Abstract:
A polyphase electrosurgical system and method are provided. In embodiments, a radiofrequency generator having the capability of delivering a plurality of independent electrosurgical signals is disclosed. An electrosurgical instrument having an array of electrodes that correspond to the plurality of signals may be used to deliver the electrosurgical signals to tissue. In embodiments, three RF signals having a phase offset of about 120° therebetween, i.e., a three-phase configuration, may be used to achieve a balanced delivery of electrosurgical energy, which may lead to increased rates of energy delivery, improved control of tissue ablation regions, and improved operative outcomes. The phase, amplitude, and/or frequency of each signal may be independently variable in response to user inputs and/or biological parameters such as tissue impedance or return electrode current.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to electrosurgical instruments for open, percutaneous, endoscopic, or laparoscopic surgical procedures. More particularly, the present disclosure relates to a radiofrequency tissue ablation system having multiple, independently-phased electrodes for providing rapid energy delivery and improved ablation control. 
     2. Background of Related Art 
     The use of electrical energy including radiofrequency and microwave energy and, in particular, radiofrequency (“RF”) electrodes or microwave antennae for ablation of tissue in the body or for the treatment of pain is known. Generally, RF electrodes (e.g., probes, resistive heating elements, and the like) include an elongated cylindrical configuration for insertion into the body to target tissue that is to be treated or ablated. The RF electrodes can further include an exposed conductive tip portion and an insulated portion. The RF electrodes can also include a method of internal cooling, such as the RF electrodes shown and described in U.S. Pat. No. 6,506,189 entitled “COOL-TIP ELECTRODE THERMOSURGERY SYSTEM” issued to Rittman, III et al., on Jan. 14, 2003 and U.S. Pat. No. 6,530,922 entitled “CLUSTER ABLATION ELECTRODE SYSTEM” issued to Cosman et al., on Mar. 11, 2003. Accordingly, when the RF electrode is connected to an external source of radiofrequency power, e.g., an electrosurgical generator (device used to generate therapeutic energy such as radiofrequency, microwave, or ultrasonic), and current is delivered to the RF electrode, heating of tissue occurs near and around the exposed conductive tip portion thereof, whereby therapeutic changes in the target tissue, near the conductive tip, are created by the elevation of temperature of the tissue. 
     In some applications, for example, tumor ablation procedures, multiple electrodes may be inserted into the body in an array to enlarge ablation volumes. In a particular application, arrays of high frequency electrodes are inserted into tumors. The electrodes are typically placed in a dispersed fashion throughout the tumor volume to cover the tumor volume with uniform heat. In one common arrangement, the electrodes are arranged in a delta (i.e., triangular) configuration. The multiple electrodes may be activated simultaneously or sequentially with high frequency energy so that each electrode heats the surrounding tissue. Simultaneous activation allows maximum energy to the applied to the tissue, but may also have drawbacks. Current from the electrodes tends to travel away from the electrode array, causing an isopotential area or volume, (i.e., a dead zone), to form between the electrodes. Such an isopotential area may result in incomplete ablation of targeted tissue because insufficient energy is delivered to the isopotential region. Series activation, wherein energy is applied to fewer than all electrodes at a time (typically one or two electrodes at a time) can prevent the formation of an isopotential region between the electrodes. However, the sequence of cycling energy through the electrodes in this maimer may also have drawbacks, because it limits the rate of energy delivery into tissue. 
     SUMMARY 
     The present disclosure provides a system and method for supplying energy to multiple electrodes in an electrode cluster, while minimizing or eliminating the isopotential area or volumes enclosed by the electrode cluster. 
     In an embodiment in accordance with the present disclosure, a polyphase RF generator having the capability of generating a plurality of independent RF signals is provided. The phase, amplitude, and/or frequency of each RF signal may be independently variable. The RF signals may be characterized by a substantially sinusoidal waveform, or may alternatively exhibit non-sinusoidal characteristics. Each independent signal may be operably coupled to a corresponding electrode of the surgical instrument. The independent RF signals may be concurrently generated. 
     In an embodiment, a polyphase RE generator according to the present disclosure may be configured to generate three RF signals having a phase offset of about 120° therebetween, i.e., a three-phase configuration. For example, phase  1  may be a reference phase having 0° phase shift, phase  2  vary from phase  1  by about 120°, and phase  3  may vary from phase  2  by about 120° and from phase  1  by about 240°. By this arrangement, a system operating in accordance with the present disclosure may operate in a balanced, or tri-polar, mode wherein a return electrode current (i.e. neutral current), is about zero because the total energy between the phases flows substantially among and between the electrodes of the instrument. 
