Patent Publication Number: US-8109927-B2

Title: Surface electrode multiple mode operation

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/271,738 filed Nov. 14, 2008, now issued as U.S. Pat. No. 7,771,423, which is a continuation of U.S. patent application Ser. No. 11/867,668, filed Oct. 4, 2007, now issued as U.S. Pat. No. 7,455,671, which is a continuation of U.S. patent application Ser. No. 11/144,848, filed Jun. 3, 2005, now issued as U.S. Pat. No. 7,278,993, which itself is a continuation of U.S. patent application Ser. No. 10/388,143, filed Mar. 13, 2003, now issued as U.S. Pat. No. 6,918,907. The above-noted Applications are incorporated by reference as if set forth fully herein. 
    
    
     FIELD OF THE INVENTION 
     The field of the invention relates generally to the use of ablation devices for the treatment of tissue, and more particularly, RF ablation devices for the treatment of tumors. 
     BACKGROUND OF THE INVENTION 
     The delivery of radio frequency (RF) energy to target regions within tissue is known for a variety of purposes. Of particular interest to the present invention, RF energy may be delivered to diseased regions in target tissue for the purpose of causing tissue necrosis. For example, the liver is a common depository for metastases of many primary cancers, such as cancers of the stomach, bowel, pancreas, kidney, and lung. Electrosurgical probes with deploying electrode arrays have been designed for the treatment and necrosis of tumors in the liver and other solid tissues. See, for example, the LeVeen™ Needle Electrode available from Boston Scientific Corporation, which is constructed generally in accord with U.S. Pat. No. 6,379,353, entitled “Apparatus and Method for Treating Tissue with Multiple Electrodes.” 
     The probes described in U.S. Pat. No. 6,379,353 comprise a number of independent wire electrodes that are deployed into tissue from the distal end of a cannula. The wire electrodes may then be energized in a monopolar or bipolar fashion to heat and necrose tissue within a defined volumetric region of target tissue. Difficulties have arisen in using the multiple electrode arrangements of U.S. Pat. No. 6,379,353 in treating tumors that lay at or near the surface of an organ, such as the liver. Specifically, some of the tips of the electrode array can emerge from the surface after deployment. Such exposure of the needle tips outside the tissue to be treated is disadvantageous in a number of respects. First, the presence of active electrodes outside of the confinement of the organ being treated subjects other tissue structures of the patient as well as the treating personnel to risk of accidental contact with the electrodes. Moreover, the presence of all or portions of particular electrodes outside of the tissue being treated can interfere with proper heating of the tissue and control of the power supply driving the electrodes. While it would be possible to further penetrate the needle electrode into the treated tissue, such placement can damage excessive amounts of healthy tissue. 
     In response to these adverse results, a device for ablating a tumor at or near the surface of tissue has been developed. Specifically, as illustrated in  FIG. 1 , an ablation assembly  20  comprises a surface electrode  22  and an electrosurgical probe  24 ; such as a LeVeen™ electrode, that can be operated in a bipolar mode to ablate the tissue in contact with, and between a needle electrode array  26  mounted to the distal end of the probe  24  and the surface electrode  22 . As illustrated, the surface electrode  22  comprises a generally flat or planar disk-shaped plate  28  and a plurality of tissue penetrating electrodes  30  that project perpendicularly from the plate  28 . The surface electrode  22  further comprises a central aperture  32  that extends through the plate  28 , so that the surface electrode  22  can be threaded over the probe  24  and locked into place about the deployed probe  24 . The ablation assembly  20  can then be operated in a monopolar or bipolar mode to ablate the tissue in contact with, and between, the electrode array  26  of the probe  24  and the needles electrodes  30  of the surface electrode  22 . Further details regarding these types of ablation devices are disclosed in U.S. Pat. No. 6,470,218, entitled “Apparatus and Method for Treating Tumors Near the Surface of an Organ,” which is hereby fully and expressly incorporated herein by reference. 
     Although the ablation assembly illustrated in  FIG. 1  is generally useful in ablating superficially oriented tumors, it cannot be used to efficiently and safely ablate such tumors in all circumstances. For example, if the tumor is quite close to the surface of the tissue, placement of the needle electrodes without exposing any metallic surface can be difficult. Also, it may not be practical to use the probe assembly when the tumors are quite shallow. In this case, the surface electrode may be used by itself. Efficient ablation of the tumor, however, may not be achieved if the tumor has a non-uniform thickness. 
     There thus is a need to provide improved systems and methods for more efficiently and safely ablating superficially oriented tumors. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present inventions, a surface electrode for ablating tissue comprises a base, a plurality of tissue penetrating needle electrodes extending from the surface of the base an adjustable distance, and an electrical interface coupled to the plurality of needle electrodes. By way of non-limiting example, the adjustability of the needle electrodes allows the depth that the needle electrodes penetrate through tissue to be adjusted. In the preferred embodiment, the needle electrodes are individually adjustable, so that the depths at which the needle electrodes penetrate the tissue can be varied. All or any portion of the needle electrodes carried by the base can be adjustable. 
     In one preferred embodiment, the base takes the form of a flat plate, but can take the form of any structure from which needle electrodes can be extended. In the preferred embodiment, the needle electrodes extend perpendicularly from the base, but can extend obliquely from the base as well. 
