Patent Publication Number: US-6663625-B1

Title: Radio-frequency based catheter system and hollow co-axial cable for ablation of body tissues

Description:
The present application is a continuation-in-part of U.S. patent application Ser. No. 09/211,188, filed Dec. 14, 1998, U.S. Pat. No. 6,190,382. 
    
    
     BACKGROUND/FIELD OF INVENTION 
     This invention relates generally to radio-frequency (“RF”) powered medical apparatus and ablation of biological tissues. More particularly, this invention concerns catheter-based RF antenna for ablating biological tissues within the body vessel of a patient and for the treatment of cardiac arrhythmias. 
     In recent years medical devices have gained significant acceptance in the medical community as an important treatment modality for heart diseases and other serious ailments, which were traditionally remedied by medication or surgical operation. Two fundamental trends have emerged in the treatment of cardiac diseases. The first has been the shift from open-heart surgical procedures to less invasive and less expensive catheter-based treatments, which are safer and less debilitating. 
     The second trend is represented by the shift from the use of anti-arrhythmic drugs to minimally invasive catheters or other device-based therapies to palliate incurable arrhythmias. For example, automatic cardioverter-defibrillator are routinely implanted in patients with lethal ventricular arrhythmias to reduce the likelihood of sudden death. Thus, radio-frequency (RF”) catheter ablation is now being performed in large number of patients suffering from cardiac arrhythmias. 
     Despite these advances in technology, atrial fibrillation (“AF”) remains a significant challenge. AF, a rapid irregular rhythm in the atria or upper chambers of the heart induced by non-uniformed electrical pulses, represents a leading cause of stroke and heart attack and a major health care burden. To date, the most effective surgical procedure for the treatment of AF has been the Maze procedure undertaken in “open-heart” surgery. In the Maze procedure, incisions are made along pre-determined lines exterior of the atrium, which are then sutured together. As healing develops, scars are formed along the incision lines thereby forming barriers to the conduction of electrical impulses. By creating such barriers, AF can no longer be sustained and regular heart rhythm is restored. However, the Maze procedure has not been widely adopted due to the morbidity and mortality associated with open-heart surgery, which involves the opening of the chest cavity and cutting of the chest bones. 
     One new approach to mimic the Maze operation is represented by catheter-based radio-frequency ablation technique, wherein, instead of surgical incisions, a catheter-electrode is applied to destroy or ablate the heart tissues inside the atrial chamber. The catheter-electrode is passed through the artery for access to the atrium, as commonly practiced in the medical field. Within the atrium, the tip of the catheter-electrode is positioned, usually with the aid of x-ray or fluoroscopic means, and is brought into contact with the heart tissue at a desired location or spot where ablation is required. At this spot, the tissue is destroyed by resistive heating generated from the catheter-electrode. Thereafter, the catheter-electrode is re-positioned to the next spot for ablation. A series of spot ablations thus mimics the lineal lesions as accomplished under the Maze procedure against the conduction of electrical impulses. 
     Existing catheter-based ablation procedures are recognizably less intrusive than “open-heart” surgery. In addition, during the ablation, disruption of cardiovascular function is reduced. However, a successful catheter-based radio-frequency ablation procedure requires the ablation of tissue spots within the spatial or proximity tolerance between adjacent spots, usually less than 2 millimeters, to prevent the passage of electrical impulses. In that connection, the task for the precise placement of the catheter-electrode represents a critical element of a successful procedure. 
     A major drawback of such existing procedures is in the time-consuming task in positioning the catheter-electrode at the desired ablation spots within the atrium while the heart chamber muscles are pulsating. Movements of atrial wall or the heart muscles often render accurate placement of the catheter-electrode difficult, and slippage of the catheter-electrode tends to occur thereby damaging portions of the atrium where ablation is not desired. As a result, placement of the catheter based RF ablation cannot be efficiently accomplished, and prolonged procedure time, in excess of 12 hours, can be expected. Further, during the procedure, x-ray or other irradiating means are routinely employed for locating and positioning the catheter-electrode, which dictates the use of heavy lead protective gear by the electro-physiologist. As a result, such inconvenience is often amplified by the prolonged procedure time, which detracts from the use of catheter-based electrode as an efficient means for tissue ablation. 