     It is also contemplated that a system operating in accordance with the present disclosure may operate in an unbalanced state, which may cause current flow through a return electrode. For example, vaporization (a.k.a. “bubble steam”) may form around an electrode altering the rate of energy flow thereat. In another instance, eschar may form at the operative site which may alter the impedance at one or more electrodes at the operative site, resulting in a change of energy transfer rate at the affected electrode(s). An unbalanced operating mode may also be achieved by altering the phase, amplitude, and/or frequency of at least one of the electrosurgical waveforms which may be useful, for example, for tailoring the shape or direction of the ablation region in accordance with therapeutic requirements. Additionally or alternatively, altering the phase, amplitude, and/or frequency of at least one of the electrosurgical waveforms may be useful to adapt the delivery of energy into tissue in response to conditions at the operative site, such as vaporization or eschar. 
     Also provided within the scope of the present disclosure is a control system for use with a polyphase electrosurgical generator which generates electrosurgical waveforms for application to tissue. At least one sensor module may be included that is configured to continually sense biometric parameters related to the operative site, including without limitation tissue impedance corresponding to the application of the electrosurgical waveforms to the tissue, and/or temperature of the operative site. The control system may include a controller configured to adjust the phase, amplitude, and/or frequency of one or more of the electrosurgical waveforms. A controller may be configured to adjust phase, amplitude, and/or frequency in response to a biometric parameter, such as, without limitation, tissue impedance, eschar formation, bubble steam formation, return pad current, and size and/or shape of the ablation region. Additionally or alternatively, a controller may be configured to adjust phase, amplitude, and/or frequency in response to a user input. For example, phase, amplitude or frequency may be independently adjusted by the user. In embodiments, the ability to select from predetermined combinations of phase, amplitude, frequency is provided. 
     In embodiments, the polyphase RF signals may be advantageously generated by digital synthesis. In embodiments, a digitized representation of a desired output waveform, which may be a sine wave, may be stored within the RF generator. The stored representation may be organized in lookup table form. The digitized representation may be converted into analog form via a digital-to-analog (D/A) converter. Each separately phased signal may be associated with a corresponding D/A converter. In embodiments, a single multiplexed D/A converter may be associated with each phased signal. The generator may be configured to include multiple pointers (i.e., an offset) to data points of the digitized waveform table corresponding to the desired phase offset between each polyphase signal. For example, a sine wave may be represented in a lookup table having 3,600 data points (i.e., samples), each point corresponding to the amplitude of a sine wave in 0.1° increments. Assuming a three-phase signal having 120° between phases, a first phase would correspond to an offset of 0 samples, a second phase would correspond to an offset of 1200 samples, and a third phase would correspond to an offset of 2400 samples. The stored data points are then processed by the D/A converters at the corresponding rate and offset to generate polyphase signals having the desired frequency and phase angle. The amplitude of each polyphase signal may be adjusted by, for example, digital scaling or a variable gain output amplifier. The phase angle of a selected signal may be adjusted by varying the table offset associated with that phase. Continuing with the present example, changing the table offset by a single location causes a phase change of 0.1°. Finer adjustments of phase may be achieved by, for example, interpolation of table values, or by providing a table having a greater number of data points. 
     The ability to control the return path of electrosurgical energy provided by the electrode cluster may be accomplished by altering the phase, amplitude, and/or frequency relationship between the waveforms. For example, in a simple monopolar configuration, RF energy will follow the path of least resistance (averaged over the three-dimensional volumetric pathway) between the electrode and return pad; there is no clear method to divert or alter the electrical current flow direction—that path is defined by the physiological parameters as the area is ablated. Similarly, in a bi-polar configuration, the current flows between the two electrodes and the surrounding normal tissue anatomy/physiology, as well as the ablation created, dictate the overall current pathway. Using a 3-phase multi-phase system, the device may be operated in a balanced mode (each electrode offset 120° in phase) to direct energy equally between the electrodes, in an unbalanced mode to deliver more current between specific electrodes and partially to the return pad, or in a single-phase mode where all the electrodes operate in the same phase (like a typical monopolar mode) which will direct the current only to the return pad. Adjusting the relative phases and amplitudes of the multiphase system may provide a level of control to current flow paths and subsequent energy deposition. 
