Abstract:
The present invention is directed to a high density mapping catheter including a number of shape memory electrode fibers and associated methods of construction ad operation. The invention ensures good electrical contact between a large number of mapping electrodes and cardiac tissue in relation to a number of cardiac tissue approach angles, including head-on approaches. In addition, the invention allows for a reduced range of deflection angles in relation to deployment and retraction of the electrode fibers, thereby reducing resistance to retraction and reducing stress on the fibers and associated concerns regarding patient safety. The catheter of the present invention allows for rapid acquisition of a large amount of mapping data and allows for a variety of different geometries in relation to sweeping of the catheter across the cardiac tissue.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 USC 119 to U.S. Provisional Application No. 60/894,144, entitled, “HIGH DENSITY MAPPING CATHETER,” filed on Mar. 9, 2007, the contents of which are incorporated herein as if set forth in full. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    a. Field of the Invention 
         [0003]    The present invention relates to electrical mapping of a patient&#39;s heart and, in particular, to a catheter that can quickly gather data for high resolution cardiac mapping and associated methodology. 
         [0004]    b. Background 
         [0005]    A number of mapping and navigation options have been developed to enable electrical mapping of a patient&#39;s heart as well as navigation of an instrument, such as an electrode catheter, to a desired site for ablation or other treatment. For example, the EnSite NavX® utility is integrated into the Ensite® Advanced Mapping System by St. Jude Medical, Inc., and provides non-fluoroscopic navigation of conventional electrophysiology catheters. The navigation methodology is based on the principle that when electrical current is applied across two surface electrodes, a voltage gradient is created along the axis between the electrodes. Although any suitable number of electrodes may be utilized, typically six surface electrodes are placed on the body of the patient and in three pairs: anterior to posterior, left to right lateral, and superior (neck) to inferior (left leg). The three electrode pairs form three orthogonal axes (X-Y-Z), with the patient&#39;s heart being at least generally at the center. 
         [0006]    The noted six surface electrodes are connected to the Ensite® Advanced Mapping System, which alternately sends an electrical signal through each pair of surface electrodes to create a voltage gradient along each of the three axes, forming a transthoracic electrical field, Conventional electrophysiology catheters may be connected to the Ensite® Advanced Mapping System and advanced to the patient&#39;s heart. As a catheter enters the transthoracic field, each catheter electrode senses voltage, timed to the creation of the gradient along each axis. Using the sensed voltages compared to the voltage gradient on all three axes, the EnSite NavX® utility calculates the three-dimensional position of each catheter electrode. The calculated position for the various electrodes occurs simultaneously and repeats many times per second (e.g., about 93 times per second). 
         [0007]    The Ensite® Advanced Mapping System displays the located electrodes as catheter bodies with real-time navigation. By tracking the position of the various catheters, the EnSite NavX® utility provides non-fluoroscopic navigation, mapping, and creation of chamber models that are highly detailed and that have very accurate geometries. In the latter regard, the physician sweeps an appropriate catheter electrode across the heart chamber to outline the structures by relaying the signals to the computer system that then generates the 3-D model. This 3-D model may be utilized for any appropriate purpose, for instance to help the physician guide an ablation catheter to a heart location where treatment is desired/required. 
         [0008]    In order to generate an accurate and highly detailed map of a patient&#39;s heart, a large amount of data is required. Accordingly, an electrode catheter may be swept across various surfaces of the heart while obtaining data as described above. In order to accelerate this mapping data acquisition and/or increase the volume of data available for mapping, a number of high-density electrode catheters have been developed or proposed. Generally, these include a number of electrodes in an array in relation to a catheter body so as to substantially simultaneously obtain many mapping data points for a corresponding surface of cardiac tissue proximate to the catheter body. For example, these electrodes may be deployed along the length of a section of the catheter body that has a coil or other three-dimensional configuration so as to provide the desired spatial distribution of the electrodes. Alternatively, the electrodes may be disposed on a number of structural elements extending from a catheter body, e.g., in the form of a basket or a number of fingers. Work continues towards developing a high density mapping electrode catheter that achieves the goal of rapidly gathering mapping information while being safe in operation and simple in construction. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention is directed to a high density mapping catheter including a number of shape memory electrode fibers and associated methods of construction and operation. The invention ensures good electrical contact between a large number of mapping electrodes and cardiac tissue in relation to a number of cardiac tissue approach angles, including head-on approaches. In addition, the invention allows for a reduced range of deflection angles in relation to deployment and retraction of the electrode fibers, thereby reducing resistance to retraction and reducing stress on the fibers and associated concerns regarding patient safety. The catheter of the present invention allows for rapid acquisition of a large amount of mapping data and allows for a variety of different geometries in relation to sweeping of the catheter across the cardiac tissue. 
