Abstract:
Tissue is treated using a radiofrequency power supply connected to an applicator having a chamber filled with an electrically non-conductive gas surrounded by a thin dielectric wall. A radiofrequency voltage is applied at a level sufficient to ionize the gas into a plasma and to capacitively couple the ionized plasma with the tissue to deliver radiofrequency current to ablate or otherwise treat the tissue.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 12/944,466 (Attorney Docket No. 37644-703.203), filed Nov. 11, 2010, which claims the benefit of U.S. Provisional Application No. 61/261,246 (Attorney Docket No. 37644-703.102), filed Nov. 13, 2009, the full disclosures of which are incorporated herein by reference. This application also is a continuation-in-part of U.S. application Ser. No. 12/541,050 (Attorney Docket No. 37644-703.202), filed Aug. 13, 2009, which claims the benefit of Provisional Application No. 61/196,870 (Attorney Docket No. 37644-703.101), filed on Oct. 21, 2008, the full disclosures of which are incorporated herein by reference. This application is being filed on the same day as application Ser. No. 12/______ (Attorney Docket No. 37644-703.501) which has a substantially identical disclosure. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to electrosurgical devices and related methods for rapid, controlled ablation of tissue. More particularly, the present invention relates to treating tissue with a radiofrequency current delivered through an electrically non-conductive gas which is ionized to capacitively couple to surrounding tissue through a thin dielectric layer surrounding the gas. 
         [0004]    The treatment of diseased organs, such as the uterus and the gallbladder, by ablation of an endometrial or mucosal layer surrounding the interior of the organ has long been proposed. Such internal surface ablation can be achieved by heating the surface, treating the surface with microwave energy, treating the surface with cryoablation, and delivering radiofrequency energy to the surface. Of particular interest to the present invention, a variety of radiofrequency ablation structures have been proposed including solid electrodes, balloon electrodes, metalized fabric electrodes, and the like. While often effective, at least most of the prior electrode designs have suffered from one or more deficiencies, such as relatively slow treatment times, incomplete treatments, non-uniform ablation depths, and risk of injury to adjacent organs. 
         [0005]    For these reasons, it would be desirable to provide methods and apparatus for the radiofrequency ablation of internal tissue surfaces which are rapid, provide for uniform ablation depths, which assure complete ablation over the entire targeted surface, and which reduce the risk of injury to adjacent organs. At least some of these objectives will be met by the inventions described hereinbelow. 
         [0006]    2. Description of the Background Art 
         [0007]    U.S. Pat. No. 4,979,948, describes a balloon filled with an electrolyte solution for distributing radiofrequency current to a mucosal layer via capacitive coupling. US 2008/097425, having common inventorship with the present application, describes delivering a pressurized flow of a liquid medium which carries a radiofrequency current to tissue, where the liquid is ignited into a plasma as it passes through flow orifices. U.S. Pat. No. 5,891,134 describes a radiofrequency heater within an enclosed balloon. U.S. Pat. No. 6,041,260 describes radiofrequency electrodes distributed over the exterior surface of a balloon which is inflated in a body cavity to be treated. U.S. Pat. No. 7,371,231 and US 2009/054892 describe a conductive balloon having an exterior surface which acts as an electrode for performing endometrial ablation. U.S. Pat. No. 5,191,883 describes bipolar heating of a medium within a balloon for thermal ablation. U.S. Pat. No. 6,736,811 and U.S. Pat. No. 5,925,038 show an inflatable conductive electrode. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    The present invention provides methods, apparatus, and systems for treating tissue of a patient. The treatment generally comprises delivering a radiofrequency current to the tissue in order to heat and usually ablate the tissue to a desired depth. Current is delivered to the tissue from a radiofrequency energy source through a first dielectric medium and a second dielectric medium in series with the first medium. The first dielectric medium will usually comprise an electrically non-conductive gas which may be ionized to form a plasma, typically by application of a high voltage radiofrequency voltage, but optionally by the direct application of heat to the gas, further optionally by the application of both the high radiofrequency voltage and heat to the gas. The second dielectric medium will separate the first medium from the target tissue, typically comprising a thin dielectric material, such as silicone or a silicone-based material, more typically comprising a thin dielectric wall which defines an interior chamber which contains the electrically non-conductive gas. The radiofrequency current is thus delivered to the tissue by applying a radiofrequency voltage across the first and second dielectric media so that the first dielectric becomes ionized, typically forming a gas plasma, and the second dielectric allows current flow to the tissue via capacitive coupling. 
         [0009]    Methods for treating tissue of a patient in accordance with the present invention comprise containing an electrically non-conductive gas in an interior chamber of an applicator having a thin dielectric wall surrounding at least a portion of the interior chamber. An external surface of the thin dielectric wall is engaged against a target region of the tissue, and a radiofrequency voltage is applied across the gas and thin wall, where the voltage is sufficient to ionize the gas to initiate a plasma in the gas and to capacitively couple the current in the gas plasma across the dielectric wall and into the engaged tissue. 
         [0010]    The electrically non-conductive gas may be held statically within the chamber, but will more often be actively flowing through the chamber of the applicator. The flow rate of the non-conductive gas will typically be in the range from about 1 ml/sec to 50 ml/sec, preferably from 5 ml/sec to 30 ml/sec. The interior chamber will have a volume in the range from 0.01 ml to 100 ml, typically from 2 ml to 10 ml. Usually, the electrically non-conductive gas will be argon or another noble gas or mixture of noble gases. 
         [0011]    The dielectric wall of the applicator may assume a variety of configurations. In a first embodiment, the dielectric wall will have a generally fixed shape that will remain constant regardless of the internal pressurization of the contained gas. Alternatively, the dielectric wall may be elastic, conformable, slack, or otherwise having a changeable shape which can conform to the engaged tissue surface. In some examples, the thin dielectric wall will comprise a balloon or other inflatable structure which is expanded by increasing an internal pressure of the electrically non-conductive gas or other medium. Alternatively, a separate frame, cage, spring, or other mechanical deployment structure could be provided within an elastic or non-elastic conformable thin dielectric wall. In the latter case, the frame or other structure can be configured and reconfigured to shape the thin dielectric wall as desired in the method. 
         [0012]    The voltage is applied to the tissue by providing a first electrode surface coupled to the non-conductive gas and a second electrode surface coupled to the patient tissue. A radiofrequency voltage is then applied across the first and second electrodes in order to both ionize the electrically non-conductive gas (forming a plasma) within the interior chamber and to capacitively couple the charged plasma with tissue across the thin dielectric wall. 
         [0013]    The voltage applied to the first and second dielectric media will depend on the distance between the first electrode surface and the dielectric wall as well as the resistance between the dielectric wall and the second electrode which is in contact with the tissue, typically being in the range between 500V (rms) and 2500V (rms). In the exemplary embodiments, the first electrode surface will usually be in or on the interior chamber or a gas flow path leading to the interior chamber, and the second electrode surface will be in contact with the patient&#39;s tissue, often being disposed on a shaft or other external surface of the treatment device. 
         [0014]    In a second aspect of the present invention, apparatus for delivering radiofrequency current to tissue comprises a body having a support end, a working end, and an interior chamber. A thin dielectric wall surrounds at least a portion of the interior chamber and has an external surface disposed at the working end of the body. A gas inlet will be provided to connect to the chamber for delivery of an electrically non-conductive gas, either in a continuously flowing mode or in a static mode. A first electrode structure is provided which has a surface exposed to either the interior chamber or the gas inlet. A second electrode structure is also provided and has a surface adapted to contact tissue, typically being somewhere on the body, more typically being on a handle or shaft portion of the device. The apparatus further includes a radiofrequency power supply connected to apply a radiofrequency voltage across the first and second electrode structures, wherein the voltage is sufficient to initiate ionization of the gas into a plasma within the chamber. The voltage will further be sufficient to capacitively couple the current in the plasma across the dielectric wall and into tissue adjacent the external surface. 
         [0015]    The specific structure of the body may vary. In a first example, the dielectric wall may comprise a rigid material, typically selected from the group consisting of a ceramic, glass, and polymer. The rigid material may be formed into a variety of geometries, including a tube, sphere, or the like. Usually, the dielectric wall will have a thickness in the range from about 0.002 in to 0.1 in, usually from 0.005 in to 0.05 in. 
         [0016]    In alternative embodiments, the dielectric wall may comprise a conformable material, typically a silicone. Such conformable dielectric walls will typically have a thickness in the range from about 0.004 in to 0.03 in, usually from 0.008 in to 0.015 in. The conformable wall may be non-distensible or may be elastic so that the wall structure may be inflated. For either non-distensible or elastic dielectric walls, the device may further comprise a frame which supports the conformable material, usually where the frame can be expanded and contracted to open and close the dielectric wall. 
