Patent Publication Number: US-2007100331-A1

Title: Systems and methods for organ tissue ablation

Description:
BACKGROUND  
      1. Field  
      The field of the application relates to medical devices, and more particularly, to systems and methods for ablating or otherwise treating tissue using electrical energy.  
      2. Background  
      Tissue may be destroyed, ablated, or otherwise treated using thermal energy during various therapeutic procedures. Many forms of thermal energy may be imparted to tissue, such as radio frequency electrical energy, microwave electromagnetic energy, laser energy, acoustic energy, or thermal conduction.  
      In particular, radio frequency ablation (RFA) may be used to treat patients with tissue anomalies, such as liver anomalies and many primary cancers, such as cancers of the stomach, bowel, pancreas, kidney and lung. RFA treatment involves the destroying undesirable cells by generating heat through agitation caused by the application of alternating electrical current (radio frequency energy) through the tissue.  
      Various RF ablation devices have been suggested for this purpose. For example, U.S. Pat. No. 5,855,576 describes an ablation apparatus that includes a plurality of wire electrodes deployable from a cannula or catheter. Each of the wires includes a proximal end that is coupled to a generator, and a distal end that may project from a distal end of the cannula. The wires are arranged in an array with the distal ends located generally radially and uniformly spaced apart from the catheter distal end. The wires may be energized in a monopolar or bipolar configuration to heat and necrose tissue within a precisely defined volumetric region of target tissue. The current may flow between closely spaced wire electrodes (bipolar mode) or between one or more wire electrodes and a larger, common electrode (monopolar mode) located remotely from the tissue to be heated.  
      Generally, ablation therapy uses heat to kill tissue at a target site. The effective rate of tissue ablation is highly dependent on how much of the target tissue is heated to a therapeutic level. In certain situations, complete ablation of target tissue that is adjacent a vessel may be difficult or impossible to perform, since significant bloodflow may draw the produced heat away from the vessel wall, resulting in incomplete necrosis of the tissue surrounding the vessel. This phenomenon, which causes the tissue with greater blood flow to be heated less, and the tissue with lesser blood flow to be heated more, is known as the “heat sink” effect. It is believed that the heat sink effect is more pronounced for ablation of tissue adjacent large vessels that are more than 3 millimeters (mm) in diameter. Due to the increased vascularity of the liver, the heat sink effect may cause recurrence of liver tumors after a radio frequency ablation.  
      Also, because of the vascularity of the liver, resection of a portion of a liver (as is required by some surgeries) may result in significant bleeding. Existing techniques in managing bleeding of a resected liver include delivering embolic material within a vessel of a liver to prevent blood flow. However, such technique is time consuming, may require complex imaging modality, and may not be effective in the case in which a relatively large portion of a liver is being resected.  
     SUMMARY  
      In accordance with some embodiments, a system for treating organ tissue includes a source of electrical energy, a first electrode coupled to the energy source, the first electrode having a surface configured for electrically coupling with a surface of an organ, and a second electrode coupled to the energy source, the second electrode having a tissue-piercing distal tip configured for piercing the organ such that the second electrode electrically couples with internal tissue of the organ.  
      In accordance with other embodiments, a method of performing an organ tissue ablation procedure includes placing a first electrode at a first position on a surface of an organ, piercing the organ with a second electrode to position the second electrode inside the organ, and applying electrical energy through a circuit formed by the first and second electrodes to ablate a portion of the organ.  
      In accordance with other embodiments, a system for treating organ tissue includes a source of electrical energy, a first electrode coupled to the energy source and having a surface configured for electrically coupling with a surface of an organ at a first position, and a second electrode coupled to the energy source and having a surface configured for electrically coupling with the surface of the organ at a second position.  
      In accordance with other embodiments, a method of performing a liver ablation procedure includes placing a first electrode at a first position on a surface of a liver, placing a second electrode at a second position on the surface the liver, and applying electrical energy through an electrical circuit formed by the first and the second electrodes to ablate a portion of the liver.  
      Other aspects and features of the embodiments will be evident from reading the following description of the embodiments.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The drawings illustrate the design and utility of embodiments of the application, in which similar elements are referred to by common reference numerals. In order to better appreciate how advantages and objects of various embodiments are obtained, a more particular description of the embodiments are illustrated in the accompanying drawings. Understanding that these drawings depict only typical embodiments of the application and are not therefore to be considered limiting its scope, the embodiments will be described and explained with additional specificity and detail through the use of the accompanying drawings.  