         [0010]    In accordance with one aspect of the present invention, a high density mapping catheter includes a plurality of thin forwardly extending electrode fibers. In this regard, the catheter includes a catheter body and a number of electrode filaments/fibers that extend from the catheter body. Free ends of these fibers extend forwardly towards the distal tip of the catheter body. Each electrode fiber supports at least one electrode thereon. In one arrangement, such electrodes are disposed on the distal ends of the fibers. 
         [0011]    In one arrangement, each electrode fiber is formed as an elongated body. In such an arrangement, each filament has a proximal end that is attached to the catheter body and a free distal end. In one arrangement, the fibers comprise a substantially cylindrical body in an undeflected state. In such an arrangement, an angle between a long axis of the cylindrical body and the longitudinal axis of the catheter body may be an acute angle. In one arrangement, such an acute angle is between about 30° and about 60°. 
         [0012]    In one arrangement, the distal ends of at least a portion of the electrode fibers extend to an axial location that is beyond the distal tip of the catheter body. In this regard, when the distal tip of the catheter is advanced axially forward, one or more of the distal ends of the electrode fibers may contact patient tissue prior to the distal tip of the catheter body contacting such tissue. In a further arrangement, all the distal ends of the fibers extend beyond the distal tip of the catheter body. Furthermore, in such an arrangement, all the distal ends may be disposed in a substantially common plane. 
         [0013]    In one arrangement, the electrode fibers may be formed of a shape memory fiber having a remembered shape. In such an arrangement, such a shape memory fiber may further include a conductive core, which may function as an electrical pathway for one or more electrodes supported by the electrode fiber. In such an arrangement, the fiber may further include an insulative coating disposed over an outside surface of at least a portion of the conductive core. Furthermore, in such an arrangement, the electrode(s) may be integrally formed with the conductive core. 
         [0014]    In one arrangement, the diameter of the catheter body is at least five times the diameter of each individual electrode fibers. In a further arrangement, the diameter of the catheter body is at least 10 times the diameter of such fibers. Correspondingly, each individual fiber may be no greater than about 0.006 inches in diameter or no greater than about 0.004 inches in diameter. In a further arrangement, an electrode disposed on the distal ends of the fibers may have a diameter that is greater than the diameter of the fiber supporting the electrode. 
         [0015]    The fibers may be spaced about the circumference of the electrode body. In this regard, such spacing may be random or predetermined. In one arrangement, the plurality of fibers are disposed in at least three axial rows disposed around the circumference of the catheter. In any arrangement, the individual fibers may be staggered to reduce the likelihood of shorting when the fibers are deflected. Electrode fibers may in one arrangement each have a common length. In another arrangement, different electrode fibers attached to the catheter body may have different lengths. 
         [0016]    In accordance with one aspect of the present invention, a high density mapping catheter is provided that utilizes thin electrode fibers. The catheter includes a catheter body and a number of fibers extending from the catheter body. The fibers have a width, along at least a portion of the length thereof that is no more than about 0.006 of an inch. In one implementation, fibers having a width of about 0.002 of an inch are utilized. At least one electrode is supported on each of the fibers for use in acquiring mapping information. For example, an electrode may be disposed at the tip of the fiber. In one embodiment, the fibers are formed from conductive core shape memory alloy wires. The electrode can be formed as a ball of the core material at the end of the fiber. 
         [0017]    In accordance with another aspect of the present invention, high density mapping catheter includes a large number of electrode fibers. More specifically, the catheter includes a catheter body and at least about 16 electrode fibers extending from the catheter body. Each of the electrode fibers includes at least one electrode for use in acquiring mapping information. In this manner, a large amount of mapping information can be rapidly acquired, and mapping information can be acquired in connection with a variety of catheter/tissue geometries. 