         [0017]    The apparatus of the present invention will typically also include a shaft or other handle structure connected to the support end of the body. Usually, the shaft will have a lumen which extends into the gas inlet of the body to deliver the electrically non-conductive gas to the chamber. The shaft or handle may also include at least a second lumen for removing the electrically non-conductive gas from the chamber so that the gas may be recirculated in a continuous flow. Often, the first electrode will be at least partly in the first lumen of the device, although it may also be within the chamber or within both the first lumen and the chamber. The second electrode will usually be disposed at least partly over an exterior surface of the device, typically over the shaft, although in certain systems the second electrode could be disposed on a separate dispersal pod. 
         [0018]    Apparatus according to the present invention will have an interior chamber volume in the range from 0.01 ml to 20 ml, typically from 1 ml to 10 ml. The dielectric wall will have an area in the range from 1 mm 2  to 100 mm 2 , typically from 5 mm 2  to 50 mm 2 . The first electrode surface will have an area in contact with the electrically non-conductive gas in the range from 0.01 mm 2  to 10 mm 2 , typically from 1 mm 2  to 5 mm 2 . Additionally, the second electrode structure will have an area available to contact tissue in the range from 0.5 mm 2  to 50 mm 2 , usually from 1 mm 2  to 10 mm 2 . 
         [0019]    The radiofrequency power supply may be of general construction as often used in electrosurgery. The power supply will typically be configured to deliver a voltage in the range from 500 V (rms) to 2500 V (rms), usually from 600 V (rms) to 1200V (rms), typically at a current in the range from 0.1 A to 1 A, typically from 0.2 A to 0.5 A, and at a frequency in the range from 450 kHz to 550 MHz, usually from 480 kHz to 500 MHz. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    In order to better understand the invention and to see how it may be carried out in practice, some preferred embodiments are next described, by way of non-limiting examples only, with reference to the accompanying drawings, in which like reference characters denote corresponding features consistently throughout similar embodiments in the attached drawings. 
           [0021]      FIG. 1  is a schematic view of an ablation system corresponding to the invention, including an electrosurgical ablation probe, RF power source and controller. 
           [0022]      FIG. 2A  is a view of the ablation probe of  FIG. 1  configured with a sharp tip for ablation of a tumor. 
           [0023]      FIG. 2B  is another view of the probe of  FIG. 2A  after being penetrated into the tumor. 
           [0024]      FIG. 3  is an enlarged schematic view of the working end of the probe of  FIG. 1  that provides a gas electrode within an interior of a thin-wall dielectric structure. 
           [0025]      FIG. 4A  is a sectional view of an alterative thin-wall cylindrical dielectric structure in which support elements are formed within the dielectric structure. 
           [0026]      FIG. 4B  is a sectional view of a portion of another thin-wall planar dielectric structure in which support elements are in a waffle-like configuration. 
           [0027]      FIG. 5A  is a sectional view of a portion of another thin-wall planar dielectric structure in which support elements comprise post-like elements. 
           [0028]      FIG. 5B  is a sectional view of a probe working end in which a thin-wall dielectric structure with post-like support elements are provided around core electrode. 
           [0029]      FIG. 6  is a block diagram of components of one an electrosurgical system corresponding to the invention. 
           [0030]      FIG. 7  is a block diagram of gas flow components of an electrosurgical system corresponding to the invention. 
           [0031]      FIG. 8  is a cut-away schematic view of a working end as in  FIG. 3  illustrating a step in a method of the invention wherein current is coupled to tissue via a spark gap (or gas electrode) and capacitive coupling through a thin-wall dielectric structure. 
           [0032]      FIG. 9A  is an enlarged schematic view of an aspect of the method of  FIG. 3  illustrating the step positioning an ionized gas electrode and thin-wall dielectric in contact with tissue. 
           [0033]      FIG. 9B  is a schematic view of a subsequent step of applying RF energy to create an arc across a gas and capacitive coupling through the thin-wall dielectric to cause current flow in a discrete path in tissue. 
           [0034]      FIG. 9C  is a schematic view similar to  FIG. 9B  depicting the scanning of current flow to another random path in the tissue. 
           [0035]      FIG. 9D  is a schematic view similar to  FIGS. 9A-9C  depicting the thermal diffusion from the plurality of scanned current flows in the tissue. 
           [0036]      FIG. 10  is a circuit diagram showing the electrical aspects and components of the energy delivery modality. 
           [0037]      FIG. 11A  is a sectional view of the working end of  FIG. 3  positioned in tissue illustrating a step in a method of using the working end wherein current is coupled to tissue via an ionized gas and capacitive coupling through a thin wall dielectric structure. 
           [0038]      FIG. 11B  is a sectional view similar to that of  FIG. 11A  illustrating another step in the method in which the ablated tissue volume is shown. 
           [0039]      FIG. 12  is a sectional view of an alternate working end similar to that of  FIG. 3  in a method of use, the dielectric structure having a central support member functioning as (i) an electrode and as (ii) a gas flow directing means. 
           [0040]      FIG. 13  is a block diagram of one method corresponding to the invention. 
           [0041]      FIG. 14  is a block diagram of another method corresponding to the invention. 
           [0042]      FIG. 15  is a block diagram of another method corresponding to the invention. 
           [0043]      FIG. 16  is a block diagram of another method corresponding to the invention. 
           [0044]      FIG. 17  is a block diagram of another method corresponding to the invention. 
           [0045]      FIG. 18A  is plan view of an alternate ablation probe that carries a plurality of extendable needle-like ablation elements from a sheath, each element having a dielectric structure with varied dielectric parameters for directional control of capacitive coupling and thus directional control of ablation. 
           [0046]      FIG. 18B  is another view of the ablation probe of  FIG. 18A  with the plurality of extendable needle ablation elements extended from the sheath. 
           [0047]      FIG. 19  is an enlarged view of a working end of the ablation probe of  FIGS. 18A-18B  with a tissue volume targeted for ablation and resection. 
           [0048]      FIG. 20  is a sectional view of ablated tissue using the working end of  FIG. 19  showing the directed capacitive coupling and directed ablation. 
           [0049]      FIG. 21  is a schematic view of a tumor ablation method using a plurality of working ends similar to that of  FIGS. 19-20  for directed capacitive coupling and directed ablation. 
           [0050]      FIG. 22  is a sectional view of an alternate working end similar to that of  FIGS. 3 and 12  with a non-uniform thickness dielectric structure for directional control of capacitive coupling and thus directional control of ablation. 
           [0051]      FIG. 23  is a sectional view of a non-uniform thickness dielectric structure for directional control of capacitive coupling to tissue. 
           [0052]      FIG. 24  is a sectional view of a uniform thickness dielectric structure with different materials for directional control of capacitive coupling to tissue. 
           [0053]      FIG. 24  is a sectional view of a uniform thickness dielectric structure with different materials for directional control of capacitive coupling to tissue. 
           [0054]      FIG. 25A  is a sectional view of a working end of an ablation probe similar to that of  FIG. 12  with an expandable thin-wall dielectric structure in a non-extended condition. 
           [0055]      FIG. 25B  is a sectional view of the working end of  FIG. 25A  with the expandable thin-wall dielectric structure in an extended condition in soft tissue, the structure configure for expansion by gas inflation pressure. 
           [0056]      FIG. 25C  is another sectional view as in  FIG. 25B  showing the capacitive coupling of energy to the tissue from a contained plasma in the expandable dielectric structure. 
           [0057]      FIG. 25D  is another sectional view as in  FIG. 25B  showing the region of ablated tissue after energy delivery. 
           [0058]      FIG. 26  is another cross-sectional view of the expandable dielectric structure in a non-extended condition folded within a translatable sheath. 
           [0059]      FIG. 27  is a cut-away schematic view of a heart and a working end of another ablation probe similar to that of  FIG. 25A  with an expandable thin-wall dielectric structure configured for ablating about a pulmonary vein to treat atrial fibrillation, with the structure configure for expansion by gas inflation pressure. 
           [0060]      FIG. 28  is an enlarged sectional schematic view of the working end of  FIG. 27  ablating a pulmonary vein. 
           [0061]      FIG. 29  is a cut-away schematic view of a heart and deflectable working end of another ablation probe configured for ablating a linear lesion to treat atrial fibrillation. 
           [0062]      FIG. 30  is a schematic perspective view of the deflectable working end of  FIG. 29  illustrating an elongate dielectric structure. 
