400  of catheter, is substantially electrically coupled to outer conductor  436  through wires  462 . Such a connection to outer conductor  436  is generally made as close to the distal end of outer conductor  436  as possible, as will be understood by those skilled in the art. 
         [0050]    In one embodiment, electrode bands  444  may each have a width of approximately 0.004 inches, or approximately 1 millimeter, although the width of each electrode band  444  may vary. As previously mentioned, electrode bands  444  may be formed from substantially any suitable biocompatible, material including, but not limited to, stainless steel and iridium platinum. Typically, the location of electrode bands  444  is such that electrode bands  444  are relatively close to monopole antenna  402 . 
         [0051]    A plug  460 , which is formed around body section  408  and intermediate section  410  of monopole antenna  402 , is arranged to hold monopole antenna  402  with respect to flexible tubing  440 . Such a plug may be molded around at least a portion of monopole antenna  402  in order to hold monopole antenna  402 . As discussed above, plug  460  may be formed from any suitable, preferably biocompatible, material, which is capable of withstanding electromagnetic fields that may be produced using monopole antenna  402 . By way of example, plug  460  may be formed from a material such as Teflon or polyethylene. The configuration of intermediate section  410 , with respect to body section  408  and tip section  414 , is arranged to hold plug  460  securely in place with respect to monopole antenna  402 . 
         [0052]    Although only a few embodiments of the present invention have been described, it should be understood that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the present invention. By way of example, an ablation catheter that includes a monopole antenna which generates a substantially deep electric field with respect to the monopole antenna has been generally described as being a microwave ablation catheter. However, such a monopole antenna may be use with various other catheters including, but not limited, to catheters which operate using radio frequency waves. 
         [0053]    While a monopole antenna has been described as being formed from a material such as stainless steel, it should be appreciated that materials used in the fabrication of a monopole antenna may vary widely. In general, monopole antenna may be formed from substantially any material having a good electrical conductivity. 
         [0054]    The sections of a monopole antenna, namely, the tip section, the intermediate section, and the body section, may take on various shapes without departing from the spirit or the scope of the present invention. By varying the shapes of the different sections, the shape of the electric field which emanates from the monopole antenna may be varied. For example, in one embodiment, the body section of a monopole antenna may not have a tapered shape. In some cases, varying the shapes associated with a monopole antenna may still enable the generated electric field to be substantially uniform. In other cases, varying the shapes may result in the generation of relatively non-uniform electric fields. The generation of relatively non-uniform electric fields may be desirable, for instance, when a monopole antenna is to be used for an ablation procedure that requires a specifically shaped electric field. That is, the tip section, the intermediate section, and the body section of a monopole antenna may be shaped to provide electric fields of particular shapes as required for specific ablation procedures. 
         [0055]    A transmission line, e.g., the center conductor of a co-axial cable, has generally been described as being crimped, or otherwise coupled, to the proximal end of a monopole antenna. It should be appreciated that a transmission line may be electrically coupled to the monopole antenna using various other methods, and at different locations with respect to the monopole antenna. Therefore, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.