       FIG. 1  illustrates an ablation system for treating tissue in accordance with some embodiments;  
       FIG. 2  illustrates a method of using the ablation system of  FIG. 1  in accordance with some embodiments;  
       FIG. 3  illustrates an ablation system for treating tissue in accordance with other embodiments;  
       FIG. 4  illustrates a variation of the ablation system of  FIG. 3  in accordance with some embodiments;  
       FIG. 5  illustrates a method of using the ablation system of  FIG. 4  in accordance with some embodiments;  
       FIG. 6  illustrates a method of using the ablation system of  FIG. 4  in accordance with other embodiments;  
       FIG. 7  illustrates the ablation system of  FIG. 4 , showing the ablation system further having a securing device for securing electrodes against a tissue surface;  
       FIG. 8  illustrates an ablation system for treating tissue in accordance with other embodiments, showing the ablation system having an electrode with an envelope configuration;  
       FIG. 9  illustrates an ablation system for treating tissue in accordance with other embodiments, showing the ablation system having two electrodes each of which having a surface for contacting an organ surface;  
       FIG. 10  illustrates a method of using the ablation system of  FIG. 9  in accordance with some embodiments; and  
       FIG. 11  illustrates a variation of the ablation system of  FIG. 9  in accordance with some embodiments.  
    
    
     DESCRIPTION OF THE EMBODIMENTS  
       FIG. 1  illustrates an ablation system  10  in accordance with some embodiments. The ablation system  10  includes a source of energy  12 , e.g., a radio frequency (RF) generator, a first device  14  carrying a first electrode  16 , and a second device  18  carrying a second electrode  20 . The source of energy  12  has a first terminal  22  and a second terminal  24 . The ablation system  10  further includes a first cable  26  for electrically coupling the first electrode  16  to the first terminal  22 , and a second cable  28  for electrically coupling the second electrode  20  to the second terminal  24 .  
      The generator  12  is preferably capable of operating with a fixed or controlled voltage so that power and current diminish as impedance of the tissue being ablated increases. Exemplary generators are described in U.S. Pat. No. 6,080,149, the disclosure of which is expressly incorporated by reference herein. The preferred generator  12  may operate at relatively low fixed voltages, typically below one hundred fifty volts (150 V) peak-to-peak, and preferably between about fifty and one hundred volts (50-100 V). Such radio frequency generators are available from Boston Scientific Corporation, assignee of the present application, as well as from other commercial suppliers. It should be noted that the generator  12  is not limited to those that operate at the range of voltages discussed previously, and that generators capable of operating at other ranges of voltages may also be used.  
      In the illustrated embodiments, the first device  14  has a structure  30  that is made from a flexible material, such as an elastic metal or a polymer. The first electrode  16 , which is also made from an elastic material (e.g., a bendable metal), is secured to the structure  30 , and has a surface  32  for contacting tissue, such as a surface of an organ. In some embodiments, the structure  30  is capable of being bent from a first configuration to a second configuration via a force, and is capable of remaining in the second configuration upon a removal of the force. Such feature allows a desired profile of the surface  32  to be created during use. Alternatively, the structure  30  and/or the first electrode  16  can be made from a rigid material that prevents the first electrode  16  from being bent. As shown in the figure, the surface  32  of the first electrode  16  has a planar configuration. As used in this specification, the term “planar configuration” refers to a configuration that can have a two dimensional characteristic (as that of a perfectly flat plane), or a three dimensional characteristic (as that of a surface having one or more portions that do not lie in a perfectly flat plane).  
      In other embodiments, the first device  14  can have other configurations. For example, in other embodiments, the first device  14  can further include a handle secured to the structure  30 , which allows a physician to press the electrode surface  32  towards a tissue surface. In further embodiments, the first device  14  can include an elongate shaft connected between the handle and the structure  30 . The shaft can be elastic (which allows a physician to bent the shaft into a desired profile during use), or rigid. During use, the elongate shaft allows a physician to reach tissue with the first electrode  16 .  