     In one envisioned embodiment, an altered phase relationship is fixed during activation of the electrodes, which may be useful to tailor the direction and shape of the ablation region in accordance with operative requirements. In another envisioned embodiment, the controller may be configured to adjust phase, amplitude, and/or frequency in accordance with predetermined time-varying contours. For example, amplitude modulation may be applied to each phase in succession to induce a quasi-rotating energy delivery pattern. In another example, the phases of two adjacent electrodes may be brought into or out of coherence over a period of time to alter the ablation area. This timed phase shift may be performed in a one-shot, multi-shot, or repeating manner. The shift pattern may be staggered or alternating, so that, for example, when the cluster electrodes are activated the phase difference between two electrode may continually vary from about 120° to about 0° and back to about 120° at a predetermined rate. In yet another embodiment, the phase, amplitude and frequency of each electrode is independently varied in according with predetermined patterns. Thus, by altering the relationships of phase, amplitude and frequency of each electrode with respect to the others, a variety of ablation effects may be achieved. 
     Other arrangements are contemplated within the scope of the present disclosure, for example, an embodiment having less than three, or more than three, independent RF signals. Such embodiments may have, for example, a generator capable of generating four RF signals having phase offsets of about 90° therebetween, or, as another example, a generator capable of generating six RF signals having phase offsets of about 60° therebetween. 
     In another aspect according to the present disclosure, a surgical instrument having a plurality of electrodes is disclosed. Each electrode may be operably coupled to a corresponding independent output of a polyphase RF generator. The electrodes may be arranged in any configuration, such as a triangle (i.e., delta) configuration or a linear configuration. In embodiments, the number of electrodes is a multiple of the number of independent RF signals provided by a polyphase RE generator. For example, a three-phase generator may be operably coupled to a surgical instrument having six electrodes configured in a hexagonal arrangement. Each RF signal, or phase, may be coupled to one or more electrodes. In embodiments, each phase may be coupled to respective electrodes of the cluster. As an example only, referring to each electrode by a letter A through F respectively around the perimeter defined by the hexagonal electrode array, phase  1  may be coupled to electrodes A and D, phase  2  to electrodes B and E, and phase  3  to C and F. In this manner, opposing electrodes of the array are commonly coupled to one phase provided by the generator. As another example, phase  1  may be coupled to electrodes A and B, phase  2  to electrodes C and D, and phase  3  to electrodes E and F. In yet another example, the phases may be coupled to an arbitrary combination electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the following detailed description when taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic representation of a polyphase electrosurgical system in accordance with an embodiment of the present disclosure; 
         FIG. 2  is an illustration of a polyphase electrosurgical system in accordance with an embodiment of the present disclosure; 
         FIG. 3  is a block diagram of a polyphase electrosurgical system in accordance with an embodiment of the present disclosure; 
         FIG. 4  is a block diagram of an example embodiment of a polyphase electrosurgical generator configured to digitally synthesize radiofrequency signals; 
         FIG. 5  is an example illustration of the relationship between a waveform table and polyphase signals generated by a polyphase electrosurgical system in accordance with the present disclosure; 
         FIG. 6A  is an example oblique view of an electrode cluster in accordance with the present disclosure illustrating balanced electric field lines between electrodes; 
         FIG. 6B  is an example oblique view of an electrode cluster in accordance with the present disclosure illustrating unbalanced electric field lines between electrodes; 
         FIG. 7A  is an example diagram showing an end view of an electrode configuration of a three-phase electrosurgical system having three electrodes in accordance with the present disclosure; 
         FIG. 7B  is an example diagram showing an end view of an electrode configuration of a three-phase electrosurgical system having six electrodes in accordance with the present disclosure; 
         FIG. 7C  is an example diagram showing an end view of an electrode configuration of a three-phase electrosurgical system having nine electrodes in accordance with the present disclosure; 
         FIG. 7D  is an example diagram showing an end view of an electrode configuration of a four-phase electrosurgical system having four electrodes in accordance with the present disclosure; 
         FIG. 7E  is an example diagram showing an end view of an electrode configuration of a four-phase electrosurgical system having eight electrodes in accordance with the present disclosure; and 
         FIG. 7F  is an example diagram showing an end view of a lattice electrode configuration of a three-phase electrosurgical system having six electrodes in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Particular embodiments of the present disclosure will be described herein with reference to the accompanying drawings. As shown in the drawings and as described throughout the following description, and as is traditional when referring to relative positioning on an object, the term “proximal” refers to the end of the apparatus that is closer to the user and the term “distal” refers to the end of the apparatus that is further from the user. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail. 