           [0063]      FIG. 31  is a cross-sectional view of the deflectable working end and dielectric structure of  FIG. 30  illustrating an interior electrode. 
           [0064]      FIG. 32  is a perspective view of another deflectable working end similar to that of  FIGS. 30-31  for creating a circumferential lesion to treat atrial fibrillation. 
           [0065]      FIG. 33  is a cut-away schematic view of a esophagus and working end of another ablation probe similar to that of  FIG. 27  with an expandable thin-wall dielectric structure configured for expansion by an interior skeletal framework. 
           [0066]      FIG. 34  is a cut-away view of the expandable thin-wall dielectric structure of  FIG. 33  showing the interior skeletal support frame that optionally functions as an electrode. 
           [0067]      FIG. 35  is a cut-away view of another expandable dielectric structure similar to  FIG. 34  showing an alternative interior skeletal support frame. 
           [0068]      FIG. 36  is a sectional schematic view of a working end of another ablation probe comprising first and second opposing jaws engaging tissue with each jaw engagement surface including a thin-wall dielectric structure, the jaws configured for sealing or coagulating tissue clamped therebetween. 
           [0069]      FIG. 37  is a schematic view of the working end of another embodiment with an expandable thin dielectric walled structure with a plurality of plasma-carrying chambers for performing another form of bi-polar ablation. 
           [0070]      FIG. 38  is a transverse sectional schematic view of the working end of  FIG. 37  taken along line  38 - 38  of  FIG. 37  rotated 90° showing the current flow in tissue. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0071]    Several embodiments of ablation systems useful for practicing an electrosurgical method corresponding to the present invention are shown in the drawings. In general, each of these embodiments utilizes a gas ionized at a first polarity and contained within a thin-wall dielectric enclosure that provides for capacitive coupling of RF current from the gas to a target tissue in contact with an electrode at a second polarity and spaced apart from, and exterior of, the dielectric enclosure. The system embodiments typically include an instrument with a working end including the thin-wall dielectric enclosure for containing an ionizable gas. Current flow to the tissue initiates when sufficient voltage is applied to ionize the contained gas into a plasma and the contemporaneous capacitive coupling through the surrounding dielectric structure occurs. The invention thus provides a voltage-based electrosurgical effect that is capable of ablating tissue to a controlled depth of 1 mm to 5 mm or more very rapidly, wherein the depth of ablation is very uniform about the entire surface of the dielectric enclosure. The instrument working end and dielectric enclosure can take a variety of forms, including but not limited to an elongated shaft portion of a needle ablation device, a dielectric expandable structure, an articulating member, a deflectable member, or at least one engagement surface of an electrosurgical jaw structure. The system embodiments and methods can be used for interstitial tissue ablation, intraluminal tissue ablation or topical tissue ablation. 
         [0072]    The system embodiments described herein utilize a thin-wall dielectric structure or wall at an instrument working end that contains an electrically non-conductive gas as a dielectric. The thin-wall dielectric structure can be a polymer, ceramic or glass with a surface configured for contacting tissue. In one embodiment, an interior chamber within the interior of the thin-wall dielectric structure carries a circulating neutral gas or static neutral gas such as argon. An RF power source provides current that is coupled to the neutral gas flow or static gas volume by an electrode disposed within the interior of the working end. The gas flow or static gas contained within the dielectric enclosure is of the type that is non-conductive until it has been transformed to a conductive plasma by voltage breakdown. The threshold voltage for breakdown of the gas will vary with variations in several parameters, including the gas pressure, the gas flow rate, the type of gas, and the distance from the interior electrode across the interior chamber to the dielectric structure. As will be seen in some of the embodiments, the voltage and other operational parameters can be modulated during operation by feedback mechanisms. 
         [0073]    The gas, which is ionized by contact with a conductive electrode in the instrument working end, functions as a switching mechanism that only permits current flow into targeted tissue when the voltage across the combination of the gas, the dielectric structure and the contacted tissue reaches a predetermined threshold potential that causes capacitive coupling across the dielectric structure. By this means of permitting current flow only at a high threshold voltage that capacitively couples current to the tissue, the invention allows a substantially uniform tissue effect within all tissue in contact with the dielectric structure. Further, the invention allows the ionized gas to be created contemporaneously with energy application to tissue by the conversion of a non-conductive gas to a plasma. 
         [0074]    In one embodiment of the apparatus, the ionized gas functions as an electrode and comprises a gas flow that can conduct current across an internal contained volume of the gas within a dielectric structure, typically from an electrode at an interior of a working end in contact with the gas flow. The gas flow is configured for the purpose of coupling energy to the dielectric structure uniformly across the surface of the dielectric structure, but that will only conduct such energy when the non-conductive gas media has been transformed to a conductive plasma by having been raised to a threshold voltage. 
       Definitions 
       [0075]    Plasma. In general, this disclosure may use the terms “plasma” and “ionized gas” interchangeably. A plasma consists of a state of matter in which electrons in a neutral gas are stripped or “ionized” from their molecules or atoms. Such plasmas can be formed by application of an electric field or by high temperatures. In a neutral gas, electrical conductivity is non-existent or very low. Neutral gases act as a dielectric or insulator until the electric field reaches a breakdown value, freeing the electrons from the atoms in an avalanche process thus forming a plasma. Such a plasma provides mobile electrons and positive ions, an acts as a conductor which supports electric currents and can form spark or arc. Due to their lower mass, the electrons in a plasma accelerate more quickly in response to an electric field than the heavier positive ions, and hence carry the bulk of the current. 
         [0076]    Dielectric and dielectric loss. The term dielectric is used in its ordinary sense meaning a material that resists the flow of electric current, that is a non-conducting substance. An important property of a dielectric is its ability to support an electrostatic field while dissipating minimal energy in the form of heat. The lower the dielectric loss (the proportion of energy lost as heat), the more effective is a dielectric material. 
         [0077]    Dielectric constant or relative permittivity. The dielectric constant (k) or relative static permittivity of a material under given conditions is a measure of the extent to which it concentrates electrostatic lines of flux, or stated alternatively is a number relating the ability of the material to carry alternating current to the ability of vacuum to carry alternating current. The capacitance created by the presence of a material is directly related to its dielectric constant. In general, a material or media having a high dielectric constant breaks down more easily when subjected to an intense electric field than do materials with low dielectric constants. For example, air or another neutral gas can have a low dielectric constant and when it undergoes dielectric breakdown, a condition in which the dielectric begins to conduct current, the breakdown is not permanent. When the excessive electric field is removed, the gas returns to its normal dielectric state. 
         [0078]    Dielectric breakdown. The phenomenon called dielectric breakdown occurs when an electrostatic field applied to a material reaches a critical threshold and is sufficiently intense so that the material will suddenly conduct current. In a gas or liquid dielectric medium, this condition reverses itself if the voltage decreases below the critical point. In solid dielectrics, such a dielectric breakdown also can occur and couple energy through the material. As used herein, the term dielectric breakdown media refers to both solid and gas dielectrics that allow current flow across the media at a critical voltage. 
         [0079]    Degree of ionization. Degree of ionization describes a plasma&#39;s proportion of atoms which have lost (or gained) electrons, and is controlled mostly by temperature. For example, it is possible for an electrical current to create a degree of ionization ranging from less than 0.001% to more than 50.0%. Even a partially ionized gas in which as little as 0.1% or 1.0% of the particles are ionized can have the characteristics of a plasma, that is, it can strongly respond to magnetic fields and can be highly electrically conductive. For the purposes of this disclosure, a gas may begin to behave like conductive plasma when the degree of ionization reaches approximately 0.1%, 0.5% or 1.0%. The temperature of a plasma volume also relates to the degree of ionization. In particular, plasma ionization can be determined by the electron temperature relative to the ionization energy. A plasma is sometimes referred to as being “hot” if it is nearly fully ionized, or “cold” or a “technological plasma” if only a small fraction (for example, less than 5% or less than 1%) of the gas molecules are ionized. Even in such a cold plasma, the electron temperature can still be several thousand degrees Celsius. In the systems according to the present invention, the plasmas are cold in this sense because the percentage of ionized molecules is very low. Another phrase used herein to describe a “cold” plasma is “average mass temperature” of the plasma which relates to the degree of ionization versus non-ionized gas and which averages the temperatures of the two gas volume components. For example, if 1% of a gas volume is ionized with an electron temperature of 10,000° C., and the remaining 99% has a temperature of 150° C., then the mass average temperature will be 149.5° C. It has been found that measuring the plasma temperature can be used to determine an approximate degree of ionization which can be used for feedback control of applied power, and as a safety mechanism for preventing unwanted high temperatures within a thin-wall dielectric structure. 
