Abstract:
An ablation system comprising a source of electrical ablation energy having first second and third power outputs it is disclosed. A first conductor is coupled to the first power output on the source of electrical energy. A second conductor is coupled to the second power output on the source of electrical energy, the source of electrical energy creates a first output ablation voltage between the first and second power outputs. The first output ablation voltage varies between a first higher average value during a first period of time and a first lower average value for a second period of time. The first lower average value is greater than or equal to zero. A third conductor is coupled to a third power output on the source of electrical energy. The source of electrical energy creates a second output ablation voltage between the first and third power outputs. The second output ablation voltage varies between a second higher average value during a third period of time and a lower average value for a fourth period of time. The lower average value is greater than or equal to zero. An ablation probe is coupled to the first conductor.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation and part of U.S. patent application Ser. No. 11/717,920 filed on Mar. 13, 2007 and entitled Ablation System and Heat Preventing Electrodes Therefor, the disclosure of which is hereby incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to methods and apparatus for preventing return electrodes in a radio frequency ablation system (such as a uterine fibroid ablation system) from overheating and causing injury or discomfort to the patient. 
       BACKGROUND OF THE INVENTION 
       [0003]    There are a number of applications in which radio frequency ablation is a preferred procedure. Such applications include the treatment of various growths and lesions, such as prostate cancer, liver cancer, and uterine fibroids. 
         [0004]    Such techniques generally involve the application of power between a return electrode and an ablation device, such as a pointed trocar tip with a plurality of extendable ablation stylets. A potential complication during the deployment and use of such devices is overheating in the area surrounding the return electrodes, with attendant discomfort and, potentially, injury. 
       SUMMARY OF THE INVENTION 
       [0005]    In accordance with the invention, an apparatus and method are provided to reduce the likelihood of return electrode overheating. 
         [0006]    In accordance with the invention, an ablation system comprises a source of electrical ablation energy having first, second and third power outputs. A first conductor is coupled to the first power output on the source of electrical energy. A second conductor is coupled to the second power output on the source of electrical energy, the source of electrical energy creates a first output ablation voltage between the first and second power outputs. The first output ablation voltage varies between a first higher average value during a first period of time and a first lower average value for a second period of time. The first lower average value is greater than or equal to zero. A third conductor is coupled to a third power output on the source of electrical energy. The source of electrical energy creates a second output ablation voltage between the first and third power outputs. The second output ablation voltage varies between a second higher average value during a third period of time and a lower average value for a fourth period of time. The lower average value is greater than or equal to zero. An ablation probe is coupled to the first conductor. 
         [0007]    An electrode provides a return path for an ablation device. The electrode comprises a first conductive ablation member which defines a first contact surface. The first contact surface defines a first active peripheral edge on a first active side of the first conductive member. A first coupling member is coupled to the second conductor and electrically connected to a first power coupling edge of the first conductive member. The second edge is an edge of the first conductive member other than the first active peripheral edge. The first coupling member is made of an electrically conductive material. A second conductive ablation member and defines a second contact surface, the second contact surface defines a second active peripheral edge on a second active side of the second conductive member. A second coupling member coupled to the third conductor is electrically connected to a second power coupling edge of the second conductive member. The second power coupling edge is an edge of the second conductive member other than the second active peripheral edge. The second coupling member is made of an electrically conductive material. The first active peripheral edge is positioned between the first power coupling edge and the second power coupling edge. 
         [0008]    In accordance with the inventive system, the first period of time may overlap a substantial portion of the fourth period of time, and the second period of time may overlap a substantial portion of the third period of time. 
         [0009]    In accordance with a preferred embodiment of the invention, the power coupling edges may be opposite the active peripheral edges. 
         [0010]    In a preferred embodiment the peripheral edges are substantially straight and have first and second ends, and a curved edge is contiguous to each of the first and second ends. 
         [0011]    In accordance with the inventive method of ablating a biological body in a mammal ablation energy is applied between an ablation stylet and, intermittently, first and second skin electrodes. 
         [0012]    Generally, in accordance with the invention, the same may also be achieved by, alternatively or additionally, monitoring electrode temperature and varying either the electrodes which are being driven with radio frequency ablation energy, and/or varying the amount of radio frequency ablation energy being coupled to the electrodes. 
         [0013]    In accordance with the invention, it is recognized that RF current flow is concentrated at the leading edge of the pad (the cephalic edge of the pad in the case of uterine fibroid ablation). Hence, temperature rises in the tissue proximate the leading edge. In accordance with the invention, there may be a plurality of pads with single or multiple electrodes on a single substrate. Each electrode is thus associated with a zone of skin positioned underneath it switching between electrode which overlie different zones may be referred to as zone switching. Zone switching is done to essentially create a new leading edge as the old one warms up. Pad zone switching is intended to distribute the heating that occurs on the skin under a return electrode by changing the zone that is being used. The majority of the heating occurs along the leading edge of a return electrode because the path from that edge to the ablation site is shortest and thus has the lowest impedance. By, in effect moving that leading edge around, local heating is reduced, minimizing the probability of a burn. 