      The second device  18  includes a handle  34  to which the second electrode  20  is secured. The second electrode  20  has a rectilinear profile, but alternatively, can have a curvilinear profile, or any of other non-linear profiles. As shown in the figure, the second electrode  20  also has a sharp distal tip  36  for piercing tissue. In other embodiments, the second device  18  can have other configurations. For example, in other embodiments, the second device  18  can include a cannula having a lumen. In such cases, the second electrode  36  can include one or more tines that assume a low profile when confined within the lumen of the cannula, and assume a relaxed and expanded profile when unconfined outside the lumen of the cannula. Examples of such device are described in U.S. Pat. No. 5,855,576, the entire disclosure of which is expressly incorporated by reference herein.  
       FIG. 2  illustrates a method of ablating tissue using the ablation system  10  of  FIG. 1  in accordance with some embodiments. First, an incision is made on a patient&#39;s skin  190  to create an opening  192 . The first device  14  is then inserted through the opening  192  (percutaneously) and the first electrode  16  is placed against a surface  200  of an organ  202  (e.g., a liver). In some embodiments, the first electrode  16  can be secured to the surface  200  using one or more hooks coupled to the electrode  16  (e.g., at the periphery of the electrode  16 ). In such cases, the hook(s) penetrate within the tissue to thereby secure the electrode  16  relative to the surface  200 . Alternatively, a suction device located next to the electrode  16  (e.g., at a periphery of the electrode  16 ) can be used to secure the electrode  16  relative to the surface  200 . In such cases, the suction device creates a suction, and pulls the organ surface  200  towards the electrode  16 , thereby stabilizing the electrode  16  relative to the surface  200 . Other methods of securing the electrode  16  relative to the surface  200  can also be used. If the first device  14  includes a handle and a shaft, these components can be used as leverage to press the first electrode  16  against the surface  200 . The second device  18  is then inserted through the opening  192 , and the second electrode  20  pierces into the organ  202  using the distal tip  36 . Alternatively, the second device  18  can be inserted through the opening  192  before the first device  14 .  
      In alternative embodiments, one or more components or elements may be provided for introducing the devices  14 ,  18  through the patient&#39;s skin  190 . For example, a conventional sheath (not shown) may be inserted through the patient&#39;s skin  190  to gain access to the organ  202 . Once properly positioned, the first and second devices  14 ,  18  may then be introduced through the sheath lumen to reach the organ  202 .  
      In some embodiments, before the first device  14  is inserted into the patient, if the structure  30  of the first device  14  is flexible, a physician can bend the structure  30  to thereby form the electrode surface  16  into a desired profile (bent configuration). For example, the electrode surface  16  can be bent such that its profile resembles a contour of a target surface of the organ  202  at which the electrode  16  will be placed.  
      Next, energy, preferably RF electrical energy, may be delivered from the generator  12  to the first electrode  16 , with the second electrode  20  functioning as a return electrode, thereby creating a lesion  204  between the first and second electrodes  16 ,  20 . Alternatively, the generator  12  may deliver energy to the second electrode  20 , with the first electrode  16  functioning as a return electrode. In some embodiments, after the lesion  204  has been created, the ablation system  10  (or another ablation device/system) can be used to ablate a target treatment site (e.g., a tumor) located on one side of the lesion  204 . In such cases, the formed lesion  204  can be used as a barrier to prevent blood from flowing from one side of the lesion  204  to the other side of the lesion  204 , thereby allowing the target treatment site located on one side of the lesion  204  to be ablated efficiently without being affected by a heat sink effect due to blood flow.  
      In some cases, if it is desired to perform further ablation to increase the lesion size or to create additional lesion(s) at different site(s) of the organ  202 , one or both of the first electrode  16  and the second electrode  20  may be positioned, and be placed at different location(s), and the same steps discussed previously may be repeated. For example, in some embodiments, after the first lesion has been created, the first electrode  16  may be placed on the other side of the organ  204  (indicated by dotted lines), with the second electrode  20  remaining in its first position. The electrodes  16 ,  20  can then be used to create a second lesion, thereby forming an ablation plane substantially across an entire cross section of the organ  202  with the first lesion. In some cases, after a lesion across a substantial cross section of the organ  202  has been created, part of the organ  202  on one side of the ablation plane can be surgically removed (resect).  