     To minimize the risk of slippage, for example, in U.S. Pat. No. 5,741,249, a catheter-based microwave antenna is disclosed wherein a distal tip is incorporated into the antenna to anchor it to the atrial wall. However, while this design reduces the likelihood of antenna or catheter-electrode slippage during each ablation step, it does not eliminate the consuming task to secure precise placement of the antenna along the desired ablation path for each ablation step. Thus after each ablation step, the antenna has to be re-positioned and anchored precisely at the next spot which must be located within the spatial or proximity tolerance on the ablation path as referenced above. 
     Accordingly, effective treatments for atrial fibrillation with catheter ablation will require the creation of long or overlapping lineal or curvilineal ablation lesions on the inner surface of the atrium. These lesions can then act as barriers to the conduction of electrical impulses, thus preventing atrial fibrillation. 
     It is also recognized that a critical requirement for the effective catheter-based ablation of atrial fibrillation is the ability to stabilize and anchor the catheter and microwave antenna inside the atrial chambers. New catheter ablation systems, preferably capable of producing long or overlapping lineal or curvilineal ablation lesions, are required for the development of minimally invasive catheter-based curative procedures for atrial fibrillation. 
     The present invention provides a design of such a catheter system, which can be used not only for atrial fibrillation but for ablation of biological tissues in other body vessels. The catheter system contains stabilizing and anchoring mechanisms employing monorail and looped antenna guide, sensors for monitoring different parameters during ablation, and handle with control slides for easy steering and manipulation of the catheters. 
     SUMMARY OF THE INVENTION 
     According to the present invention, an improved radio-frequency catheter system is provided for ablating biological tissues of a body vessel, including the atrium of a patient. The catheter system comprises a catheter that is adaptable for insertion into the body vessel and a deployable antenna guide disposed within the catheter lumen. A deployable radio-frequency antenna, together with a hollow co-axial cable, is provided at the distal portion of the catheter to receive and transmit radio-frequency energy for tissue ablation. In a representative embodiment of the invention, the antenna includes a helical coil and has an axial passageway to accommodate the antenna guide, which, upon deployment prescribes the ablation pathway of the antenna for tissue ablation. The antenna guide includes elongated portions which are secured to control slides for positioning and deployment control. The antenna guide is deployable within a body vessel to form a loop configuration that is conformable to the contour of the body vessel. Alignment of the loop with the desired tissue ablation pathway is facilitated with the use of radio-opaque markers and intracardiac electrodes mounted along the antenna guide. After the loop is formed within the body vessel, the radio-frequency antenna will be deployed along the antenna guide for tissue ablation. 
     In an alternate embodiment of the present invention, one of the elongated portions of the antenna guide is secured to a positioning control slide, and the other portion is secured to the distal portion of the catheter. As a further alternate embodiment of the invention, the antenna guide is formed as an elongated flexible member having a detached distal end portion that is terminated with a distal tip. 
     The radio-frequency catheter system of the present invention can also incorporate various alternate radio-frequency antenna designs. In one such alternate embodiment of the present invention, the radio-frequency antenna comprises a monopole bead disposed at the distal portion of the catheter for delivering an optimal radiation pattern while minimizing refection and voltage standing wave ratios. In another alternate embodiment of the present invention, a microstrip flexcircuit is provided. 
     In application, the antenna guide is deployed out of the catheter lumen to establish contact with the interior surface of the body vessel. The flexibility of the antenna guide enables it to flex to conform to the contour of the body vessel to define the ablation pathway for the radio-frequency antenna. 
     The present invention effectively reduces if not avoids the need for repetitive pin-point precision placement of the ablation catheter electrode of the prior art. The present invention conveniently places the radio-frequency antenna along the locus of an antenna guide which defines the tissue ablation pathway. At the same time, the present invention ensures a continuous ablation pathway and substantially reduces the risk of electrical impulse leakage between ablated spots of the prior art. Accordingly, the present invention substantially accomplishes the objective of the Maze procedure in achieving curvilineal lesions yet without the need for open-heart surgery. These and other aspects and advantages of the invention will become apparent from the following detailed description, and the accompanying drawings, which illustrate by way of example the features of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a conceptual diagram of a radio-frequency catheter ablation system of the present invention, together with radio-frequency power module, computer control and data recording device. 
     FIG. 2 is a perspective view of the radio-frequency catheter ablation system of the present invention. 