         [0014]    In a typical arrangement return electrode pads having one or more electrodes are located on both of the patient&#39;s legs. If each leg has a single return electrode pad divided into two or three electrically isolated electrodes and corresponding zones, in accordance with the invention the zones may be oriented across the leg, like stripes. If at one point in time the first (e.g. cephalic) zone is in use as the return electrode, heating occurs primarily on the leading edge of that zone. A short time later (e.g. ten seconds) the system can switch to another electrode associate with a different zone. That moves the localized heating to the leading edge of the new zone and gives the body time to cool the previously-used zone. Blood flow and conduction will tend to bring the skin under the “old” leading edge back to normal body temperature. In effect we are moving the return electrode from one place to another and are avoiding prolonged heating of a single patch of skin. 
         [0015]    Generally, effective cooling is achieved because power is maintained at levels where the time it takes for a zone to increase in temperature by a certain amount is reliably a little longer than the time it takes for the body to cool that zone back down after the zone is changed in the case of a two electrode arrangement. If there are more than two electrodes, more elaborate methodologies may be employed, as detailed below. 
         [0016]    One possible algorithm involves, for each leg, starting to apply RF using (as the return electrode) the coldest of the N zones on that leg. After a certain time (e.g. five seconds), the system starts looking at the temperature along the leading edge of the zone(s) in use. If the temperature reaches the “switching” level (e.g. 40° C.) the system then switches to the coldest zone. In any case, if a maximum time limit (e.g. thirty seconds) is reached and the temperature has not yet reached the switching level, then the system switches to the coldest zone. Note that if the zone being currently excited is still the coldest of the N zones there is no change in zone, as the object is to use the coldest zone at all times. 
         [0017]    As an alternative to using the coolest zone or other selection strategy, the system may be designed to periodically change zones, for example every thirty seconds to always implement a switch in active pad to act as a fail safe for a pad that happens to lift right where a sensing thermocouple is placed. The thermocouples are preferably placed at the cephalic midline of the zone to make it less likely that it will peel up there, but if it did and there is not enough of a change in impedance to trigger a contact quality alarm threshold, by moving the active zone every set number of seconds, a risk of injury is mitigated. 
         [0018]    The inventive method for ablating a tissue mass associated with a human or animal patient being treated comprises positioning an ablating electrode in a tissue mass to be ablated. A plurality of return electrodes are positioned on the patient. Electrical energy is applied between the return electrodes and the ablating electrode. The temperature of the return electrodes is measured to generate a temperature measurement signal. 
         [0019]    The electrical energy applied between the return electrodes and the ablating electrode may be varied in response to the temperature measurement signal. 
         [0020]    The electrical energy may be radio frequency energy. 
         [0021]    The temperature may be measured on a portion of the return electrode which may be more proximate to the tissue mass than other portions of the return electrode. The temperature may be measured on an edge of the return electrode. 
         [0022]    The varying of electrical energy may comprise shutting off one or more electrodes. The varying of electrical energy may comprise apportioning electrical energy between the electrodes. The apportionment may be made by sending more electrical energy to electrodes which are less likely to become overheated. The electrical energy may be apportioned by varying the duty cycle of electrical energy sent to each of the return electrodes. 
         [0023]    The apportionment may be made by sending more electrical energy to electrodes which are more likely to cool rapidly, proportionately on the basis of cooling rate. The apportionment may be made by sending more electrical energy to electrodes which are less likely to become overheated and by sending more electrical energy to electrodes which are more likely to cool rapidly, proportionately on the basis of cooling rate. 
         [0024]    The impedance path of return electrodes may be measured to determine the existence of a poor electrical connection. 
         [0025]    In accordance with the invention, a device is provided for ablating tissue masses associated with a human or animal patient, the invented device comprises an electrical source for generating an ablation current. A coupling circuit is coupled to the electrical source. An ablation electrode is coupled to the coupling circuit. A plurality of return electrodes are coupled to the coupling circuit. A plurality of temperature measurement transducers are each associated with a respective return electrode. The temperature transducers each provide a temperature measurement signal. They computing device is coupled to the temperature measurement signals for producing signals to control the coupling of the return electrodes to the coupling circuit. 
         [0026]    An initial set of operating parameters may be deployed. These operating parameters may be varied in response to heating and/or cooling of the electrodes. 
         [0027]    The inventive apparatus may include software for storing information for operation type, or surgeon identity, or another factor to generate initial parameters for the system. 
         [0028]    In accordance with the preferred embodiment, the amount of energy sent to a return electrode is varied individually as a function of its temperature history. 
         [0029]    The temperature measurement transducers may be secured to their respective return electrodes. 
         [0030]    In accordance with a preferred embodiment, the amount of energy sent to a return electrode may be varied individually. 
         [0031]    The inventive apparatus may provide a display of electrode temperature in a first color, such as green or blue, when the return electrode is cool, and a second color, such as amber, when the return electrode is becoming significantly warmed, and in a third color, such as red, when the return electrode has exceeded acceptable threshold temperature. 
         [0032]    In accordance with the invention, the inventive apparatus may provide for the amount of ablation current being sent to one of the return electrodes to be varied as a function of temperature in accordance with a first algorithm, and the amount of ablation current being sent to another of the return electrodes to be varied as a function of temperature in accordance with a second algorithm, with the second algorithm being different from the first algorithm. 
         [0033]    The inventive apparatus may include software for storing information for operation type, or surgeon identity, or another factor to generate initial parameters for the system. 
         [0034]    In accordance with the preferred embodiment, the amount of energy sent to a return electrode is varied individually as a function of its temperature history. 