         [0018]    In accordance with yet another aspect of the present invention, different length electrode fibers are used in connection with a high density mapping catheter. The catheter includes a catheter body and a number of mapping electrode elements extending from the catheter body, where each of the elements is formed from a conductive core shape memory fiber. The elements include a first element and a second element where the first element has a length different than that of the second element. For example, such differing lengths may allow for a desired spatial configuration of the tip electrodes of the various fibers when unconstrained. 
         [0019]    In accordance with another aspect of the present invention, a method is provided for use in constructing a high-density mapping catheter. The method involves providing a shape memory fiber with a conductive core. An end portion of the shape memory material of the fiber is then stripped back to expose the conductive core. The exposed portion of the conductive core can then be melted to form a generally spherical tip electrode. For example, the core may be melted by a laser or by exposure to another heat source. This allows for simple construction of electrode fibers having an enlarged rounded tip. Such a tip shape is desirable to avoid puncturing tissue and to enhance visibility of the tip electrodes in relation to various visualization modalities. 
         [0020]    In accordance with a further aspect of the present invention, a method is provided for use in mapping cardiac tissue. The method includes the steps of: providing an electrode catheter, including a catheter body with a number of electrode elements extending therefrom, where each of the element is formed from a shape memory fiber having a conductive core, and the electrode catheter further includes a sheath; introducing the electrode catheter into a chamber of a patient&#39;s heart to be mapped; extending the catheter body from the sheath such that the mapping electrode elements extend from the catheter body in a mapping configuration; and sweeping the mapping electrode elements across a cardiac surface. The noted method allows for acquisition of a large volume of mapping information in a short time. 
         [0021]    In accordance with another aspect of the present invention, a further method for use in mapping cardiac tissue is provided. The method involves providing an electrode catheter including a catheter body having a tip electrode disposed on a distal end thereof and a number of mapping electrode elements extending from the catheter body. Each of the mapping electrode elements is formed from a shape memory fiber having a conductive core. The method further involves operating a number of the mapping electrode elements, disposed circumferentially around the catheter tip electrode to obtain position information, and substantially simultaneously operating the catheter tip electrode to perform a desired medical procedure. For example, the catheter tip electrode may be a mapping electrode, and the medical procedure may involve mapping using the catheter tip electrode and the mapping electrode elements. Alternatively, the catheter tip electrode may be an ablation electrode, and the desired medical procedure may be an ablation procedure. In this regard, the mapping electrode elements may be used to guide the ablation electrode to the desired site or locus of ablation points. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a schematic diagram of a navigation and mapping system in accordance with the present invention. 
           [0023]      FIG. 2  illustrates a catheter constructed in accordance with the present invention being introduced into a patient&#39;s heart. 
           [0024]      FIG. 3  illustrates a display provided by a navigation and mapping system in accordance with the present invention. 
           [0025]      FIGS. 4-6F  illustrate various embodiments of a high density mapping catheter in accordance with the present invention. 
           [0026]      FIGS. 7A-7C  illustrate operation of a high density mapping catheter in accordance with the present invention. 
           [0027]      FIGS. 8A-8C  illustrate construction of an electrode fiber for use in a high density mapping catheter in accordance with the present invention. 
           [0028]      FIGS. 9A-9C  show a high density mapping catheter with curved electrode fibers in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0029]      FIG. 1  presents a schematic of one embodiment of a medical navigation/visualization system  5 . The medical navigation/visualization system  5  will be briefly addressed herein, as it is one such system that may utilize the mapping electrode functionality that will be addressed in detail below. The medical navigation/visualization system  5  is also discussed in detail in U.S. Patent Application Publication No. US 2004/0254437, that is entitled “METHOD AND APPARATUS FOR CATHETER NAVIGATION AND LOCATION AND MAPPING IN THE HEART,” that published on Dec. 16, 2004, that is assigned to the assignee of this patent application, and the entire disclosure of which is incorporated by reference in its entirety herein. 
