Patent Publication Number: US-7582084-B2

Title: Systems and methods for controlling power in an electrosurgical probe

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 09/625,533, filed Jul. 26, 2000, now U.S. Pat. No. 6,488,679, which is a continuation of U.S. application Ser. No. 09/150,830, filed Sep. 10, 1998, now U.S. Pat. No. 6,123,702. 

   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention relates generally to structures for positioning one or more diagnostic or therapeutic elements within the body and, more particularly, to power control systems for use with the same. 
   2. Description of the Related Art 
   There are many instances where diagnostic and therapeutic elements must be inserted into the body. One instance involves the treatment of cardiac conditions such as atrial fibrillation and atrial flutter which lead to an unpleasant, irregular heart beat, called arrhythmia. 
   Normal sinus rhythm of the heart begins with the sinoatrial node (or “SA node”) generating an electrical impulse. The impulse usually propagates uniformly across the right and left atria and the atrial septum to the atrioventricular node (or “AV node”). This propagation causes the atria to contract in an organized way to transport blood from the atria to the ventricles, and to provide timed stimulation of the ventricles. The AV node regulates the propagation delay to the atrioventricular bundle (or “HIS” bundle). This coordination of the electrical activity of the heart causes atrial systole during ventricular diastole. This, in turn, improves the mechanical function of the heart. Atrial fibrillation occurs when anatomical obstacles in the heart disrupt the normally uniform propagation of electrical impulses in the atria. These anatomical obstacles (called “conduction blocks”) can cause the electrical impulse to degenerate into several circular wavelets that circulate about the obstacles. These wavelets, called “reentry circuits,” disrupt the normally uniform activation of the left and right atria. 
   Because of a loss of atrioventricular synchrony, the people who suffer from atrial fibrillation and flutter also suffer the consequences of impaired hemodynamics and loss of cardiac efficiency. They are also at greater risk of stroke and other thromboembolic complications because of loss of effective contraction and atrial stasis. 
   Although pharmacological treatment is available for atrial fibrillation and flutter, the treatment is far from perfect. Many believe that the only way to treat the detrimental results of atrial fibrillation and flutter is to actively interrupt all of the potential pathways for atrial reentry circuits. 
   One surgical method of treating atrial fibrillation by interrupting pathways for reentry circuits is the so-called “maze procedure” which relies on a prescribed pattern of incisions to anatomically create a convoluted path, or maze, for electrical propagation within the left and right atria. The incisions direct the electrical impulse from the SA node along a specified route through all regions of both atria, causing uniform contraction required for normal atrial transport function. The incisions finally direct the impulse to the AV node to activate the ventricles, restoring normal atrioventricular synchrony. The incisions are also carefully placed to interrupt the conduction routes of the most common reentry circuits. The maze procedure has been found very effective in curing atrial fibrillation. However, the maze procedure is technically difficult to do. It also requires open heart surgery and is very expensive. Thus, despite its considerable clinical success, only a few maze procedures are done each year. 
   More recently, maze-like procedures have been developed utilizing catheters and probes which can form lesions on the endocardium to effectively create a maze for electrical conduction in a predetermined path. Exemplary catheters are disclosed in commonly assigned U.S. Pat. No. 5,582,609. Exemplary surgical soft tissue coagulation probes employing a relatively shorter and stiffer shaft than a typical catheter are disclosed in commonly assigned U.S. patent application Ser. No. 08/949,117, filed Oct. 10, 1997, and U.S. patent application Ser. No. 09/072,835, filed May 5, 1998, both of which are incorporated by reference. Such probes may, for example, be used to treat atrial fibrillation in procedures wherein access to the heart is obtained by way of a thoracostomy, thoracotomy or median sternotomy. 
   Typically, the lesions are formed by ablating tissue with an electrode carried by the catheter or ablation probe. Electromagnetic radio frequency (“RF”) energy applied by the electrode heats, and eventually kills (or “ablates”) the tissue to form a lesion. During the ablation of soft tissue (i.e. tissue other than blood, bone and connective tissue), tissue coagulation occurs and it is the coagulation that kills the tissue. Thus, references to the ablation of soft tissue are necessarily references to soft tissue coagulation. “Tissue coagulation” is the process of denaturing proteins in tissue and heating the fluid within the tissue cell membranes which causes it to jell, thereby killing the tissue. 
   A primary goal of many soft tissue coagulation procedures is to create contiguous lesions (often long, curvilinear lesions) without over-heating tissue and causing coagulum and charring. Soft tissue coagulation occurs at 50° C., while over-heating occurs at 100° C. A problem in the related art is the issue of rapid turning on and off power when a coagulation electrode loses contact with tissue. Tissue in contact with a coagulation electrode acts as a load to the power circuit powering the electrode, usually an RF power circuit. When the coagulation electrode is pulled away from tissue or efficacious contact is lost, the load is removed, and the voltage output of the power circuit may change. Voltage may rise suddenly, which can cause problems when the electrode is reintroduced into contact with tissue, such as arcing or charring. As a safety consideration, the circuit in conventional systems is powered off for a predetermined period by turning off the power to the RF coagulation electrode when contact is lost. 
   However, the inventors herein have determined that powering a circuit completely off can result in a number of problems. For example, abrupt powering on of a coagulation electrode can char tissue if the voltage rise is too rapid. Additionally, powering a circuit completely off introduces the delay associated with powering the circuit back on into the procedure. Not only is this delay inconvenient, it can also be detrimental to the patient, especially since the loss of contact can happen many times during a procedure. For example, soft tissue coagulation probes can be used to perform a maze procedure during a mitral valve replacement, which requires cardiopulmonary bypass. The longer the patient is on bypass, the greater the likelihood of morbidity and mortality. Consequently, there is a need to quickly recover from a loss of electrode-issue contact, without completely shutting off the power supply. 
   Another problem identified by the present inventors has been verifying that tissue is in contact with a coagulation electrode prior to or during a surgical or catheter-based procedure, which is generally termed electrode contact verification. This is a problem pervasive throughout all surgery being performed remotely, especially when direct visual line-of-sight is not present. The use of fluoroscopic techniques is somewhat inaccurate, and requires the use of human feedback. Accordingly, a need exists for an automated control system for electrode contact verification, and optionally with visual and/or audio feedback when there is loss of contact between the electrode and tissue. 
   Yet another problem identified by the present inventors is associated with tissue treatment efficacy when coagulating tissue. Specifically, because different tissues in the human body and between patients absorb energy at different rates, it is difficult to ascertain when proper tissue coagulation has been completed. Heretofore, ad hoc techniques have been used to determine when the soft tissue coagulation process has been completed. One technique is visual inspection. Another is applying coagulation energy for a predetermined period based on an estimate of the amount of time required to produce a therapeutic lesion. Such techniques are not always as reliable as desired. Thus, there is a need for accurately determining when tissue has been properly coagulated, so that coagulation may be automatically stopped. 