     
    
     
       BRIEF DESCRIPTION THE DRAWINGS 
         [0035]    The operation of the invention will become apparent from the following description taken in conjunction with the drawings, in which: 
           [0036]      FIG. 1  is a flow chart generally illustrating one implementation of the present invention; 
           [0037]      FIG. 2  is a block diagram illustrating an alternative embodiment of the method of the present invention; 
           [0038]      FIG. 3  illustrates another alternative method in accordance with the present invention; 
           [0039]      FIG. 4  is a flow chart of yet another alternative embodiment of the method according to the present invention; 
           [0040]      FIG. 5  illustrates still yet another alternative embodiment of the present invention in which both power and the number of electrodes is varied; 
           [0041]      FIG. 6  illustrates an algorithm for varying and allocating power between electrodes in accordance with the present invention; 
           [0042]      FIG. 7  illustrates an apparatus for implementing the methods of the present invention; 
           [0043]      FIG. 8  illustrates a top plan view of an electrode useful for practicing the method of the present invention; 
           [0044]      FIG. 9  is a side view along lines  9 - 9  of  FIG. 8  illustrating an electrode useful for practicing the method of the present invention; 
           [0045]      FIG. 10  illustrates the placement of electrodes in accordance with the present invention; 
           [0046]      FIGS. 11-14  illustrate alternative embodiments of electrodes useful in connection with the implementation of the present invention; and 
           [0047]      FIG. 15  illustrates an alternative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0048]    Typically, ablation is performed using a device comprising an elongated handle which may be gripped by the surgeon. Extending from the handle is a thin elongated, often pointed sometimes stiff but somewhat flexible catheter which may be used to puncture the skin and enter the region where the particular growth or lesion to be removed is located. 
         [0049]    The above device is of particular value with respect to the ablation of uterine fibroids. Alternatively, a device with a rounded tip may be used to implement ablation in parts of the body that allow entrance without piercing, for example the urethra. Other areas of application include liver lesions, prostate cancer, and so forth. The present invention applies to all of the above and similar devices. 
         [0050]    In such devices a plurality of stylets extends from the pointed tip and may be deployed into a growth to be removed. For example, the tip, under the guidance of ultrasound imaging may be advanced into a liver lesion. Stylets extending from the tip are then deployed through holes in the tip and positioned in various parts of the lesion to be ablated. Typically, the stylets are electrically conductive wires which are housed within the tip and under the action of a lever, slider or other device in the handle of the ablation instrument are advanced through those holes and into the growth to be ablated. 
         [0051]    The application of radio frequency energy to the stylets is controlled by another control located on the handle of the ablation device, such as a radio frequency energy actuation button. For example, this may be a push button control. The application of RF energy may be continuous, or pulsating with a fixed or variable duty cycle. 
         [0052]    Referring to  FIG. 1 , a method  10  for controlling the temperature of electrodes and preventing overheating is illustrated. In accordance with method  10 , power is applied to a return electrode at step  12 . The temperature of the electrode is measured at step  14 . If the temperature of the electrode exceeds an acceptable threshold value as determined at step  16 , the system removes power from the electrode at step  18 . 
         [0053]    If the temperature of the electrode is not in excess of the maximum acceptable electrode temperature, the system proceeds back to step  14  to measure electrode temperature. 
         [0054]    Periodically, the electrode temperature is measured by the system automatically at step  20 . In accordance with the preferred embodiment of the invention, measurement of the temperature of electrode is done by briefly removing ablation power from the return electrode. During the period in which ablation power has been removed from the return electrode, the system reads the temperature measured by a temperature transducer, whose structure is more fully described below. The removal of power allows the relatively weak signals produced by the temperature transducer to be read, without the interference of the relatively high power RF ablation signal applied to the return electrode. If the measured temperature is still above the threshold, at step  22  the system proceeds back to step  20  to repeat the measurement. If, on the other hand, the temperature is below the threshold, at step  22  the system returns power to the electrode at step  12 . 
         [0055]    In accordance with the present invention it is contemplated that the above control scheme may be applied in sequence and repeatedly to each of the return electrodes, thus ensuring that none of them overheat. 
         [0056]    An alternative method  110  in accordance with the present invention is illustrated in  FIG. 2 . The system starts at step  112  to apply power. At step  114 , the surgeon is given the option of using all electrodes or selecting only certain electrodes. At step  116 , equal power is applied to all electrodes. At step  118  one electrode is selected for temperature measurement. Such measurement is made at step  120  and sent to the computer at step  122 . The measured temperature is displayed at step  124 . The surgeon is also given the opportunity, which is implemented at step  126 , to change or select a maximum operating temperature for the return electrodes. The selection is illustrated at step  128 . Different threshold temperatures, optionally, may be selected for different electrodes. 
         [0057]    If the temperature measured at step  120  is not above the selected threshold, at step  130 , the system proceeds to step  132 , where it increments to the next electrode and the temperature of that electrode is measured at step  120 . In accordance with the present invention, it is contemplated that temperature measurements may be made when all RF power is removed from all of the return electrodes and the system is in a measurement mode. While, in principle, it is possible to take a temperature measurement most easily with only one electrode, namely that electrode whose temperature is to be measured, disconnected from the RF ablation power source, additional reliability is provided by disconnecting all ablation power during measurement. Because measurements may be taken in a very short period of time, for example on the order of milliseconds, there is no noticeable effect on the ablation procedure on account of the disablement of ablation power during measurement. 
