Abstract:
Electrosurgical apparatus including an electrode adapted to deliver radio frequency (RF) energy to a tissue, an RF energy source in communication with the electrode, a manipulator in operable connection with the electrode, adapted to move the electrode, and a controller in communication with the RF energy source and the manipulator, adapted to control operation of the manipulator and the electrode in accordance with a mode of operation that includes at least the following steps:  
     a) delivering RF energy to the electrode from the RF energy source sufficient to cause ablation of a tissue, and  
     b) moving the electrode so as to at least diminish adherence of tissue to the electrode.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to electrosurgical apparatus for tissue ablation generally, and particularly to electrosurgical apparatus for radio frequency (RF) tissue ablation.  
         BACKGROUND OF THE INVENTION  
         [0002]    Radio frequency (RF) tissue ablation is a well-known technique for making thermal lesions in the vicinity of an uninsulated tip of an electrode due to tissue coagulation caused by resistive heating. The electrode can be applied directly on superficial structures, surgically, endoscopically, laparascopically, or even via a transcatheter access such as a treatment for symptomatic cardiac arrhythmias. If the electrode is formed as a needle, then the electrode may be inserted interstitially, and guided by imaging.  
           [0003]    As is known in the art, resistive heating is proportional to the square of the current density, the latter being inversely proportional to the square of the distance from the ablation electrode. Therefore, resistive heating decreases from the ablation electrode with the distance to the fourth power. In other words, significant resistive heating only occurs within a narrow rim (of a few millimeters) of tissue in direct contact with the ablation electrode. Deeper tissue heating occurs as a result of passive heat conduction from that rim.  
           [0004]    A general problem in RF tissue ablation is limitation in lesion size. Increasing the power to the electrode or exposure time to the tissue increases the amount of energy delivered around the electrode and thereby increases the lesion size. However, at high temperatures (e.g., above 100° C.) at the electrode-tissue interface, the impedance increases significantly because of desiccation followed by charring around the electrode tip. This leads to an abrupt fall in lesion current (and delivered effect) and no further energy is delivered around the electrode, and no further tissue heating occurs. This phenomenon tends to limit lesion size in the transverse direction around the electrode. The longitudinal dimension of the lesion is basically dependent on the length of the uninsulated part of the electrode.  
           [0005]    Tissue adhesion and limited lesion size are just two examples of problems associated with RF tissue ablation. Another problem occurs when attempting to ablate tissue in difficult-to-reach locations, such as the lower esophagus. It is difficult to control the depth of the ablation into the tissue and to obtain a uniform ablation over the entire surface area that needs to be ablated. Surface irregularities, folds into the tissue, and variations in the anatomical configurations of body cavities increase the difficulty of achieving uniform RF tissue ablation. As a result, only a portion of the tissue to be ablated may be destroyed, and in some areas, more tissue may be ablated than was intended.  
           [0006]    Attempts have been made in the prior art to overcome the abovementioned problems. One known solution is that of “electrolyte-assisted” ablation. This form of ablation relies on contacting the tissue to be ablated with an electrolyte, such as a fluid or gel. Electrical energy is applied through the electrolyte to the tissue in contact with the electrolyte. Because the electrical resistance of the electrolyte-tissue interface is significantly high relative to the resistance of the electrolyte itself, most of the energy will be dissipated at this interface in the form of heat, leading to thermal ablation of the superficial tissue at this interface. Introduction of a conducting and/or cooling liquid into the treated area generally increases the coagulated volume and tends to reduce tissue adhesion. Electrolyte-assisted ablation is also effective in difficult-to-reach locations, because the liquid or gel electrolyte effectively bathes the entire surface area of the tissue that is to be ablated.  
           [0007]    One example of electrolyte-assisted ablation is discussed in U.S. Pat. No. 6,112,123 to Kelleher et al., assigned to Endonetics, Inc., San Diego, Calif., USA, the disclosure of which is incorporated herein by reference. Kelleher et al. describes electrolyte-assisted tissue ablation of metaplasia in the esophagus by means of a shaft with an expandable barrier that is deployable from the shaft&#39;s distal end. An electrode and a temperature/impedance sensor are mounted on the shaft. The shaft may be a catheter formed with several lumens that are used independently for housing optical elements and for transferring fluid.  