   SUMMARY OF THE INVENTION 
   Accordingly, the object of the present invention is to provide an apparatus for positioning an operative element (such as a coagulation electrode) within the body that avoids the aforementioned problems. Other operative elements include lumens for chemical or cryogenic coagulation, laser arrays, ultrasonic transducers, microwave electrodes, and D.C. hot wires. 
   In accordance with one advantageous aspect of a present invention, a power control system is provided which, in response to an indication that mechanical or efficacious contact between an electrode and tissue has been lost, will merely reduce the power to the electrode, rather than completely shut it off. In one embodiment, this is accomplished by ramping down the amplitude of a RF waveform supplied to the electrode to a lower level. Adjustment of the RF signal amplitude need not involve shutting off the RF power supply. 
   One exemplary method of determining when contact is lost involves the use of impedance measurements. Here, the impedance is measured and compared to an expected impedance. When the impedance is greater than the expected impedance, the RF source is driven to a relatively low voltage level, such as 5V, that allows safe continued operation and impedance measurement. When the measured impedance is less than the expected impedance, the RF source ramps up to the levels needed to reach the set temperature. 
   There are a number of advantages associated with such a system. For example, merely reducing voltage, as opposed to shutting it off completely, increases the speed at which probe-based maze procedures proceed by reducing the down time resulting from a loss of mechanical or efficacious electrode-tissue contact. In addition, when tissue contact resumes, voltage is ramped back up. As such, tissue damage due to an abrupt voltage rise is avoided. 
   In accordance with another present invention, a control system is provided which verifies electrode-tissue contact. Preferably, data are collected relating to variables that can be used by a processor for electrode contact verification. The data, which are sampled by the controller before and/or during the soft tissue coagulation process, may be temperature or tissue impedance data. For example, a rise in temperature over a predetermined period of time usually means that the electrode is in contact with tissue and is heating tissue rather than blood. Such a temperature rise may be measured prior to coagulation by applying a small amount of energy (less than that required for coagulation) to the tissue to verify contact. Conversely, a drop-off in temperature during coagulation may mean contact has been lost. In the case of impedance, a flat profile of impedance over frequency indicates that there is no tissue-electrode contact. 
   When it is determined from the data that efficacious electrode-tissue contact has been lost, the processor instructs the console to sound a visual or audio alarm. Voltage may also be ramped down as described above. In addition to the advantages related to power control and audio/visual feedback, this invention also reduces reliance on fluoroscopic techniques. 
   In accordance with another present invention, a control system is provided which determines when the soft tissue coagulation process is completed. In one embodiment, tissue impedance measurements are used to determine efficacious lesion formation. A change in the impedance versus frequency curve, from a sloping curve to a flat curve, indicates that tissue coagulation is completed. Temperature can also be used. Specifically, for a given coagulation energy level and time period, a predetermined temperature profile over time indicates that a lesion has been formed. As a result, coagulation procedures, especially those involving the formation of multiple therapeutic lesions, may be performed more efficiently. 
   In accordance with still another present invention, a control system is provided which brings the temperature at the electrode to a temperature that is less than the maximum set temperature, maintains that temperature at the electrode at this temperature for a predetermined period, and then increases the temperature at the electrode to the set temperature. In a preferred embodiment, the temperature at which tissue is maintained prior to ramping up may only be sufficient to create a transmural lesion in a relatively thin anatomical structure, while the set temperature is sufficient to create a transmural lesion in a relatively thick structure. Such a control system provides a number of important benefits. For example, in those instances where the tissue structure turns out to be relatively thin, a transmural lesion may be completed (and power delivery stopped) before the temperature reaches the set temperature because lower temperatures will be automatically used prior to reaching the set temperature. In other words, the system automatically attempts to form a lesion at a lower temperature before ramping up to the higher temperature. As many lesions will be formed at the lower temperature, coagulation procedures performed using the present control system are less likely to cause tissue charring and coagulum formation than procedures performed with conventional control systems. 
   The variable temperature set point system described in the preceding paragraph is also useful in epicardial applications where electrodes are placed on the epicardial surface of a heart chamber. Here, blood flow within the heart chamber produces a convective cooling effect on the heart surface and makes the creation of transmural lesions from the epicardial surface more difficult. As a result, higher temperatures (measured at the electrodes) or increased energy delivery duration is required to create a transmural lesion. Ramping the temperature to a temperature that is less than the maximum set temperature and maintaining that temperature for a predetermined period causes desiccation of the epicardial tissue and improves electrode/tissue contact. Then, when the temperature is increased to the maximum set temperature, tissue vaporization is less likely because the tissue is already desiccated. Conversely, when the temperature of tissue that has not been desiccated is suddenly increased from body temperature to the maximum set temperature required to make transmural lesions on the epicardial surface, vaporization commonly occurs. This can lead to perforation of the myocardium or the dislodgment of tissue. 
   Tissue coagulation depth can also be controlled by the varying the length of RF delivery. Longer RF applications usually produce deeper tissue coagulation. 
   In accordance with still another present invention, an interface is provided which audibly or visually indicates the status of the various aspects of the system such as, for example, ablation power, temperature, tissue/electrode contact, tissue impedance, time elapsed, type of electrode, and type of probe. In a preferred embodiment, the console may be driven by software that is modular and upgradeable to allow for new parameters to be displayed and monitored. 
   The above described and many other features and attendant advantages of the present invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Detailed description of preferred embodiments of the invention will be made with reference to the accompanying drawings. 
       FIG. 1  is a schematic circuit block diagram of one embodiment of the present invention. 
       FIG. 2  is a perspective view of a tissue coagulation system in accordance with one embodiment of the present invention. 
       FIGS. 3 and 4  are schematic views of a system for controlling the application of ablating energy to multiple electrodes using multiple temperature sensors. 
       FIG. 5  is a schematic flow chart showing an implementation of the temperature feedback controller shown in  FIGS. 3 and 4 , using individual amplitude control with collective duty cycle control. 
       FIG. 6  is a schematic flow chart showing an implementation of the temperature feedback controller shown in  FIGS. 3 and 4 , using individual duty cycle control with collective amplitude control. 
       FIG. 7  is a schematic flow chart showing an implementation of the temperature feedback controller shown in  FIGS. 3 and 4 , using temperature control with hysteresis. 
       FIG. 8  is a schematic flow chart showing an implementation of the temperature feedback controller shown in  FIGS. 3 and 4 , using variable amplitude and differential temperature disabling. 