         [0080]    Referring to  FIG. 1 , a first embodiment of a tissue ablation system  100  utilizing principles of the present invention is shown. The system  100  includes a probe  110  having a proximal handle  112  and an elongated shaft or extension member  114  that extends along axis  115 . The handle  110  is fabricated of an electrically insulative material such as a plastic, ceramic, glass or combination thereof. The extension member  114  has a proximal end  116  coupled to handle  112 . The extension member  114  extends to a distal working end  120  that includes a dielectric member or structure  122  that is configured for contacting tissue that is targeted for ablation. 
         [0081]    In the embodiment of  FIG. 1 , the working end  120  and dielectric structure  122  is elongated and cylindrical with a cross-section ranging from about 0.5 mm to 5 mm or more with a length ranging from about 1 mm to 50 mm. The cross-section of the working end  120  can be round, oval, polygonal, rectangular or any other cross-section. As can be seen in  FIGS. 2A-2B , in one embodiment, the working end  120  has a sharp tip  124  for penetrating tissue to perform an ablation procedure, such as ablating a tumor indicated at  125  in a tissue volume  130 . In other embodiment, the distal tip of a working end  120  can be blunt. In yet other embodiment, the entire working end can have a guide channel therein for advancing the working end over a guide wire. 
         [0082]    Now turning to  FIG. 3 , an enlarged view of the working end  120  of  FIGS. 1 ,  2 A and  2 B is shown. It can be seen that the dielectric structure  122  has a thin wall  132  that provides an enclosure about an interior chamber  135  that contains a gas media indicated at  140  in  FIG. 3 . In one embodiment, the dielectric structure  122  can comprise a ceramic (e.g., alumina) that has a a dielectric constant ranging from about 3 to 4. The thickness of wall  132  can range from 0.002″ to 0.10″ depending on the diameter, or more typically 0.005″ to 0.050″ in a diameter ranging from 1 to 4 mm. In other embodiment shown in  FIG. 4A , the dielectric structure  122  can comprise a ceramic, glass or polymer in a molded form with strengthening support portions  142  or ribs that end axially, radially, helically or a combination thereof. The support portions  142  alternatively can comprise members that are independent of a thin-wall  132  of a dielectric material. In such an embodiment ( FIG. 4A ) as will be described below, the thin wall portions  144  of the dielectric structure  122  permit capacitive coupling of current to tissue while the support portions  142  provide structural strength for the thin-wall portions  144 . In another embodiment, a portion of which is shown in  FIG. 4B , the dielectric structure  122  has support portions  142  in a waffle-like configuration wherein thin-wall portions  144  are supported by thicker wall support portions  142 . The waffle-like structure can be substantially planar, cylindrical or have any other suitable configuration for containing a gas dielectric in a chamber indicated at  135  on one side of the dielectric structure  122 . In another embodiment of  FIGS. 5A and 5B , the dielectric structure  122  can have support portions  142  comprising posts that support the thin-wall portions  144  over another supporting member  145 . The planar dielectric structure  122  can be used, for example, in planar jaw members for applying RF energy to seal tissue. In another example,  FIG. 5B  shows a blunt-tipped, cylindrical thin-wall  132  of a dielectric structure  122  supported by a core supporting member  145 . In the embodiment of  FIG. 5B , the interior chamber  135  which can contain a plasma comprises a space between the thin wall portions  144  and the core support member  145 . 
         [0083]    Referring again to  FIG. 3 , the extension member  114  is fabricated of an electrically non-conductive material such as polymer, ceramic, glass or a metal with an insulative coating. The dielectric structure  122  can be bonded to extension member  114  by glues, adhesives or the like to provide a sealed, fluid-tight interior chamber  135 . In one embodiment, a gas source  150  can comprise one or more compressed gas cartridges ( FIGS. 1 and 6 ). As will be described below ( FIG. 6 ), the gas source is coupled to a microcontroller  155  that includes a gas circulation subcontroller  155 A which controls a pressure regulator  158  and also controls an optional negative pressure source  160  adapted for assisting in circulation of the gas. The RF and controller box  162  in  FIG. 1  can include a display  164  and input controls  165  for setting and controlling operational parameters such as treatment time intervals, gas flows, power levels etc. Suitable gases for use in the system include argon, other noble gases and mixtures thereof 
         [0084]    Referring to  FIG. 3 , the gas source  150  provides a flow of gas media  140  though a flexible conduit  166  to a first flow channel  170  in extension member  114  that communicates with at least one inflow port  172  interfacing with interior chamber  135 . The interior chamber  135  also interfaces with an outflow port  174  and second flow channel  180  in extension member  114  to thereby allow a circulating flow of gas media  140  within the interior of dielectric structure  122 . 
         [0085]    Still referring to  FIG. 3 , a first polarity electrode  185  is disposed about flow channel  170  proximate to the inflow port  172  thus being in contact with a flow of gas media  140 . It should be appreciated that electrode  185  can be positioned in any more proximal location in channel  170  in contact with the gas flow, or the electrode  185  can be within interior chamber  135  of dielectric structure  122 . The electrode  185  is electrically coupled to a conductor or lead  187  that extends through the extension member and handle  112  and is coupled to a first pole of a high frequency RF generator  200  which is controlled by controller  155  and RF subcontroller  155 B. An opposing polarity electrode  205  is disposed on the exterior surface of extension member  114  and is electrically coupled by lead  207  to a second pole of RF generator  200 . 
         [0086]    The box diagrams of  FIGS. 6 and 7  schematically depict the system, subsystems and components of one embodiment that is configured for delivering ablative electrosurgical energy to tissue. In the box diagram of  FIG. 6 , it can be seen that an RF power source  200  and circuit is controlled by RF subcontroller  155 B. Feedback control subsystems (described below) based on systems and probe pressure feedback, probe temperature feedback, and/or gas flow rate feedback are also operatively coupled to controller  155 . The system can be actuated by footswitch  208  or another suitable switch.  FIG. 7  shows a schematic of the flow control components relating to the flow of gas media through the system and probe  110 . It can be seen that a pressurized gas source  150  in linked to a downstream pressure regulator  158 , an inflow proportional valve  210 , flow meter  212  and normally closed solenoid valve  220 . The valve  220  is actuated by the system operator which then allows a flow of gas media  140  to circulate through flexible conduit  166  and probe  110 . The gas outflow side of the system includes a normally open solenoid valve  225 , outflow proportional valve  226  and flowmeter  228  that communicate with negative pressure source  160 . The exhaust of the gas can be into the environment or into a containment system. A temperature sensor  230  (e.g., thermocouple) is shown in  FIG. 7  for monitoring the temperature of outflow gases. 
         [0087]    FIGS.  8  and  9 A- 9 D schematically illustrate a method of the invention wherein (i) the dielectric structure  122  and (ii) the contained neutral gas volume  140  function contemporaneously to provide first and second dielectric media that cooperatively function as independent mechanisms to optimize very high voltage current delivery to engaged tissue volumes. The two dielectric components can be characterized as having complementary voltage thresholds levels at which only high voltage current can couple through a filament  235  of a plasma  240  within chamber  135  and capacitively couple through the thin-wall dielectric  132  to allow a current to further pass through a least resistive path  245  in the engaged tissue. In  FIG. 8 , the engaged tissue is assumed to be surrounding the dielectric structure  122  and is transparent. In the embodiment of  FIG. 8 , the electrode  185  also functions and a gas delivery sleeve wherein a neutral gas  140  can exit ports  250  in chamber  135 . The high voltage current paths  245  in tissue are effectively “scanned” across and about the inner surface  252  of the dielectric structure  122  and within the contacted tissue to cause a voltage-maximized form of electrosurgical ablation.  FIG. 8  provides a schematic view of what is meant by the term “scanned”, wherein high intensity electrical fields are produced in the interior chamber  135  of the dielectric structure  122  by capacitive coupling through the dielectric wall  132  until a voltage threshold is reached in the neutral gas media  140  to convert the gas into a plasma  240  (see  FIG. 8 ) which in turn allows plasma filaments  235  to form within the chamber  135  which randomly jump or scan about the interior surface  248  of the dielectric wall. The random jump of plasma filaments  235  within the dielectric chamber  135  (from electrode  185  to inner surface  248  of dielectric wall  132 ) occurs where there is a transient, reversible voltage breakdown in a localized portion  252  of the dielectric wall  132 , which is determined by a transient highest conduction path  240  in engaged tissue to the second polarity electrode  205  ( FIG. 3 ). An instant after the flow of current through the plasma  240  and path  245  in tissue, the localized portion  252  dissipates the electrical field and another capacitive coupling occurs through another plasma filament  235 ′ and current path  245 ′ in tissue to cause electrosurgical ablation in another random, discrete location. 
