Abstract:
Bipolar belt ablation systems and methods for treating patient tissue involve the use of an ablation an ablation apparatus with a plurality of ablation elements carried by a flexible tube structure, a radiofrequency generator capable of delivering a plurality of differing radio frequency power signals to the ablation elements of the ablation device, wires transmitting the radiofrequency power signals from the radiofrequency generator to the ablation elements, and a control mechanism to enable temperature-based power control to each of the powered ablation elements.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/850,944 filed Aug. 5, 2010, which is a nonprovisional of U.S. Provisional Patent Application No. 61/231,613, filed Aug. 5, 2009, each of which is incorporated herein by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Embodiments of the present invention relate in general to systems and methods for ablating tissue, and in particular instances to bipolar belt ablation systems and methods. 
         [0003]    Several types of unipolar wand or belt type devices have been described. A suction applied bipolar ablation device with two adjacent ablation elements to produce an arcing energy delivery pattern has been described. 
         [0004]    Although these and other proposed treatments may provide real benefits to patients in need thereof, still further advances would be desirable. Embodiments of the present invention provide solutions that address the problems which may be associated with the techniques described above, and hence provide answers to at least some of these outstanding needs. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    Embodiments of the present invention provide systems and methods for encircling tissue structures such as pulmonary veins, and applying ablative energy thereto. Systems for ablating tissue can be used with any preferred surgical access technique, including without limitation sternotomy and thoracotomy procedures. 
         [0006]    In one aspect, embodiments of the present invention encompass radiofrequency ablation systems for treating tissue. Exemplary systems may include an ablation apparatus with a plurality of ablation elements carried by a flexible tube structure, and a radiofrequency generator Capable of delivering a plurality of differing radiofrequency power signals to the ablation elements of the ablation device. Systems may also include transmission elements such as wires that transmit the radiofrequency power signals from the radiofrequency generator to the ablation elements, and a control mechanism to enable temperature-based power control to each of the powered ablation elements. In some cases, the plurality of power signals include the output of the radio frequency generator and the return path of the generator. In some cases, the plurality of power signals include at least two separate outputs of the radio frequency generator. Optionally, at least two separate outputs of the radiofrequency generator can have the same frequency but differ in at least one of amplitude or phase from other power outputs. According to some embodiments, two separate outputs of the radio frequency generator can have different frequencies. In some cases, one or more electrode pads placed on the skin of the patient can provide a return path to the generator. In some cases, one or more electrodes on the ablation apparatus may provide the only return path to the generator. Optionally, the ablation apparatus can be configured to enable placement of the ablation elements on the epicardium of the heart. Exemplary embodiments also encompass a radiofrequency ablation apparatus that enables placement of the ablation elements near the pulmonary veins. In some cases, the ablation apparatus can be adapted to enable ablation of atrial tissue at least partially encircling the pulmonary veins. Optionally, an ablation apparatus can be adapted to enable ablation of atrial tissue to isolate tissue activation within at least one pulmonary vein from the rest of the atrium. In some instances, operation of an ablation apparatus provides a contiguous lesion around all pulmonary veins without needing to move the ablation elements during the lesion-making process. According to exemplary embodiments, the ablation apparatus may form a loop with more proximal ablation elements in close proximity to one or more distal ablation elements. In some cases, the ablation apparatus may provide a contiguous lesion around all pulmonary veins without needing to move the ablation elements during the lesion-making process. Optionally, a loop can be formed using a cinching structure or mechanism. In some cases, a cinching structure uses one or more magnets to form the loop. Optionally, the cinching structure may use the interference fit of a snap connection to form the loop. In some cases, the cinching structure uses suction to form the loop. In some instances, the ablation apparatus includes one or more suction pods to enhance tissue contact to the ablation elements. 