      In the above embodiments, the first and second electrodes  16 ,  20  are used to create the lesion  204  in a bipolar configuration. Alternatively, the lesion  204  can be created in a monopolar configuration. In such cases, the first and the second electrodes  16 ,  20  may be connected to the active terminal  22  of the generator  12  using a “Y” cable, and a common ground pad electrode (not shown) is electrically coupled to the terminal  24 . The first and second electrodes  16 ,  20  then deliver energy to the common ground pad electrode, which is generally placed on a patient&#39;s skin, in a monopolar mode.  
       FIG. 3  illustrates an ablation system  10  in accordance with other embodiments. The ablation system  10  is the same as that described with reference to  FIG. 1 , except that the ablation system  10  of  FIG. 3  further includes a third device  300  having a structure  302  for carrying a third electrode  306 . Similar to the first electrode  16 , the third electrode  306  has a surface  308  for contacting tissue surface (e.g., surface of an organ). The ablation system  10  further includes a third cable  310  that electrically couples the third electrode  306  to a third terminal  312  on the source of energy  12 . The output terminals  22 ,  312  of the generator  12  may be coupled to common control circuits (not shown) within the generator  12 . Alternatively, the generator  12  may include separate control circuits coupled to each of the output terminals  22 ,  312 . The control circuits may be connected in parallel with one another, yet may include separate impedance feedback to control energy delivery to the respective output terminals  22 ,  312 . In some embodiments, the output terminals  22 ,  312  may be connected in parallel to an active terminal of the generator  12  such that the first and third electrodes  16 ,  306  can deliver energy to a common ground pad electrode (not shown) in a monopolar mode, or to the second electrode  20  in a bipolar mode. Alternatively, the output terminals  22 ,  312  may be connected to opposite terminals of the generator  12  for delivering energy between the first and third electrodes  22 ,  312  in a bipolar mode.  
      In further embodiments, the generator  12  does not have the third terminal  312 . Instead, the first and the third electrodes  16 ,  306  are electrically coupled to each other via a cable. In such cases, the cable is electrically coupled to the first terminal, which supplies electrical energy to the first and the third electrodes  16 ,  306 . The first and the third electrodes  16 ,  306  form a first pole of a circuit, and the second electrode  20  form a second pole of the circuit.  
      In other embodiments, if the source of energy  12  has only two terminals  22 ,  24 , a “Y” cable  400  can be provided to electrically couple the first and third electrodes  16 ,  306  to the first terminal  22  ( FIG. 4 ).  
       FIG. 5  illustrates a method of ablating tissue using the ablation system  10  of  FIG. 4  in accordance with some embodiments. First, an incision is made on a patient&#39;s skin  190  to create an opening  192 . The first device  14  is then inserted through the opening  192  (percutaneously) and the first electrode  16  is placed at a first location  502  against a surface  200  of an organ  202  (e.g., a liver). The second device  18  is then inserted through the opening  192 , and the second electrode  20  pierces into the organ  202  using the distal tip  36 . The third device  300  is then inserted through the opening  192  and the third electrode  306  is placed at a second location  504  against the surface  200  of the organ  202 . Alternatively, the order of inserting the first, second, and third devices  14 ,  18 ,  300  can be different from that described previously. In the illustrated embodiments, the first, second, and third electrodes  16 ,  20 ,  306  are positioned such that they lie approximately within a flat (or linear) plane.  
      In alternative embodiments, one or more components or elements may be provided for introducing the devices  14 ,  18 ,  300  through the patient&#39;s skin  190 . For example, a conventional sheath (not shown) may be inserted through the patient&#39;s skin  190  to gain access to the organ  202 . Once properly positioned, the first, second, and third devices  14 ,  18 ,  300  may then be introduced through the sheath lumen to reach the organ  202 .  
      In some embodiments, before the first device  14  is inserted into the patient, if the structure  30  of the first device  14  is flexible, a physician can bend the structure  30  to thereby form the electrode surface  16  into a desired profile (bent configuration). For example, the electrode surface  16  can be bent such that its profile resembles a contour of a portion of the surface  200  (e.g., the surface portion at the first location  502 ) at which the first electrode  16  will be placed. Similarly, before the third device  300  is inserted into the patient, if the structure  302  of the third device  300  is flexible, a physician can bend the structure  302  to thereby form the electrode surface  308  into a desired profile (bent configuration). For example, the electrode surface  308  can be bent such that its profile resembles a contour of a portion of the surface  200  (e.g., the surface portion at the second location  504 ) at which the third electrode  306  will be placed.  