           [0008]    In the operation of the device, the distal end of the shaft or catheter is placed in the esophagus proximal the cardia. The barrier is then deployed to the cardia and expanded to seal the esophagus from the stomach. The esophageal volume between the barrier and the catheter is partially flooded with a conducting medium and the electrode is deployed into the conducting medium. The sensor is also deployed into contact with the tissue to be ablated. Using open-loop control, or using temperature, impedance, or visual monitoring for closed-loop control, the metaplasia is ablated by passing RF energy from the electrode and through the conducting medium for contact with the tissue being ablated.  
           [0009]    Kelleher et al. limits the RF power to a moderate level by using the closed-loop control, wherein the output of the temperature sensors near the electrode is used to control power delivery. However, a disadvantage of Kelleher et al. is that enlargement of the coagulated volume is at the expense of slower coagulation speed and prolonged treatment time.  
           [0010]    Another example of electrolyte-assisted ablation is described in U.S. Pat. No. 5,348,554 to Imran et al., assigned to Cardiac Pathways Corporation, Sunnyvale, Calif., USA, the disclosure of which is incorporated herein by reference. Imran et al. includes a catheter constructed of an elongate member having proximal and distal extremities. A metal conducting electrode is secured to the distal extremity of the elongate member and has a chamber formed therein. A conductor extends through the elongate member from the proximal to the distal extremity for supplying RF energy to the electrode. The elongate member has a lumen in the distal extremity, which is in communication with the chamber. A coolant is disposed in the chamber and in contact with the electrode for dissipating heat created in the electrode by the application of RF energy thereto. A disadvantage of Imran et al. is that the coagulated volume is limited due to the relatively small diameter of the electrode used.  
           [0011]    Other examples of cooled electrodes include U.S. Pat. No. 5,100,388 to Behl et al., which describes a catheter having a conductive material delivery lumen and a distal tip heating element, suitable for hollow body organs, such as the gall bladder. U.S. Pat. No. 5,304,214 to DeFord et al., describes an RF ablation device specifically designed to selectively ablate prostatic tissue about the prostatic urethra. The disclosures of these patents are incorporated herein by reference. A disadvantage of Behl et al. or DeFord et al. is that the structure of these devices is custom-made to fit a particular anatomy and is not generally suitable for any other anatomy in the body. They also suffer from limited coagulated volume that is possible with the electrode.  
           [0012]    In order to overcome the limitation of the small coagulated volume associated with a fine linear needle electrode, U.S. Pat. Nos. 5,431,649 and 6,016,809 to Mulier et al., assigned to Medtronic, Inc., Minneapolis, Minn., USA, the disclosures of which are incorporated herein by reference, propose a hollow helical electrode, with injection of a cooled conducting liquid.  
           [0013]    In general, Mulier et al. is directed to treatment of tachyarrhythmias, wherein one or more chambers of the heart exhibit an excessively fast rhythm. In particular, Mulier et al. is directed to treatment of tachycardias, which are due to the presence of ectopic foci within the cardiac tissue or due to the presence of aberrant condition pathways within the cardiac tissue. In Mulier et al., a catheter is provided with a hollow, helical electrode, which is screwed into cardiac tissue at a desired ablation site and connected to a source of RF electrical energy to ablate the tissue adjacent the electrode. Prior to ablation, it is essential to inject a conductive fluid through the hollow needle, which cools the tissue adjacent the needle and increases the conductivity of the tissue in the area of the electrode.  
           [0014]    The helical electrode provides an enlarged surface area as compared to relatively straight or needle-like electrodes for insertion into the endocardium, and also serves to stabilize the location of the catheter during the application of the RF signal. In addition, there is essentially no bleeding following removal of the helical electrode, so it can safely be placed in multiple locations for mapping and ablation purposes.  
           [0015]    Mulier et al. uses a non-toxic, non-arrhythmogenic, conductive solution such as Ringer&#39;s solution to the area of the electrode, before and during application of RF energy. The helical electrode is hollow, and the conductive solution is applied through one or more apertures in the electrode. The conductive solution injected prior to application of the RF signal is believed to displace blood in the vicinity of the electrode. Ringer&#39;s solution, for example, has a much higher conductivity than blood (approximately 3-4 times higher) or cardiac muscle (approximately 7 times higher). Overall resistance to the induced electrical current is reduced, which is believed to assist in expanding the size of the lesion, by spreading the effective area of application of the electrical current over a wider area. Application of the conductive solution during the burn helps prevent overheating of the tissue, allowing for a prolonged application of the RF signal, extending beyond the point at which burning or charring would otherwise normally occur.  