         [0058]    If the temperature measured at step  120  is above the selected threshold, at step  130  the system proceeds to step  134  where control signals to remove power from the electrode are generated by the system. The power distribution between the electrodes is thus reorganized at step  136 . The system then proceeds periodically to step  138  where the temperature of the disabled electrode is periodically checked. As noted above, the temperature of all return electrodes may be periodically checked during a single measurement period. This temperature is sent to the computer at step  140 , for example, for display, and to provide the surgeon with an opportunity to override the same. If the temperature is found to be above the threshold at step  142 , the system proceeds back to step  138 . The system also returns to step  118  to repeat the cycle. 
         [0059]    On the other hand, if the temperature is not above the threshold, the system proceeds to step  144  where power between the electrodes is distributed equally and, at step  146  power is applied to the previously disabled electrode, allowing the system to return to step  118  to repeat the cycle. Optionally, power may be distributed unequally, as described below. 
         [0060]    Turning to  FIG. 3 , another alternative embodiment of a method  210  in accordance with the present invention is illustrated. Method  210  begins at step  212  and provides the opportunity of selecting electrodes at step  214 . Power is then applied to the selected electrodes at step  216 . 
         [0061]    An electrode is then selected for temperature measurement at step  218 . Such measurement is made at step  220  and the measured temperature is sent to a computer which controls operation of the system at step  222 . If there is no change in the temperature of any electrode, at step  224  the system increments to the next electrode at step  226  and returns to step  220  to repeat the process for all electrodes. 
         [0062]    If it is determined at step  224  that there has been a rise in temperature, the system proceeds to step  228  to adjust the power sent to the electrodes in accordance with a power adjustment algorithm. Such adjustment may simply mean applying power only to the two coolest of four available operating electrodes. Alternatively, the algorithm may simply provide for the hottest electrode to be disabled, thus allowing blood flow to cool the area before discomfort or injury occurs. Alternatively, power may only be applied to the half or two thirds coolest electrodes. 
         [0063]    After the adjustment has been made, the same may be implemented at step  230 , after which the system proceeds to the next electrode at step  226 . Alternatively, it is possible for power to the electrodes to be varied, by, for example, driving the electrodes with an intermittent RF ablation signal reducing the duty cycle of the intermittent RF signal to reduce power. Another way to reduce power is to vary the amplitude of a continuous RF signal. 
         [0064]    In accordance with the method illustrated in  FIG. 3 , the temperature of the electrodes may also be optionally assessed at step  232 . If it is determined that the temperature of an electrode which has been disabled has dropped below the threshold at step  234 , power is restored to the disabled electrode at step  236  and the system is returned to step  228  to adjust the allocation of power between electrodes. 
         [0065]    If the temperature is still above the threshold, the system returns to step  232  to continue its periodic checks in accordance with a determined schedule. In accordance with the invention, it is contemplated that such a schedule may be as simple as checking temperature every five or 10 seconds. In such a method, all RF energy would remove from all return electrodes for a period of, for example, 100 ms, during which the system would sequentially, or simultaneously remove all RF ablation power and measured the temperature of all electrodes, and reallocate the distribution as described herein. Alternatively, this period may be varied to become more frequent if prior readings indicated relatively large temperature rises. 
         [0066]    In accordance of the invention, the temperature assessed at step  232  is sent to the computer controlling the system at step  238  allowing display of temperature at step  240 . Likewise, if desired, the physician may vary the threshold at step  242  with the system displaying the threshold value at step  244 . 
         [0067]    Referring to  FIG. 4 , yet another alternative embodiment of a method  310  in accordance with the present invention is illustrated. The method begins at step  312  upon the activation of the system. The surgeon then selects electrodes for measurement at step  314  and applies power to the selected electrodes at step  316 . 
         [0068]    At step  318  the temperatures of all electrodes are measured. At step  320  this information is sent to the computer controlling the system. At step  322  the system determines which electrodes are coolest and drives the coolest or the two coolest electrodes at step  324 , provided that they are below the maximum tolerable return electrode temperature. The system then returns to step  318  to check electrode temperatures. 
         [0069]    In accordance with the invention, the system displays electrode temperatures at step  326 . The system also provides for the adjustment of the maximum tolerable electrode temperature at step  328 . This temperature setting is displayed at step  330 . 
         [0070]    Periodically the system checks electrode temperature at step  332 . If all the electrodes have exceeded the maximum tolerable electrode temperature threshold, at step  334 , the system determines this and causes the actuation at step  336  of an alarm. After the alarm is actuated the system continues to assess temperature at step  332  and proceed through the temperature checking loop, as described above, until at least one electrode has dropped below the temperature threshold. The fact that the electrode is cool enough to receive energy is determined at step  334  which causes the system to signal that the system is again ready to deliver RF ablation energy. The signal is produced at step  338 , after which the system proceeds to step  322  where the coolest one or two electrodes are found and identified as suitable for conducting current, which occurs in step  324 . 
         [0071]      FIG. 5  illustrates, an embodiment of the inventive method  410 , which may be implemented on a personal or other general-purpose computer, as will be described below. This system both selects electrodes and varies the effective power with which they are driven. The inventive method and apparatus begin operation at step  412  which triggers the application of power to all electrode systems at step  414 . 