         [0030]    The patient  11  is only schematically depicted as an oval for clarity. Three sets of surface or patch electrodes are shown as  18 ,  19  along a Y-axis; as  12 ,  14  along an X-axis; and  16 ,  22  along a Z-axis, Patch electrode  16  is shown on the surface closest to the observer, and patch electrode  22  is shown in outline form to show its placement on the back of patient  11 . An additional patch electrode, which may be referred to as a “belly” patch, is also seen in the figure as patch electrode  21 . Each patch electrode  18 ,  19 ,  12 ,  14 ,  16 ,  22 ,  21  is independently connected to a multiplex switch  24 . The heart  10  of patient  11  lies between these various sets of patch electrodes  18 ,  19 ,  12 ,  14 ,  16 ,  22 . Also seen in this figure is a representative catheter  13  having a number of electrodes  17 . The electrodes  17  may be referred to as the “roving electrodes” or “measurement electrodes” herein. In the embodiments described below, many electrodes on fiber elements are used for high-density mapping. It should be appreciated that in use the patient  11  will have most or all of the conventional  12  lead ECG system in place as well, and this ECG information is available to the system although not illustrated in the figure. 
         [0031]    Each patch electrode  18 ,  19 ,  12 ,  14 ,  16 ,  22 ,  21  is coupled to the switch  24 , and pairs of electrodes  18 ,  19 ,  12 ,  14 ,  16 ,  22  are selected by software running on computer system  20 , which couples these electrodes  18 ,  19 ,  12 ,  14 ,  16 ,  22  to the signal generator  25 . A pair of electrodes, for example electrodes  18  and  19 , may be excited by the signal generator  25  and they generate a field in the body of the patient and the heart  10 . During the delivery of the current pulse, the remaining patch electrodes  12 ,  14 ,  16 ,  22  are referenced to the belly patch electrode  21 , and the voltages impressed on these remaining electrodes  12 ,  14 ,  16 ,  22  are measured by the analog-to-digital or A-to-D converter  26 . Suitable lowpass filtering of the digital data may be subsequently performed in software to remove electronic noise and cardiac motion artifact after suitable low pass filtering in filter  27 . In this fashion, the various patch electrodes  18 ,  19 ,  12 ,  14 ,  16 ,  22  are divided into driven and non-driven electrode sets. While a pair of electrodes is driven by the signal generator  25 , the remaining non-driven electrodes are used as references to synthesize the orthogonal drive axes. 
         [0032]    The belly patch electrode  21  is seen in the figure is an alternative to a fixed intra-cardiac electrode. In many instances, a coronary sinus electrode or other fixed electrode in the heart  10  can be used as a reference for measuring voltages and displacements. All of the raw patch voltage data is measured by the A-to-D converter  26  and stored in the computer system  20  under the direction of software. This electrode excitation process occurs rapidly and sequentially as alternate sets of patch electrodes  18 ,  19 ,  12 ,  14 ,  16 ,  22  are selected, and the remaining members of the set are used to measure voltages. This collection of voltage measurements may be referred to herein as the “patch data set”. The software has access to each individual voltage measurement made at each individual patch electrode  18 ,  19 ,  12 ,  14 ,  16 ,  22  during each excitation of each pair of electrodes  18 ,  19 ,  12 ,  14 ,  16 ,  22 . 
         [0033]    The raw patch data is used to determine the “raw” location in three spaces (X, Y, Z) of the electrodes inside the heart  10 , such as the roving electrodes  17 . The patch data set may also be used to create a respiration compensation value to improve the raw location data for the locations of the electrodes  18 ,  19 ,  12 ,  14 ,  16 ,  22 . 
         [0034]    If the roving electrodes  17  are swept around in the heart chamber while the heart  10  is beating, a large number of electrode locations are collected. These data points are taken at all stages of the heartbeat and without regard to the cardiac phase. Since the heart  10  changes shape during contraction, only a small number of the points represent the maximum heart volume. By selecting the most exterior points, it is possible to create a “shell” representing the shape of the heart  10 . The location attribute of the electrodes within the heart  10  are measured while the electric field is impressed on the heart  10  by the surface patch electrodes  18 ,  19 ,  12 ,  14 ,  16 ,  22 . 