     The needle electrodes can be mounted to the base in any variety of manners, so that the distance that the needle electrodes extend from the base can be adjusted. For example, the needle electrodes can be mounted to the surface of the base in a threaded arrangement, such that rotation of each needle electrode in one direction increases the distance that the needle electrode extends from the surface of the base, and rotation of each needle electrode in the other direction decreases the distance that the needle electrode extends from the surface of the base. As another example, the needle electrodes can be mounted to the surface of the base in a sliding arrangement, such that displacement of each needle electrode in a distal direction increases the distance that the needle electrode extends from the surface of the base, and displacement of each needle electrode in a proximal direction decreases the distance that the needle electrode extends from the surface of the base. In this case, the surface electrode may further comprise one or more locking mechanisms (e.g., thumb screws) for fixing displacement of the needle electrodes relative to the surface of the base. 
     The electrical interface may be configured, such that the ablation energy is delivered to the needle electrodes in the desired manner. For example, the electrical interface may couple the needle electrodes in a monopolar or bipolar arrangement. The electrical interface may optionally be adjustable, so that certain needle electrodes can be selectively activated or combinations of needle electrodes can be selectively placed in a bipolar arrangement with respect to each other. 
     The surface electrode may optionally comprise insulation to minimize inadvertent ablation of healthy tissue. For example, the surface of the base from which the needle electrodes extend may be electrically insulated. Or the surface electrode may further comprise a plurality of electrically insulating sleeves extending from the surface of the base, wherein the insulating sleeves encompass portions of the respective needle electrodes. The insulating sleeves may be optionally extendable from the surface of the base an adjustable length. By way of non-limiting example, the adjustability of the insulating sleeves allows the depth at which healthy tissue is protected to be adjusted. 
     One or more of the plurality of needle electrodes can comprise a liquid conveying lumen to carry a medium for cooling, therapeutic, or other purposes. For example, the lumen(s) can be configured to perfuse a medium from, and/or internally convey a medium within, the respective needle electrode(s). 
     In accordance with a second aspect of the present inventions, a tissue ablation system comprises a surface electrode comprising a base and a plurality of tissue penetrating needle electrodes extending from the surface of the base an adjustable distance, and an ablation source (e.g., a radio frequency generator) coupled to the plurality of needle electrodes. The surface electrode can be configured in the same manner described above. 
     The tissue ablation system can be operated in a monopolar mode or a bipolar mode. For example, the tissue ablation system can further comprise a dispersive electrode, wherein the radio frequency generator comprises a first pole electrically coupled to the surface electrode and a second pole electrically coupled to the dispersive electrode. As another example, the first pole of the radio frequency generator can be electrically coupled to a first set of the plurality of needle electrodes and the second pole can be electrically coupled to a second set of the plurality of needle electrodes. If any of the needle electrodes is configured to convey a medium, the tissue ablation system can further comprise a source of medium (e.g., a pump) in fluid communication with the lumen of the respective needle electrode. 
     In an optional embodiment, the tissue ablation system can further comprise a clamping device having first and second opposing arms. In this case, the surface electrode can be mounted to one of the arms, and a similar surface electrode can be mounted to the other of the arms. The tissue ablation system can be operated in a bipolar mode by connecting the first pole of the radio frequency generator to the first surface electrode and the second pole to the second surface electrode. Alternatively, a second surface electrode is not provided, but rather a support member with or without tissue penetrating needles. In this case, the support member is used merely to stabilize contact between the surface electrode and the tissue. 
     In accordance with a third aspect of the present inventions, a method of ablating tissue using a surface electrode with a plurality of needle electrodes is provided. The method comprises adjusting distances that the needle electrodes extend from a base of the surface electrode, penetrating the tissue with the needle electrodes, and conveying ablation energy from the needle electrodes into the tissue (e.g., in a monopolar or bipolar mode) to create a lesion on the tissue. The needle electrode distances can be adjusted prior to, and/or subsequent to, penetrating the tissue with the needle electrodes. Optionally, the needle electrodes from which the ablation energy is conveyed can be dynamically selected. 
     In the preferred method, the needle electrode distances are individually adjusted, in which case, the needle electrode distances may differ from each other. These needle electrode distances can be adjusted in a variety of manners, e.g., by rotating the needle electrodes or longitudinally sliding the needle electrodes relative to the base. 
     In order to, e.g., protect healthy tissue, the surface electrode can be insulated by insulating the base and/or insulating portions of the needle electrodes that would otherwise be in contact with the tissue, e.g., by insulating the needle electrode portions with insulation sleeves that extend from the surface of the base. In this case, the distances from which the insulation sleeves extend from the surface of the base can be adjusted. 
     The method optionally comprises conveying a medium through one or more of the plurality of needle electrodes. For example, the medium can be perfused from the needle electrode(s) into the tissue (e.g., to cool the tissue and/or deliver a therapeutic agent to the tissue) and/or internally conveyed within the needle electrode(s) to cool the needle electrode(s). 