       FIG. 9  is a schematic flow chart showing an implementation of the temperature feedback controller shown in  FIGS. 3 and 4 , using differential temperature disabling. 
       FIG. 10  is a schematic view of a neural network predictor, which receives as input the temperatures sensed by multiple sensing elements at a given electrode region and outputs a predicted temperature of the hottest tissue region. 
       FIGS. 11(   a ),  11 ( b ) and  11 ( c ) are graphs illustrating impedance versus frequency. 
       FIG. 11(   d ) is a schematic diagram of an impedance measurement technique. 
       FIG. 12  is a flowchart illustrating the steps for software operation of certain components in accordance with one embodiment of the present invention. 
       FIG. 13  is graph showing a variable temperature setpoint in accordance with a preferred embodiment of a present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following is a detailed description of the best presently known mode of carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention. 
   The detailed description of the preferred embodiments is organized as follows: 
   I. Overview of the System 
   II. Electrosurgical Probe 
   III. Power Control 
   IV. Monitoring Tissue Contact, Coagulation Efficacy and Tissue Type 
   V. Variable Temperature Set Point 
   VI. User Interface System 
   The section titles and overall organization of the present detailed description are for the purpose of convenience only and are not intended to limit the present invention. 
   I. Overview of the System 
   A schematic block circuit diagram of one preferred embodiment of the present invention is shown in  FIG. 1 . The exemplary system  100  employs a processor  102  to command the overall system. In the preferred embodiment, the processor is a microprocessor. However, any suitable controller, microcomputer, hardwired or preconfigured dedicated processor, or ASIC may be used for command and control. The processor  102  can have a separate and isolated power supply for safety purposes. 
   In the illustrated embodiment, the processor  102  is connected to an RF power supply controller  104 , which regulates an AC power supply  106  that supplies RF power. RF power is preferably produced as a continuous sinusoidal waveform. However, other waveforms such as non-sinusoidal or pulsed can be used. RF power is received by a one-to-many power switching unit  112  for transmitting RF power to one or more electrode leads  121  that are attached to electrodes  115  on a probe  108 . The selection of which electrodes  115  are supplied with RF energy by the power switching unit  112  is under the control of processor  102 . 
   The processor  102  is also connected through suitable I/O lines to a console (or “user interface”)  103 , which may be part of a single unit that also includes the processor and power supply/control apparatus (see  FIG. 2 ) or, alternatively, a separate device. The console  103  preferably includes keys, LED readouts, indicator lights, and audio and visual alarms. One example of a suitable user interface is illustrated in  FIG. 2 . A plurality of ports located on the console  103 , and operably connected to the processor  102 , may be used to connect the system  100  with other devices that may be used in a medical procedure. 
   The probe  108  may be any instrument capable of applying electro-magnetic energy, especially in the RF frequency range, including catheters, but preferably is a surgical probe such as those disclosed in U.S. patent application Ser. No. 08/949,117, filed Oct. 10, 1997, and Ser. No. 09/072,835, filed May 5, 1998, both incorporated herein by reference. Ablation probe  108  has a distal end  111  and a proximal end  113  attached to a handle (shown in  FIG. 2 ). Initiation of RF power to the electrodes on the probe can be accomplished through the use of a footswitch, a button on the probe handle, or a remote control device (as shown in  FIG. 2  and discussed in Section II). 
   A number of sensor elements  120  reside on the probe  108  to measure data parameters relevant to the tissue being treated, such as tissue temperature. The sensors  120  are spaced apart from one another and are located on or near the electrodes  115 . The sensors may be in the form of transducers, thermocouples, thermistors, or other collection electrodes. Sensors  120  receive data in analog form, which is converted to digital form. The data signals collected from sensors  120  are interfaced, signal conditioned, calibrated, range checked, converted from analog to digital and multiplexed at a signal conditioner, A/D converter and multiplex circuit unit  118 , and received by processor  102  via I/O data signal line  130 . The leads  121 ,  122  shown outside the probe  108  in  FIG. 1  would in actual manufacture reside within the probe. For enhanced signal reconstruction, the processor  102  samples the sensors  120  to collect data at a frequency at least greater than the Nyquist sampling frequency. 
   With respect to the use of temperature sensors, details of a temperature controller and neural network for predicting temperature are described in Section III below. Once the coagulation procedure has started, the temperature sensors may be used to monitor the temperature setpoint, which is typically in a range from 50° to 100° C., and maintain the tissue temperature at the setpoint by increasing and decreasing power to the electrodes as needed. As described in detail below, data from the temperature sensors may also determine electrode-tissue contact and when a coagulation procedure is complete. 
   Turning to impedance, a flattening of the impedance versus frequency curve is either an indication of a lack of tissue contact, or an indication that the tissue is coagulated. Dedicated impedance electrodes may be provided on the probe. However, impedance is preferably monitored by simply measuring the voltage and current at different frequencies through the coagulating electrodes and dividing the measured voltage by the measured current. The level of current supplied to each electrode may also be used as a control parameter. These aspects of the exemplary embodiment are described in Sections IV and V below and in U.S. Pat. No. 5,577,509, which is incorporated herein by reference. 
   The RF source  106 , processor  102 , and sensors  120  form part of a negative feedback system for regulating RF power output to the electrodes. In the closed loop negative feedback system, the controller compares the sensor data (such as temperature, impedance or current data) with reference data stored in memory to generate an error signal. The processor interfaces with power supply controller  104  and RF source  106  (which may be combined into one circuit) to supply varying amounts of energy and power to the electrodes in order to decrease the error signal. Specifically, the RF source  106  is regulated to decrease the error signal. 
   Processes to determine whether the electrodes are in contact with tissue are preferably performed both before and during a coagulation procedure. These processes are discussed in Section IV below. Should the processor determine that there is a lack of sufficient contact, the level of power to the electrodes  115  will not be increased to the level required for ablation and, if desired, an audible and/or visual alarm will be activated by the processor  102  in order to inform the physician that the electrodes should be repositioned. 
   Upon detection of suitable contact between the electrodes  115  and tissue, the processor  102  instructs the RF power supply  106  and/or RF power supply controller  104  to supply power to the electrodes. In one embodiment the initial power setting for the apparatus is at 100 W power at about 500 kHz, with a maximum of about 150 W power. 
   When processor  102  senses that the electrodes  115  have lost contact with tissue or are no longer in close enough proximity for efficacious treatment, power to the electrodes is reduced (but not completely cut-off). Power may either be reduced to all of the electrodes, or only to those electrodes that have lost contact with tissue. The processor may use either temperature or impedance measurements to make this determination. Preferably, power is reduced by ramping down the amplitude of the RF energy source. Additionally, a visual or audible alarm may be provided to inform the physician that contact has been lost and power reduced. Once it is determined that contact or close proximity has been reestablished, the power level may be ramped back up. 