     FIG. 3A is a sectional view of the antenna guide and the radio-frequency antenna in a deployed position at the distal portion of the radio-frequency catheter ablation system. 
     FIG. 3B is a sectional view of the antenna guide and the radio-frequency antenna in a retracted position at the distal portion of the radio-frequency catheter ablation system. 
     FIG. 4A is a partial sectional view of the distal portion of the radio-frequency catheter ablation system. 
     FIG. 4B is a partial sectional view of the distal portion of another embodiment of the radio-frequency catheter ablation system. 
     FIG. 5 is sectional view of the radio-frequency antenna and a partial view of the antenna guide. 
     FIG. 6 is a cross-sectional view taken from line  6 — 6  of FIG.  5 . 
     FIG. 7 is a perspective view of another embodiment of the present invention. 
     FIG. 8 is a typical cross-sectional view of distal portion of the catheter system. 
     FIG. 9 is a plan view of a microstrip used for electrical connection between the radio-frequency antenna and a source of the radio-frequency energy. 
     FIG. 10 is an elevational view of the microstrip of FIG.  9 . 
     FIG. 11 is a partial sectional view of the radio-frequency catheter ablation system. 
     FIG. 12 is a partial sectional view of a handle chassis used in the radio-frequency catheter ablation system. 
     FIG. 13 is a cross-sectional view of the microstrip disposed within the handle chassis of FIG.  12 . 
     FIG. 14 is a partial sectional view of another embodiment of present invention incorporating a design of a monopole radio-frequency antenna. 
     FIG. 15 is a partial sectional view of another embodiment of the present invention incorporating a design of a microstrip flexcircuit radio-frequency antenna. 
     FIG. 16 is a cross-sectional view of the microstrip flexcircuit taken from line  16 — 16  of FIG.  15 . 
     FIG. 17 is a partial cut-away view of the distal portion of a hollow cable of the present invention for use in a radio-frequency catheter ablation system. 
     FIG. 18 is a partial sectional view of the distal portion of the catheter of the present invention incorporating one or more steering wires. 
     FIG. 19 is another partial sectional view of the distal portion of the catheter of the present invention deflected by one or more steering wires. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The current invention provides an improved radio frequency-based catheter system for ablating biological tissues within the body vessel of a patient. The system includes a catheter that is adaptable for insertion into a body vessel of patient. It incorporates a deployable radio-frequency antenna for delivering electromagnetic energy to the treatment site. A monorail guide is provided for precise positioning of the antenna along a desired ablation pathway. The present invention also provides a hollow co-axial cable for conducting electromagnetic energy. 
     As seen in FIGS. 1,  2 , and  3 , the present invention comprises a catheter  3 , which is adapted for insertion into a body vessel of the patient. The catheter has a flexible elongated tubular body  10  with a proximal portion  12  and a distal portion  14 . A lumen  16  extends from the proximal portion of the catheter to the distal portion with a distal opening  18  (FIGS.  3  and  4 ). Located at the proximal portion  12  of catheter  3  is a handle chassis  20  for housing necessary steering and positioning controls, as will be described in further details below. Incorporated at the proximal end of the catheter  3  is a coupling  22  for connecting various electrodes (not shown) in support of the ablation procedure. 
     The dimensions of catheter  3  are adapted as required to suit the particular medical procedure, which are well known in the medical art. The tubular body  10  of the catheter is generally constructed of polymer materials that are bio-compatible within the body vessel environment. Examples of these materials include Pebax from Autochem Germany, polyethylene, polyurethane, polyester, polyimide and polyamide, with varying degrees of radio-pacificity, hardness and elasticity. 
     In one embodiment of the present invention, the catheter  3  is formed with a plurality of segments using one or more of the afore-mentioned materials such that the catheter body is progressively more flexible toward its distal end. The segments are joined together via thermal bonding, butt joint, or adhesive bonding. Braiding reinforcement can also be added to the circumferential surface of tubular body  10  to attain the desirable level of stiffness and torsional strength for the catheter. This allows the catheter to advance and negotiate through the body vessel of a patient, and to enable torque transfer along the length of the catheter from the proximal portion to the distal portion. 