         [0035]      FIG. 2  shows a catheter  13 , which may be a high-density mapping catheter, as described in more detail below, in the heart  10 . The catheter  13  has a tip electrode  51  and additional electrodes  52 . Since these electrodes  51  and  52  lie in the heart  10 , the location process detects their location in the heart  10 . While they lie on the surface and when the signal generator  25  is “off”, each patch electrode  18 ,  19 ,  12 ,  14 ,  16 ,  22  ( FIG. 1 ) can be used to measure the voltage on the heart surface. The magnitude of this voltage, as well as the timing relationship of the signal with respect to the heartbeat events, may be measured and presented to the cardiologist through the display  23 . The peak-to-peak voltage measured at a particular location on the heart wall is capable of showing areas of diminished conductivity, and which may reflect an infracted region of the heart  10 . The timing relationship data are typically displayed as “isochrones”. In essence, regions that receive the depolarization waveform at the same time are shown in the same false color or gray scale. 
         [0036]      FIG. 3  shows an illustrative computer display from the computer system  20 . The display  23  is used to show data to the physician user and to present certain options that allow the user to tailor the system configuration for a particular use. It should be noted that the contents on the display  23  can be easily modified and the specific data presented is only of a representative nature. An image panel  60  shows a geometry of the heart chamber  62  that shows “isochrones” in false color or grayscale together with guide bar  64  to assist in interpretation. In this hypothetical image, the noted mapping methodology has been used with a high-density catheter to create a chamber representation that is displayed as a contoured image. 
         [0037]    The guide bar  64  is graduated in milliseconds and it shows the assignment of time relationship for the false color image in the geometry. The relationship between the false color on the geometry image  62  and the guide bar  64  is defined by interaction with the user in panel  66 . As shown, the display may also provide traces and other information related to the ECG electrodes, mapping electrodes and reference electrodes, as well as other information that may assist the physicians. 
         [0038]    As noted above, a significant amount of data is required to generate a detailed image of the cardiac tissue of interest. In order to gather adequate data more quickly, it is desirable to provide a high density mapping electrode catheter having a plurality of electrodes. Once such catheter in accordance with the present invention is illustrated in  FIGS. 5A and 5B . The illustrated catheter  500  includes a catheter body or shaft  502  having an electrode tip  508  disposed at a distal end thereof. The catheter  500  further includes a number of mapping electrode fibers  504  extending from the catheter shaft  502 . Each of the illustrated mapping electrode fibers  504  terminates in a tip electrode  506 . The electrodes  506  and  508  can be used to map cardiac tissue, as discussed above. More specifically, a physician can sweep the electrodes  506  and  508  across tissue to be mapped. In this regard, a large volume of mapping information can be obtained quickly due to the large number of electrodes  506  and  508  that can be maintained in contact with the tissue as the catheter  500  is swept across the tissue. 
         [0039]    As will be described in more detail below, each of the mapping electrode fibers  504  may be formed from a shape memory fiber with a conductive core. For example, the fibers may be formed from a nickel titanium shaped memory fiber such as Nitinol with a conductive metallic core such as platinum. In addition, the fibers may be coated with an insulating material, e.g., Polyimide, to prevent shorts. The conductive core of the illustrated fibers  504  serves as the electrical pathway for the tip electrodes  506 . In addition, the tip electrodes  506  may be formed, as discussed below, by melting an exposed section of the conductive core. Alternatively, the tip electrodes may be formed separately and then tightly secured to the fibers. 
         [0040]    Each of the electrode fibers  504  may be threaded through an inner lumen of the catheter shaft  502 . The fibers  504  then extend through holes formed in the catheter shaft  502  at the desired location. As is well known, shape memory materials such as Nitinol can be processed to remember a desired shape. When the shape memory materials are deflected from this remembered shape, the shape memory properties of the material tend to return the material to the remembered shape. In this case, the fibers  504  are processed to extend outwardly and forwardly from the catheter shift when unconstrained. The fibers may be bonded to the shaft  502  at the openings or may be maintained in a substantially fixed relationship with respect to the shaft  502  due to the configuration of the fibers  504 . In one construction implementation, platinum core Nitinol fibers with a Polyimide coating are threaded through the inner lumen of the catheter shaft  502 . The distal ends of the fibers are then pulled through openings in the catheter shaft, and a desired length of the fiber is pulled through the opening. The fibers are then processed to remember a particular configuration in relation to the angle formed between the catheter shaft  502  and the extending fibers  504 , as will be discussed in more detail below. Thereafter, a first length of the Polyimide coating and a second length of the Nitinol material are stripped from the end of the fibers to expose a portion of the platinum core. This platinum core is then melted to form a general spherical electrode tip  508 . It will be appreciated that other production sequences are possible. For example, the electrodes need not be integrally formed. 