         [0072]    In accordance with the invention each time the surgeon activates the application of radio frequency energy, for example by pushing a button on the handle of the ablation apparatus, RF power is applied to the stylets, causing them to apply energy to the tissue surrounding the stylets, with the objective of heating that tissue and killing the cells in the tissue mass to be destroyed, thus allowing the body to remove the dead cells that comprised the growth. Actual generation of RF ablation energy to cause such cell necrosis is triggered by the surgeon at step  416 . 
         [0073]    The application of radio frequency energy to a growth to be ablated is achieved by the connection of the ablation stylet and a return electrode to the RF energy source. Typically, the return electrode is a planar conductive member which is adhered to the skin of a patient. An electrical path thus exists from the ablation stylet or stylets, through the body to the return path electrode which is typically placed at a position remote from that being treated, for example, on the thighs. In such an arrangement, the much greater area of the return electrode results in fairly diffuse RF energy flow, and, accordingly, minimal to nonexistent heating and cell damage adjacent the return electrode. 
         [0074]    In accordance with the preferred embodiment of the invention, the turning on and turning off of ablation energy is monitored by the system through the storage of time on and time off information at step  418 . In accordance with the invention, the system collects this data as well as skin temperature data to determine what distribution of RF energy between a plurality of return electrodes will minimize the likelihood of patient discomfort and/or injury, as will be discussed in detail below. 
         [0075]    In accordance with the present invention, the amount of power applied to each electrode may be varied, as may be the number of electrodes being excited with RF energy. Generally, enough energy must be passed through the return electrodes as is needed to ablate the particular tissue mass to be destroyed. Initially, the system may allocate the current to be passed through the return electrodes equally between all the return electrodes. Likewise, the system may automatically apply current to return electrodes. These initial operating parameters are subject to amendment as operating data is gathered and the operating parameters are adjusted to maximize the likelihood of patient comfort by avoiding overheating. 
         [0076]    Alternatively, choices may be based on operation type, surgeon identity (to accommodate for expected surgeon specific customary operating procedures), or other surgeon set preferences. 
         [0077]    It is useful to monitor the impedance of the ablation system, and in particular the quality of the connection of the return electrode to the skin. This may be done in a number of ways. For example, upon the application of RF energy to an ablation device at step  416 , the system may proceed to measure the current through one of the return electrodes at step  420 . At the same time, the system captures voltage information from the radio frequency source and uses this information to calculate the impedance of the path between the ablation stylet and the return electrode. Generally, low impedance in the path between the ablation stylet and the return electrode signifies that a good connection has been made at the return electrode. If a bad connection is indicated, and an alarm sounded, the surgeon or an assistant then has the opportunity to reset or replace the electrode. Poor connection may be a primary cause of overheating. The high ohmic resistance of a poor connection tends to increase the amount of power sent to and dissipated by the electrode/skin interface. 
         [0078]    The quality of an electrical connection at a return electrode is a function of numerous factors. For example, the skin may be dry, or moist. Likewise, oily skin or contaminant materials may tend to prevent adhesion Muscle movement and other mechanical factors may also affect the quality of the connection initially, and over time. Mechanical delamination of an otherwise good electrical connection may also result in a poor electrical connection. 
         [0079]    Even for good connections, path impedance can vary depending upon numerous factors. For example, some areas of the body have relatively large fat deposits under the skin. Other areas may have mostly venous or muscle tissue. Still other areas will have bony structures, cartilage or the like. In addition, all of these factors will vary from individual to individual. Moreover, dependent upon the electrode placement the path length will also change. All these factors will affect the ability of a return electrode to operate effectively and pass a desired magnitude of current. 
         [0080]    At step  422 , the resistance which results from all of these factors are measured and a determination made as toward whether the impedance is within an acceptable range. If it is determined that the impedance is outside the acceptable range, at step  424  an alarm is sounded. In addition, the particular electrode or electrodes with temperatures outside the range may be displayed on the display of the computer used to control the system. If desired, the physician may override the alarm and choose to accept the poor connection at step  426 . Alternatively, all electrode temperatures may be displayed. 
         [0081]    In either case, the system proceeds to step  428  where it is determined whether or not all electrodes have been tested. If they have not the system increments to the next electrode at step  430  and measures the current through that electrode at step  431 , after which the system returns to step  422  to repeat the sequence until all electrodes have been measured. 
         [0082]    At the point where it is determined at step  428  that all electrodes have been measured, the system operates to apply RF ablation energy to the ablation electrodes. After a period of time, for example something in the range of 10 seconds to a few minutes, for example three minutes, the system proceeds again to step  432 , where the temperature of the first return electrode is measured. As there has been a previous measurement the determination is made at step  434  as toward whether there has been a temperature change. If there has been no temperature change, the system proceeds to step  436  where it is determined whether the temperatures of all electrodes have been measured. If the temperature of all electrodes have not been measured, the system proceeds to step  438 , where it increments to the next electrode and returns to step  432  to repeat the temperature measurement. This cycle is performed until all electrodes have had their temperature measured, as is determined at step  436 . 
         [0083]    Once all electrodes have again had their temperature measured, an initial set of data is established, and the system proceeds, after a period of applying ablation current, at step  436  to step  432  to repeat the sequence of impedance and temperature measurement. 
         [0084]    Each time that a temperature is measured at step  432 , the system sends such measurement information to the computer at step  440 . Likewise, each time the system determines that there has been a rise in temperature at step  434 , the system proceeds to step  442 , where the inventive system adjusting algorithm is implemented to control optionally the amount of RF ablation energy sent to each electrode and optionally which electrodes are receiving RF ablation energy. 