     The distal portion  14  of catheter  3  consists of a softer polymer compound with little or no braiding to provide the desired flexibility to accommodate distal deflection or steering of the catheter  3  when it is maneuvered through the narrow passageways of body vessels such as arteries or veins. In the present invention, steering of the catheter is implemented by a pull wire  30 , which extends from the control handle chassis  20  to the distal portion  14  of the catheter  3 , as shown in FIG.  11 . At the distal end of catheter  3 , pull wire  30  is affixed to the inner wall of the catheter lumen  16  by soldering or other suitable means. 
     Pull wire  30  is proximally fastened to deflection control grip or thumb slide  32 , which is slidably engaged along a longitudinal slot  34  of the handle chassis  20 . Longitudinal movement of the thumb slide  32  along slot  34 , together with the torsional movement of the catheter  3  enables a physician to bend or straighten the catheter  3  as needed in order to negotiate through the passageways of the body vessel. Incorporated in the thumb slide  32  is frictional capture means for affixing the grip position in the slot  34 . Many such means are commercially available. Examples of such means include set-release, pressure switch or self-locking mechanisms. 
     The catheter system  1  of the present invention provides an effective means for guiding a RF antenna for tissue ablation along a predetermined ablation pathway. FIGS. 1,  3 A,  4 A and  4 B show an antenna guide or monorail  36 , which is deployed in an extended position adjacent the distal portion  14  of catheter  3 . The antenna guide or monorail  36  is also adaptable to be retracted within the catheter lumen  16  as shown in FIG.  3 B. 
     In one embodiment of the present invention, monorail  36  comprises a flexible elongated member, which can be constructed of a strip-like material. Alternatively, monorail  36  can also be made of small-diameter tubing, as shown in the drawings. Monorail  36  has extended portions  42  and  44  which extend proximally within the catheter lumen  16  (FIGS. 4A,  8 - 10 ). At the handle chassis  20 , monorail extension portions are secured to respective control slides  46  and  48 . Similar to the catheter deflection pull wire  30 , control slides  46  and  48  are slidably engaged within longitudinal slots on the handle chassis  20 , as shown in FIG. 2, and are moveable distally or proximally along the longitudinal axis of the catheter  3 . Thus by moving one or both control slides, the monorail guide can establish a deployed position, as shown in FIGS. 2, and  3 A, or a retracted position, FIG.  3 B. For deployment of the monorail  36 , either one or both control slides  46  and  48  are moved distally relative to the handle chassis  20 . For retraction, the control slides are moved proximally. The positions of the control slides can be secured with appropriate means such as spring-loaded frictional capture means or the like, as similar to those used for the deflection control or thumb slide  32 . 
     FIG. 3B shows the monorail  36  in a substantially retracted position, where it is arranged in a compact U-shaped fashion within the catheter lumen  16  at the distal portion  14  of the catheter  3 . A smooth or curved tip  40  is provided at the monorail  36  such that in the retracted position, tip  40  substantially closes the distal opening  18  of catheter  3  to isolate the catheter lumen  16  from the biological environment. The tip  40  also renders the catheter “atraumatic” and provides a smooth distal profile for the catheter to reduce the risks of body vessel puncture as it is navigated through the passageways of the body vessels. 
     Tip  40  can be made of bio-compatible materials which are commonly used for the construction of catheters. Further it can incorporate a radio-opaque material to aid in the identification of its location within the body vessel by X-rays or other fluoroscopic means, as commonly practiced in the art. 
     The monorail  36  is made of materials in either metallic or in the polymer group having appropriate degree of memory, bio-compatibility, and spring-like structural properties. Examples of such materials include nitinol (nickel-titanium), stainless steel, polyamide and polytetrafluroethylene (“PTFE”). Metallic materials used can also be heat treated or cold worked as necessary to provide the desirable structural properties, such as stiffness and flexibility. These structural properties allow monorail  36  to be moved without crinkles within the catheter lumen  16 . However, in its deployed position outside the catheter lumen  16 , the monorail  36  is adaptable to flex. 
     Monorail  36  is deployable beyond the distal opening  18  of the catheter  3  within a body vessel to form a substantially continuous loop  50  as shown in FIGS. 3A,  4 A and  4 B. Monorail deployment is enabled with the longitudinal advancement of control slides  46  and  48  toward the distal end of catheter  3  such that the monorail extends beyond the catheter distal opening  18  to establish contact with the interior wall of the body vessel. Upon such contact, the extended portion of monorail  36  will flex to acquire a loop configuration. Depending on the interior contour the body vessel where treatment is desired, the size of the loop  50  can be adapted by adjusting the amount of distal displacement of the control slides such that the monorail conforms to the contour of the body vessel. The spring-like properties of the monorail  36  make possible that at least a portion of the loop  50  bears against the wall of the body vessel thereby acquiring line contact with the interior wall of the body vessel in spite of its possible movements. The tip  40  further helps in anchoring the monorail  36  at crevices or minor depressions on the interior wall of the body vessel without the risks of causing puncture on the body vessel. 