     With reference to  FIG. 1 , a polyphase electrosurgical system  100  in accordance with an embodiment of the present disclosure includes a polyphase RF generator  110  that is operably coupled to an electrosurgical instrument  120 . A cluster electrode array  130  is provided at distal end  121  of electrosurgical instrument  120  for delivering electrosurgical energy to tissue of a patient P. Polyphase RF generator  110  may be operably coupled to an electrosurgical instrument  120  by a cable  115 , which may be a multi-conductor cable. Polyphase electrosurgical system  100  may optionally be configured with at least one return electrode  140  to provide a return path for electrosurgical energy to polyphase RF generator  110  via conductor  150  operably coupled therebetween. As can be seen in  FIG. 1 , return electrode  140  may be positioned on the body of a patient at, for example, the leg, buttocks, or other medically-suitable location. 
     As shown in  FIG. 2 , aspects of a polyphase electrosurgical system in accordance with an embodiment of the present disclosure are shown as system  200  and include an electrosurgical instrument  210  having a housing  211  adapted for use by a user for performing open, percutaneous, endoscopic, or laparoscopic surgical procedures. The distal end  212  of instrument  210  may include electrodes  221 ,  222 , and  223  that may be operably and/or independently coupled to separate sources of electrosurgical energy provided by polyphase electrosurgical generator  260 . Instrument  210  may additionally include at least one user interface element  230 , which may be used to facilitate control of the generator  260 . For example, user interface element  230  may be a momentary pushbutton (e.g., push-on/release-off), toggle pushbutton (e.g., push-on/push-off) or sequence pushbutton (e.g., for stepping through alternative selections). Additionally, or alternatively, user interface element  230  may be a slide switch or continuously variable control, such as a potentiometer. 
     Instrument  210  may be operably coupled to polyphase generator  260  by a cable  235 . In some embodiments, cable  235  may be detachably coupled to the instrument  210 . Additionally, or alternatively, cable  235  may be detachably coupled to polyphase generator  260  via connector  237 . A strain relief  236  may be included at the proximal end  213  of instrument  210 . 
     Optionally, a return electrode pad  240  having a return electrode  245  incorporated therewith may be provided. Return electrode pad  240  may be operably coupled to polyphase generator  260  by a cable  250 . In some embodiments, cable  250  may be detachably coupled to the instrument  210  and, additionally or alternatively, cable  250  may be detachably coupled to polyphase generator  260  via connector  251 . Return electrode pad  240  may additionally include a connector  246  configured to couple cable  250  thereto. 
     Polyphase generator  260  may include at least one user input element  270 ,  275  and display element  265  to facilitate user interaction with the system. Display element  265  may be any suitable display device, including without limitation an LED display, an LCD display, a graphics display (e.g., flat panel), or an electromechanical indicator. 
     In  FIG. 3  there is illustrated a block diagram of a polyphase electrosurgical generator system  300  in accordance with an embodiment of the present disclosure. The system  300  includes a controller module  310  that is operably coupled to a three-phase generator module  330 . Additionally, controller module  310  may be operably coupled to a user interface module  320  and/or an activation control, such as a footswitch  315 . The controller  310 , user interface module  320 , and/or generator module  330  may conveniently be arranged in a common housing  305 . Generator module  330  provides three RF outputs  331 ,  332 , and  333 , corresponding to phase  1 , phase  2 , and phase  3 , respectively, of a polyphase electrosurgical signal. Each RF output  331 ,  332 , and  333  may be operably coupled to electrodes  351 ,  352 , and  353  by conductors  341 ,  342 , and  343 , respectively. Electrodes  351 ,  352 , and  353  are disposed within an instrument housing  350  configured to position electrode tips  356 ,  357 , and  358 , at or adjacent to the distal end thereof. Instrument housing  350  may be constructed from materials that may include electrically non-conductive material, and may by be configured to electrically insulate electrodes  351 ,  352 , and  353  from each other and from non-targeted tissue. 