         [0007]    In another aspect, embodiments of the present invention encompass methods of administering a radiofrequency ablation treatment to a patient. Exemplary methods include delivering a plurality of differing radiofrequency power signals to a plurality of ablation elements of an ablation apparatus. The plurality of ablation elements can be coupled with a flexible tube structure. Methods may also include sensing a temperature of a tissue of the patient, and controlling power to one or more of the ablation elements based on the sensed temperature. Optionally, methods may include using an introducer during the course of the treatment. In some cases, methods involve using an endoscopic visualization apparatus during the course of the treatment. 
         [0008]    In a further aspect, embodiments of the present invention encompass a radiofrequency ablation system for treating tissue. The system can include an ablation apparatus having a first ablation element and a second ablation element carried by a flexible tube structure. The system can also include a radiofrequency generator capable of generating a first radiofrequency power signal for the first ablation element and a second radio frequency power signal for the second ablation element. Exemplary systems also include a first transmission element configured to transmit the first radiofrequency power signal from the radiofrequency generator to the first ablation element and a second transmission element configured to transmit the second radiofrequency power signal from the radiofrequency generator to the second ablation element. Some systems may include a control mechanism to enable temperature-based power control to each of the powered ablation elements. In some instances, the first radiofrequency power signal is equivalent to the second radiofrequency power signal. In some instances, the first radiofrequency power signal is different from the second radiofrequency power signal. 
         [0009]    In yet another aspect, embodiments of the present invention encompass methods of administering a radiofrequency ablation treatment to a patient. Exemplary methods may include delivering a first radiofrequency power signal to a first ablation element of an ablation apparatus and delivering a second radio frequency power signal to a second ablation element of the ablation apparatus. Optionally, methods may include sensing a temperature of a tissue of the patient, and controlling power to at least one of the first ablation element or the second ablation element based on the sensed temperature. According to some methods, the first radiofrequency power signal is equivalent to the second radiofrequency power signal. In some instances, the first radiofrequency power signal is different from the second radiofrequency power signal. 
         [0010]    For a fuller understanding of the nature and advantages of the present invention, reference should be had to the ensuing detailed description taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  illustrates aspects of tissue treatment system according to embodiments of the present invention. 
           [0012]      FIG. 2  illustrates aspects of tissue treatment system according to embodiments of the present invention. 
           [0013]      FIG. 3  illustrates aspects of tissue treatment system according to embodiments of the present invention. 
           [0014]      FIGS. 4A, 4B, and 4C  illustrates aspects of tissue treatment system according to embodiments of the present invention. 
           [0015]      FIG. 5  illustrates aspects of tissue treatment system according to embodiments of the present invention. 
           [0016]      FIG. 6  illustrates aspects of tissue treatment system according to embodiments of the present invention. 
           [0017]      FIG. 7  illustrates aspects of tissue treatment system according to embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Embodiments of the present invention encompass multi-element belt type ablation devices for encircling tissue structures such as one or more pulmonary veins. Ablation elements can deliver RF energy and can be configured such that certain elements deliver the RF and other elements provide the return path. Various configurations are possible and two particular embodiments are described including alternating and grouped opposing. For example,  FIG. 1  illustrates an embodiment of a treatment system  100  which is configured to deliver an alternating treatment protocol. Treatment system  100  includes active or delivery electrodes  110  and return electrodes  120 .  FIG. 2  shows a treatment system  200  which is configured to deliver a grouped or opposing treatment protocol. Treatment system  200  includes active or delivery electrodes  210  and return electrodes  220 . As shown in  FIG. 3 , a treatment system  300  which is configured to deliver a grouped or opposing treatment protocol can also be used as a monopolar ablation device to deliver a monopolar ablation. As depicted here, treatment system  300  includes active or delivery electrodes  310 , and return path electrodes  320  which have been withdrawn. 
         [0019]    According to some embodiments, devices may include a cinching mechanism to allow a circumference to be adjustable to the tissue structure to be ablated, and also to allow the active and return path electrodes on opposing sides of a tissue structure to be brought into closer proximity to one another. A cinching mechanism may be facilitated or constructed by fixing the distal end of the belt and retracting the proximal end or by a separate mechanism that cinches both distal and proximal ends. 