     In another optional method, the tissue can be penetrated with a plurality of needle electrodes of another surface electrode opposite the needle electrodes of the first surface electrode, in which case, the ablation energy may be conveyed from the other needle electrodes into the tissue. If RF energy is used as the ablation energy, the ablation energy can be conveyed between the first and second surface electrodes. In addition, the needle electrode distances of the second surface electrode can be adjusted in the same manner as the needle electrode distances of the first surface electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate the design and utility of a preferred embodiment of the present invention, in which similar elements are referred to by common reference numerals. In order to better appreciate the advantages and objects of the present invention, reference should be made to the accompanying drawings that illustrate this preferred embodiment. However, the drawings depict only one embodiment of the invention, and should not be taken as limiting its scope. With this caveat, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  is a perspective view of a prior art ablation assembly; 
         FIG. 2  is plan view of a tissue ablation system constructed in accordance with one preferred embodiment of the present inventions; 
         FIG. 3  is a perspective view of one preferred embodiment of a surface electrode that can be used in the tissue ablation system of  FIG. 2 , wherein an electrical interface that can configure the needle electrodes in a monopolar arrangement is particularly shown; 
         FIG. 4  is a cross-sectional view of the surface electrode of  FIG. 3 , wherein adjustment of needle electrodes are accomplished using a threaded arrangement; 
         FIG. 5  is a cross-sectional view of the surface electrode of  FIG. 3 , wherein adjustment of needle electrodes are accomplished using a sliding arrangement; 
         FIG. 6  is a perspective view of another preferred embodiment of a surface electrode that can be used in the tissue ablation system of  FIG. 2 , wherein an electrical interface that can configure the needle electrodes in a bipolar arrangement is particularly shown; 
         FIG. 7  is a perspective view of a still another preferred embodiment of a surface electrode that can be used in the tissue ablation system of  FIG. 2 , wherein adjustable insulated sleeves are used to insulate portions of the needle electrodes; 
         FIG. 8  is a cross-sectional view of the surface electrode of  FIG. 7 , wherein adjustment of needle electrodes is accomplished using a threaded arrangement; 
         FIG. 9  is a cross-sectional view of the surface electrode of  FIG. 7 , wherein adjustment of needle electrodes is accomplished using a sliding arrangement; 
         FIGS. 10-13  are plan views illustrating one preferred method of using the tissue ablation system of  FIG. 1  to ablate a treatment region within tissue of a patient; 
         FIG. 14  is plan view of a tissue ablation system constructed in accordance with another preferred embodiment of the present inventions; 
         FIG. 15  is a cross-sectional view of one preferred embodiment of a needle electrode that can be used in the tissue ablation system of  FIG. 14 ; 
         FIG. 16  is a cross-sectional view of another preferred embodiment of a needle electrode that can be used in the tissue ablation system of  FIG. 14 ; 
         FIG. 17  is a perspective view of a preferred embodiment of a surface electrode that can be used in the tissue ablation system of  FIG. 15 ; 
         FIG. 18  is plan view of a tissue ablation system constructed in accordance with still another preferred embodiment of the present inventions; and 
         FIGS. 19-20  are plan views illustrating one preferred method of using the tissue ablation system of  FIG. 18  to ablate a treatment region within tissue of a patient. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  illustrates a tissue ablation system  100  constructed in accordance with a preferred embodiment of the present invention. The tissue ablation system  100  generally comprises a surface electrode  102 , which is configured to be applied to the surface of tissue, e.g., an organ, in order to ablate target tissue therein, and a radio frequency (RF) generator  104  configured for supplying RF energy to the surface electrode  102  via a cable  106  in a controlled manner. In the embodiment illustrated in  FIG. 2 , only one surface electrode  102  is shown. As will be described in further detail below, however, multiple surface electrodes  102  can be connected to the RF generator  104  depending upon the specific ablation procedure that the physician selects. 
     Referring further to  FIG. 3 , the surface electrode  102  generally comprises a base  108  having a flat surface  110 , an array of needle electrodes  112  extending from the surface  110  of the base  108 , and an electrical interface  114  for electrically coupling the needle electrodes  112  to the RF generator  104 . 
     In the illustrated embodiment, the base  108  takes the form of a flat rectangular plate, but other shapes (e.g., circular, oval, quadrangular) can be used, depending upon the nature of the tissue to be ablated. Exemplary bases will have a length in the range of 10 mm to 100 mm, preferably from 30 mm to 60 mm, a width in the range of 10 mm to 100 mm, preferably from 30 mm to 60 mm, and a thickness in the range of 1 mm to 20 mm, preferably from 5 mm to 15 mm. Preferably, the base  108  is composed of a rigid material, such as, e.g., Acrylonitrile Butadiene Styrene (ABS), polycarbonate, polyvinylchloride (PVC), aluminum, or stainless steel. In this manner, application of force on any flat portion of the base  108  will uniformly distribute the force along the surface  110  of the base  108 , so that all of the needle electrodes  112  will penetrate the tissue. In alternative embodiments, however, the base  108  may be composed of a semi-rigid or flexible material. In this manner, the base  108  can be conveniently conformed to the curved surface of tissue. In this case, the force needed to penetrate the tissue with the needle electrodes  112  can be applied to the base  108  in a uniformly distributed manner, e.g., using the entire palm of a hand, or can be serially applied to portions of the base  108  so that the needle electrodes  112  penetrate the tissue in a piece-meal fashion. 
     In any event, if the base  108  is composed of an electrically conductive material, a layer of insulation  116 , e.g., rubber, is disposed on the surface  110  of the base  108  using suitable means, such as bonding. In this manner, any ablation energy will be concentrated within the needle electrodes  112 , so that only the target tissue is ablated, thereby preventing non-target tissue, e.g., the surface of the tissue in contact with the surface  110  of the base  108 , from being ablated. 