           [0016]    However, liquid handling and adjusting the electrode length to that of the treated area are shortcomings, especially for a relatively large volume. Moreover, a monopolar helical electrode is limited in its ability to deliver power inside the helix and to control the treated boundary outside the helix. Furthermore, local temperature along a long electrode is generally difficult to control, because local temperature varies according to local variations of tissue impedance and current density.  
           [0017]    U.S. Pat. No. 6,132,426 to Kroll, assigned to Daig Corporation, Minnetonka, Minn., USA, describes an RF ablation catheter that incorporates therein a positive temperature coefficient (PTC) temperature sensor. The PTC sensor provides a built-in fail-safe current limiter to avoid over-ablating the target tissue, and to prevent coagulation adherence to the catheter electrode tip and damage to the catheter and patient from overheating. However, disadvantages of Kroll include slower coagulation speed and prolonged treatment time.  
           [0018]    Some prior art mentions rotation of the ablating electrode. For example, U.S. Pat. No. 6,010,476 to Saadat, assigned to AngioTrax, Inc., Sunnyvale, Calif., USA, describes a device for creating transmural channels for transmyocardial revascularization. The device includes a cutting head that may ablate or cut tissue with RF energy. The cutting head may be coupled to a drive tube and rotated at high speeds by a motor and gearing. However, the rotation of the cutting head is not used to prevent tissue adhesion, rather rotation is used to facilitate entry of the cutting head into the cardiac tissue and to help form transmural channels. Once the electrode has arrived at the desired location, RF energy is applied to the electrode to ablate tissue. Saadat does not mention ablating while rotating the electrode.  
           [0019]    U.S. Pat. No. 6,066,134 to Eggers et al., assigned to ArthroCare Corporation, Sunnyvale, Calif., USA, the disclosure of which is incorporated herein by reference, describes an electrosurgical probe that includes a shaft having an electrode array at its distal end and a connector at its proximal end for coupling the electrode array to a high frequency power supply. The shaft includes a return electrode recessed from its distal end and enclosed within an insulating jacket. The return electrode defines an inner passage electrically connected to both the return electrode and the electrode array for passage of an electrically conducting liquid. By applying high frequency voltage to the electrode array and the return electrode, the electrically conducting liquid generates a current flow path between the return electrode and the electrode array so that target tissue may be cut or ablated.  
           [0020]    Eggers et al. mentions manually reciprocating or rotating the probe in a light brushing motion, so as to maintain the supply of electrically conducting fluid in the region between the active electrodes and the tissue. The dynamic movement of the active electrodes over the tissue site is used by Eggers et al. to enable the electrically conducting liquid to cool the tissue surrounding recently ablated areas to minimize damage to this surrounding tissue. Eggers et al does not mention ablating while reciprocating or rotating the electrode. Moreover, the dynamic movement is not used by Eggers et al. to prevent tissue adhesion. Identical use of reciprocation of the probe during electrosurgical tissue revascularization is discussed in U.S. Pat. No. 6,102,046 to Weinstein et al., also assigned to ArthroCare Corporation, the disclosure of which is incorporated herein by reference.  
         SUMMARY OF THE INVENTION  
         [0021]    The present invention seeks to provide novel electrosurgical apparatus for RF tissue ablation that solves the abovementioned problems of the prior art.  
           [0022]    The electrosurgical apparatus of the present invention comprises a manipulator that moves electrodes in a controlled manner while RF power is delivered to the electrodes. Unlike the prior art, the RF electrode may ablate during controlled motion thereof. This motion helps prevent tissue-electrode adhesion. The motion may be unidirectional along the length of the treated volume, or alternatively may comprise reciprocating motion, vibration or combinations of different kinds of motion. The motion of the electrode is preferably along the electrode trajectory, i.e., linear motion for linear electrodes and helical motion for helical electrodes.  