      Next, energy, preferably RF electrical energy, may be delivered from the generator  12  to the first and third electrodes  16 ,  306 , with the second electrode  20  functioning as a return electrode, thereby creating a first lesion  510  between the first and second electrodes  16 ,  20 , and a second lesion  512  between the second and third electrodes  20 ,  306 . Alternatively, the generator  12  may deliver energy to the second electrode  20 , with the first and third electrodes  16 ,  306  functioning as return electrodes. In some embodiments, after the lesion  514  has been created, the ablation system  10  (or another ablation device/system) can be used to ablate tissue at a target treatment site  520  (e.g., a tumor) located on one side of the lesion  514 . In such cases, the formed aggregate lesion  514  (formed by lesions  510 ,  512 ) can be used as a barrier to prevent blood from flowing from one side of the lesion  514  to the other side of the lesion  514 , thereby allowing the target treatment site  520  located on one side of the lesion  514  to be ablated efficiently without being affected by a heat sink effect due to blood flow.  
      In some embodiments, if the first and third electrodes  16 ,  306  are sufficiently large, the above technique will result in an ablation plane formed substantially across an entire cross section of the organ  202 . Alternatively, if the first and third electrodes  16 ,  306  are not sufficiently large, one or both of the first and third electrodes  16 ,  306  can be positioned, and the above technique is repeated until a lesion substantially across an entire cross section of the organ  202  is formed. In some cases, after a lesion across a substantial cross section of the organ  202  has been created, part of the organ  202  on one side of the ablation plane can be surgically removed, e.g., by cutting through the ablated region. The ablated region acts as a shield to prevent, or at least reduce, bleeding after the resection of the organ  202 .  
       FIG. 6  illustrates another method of ablating tissue using the ablation system  10  of  FIG. 4  in accordance with other embodiments. As shown in the figure, the first, second, and third electrodes  18 ,  20 ,  306  are positioned relative to each other such that a first line  600  extending between the first electrode  16  and the second electrode  20 , and a second line  602  extending between the second electrode  20  and the third electrode  306 , form an non-180° angle. In some embodiments, such arrangement of the electrodes  18 ,  20 ,  36  can be used to perform a wedge resection in which a first resection (or ablation) plane is created between the first and second electrodes  18 ,  20 , and a second resection (or ablation) plane is created between the second and third electrodes  20 ,  306 , thereby resecting tissue that contains a tumor  606 .  
      In the above embodiments, the first electrode  16  (and the third electrode  306 ) are secured to tissue surface by a physician applying a force to press the electrode  16  (and electrode  306 ) against the tissue surface. In other embodiments, any of the ablation systems  10  described herein can further include a securing device for securing the first electrode  16  and the third electrode  306  against tissue surface (e.g., surface of an organ).  FIG. 7  illustrates the ablation system  10  of  FIG. 4 , which further includes two elastic bands  700 ,  702  for securing the first electrode  16  and the third electrode  306  against the surface  200  of the organ  202 . The elastic bands  700 ,  702  can be a rubber band, a spring, or any of other elastic structures (including structures made from nylon, elastic polymers, or any of other elastic materials). During use, the first electrode  16  and the third electrode  306  are placed at different locations along the surface  200  of the organ  202 , with the elastic bands  700 ,  702  wrapped at least partially around parts of the organ  202 . The elastic bands  700 ,  702  pull the first and the third electrodes  16 ,  306  towards each other, thereby applying a compression force to push the first electrode  16  and the third electrode  306  towards the surface  200 .  
      In other embodiments, the ablation system  10  can include other types of securing devices for securing the first electrode  16  (and the third electrode  306 ) against a tissue surface. For example, in other embodiments, the ablation system  10  can further include a suction device (not shown), and a tube (not shown) having a first end connected to the suction device, and a second end connected to the first device  14 . In some embodiments, the second end of the tube can be located adjacent to the first electrode  16 . In other embodiments, the first electrode  16  can include an opening, which is in fluid communication with the lumen of the tube. During use, the suction device applies a suction through the tube, thereby pulling a tissue surface towards the first electrode  16  to secure the first electrode  16  relative to the tissue surface.  