     The needle electrodes  112  are composed of a rigid electrically conductive material, e.g., stainless steel. The needle electrodes  112  preferably have circular cross-sections, but may have non-circular cross-sections, e.g., rectilinear cross-sections. Exemplary needle electrodes will have a diameter in the range of 0.6 mm to 3.4 mm, preferably from 1.1 mm to 1.6 mm, and a length in the range of 10 mm to 70 mm, preferably from 20 mm to 50 mm. The distal ends of the needle electrodes  112  may be honed or sharpened to facilitate their ability to penetrate tissue. The distal ends  118  of these needle electrodes  112  may be hardened using conventional heat treatment or other metallurgical processes. The needle electrodes  112  are covered with insulation (not shown), although they will be at least partially free from insulation over their distal ends  118 . As will be described in further detail below, the portion of the needle electrodes  112  that can be placed in contact with the base  108  are also free of insulation. 
     In the illustrated embodiment, the RE current is delivered to the needle electrodes  112  in a monopolar fashion. Therefore, the current will pass through the needle electrodes  112  and into the target tissue, thus inducing necrosis in the tissue. To this end, the needle electrodes  112  are configured to concentrate the energy flux in order to have an injurious effect on tissue. However, there is a dispersive electrode (not shown) which is located remotely from the needle electrodes  112 , and has a sufficiently large area—typically 130 cm 2  for an adult—so that the current density is low and non-injurious to surrounding tissue. In the illustrated embodiment, the dispersive electrode may be attached externally to the patient, using a contact pad placed on the patient&#39;s skin. 
     Alternatively, the RF current is delivered to the needle electrodes  112  in a bipolar fashion, which means that current will pass between “positive” and “negative” electrodes  112 . As will be described in further detail below, RF current can also pass between needle electrodes  112  of two or more surface electrodes  102  in a bipolar fashion. 
     The lengths of the needle electrodes  112  are adjustable, i.e., the needle electrodes  112  extend from the surface  110  of the base  108  an adjustable distance. Specifically, in the illustrated the proximal ends  120  of the needle electrodes  112  are mounted within apertures  122  formed through the base  108  in a threaded arrangement. As best shown in  FIG. 4 , the shafts of the needle electrodes  112  and the respective apertures  122  formed through the base  108  include threads  124 , such that rotation of the needle electrodes  112  in one direction  126  (here, in the counterclockwise direction) decreases the distances that the needle electrodes  112  extend from the surface  110  of the base  108 , and rotation of the needle electrodes  112  in the opposite direction  128  (here, in the clockwise direction) increases the distances that the needle electrodes  112  extend from the surface  110  of the base  108 . As can be appreciated, the lengths of the needle electrodes  112  are individually adjustable, so that the distances that the respective needle electrodes  112  extend from the surface  110  of the base  108  may differ, as clearly shown in  FIG. 4 . 
     Alternatively, as illustrated in  FIG. 5 , the shafts of the needle electrodes  112  can be slidably mounted within the apertures  122  formed through the base  108 , so that longitudinal translation of the needle electrodes  112  relative to the apertures  122  in the distal direction  130  increases the distances that the needle electrodes  112  extend from the surface  110  of the base  108 , and longitudinal translation of the needle electrodes  112  relative to the apertures  122  in the proximal direction  132  decreases the distances that the needle electrodes  112  extend from the surface  110  of the base  108 . In order to fix the needle electrodes  112  relative to the base  108 , the base  108  comprises bosses  134  formed along its opposite surface  111 . Each of the bosses  134  comprises a threaded hole  136  with an associated thumb screw  138 . Thus, the shafts of the needle electrodes  112  can be affixed within the bosses  134  by tightening the thumb screws  138 , and released within the bosses  134  by loosening the thumb screws  138 . 
     Referring back to  FIG. 3 , the electrical interface  114  provides a means for selectively configuring the needle electrodes  112 . For example, in a monopolar arrangement where all of the needle electrodes  112  are coupled to a first pole of the RF generator  104  and a separate dispersive electrode (not shown) is coupled to a second pole of the RF generator  104 , the electrical interface  114  can comprise a single dip switch  140  coupled to the first pole of the RF generator  104 . In this manner, the dip switch  140  can be operated to selectively turn each needle electrode  112  on or off, so that RF energy will be conveyed between some of the needle electrodes  112  and the dispersive electrode, and will not be conveyed between others of the needle electrodes  112  and the dispersive electrode. 
     In a bipolar arrangement where some of the needle electrodes  112  are coupled to the first pole of the RF generator  104  and others of the needle electrodes  112  are coupled to the second pole of the RF generator  104 , the electrical interface  114  can take the form of a pair of dip switches  140 ′ and  140 ″ respectively coupled to the two poles of the RF generator  104 , as illustrated in  FIG. 6 . In this manner, the first dip switch  140 ′ can be operated to turn a first subset of needle electrodes  112  on, and the second dip switch  140 ″ can be operated to turn a second different subset of needle electrodes  112  on, so that RF energy will be conveyed between the first and second subsets of needle electrodes  112 . 