           [0023]    The electrode is at the distal tip of an electrically insulated support. The electrode may be monopolar or bipolar, hollow or non-hollow. The electrode may be linear or may be a short segment at the tip of a helical, insulated support. The electrode itself may be helical. For example, the electrode may be configured as a monopolar electrode, with a single or multiple-helix as the active (current emitting) electrode, or as a bipolar electrode with single or multiple helix-pairs. Alternatively, a bipolar electrode may comprise a single or multiple-helix as one electrode and a conducting central rod as the other electrode.  
           [0024]    In one embodiment of the present invention, the electrodes are configured as a pair of bipolar concentric (sharing a common center) or eccentric (off-center) helices. A plurality of electrodes may be mounted on the same helical insulated support. A central insulated rod may be added to helical electrodes for motion stabilization. Additional possible electrodes configurations may be obtained by using straight needles instead of helices.  
           [0025]    The helical electrode assembly may coagulate a cylindrical envelope of tissue, while at the same time sparing a cylinder of tissue at the center of the helix. For example, the helical arrangement may be used to coagulate prostate tissue around the urethra without causing coagulation of the urethra itself. In other treatment plans, it may be desirable to cause necrosis of the inner cylindrical volume of the helical electrode assembly. In such a case, the helical electrode assembly may coagulate the tissue surrounding the inner cylindrical volume in such a way such that the blood supply to the inner non-coagulated cylindrical tissue is cut off. The non-coagulated cylindrical tissue is then left to die due to the absence of a sufficient blood supply from the coagulated cylindrical envelope, thereby increasing the amount of tissue that undergoes necrosis and shortening treatment time.  
           [0026]    The manipulator, controlled by a controller, preferably inserts electrodes into the tissue target site (X-direction) at a predetermined YZ position using linear and/or rotational motion. The manipulator may then impart reciprocating motion to the electrodes by reversing the insertion motion. The velocity of the electrode motion may be controlled in various manners, such as by an impedance measurement in a closed-loop fashion. The manipulator may translate the electrodes to a different YZ position prior to subsequent insertion. The electrode assembly may be disposable, and may be easily replaced and attached to the manipulator.  
           [0027]    Treatment planning software, based on user interaction with a display of registered anatomical images obtained prior to the treatment, may generate a treatment plan that includes positioning and power delivery instructions to the controller and manipulator.  
           [0028]    Imaging apparatus, mechanically or otherwise coupled to the manipulator, determines electrode position during intra-corporeal motion in order to facilitate power delivery according to the treatment plan.  
           [0029]    There is thus provided in accordance with a preferred embodiment of the present invention electrosurgical apparatus including an electrode adapted to deliver radio frequency (RF) energy to a tissue, an RF energy source in communication with the electrode, a manipulator in operable connection with the electrode, adapted to move the electrode, and a controller in communication with the RF energy source and the manipulator, adapted to control operation of the manipulator and the electrode in accordance with a mode of operation that includes at least the following steps:  
           [0030]    a) delivering RF energy to the electrode from the RF energy source sufficient to cause ablation of a tissue, and  
           [0031]    b) moving the electrode so as to at least diminish adherence of tissue to the electrode.  
           [0032]    In accordance with a preferred embodiment of the present invention the electrode has a longitudinal axis and the manipulator is adapted to move the electrode generally along the longitudinal axis.  
           [0033]    Further in accordance with a preferred embodiment of the present invention the manipulator is adapted to move the electrode in a reciprocating movement.  
           [0034]    Still further in accordance with a preferred embodiment of the present invention the manipulator is adapted to vibrate the electrode in at least two directions.  
           [0035]    Additionally in accordance with a preferred embodiment of the present invention the electrode includes an insulating support, and the electrode and insulating support are configured as an electrode assembly that has a generally helical shape.  
           [0036]    In accordance with a preferred embodiment of the present invention the manipulator is adapted to screw the electrode assembly into a tissue, wherein the electrode is adapted to cut a helical path into the tissue. The manipulator is preferably adapted to move the electrode back and forth along the helical path.  
           [0037]    Further in accordance with a preferred embodiment of the present invention a plurality of the electrode assemblies are provided. A helical pitch of one of the electrode assemblies may be shifted axially with respect to a helical pitch of another of the electrode assemblies.  