         [0020]      FIGS. 4A-4C  show aspects of ablation systems according to embodiments of the present invention. An ablation system  400   a  is depicted in  FIG. 4A . Ablation system  400   a  includes an ablation assembly  415   a  and a cinching device  410   a.  Ablation assembly  415   a  includes an ablation member  416   a  and a stabilizer member  417   a.  Cinching device  410   a  includes or is coupled with an ablation segment  420   a  such as a corner electrode. Cinching device  410   a  includes a support  435   a  and one or more rollers  430   a.  Ablation segment  420   a  can be coupled with support  435   a  as part of cinching device  410   a.  Ablation segment  420   a  may be positioned on support  435   a  toward a distal end of cinching device  410   a,  so that when an operator or surgeon advances cinching device  410   a  towards pulmonary veins (PV) of patient tissue  405   a,  ablation segment  420   a  can make contact with the atria adjacent to the pulmonary veins. In some embodiments, ablation segment  420   a  can be used to help insure or increase the likelihood that the ablation system ablates the patient tissue  405   a  in an approximate or complete circle or closed path around the patient tissue. Ablation segment  420   a  can be designed to bridge the gap between opposing sides of ablation element  416   a,  so that a loop structure is formed in more of a smooth circumferential path and in less of a teardrop shaped path. Ablation segment  420   a  can thus be used to bridge a gap that might otherwise exist between ablation member  415   a  and patient tissue  405   a.  In some embodiments, ablation member  416   a  may be coupled with ablation segment  420   a.  In some embodiments, the ablation segment or member may include a material that is capable of transmitting different forms of energy, including but not limited to RF, thermoelectric, cryogenic, microwave, laser, ultrasound, or the like. In some embodiments, ablation segment  420   a  can be positioned in order to inhibit the ablation assembly  415   a  from pinching the patient tissue  405   a.  Ablation member  416   a  may include active and return delivery electrode configurations as discussed elsewhere herein, for example with reference to  FIGS. 1 to 3 . 
         [0021]    In some embodiments, the ablation segment  420   a  may be shaped in order to maximize contact with the patient tissue  405   a.  In some embodiments, the ablation segment  420   a  may be shaped in order to minimize or reduce any pinching of the patient tissue  405   a.  Ablation segment  420   a  may be rigid in some embodiments. Ablation segment  420   a  may be flexible in some embodiments. Ablation segment  420   a  may make contact with the ablation member  416   a.  Optionally, ablation segment  420   a  may not make contact with the ablation member  416   a.  Cinching device  410   a  can be advanced in a direction toward the patient tissue  405   a  as indicated by arrow A 1 , to increase the amount of contact between ablation member  417   a  and the patient tissue  405   a  or to help secure the position of ablation member  417   a  relative to patient tissue  405   a.  Similarly, by advancing cinching device  410   a  in this direction, an operator can increase the amount of contact between ablation segment  420   a  and the patient tissue  405   a  or help secure the position of ablation segment  420   a  relative to patient tissue  405   a.  The operator can also establish or apply an opposing force by grasping or pulling sections  418   a,    419   a  of ablation assembly  415   a  in an opposing direction, as indicated by arrows A 2 . The operator can position ablation assembly  415   a  to make contact with selected parts of patient tissue  405   a  such that when ablative energy is transmitted through the ablation assembly, it is possible to create an approximately circular or closed ablation pattern or lesion on the tissue. In this way, energy can be applied by the ablation system to the tissue. The position of cinching device  410   a  relative to ablation assembly  415   a  can be adjusted by the operator. For example, the cinching device may be advanced or retracted to differing degrees in order to increase or decrease an amount of contact between the ablation assembly and the patient tissue. Ablation segment  420   a  in combination with ablation member  416   a  can form a continuous circumferential loop that can be used to ablate a circumferential lesion on the patient tissue. 