         [0088]      FIGS. 9A-9D  are enlarged schematic illustrations of the electrosurgical ablation method of  FIG. 8  that depict other aspects of the ablation method. In  FIG. 9A , it can be seen that the system and method is generalized to show clearly the first and second dielectric current transmission mechanisms characterized by selected voltage parameters to cause an electron avalanche in the gas and capacitive coupling in the thin-wall enclosure to optimize and maximize a form of high voltage current delivery to an exemplary tissue  260 . As described previously, the voltage threshold or dielectric breakdown mechanisms occur within (i) the gas dielectric or neutral gas volume  140  that is contained within an interior chamber  135  of dielectric structure  122  and (ii) the non-gas dielectric or structure  122  shown as a plane in  FIGS. 9A-9D . 
         [0089]      FIG. 9A  illustrates the working end components and tissue  260  prior to the actuation and delivery of energy to the tissue. It can be seen that the gas media  140  is neutral and not yet ionized. The first polarity electrode  185  positioned in the interior chamber  135  in contact with neutral gas  140  is shown schematically. The second polarity electrode  205  in contact with tissue is also shown schematically, but the illustration represents another aspect of the invention in that the second electrode  205  can have a small surface area compared to the surface areas of return electrodes/ground pads as in conventional electrosurgical systems. It has been found that the capacitively coupled energy delivery mechanism of the invention does not cause tissue heating at or about the surface of the second polarity electrode  205  as would be expected in a conventional electrosurgical device. As will be described below, it is believed that the constant flux in voltage breakdown-initiated and capacitive coupling-initiated current paths in the tissue  260  greatly reduces heat built up at or about the return electrode  205 . 
         [0090]      FIG. 9B  illustrates the working end components and tissue  260  at an instant in time immediately after the operator actuates the system and delivers power to the probe working end. Several aspects of the voltage-initiated breakdown ablation method are represented in  FIG. 9B , including (i) in one aspect of the instant in time, the neutral gas  140  is converted to a plasma  240  by potential between the first and second polarity electrodes  185  and  205 ; and contemporaneously (ii) current flow defines a least resistive path  245  in the tissue  260 ; (iii) a portion  252  of dielectric structure  122  adjacent current path  245  allows capacitive coupling to the tissue; (iv) the plasma filament  235  arcs across a high intensity plasma stream  262  between electrode  185  and the portion  252  of the dielectric structure. In other words, when the a selected voltage potential is reached, the voltage breakdown of the gas  140  and capacitively coupling through the dielectric  122  causes a high voltage current to course through path  245  in the tissue  260 . An instant later, thermal diffusion indicated by arrows  265  causes thermal effects in a tissue volume  270   a  outward from the transient current path  245 . The thermal effects in and about path  245  elevates tissue impedance, which thus causes the system to push a conductive path to another random location. 
         [0091]      FIG. 9C  illustrates the working end components and tissue  260  an instant after that of  FIG. 9B  when continued voltage potential causes voltage breakdown in plasma filament  235 ′ together with capacitively coupling through dielectric  122  to provide another high voltage current to course through path  245 ′ after which heat diffusion  265 ′ causes thermal effects indicated at  270   b.  The “scanning” aspect of the ablation method can be understood from  FIGS. 9A-9B  wherein the plasma filaments  235 ,  235 ′ and current paths very rapidly jump or scan about the interior chamber  135  to thereby deliver current in a path of least resistance  245 ,  245 ′ in the tissue  260 . 
         [0092]    Now turning to  FIG. 9D , another schematic is shown following an interval of energy delivery in which a multiplicity of current paths through the pre-existing plasma and dielectric  122  have provided thermal effects diffused throughout a multiplicity of regions indicated at  270   a - 270   f.  By this method, it has been found that ablation depths of 3 mm to 6 mm can be accomplished very rapidly, in for example 30 seconds to 90 seconds dependent upon the selected voltage. 
         [0093]    In one aspect of the invention,  FIG. 10  is a circuit diagram representing the steps of the method of  FIGS. 9A-9D  which explains the discovery that return electrode  205  can have a small surface area and not be subject to significant heating. In  FIG. 10 , it can be seen that voltage potential can increase until a dielectric breakdown occurs in both the neutral gas  140  and the dielectric structure  122  which cause a high voltage current through path P 1  to electrode  205 , followed by that path impeding out, thus causing the current to shift to current path P 2 , then current path P 3  ad infinitum to current path indicated at Pn. The tissue  260  in  FIG. 10  thus is shown as variable resistor in each current path as the current path is in continual flux based on the path increasing in resistance. 
         [0094]      FIGS. 11A and 11B  are enlarged schematic illustrations of the method of using the embodiment of  FIG. 3  to capacitively couple current to tissue with a gas dielectric  140  in interior chamber  135  (i.e., plasma indicated at  240 ). Referring to  FIG. 11A , the system is actuated, for example by a footswitch  208  ( FIG. 1 ) coupled to RF power source  200  and controllers  155 A and  155 B which initiates a gas flow from source  150  to provide circulating flow through the first (inflow) channel  170 , interior chamber  135  and the second (outflow) channel  180 . For convenience, the embodiments utilizing such a circulating gas flow will be described herein as using one preferred gas, which is argon. In one embodiment, the gas flow rate can be in the range of 1 ml/sec to 50 ml/sec, more typically from 5 ml/sec to 30 ml/sec. In  FIG. 11A , the working end  120  of the probe is introduced into tissue  260 , for example to ablate a tumor as in  FIGS. 2A-2B . The dielectric structure  122  is positioned in a desired location to ablate tissue adjacent thereto. The actuation of the system contemporaneously applies RF energy to electrode  185  and the gas flow which instantly converts the non-conductive argon  140  to a plasma indicated at  240  in  FIG. 11A . The threshold voltage at which the argon becomes conductive (i.e., converted in part into a plasma) is dependent upon a number of factors controlled by the controller, including the pressure of the argon gas, the volume of interior chamber  135 , the flow rate of the gas  140 , the distance between electrode  185  and interior surfaces of the dielectric surface  122 , the dielectric constant of the dielectric structure  122  and the selected voltage applied by the RF power source  200 . It should be appreciated that the actuation of the system can cause gas flows for an interval of 0.1 to 5 seconds before the RF generator powers on to insure circulatory gas flows. 
         [0095]      FIG. 11A  schematically depicts current indicated at  280  being capacitively coupled through the wall  132  of the dielectric structure  122  to tissue  260 , with the electric field lines indicating that high energy densities do not occur about electrode  205 . Rather, as described above, the high resistance developed in tissue about the current path dielectric structure  122  causes rapidly changing current paths and ohmic heating. In one aspect of the invention, the capacitive coupling allows for rapid, uniform ablation of tissue adjacent the dielectric structure.  FIG. 11B  schematically depicts the tissue after the RF energy delivery is terminated resulting in the ablated tissue indicated at  285 . 
         [0096]    Now turning to  FIG. 12 , an alternate working end  120 ′ is shown in a method of use. In this embodiment, the dielectric structure  122  is similar to that of  FIG. 3  except the working end  120 ′ includes a central support member  290  that extends from extension member  214  to a distal tip portion  292 . In this embodiment, the central support member  290  can comprise, or carry, a conductive electrode surface indicated at  295  to delivery energy to the gas  140  in interior chamber  135  for creating the plasma. The embodiment of  FIG. 12  also includes concentric gas inflow and outflow channels,  170  and  180 , wherein the first (inflow) channel  170  comprises a lumen in support member  290  that communicates with a plurality of flow outlets  250  in a distal portion of interior chamber  135 . The gas outflow port  174  is again disposed in a proximal portion of interior chamber  135 . The placement of gas inflow and outflow ports in opposing ends of interior chamber allows for effective gas circulation which assists in maintaining a predetermined plasma quality. In  FIG. 12 , the ablative currents and ohmic heating in tissue are indicated at  200 . 