         [0041]    Generally, the catheter shaft  502  will have a diameter and stiffness that is significantly greater than the diameter and stiffness of the individual fibers  504 . For instance, the catheter shaft  502  may be a 5 or 7 French (i.e., 0.065 in. or 0.092 in.) catheter. In such embodiments, the catheter shaft may have a diameter that is at least five to ten times (or more) the diameter of the individual fibers. Such a difference in the relative sizes of the fibers  504  and the catheter shaft  502  may allow the fibers  504  to readily deflect when they are moved (e.g., brushed) over an internal tissue surface without significant deflection of the catheter shaft. For instance, each individual fiber may have a buckle strength (e.g., where bending is initiated) of no more than about 5 grams and more preferably no more than about 1-2 grams. Use of such low buckling strength allows the ends of the fibers  504  to readily conform to a tissue surface without significantly deflecting or otherwise penetrating the tissue surface. In contrast, when the catheter shaft contacts such an internal tissue surface, the stiffness of the shaft alerts an operator (e.g., physician) that the catheter shaft is in contact with patient tissue. 
         [0042]    The inner lumen of the catheter shaft  502  may also be used to thread wiring for the tip electrode  508 . In addition, for certain procedures, it may be desired to irrigate the electrodes  506  and/or  508  with saline solution, for example, to prevent undesired heating or clotting. A lumen for such irrigation fluid may be formed within catheter shaft  502  (which can include openings to allow for flow of the irrigation fluid), or the irrigation fluid may be delivered via a separate lumen associated with other structure of the catheter. 
         [0043]    The tip electrode  508  can be any of various types of electrode tips including an ablation tip or a mapping tip. The illustrated electrode tip  508  is a mapping tip, as best shown in  FIG. 4 . The mapping tip  508  is divided into a number of electrically isolated sections  510 , in this case, defining four quadrants. Because the sections  510  are electrically isolated, independent positioning signals can be obtained with regard to each of the sections  510 . In this manner the signals from the sections  510  can be processed to define references, e.g., North, South, East and West, which are useful in guiding movement of the catheter during a medical procedure. It will be appreciated in this regard that it may be useful to press the catheter tip directly into cardiac tissue in a head-on configuration. In this regard, it is advantageous to configure the electrode fibers  504  in a forwardly extending configuration, as illustrated in  FIGS. 5A and 5B , so as to obtain positioning data from a number of electrodes that are circumferentially disposed in relation to the tip electrode  508 . Similar advantages are obtained in relation to guidance of the tip electrode in ablation applications. 
         [0044]    While the catheter  500  of  FIGS. 4-5B  thus represents an advantageous implementation of the present invention, it will be appreciated that many other implementations are possible. Some examples in this regard are illustrated in  FIGS. 6B-6F . Referring first to  FIG. 6A , the illustrated catheter  600  includes a catheter shaft  602  having an electrode tip  608  at a distal end thereof. In this case, the catheter  600  includes four mapping electrode fibers  604  formed from conductive core shape memory fibers, as described above. When unconstrained, each of the electrode fibers  604  extends outwardly and forwardly from the catheter shaft  602  so as to define an angle θ therebetween. A number of factors may be considered in determining a value of θ for a particular application. Some of these factors include the following: 
         [0045]    1. The angle θ may be selected to provide a desired lateral spacing of the electrode tips  606  for a given length of the electrode fibers  604  extending from the shaft  602 ; 
         [0046]    2. The angle θ is preferably greater than zero but less than 90 degrees in order to provide the desired forwardly extending configuration; and 
         [0047]    3. The angle θ may be selected to allow the fibers  604  to be retracted within a sheath and extended therefrom without undue resistance or stress on the fibers  604 . 
         [0048]    It will be appreciated that other factors may be considered in this regard. In the illustrated embodiment, the angle θ is preferably between about 30 degrees and 60 degrees, for example, about 45 degrees. It will be appreciated that different angles may be used for different fibers if desired. 