   II. Electrosurgical Probe 
   A preferred electrosurgical probe is shown by way of example in  FIG. 2 , and described in detail in the aforementioned U.S. patent application Ser. Nos. 08/949,117 and 09/072,835. The probe  208  includes a shaft  210  and a handle  212  towards the proximal section of the shaft. The shaft  210  consists of a hypo-tube, which is preferably either rigid or relatively stiff, and an outer polymer coating  216  over the hypo-tube. The handle  212  preferably consists of two molded handle halves and is also provided with strain relief element  222 . An operative element  220  is provided on the distal section  214  of the shaft  210 . In the illustrated embodiment, the operative element is in the form of a plurality of spaced electrodes  224 , which are preferably either rigid ring-shaped electrodes or flexible helical electrodes. 
   The probe  208  may be used in a conventional electrosurgical system configuration, where power transmission from an RF generator (or other energy source) to the electrodes  224  is controlled by a footswitch. Nevertheless, and shown by way of example in  FIG. 2 , a manually operable remote control  213  having individual on-off switches  215  is preferably provided in conjunction with the probe  208 . Similar switches may also be provided on the console  103 . A global on-off switch (such as a footswitch, a switch on the handle  212 , or a switch on the remote control  213 ) may also be provided to allow the physician using the apparatus to selectively enable and disable the supply of RF energy to electrodes  224 . The individual on-off switches  215  allow the physician to selectively control the supply of power to individual electrodes. The exemplary remote control  213 , which has seven individual on-off switches  215 , is preferably used in conjunction with a probe having seven electrodes. If, for example, the physician intends to ablate tissue with only three of the electrodes, then the three chosen electrodes may be enabled by way of the corresponding switches  215  prior to placing the global on-off switch in the ON position. 
   A plurality of indicator elements  217  are also provided on the exemplary remote control  213 , as shown in  FIG. 2 . Preferably, there is one indicator element for each of the on-off switches  215 . The indicator elements may also be in the form of indicator lights. Sound-based indications of the on-off status of the switches  215  may also be used. For example, a speaker on the handle or the power control apparatus may be employed to periodically indicate which of the switches  215  are in the ON position. 
   A plurality of temperature sensors (not shown in  FIG. 2 ) may be located on, under, abutting the edges of, or in between, the electrode elements in any of the exemplary devices disclosed herein. Additionally, a reference temperature sensing element may be provided on the handle  212 , the shaft  210 , or within the power supply and control apparatus. 
   III. Power Control 
   A. General 
     FIG. 3  shows, in schematic form, an exemplary system  300  for applying soft tissue coagulating (or “ablating”) energy by multiple emitters based, at least in part, upon local temperature conditions sensed by multiple sensing elements. 
   In  FIG. 3 , the multiple sensing elements comprise thermocouples  308 ,  309 , and  310  individually associated with the multiple emitters of energy, which comprise electrode regions  301 ,  302 , and  303  (such as those located on the distal portion of the probe shown in  FIG. 2 ). The system  300  also includes a common reference thermocouple  311  carried for exposure to the blood pool. The reference thermocouple can also be located on the probe handle or within the power control hardware. Alternatively, other kinds of temperature sensing elements can be used, like, for example, thermistors, fluoroptic sensors, and resistive temperature sensors, in which case the reference thermocouple  311  would typically not be required. 
   The system  300  further includes an indifferent electrode  319  for operation in uni-polar mode. 
   The electrode regions  301 ,  302 ,  303  can comprise the rigid electrode segments previously described. Alternatively, the electrode regions  301 ,  302 ,  303  can comprise a continuous or segmented flexible electrode of wrapped wire or ribbon. It should be appreciated that the system  300  can be used in association with any energy emitting element that employs multiple, independently actuated emitting elements. 
   The system  300  includes a source of energy  317 , such as the RF energy sources described above with reference to  FIGS. 1 and 2 . The source  317  is connected (through a conventional isolated output stage  316 ) to an array of power switches  314 , one for each electrode region  301 ,  302 , and  303 . A connector  312  (carried by the probe handle) electrically couples each electrode region  301 ,  303 ,  303  to its own power switch  314  and to other parts of the system  300 . 
   The system  300  also includes a microcontroller  331  coupled via an interface  330  to each power switch  314 . The microcontroller  331 , which preferably corresponds to the processor  102  shown in  FIG. 1 , turns a given power switch  314  on or off to deliver RF power from the source  317  individually to the electrode regions  301 ,  302 , and  303 . The delivered RF energy flows from the respective electrode region  301 ,  302 , and  303 , through tissue, to the indifferent electrode  319 , which is connected to the return path of the isolated output stage  316 . 
   The power switch  314  and interface  330  configuration can vary according to the type of energy being applied.  FIG. 4  shows a representative implementation for applying RF energy. 
   In this implementation, each power switch  314  includes an N-MOS power transistor  335  and a P-MOS power transistor  336  coupled in between the respective electrode region  301 ,  302 , and  303  and the isolated output stage  316  of the power source  317 . 
   A diode  333  conveys the positive phase of RF energy to the electrode region. A diode  334  conveys the negative phase of the RF energy to the electrode region. Resistors  337  and  338  bias the N-MOS and P-MOS power transistors  335  and  336  in conventional fashion. 
   The interface  330  for each power switch  314  includes two NPN transistors  339  and  340 . The emitter of the NPN transistor  339  is coupled to the gate of the N-MOS power transistor  335 . The collector of the NPN transistor  340  is coupled to the gate of the P-MOS power transistor  380 . 
   The interface for each power switch  314  also includes a control bus  343  coupled to the microcontroller  331 . The control bus  343  connects each power switch  314  to digital ground (DGND) of the microcontroller  331 . The control bus  343  also includes a (+) power line (+5V) connected to the collector of the NPN transistor  339  and a (−) power line (−5V) connected to the emitter of the NPN interface transistor  340 . 
   The control bus  343  for each power switch  314  further includes an E SEL  line. The base of the NPN transistor  339  is coupled to the E SEL  line of the control bus  343 . The base of the NPN transistor  340  is also coupled the E SEL  line of the control bus  343  via the Zener diode  341  and a resistor  332 . E SEL  line connects to the cathode of the Zener diode  341  through the resistor  332 . The Zener diode  341  is selected so that the NPN transistor  340  turns on when E SEL  exceeds about 3 volts (which, for the particular embodiment shown, is logic 1). 
   It should be appreciated that the interface  330  can be designed to handle other logic level standards. In the particular embodiment, it is designed to handle conventional TTL (transistor transfer logic) levels. 