           [0038]    Still further in accordance with a preferred embodiment of the present invention the electrode has a generally hollow lumen formed therein.  
           [0039]    Additionally in accordance with a preferred embodiment of the present invention a fluid source is in fluid communication with the lumen, wherein the lumen is formed with at least one outlet for passage therethrough of the fluid.  
           [0040]    In accordance with a preferred embodiment of the present invention imaging apparatus is in communication with the manipulator and/or the controller, the imaging apparatus being adapted to sense motion of the electrode.  
           [0041]    Further in accordance with a preferred embodiment of the present invention a central rod passes through a helix of the electrode. The central rod may be electrically conducting or insulating.  
           [0042]    In accordance with a preferred embodiment of the present invention a sensor is placed at a distal portion of the electrode, the sensor being in communication with the controller.  
           [0043]    Further in accordance with a preferred embodiment of the present invention one of the electrode assemblies is disposed within an inner volume of another of the electrode assemblies. One of the electrode assemblies may be concentric or eccentric with another of the electrode assemblies. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0044]    The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:  
         [0045]    [0045]FIG. 1 is a simplified illustration of electrosurgical apparatus constructed and operative in accordance with a preferred embodiment of the present invention, comprising generally straight fine needle electrodes;  
         [0046]    [0046]FIG. 2 is a simplified illustration of electrosurgical apparatus constructed and operative in accordance with another preferred embodiment of the present invention, comprising helical electrode assemblies;  
         [0047]    [0047]FIG. 3 is a more detailed, partially sectional illustration of the helical electrode assembly of FIG. 2;  
         [0048]    [0048]FIG. 4 is a simplified illustration of insertion points and helical path limits cut by the helical electrode assemblies of FIG. 2;  
         [0049]    [0049]FIG. 5 is a simplified illustration of a helical electrode assembly with a central rod passing therethrough, constructed and operative in accordance with a preferred embodiment of the present invention;  
         [0050]    [0050]FIGS. 6A and 6B are simplified side-view and end-view illustrations, respectively, of electrosurgical apparatus constructed and operative in accordance with yet another preferred embodiment of the present invention, comprising electrodes configured as a pair of bipolar concentric helices;  
         [0051]    [0051]FIG. 7 is a simplified end-view illustration of electrosurgical apparatus constructed and operative in accordance with still another preferred embodiment of the present invention, comprising electrodes configured as a pair of bipolar eccentric helices;  
         [0052]    [0052]FIGS. 8A, 8B and  8 C are simplified pictorial, side-view and end-view illustrations, respectively, of electrosurgical apparatus constructed and operative in accordance with still another preferred embodiment of the present invention, comprising electrodes configured as a pair of bipolar helices. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0053]    Reference is now made to FIG. 1, which illustrates electrosurgical apparatus  10 , constructed and operative in accordance with a preferred embodiment of the present invention.  
         [0054]    Electrosurgical apparatus  10  preferably includes one or more electrodes  12  adapted to deliver radio frequency (RF) energy to a tissue  14 . In FIG. 1, a pair of electrodes  12  are shown, shaped as fine needles and configured to operate as bipolar electrodes. Alternatively, electrode  12  may be used as a monopolar electrode in conjunction with an external electrode  16 .  
         [0055]    Each electrode  12  is preferably carried by or attached to an insulating support  18 . Insulating support  18  may be a non-conducting support, or an insulating coating formed on a support. As another alternative, insulating support  18  may comprise an insulating outer sheath, which may be made of polyurethane, polytetrafluoroethylene (e.g., TEFLON), or any other biocompatible plastic. The electrode  12  is preferably connected at a proximal end thereof to an RF connector  20 , which in turn is connected to an RF energy source  22 .  
         [0056]    In one embodiment of the invention, electrode  12  has a generally hollow lumen  24  formed therein (not seen in FIG. 1, but shown in FIG. 3). A fluid source  26  is in fluid communication with lumen  24 , such as by means of a fluid connector  28  (and any suitable valving not shown for the sake of simplicity) positioned at the proximal portion of electrode  12 . Lumen  24  is preferably formed with at least one outlet  30  (FIG. 3) for passage therethrough of the fluid. The fluid may be a liquid or gel electrolyte, for example.  