         [0022]    As shown in  FIGS. 4B and 4C , an ablation system  400   b  can include a cinching device  420   b  and an ablation assembly  415   b.  Cinching device  420   b  can include one or more guides  450   b,  and can be used in operative association with an ablation assembly  415   b  having an ablation member  416   b  and a stabilizer member  417   b.  Cinching device  420   b  can also include an ablation member protection mechanism  445   b  which can be disposed between portions  470   b  and  471   b  of ablation member  416   b.  Hence, it is possible to avoid contact between different portions of an ablation member such as an electrode. One or more guides  450   b,    451   b  of cinching device  420   b  may include a flat or curved retaining wall perpendicularly attached to, or formed along with, a support  440   b  of cinching device  420   b.  In some embodiments, one or more guides  450   b,    451   b  may have a curved top edge. The shape of guides  450   b,    451   b  can be designed in order to help facilitate positioning an ablation assembly  415   b  around the patient tissue  405   b  by keeping distal  471   b  and proximal  470   b  segments of the ablation member  416   b  close to each other. In an embodiment with two or more guides, the guides may be located on opposite sides of a support plate of cinching device  420   b.  in one method for using the cinching device  420   b  with guides, ablation assembly  415   b  can be passed through the cinching device along one guide  450   b  and then wrapped around the patient tissue  405   b.  The ablation assembly  415   b  can then be pulled through cinching device  420   b  in the opposed direction, passing, along another guide  451   b.  The guides may be made of or include various semi-rigid or rigid materials. Cinching device  420   b  can be advanced in a direction toward the patient tissue  405   b  as indicated by arrow B 1 , to increase the amount of contact between the ablation member  417   b  and the patient tissue  405   b  or to help secure the position of ablation member  417   b  relative to patient tissue  405   b.  Similarly, by advancing cinching device  420   b  in this direction, an operator can increase the amount of contact between ablation segment  435   b  and the patient tissue  405   b  or help secure the position of ablation segment  435   b  relative to patient tissue  405   b.  The operator can also establish or apply an opposing force by grasping or pulling sections  418   b,    419   b  of ablation assembly  415   b  in an opposing direction, as indicated by arrows B 2 . 
         [0023]    In some embodiments, as cinching device  420   b  is advanced towards the patient tissue  405   b,  a loop structure  411   b  of the ablation assembly  415   b  is reduced in diameter or otherwise contracted. As shown in  FIG. 4B , in some embodiments, an ablation member protection mechanism  445   b  can be utilized along with the cinching device  420   b.  Cinching device  420   b  can include ablation member protection mechanism  445   b  disposed between guides  450   b,    451   b.  Ablation member protection mechanism  445   b  can act to keep a first segment  470   a  and a second segment  471   b  of the ablation member from making contact with each other. Ablation member protection mechanism  445   b  may either be fixed or integral to cinching device  420   b  or separate from cinching device  420   b.  In some embodiments, an ablation segment  435   b  may be attached with cinching device  420   b,  such as with a first or distal section  446   b  of ablation member protection mechanism  445   b.  Ablation segment  435   b  can be utilized to increase the amount of contact between the ablation assembly  415   b  and the patient tissue  405   b.  Ablation segment  435   b  can also be utilized to reduce pinching of the patient tissue  405   b  that might otherwise occur if ablation segment  435   b  were not disposed between opposing segments of ablation member  416   b.  The operator can position ablation assembly  415   b  to make contact with selected parts of patient tissue  405   b  such that when ablative energy is transmitted through the ablation assembly, it is possible to create an approximately circular or closed ablation pattern or lesion on the tissue. In this way, energy can be applied by the ablation system to the tissue. The position of cinching device  420   b  relative to ablation assembly  415   b  can be adjusted by the operator. For example, the cinching device may be advanced or retracted to differing degrees in order to increase or decrease an amount of contact between the ablation assembly and the patient tissue. Ablation segment  435   b  in combination with ablation member  416   b  can form a continuous circumferential loop that can be used to ablate a circumferential lesion on the patient tissue. Ablation member  416   b  may include active and return delivery electrode configurations as discussed elsewhere herein, for example with reference to  FIGS. 1 to 3 .  FIG. 4C  shows a perspective view of cinching device  420   b  which includes ablation member protection mechanism  445   b,  guides  450   b,    451   b,  and ablation segment  435   b.  In is understood that any of a variety of cinching techniques can be used in conjunction with the active and return delivery electrode configurations described herein, including those. cinching systems and methods described in U.S. Patent Application No. 60/939,201 filed May 21, 2007, Ser. No. 12/124,743 filed May 21, 2008, and Ser. No. 12/124,766 filed May 21, 2008, the content of each of which is incorporated herein by reference. 