         [0097]    In another aspect of the invention,  FIG. 12  illustrates that at least one temperature sensor, for example thermocouples  300 A and  300 B, are provided within or adjacent to interior chamber  135  to monitor the temperature of the plasma. The temperature sensors are coupled to controllers  155 A and  155 B to thus allow feedback control of operating parameters, such a RF power delivered, neutral gas inflow rate, and negative pressure that assists outflow. By measuring the mass average temperature of the media in chamber  135 , the degree of ionization of the ionized gas  240  can be determined. In one aspect of the invention, the measured temperature within chamber  135  during operation can provide feedback to gas circulation controller to thereby modulate the flow of neutral gas to maintain a degree of ionization between 0.01% and 5.0%. In another aspect of the invention, the measured temperature within chamber  135  during operation can provide feedback to modulate flow of neutral gas to maintain a temperature of less than 200° C., 180° C., 160° C., 140° C., 120° C., or 100° C. In several embodiments of polymeric dielectric structures, it is important to maintain a cold or technological plasma to prevent damage to the dielectric. In another aspect of invention, the system operating parameters can be modulated to maintain the mass average temperature within a selected range, for example a 5° C. range, a 10° C. range or a 20° C. range about a selected temperature for the duration of a tissue treatment interval. In another aspect of invention, the system operating parameters can be modulated to maintain a degree of ionization with less than 5% variability, less than 10% variability or less than 20% variability from a selected “degree of ionization” target value for a tissue treatment interval. While  FIG. 12  shows thermocouples within interior chamber  135 , another embodiment can position such temperature sensors at the exterior of the wall  132  of the dielectric structure to monitor the temperature of the wall. It also should be appreciated that multiple electrodes can be provided in the interior chamber to measure impedance of the gas media to provide an additional from of feedback signals. 
         [0098]    In another embodiment similar to  FIG. 13 , the working end or flow channel in communication with the interior chamber  135  can carry at least one pressure sensor (not shown) and pressure measurement can provide feedback signals for modulating at least one operational parameter such as RF power delivered, neutral gas inflow rate, negative pressure that assists outflow, degree of ionization of the plasma, or temperature of the plasma. In another aspect of invention, the system operating parameters can be modulated to maintain a pressure within chamber  135  less than 5% variability, less than 10% variability or less than 20% variability from a selected target pressure over a tissue treatment interval. 
         [0099]    In general,  FIG. 13  represents the steps of a method corresponding to one aspect of the invention which comprises containing a non-conductive gas in an interior of an enclosure having a thin dielectric wall, engaging and external surface of the dielectric wall in contact with a target region of tissue, and applying a radiofrequency voltage across the gas and the dielectric wall wherein the voltage is sufficient to initiate a plasma in the gas and capacitively couple current in the gas plasma across the dielectric wall and into the engaged tissue. This method includes the use of a first polarity electrode in contact with the gas in the interior of the thin dielectric wall and a second polarity electrode in contact with the patient&#39;s tissue. 
         [0100]      FIG. 14  represents aspects of a related method corresponding to the invention which comprises positioning a dielectric structure on a tissue surface, containing a non-conductive, ionizable gas within the dielectric structure, and applying RF voltage actoss the gas and tissue to to ionize the gas and deliver current through the dielectric structure to the tissue to ohmically heat the tissue. 
         [0101]    In general,  FIG. 15  represents the steps of a method corresponding to another aspect of the invention which comprises providing an electrosurgical working end or applicator with a first gas dielectric and a second non-gas dielectric in a series circuit, engaging the non-gas dielectric with tissue, and applying sufficient RF voltage across the circuit to cause dielectric breakdown in the gas dielectric to thereby apply ablative energy to the tissue. The step of applying ablative energy includes capacitively coupling RF current to the tissue through the second non-gas dielectric media. 
         [0102]      FIG. 16  represents steps of another aspect of the invention which comprises positioning a dielectric structure enclosing an interior chamber in contact with targeted tissue, providing a gas media in the interior chamber having a degree of ionization of at least 0.01%, and applying RF current through the gas media to cause capacitive coupling of energy through the dielectric structure to modify the tissue. In this aspect of the invention, it should be appreciated that an ionized gas can be provided for inflow into chamber  135 , for example with a neutral gas converted to the ionized gas media prior to its flow into chamber  135 . The gas can be ionized in any portion of a gas inflow channel intermediate the gas source  150  and the interior chamber  135  by an RF power source, a photonic energy source or any other suitable electromagnetic energy source. 
         [0103]      FIG. 17  represents the steps of another method of the invention which comprises positioning a dielectric structure enclosing a gas media in contact with targeted tissue, and applying RF current through the gas media and dielectric structure to apply energy to tissue, and sensing temperature and/or impedance of the ionized gas media to provide feedback signals to thereby modulate a system operational parameter, such as RF power delivered, neutral gas inflow rate, and/or negative pressure that assists gas outflows. 
         [0104]    Now turning to  FIGS. 18A-22 , other embodiments of electrosurgical working ends are shown that adapted to apply energy to tissue as described above, except that the dielectric structures have differing dielectric portions each having a different relative permittivity to thus cause differential effects (greater or lesser capacitive coupling) in tissue regions in contact with the different portions of the dielectric structure. In one probe embodiment  400  shown in  FIGS. 18A-18B , a working end carries multiple tissue-penetrating elements  405  that are similar to the needle-like working end  120  of  FIGS. 1-3 . The tissue-penetrating elements  405  can be extendable from a shaft  410  of an endoscopic instrument  412  by actuation of lever  414 . Each tissue-penetrating elements  405  has a working end with a dielectric structure  422  as described above and one or more return electrodes indicated at  425 . As can be seen in  FIG. 19 , the tissue-penetrating elements  405  are adapted for penetrating tissue  260 , such as a liver, on either side of a target line  430  that is to be a resection line or plane. Thus, the tissue-penetrating elements  405  can coagulate tissue on either side of line  430 , and thereafter the tissue can be cut and bleeding will be prevented or reduced. Such an instrument  412  can be used in liver resections, lung resections and the like.  FIG. 20  illustrates a cross-section of the multiple tissue-penetrating elements  405  of  FIG. 19  in tissue wherein it can be seen that the wall  432  of the dielectric structure varies from a thin-wall portion  435  to a thicker wall portion  436  with each portion extending axially along the length of the dielectric structure. As can easily be understood, the thin-wall portion  435  allows a greater coupling of current to adjacent tissue  260  when compared to the thicker wall portion  436 . For this reason, the depth of ablated or cauterized tissue regions  440  will vary depending on whether it is adjacent to thin-wall portion  435  or the thicker wall portion  436 . Thus, the instrument can control the depth of ablation by varying the volume resistivity of the dielectric wall. For example, the thin-wall portion  435  can have a volume resistivity in the range of 1×10 14  Ohm/cm as described above which can then transition to thicker wall portion  438  having a volume resistivity of 1.5×, 2× or 3× triple the 1×10 14  Ohm/cm range. As depicted in  FIG. 20 , the energy delivery converges to ablate or cauterize tissue regions  440  inwardly toward line  430  that is targeted for cutting. Outwardly from line  430  there is less collateral damage due to reduced ohmic heating. 
         [0105]      FIG. 21  illustrate a plurality of probes  450 A- 450 D that demonstrate a similar use of “directional” dielectric structures  422  for directional control of energy delivery to tissue, in this case to provide converging regions of ablation to ablate tumor  452  as in the working ends of the device of  FIG. 18A-20 . In this embodiment, it can be seen that the probe handles include an indicator mark  455  that indicates the orientation of the thin-wall portion  435  or thick wall portion  436  to thus selectively direct RF energy delivery. In another embodiment, it should be appreciated that the proximal and distal ends of a dielectric structure  422  can be marked with any suitable imageable marker, for example radiopaque markings. In another aspect of the invention shown in  FIG. 20 , any probe can carry at least one thermocouple, for example thermocouples  456   a  and  456   b,  at locations proximal and distal to the dielectric structure  422  to measure tissue temperatures to provide an endpoint for terminating the delivery of energy. The thermocouples provide signal to controllers  155 A and  155 B to terminate the ablation procedure. The ablation probes  450 A- 450 D can each carry a return electrode as in the working end of  FIG. 19 , or alternatively there can be a remote return electrode as indicated at  458  in  FIG. 21 . 
         [0106]      FIG. 22  illustrates another embodiment of electrosurgical working end  460  wherein wall  432  of the dielectric structure  422 ′ varies from thin-wall portion  435 ′ to a proximal and distal thicker wall portions  436 ′ with each portion extending radially about the dielectric structure. As can easily be understood as shown in  FIG. 22 , the central thin-wall portion  435 ′ thus allows a greater coupling of current to adjacent tissue  260  to cause a deeper ablated tissue  440 ′ as compared to the thicker wall portions  436 ′ at the ends of the dielectric structure (cf. ablated tissue in  FIG. 11B ). In all other respects, the working end  460  operates as previously described embodiments. 