         [0057]    A manipulator  32  is in operable connection with each electrode  12 . Manipulator  32  may be electrically, pneumatically, hydraulically or mechanically driven, and may include without limitation, a servomotor, step motor, linear actuator, rotary actuator, vibrator or solenoid, for example. Manipulator  32  may be coupled to all electrodes at once so as to move all electrodes generally simultaneously in synchronization with one another. Alternatively, manipulator  32  may be coupled individually to the electrodes so as to be capable of moving the electrodes independently of one another.  
         [0058]    A controller  34  is in communication with RF energy source  22  and manipulator  32 , and is adapted to control operation of manipulator  32  and electrodes  12  in accordance with a treatment plan or mode of operation, described further hereinbelow.  
         [0059]    Imaging apparatus  36  is in communication with controller  34 , and may also be in communication with manipulator  32 , either by direct coupling therewith or via controller  34 . Imaging apparatus  36  determines electrode position during intra-corporeal motion in order to facilitate power delivery according to the treatment plan. For example, imaging apparatus  36  may be a trans-rectal ultrasound probe used for imaging the prostate in conjunction with treatment of benign prostatic hypertrophy (BPH) or a malignant condition. Other examples include fiber optic imaging apparatus, fluoroscopic imaging apparatus, endoscopic imaging apparatus or laparoscopic imaging apparatus, which may be useful in RF tissue ablation to treat gastroesophageal reflux disease (GERD) or as part of transmyocardial revascularization, for example.  
         [0060]    In addition to imaging, electrodes  12  may be coupled to other medical sensors or equipment. For example, in RF ablation of cardiac tissue such as in the treatment of tachyarrhythmias, electrodes  12  may be coupled to EKG monitoring equipment (not shown) to assist in determining whether tachycardia persists and whether the tissue in the vicinity of electrodes  12  is still participating in aberrant conduction or ectopic activity, associated with the tachycardia.  
         [0061]    It is noted that electrodes  12  may be brought to tissue  14  in any convenient manner, such as by directly piercing into tissue  14  or by delivery by a catheter system (not shown) to the desired site.  
         [0062]    In one example of a treatment plan possible with the present invention, manipulator  32 , controlled by controller  34 , inserts electrodes  12  into the tissue target site generally along a longitudinal axis  38  of the electrodes  12  (referred to as the X axis) at a predetermined YZ (referring to the plane perpendicular to X axis) position using linear and/or rotational motion. RF energy source  22  delivers RF energy to electrodes  12  to perform tissue ablation. Controller  34  may implement treatment planning software  40 , based on user interaction with a display of registered anatomical images obtained prior to the treatment, in order to control positioning of and power delivery to electrodes  12 .  
         [0063]    During RF ablation, manipulator  32  may move electrodes  12  proximally along longitudinal axis  38  (i.e., retracting the electrodes from their inserted position) or may move electrodes  12  in a reciprocating motion generally along longitudinal axis  38 . Alternatively or additionally, manipulator  32  may vibrate or oscillate electrodes  12  in at least two directions (along or about the X, Y and Z axes or any combination thereof). (A proximal position of electrodes  12  during reciprocating motion thereof is shown in solid lines in FIG. 1, whereas a distal position of electrodes  12  during reciprocating motion thereof is shown in broken lines in FIG. 1.) The proximal or reciprocating motion or vibration may help prevent tissue adhesion during ablation. The electrodes may then be translated to a different YZ position prior to subsequent insertion into a different tissue site.  
         [0064]    Reference is now made to FIGS. 2 and 3, which illustrate electrosurgical apparatus  50  constructed and operative in accordance with another preferred embodiment of the present invention. Electrosurgical apparatus  50  comprises one or more electrodes  52 , each of which is preferably carried by or attached to an insulating support  51  that has a generally helical shape. Electrode  52  is preferably a short segment at the tip of the helical support  51 . Alternatively, electrode  52  may be helical as well. Insulating support  51  may be constructed of a nonconducting material, coating or sheath as described hereinabove for insulating support  18 . Like electrosurgical apparatus  10 , electrosurgical apparatus  50  preferably comprises manipulator  32 , RF energy source  22 , controller  34  (with treatment planning software  40 ) and imaging apparatus  36 , and may also comprise fluid source  26 , all of which are omitted in FIGS. 2 and 3 for the sake of simplicity. The combination of electrode  52  and insulating support  51  is also referred to as an electrode assembly.  