         [0024]    As shown in  FIG. 5 , in some embodiments, a treatment system  500  can include a first opposing member  510  and a second opposing member  520 . System  500  can also include a first electrode assembly  512  disposed on first opposing member  510  and a second electrode assembly  522  disposed on second opposing member  520 . As shown here, a first coupling mechanism  514  of first opposing member  510  is configured to couple with a second coupling mechanism  524  of second opposing member  520 . In some cases, distal coupling mechanisms  514 ,  524  may include a magnet. Treatment system  500  can also include a first compressing member  530  and a second compressing member  540 . First and second compressing members  530 ,  540  can be semi-rigid members that operate to compress first and second opposing members  510 ,  520  together or toward each other, into closer proximity, as indicated by arrows A. Hence, two flexible ablation elements, each optionally coupled with an attachment mechanism to a stiffening/positioning member, could be delivered and potentially coupled at their distal end. Coupling mechanisms  514 ,  524  could include snap fit mechanisms, magnets, suction mechanisms, and the like. Treatment system  500  can also include active and return delivery electrode configurations as discussed elsewhere herein, for example with reference to  FIGS. 1 to 3 . 
         [0025]    As shown in  FIG. 6 , in some embodiments, a treatment system  600  can include a first stiffening or compressing member  610  and a second stiffening or compressing member  620 . System  600  can also include a first electrode assembly  612  disposed on first stiffening or compressing member  610  and a second electrode assembly  622  disposed on second stiffening or compressing member  620 . As shown here, stiffening or compressing members  610 ,  620  can be integrated into the ablation device or attachable and include stylets either externally attached or inserted into a channel. For example, first stiffening member  610  can include a first vacuum channel  614  and second stiffening member  620  can include a second vacuum channel  624 . In some cases, stiffening or compressing member  610 ,  620  can include a collapsible vacuum chamber. For instance, stiffening or compressing members  610 ,  620  can include a thin flexible material that collapses under a vacuum. Suction applied along the length of the ablation member or electrode assembly can enhance tissue contact. As shown here, first electrode assembly  612  can include a return electrode assembly, and second electrode assembly  622  can include an active electrode assembly. Treatment system  600  can also include active and return delivery electrode configurations as discussed elsewhere herein, for example with reference to  FIGS. 1 to 3 . 
         [0026]    As shown in  FIG. 7 , in some embodiments, a treatment system  700  can include a first opposing member or portion  710  and a second opposing member or portion  720 . System  700  can also include a first electrode assembly  712  disposed on first opposing member  710  and a second electrode assembly  722  disposed on second opposing member  720 . Treatment system  700  can also include a first compressing member  730  and a second compressing member  740 . First and second compressing members  730 ,  740  can be balloon or expandable members that operate to compress first and second opposing members  710 ,  720  together or toward each other, into closer proximity, as indicated by arrows A. Treatment system  700  can also include active and return delivery electrode configurations as discussed elsewhere herein, for example with reference to  FIGS. 1 to 3 . 
         [0027]    Belt devices may include visualization and delivery systems including scopes with protective lenses and introducers with stylets, sheaths, and or magnets. Relatedly, belt systems according to embodiments of the present invention may incorporate features of treatment or ablation systems such as those described in U.S. Patent Application No. 61/179,564 filed May 19, 2009, Ser. No. 61/015,472 filed Dec. 20, 2007, and Ser. No. 12/339,331 filed Dec. 19, 2008, the content of each of which is incorporated herein by reference. Belt devices can also include a marking system for identifying to the user which electrodes are or are not in contact with tissue for ablation. 