         [0107]    In the electrosurgical ablation working ends of  FIGS. 19-21  above, the dielectric structures  422  and  422 ′ provide differential or energy transmissibility by means of varying the thickness of a dielectric such as silicone. A portion of an exemplary dielectric wall  470  with varying thickness portions  435 ′ and  436 ′ is shown in  FIG. 23  which represents the dielectric of  FIG. 22 . In other words, a varied thickness wall with a uniform dielectric constant or volume resistivity of the material can provide varied coupling of RF current to tissue about the surface of the dielectric. It should be appreciated that an objective of the invention is controlled depth of ablation which can be accomplished equally well by having a uniform thickness dielectric but varying the electrical properties of the material.  FIG. 24  illustrates a constant thickness dielectric wall  475  with first and second dielectric materials  477  and  480  that provides for higher capacitive coupling through material  480 . The number of layers of materials, or material portions, and their dielectric properties can range from two to ten or more. Further, combinations of varying material thickness and dielectric properties can be utilized to control capacitive coupling of current through the dielectric. 
         [0108]      FIGS. 25A-25D  illustrate another embodiment of electrosurgical system  500  and working end  520  and method of use that is similar to the device of  FIG. 12  except that the dielectric structure  522  of  FIGS. 25A-25D  is fabricated of a thin-wall dielectric that can be moved from a first non-expanded condition to an expanded condition. In  FIG. 25A , the working end is shown with a distally-extended sheath  524  that can be of plastic or metal. A first step of a method thus comprises introducing the working end into tissue interstitially or into a body lumen with the sheath protecting the dielectric structure  522 . The dielectric structure  522  is then expanded by gas inflows which causes compression of surrounding tissue and increases the surface area of the thin dielectric wall in contact with tissue. As can be seen in  FIG. 25A , the expandable dielectric  522  can be fabricated of a distensible or non-distensible material, such as a stretchable silicone or a braided, reinforced non-stretch silicone. The wall thickness of a silicone structure can range from 0.004″ to 0.030″, and more typically from 0.008″ to 0.015″ with an interior volume ranging from less that 5 ml to more than 100 ml. The dielectric structure can have any suitable shape such as cylindrical, axially tapered, or flattened with interior baffles or constraints.  FIG. 26  depicts a cross-section of the sheath  524  and a non-distensible expandable dielectric  522  with a method of folding the thin dielectric wall. 
         [0109]      FIG. 25B  illustrates multiple subsequent steps of the method wherein sheath  524  is retracted and the physician actuates the gas source  150  and controller to expand the expandable dielectric structure  522 . The structure  522  or balloon can be expanded to any predetermined dimension or pressure in soft tissue or in any body lumen, cavity, space or passageway. Radiopaque marks on the dielectric structure (not shown) can be viewed fluoroscopically to determine its expanded dimension and location. The gas circulation controller  155 A can circulate gas flow after a predetermined pressure is achieved and maintained. 
         [0110]      FIG. 25C  depicts a subsequent step of the method in which the physician actuates the RF power source  200  and controller  155 B to develop high voltage potential between central support electrode  295  and return electrode  205  which, as described previously, can cause a voltage breakdown in the gas dielectric  140  ( FIG. 25B ) to create plasma  240  and contemporaneously capacitively couple current to tissue  260  as indicated by current flows  530 .  FIG. 25D  depicts the termination of RF energy delivery so that the voltage breakdown and resulting plasma is extinguished—leaving uniform ablated tissue  540  similar to that shown in  FIG. 11B . 
         [0111]    In one embodiment, the dielectric structure  522  was made from NuSil MED-6640 silicone material commercially available from NuSil Technology LLC, 1050 Cindy Lane, Carpinteria, Calif. 93013. The dielectric structure  522  was fabricated by dipping to provide a length of 6 cm and a uniform wall thickness of 0.008″ thereby providing a relative permittivity in the range of 3 to 4. The structure ends were bonded to a shaft having a diameter of approximately 4 mm with the expanded structure having an internal volume of 4.0 cc&#39;s. The gas used was argon, supplied in a pressurized cartridge available from Leland Limited, Inc., Post Office Box 466, South Plainfield, N.J. 07080. The argon was circulated at a flow rate ranging between 10 ml/sec and 30 ml/sec. Pressure in the dielectric structure was maintained between 14 psia and 15 psia with zero or negative differential pressure between gas inflow source  150  and negative pressure (outflow) source  160 . The RF power source  200  had a frequency of 480 KHz, and electrical power was provided within the range of 600 Vrms to about 1200 Vrms and about 0.2 Amps to 0.4 Amps and an effective power of 40 W to 80 W. 
         [0112]      FIGS. 27 and 28  illustrate another embodiment of electrosurgical system  600  that comprises a catheter having working end  610  for treating atrial fibrillation by means of ablation about pulmonary veins PV. Various methods of using conventional RF catheters for such treatments are known. Catheter  610  is configured with a guidewire channel  612  and can be navigated to a site shown in  FIGS. 27-28 . The catheter working end  620  included an expandable dielectric structure  622  similar to that of  FIGS. 25A-25D  that can be expanded to apply pressure between the balloon wall and the tissue to thereafter create a circumferential lesion in a pulmonary vein PV.  FIG. 28  is a schematic illustration that again show gas source  150  and gas circulation controller  155 A that can expand chamber  635  in the thin-wall dielectric structure  622  to engage the wall of the pulmonary vein PV. In the embodiment of  FIG. 28 , it can be seen that the wall of dielectric  622  includes a first (lesser) energy-transmissible region  636  and a second (greater) energy-transmissible region  638  thus allowing a focused circumferential ablation—which corresponds to the configuration of dielectric wall shown in  FIG. 24 . Thereafter, the RF power source  200  and controller  155 B can be actuated to convert the neutral gas flow to plasma  240  and contemporaneously ablate tissue indicated at  640 . In this embodiment, a first polarity electrode  645  is provided on the catheter shaft in chamber  635  that can cooperate with a second polarity electrode on the catheter shaft remote from balloon  622  or any other type of ground pad may be used (not shown). In all other respects, the method of the invention for ablation of cardiac tissue follows the steps described above. The balloon can have radiopaque markings, and the system can be operated by an algorithm to expand the dielectric structure  622  or balloon to a pre-determined pressure, then delivery RF energy and terminate delivery automatically. It should be appreciated that additional electrodes can be provided in the balloon surface (not shown) for mapping conduction in the cardiac tissue. 
         [0113]    While  FIG. 27-28  illustrate an expandable dielectric  622  for treating cardiac tissue, it should be appreciate that the scope of the invention includes using a similar apparatus and method to controllably apply ablative RF energy to any body lumen, vessel, body cavity or space such as in a stomach, gall bladder, esophagus, intestine, joint capsule, airway, sinus, a blood vessel, an arteriovascular malformation, heart, lung, uterus, vaginal canal, bladder or urethra. 
         [0114]      FIGS. 29-31  schematically illustrate another embodiment of electrosurgical system  700  and catheter having working end  710  for treating atrial fibrillation with linear lesions within a heart chamber to block aberrant conduction pathways. Catheter  710  can have a guidewire channel (not shown) and can be navigated to perform an elongated ablation in a heart chamber as in  FIG. 29 . In this embodiment, the catheter working end  720  has a flexible shaft portion  721  that included an axially-extending thin-wall dielectric  722  in one surface for engaging tissue to provide a linear lesion as depicted in  FIG. 31 . The catheter shaft  721  is deflectable by means of a pull-wire  728  that can be actuated from a catheter handle.  FIG. 30  is another schematic illustration that shows the gas source  150  and gas circulation controller  155 A that can provide gas circulation within interior chamber  735  interior of the thin-wall dielectric  722 . The RF power source  200  is coupled to a lead  738  and elongated first polarity electrode  740  in the interior chamber  735 . The RF power source  200  and controller  155 B can be actuated to convert the neutral gas flow to a plasma and contemporaneously ablate tissue engaged by dielectric  722  as described above. The second polarity electrode can be provided on the catheter shaft remote from dielectric  722  or any type of ground pad may be used (not shown). In all other respects, the method of the invention for ablation of cardiac tissue follows the steps described above. The working end can have radiopaque markings, and the system can be operated in accordance with algorithms. It should be appreciated that additional electrodes can be provided in the catheter working end (not shown) for mapping conduction in the heart pre-and post ablation. 