   The microcontroller  331  sets E SEL  of the control bus  343  either at logic 1 or at logic 0. At logic 1, the gate of the N-MOS transistor  335  is connected to (+) 5 volt line through the NPN transistors  339 . Similarly, the gate of the P-MOS transistor  336  is connected to the (−) 5 volt line through the NPN transistor  340 . This conditions the power transistors  335  and  336  to conduct RF voltage from the source  317  to the associated electrode region. The power switch  314  is “on.” 
   When the microcontroller  331  sets E SEL  at logic 0, no current flows through the NPN transistors  339  and  340 . This conditions the power transistors  335  and  336  to block the conduction of RF voltage to the associated electrode region. The power switch  314  is “off.” 
   The system  300  (see  FIG. 3 ) further includes two analog multiplexers (MUX)  324  and  325 . The multiplexers  324  and  325  receive voltage input from each thermocouple  308 ,  309 ,  310 , and  311 . The microcontroller  331  controls both multiplexers  324  and  325  to select voltage inputs from the multiple temperature sensing thermocouples  308 ,  309 ,  310 , and  311 . 
   The voltage inputs from the thermocouples  308 ,  309 ,  310 , and  311  are sent to front end signal conditioning electronics. The inputs are amplified by differential amplifier  326 , which reads the voltage differences between the copper wires of the thermocouples  308 / 309 / 310  and the reference thermocouple  311 . The voltage differences are conditioned by element  327  and converted to digital codes by the analog-to-digital converter  328 . The look-up table  329  converts the digital codes to temperature codes. The temperature codes are read by the microcontroller  331 . 
   The microcontroller  331  compares the temperature codes for each thermocouple  308 ,  309 , and  310  to preselected criteria to generate feedback signals. The preselected criteria are inputted through a user interface  332 . In addition to temperature, and as discussed below, criteria such as power, impedance and current may also be used to generate feedback signals. These feedback signals control the interface power switches  314  via the interface  330 , turning the electrodes  301 ,  302 , and  303  off and on. 
   The other multiplexer  325  connects the thermocouples  308 ,  309 ,  310 , and  311  selected by the microcontroller  331  to a temperature controller  315 . The temperature controller  315  also includes front end signal conditioning electronics, as already described with reference to elements  326 ,  327 ,  328 , and  329 . These electronics convert the voltage differences between the copper wires of the thermocouples  308 / 309 / 310  and the reference thermocouple  311  to temperature codes. The temperature codes are read by the controller and compared to preselected criteria to generate feedback signals. These feedback signals control the amplitude of the voltage (or current) generated by the source  317  for delivery to the electrodes  301 ,  302 , and  303 . 
   Based upon the feedback signals of the microcontroller  331  and the temperature controller  315 , the system  300  distributes power to the multiple electrode regions  301 ,  302 , and  303  to establish and maintain a uniform distribution of temperatures along a lesion-forming element, such as the distal portion of the exemplary probe illustrated in  FIG. 2 . In this way, the system  300  obtains safe and efficacious lesion formation using multiple emitters of energy. 
   The system  300  can control the delivery of ablating energy in different ways. Representative modes will now be described. 
   B. Individual Amplitudes/Collective Duty Cycle 
   The electrode regions  301 ,  302 , and  303  will be symbolically designated E(J), where J represents a given electrode region (J=1 to N). 
   As before described, each electrode region E(J) has at least one temperature sensing element  308 ,  309 , and  310 , which will be designated S(J,K), where J represents the electrode region and K represents the number of temperature sensing elements on each electrode region (K=1 to M). 
   In this mode (see  FIG. 5 ), the microcontroller  331  operates the power switch interface  330  to deliver RF power from the source  317  in multiple pulses of duty cycle 1/N. 
   With pulsed power delivery, the amount of power (P E(J) ) conveyed to each individual electrode is as follows:
 
P E(J) ˜AMP E(J)   2 ×DUTYCYCLE E(J)  
 
   where: 
   AMP E(J)  is the amplitude of the RF voltage conveyed to the electrode region E(J), and 
   DUTYCYCLE E(J)  is the duty cycle of the pulse, expressed as follows:
 
DUTYCYCLE E(J)   =T ON E(J)   /[T ON E(J)   +T OFF E(J) ]
 
   where: 
   TON E(J)  is the time that the electrode region E(J) emits energy during each pulse period, 
   TOFF E(J)  is the time that the electrode region E(J) does not emit energy during each pulse period. 
   The expression TON E(J) +TOFF E(J)  represents the period of the pulse for each electrode region E(J). 
   In this mode, the microcontroller  331  collectively establishes duty cycle (DUTYCYCLE E(J) ) of 1/N for each electrode region (N being equal to the number of electrode regions). 
   The microcontroller  331  may sequence successive power pulses to adjacent electrode regions so that the end of the duty cycle for the preceding pulse overlaps slightly with the beginning of the duty cycle for the next pulse. This overlap in pulse duty cycles assures that the source  317  applies power continuously, with no periods of interruption caused by open circuits during pulse switching between successive electrode regions. 
   In this mode, the temperature controller  315  makes individual adjustments to the amplitude of the RF voltage for each electrode region (AMP E(J) ), thereby individually changing the power P E(J)  of energy conveyed during the duty cycle to each electrode region, as controlled by the microcontroller  331 . 
   In this mode, the microcontroller  331  cycles in successive data acquisition sample periods. During each sample period, the microcontroller  331  selects individual sensors S(J,K), and voltage differences are read by the controller  315  (through MUX  325 ) and converted to temperature codes TEMP(J). 
   When there is more than one sensing element associated with a given electrode region, the controller  315  registers all sensed temperatures for the given electrode region and selects among these the highest sensed temperature, which constitutes TEMP(J). 
   In this mode, the controller  315  compares the temperature TEMP(J) locally sensed at each electrode E(J) during each data acquisition period to a set point temperature TEMP SET  established by the physician. Based upon this comparison, the controller  315  varies the amplitude AMP E(J)  of the RF voltage delivered to the electrode region E(J), while the microcontroller  331  maintains the DUTYCYCLE E(J)  for that electrode region and all other electrode regions, to establish and maintain TEMP(J) at the set point temperature TEMP SET . 
   The set point temperature TEMP SET  can vary according to the judgment of the physician and empirical data. A representative set point temperature for cardiac ablation is believed to lie in the range of 40° C. to 95° C., with 70° C. being a representative preferred value. 
   The manner in which the controller  315  governs AMP E(J)  can incorporate proportional control methods, proportional integral derivative (PID) control methods, or fuzzy logic control methods. 