         [0085]    As illustrated in  FIG. 6 , the inventive power adjustment algorithm which begins at node  444  is implemented at step  434  upon the determination that there has been a change in electrode temperature. Alternatively, application of the power adjustment algorithm may be reserved for those occasions where there has been an increase (or, alternatively, a problematic increase) in electrode temperature. 
         [0086]    In accordance with the invention, the times at which the system turns on various return electrodes are noted at step  446 . The times at which the system turns off are noted at step  448 . This enables the calculation of an overall on time during performance of a procedure which is specific to the particular operation and potentially, from a database standpoint, this is storable as characteristic of the operating style of a particular surgeon. This calculation is done at step  450 . 
         [0087]    It is also an object of the present invention to measure the effect on electrode temperature of the presence or absence of RF ablation energy. Such effect may be difficult to predict, as it relates to the quality of connection, the nature of underlying tissue, the cooling effect of the vasculature, the insulative nature of tissue between the return electrode, for example, cooling vasculature, and the quality of the thermal coupling to, the thermal conductivity of and heat capacity of the tissue coupled to the return electrode. 
         [0088]    In accordance with the present invention, the actual effect of the application of RF energy and its removal are used to generate information for the allocation of ablation energy between return electrodes. 
         [0089]    More particularly, by associating time periods during which electrodes are on at step  452 , with the duration of those time periods at step  454 , and with the magnitude of a temperature rise at step  456 , the system may calculate the rate of temperature rise during the application of RF ablation energy at step  458 . 
         [0090]    Using this information, the system, at step  460 , can calculate the time it will take for the temperature to rise to an unacceptable threshold level. This may be done for all electrodes. On this basis, the amount of excitation energy applied to each electrode may be apportioned in such a manner so as to make it more likely that the electrodes will all rise in temperature at the same rate. This criteria may be used as a sole criteria for controlling the amount of energy applied to the electrodes. 
         [0091]    Such amount of energy may be controlled, for example, by varying the duty cycle of energy applied to the electrodes. By duty cycle is meant the sequential turning on and off of RF ablation power. It is also possible to vary an absolute amount of steady energy applied to the electrodes by varying the amplitude of the RF signal applied to the return electrode. 
         [0092]    However, in accordance with the preferred embodiment amplitude is maintained constant, but during the period during which the surgeon is causing ablation energy to enter a tissue mass to be destroyed, the system is rapidly turning on and off, for example at the rate of 10 Hz, with on-time constituting between, for example, 10% and 90% of the cycle, with 10% corresponding to a 10% application of the full power of the RF ablation source and 90% on-time corresponding to application of RF energy equal to 90% of the total power of the RF ablation source. 
         [0093]    Alternatively or in addition, the assessment steps reflected by steps  462 - 470 , by associating time periods during which electrodes are instructed to be off on account of the release of the RF actuating switch by the physician on the ablation device, at step  462 , with the duration of those time periods at step  464 , and with the magnitude of a temperature drop during such periods at step  466 , the system may calculate the rate of temperature rise during the application of RF ablation energy at step  468 . 
         [0094]    Using this information, the system, at step  470 , can calculate the time it will take for the temperature to fall to an acceptable threshold level. This may be done for all overheated electrodes. On this basis, the amount of excitation energy applied to each electrode may be apportioned in such a manner so as to make it more likely that the electrodes will all cool at the same rate, thus tending to ensure that they will not become overheated beyond the acceptable threshold. 
         [0095]    As alluded to above, the allocations which the system begins to calculate at steps  462 - 470  may be based on an assumption that the electrodes are activated only 50% of the time by the surgeon. This initial assumption may be varied as information is gathered in steps  446 - 450  and combined with the rates calculated at steps  458  and  468  to calculate time to the threshold temperature. 
         [0096]    As described generally above, the assessment of positive changes in electrode temperature is repeated by determining whether all electrodes have been assessed at step  472 , incrementing to the next electrode at step  474  and repeating the process starting at step  452 . 
         [0097]    Also as described generally above the assessment of negative changes in electrode temperature is repeated by determining whether all electrodes have been assessed at step  476 , incrementing to the next electrode at step  478  and repeating the process starting at step  462   
         [0098]    Apportionment of duty cycles is performed with respect to time to heat at step  480 . In addition or in the alternative an apportionment may be performed at step  482  with respect to time to cool. Results may be combined and averaged at step  484 . 
         [0099]    Referring back to  FIG. 5 , after the power adjustment algorithm has been run at step  442 , the system proceeds to step  486  where the new operating parameters are applied. The system then determines whether, at step  436 , all electrodes have been measured and proceeds with the above described process until such measurements have been completed, after which the system proceeds to repeat the above process. 
         [0100]    In connection with the same, it is noted that each time a temperature is sent to the computer at step  440 , the system proceeds at step  488  to determine whether a particular electrode has exceeded the highest acceptable electrode temperature. If such is not the case, the system proceeds as discussed above. 
         [0101]    However, if the threshold temperature has been exceeded, as determined at step  488 , the system proceeds to disable the electrode at step  490 . At the same time an alarm is sounded and the display on the computer indicates which electrode has been disabled at step  492 . At step  494 , the surgeon may override the automatic shutoff of the offending electrode. This is recognized by the system at step  496 , after which the system proceeds to step  442  to restore the electrode to the system. If no override is received, at step  494  the system proceeds to adjust power allocations in accordance with the disabling of the subject electrode. During such a period ablation energy to other electrodes may be increased if their temperature permits this without danger or discomfort to the patient. 