         [0115]      FIG. 32  illustrates another catheter working end  720 ′ that is similar to that of  FIGS. 29-31  that is deflectable by a pull-wire  738  to provide all or part of circumferential lesion in a pulmonary vein (see  FIGS. 28-29 ). In this embodiment, the thin-wall dielectric  722 ′ extends around the exterior surface of the articulated working end. 
         [0116]      FIGS. 33 and 34  illustrate another embodiment of electrosurgical system  800  that comprises a catheter having working end  810  for treating an esophagus  811 , for example to ablate Barrett&#39;s esophagus, to apply energy to lower esophageal sphincter or for other disorders. The system operates as previously described in  FIGS. 25A-28  in embodiments that have an expandable dielectric structure. In the dielectric structure  822  of  FIGS. 33-34 , the expansion of the structure is provided by a skeletal support member such as an interior spring-like member, with an optional pull-cable actuation mechanism. As can be seen in  FIG. 34 , a helical support member  825  is provided that is capable of a contracted cross-section (axially-stretched) or an expanded cross-section in chamber  835  which is assisted by pulling central cable  828  in catheter shaft  830 . In this embodiment, the dielectric can again comprise a thin-wall silicon as described above. In this embodiment, it has been found that the support member  825  can be of a conductive metal and coupled to RF power source to function as a first polarity electrode  840 . The second polarity electrode (not shown) can be located elsewhere on the catheter is a location in contact with tissue, or a ground pad can be used. 
         [0117]      FIG. 35  illustrate another embodiment of electrosurgical system  800 ′ that is similar to that of  FIG. 34  with a dielectric structure  822  that is supported in an expanded condition by a plurality of bowed-out skeletal support members  825 ′ that are assisted by pull-cable  828 . In this embodiment, the portion of the pull-cable within chamber  835  functions as a first polarity electrode  840 ′. In operation in any of the embodiments above, it has been found that the first polarity electrode can provide sufficient voltage to create a substantially uniform plasma in an interior chamber (see  FIGS. 2 ,  8 ,  11 A,  28 ,  30 ,  34 ,  35 ) of a non-expandable or expandable dielectric when the surface of the electrode is less than 15 mm, less than 10 mm or less than 5 mm from the interior wall of the dielectric. This maximum dimension from the dielectric wall to the electrode  840 ′ is indicated at D in  FIG. 35 . In has also been found that, in operation, the first polarity electrode can provide voltage to create a substantially uniform plasma in an interior chamber of a non-expandable or expandable dielectric wall when the electrode contacts the surface of the dielectric  822  as in  FIG. 34 , but the electrode surface should engage less than about 10% of the interior surface of the dielectric wall. If the first polarity electrode engages greater than about 10% of the interior surface of the dielectric wall, then the “flux” of energy delivery through tissue as schematically depicted in  FIG. 8  will be reduced, a greater capacitive coupling may occur about the regions of the electrode(s) in contact with the wall which can reduce the uniformity of tissue ablation. 
         [0118]      FIG. 36  illustrates another embodiment of electrosurgical system  900  wherein the working end  910  comprises first and second opposable jaws  912 A and  912 B that are adapted for clamping tissue for coagulation, sealing or welding tissue  914 . In one embodiment, both jaws have a tissue-engaging surface that comprises a dielectric structure  922 A,  922 B that is similar in function to all other such dielectric structures described above.  FIG. 36  is a schematic illustration that again shows gas source  150  and gas circulation controller  155 A that can deliver gas to chambers  935 A,  935 B in the jaws. The RF power source  200  and controller  155 B can be actuated to convert the neutral gas flows in the chambers  935 A,  935 B into plasma  240  and contemporaneously to apply energy to engaged tissue  914 . In this embodiment, the jaws carry first and second polarity electrodes  945 A and  945 B, respectively, to thus make jaw function by means of a contained ionized gas and capacitive coupling, which differs from previous embodiments. It should be appreciated that one jaw can comprise a single electrode surface, as opposed to the plasma-initiated capacitive coupling system of  FIG. 36 . The dielectric structure of  FIG. 36  are of the type described in  FIGS. 4B and 5A  wherein the thin-wall dielectric material is supported by support columns, posts, channels of the like. 
         [0119]      FIGS. 37 and 38  illustrate another embodiment of electrosurgical system  1000  again includes a catheter or probe shaft  1002  extending to a working end  1010  that carries an expandable dielectric structure  1022 . In this embodiment, the dielectric structure  1022  includes a plurality of interior chambers, for example first and second chambers  1024 A and  1024 B. The expansion of the dielectric structure  1022  can be provided by skeletal support members such as interior spring-like members as described above or by expansion by fluid pressure of gas inflows or a combination thereof. Each chamber is configured to carry a flexible interior electrode, with adjacent chambers having opposing polarity interior electrodes, such as electrodes  1040 A and  1040 B indicated at (+) and (−) polarities in  FIGS. 37 and 38 , to allow another form a bi-polar ablation. In this embodiment, the electrodes and support members can comprise the same members. As can be seen in  FIG. 37 , the external wall of dielectric structure  1022  has thin wall portions  1032 A and  1032 B for capacitively coupling energy to tissue, and a thicker wall portion  1042  that insulates and separates the first and second chambers  1024 A and  1024 B. The flexible electrodes  1040 A and  1040 B are operatively coupled to RF power source  200 . The gas inflow source  150  and negative pressure source  160  are coupled to in inflow and outflow channels communicating with each interior chamber,  1024 A and  1024 B, independently. In the transverse sectional view of  FIG. 38 , the open terminations  1046  and  1048  of the inflow and outflow channels can be seen in each interior chamber,  1024 A and  1024 B. Thus, each chamber is provided with a circulating gas flow (indicated by arrows in  FIG. 37 ) similar to that described in previous embodiments with respect to single chamber working ends. 
         [0120]      FIG. 38  is a schematic sectional view of the dielectric structure  1022  deployed in a targeted tissue  1050 . It can be understood that the system can be actuated to circulate gas in the chambers  1024 A and  1024 B which then is converted to a plasma  240  in each chamber as described previously. In this embodiment and method of use, the capacitive coupling occurs through the thin dielectric walls  1032 A and  1032 B in paths of current flow indicated at  280  in  FIG. 38 . Whereas the previous embodiments illustrated a single chamber containing a plasma that capacitively coupled current to a non-gas electrode, the embodiment of  FIGS. 37 and 38  depicts the use of at least two contained plasma electrodes and capacitive coupling therebetween. It should be appreciated that the number of adjacent chambers carrying opposing polarity electrodes can be utilized in a thin-wall dielectric structure, for example 2 to 10 or more, with the chambers having any suitable dimensions or orientations relative to one another. 
         [0121]    In another aspect of the invention, it has been found that the operating parameters of the system can be adjusted to provide cooperating mechanisms for causing thermal effects in tissue, that is, (i) Joule heating of tissue as described above and (ii) passive heat conduction from the dielectric wall which is heated by the plasma (see co-pending U.S. patent application Ser. No. 12/944,466). 
         [0122]    In one embodiment, the system parameters that can be adjusted to minimize Joule heating of tissue and to correspondingly maximize passive conductive heating of tissue. The operating parameters that can be adjusted consist of (i) the dielectric constant of the dielectric wall; (ii) the surface area of dielectric wall in contact with tissue; (iii) the thickness of the dielectric wall; (v) the circulation of neutral gas through the interior chamber which controls plasma resistance; (vi) the frequency of the RF current; (vii) the pressure within the interior chamber; and (viii) the field strength which relates to voltage and spacing between the electrode in the interior chamber and the dielectric wall. In this aspect of the invention, it has been found that a high plasma resistance, compared to the tissue resistance, can result in plasma filaments or streamers coupling energy to the focal points of the dielectric wall without coupling as much energy through the dielectric to cause Joule heating of the tissue. In this aspect of the invention, utilizing a dielectric wall with a lower dielectric constant will provide for greater passive tissue heating from the heated of focal points on the dielectric and lesser Joule heating along current paths in tissue. 
         [0123]    Although particular embodiments of the present invention have been described above in detail, it will be understood that this description is merely for purposes of illustration and the above description of the invention is not exhaustive. Specific features of the invention are shown in some drawings and not in others, and this is for convenience only and any feature may be combined with another in accordance with the invention. A number of variations and alternatives will be apparent to one having ordinary skills in the art. Such alternatives and variations are intended to be included within the scope of the claims. Particular features that are presented in dependent claims can be combined and fall within the scope of the invention. The invention also encompasses embodiments as if dependent claims were alternatively written in a multiple dependent claim format with reference to other independent claims.