         [0028]    According to some embodiments, the treatment systems and methods described herein may be used in conjunction or combined with aspects of introducer systems and methods such as those described in U.S. Patent Application No. 60/337,070 filed Dec. 4, 2001; Ser. No. 10/272,446 filed Oct. 15, 2002; Ser. No. 10/310,675 filed Dec. 4, 2002; Ser. No. 10/410,618 filed Apr. 8, 2003; Ser. No. 11/148,611 filed Jun. 8, 2005; 60/939,201 filed May 21, 2007; 61/015,472 filed Dec. 20, 2007; 61/051,975, filed May 9, 2008; Ser. No. 12/124,743 filed May 21, 2008; Ser. No. 12/124,766 filed May 21, 2008; Ser. No. 12/255,076 filed Oct. 21, 2008; Ser. No. 12/273,938 filed Nov. 19, 2008; Ser. No. 12/339,331 filed Dec. 19, 2008; Ser. No. 12/463,760 filed May 11, 2009; 61/179,564 filed May 19, 2009; 61/231,613 filed Aug. 5, 2009; and 61/241,297 filed Sep. 10, 2009. The entire content of each of these filings is incorporated herein by reference for all purposes. 
         [0029]    Relatedly, in some instances, the treatment systems and methods described herein may include elements or aspects of the medical systems and methods discussed in U.S. Patent Application Nos. 60/337,070 filed Dec. 4, 2001; Ser. No. 10/080,374 filed Feb. 19, 2002; Ser. No. 10/255,025 filed Sep. 24, 2002; Ser. No. 10/272,446 filed Oct. 15, 2002; Ser. No. 10/310,675 filed Dec. 4, 2002; Ser. No. 10/410,618 filed Apr. 8, 2003; Ser. No. 11/067,535 filed Feb. 25, 2005; Ser. No. 11/148,611 filed Jun. 8, 2005; 60/939,201 filed May 21, 2007; 61/015,472 filed Dec. 20, 2007; 61/051,975, filed May 9, 2008; Ser. No. 12/124,743 filed May 21, 2008; Ser. No. 12/124,766 filed May 21, 2008; Ser. No. 12/255,076 filed Oct. 21, 2008; Ser. No. 12/273,938 filed Nov. 19, 2008; Ser. No. 12/339,331 filed Dec. 19, 2008; Ser. No. 12/463,760 filed May 11, 2009; 61/179,564 filed May 19, 2009 61/231,613 filed Aug. 5, 2009; 61/241,297 filed Sep. 10, 2009, 61/288,031 filed Dec. 18, 2009; 61/318,474 filed Mar. 29, 2010; Ser. No. 12/781,072 filed May 17, 2010; and Ser. No. 12/822,616 filed Jun. 24, 2010. The entire content of each of these filings is incorporated herein by reference for all purposes. 
         [0030]    Power Configurations 
         [0031]    Certain known treatment systems or methods may require the application of RF ablations at least three separate times to the patient to achieve a box lesion, that is a contiguous lesion around all pulmonary veins. It is noted that a typical patient presents four pulmonary veins, but some patients have a. different number of veins, the most common of those variants being five pulmonary veins. More rarely patients may have 3 or 6 pulmonary veins emptying into the left atrium. Advantageously, embodiments of the present invention can involve a reduced number of RF applications with effective results. For example, it is possible to achieve a contiguous box lesion with a reduced number of RF applications by using a device that forms a closed belt-like structure around all pulmonary veins on the left atria. In some cases, such techniques may involve two RF applications. In some cases, two applications may be helpful or required due to possible power limitations of an electrosurgical unit. For example, a reduced number of applications may require an increase in output power (e.g. double the amount of output power). Further, two applications may be helpful or required because of the very high current levels needed to power all the ablation electrodes simultaneously when using monopolar ablation to create an entire box lesion. RF ablation electrode technology may involve the application of about 3.5 to about 4.0 amperes of current to achieve and maintain the tissue temperatures needed to achieve transmural lesions for the box lesion in patients. In some cases, the maximum safe current that can be returned to the ESU through each return pad attached to the patient&#39;s skin may be about 1.0 ampere, so at least four return pads would be required. A technique involving fewer than four pads may be desirable. 