     To ascertain the position of the monorail  36  when it is being advanced within a body vessel, one or more radio-opaque markers can be installed on the monorail  36 . As shown in FIGS. 1-4, a radio-opaque marker is incorporated into tip  40  of the monorail  36 . With the radio-opaque material, tip  40  becomes opaque under x-ray or fluoroscopic examination, thereby aiding the identification of its position during catheter insertion or tissue ablation. The structure and use of radio-opaque markers are well-known in the art, and are not detailed here. 
     As a variation in design, the antenna guide can be constructed of two separate elongated members joined at the distal tip to form a unitary monorail. The joint angle between the elongated members can be pre-determined based on the profile of the monorail as it is needed for the particular application. Thus by way of example, a low profile (having ultra small cross-section) catheter used in operation within a narrow lumen of a body vessel could require a relatively small joint angle for the elongated members so as to facilitate the monorail retraction and deployment. FIG. 4B shows another embodiment of the present invention, wherein one end of the monorail guide  36   a  is secured to the catheter  3  adjacent the distal opening  18 . 
     The other end of the monorail  36   a,  which incorporates an extension portion  44   a,  is attached to a control slide (not shown) at the handle chassis. This embodiment enables the deployment and retraction of the monorail with the use of a single control slide at the handle chassis. 
     The present invention includes a radio-frequency (RF) antenna  54  disposed adjacent the distal portion  14  of the catheter  3 , as shown in FIGS. 2-7, for tissue ablation. In an representative embodiment of the present invention, the RF antenna  54  includes an electrically conductive material or wire strip that is wound in a helical fashion to form a helical coil  56 . The appropriate diameter, pitch and length of the coil winding, and the selection of the conductive material or wire strip are a matter of design choice, which can vary according to the particular procedure requirements as known in the art. Thus these design elements and considerations are not detailed here. 
     As shown in FIGS. 2,  3  and  4 A and  4 B, the RF antenna  54  includes the helical coil  56 , which defines an axial passageway  58  for accommodating the monorail  36 . The RF antenna  54  is slidably mounted over the monorail  36 . Thus its movement will be prescribed by the monorail. 
     To enhance its shape integrity, RF antenna  54  is provided with a tubular liner or sleeve  60 , which has a flexible extended body extending from the helical coil  56  proximally toward the proximal portion  12  of the catheter  3 . Sleeve  60  is constructed of a dielectric material, which reduces the likelihood of electrical short between the metallic surfaces of helical coil  56  and body fluids in the passageway  58 , and to help confine the electro-magnetic field to the outside of the passageway. 
     As shown in FIGS. 5 and 6, helical coil  56  is coupled at contact points  65  to a first or inner electrically conductive member or conductor  64 , which is in turn electrically coupled to a source of RF energy provided by the RF power control source  5 . In the embodiment shown in FIGS. 5,  6 ,  11  and  17 , inner conductor  64  is made of a flexible mesh or braided wired construction, or made of a thin-film electrically conductive material, which circumscribes the outer surface  62  of sleeve  60  and extends proximally from the helical coil  56  to the handle chassis  20 . In this embodiment, inner conductor  64  assumes an elongated tubular configuration. 
     Inner conductor  64  is coated with a polymeric dielectric protective coating  68  along its outer circumferential surface and extends proximally to the handle chassis. Protective coating  68  serves as a substrate for the helical coil  56  and for a second electrically conductive member or outer conductor  66 . Protective coating  68  also electrically isolates the inner conductor  64  from the outer conductor  66 . 
     As shown in FIGS. 5 and 6, helical coil  56  is wound around the outer circumferential surface of the protective coating  68  and is connected to outer conductor  66  at contact point  67 . In turn, outer conductor  66  is electrically coupled to the source of RF energy provided by the RF power control source  5 . 