   For example, using proportional control methods, if the temperature sensed by the first sensing element TEMP( 1 )&gt;TEMP SET , the control signal generated by the controller  315  individually reduces the amplitude AMP E(1)  of the RF voltage applied to the first electrode region E( 1 ), while the microcontroller  331  keeps the collective duty cycle DUTYCYCLE E(1)  for the first electrode region E( 1 ) the same. If the temperature sensed by the second sensing element TEMP( 2 )&lt;TEMP SET , the control signal of the controller  315  increases the amplitude AMP E(2)  of the pulse applied to the second electrode region E( 2 ), while the microcontroller  331  keeps the collective duty cycle DUTYCYCLE E(2)  for the second electrode region E( 2 ) the same as DUTYCYCLE E(1) , and so on. If the temperature sensed by a given sensing element is at the set point temperature TEMP SET , no change in RF voltage amplitude is made for the associated electrode region. 
   The controller  315  continuously processes voltage difference inputs during successive data acquisition periods to individually adjust AMP E(J)  at each electrode region E(J), while the microcontroller  331  keeps the collective duty cycle the same for all electrode regions E(J). In this way, the mode maintains a desired uniformity of temperature along the length of the distal portion of the exemplary probe shown in  FIG. 2  or other lesion-forming element. 
   Using a proportional integral differential (PID) control technique, the controller  315  takes into account not only instantaneous changes that occur in a given sample period, but also changes that have occurred in previous sample periods and the rate at which these changes are varying over time. Thus, using a PID control technique, the controller  315  will respond differently to a given proportionally large instantaneous difference between TEMP (J) and TEMP SET , depending upon whether the difference is getting larger or smaller, compared to previous instantaneous differences, and whether the rate at which the difference is changing since previous sample periods is increasing or decreasing. 
   C. Collective Amplitude/Individual Duty Cycles 
   In this feedback mode (see  FIG. 6 ), the controller  315  governs the source  317  to collectively control the RF voltage amplitude AMP E(J)  for all electrode regions based upon the lowest local temperature sensed TEMP SMIN . At the same time, in this feedback mode, the microcontroller  331  individually alters the power conveyed to the electrode regions where temperatures greater than TEMP SMIN  are sensed, by adjusting the duty cycle DUTYCYCLE E(J)  of these electrode regions. 
   In this mode, as in the previous mode, the microcontroller  331  separates the power into multiple pulses. Initially, each pulse has the same duty cycle (DUTYCYCLE E(J) ) of 1/N. As in the previous mode, the application of successive RF pulses to adjacent electrode regions may be timed to overlap so that the source  317  applies power continuously to the electrode regions E(J). 
   The controller  315  cycles in successive data acquisition periods to sequentially read the temperature sensed by each sensing element TEMP(J). When there are multiple sensing elements associated with each electrode region, the controller  315  registers all sensed temperatures for the particular electrode and selects among these the highest sensed temperature, which is TEMP(J). 
   In this mode, the controller  315  compares, during each data acquisition period, the individual temperatures sensed TEMP(J) to the set point temperature TEMP SET . The controller  315  also selects the lowest sensed temperature TEMP SMIN . The controller  315  adjusts AMP E(J)  to maintain TEMP SMIN &gt;&gt;TEMP SET , using proportional, PID, or fuzzy logic control techniques. At the same time, the microcontroller  331  adjusts DUTYCYCLE E(J)  of the electrode regions where TEMP(J)&gt;TEMP SMIN  to maintain TEMP(J)&gt;&gt;TEMP SET . 
   For example, using only proportional control techniques, if TEMP SMIN &lt;TEMP SET , the controller  315  collectively increases the amplitude of the RF voltage of all electrode regions, based upon the difference between TEMP SMIN  and TEMP SET (ΔTEMP SMIN/SET ), until TEMP SMIN &gt;TEMP SET . 
   During this time (when TEMP SMIN  remains below TEMP SET ), the microcontroller  331  also controls the application of power to the other electrode regions E(J) where the local sensed temperature TEMP(J) is above TEMP SMIN , as follows:
         (i) if TEMP(J)&lt;TEMP SET , the microcontroller  331  increases the duty cycle of the power applied to the electrode region E(J) at the RF voltage amplitude established by ΔTEMP SMIN/SET ,   (ii) if TEMP(J)&gt;TEMP SET , the microcontroller  331  decreases the duty cycle of the power applied to the electrode region E(J) at the RF voltage amplitude established by ΔTEMP SMIN/SET ,   (iii) if TEMP(J) S(N) =TEMP SET , the microcontroller  331  maintains the duty cycle for the given electrode region E(N) at the RF voltage amplitude established by ΔTEMP SMIN/SET .       

   When TEMP SMIN &gt;TEMP SET , the controller  315  collectively reduces the RF voltage amplitude delivered to all electrode regions. When TEMP SMIN =TEMP SET , the controller  315  collectively maintains the then-established RF voltage amplitude delivered to all electrode regions. 
   D. Temperature Control with Hysteresis 
   In this mode (see  FIG. 7 ), as in the previous modes, the system  300  cycles in successive data acquisition periods to sequentially register the temperature sensed by the sensing elements TEMP(J) for the electrode regions E(J). As before, when there are multiple sensing elements associated with each electrode region, the system  300  registers all sensed temperatures for the particular electrode region and selects among these the highest sensed temperature, which becomes TEMP(J). 
   In this mode, the microcontroller  331  compares the temperature sensed locally at each electrode region TEMP(J) during each data acquisition period to high and low threshold temperatures TEMP HITHRESH  and TEMP LOWTHRESH , where
 
TEMP HITHRESH =TEMP SET +INCR
 
TEMP LOWTHRESH =TEMP SET −INCR
 
   where 
   TEMP SET  is the set point temperature, and 
   INCR is a preselected increment. 
   When operated in this mode, the microcontroller  331  operates the power switch interface  330  to turn a given electrode region E(J) off when the local temperature sensed at that electrode region TEMP(J)&gt;TEMP HITHRESH . The microcontroller  331  keeps the electrode region turned off until the locally sensed temperature TEMP(J) drops below TEMP LOWTHRESH . The microcontroller  331  turns a given electrode region E(J) on and supplies power at a selected voltage amplitude when the local temperature sensed at that electrode region TEMP(J)&lt;TEMP LOWTHRESH . 
   The values for TEMP SET  and INCR can vary according to the judgment of the physician and empirical data. As before stated, a representative value for TEMP SET  is believed to lie in the range of 40° C. and 95° C., with a preferred value of 70° C. A representative value of INCR is believed to lie in the range of 2° C. to 8° C., with a preferred representative value of around 5° C. 