         [0065]    Manipulator  32  is adapted to screw each electrode  52  into tissue  14  (FIG. 1), wherein electrodes  52  cut a helical path  53  into tissue  14 . One way of transferring torque to the electrode  52  is by means of a torque cable  54 , shown in FIG. 3. Torque cable  54  may include two coils  56  and  58 , which are wound in opposite directions about a tube  60  housed in an insulating catheter sleeve  62 . Such a torque cable is commercially available from Lake Region Manufacturing Company of Chaska, Minn., USA, and is described in U.S. Pat. No. 5,165,421, the disclosure of which is incorporated herein by reference. A proximal portion  64  of the electrode assembly is tightly fit into tube  60  through an end cap  66  of sleeve  62 . Coils  56  and  58  may be constructed of metal and serve as conductors.  
         [0066]    As mentioned hereinabove, the devices of U.S. Pat. Nos. 5,431,649 and 6,016,809 to Mulier et al., do not move the electrodes during ablation, but rather rely upon the application of a conductive solution to the ablated area to prevent over-ablation. In contrast, in the present invention, manipulator  32  may move electrodes  52  back and/or forth along helical path  53  during ablation. This motion tends to prevent tissue adherence to the electrodes  52 . (Of course the present invention may also be carried out by moving the electrode assemblies back and/or forth along helical path  53  even not during ablation.)  
         [0067]    In one embodiment of the invention, a helical pitch of one of the electrode assemblies is shifted axially, such as by a distance d (FIG. 3), with respect to the helical pitch of another of the electrode assemblies. The distance d may one one-half pitch, for example. Electrode assemblies with uniform or different pitches may be used.  
         [0068]    Reference is now made to FIG. 4, which illustrates insertion points  57  and helical path limits  59  cut by electrode  52 . The use of axially shifted helices may increase the ablated area around the electrodes  52 . As seen in FIG. 4, a plurality of helical electrode assemblies may ablate a relatively large volume of tissue in a plurality of regions, not possible heretofore with the prior art. The electrode assemblies may be configured as monopolar electrodes, with a single or multiple-helix as the active electrode, or as bipolar electrodes with single or multiple helix-pairs.  
         [0069]    Reference is now made to FIG. 5, which illustrates a central rod  70  passed through the helix of electrode  52 . Central rod  70  may be electrically conducting, in which case electrode  52  may be bipolar, wherein electrode  52  is the current emitting electrode and the conducting central rod  70  serves as the other electrode. Alternatively, central rod  70  may be insulated. In such an embodiment, rod  70  may be added to electrodes  52  for motion stabilization.  
         [0070]    As seen in FIG. 5, a sensor  72  may be placed at a distal portion of electrode  52  (or electrode  12  of FIG. 1). Sensor  72 , which may be in communication with controller  34  via a wire  74  running through electrode  52 , may be used in a closed-loop control of apparatus  50  (or  10 ), by feeding back information to controller  34 . For example, sensor  72  may be a temperature sensor, such as a thermocouple or thermistor, and controller  34  controls RF energy levels or movement of the electrode in accordance with the sensed temperature so as to avoid over-ablation or tissue adherence. As another example, sensor  72  may be a capacitance or resistance sensor, which may sense the electrical capacitance or resistance between the electrode and the tissue being ablated. Controller  34  may control RF energy levels or movement of the electrode in accordance with the sensed electrical parameters so as to avoid over-ablation or tissue adherence. For example, the velocity of the electrode motion may be controlled in a closed-loop fashion with an impedance measurement made by sensor  72 .  
         [0071]    Reference is now made to FIGS. 6A and 6B, which illustrate electrosurgical apparatus  80  constructed and operative in accordance with yet another preferred embodiment of the present invention. Electrosurgical apparatus  80  differs from electrosurgical apparatus  50  in that the electrode assemblies (comprising the electrodes and the insulating supports) are configured as a pair of bipolar concentric helices.  