     In the embodiment as shown in FIGS. 5 and 6, outer conductor  66  is made of an electrically conductive material circumscribing the dielectric protective coating  68 , and extends from the helical coil  56  proximally toward the handle chassis  20 . The outer conductor can be made of braided wired construction or thin film electrically conductive material. 
     As shown in FIG. 5, the helical coil  56  is coated with a polymeric dielectric encapsulant  70  along its outer circumferential surface to ensure the structural integrity of the helical coil and to protect same from the biological environment. Encapsulant  70  is made of suitable materials such as silicon or polymer-based materials or rubber compounds. Similarly, an outer jacket  72  made of similar materials is provided to encase the outer conductor  66  and to provide electromagnetic and thermal isolation from the biological environment. 
     As shown in FIG. 11, outer jacket  72  is coupled to a microstrip  80 , which is slidably secured to the handle chassis  20  for the axial displacement of the RF antenna at the proximal portion, as will be discussed in more details below. The extended portion  44  of monorail  36  extends proximally within the passageway  58  to the proximal portion  12  of the catheter  3 . Thus the present invention provides for a set of electrical conductors each of which is formed in an elongated tubular configuration and arranged in a substantially coaxially aligned relationship with each other to form a hollow cable which extends from the helical coil  56  proximally to the handle chassis  20  for the delivery of RF energy. 
     The RF antenna  54  is adapted to receive and radiate electromagnetic energy from a source of radio-frequency energy (not shown). An example of suitable spectrum of radio frequency is that of the microwave frequency ranging from approximately 300 mHz and up. The RF antenna is capable of applying substantially uniformly distributed electromagnetic field energy transmitted by the helical coil. The power of the electromagnetic field transmitted is substantially normal to the longitudinal axis of the RF antenna, and therefore producing uniform energy field circularly about and bounded by the antenna. The energy delivered for the ablation will be uniformly distributed along the antenna, which is independent of the contact between the antenna and the tissue to be ablated. As a result, the present invention reduces the likelihood of creating hot spots in tissue and blood in close proximity or in contact during ablation in comparison to the spot conductive or resistive ablation catheter of the prior art. 
     At the handle chassis  20 , the inner conductor  64  and outer conductor  66  are terminated with coupling to respective junction plates  74  and  76  of an impedance matching microstrip  80  (FIGS.  11 - 13 ). Junction plates in turn are coupled to an electrical conductor  82 , for example solid co-axial cable, which extends from the handle chassis  20  to a source of electromagnetic energy (not shown) via wire connector  22 . At the microstrip, monorail  36  exits the sleeve  60  of the RF antenna, which enables it to be connected to one of control slides. 
     Microstrip  80  is slidably engaged along the side channels  84  and  86  on the opposite side walls  88  and  90  of a mounting blocks  92   a  and  92   b  housed with the handle chassis  20 . To provide for the axial movement of the RF antenna, cable  82  can be moved distally or proximally relative to the handle chassis for the deployment or retraction of the RF antenna. Alternatively, microstrip  80  can be secured to a positioning slide which is moveable along a longitudinal slot on the handle chassis  20  (not shown). 
     Proper placement of the guide member is aided by the radio-opaque marker  40  as discussed above. In addition, monorail  36  can be provided with one or more intracardiac electrocardiogram (“ECG”) electrodes  96  for the physicians to obtain both optimum tissue proximity and electrical conductive activities before and after tissue ablation, as well as to obtain feedback of their actions. These electrodes are secured along the length of the monorail  36 . FIG. 3A shows a typical arrangement of intracardiac electrodes  96 , which are electrically coupled to conductors disposed within the monorail  36  to terminate into the signal pins (not shown) provided for in the wire connector  22 . 
     The catheter is adaptable to be inserted through an opening into a body vessel of a patient where it is brought into the proximity of target tissue for ablation. Prior to the insertion, both the guide member  36  and the RF antenna  54  are retracted within the catheter lumen  16  with the radio-opaque marker  40  to attain an atraumatic tip configuration for the catheter to facilitate smooth passage. The distal portion  14  of the catheter  3  is then inserted into the body opening and is manipulated to reach within the proximity of the location where ablation is needed. Directional control is accomplished with rotational action on the handle chassis and the use of the deflection control  32 . 