         [0049]      FIG. 6B  shows an embodiment of a catheter  610  where a larger number of fibers  614  extend from the catheter shaft  612 . In addition, the illustrated fibers  614  are configured in a number of rows at different distances from the distal end of the shaft  612 . The fibers  614  in adjacent rows may be staggered so as to reduce the likelihood of shorts due to contact between electrode tips  616 S. In the embodiment of  FIG. 6B  (as well as that of  FIG. 6A ), the tip electrodes  616  are arranged in a generally planar configuration slightly forward of the tip electrode  618  when unconstrained. It will thus be appreciated that the fibers  614  of different rows have different lengths. Such a configuration may be desirable in order to promote good contact by as many tip electrodes  616  as possible in relation to a head-on approach to cardiac tissue. That is, in connection with axial advancement of the shaft  612  towards cardiac tissue, it is expected that the tip electrodes  616  will first come into contact with the tissue. As advancement of the shaft  612  progresses, the fibers  616  deflect slightly to allow contact of the tip electrode with the tissue. Due to the shape memory properties of the fibers  614 , the tip electrode  616  will then be urged into good contact with the tissue and can accommodate a range of tissue contours. In addition,  FIG. 6B  also shows use of an optional webbing  613  that extends between adjacent fibers. Such webbing may be formed of a thin elastomeric material and provides a redundancy means for retaining an electrode fiber connected to the catheter shaft  612  in the event that the proximal end of the fiber  614  were to become disconnected from the catheter shaft  612 . 
         [0050]      FIG. 6C  shows a further alternative embodiment of a catheter  620  in accordance with the present invention. In this case, a number of electrode fibers  624  extend from the catheter shaft  622  at different positions along the length of the catheter shaft  622 . Again, fibers  624  of adjacent rows may be staggered, as discussed above. However, in this case, the tip electrodes  626  do not define a planar configuration. Rather, some of the tip electrodes  626  extend beyond the tip electrode  628  of the catheter shaft  622 , but others do not. Thus, the illustrated catheter  620  provides good mapping electrode contact for head-on approaches to cardiac tissue but also provides good contact in cases of dragging the catheter  620  across cardiac tissue with a side surface of the shaft  622  laying on the cardiac tissue as may be desired or otherwise occur. Moreover, in this configuration, there is a reduced likelihood of shorts due to contact between electrode tips  626 . It should be noted that any such shorts are not hazardous as the tip electrodes  626  are essentially receiving electrodes. Moreover, such shorts can be readily recognized and disregarded by the mapping processing logic. Nonetheless, avoiding such shorts enhances the amount of data that can be acquired. 
         [0051]    In certain embodiments described above, the mapping tip electrodes were shown and described as defining a planar configuration when unconstrained. In some cases, a different special configuration may be desired. For example, when the catheter is expected to be deployed against a concave cardiac wall surface, a complementary spatial configuration (i.e., convex) of the tip electrodes may be desired. Conversely, when it is expected that the catheter will be deployed against a convex surface, a concave special configuration of the tip electrodes may be desired.  FIGS. 6D and 6E  illustrate concave and convex configurations of the tip electrodes  636  and  646  in this regard. 
         [0052]    In connection with certain embodiments above, the mapping electrode fibers have been described as being configured in rows in relation to the length of the catheter shaft. It will be appreciated that it is unnecessary to deploy the electrode fibers in rows. This is illustrated in  FIG. 6F . There, the illustrated catheter  650  includes a number of electrode fibers  654  terminating in fiber end  656 . The illustrated fibers  654  extend from the catheter shaft  652  at various locations along the shaft  652 , but they are not arranged in rows defined by a common location along the length of the shaft  652 . Similar to certain embodiments above, some of the tip electrodes  656  extend beyond the tip electrode  658 , but others do not. Moreover, different ones of the fibers  654  may extend different lengths from the shaft  652 . 
         [0053]    As a further alternative, the electrode fibers may be cured rather than straight. This is generally shown in  FIGS. 9A-9C . The illustrated catheter includes a core  900  with a number of curved electrode fibers  904  extending therefrom. Each of the electrode fibers  904  terminates in a tip electrode  9 - 6  as discussed above. The catheter is delivered to the procedure site in an introducer or sheath  902 . 
         [0054]    In the illustrated embodiment, each of the electrode fibers  904  has a slightly convex curve. When the core  900  is retracted into the sheath  902 , as shown in section  9 B, the tip electrodes extend inwardly from the sheath  902 . This is best seen in the enlarged view of  FIG. 9C . This reduces concerns about the enlarged tip electrode  906  snagging on the end of the sheath  902 . 