         [0102]    In accordance with the present invention, the system also displays at step  497 , the temperature of the electrodes as measured at step  432 . In accordance with the invention, it is also contemplated that in response to such display, the surgeon may elect to change threshold temperature and this information is input into the system at step  498 . Likewise, the computer with which the system is used may display the temperature setting for the highest acceptable electrode temperature at step  499 . 
         [0103]    Referring to  FIG. 7 , an apparatus for carrying out the method of the present invention, such as the method of  FIGS. 5-6 , is illustrated. System  510  comprises a plurality of electrodes  512 - 518 . Upon the actuation of, for example, a push button  520  on the ablation apparatus, energy is coupled to a plurality of ablation switches  522 - 528 . Switches  522 - 528  act as gates to couple the application of power in response to controls from computer  530  which provides such controls through an interface board  538 . 
         [0104]    Power is coupled from the ablation switches to respective duty cycle modulators  540 - 546 , which output an intermittent RF signal whose duty cycle is controlled by computer  530  coupled to modulators  540 - 546  by interface board  538 . This allows computer  530  to control the power sent to electrodes  512 - 518 . 
         [0105]    Such power is controlled in response to electrode temperature. The temperature of electrodes  512 - 518  is measured by temperature detectors  548 - 554 , whose outputs are provided by interface board  538  to computer  530 . Such temperature may be displayed on computer screen  556 , along with other information, as described above in connection with the description of the embodiment of  FIGS. 5-6 . 
         [0106]    In addition, any alarms which need to be provided by the system, including audible alarms, may be provided by computer  530 . At the same time visual alarms may be provided on screen  558 , together with the other information described above in connection with the embodiment of  FIGS. 5-6 . 
         [0107]    Referring to  FIGS. 8-9 , an electrode assembly  610  useful in practicing the method of the present invention is illustrated. More particularly, it is contemplated that electrode pads may include more than one electrode and that the application of power to return electrodes may be varied as described above, with each electrode on a pad given its own separate temperature measurement transducer. Electrode assembly  610  is provided to the user with an adhesive protective member  612  whose surface  614  is coated with a release agent such as wax  615 . The underside  616  of electrode support member  618 , made for example of plastic film, foam or paper, is coated with adhesive material  619  which is protected by protective member  612 . 
         [0108]    A foil electrode  620  is secured to support member  618 . A temperature sensing transducer  622  is, in turn, disposed over and secured to electrode  620  by adhesive  619 . Foil and adhesive are of conventional thickness being about the thickness of common paper, but are exaggerated in size for purposes of illustration in the figures. In similar fashion, electrodes  624  and  626  are secured to and coupled to temperature sensing transducers  628  and  630 . Transducers  622 ,  628  and  630  are coupled respectively to lead wires  632 ,  634  and  636 , and are adhered in position by a layer of hydrogel  637 , a conductive material used in skin contacting electrodes, and which overlies the entire surface of the electrodes that contact the skin during use. 
         [0109]    Placement of four electrode assemblies  640 ,  642 ,  644  and  646  is illustrated in a typical configuration in  FIG. 10 . An ablation device  648  comprising a handle  650  and Trocar  652  is inserted through a hole  654  in the skin  656  of a patient with its point  658  in a tissue mass  660  which is to be ablated. Typically, stylets are caused to exit from Trocar point  658 . Radiofrequency ablation energy is then applied to the electrode assemblies and the stylets, causing a current path between the stylets adjacent point  658  and the electrode assemblies. 
         [0110]    In accordance with the present invention it is contemplated that temperature sensing transducers, such as transducers  622 ,  628  and  630  are positioned at the edge of the electrodes closest to the point of application of RF energy by the stylets. Accordingly, transducers are positioned adjacent edges  662 ,  664  and  666  of assembly  640 , as can be seen most clearly in  FIG. 8 . Transducers are shown as twisted pair members, but untwisted transducer comprising copper and constantan, side by side in an untwisted configuration may be used. The transducer may be on either side of the electrode. Accordingly, a transducer may alternatively be glued to the side of the foil electrode opposite the layer of hydrogel to be contacted with the skin. 
         [0111]    The illustrated placement at the leading edge of the current path is made because the edge of each of the electrodes closest to the point of application of ablation energy adjacent point  658  tends to be the edge which conducts most of the RF ablation energy and thus tends to be the edge that heats up. Thus, temperature measurement is made of the hottest part of the electrode and the possibility of discomfort or injury minimized. 
         [0112]    Referring to  FIG. 11 , an alternative embodiment of an electrode  710  in accordance with the present invention is illustrated. Electrode  710  is similar to electrode  610  illustrated in  FIG. 8 , except that the temperature transducers  722 ,  728  and  730  are small circular members which are preferably, placed at the center of the leading edge of their respective electrodes. 
         [0113]    Referring to  FIG. 12 , a multielectrode pad  810  including elongated electrodes  822 ,  824  and  826  is illustrated. The elongated electrodes are of particular value, insofar as they provide a very long leading edge. In accordance with the preferred embodiment of the invention, each electrode  822 ,  824  and  826  may be, for example, in the range of approximately 4 cm in width and 20 cm long. Otherwise, his structure is similar to that of  FIG. 8 . 