       FIG. 8  illustrates a variation of the ablation system  10  in accordance with other embodiments. The ablation system  10  is similar to that described with reference to  FIG. 1 , except that the structure  30  of the first device  14  is an envelope  800  having an opening  802  at one end, and a lumen  808  for accommodating a portion of the organ  202 . In some embodiments, the envelope  800  itself is made from a conductive material, thereby allowing the structure  30  to function as the electrode  16 . For example, the envelope  800  can be made from a plurality of metallic wires/strands that are weaved into a sock-like structure. In other embodiments, the structure  30  can be made from a non-conductive material. In such cases, at least part of the structure  30  can be covered with a conductive material (e.g., strands of metallic wires, metallic particles, or conductive pads) to form the electrode  16 . In the illustrated embodiments, the envelope  800  has a closed end  804 . In other embodiments, the structure  30  can have an opening at the end  804 , and resembles a tube or a ring.  
       FIG. 9  illustrates a variation of the ablation system  10  of  FIG. 4  in accordance with other embodiments. The ablation system  10  is similar to that described with reference to  FIG. 4 , except that it does not include the second device  18  and the second electrode  20 . In such cases, the first electrode  16  is electrically coupled to the first terminal  22  of the energy source  12 , and the third electrode  306  is electrically coupled to the second terminal  24  of the energy source  12 . During use, the electrodes  16 ,  306  are used to ablate tissue in a bipolar configuration.  
      In some embodiments, the ablation system  10  of  FIG. 9  can be used to create a lesion (a transmural lesion) across a thickness of an organ. As shown in  FIG. 10 , the first electrode  16  can be placed on one side  850  of the organ  202 , with the third electrode  306  placed on the opposite side  852  of the organ  202 . The first and the third electrodes  16 ,  306  can then be used to deliver ablation energy to ablate tissue  900  between the electrodes  16 ,  306  (e.g., to ablate a tumor  854 ).  
      In any of the embodiments described herein, the structure  30  and the electrode  16  can be made from a material, and have respective thicknesses that are thin enough, such that a physician can cut (e.g., using a scissor, a knife, or any of other known cutting devices) the structure  30  and the electrode  16  into a desired shape during use. For example, in some embodiments, the structure  30  can be made from a polymer, and has a thickness that is less than 10 millimeters (mm). Also, in some embodiments, the electrode  16  can include a substrate made from a material (e.g., a polymer) that can be cut, with at least a portion of the substrate covered by a conductive material. In other embodiments, the electrode  16  can be made from a metal that can be cut. For example, in some embodiments, the electrode  16  can be a foil.  FIG. 11  illustrates an embodiments of the ablation system  10  of  FIG. 9 , with the first electrode  16  and the third electrode  306  each cut into a “C” shape. During use, the first and the third electrodes  16 ,  306  are placed on different sides of the organ  202 , and a “C” shape ablation plane  902  can be created between the first and the third electrodes  16 ,  306 . In other embodiments, each of the first and the third electrodes  16 ,  306  can be cut into other shapes, such as a “V” shape or an “O” shape.  
      It should be noted that the ablation system  10  is not necessarily limited to the configurations described previously, and that the ablation system  10  can have other configurations in other embodiments. For example, in other embodiments, the first electrode  16  and the third electrode  306  can have different shapes and/or sizes. Also, in other embodiments, instead of having the electrodes  16 ,  20 ,  306 , for delivering RF energy, the ablation system  10  can include other types of ablation devices. For example, in other embodiments, the ablation system  10  can include ablation devices connected to the energy source  12 , wherein each of the ablation devices is configured for delivering other form of energy, such as ultrasound energy, or microwave energy, for the purpose of ablation.  
      Also, instead of delivering ablation energy in a bipolar configuration, any of the embodiments of the ablation systems  10  described herein can be modified to allow delivery of ablation energy in a monopolar configuration. For example, in the embodiments of  FIG. 9 , the first and third electrodes  16 ,  306  can be electrically coupled to the first terminal  22  using a “Y” cable, and a neutral or ground electrode (e.g., an external electrode pad) may be electrically coupled to the opposite terminal  24  of the generator  12 . In such cases, the ground electrode can be coupled to the patient, e.g., be placed on the patient&#39;s skin, and the electrodes  16 ,  306  can then be used to deliver ablation energy in a monopolar configuration.  
      Thus, although several embodiments have been shown and described, it would be apparent to those skilled in the art that many changes and modifications may be made thereunto without the departing from the scope of the invention, which is defined by the following claims and their equivalents.