     Placement of the RF antenna guide member or monorail  36  is facilitated by the radio-opaque marker  40 , whose position can be detected by suitable x-ray or fluoroscopic means, as practiced in the art. After the distal portion  14  of the catheter  3  is placed within the proximity of the tissue ablation site, the monorail is moved distally by the control slides so it exits the catheter lumen opening  16  to acquire an extended or a deployed position loop configuration as described above. 
     Depending on the internal shape and dimensions of the body vessel, one or both of the monorail control slides can be manipulated to acquire the desired monorail loop size or profile. Acquisition of the loop size or profile is further aided with the use of the intracardiac ECG electrodes  96  for the physician to align the RF antenna guide or monorail  36  with the desired ablation pathway. 
     By way of example, in the case of an atrium of the heart, the size of loop  50  can be adjusted to conform to the contour of the interior wall of the atrium to allow at least a portion of the loop  50  to rest upon the atrial wall, which establishes line contact between the atrium and the monorail. The flexibility of the monorail  36  allows at least a portion of the loop to conform to the internal contour of body vessel and to rest against its internal wall. As the atrial wall pulsates, the monorail, which is in contact with the atrial wall, will also move in concert, thereby achieving an affixed and stable relationship with the location of the body vessel where treatment is desired. 
     Once the loop profile for the monorail has been acquired and aligned in parallel with the desired ablation pathway, the control slides  46  and  48  are secured in position at the handle control. The RF antenna  54  is then moved distally to exit the distal end opening of the catheter and slidably guided by the monorail to reach the precise location where ablation is needed. Thereafter, tissue ablation can be accomplished with the application of radio-frequency energy. Depending on the particular procedure requirements, the length of the ablation can be adjusted by positioning the RF antenna along various locations along the loop followed by applications of the RF energy. Thus, long and contiguous ablation lines can be established to substantially eliminate the risk of electrical impulse leakage between ablated tissue pathways. The above steps can be repeated for other locations within the atrium as necessary depending on the particular procedure requirements. 
     FIG. 7 shows another embodiment of the present invention which incorporates a variation of the antenna guide design. In this embodiment, the antenna guide  102  comprises an elongated flexible member having a detached distal end portion  104  that is terminated with a distal tip  106 . The distal tip  106  is incorporated with a radio-opaque material to aid in the placement of the catheter as described above. The other end portion of the guide  102  extends proximally to a handle chassis (not shown) and is secured to a positioning control slide (not shown) in a similar fashion as the embodiments described above. Similarly, the antenna guide  102  can be retracted within the lumen of the catheter  100  prior to its deployment, together with a RF antenna  110 . 
     In application, after the catheter  100  is placed within the proximity of the tissue to be ablated, the antenna guide  102  is deployed out of the catheter lumen  108  where the distal tip  106  is allowed to anchor within crevices on the surface of the body vessel. The flexibility of the antenna guide  102  enables it to flex to conform to the contour of the body vessel and to establish line contact between the guide  102  and the body vessel. As a result, any relative movement between the guide  102  and the body vessel can be minimized. Thereafter, the RF antenna  110  is carried by the antenna guide  102  to be extended out of the catheter lumen  108  for the ablation along a pathway that is substantially aligned in parallel with the line contact between the antenna guide  102  and the body vessel. 
     As alternative embodiments, the radio-frequency antenna of the present invention can incorporate various designs of radio-frequent cy antennas. FIG. 14 illustrates one such alternative embodiment. As shown in FIG. 14, in lieu of the helical coil configuration as described above and in lieu thereof, the catheter system is provided with an antenna  120  wich comprises a monopole bead  122 . The monopole bead is disposed circumferentially over sleeve  60  at the distal end portion of antenna  120 . Sleeve  60  has a lumen  58  to accommodate a guide member, such as the guide member or monorail  36  or antenna guide  102 , as described above. 
     The monopole bead is connected to inner conductor  64 , which is electrically isolated from outer conductor  66 . When the inner conductor  64  and outer conductor  66  are energized, as described above, an electromagnetic field is generated between the monopole bead  122  and the outer conductor  66  external to the antenna, which can be applied for tissue ablation. Thus, though there is no physical contact between the monopole bead  122  and the outer conductor  66 , they are considered being electrically coupled for generating an electromagnetic field. 