         [0055]    As discussed above, the electrode fibers preferably extend outwardly and forwardly in relation to the catheter shaft. A number of advantages associated with this geometry were noted above. A further advantage is illustrated with reference to  FIGS. 7A-7C , which also illustrate the operation of the mapping catheter. As shown in  FIG. 7A , as the catheter  700  is threaded through a vessel of a patient to the patient&#39;s heart, the catheter shaft  702  and electrode fibers  704  may be in a retracted configuration in relation to a sheath  706 . It will be appreciated that this provides a compact profile, which facilitates passage of the catheter through the patient&#39;s vessel. Once the catheter has reached the desired site for medical procedure, the catheter shaft  702  can be advanced in relation to the sheath  706 , as shown in  FIG. 7B . 
         [0056]    Once the electrode fibers  704  extend beyond the end of the sheath  706  and are unconstrained, they spring into the deployed configuration due to the operation of the shape memory alloy. When the procedure is completed, the catheter shaft  702  can be retracted back into the sheath  706 , as shown in  FIG. 7C . As this occurs, the electrode fibers  704  deflect and are constrained by the sheath  706 . It will be appreciated that the forwardly extending configuration of the fibers  704  facilitates the deployment and retraction of the catheter shaft  702 , as shown in  FIGS. 7A-7C . In particular, the forwardly extending configuration reduces the resistance of the fibers to retraction of the shaft  702 . Moreover, the angular range of deflection associated with advancement and withdraw of the shaft  702  in relation to the sheath  706  is minimized. This reduces stress to the fibers  704 . 
         [0057]      FIGS. 8A-8C  illustrate a process for forming an electrode fiber as utilized in the various embodiments described above. In particular, it is desirable to provide an enlarged, generally spherical tip electrode in connection with the electrode fibers. This tip electrode configuration has a number of advantages. First, it is desirable to avoid puncturing of the cardiac tissue in connection with contact by the mapping electrodes. The enlarged and rounded configuration of the tip electrodes in this regard provides a larger surface contact area and reduces the pressure on and likelihood of puncturing any cardiac tissue contacted. In addition, it is desirable to enhance the visibility of the tip electrodes, both on the mapping display and in connection with any fluoroscopic images obtained in connection with the procedure. The enlarged tip electrode improves impedance and, therefore, visibility with respect to the electrical navigation system. The increased cross-section also improves visibility with respect to the fluoroscopic images. 
         [0058]    Referring to  FIGS. 8A-8C , the electrode fibers may be formed from commercially available conductive core shape memory fibers. For example, the electrode fibers may be formed from platinum core nickel titanium fibers. Such a commercially available fiber is illustrated in  FIG. 8A . The fiber  800  includes a conductive core  802  that may be formed, for example, from a metallic conductor such as platinum. The core is surrounded by a tube of shape memory alloy material  804  such as a nickel-titanium material. An insulating coating  806  may be provided around the shape memory alloy  804  (which is also conductive). 
         [0059]    To form the electrode fiber, the shape memory alloy material  804  and insulative coating  806  are stripped back from the distal end  808  of the fiber  800 . More specifically, the shape memory material is stripped back a distance L 1 , and the insulating coating  806  is striped back a distance L 2  that is greater than the distance L 1 . This leaves a length of L 3  where the shape memory material  804  is exposed. In one embodiment, the distance L 3  is between about 0.020 and 0.060 of an inch, for example, 0.040 of an inch. 
         [0060]    The exposed core  802  is then melted to form a generally spherical tip electrode  810 , as shown in  FIG. 8C . For example, a laser may be used to melt the core, or the core may be exposed to another heat source. The result is a tip electrode  8110  that has a diameter or width w 2  that is greater than the width w 1  of the fiber  800 . In this regard, the fiber preferably has a width w 1  of between about 0.002 to 0.006 of an inch, for example, 0.002 of an inch. The tip electrode  810  has a width w 2  of between about 0.003 and 0.012 of an inch, for example, 0.006 of an inch. This fiber, in combination with the geometries described above, provides a suitable stiffness or resistance to retraction of the catheter shaft into the sheath. That is, there is not undue resistance or stress on the electrode fibers. 
         [0061]    The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other embodiments and with various modifications required by the particular application(s) or use(s) of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.