     Whether the needle electrodes  112  are operated in a monopolar mode or bipolar mode, the electrical interface  114  is coupled to the respective needle electrodes  112  by electrical paths  144  that are preferably electrically isolated from each other. This can be accomplished depending on the means that is used for adjusting the lengths of the needle electrodes  112 . For example, if the lengths of the needle electrodes  112  are adjusted by rotating the needle electrodes  112  within the respective apertures  122 , the electrical paths can comprise electrical traces (shown in  FIG. 3 ) that extend along the opposite surface  111  of the base  108  between the electrical interface  114  and the apertures  122  in which the shafts of the needle electrodes  112  are mounted. In this case, the base  108  can be composed of an electrically non-conductive material, in which case, the apertures  122  can be coated with an electrically conductive material, such as, e.g., gold or copper. If the lengths of the needle electrodes  112  are adjusted by longitudinally translating the needle electrodes  112  within the respective apertures  122 , the electrical paths  144  can comprise insulated wires (not shown) that extend between the electrical interface  114  and the proximal ends  120  of the needle electrodes  112 . 
     It should be noted, however, that if selective arrangement of the needle electrodes  112  is not desired, and the needle electrodes  112  are not operated in a bipolar mode, the base  108  can be totally composed of an electrically conductive material, in which case, the electrical interface  114  can take the form of a simple connection that operates to couple the cable  106  of the RF generator  104  to the apertures  122  (and thus the needle electrodes) through the base  108  itself. 
     In an optional embodiment of a surface electrode, the electrical insulation associated with the needle electrodes  112  can be adjustable, so that the length of the electrically conductive portion of the needle electrodes  112  that will be in contact with the tissue can be adjusted. For example, as illustrated in  FIG. 7 , an optional surface electrode  152  is similar to the previously described surface electrode  102 , with the exception that it comprises insulating sleeves  154  that are mounted within apertures  122  formed through the base  108 . 
     The lengths of the insulating sleeves  154  are adjustable, i.e., the distances that the insulating sleeves  154  extend from the surface  110  of the base  108  can be varied. Specifically, in the illustrated embodiment, the insulating sleeves  154  are mounted within apertures  122  formed through the base  108  in a threaded arrangement. As best shown in  FIG. 8 , the insulating sleeves  154  and the respective apertures  122  formed through the base  108  include threads  156 , such that rotation of the insulating sleeves  154  in the direction  126  decreases the distances that the insulating sleeves  154  extend from the surface  110  of the base  108 , and rotation of the insulating sleeves  154  in the opposite direction  128  increases the distances that the insulating sleeves  154  extend from the surface  110  of the base  108 . 
     As with the previously described surface electrode  102 , the lengths of the needle electrodes  112  are also adjustable. In this case, the proximal ends of the needle electrodes  112  are mounted within insulating sleeves  154  in a threaded arrangement. The shafts of the needle electrodes  112  and the insulating sleeves  154  include threads  158 , such that rotation of the needle electrodes  112  relative to the insulating sleeves  154  in direction  126  decreases the distances that the needle electrodes  112  extend from the surface  110  of the base  108 , and rotation of the needle electrodes  112  relative to the insulating sleeves  154  in the opposite direction  128  increases the distances that the needle electrodes  112  extend from the surface  110  of the base  108 . 
     Alternatively, as illustrated in  FIG. 9 , the shaft of the needle electrodes  112  can be slidably mounted within the insulating sleeves  154 , so that longitudinal translation of the needle electrodes  112  relative to the insulating sleeves  154  in the distal direction  130  increases the distances that the needle electrodes  112  extend from the surface  110  of the base  108 , and longitudinal translation of the needle electrodes  112  relative to the insulating sleeves  154  in the proximal direction  132  decreases the distances that the needle electrodes  112  extend from the surface  110  of the base  108 . In order to fix the needle electrodes  112  relative to the base  108 , each of the insulating sleeves  154  comprises a threaded hole  160  with an associated thumb screw  162 . Thus, the shafts of the needle electrodes  112  can be affixed within the insulating sleeves  154  by tightening the thumb screws  162 , and released within the insulating sleeves  154  by loosening the thumb screws  162 . 
     Whichever means is used to adjust the needle electrodes  112  within the insulating sleeves  154 , the lengths of the insulating sleeves  154  are individually adjustable, so that the distances that the respective insulating sleeves  154  extend from the surface  110  of the base  108 , and thus, the lengths of the electrical portions of the needle electrodes  112  that will be in contact with the tissue, may differ. In a similar manner, the needle electrodes  112  are individually adjustable, so that the distances that the respective needle electrodes  112  extend from the surface  110  of the base  108 , and thus, the depths that they penetrate tissue, may differ. 
     The insulating sleeves  154  are composed of a rigid electrically non-conductive material, such as, e.g., fluoropolymer, polyethylene terephthalate (PET), polyetheretherketon (PEEK), polyimide, and other like materials. Alternatively, the insulating sleeves  154  may be composed of an electrically conductive material that is coated within an electrically non-conductive material. If the lengths of the needle electrodes  112  are adjusted by rotating the needle electrodes  112  within the respective apertures  122 , the threaded portions of the insulating sleeves  154  are preferably composed of an electrically conductive material to provide an electrical path between the shafts of the needle electrodes  112  and the traces  144  extending along the opposite surface  111  of the base  108 . If the lengths of the needle electrodes  112  are adjusted by longitudinally translating the needle electrodes  112  within the insulating sleeves  154 , the insulating sleeves  154  can be composed entirely of electrically non-conductive material, since the wires leading from the electrical interface  114  can be coupled directly to the proximal ends  120  of the needle electrodes  112 . 