     The shape and dimensions of the monopole bead  122  are designed to optimize radiation pattern while minimizing reflection and voltage standing wave ratio (“VSWR”), as known in the art to effect an impedance matching function that provides smooth impedance transition between the transmission line that supplies the RF energy and the medium to which the RF energy is radiated. Preferably, the shape and dimensions of the monopole bead  122  are designed to minimize RF reflection coefficient of the antenna system and therefore minimizing VSWR to approximately 1:1. By way of illustration, the diameter of the monopole bead  122  gradually increases distally and terminated with a decrease in its diameter at the distal end opening of sleeve  60  to form a generally tear-drop shape, as shown in FIG.  14 . 
     As described above, both inner and outer conductors are adaptable to be connected to a source of RF energy. When energized, an electromagnetic field is generated extending from the outer conductor to the tip of the monopole bead that is normal and omnidirectional to the surface of the monopole bead. The power of the electromagnetic field transmitted is substantially normal to the longitudinal axis of the RF antenna, and therefore producing uniform energy field circularly about and bounded by the antenna. 
     As a further alternative embodiment of the present invention, the radio-frequency antenna can incorporate the design of a microstrip flexcircuit in lieu of the helical coil or monopole bead as described above. As shown in FIGS. 15 and 16, microstrip flexcircuit antenna  132  includes a pair of spaced apart electrically conductive microstrips  134  and  136  disposed at the distal portion of the antenna on a dielectric backing  69  as part of the dielectric coating material  68  used as coating for the inner conductor  64 , or of an extension of sleeve  60 . The microstrips  134  and  136  are electrically coupled to the inner conductor  64  and outer conductor  66  respectively such that when energized, an electromagnetic field is generated between the microstrips, which can be used for tissue ablation. 
     The spacing between the microstrip  134  and  136  and the dimensions are determined such that a substantially smooth impedance transition from the electrical conductors  64  and outer conductor  66  and the body tissue to be ablated can be achieved in the art. Accordingly, it is preferable that the microstrip flexcircuit antenna be designed to minimize the reflection VSWR, as known in the art. The dimensions of the microstrips conductors are sufficiently small to accommodate bending and deflections as necessitated in tissue ablation in a manner as described above. 
     The materials used in the construction of the monopole or the microstrip conductors comprise bio-compatible electrically conductive materials, Examples of such materials are platinum, gold, or silver, or any combination thereof, which are bio-compatible. Alternatively, other electrically conductive materials coated with bio-compatible materials can be employed in forming the monopole bead or the microstrip conductors. 
     Optionally, as shown in FIGS. 18 and 19, one or more ECG wires  140  can be installed in connection to one or more electrodes  142  at the distal portion of the RF antenna  138  of the present invention to provide a means for obtaining optimal tissue proximity and electrical conductivity measurements before and after tissue ablation. 
     Additionally, the antenna of the present invention can include one or more antenna deflection or steering wires affixed at distal portion of the antenna to achieve more pronounced shape or curvature of the antenna. FIGS. 18 and 19 illustrate an exemplary embodiment where a deflection wire  144  is affixed to the distal end portion  146  of the antenna  138  and extends proximally within the internal lumen of the hollow coaxial cable to be attached to and controlled by a deflection control mechanism at the handle (not shown). The attachment of the deflection wire  144  at the end portion  146  of the antenna permits amplified deflection of the antenna at that location, as illustrated in FIG.  19 . In application, the antenna  138 , provided with such deflection means, can be deployed into the body lumen with the aid of the guide member or monorail  36  as described above. Where necessary, the guide member  36  can be retrieved, followed by the actuation of the deflection wire  144  to effect an amplified deflection in the radio-frequency antenna. As a result, the antenna can be shaped in such a way so as to be adaptable to gain access to the otherwise inaccessible regions of atrium or other body vessels. 
     From the above description, it is apparent that the present invention effectively reduces if not eliminates the need for repetitive pin-point precision placement of the ablation catheter electrodes of the prior art. The present invention conveniently places the RF antenna along the locus of an antenna guide which defines the tissue ablation pathway. At the same time, the present invention ensures a continuous ablation pathway and substantially reduces the risk of electrical impulse leakage between ablated spots of the prior art. Accordingly, the present invention substantially accomplishes the objective of the Maze procedure in achieving lineal lesions yet without the need for open-heart surgery. 
     While the above description of the invention is directed to the present embodiments or examples of applications, various modifications and improvements can be made without departing from the spirit and scope of the invention.