   In this implementation, the controller  315  establishes a constant RF voltage amplitude sufficiently high to maintain the desired temperature conditions during hysteresis. Alternatively, the controller  315  can have the capability to adjust voltage should the coolest sensed temperature TEMP SMIN  decrease below a selected lower limit below TEMP LOWTHRESH , or should the longest duty cycle exceed a predetermined value. It should be appreciated that there are other ways of adjusting and maintaining the amplitude while the hysteresis control method is carried out. 
   E. Differential Temperature Disabling 
   In this mode (see  FIG. 8 ), the temperature controller  315  selects at the end of each data acquisition phase the sensed temperature that is the greatest for that phase (TEMP SMAX ). The temperature controller  315  also selects for that phase the sensed temperature that is the lowest (TEMP SMIN ). 
   The controller  315  compares the selected hottest sensed temperature TEMP SMAX  to a selected high set point temperature TEMP HISET . The comparison generates a control signal that collectively adjusts the amplitude of the RF voltage for all electrodes using proportional, PID, or fuzzy logic control techniques. 
   In a proportion control implementation scheme:
         (i) If TEMP SMAX &gt;TEMP HISET , the control signal collectively decreases the amplitude of the RF voltage delivered to all electrode regions;   (ii) If TEMP SMAX &lt;TEMP HISET , the control signal collectively increases the amplitude of the RF voltage delivered to all electrode regions;   (iii) If TEMP SMAX =TEMP HISET , no change in the amplitude of the RF voltage delivered to all electrode regions is made.       

   It should be appreciated that the temperature controller  315  can select for amplitude control purposes any one of the sensed temperatures TEMP SMAX , TEMP SMIN , or temperatures in between, and compare this temperature condition to a preselected temperature condition. 
   Working in tandem with the amplitude control function of the temperature controller  315 , the microcontroller  331  governs the delivery of power to the electrode regions based upon difference between a given local temperature TEMP (J) and TEMP SMIN . This implementation computes the difference between local sensed temperature TEMP(J) and TEMP SMIN  and compares this difference to a selected set point temperature difference D TEMP SET . The comparison generates a control signal that governs the delivery of power to the electrode regions. 
   If the local sensed temperature TEMP(J) for a given electrode region E(J) exceeds the lowest sensed temperature TEMP SMIN  by as much as or more than ΔTEMP SET  (that is, if TEMP(J)−TEMP SMIN &gt;ΔTEMP SET ), the microcontroller  331  turns the given electrode region E(J) off. The microcontroller  331  turns the given electrode E(J) back on when TEMP(J)−TEMP SMIN &lt;ΔTEMP SET . 
   Alternatively (see  FIG. 9 ), instead of comparing TEMP(J) and TEMP SMIN , the microcontroller  331  can compare TEMP SMAX  and TEMP SMIN . When the difference between TEMP SMAX  and TEMP SMIN  equals or exceeds a predetermined amount ΔTEMP SET , the controller  331  turns all electrode regions off, except the electrode region where TEMP SMIN  exists. The controller  331  turns these electrode regions back on when the temperature difference between TEMP SMAX  and TEMP SMIN  is less than ΔTEMP SET . 
   Some of the above-described temperature-based control schemes alter power by adjusting the amplitude of the RF voltage. It should be appreciated that, alternatively, power can be altered by the adjusting the amplitude of RF current. Therefore, the quantity AMP E(J)  used in this Specification can mean either RF voltage amplitude or RF current amplitude. 
   F. Deriving Predicted Hottest Temperature 
   As previously described, a given electrode region can have more than one temperature sensing element associated with it. In the previously described control modes, the controller  315  registers all sensed temperatures for the given electrode region and selects among these the highest sensed temperature, which constitutes TEMP(J). There are alternative ways of making this selection. One way is to derive the predicted hottest temperature. 
   Because of the heat exchange between the tissue and the electrode region, the temperature sensing elements may not measure exactly the maximum temperature at the region. This is because the region of hottest temperature occurs beneath the surface of the tissue at a depth of about 0.5 to 2.0 mm from where the energy emitting electrode region (and the associated sensing element) contacts the tissue. If the power is applied to heat the tissue too quickly, the actual maximum tissue temperature in this subsurface region may exceed 100° C. and lead to tissue desiccation and/or micro-explosion. 
     FIG. 10  shows an implementation of a neural network predictor  400 , which receives as input the temperatures sensed by multiple sensing elements S(J,K) at each electrode region, where J represents a given electrode region (J=1 to N) and K represents the number of temperature sensing elements on each electrode region (K=1 to M). The predictor  400  outputs a predicted temperature of the hottest tissue region T MAXPRED (t). The controller  315  and microcontroller  331  derive the amplitude and duty cycle control signals based upon T MAXPRED (t), in the same manners already described using TEMP(J). 
   The predictor  400  uses a two-layer neural network, although more hidden layers could be used. As shown in  FIG. 10 , the predictor  400  includes a first and second hidden layers and four neurons, designated N (L,X) , where L identifies the layer 1 or 2 and X identifies a neuron on that layer. The first layer (L=1) has three neurons (X=1 to 3), as follows N (1,1) ; N (1,2) ; and N (1,3) . The second layer (L=2) comprising one output neuron (X=1), designated N (2,1) . 
   Temperature readings from the multiple sensing elements, only two of which—TS 1 ( n ) and TS 2 ( n )—are shown for purposes of illustration, are weighed and inputted to each neuron N (1,1) ; N (1,2) ; and N (1,3)  of the first layer.  FIG. 10  represents the weights as W L   (k,N) , where L=1; k is the input sensor order; and N is the input neuron number 1, 2, or 3 of the first layer. 
   The output neuron N (2,1)  of the second layer receives as inputs the weighted outputs of the neurons N (1,1) ; N (1,2) ; and N (1,3) .  FIG. 10  represents the output weights as W L   (O,X) , where L=2; O is the output neuron 1, 2, or 3 of the first layer; and X is the input neuron number of the second layer. Based upon these weighted inputs, the output neuron N (2,1)  predicts T MAXPRED (t). Alternatively, a sequence of past reading samples from each sensor could be used as input. By doing this, a history term would contribute to the prediction of the hottest tissue temperature. 
   The predictor  400  must be trained on a known set of data containing the temperature of the sensing elements TS 1  and TS 2  and the temperature of the hottest region, which have been previously acquired experimentally. For example, using a back-propagation model, the predictor  400  can be trained to predict the known hottest temperature of the data set with the least mean square error. Once the training phase is completed the predictor  400  can be used to predict T MAXPRED (t). 