     Referring back to  FIG. 2 , as previously noted, the RF generator  104  is electrically connected, via the electrical interface  114 , to the needle electrodes  112 . The RF generator  104  is a conventional RF power supply that operates at a frequency in the range of 200 KHz to 1.25 MHz, with a conventional sinusoidal or non-sinusoidal wave form. Such power supplies are available from many commercial suppliers, such as Valleylab, Aspen, and Bovie. Most general purpose electro-surgical power supplies, however, operate at higher voltages and powers than would normally be necessary or suitable for controlled tissue ablation. 
     Thus, such power supplies would usually be operated at the lower ends of their voltage and power capabilities. More suitable power supplies will be capable of supplying an ablation current at a relatively low voltage, typically below 150V (peak-to-peak), usually being from 50V to 100V. The power will usually be from 20 W to 200 W, usually having a sine wave form, although other wave forms would also be acceptable. Power supplies capable of operating within these ranges are available from commercial vendors, such as Boston Scientific of San Jose, Calif., which markets these power supplies under the trademarks RF2000™ (100 W) and RF3000™ (200 W). 
     Having described the structure of the tissue ablation system  100 , its operation in treated targeted tissue will now be described. The treatment region may be located anywhere in the body where hyperthermic exposure may be beneficial. Most commonly, the treatment region will comprise a solid tumor within an organ of the body, such as the liver, kidney, pancreas, breast, prostrate (not accessible via the urethra), and the like. The volume to be treated will depend on the size of the tumor or other lesion, typically having a total volume from 1 cm 3  to 150 cm 3 , and often from 2 cm 3  to 35 cm 3 . The peripheral dimensions of the treatment region may be regular, e.g., spherical or ellipsoidal, but will more usually be irregular. The treatment region may be identified using conventional imaging techniques capable of elucidating a target tissue, e.g., tumor tissue, such as ultrasonic scanning, magnetic resonance imaging (MRI), computer assisted tomography (CAT) fluoroscopy, nuclear scanning (using radiolabeled tumor-specific probes), and the like. Preferred is the use of high resolution ultrasound of the tumor or other lesion being treated, either intraoperatively or externally. 
     Referring now to  FIG. 10-12 , the operation of the tissue ablation system  100  is described in treating a treatment region TR, such as a tumor, located below the surface S of tissue T, e.g., an organ. The treatment region TR has a proximal surface S 1  and a distal surface S 2  opposite the proximal surface S 1 . As can be seen, the depth of the treatment region TR varies below the surface S of the tissue. Thus, by itself, a surface electrode having needle electrodes with uniform lengths will typically not optimally ablate the treatment region TR. The previously described surface electrode  102 , however, can be used to optimally ablate the treatment region TR when properly configured. 
     First, the electrical interface  114  on the surface electrode  102  is configured, based on the shape of the treatment region TR and whether a monopolar or a bipolar arrangement is desired. The surface electrode  102  is then positioned onto the surface S of the tissue T directly above the treatment region TR, and pressure is applied so that the needle electrodes  112  penetrate into the tissue T, as illustrated in  FIG. 11 . Preferably, access to the tissue T is gained through a surgical opening made through the skin of the patient. If the surface electrode  102  is small enough or flexible enough, it can alternatively be introduced into contact with the tissue T laparoscopically. Once the needle electrodes  112  are embedded into the tissue T, the distances that the needle electrodes  112  extend from the base  108  of the surface electrode  102  are individually adjusted, so that the distal ends  118  of the needle electrodes  112  penetrate through, or almost penetrate through, the distal surface S 2  of the treatment region TR, as illustrated in  FIG. 12 . As can be appreciated, the deeper a region of the distal surface S 2  is below the surface of the tissue T, the greater the distance that the needle electrode  112  associated with that region must extend from the base  108  of the surface electrode  102 . 
     If the surface electrode  152  with adjustable insulating sleeves  154  is used, the distances that the insulating sleeves  154  extend from the base  108  of the surface electrode  102  are individually adjusted, so the distal ends of the insulating sleeves  154  are just above the proximal surface S 1  of the treatment region TR, as illustrated in  FIG. 13 . As can be appreciated, the deeper a region of the proximal surface S 1  is below the surface of the tissue T, the greater the distance that the insulating sleeve  204  associated with that region must extend from the base  108  of the surface electrode  102 . Thus, as can be seen, the electrically conductive portions of the needle electrodes  112  are only in contact with the treatment region TR. 
     It should be noted that whichever surface electrode is used, the treatment region TR can be monitored using suitable imaging means to ensure that the needle electrodes  112  and/or insulating sleeves  154  are properly adjusted. It should also be noted that gross adjustments of the needle electrodes  112  and/or insulating sleeves  154  can be accomplished prior to introducing the surface electrode  102  within the patient&#39;s body to minimize adjustments to the surface electrode  102  while it is in the patient&#39;s body. In this case, fine adjustments of the needle electrodes  112  and/or insulating sleeves  154  can be performed after the needle electrodes  112  have been embedded into the tissue T. 
     Next, the RF generator  104  is connected to the electrical interface  114  of the surface electrode  102 , and then operated to ablate the treatment region TR, resulting in the formation of a lesion that preferably encompasses the entirety of the treatment region TR. If the treatment region TR is substantially larger than that which the surface electrode  102  can cover, thereby resulting in a treatment region TR that is only partially ablated, the surface electrode  102  can be moved to the non-ablated portion of the treatment region TR, and the process can then be repeated. Alternatively, multiple surface electrodes  102  can be used, so that the large treatment region TR can be ablated in one step. 