         [0114]    In accordance with the present invention, it is contemplated that small electrodes, such as electrodes  926 ,  928 , and  930  may be incorporated into an electrode  910 . 
         [0115]    The length of an electrode  1010  may be varied as required by the power output of the system. For example, a very wide electrode  1010 , such as that illustrated in  FIG. 14  may be used in applications where it is desirable to use a relatively high powered ablation device without too much off time. 
         [0116]    In accordance with the method and apparatus developed by the inventors herein, it is contemplated that the distribution of power between return electrodes may be varied in accordance with return electrode temperature, electrode impedance, and the combination of the return electrode temperature and impedance. Also in accordance with the present invention power may be intermittently applied in accordance with a fixed function independent of return electrode temperature and impedance. Alternatively, intermittent application of power may be added to any of the above strategies to provide a failsafe in the event of selective electrode delamination, temperature measurement failure and/or impedance measurement failure. 
         [0117]    Referring to  FIG. 15 , a system in which the distribution of power between return electrodes may be varied in accordance with the combination of the return electrode temperature and impedance, is illustrated. The operation of the system illustrated in  FIG. 15  is similar to that of the system illustrated in  FIG. 5 , except that in addition to monitoring impedance for the purpose of alarming the user with respect to, for example, delamination, this information is, additionally, used to generate control information for controlling the distribution of power between the return electrodes. Similar or analogous operations, for clarity of understanding, are given numbers which vary by a multiple of 100 with respect to those of the embodiment of  FIG. 5 , where practical. 
         [0118]    In accordance with the system illustrated in  FIG. 15 , after the start of the system at step  1112  followed by the application of impedance testing power, at operation  1114 , between, for example, separate electrode members of a single pair, the system proceeds to measure current through the first electrode at operation  1120 . 
         [0119]    The system then proceeds through a loop of impedance measurement at operation  1122 , a determination whether all electrodes have been measured at operation  1128 , incrementation at operation  1130  and repeated measurement of current at operation  1131  until all electrodes have been measured. This is determined at operation  1128 , and upon the determination that all return electrodes have had their impedance measured, the system proceeds to a second loop involving temperature measurement at operation  1132 , storage of temperature and time within a computer at operation  1140 , comparison to a threshold at operation  1188 , a determination whether all electrodes have been measured at operation  1136  and incrementation to the next electrode at operation  1138 . 
         [0120]    Temperature display, temperature setting, and a display of temperature setting is provided at operations  1197 ,  1198  and  1199 . 
         [0121]    In the event that it is determined at operation  1188  that a temperature is above the threshold, the system proceeds to disable the electrode at operation  1190  and sound an alarm at operation  1192 . In the manner of the embodiment of  FIG. 5 , an override may be detected at operation  1194 , resulting in a reversal of the command to disable electrode power. In either alternative, the system then proceeds to continue the return electrode temperature measurement cycle. 
         [0122]    When the temperature measurement cycle has been completed, as determined at operation  1136 , the system proceeds to operation  1142  where a power adjustment algorithm is used to determine the distribution of power between the various return electrodes. The system then operates with the determined power allocation for a period of time, after which the impedance measurement and electrode temperature measurements may be repeated, and power reallocated, if necessary. 
         [0123]    In accordance with the invention, a number of different methodologies may be employed to determine the allocation of power between return electrodes. Generally, it is contemplated, where possible (for example possible without causing a patient undue pain or injury) at least one electrode will always be receiving power, and thus all the ablation stylets are substantially continuously receiving ablation energy. Typically, it is contemplated that return electrode power will be alternated between various return electrodes, thus giving them the possibility of being driven with an ablation current which, for example, during alternating periods of time, is low enough, optionally zero, to give the body a chance to cool through such mechanisms as blood flow and conduction. 
         [0124]    In accordance with the invention it is contemplated that the algorithm for adjusting power in response to temperature measurements, for example temperature measurements over time, may be any of the methodologies and algorithms described above. 
         [0125]    In the case of impedance, power may be allocated inversely in accordance with impedance, for example, with a proportional distribution of power impedance, with the lowest impedance electrodes receiving the most return electrode power and the highest impedance electrodes receiving the least return electrode power. This power adjustment algorithm based on impedance may be used, advantageously, on systems which do not incorporate temperature sensitive transducers for monitoring temperature. Such a system would have the advantage of using inexpensive electrode pads without temperature transducers. 
         [0126]    Alternatively, the allocation of return electrode power may be responsive to both temperature and impedance. Optionally, the allocation of power may be responsive to both temperature and impedance and subjected to predetermined off periods for electrodes regardless of temperature or impedance as a failsafe measure. 
         [0127]    In accordance with the present invention, if it is desired to control the allocation of power between return electrodes in response to both electrode temperature and the impedance of the connection between the electrode and the skin, the above algorithms for determination of allocation in response to temperature may be separately calculated to obtain a first scaling factor for each electrode and the allocation in response to skin electrode impedance noted above also be separately calculated to obtain an impedance responsive scaling factor for each electrode. The individual temperature and impedance scaling factors for each electrode may then be multiplied by each other to determine the actual scaling factor to be employed. 
         [0128]    Optionally, the allocation of power between electrodes may be between pads, or, alternatively, between individual electrodes on a single pad or, as a yet further alternative, between individual electrodes regardless of pad location. 
         [0129]    In addition to this, cut offs may be employed with electrodes having an impedance above a certain value being shut down and resulting in an alarm.