         [0032]    According to embodiments of the present invention, it is possible to return some or all the applied current to a selected set of ablation electrodes to another set of selected ablation electrodes rather than to non-ablating electrodes so as to provide an improvement to some purely monopolar systems creating a box lesion. For example, current can be returned to selected ablation electrodes rather than to the return pads on the patient&#39;s skin. In some cases, this technique can be described as a bipolar ablation. In some cases, this approach may involve placement of the return ablation electrodes at about four or more centimeters distant from the “active” ablation electrodes. As described elsewhere herein, there are several possible or preferred patterns for active and return electrodes in which one set of electrodes is connected to an ESU power source and other electrodes return the RF current to the ESU. 
         [0033]    It may be helpful to separately control power to all ablation electrodes, whether they are active or return electrodes, since the power requirements can vary widely at different locations on the atrium. In some instances, for the same sized electrodes, active and return electrodes can be equally effective ablation electrodes and the heating patterns produced by each can be the same. For the simplest case of one RF output and one return path to the isolated ground of the RF generator, RF power may be controlled by the average or median temperature of the selected active electrodes. Power delivery can be delivered preferentially to electrodes with the lower temperatures to maintain the electrode temperature of the selected active electrodes at nearly the same temperature. One algorithm to accomplish this is to temporarily disconnect electrodes that are hotter than the average or median temperature of the selected active electrodes by a predetermined amount (2-5° C.); an alternatively algorithm only disconnects electrodes that are hotter than the selected control temperature by a predetermined amount (2-5° C.). In the latter case all selected active electrodes can be connected to the RF power source until they individually exceed the set temperature. In a similar manner all selected return electrodes can be powered to maintain those electrodes at nearly the same temperature. As in the active electrode set, the hotter electrodes may be temporarily disconnected, for example if the electrodes exceed set temperature or if one or more exceed average or median return electrode temperatures by 2-5° C. 
         [0034]    If the only return path for the RF current applied to the set of active ablation electrodes is through the set of return ablation electrodes, then the median or average temperatures of the active and return electrodes will often be quite different from one another even if the same size and number of electrodes are used for the active and return set. This effect can occur because ablation conditions vary by location on the heart. One method to achieve improved ablation at both active and return electrodes is to provide more ablation surface area on the active ablation sites than on the return electrode sites and return some of the RF current to return electrode pads on the skin when the return electrodes exceed a set temperature. In one algorithm, the return pads are connected to the ESU when the return ablation electrodes are disconnected. In alternative algorithm, the return pads on the patient&#39;s skin are returned to isolated ground through a path of relatively high resistance, for example 100-200 ohm, while the return ablation electrodes are connected to the isolated RF ground via a low impedance path. A higher ablation surface electrode area associated with this method can be achieved by using active ablation electrodes that have a greater surface area than the return electrodes, for example by using longer active electrodes, or using a greater number of active electrodes than return electrodes. 