       FIG. 14  illustrates a tissue ablation system  200  constructed in accordance with another preferred embodiment of the present invention. The tissue ablation system  200  is similar to the previously described tissue ablation system  100 , with the exception that it provides cooling functionality. Specifically, the tissue ablation system  200  comprises the previously described RF generator  104 , a surface electrode  202  additionally configured to cool the target tissue while it is being ablated, and a pump assembly  204  configured for delivering a cooling medium to the surface electrode  102 . 
     The surface electrode  202  is similar to the surface electrode  102  illustrated in  FIG. 5 , with the exception that it has cooling functionality. Specifically, the surface electrode  202  comprises needle electrodes  212  that comprise cooling lumens through which a cooling medium can be pumped. In the illustrated embodiment, two types of needle electrodes  212  are used: an irrigated needle electrode  212 ( 1 ) and an internally cooled needle electrode  212 ( 2 ). As illustrated in  FIG. 15 , the irrigated needle electrode  212 ( 1 ) comprises a cooling lumen  218  that originates at an entry port  220  at the proximal end  214  of the needle electrode  212 ( 1 ) and terminates at an exit port  222  at the distal end  216  of the needle electrode  212 ( 1 ). As a result, a cooling medium that is conveyed through the entry port  220  and distally through the cooling lumen  218  is perfused out from the exit port  222  into the surrounding tissue, thereby cooling the tissue while it is being ablated. As illustrated in  FIG. 16 , the internally cooled needle electrode  212 ( 2 ) comprises a cooling lumen  224  that originates at an entry port  228  at the proximal end  214  of the needle electrode  212 ( 2 ) and a return lumen  226  that terminates at an exit port  230  at the proximal end  214  of the needle electrode  212 ( 2 ). The cooling and return lumens  224  and  226  are in fluid communication with each other at the distal end  216  of the needle electrode  212 ( 2 ). As a result, a cooling medium is conveyed into the entry port  228  and distally through the cooling lumen  224  to cool the shaft of the needle electrode  212 ( 2 ), with the resultant heated medium being proximally conveyed through the return lumen  226  and out through the exit port  230 . It should be noted that for the purposes of this specification, a cooling medium is any medium that has a temperature suitable for drawing heat away from the surface electrode  202 . For example, a cooling medium at room temperature or lower is well suited for cooling the surface electrode  202 . 
     Referring to  FIG. 17 , the surface electrode  202  further comprises a fluid manifold  232  having an inlet fluid port  234  and an outlet fluid port  236  that are configured to be connected to the pump assembly  204 , as will be described in further detail below. The fluid manifold  232  further comprises an array of branch ports  238  that are in fluid communication with the inlet and outlet ports  234  and  236 . 
     The surface electrode  202  further comprises an array of conduits  240  that are respectively mounted at their proximal ends to the branched ports  238  of the cooling manifold  232  and at their distal ends to proximal ends  214  of the needle electrodes  212  in fluid communication with the lumens therein. The conduits  240  that are associated with the irrigated needle electrodes  212 ( 1 ) each comprises a single cooling lumen (not shown) that is in fluid communication between the inlet port  234  of the cooling manifold  232  and the entry port  220  (shown in  FIG. 15 ) of the respective needle electrode  212 ( 1 ). The conduits  240  that are associated with the internally cooled needle electrodes  212 ( 2 ) each comprises a cooling lumen (not shown) that is in fluid communication between the inlet port  234  of the cooling manifold  232  and the entry port  228  (shown in  FIG. 16 ) of the respective needle electrode  212 ( 2 ), and a return lumen (not shown) that is in fluid communication between the outlet port  236  of the cooling manifold  232  and the exit port  230  (shown in  FIG. 16 ) of the respective needle electrode  212 ( 2 ). 
     Referring back to  FIG. 14 ; the pump assembly  204  comprises a power head  242  and a syringe  244  that is front-loaded on the power head  242  and is of a suitable size, e.g., 200 ml. The power head  242  and the syringe  244  are conventional and can be of the type described in U.S. Pat. No. 5,279,569 and supplied by Liebel-Flarsheim Company of Cincinnati, Ohio. The pump assembly  204  further comprises a source reservoir  246  for supplying the cooling medium to the syringe  244 , and a discharge reservoir  248  for collecting the heated medium from the surface electrode  202 . The pump assembly  204  further comprises a tube set  250  removably secured to an outlet  252  of the syringe  244 . Specifically, a dual check valve  254  is provided with first and second legs  256  and  258 . The first leg  256  serves as a liquid inlet connected by tubing  260  to the source reservoir  246 . The second leg  258  serves as a liquid outlet and is connected by tubing  262  to the inlet fluid port  234  on the cooling manifold  232  of the surface electrode  202 . The discharge reservoir  248  is connected to the outlet fluid port  236  on the cooling manifold  232  of the surface electrode  202  via tubing  264 . 