         [0072]    Specifically, referring to FIG. 6A, electrosurgical apparatus  80  comprises an inner electrode assembly  82  that includes an electrode  84 A carried by or attached to an insulating helical support  86 A, and an outer electrode assembly  88  that includes an electrode  84 B carried by or attached to an insulating helical support  86 B. Like electrosurgical apparatus  10  and  50 , electrosurgical apparatus  80  preferably comprises manipulator  32 , RF energy source  22 , controller  34  (with treatment planning software  40 ) and imaging apparatus  36 , and may also comprise fluid source  26 , all of which (except manipulator  32 ) are omitted in FIGS. 6A and 6B for the sake of simplicity.  
         [0073]    Inner electrode assembly  82  is generally concentric with outer electrode assembly  88 , meaning that the two assemblies generally share a common center lying on a longitudinal axis  87 . The concentric arrangement, like an individual electrode, is rotatable like a corkscrew. Proximal ends  85 A and  85 B of both the inner  82  and outer  88  electrode assemblies, respectively, may be attached to a small plate or disc  83 . Plate  83  is preferably generally perpendicular to longitudinal axis  87 , and is attachable to manipulator  32  for rotating the electrode assemblies  82  and  86 .  
         [0074]    Reference is now made to FIG. 7, which illustrates electrosurgical apparatus  90  constructed and operative in accordance with still another preferred embodiment of the present invention. Electrosurgical apparatus  90  differs from electrosurgical apparatus  80  in that the electrode assemblies (comprising the electrodes and the insulating supports) are configured as a pair of bipolar eccentric helices.  
         [0075]    Specifically, referring to FIG. 7, electrosurgical apparatus  90  comprises an inner electrode assembly  92  disposed in an inner volume of an outer electrode assembly  94 . The rest of the construction of electrosurgical apparatus  90  and its operation is preferably similar to that described for electrosurgical apparatus  80 . It is appreciated that other arrangements of helical pairs of electrode assemblies are also within the scope of the invention.  
         [0076]    Reference is now made to FIGS. 8A, 8B and  8 C, which illustrate electrosurgical apparatus  100  constructed and operative in accordance with still another preferred embodiment of the present invention. Electrosurgical apparatus  100  employs a helical electrode assembly  102 , constructed like any of the helical electrode assemblies described hereinabove (monopolar, bipolar, concentric, eccentric and the like).  
         [0077]    A proximal end of helical electrode assembly  102  is preferably coupled by means of a coupling  104  to manipulator  32  for rotating the electrode assemblies and screwing them into and out of a tissue (not shown). The electrode assemblies  102  may be energized with RF energy from RF energy source  22  (not shown in FIGS.  8 A- 8 C) via slip rings  106 . Slip rings  106  may have a pair of stationary terminals  108  for connection to RF energy source  22 .  
         [0078]    A distal end of helical electrode assembly  102  is preferably guided through an aperture  109  of a nut plate  110 . Nut plate  110  is preferably secured, such as by means of a pin  112 , to a longitudinal guide rail  114 . Manipulator  32  may be also secured to guide rail  114  by means of a bracket  116  that has a U-shaped channel that receives guide rail  114 . As manipulator  32  advances or retracts the helical electrode assembly  102  along a longitudinal axis  118  of the assembly  102 , the manipulator  32  travels together with assembly  102 . The movement of assembly  102  along longitudinal axis  118  may be measured by a linear displacement measuring device  120 , such as a potentiometer or linear encoder. Linear displacement measuring device  120  may be provided with a cover  122 .  
         [0079]    The helical electrode assembly  102  may be moved or adjusted along a transverse axis  124 , generally perpendicular to longitudinal axis  118 . This may be accomplished by attaching the electrode assembly  102  to a transverse adjustment assembly  126 , which may comprise a threaded rod  128  to which assembly  102  is attached. Rod  128  may be turned by means of a handle  130  to adjust the position of assembly  102  along axis  124 . Assembly  102  may slide along a bar  132  during the transverse movement.  
         [0080]    Electrosurgical apparatus  100  and imaging apparatus  36  (which may be an ultrasound probe) are preferably mounted on a translator assembly  134 . Translator assembly  134  is adapted to move electrosurgical apparatus  100  and imaging apparatus  36  generally along axis  118  or another axis  136 , generally perpendicular to longitudinal axis  118 .  
         [0081]    It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of the features described hereinabove as well as modifications and variations thereof which would occur to a person of skill in the art upon reading the foregoing description and which are not in the prior art.