     In use, controller module  310  may receive a user input from user interface module  320 . For example, a user may select an amplitude, frequency, and/or phase relationship characterizing the desired electrosurgical signal. In some embodiments, a user may select a static electrosurgical signal, e.g., one characterized by a continuous steady-state delivery of energy. In other embodiments, the user may select a dynamically-changing signal, wherein at least one signal parameter changes on a temporal basis, for example without limitation, periodic modulation (e.g., pulse width modulation) and aperiodic modulation (e.g., altering a parameter in a predetermined or arbitrary manner over time). 
     Controller module  310  may additionally receive an activation signal from an activation control, such as a footswitch  315  or an activation control included with the instrument housing  350  (not explicitly shown). Upon receipt of an activation signal, controller module causes generator module  330  to begin outputting a polyphase RF signal at output  331 ,  332 , and  333 . The polyphase energy is conducted via conductors  341 ,  342 , and  343  to electrodes  351 ,  352 , and  353  for performing an electrosurgical procedure, such as ablation, on tissue T. 
     Shown in greater detail in  FIG. 4 , embodiments of the disclosed polyphase electrosurgical system are envisioned and are illustrated as system  400  that includes a processor  410  that is operably coupled to at least one storage device  415 . The storage device may be of any type of suitable storage device, including without limitation fixed and/or removable solid state memory devices (such as dynamic RAM, flash memory, or read-only memory), or disk drives (i.e., magnetic, magneto-optical, or optical drives). Storage device  415  may contain a waveform table  430  that includes a digitized waveform representation, and may contain a set of programmable instructions configured for execution by processor  410 . 
     Processor  410  is operably coupled to digital-to-analog (D/A) converters  441 ,  442 , and  443  for converting a digital representation of a waveform into analog form. The outputs of D/A converters  441 ,  442 , and  443  are operably coupled to output stages  451 ,  452 , and  453 , which collectively may form an energy module  450 . Output stages  451  et seq. may be an amplifier for amplifying the output of D/A converters  441  et seq. to the power level required for electrosurgical procedures. Output stages  451  may include a low-pass filter. The gain of output stages  451  et seq. may be controlled by processor  410 , by, for example without limitation, direct control of the output stage  451  et seq. by processor  410  via a control signal (not explicitly shown), or by scaling of the digital signal by the processor  410  prior to D/A conversion. Outputs of the output stages  451  et seq. are operably coupled to electrodes  471 ,  472 , and  473  via conductors  461 ,  462 , and  463  for delivering electrosurgical energy to the operative site. A return electrode  480  and corresponding return conductor  481  may be operably coupled to energy module  450 . 
     Continuing with respect to  FIG. 4 , processor  410  may be operably coupled to a display device  420 , a user interface  425 , and at least one activation control  435  that may be, for example, a footswitch or a handswitch. Processor  410  may be configured to perform a set of programmable instructions for receiving inputs from user interface  425  and/or activation control  435 , and for causing operational information to be displayed on display device  420 . In response to inputs received from user interface  425  and/or activation control  435 , processor  410  may be configured to read waveform data from waveform table  430  and cause the waveform data to be converted into analog form by D/A converters  441  et seq. The maimer in which data is read and/or caused to be converted into analog form is dependent upon the desired phase, amplitude and frequency relationships among and between the individual polyphase signals. For example, in an embodiment that provides a balanced three-phase polyphase signal, processor  410  may retrieve waveform data from waveform table  430  from three separate table locations. The offset between the separate table locations from which waveform data is retrieved may be the number of waveform samples corresponding to the desired phase difference, which, in the present example, is 120°. The phase difference among and between the individuals may thus be tailored by varying this offset. 
     Advantageously, the rate at which waveform samples are read from waveform table  430  and consequently converted to analog form is in direct proportion to the desired frequency of the polyphase signal and the number of samples that represent a single period of the waveform. For example, assume a 500 kHz signal is desired, and the waveform table  430  contains 120 samples that collectively represent a single period of the waveform. Since the period of a waveform is expressed as the reciprocal of the frequency, i.e., p=1/f, the period of a 500 kHz signal is 2 μs. Therefore, to generate a 500 kHz signal, the 120 samples representing a single period of the electrosurgical polyphase signal must be read and converted in a 2 μs time interval, which correspond to a rate of 16.7 ns per sample, or a sample frequency of 60 mHz. 
     The frequency of one or more individual polyphase signal may be adjusted by altering the rate at which waveform data is delivered to, and converted by, a D/A converter  441  et seq. 