     Thus, it can be appreciated that the pump assembly  204  can be operated to periodically fill the syringe  244  with the cooling medium from the source reservoir  246 , and convey the cooling medium from the syringe  244 , through the tubing  262 , and into the inlet fluid port  232  on the cooling manifold  232 . The cooling medium will then be conveyed through the branched ports  238  of the cooling manifold  232 , through the cooling lumens on the conduits  240  (shown in  FIG. 17 ), and into the cooling lumens  218  and  224  in the respective needle electrodes  212 ( 1 ) and  212 ( 2 ) (shown in  FIGS. 15 and 16 ). With respect to the irrigated needle electrodes  212 ( 1 ), the cooling medium will be perfused into the tissue to cool it. With respect to the internally cooled needle electrodes  212 ( 2 ), the cooling medium will be internally circulated within the needle electrode  112  to cool it. The resultant heated medium will be conveyed from the return lumens  226  of the needle electrodes  212 ( 2 ), through the return lumens of the conduits  240 , and into the branched ports  238  of the cooling manifold  232 . The heat medium will then be conveyed from the outlet fluid port  234  on the cooling manifold  232 , through the tubing  264 , and into the discharge reservoir  248 . 
     Operation of the tissue ablation system  200  will be similar to that of the tissue ablation system  100 , with the exception that the pump assembly  204  will be operated to cool the target tissue and needle electrodes  212 . Optionally, the cooling medium can contain a therapeutic agent that can be delivered to the tissue via the irrigated needle electrodes  212 ( 2 ). The pump assembly  204 , along with the RF generator  104 , can include control circuitry to automate or semi-automate the cooled ablation process. 
       FIG. 18  illustrates a tissue ablation system  300  constructed in accordance with still another preferred embodiment of the present invention. The tissue ablation system  300  is similar to the previously described tissue ablation system  100 , with the exception that it comprises an ablative clamping device  302  that incorporates two surface electrodes  102 . In particular, the clamping device  302  comprises two opposing clamping members  304  that are coupled to each other via a pivot point  306 . The surface electrodes  102  are mounted to arms  308  of the respective clamping members  304 , such that the needle electrodes  112  of the surface electrodes  102  oppose each other. The clamping members  304  include handles  310  that, when closed, move the opposing needles  112  of the respective surface electrodes  102  away from each other, and when opened, move the opposing needles  112  of the respective surface electrodes  102  towards each other. Thus, it can be appreciated that the clamping device  302  can engage and penetrate tissue in an opposing manner when the handles  310  of the clamping device  302  are opened, and disengage the tissue when the handles  310  are closed. 
     The cables  106  of the RF generator  104  are coupled to the electrical interfaces (not shown in  FIG. 18 ) of the respective surface electrodes  102  via RF wires (not shown) extending through the clamping members  304 . In the illustrated embodiment, the respective surface electrodes  102  are coupled to the RF generator  104  in a bipolar arrangement, i.e., one surface electrode  102  is coupled to the first pole of the RF generator  104 , and the other surface electrode  102  is coupled to the second pole of the RF generator  104 . The needle electrodes  112  on each of the surface electrodes  102  can be selectively activated using the electrical interface on the respective surface electrode  102 . Alternatively, only one of the surface electrodes  102  is activated, in which case, the other surface electrode merely serves as a means for stabilizing the other surface electrode  102  within the tissue. The needle electrodes  112  of the active surface electrode  102  can be designed to be placed in a bipolar arrangement or a monopolar arrangement. Deactivation of the surface electrode  102  can be accomplished via the electrical interface, or alternatively, a stabilizing member with non-active needles, or a stabilizing member with no tissue penetrating needles, can be used instead of a potentially active surface electrode  102 . 
     Referring now to  FIGS. 19 and 20 , operation of the tissue ablation system  300  is described in treating a treatment region TR, such as a tumor, located between opposing surfaces S 1  and S 2  of tissue T, e.g., an organ. The treatment region TR also has opposing surfaces S 3  and S 4 . 
     First, the electrical interface(s) on the surface electrodes  102  are configured, based on the shape of the treatment region TR and whether a monopolar or a bipolar arrangement is desired. The surface electrodes  102  are then positioned, such that they are respectively adjacent the opposing surfaces S 1  and S 2  of the tissue T, as illustrated in  FIG. 19 . Preferably, access to the tissue T is gained through a surgical opening made through the skin of the patient. The clamping device  302  can be closed, while introduced through the opening, and then opened in order to place the tissue T between the opposing surface electrodes  102 . Alternatively, each individual clamping member  304  and associated surface electrode  102  can be introduced through two respective laparoscopes and then subsequently assembled at the pivot point  306 . 
     Next, the clamping device  302  is closed, such that the needle electrodes  112  of the respective surface electrodes  102  penetrate through the surfaces S 1  and S 2  into the tissue T, as illustrated in  FIG. 20 . As illustrated, the application of pressure by the clamping device  302 , causes the tissue T in contact with the surface electrodes  102  to compress. Once the needle electrodes  112  are embedded into the tissue T, the distances that the needle electrodes  112  extend from the base  108  of the surface electrode  102  are individually adjusted in the manner previously described. If provided, the optional insulating sleeves can also be adjusted. The RF generator  104  is then connected to the clamping device  300 , and then operated to ablate the treatment region TR, resulting in the formation of a lesion that preferably encompasses the entirety of the treatment region TR. If the treatment region TR is substantially larger than that which the surface electrode  102  can cover, thereby resulting in a treatment region TR that is only partially ablated, the clamping device  302  can be opened to release the needle electrodes  102  from the tissue T, and then reapplied to a different portion of the tissue T. 
     Although particular embodiments of the present invention have been shown and described, it should be understood that the above discussion is not intended to limit the present invention to these embodiments. It will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. Thus, the present invention is intended to cover alternatives, modifications, and equivalents that may fall within the spirit and scope of the present invention as defined by the claims.