         [0035]    An alternative approach of powering all electrodes in the box lesion ablation catheter involves delivering power to all electrodes by one of two RF channels, with the second channel waveform being inverted relative to the first channel, i.e. the second channel is 180° out of phase with the first channel. The RF outputs of each channel can both source and sink RF power for example to a maximum rating of at least 150 Watts. The connection pattern can include the patterns as illustrated elsewhere herein. In some cases, the electrodes shown as being returned to the isolated ground of the RF power generator can instead be connected to the second RF channel. For each RF output, RF power can be controlled by the average or median temperature of the selected active electrodes connected to that output channel. Power delivery can be delivered preferentially to electrodes with the lower temperatures to maintain the electrode temperature of the selected active electrodes at nearly the same temperature. One algorithm to accomplish this is to temporarily disconnect electrodes that are hotter than the average or median temperature of the selected active electrodes by a predetermined amount (2-5° C.); alternatively an algorithm can be used that only disconnects electrodes that are hotter than the selected control temperature by a predetermined amount (2-5° C.) In the latter case all selected active electrodes can be connected to the RF power source until they individually exceeded the set temperature. As with the bipolar situation with a single output channel, temperatures of the ablation electrodes connected to the two output channels may not be the same if return pads on the patients skin are not used, since currents sourced from one channel are typically sunk in the other channel. In other words, according to such embodiments, with no return pads, the total current from all electrodes connected to channel  1  is equal to the total current through electrodes connected to channel  2 . With return pads connected to the isolated ground of the channel outputs, the current amplitudes on channel  1  and channel  2  can differ. The difference in current levels flowing through channels  1  and  2  flows to the return pads and is returned to the RF generator. Usually the current required to ablate tissues at the electrodes connected to channel  1  is similar to the current required to ablate tissue in the electrodes connected to channel  2 . Therefore, the difference in the current amplitudes provided by channels  1  and  2  is much smaller than the currents in each channel and is smaller than the sum of the currents flowing in each channel, which is the current amplitude that is ablating tissue in the box lesion. For these reasons, the current flowing through the return pads is usually manifold lower than currents that are flowing through all the selected ablation electrodes (or the sum of the current amplitudes provided by channels  1  and  2 ). In some cases, systems and methods involve the use of equivalent RF frequencies or phases applied to different electrodes. 
         [0036]    With three or more separate RF output channels, separate power control to the differing ablation electrodes can be achieved without use of grounding pads on the patient&#39;s skin. If a single RF frequency is used, then signals with 120° phase angle differences among the three channels can be used, and other phase choices can provide acceptable performance as well. If no return pad is used, it may be helpful if phase angles for three output channels using the same frequency differ by more than 30°. If more than one electrode is connected to a single output, then the methods previously described to maintain those electrodes at similar temperatures can be used. If four or more output channels are used, then equal phase separation among the output channels may be used, i.e. 360°/N where N. is the number of output channels used. 
         [0037]    The frequencies used for the output channels need not be at the same. Use of differing frequencies can have advantages, for example if three or more output channels are used, and the output waveforms from each channel are uncorrelated to each other. For such waveforms, changes in one channel has no impact on the other channels, which simplifies power control for the entire system. 
         [0038]    According to some ablation system embodiments, co-axial electrode separation can be about 2 mm to enable regional control of the ablation process while preventing gaps in the ablation pattern between electrodes. Such spacing can be optimized based on the methods of activating those electrodes: the electrodes can be actively maintained at the same potentials during ablation, or they can be disconnected from the RF generator. For activation patterns that have co-axial electrodes actively maintained at different potentials, electrode distances can be increased to 4-8 mm to prevent char formation between electrodes. Since current densities are high in the tissue between closely spaced electrodes for bipolar or other activation schemes described in this disclosure, when those co-axial electrodes are selected for ablation, the region between the electrodes are overheated, resulting in char formation between the electrodes. On the other hand, if the co-axial electrodes are never activated simultaneously, or if the same potentials are applied to each adjacent co-axial electrode, then a gap may occur between the electrodes separated by 4-8 mm. In some instances, systems and methods may not involve the use of different RF frequencies or phases applied to different electrodes. 
         [0039]    It is appreciated that system embodiments of the present in invention can be configured to carry out an of the various method aspects disclosed herein. Likewise, embodiments of the present invention encompass methods which are performed by any of the systems described herein. 
         [0040]    While the exemplary embodiments have been described in some detail, by way of example and for clarity of understanding, those of skill in the art will recognize that a variety of modification, adaptations, and changes may be employed. Hence, the scope of the present invention should be limited solely by the claims.