     In some embodiments, the phase and amplitude relationships among and between the polyphase signals may be adjusted by, for example, processor  410  and/or energy module  450  to achieve a target return electrode  480  current. In some embodiments, a target return electrode current may be a minimal or nearly zero current. For example, a software algorithm (not explicitly shown) adapted to be executed by processor  410  receives an input corresponding to the return electrode  480  current. In response thereto, the algorithm may alter one or more of the phase and/or amplitude of a polyphase signal to minimize or nearly eliminate the return electrode  480  current. 
     The relationship between waveform data and polyphase signals is illustrated by example in  FIG. 5  Waveform table  530  includes waveform data representing one period of a sine wave having twelve samples  501 - 512 , and having maximum and minimum values scaled to +64 and −64 units, respectively. Phases  1 ,  2 , and  3  of the polyphase signal are represented at  510 ,  515 , and  520 , respectively. Since twelve samples are shown in waveform table  530 , it follows that the phase difference between each sample represents a phase shift of 30 degrees (360/12=30). Thus, in the present example, in order to generate polyphase signals having a phase difference of 120°, an offset of 4 samples between phases is indicated (120/30=4). Accordingly, phase  1  is generated by D/A converter  441  from samples beginning at sample  501 , phase  2  is generated by D/A converter  442  from samples initially indexed at  505 , and phase  3  is generated by D/A converter  443  from samples indexed beginning at  509 . Waveform table  530  may be organized as a circular table, that is, after the final sample is reached, an index “wraps” back to the beginning of the table, i.e., sample  501 . In this manner a continuous waveform may be generated. 
     In some embodiments, the respective phase difference between individual polyphase signals may be altered by altering the table offset accordingly. In some embodiments, table  530  may include greater or fewer samples, which may be scaled in any suitable manner, to correspond with, for example, the desired minimum phase angle resolution, or the amplitude resolution of D/A converter  441  et seq. 
     Turning now to  FIGS. 6A and 6B , example embodiments in accordance with the present disclosure are presented wherein electric field lines representative of current flow are illustrated. As illustrated in FIGS. 6 A and  6 B return electrode  625  may be coupled to polyphase RF generator (not shown) via conductor  626 .  FIG. 6A  depicts a polyphase electrosurgical probe  600  having a three-phase electrode cluster that includes electrodes  611 ,  612 , and  613  that are shown delivering polyphase electrosurgical energy to tissue T in a balanced mode. As can be seen by representative current flow lines  621 ,  622 , and  623 , in a balanced mode energy flows among and between three-phase electrode cluster formed by electrodes  611 ,  612 , and  613 , while negligible or no current flow exists between the three-phase electrode cluster and return electrode  625 . In  FIG. 6B , the polyphase electrosurgical probe  600  is shown operating in an unbalanced mode, wherein energy flows among and between the three-phase electrode cluster as shown by representative current flow lines  631 ,  632 , and  633  and between the electrode cluster and return electrode  625  as shown by representative current flow lines  634 . 
     In  FIGS. 7A-7F , various illustrative arrangements of cluster electrodes and attendant current paths are shown. As can be seen in  FIG. 7A , a polyphase electrosurgical system accordance with the present disclosure having a three-phase topology may have a cluster electrode array that includes a triangular or delta electrode configuration  710 . In  FIGS. 7B and 7C , an electrode configuration of a three-phase electrosurgical system having six electrodes  720  and nine electrodes  730 , respectively, is shown. In  FIGS. 7D and 7E , there is shown an exemplary configuration of a four-phase electrosurgical system having four electrodes  740  and eight electrodes  750 , respectively, in accordance with the present disclosure. Other arrangements are contemplated and within the scope of the present disclosure, for example, as seen in the three-phase embodiment of  FIG. 7F , a polyphase cluster electrode  760  may be configured in a linear or lattice-type arrangement. In each of  FIGS. 7A, 7B, 7C, 7D, 7E, and 7F , the current paths between electrodes operating in a substantially balanced mode are depicted illustratively by  711 ,  721 ,  731 ,  741 ,  751 , and  761 , respectively. In embodiments, the phase relationship among and between multiple electrodes may be arbitrarily configured or patterned to tailor energy delivery patterns to achieve a particular ablation region shape. 
     The described embodiments of the present disclosure are intended to be illustrative rather than restrictive, and are not intended to represent every embodiment of the present disclosure. Further variations of the above-disclosed embodiments and other features and functions, or alternatives thereof, may be made or desirably combined into many other different systems or applications without departing from the spirit or scope of the disclosure as set forth in the following claims both literally and in equivalents recognized in law.