   The predicted tissue temperature can also be used to adjust the temperature set curve. For example, if the predictor  400  predicts a relatively high tissue temperature, then the temperature set curve can be adjusted downwardly and vice versa. The duration of an ablation procedure can also be adjusted based on predicted tissue temperature. 
   Other types of data processing techniques can be used to derive T MAXPRED (t). See, e.g., co-pending patent application Ser. No. 08/801,484, filed Feb. 18, 1997, and entitled “Tissue Heating and Ablation Systems and Methods Using Predicted Temperature for Monitoring and Control.” 
   In addition to being used in the previously described temperature control systems, the predicted temperature of the hottest tissue region T MAXPRED (t) can be used in other power control systems or to adjust other power control parameters. 
   For example, T MAXPRED (t) may be used to adjust a temperature set point curve upwardly or downwardly. Either the entire curve, or just a portion thereof, may be adjusted. One example of such a temperature set point curve is the variable temperature set point curve discussed in Section V below. Another example is disclosed in U.S. Pat. No. 5,755,715, entitled “Tissue Heating and Ablation Systems and Methods Using Time-Variable Set Point Curves for Monitoring and Control.” Adjusting a temperature set point curve upwardly will occur when T MAXPRED (t) is lower than the assumed maximum temperature that formed the basis for the set point curve. Such an adjustment will decrease the duration of the coagulation procedure, as compared to duration without the adjustment. The increase in speed is especially useful when the patient is on cardiopulmonary bypass. Conversely, when T MAXPRED (t) is higher than the assumed maximum temperature, the temperature set point curve will be adjusted downwardly, thereby reducing the likelihood of undesired damage to ancillary tissue as well as charring or popping of the targeted tissue. 
   T MAXPRED (t) may also be used to increase or decrease the duration of a coagulation procedure. Adjusting a duration upwardly will occur when T MAXPRED (t) is lower than the assumed maximum temperature that formed the basis for the original duration estimation. This will insure that a therapeutic lesion will be formed. The duration of a coagulation procedure will be reduced when T MAXPRED (t) is higher than the assumed maximum temperature, thereby reducing the length of time that a patient is on bypass as well as the likelihood of undesired tissue damage. 
   IV. Monitoring Tissue Contact, Coagulation Efficacy and Tissue Type 
   In addition to maintaining a set temperature, electrode-tissue contact may also be monitored using the same process of sensing a variable (such as temperature, impedance or current), comparing it with a reference signal, and generating an error signal that may be used to control power output and audio/visual indicators on the console. The sensed data may also be used to make other determinations, such tissue viability and tissue type. Preferably, all three of the determinations (i.e. contact, viability and type) will be made at at least one point during the coagulation procedure. 
   Turning first to impedance, the graph illustrated in  FIG. 11(   a ) is representative of the impedance magnitude versus frequency curve for viable tissue, while the graph illustrated in  FIG. 11(   b ) is representative of the curve for coagulated tissue or an instance where there is insufficient contact between the electrodes and tissue and the electrode is in blood, and the graph illustrated in  FIG. 11(   c ) is representative of the curve when the electrodes are in air. 
   For viable tissue, there is a generally downward sloping curve as frequency increases, having a relative maximum point MAX 1  at frequency f 1  (about 1 kHz) and a relative minimum point MIN 2  at frequency f 2  (about 100 kHz). Under normal circumstances, the curve flattens out, as shown in  FIG. 11(   b ), when the tissue is coagulated. However, the relatively flat curve is also indicative of a situation where the electrode tissue contact has been lost and the sensing electrodes are surrounded by blood. The processor could be used to take derivatives of the impedance curve to ascertain whether a change in the curve has occurred. Alternatively, the processor may be used to compare the difference between the values of MAX1 and MIN2 (or the MAX1/MIN2 ratio) with sensed values at the same frequencies until the difference (or ratio) is smaller than a predetermined threshold value, which indicates coagulation has been achieved. In addition, or alternatively, a table of impedance versus frequency curve points may be stored in memory and used for comparison purposes. 
   As noted above, one method of measuring impedance involves measuring the voltage and current and dividing the measured voltage by the measured current. When using a probe having four spaced electrodes E 1 -E 4 , the technique illustrated in  FIG. 11(   d ) may be used. 
   The particular f1 and f2 values and MAX1 and MIN2 values can vary according to tissue characteristics, as can the f1/f2 ratio and the MAX1/MIN2 ratio. For example, cardiac tissue and liver tissue may have different f1, f2, MAX1 or MIN2 values. This phenomenon is discussed in more detail in Rabbat A: Tissue resistivity. In Webster J G, ed: Electrical Impedance Tomography. Adam Hilger, Bristol, 1990. As such, impedance measurements may be used to determine tissue type. The tissue type determination may in turn be used both before and during a coagulation procedure to insure that the intended type of tissue will be and is being coagulated. 
   Additionally, if the impedance level exceeds 300 ohms, which is indicative of a situation where the electrodes are in air, then the energy level can be reduced to 5 V for a period of up to 10 seconds. The reduction can be either an immediate reduction or a ramping reduction. If the impedance level continues to exceed 300 ohms at the end of the period, then energy delivery will be stopped. 
   With respect to current, it can be used to both measure tissue contact and to prevent tissue charring and the formation of coagulum. For a given input maximum power level, there is a current level which, if reached, is either indicative of poor contact or of tissue charring and coagulum formation. For example, using 12.5 mm electrodes and a set power level of 70 W, 0.9 A is a preferable current limit, while 0.7 A is a preferable limit when 6 mm electrodes are used with the same power setting. 
   Turning to temperature, prior to the coagulation procedure, a relatively small amount of energy (preferably about 20 V for 3 seconds) may be delivered to the tissue prior to the application of the larger amount of power necessary to coagulate tissue. The relatively small amount of power will increase the temperature of the tissue by a predetermined amount when the electrodes are in efficacious contact with the tissue. By comparing the sensed temperature rise to the expected temperature rise associated with efficacious contact (greater than or equal to about 1° C. per second), the processor can determine whether there is sufficient contact for the procedure to continue. 
   A temperature-based process may also be used to ascertain when tissue has been properly coagulated. Here, the controller monitors the sensed tissue temperature profile history (i.e. temperature over time) for a given power level. The temperature profile history is compared to a predetermined history for that power level stored in memory to determined when the coagulation process has ended. The controller will then cause the power circuit to end the delivery RF energy to the ablation electrode and, if desired, cause the console to provide an audio and/or visual indication that coagulation is complete. 
   As described above, certain aspects of the disclosed embodiments illustrated in  FIGS. 1-10  combine negative feedback techniques with an automatic process controller. The controller (element  102  in  FIG. 1  and element  331  in  FIG. 3 ) compares sensed controlled variable data (such as temperature, impedance or current) to reference signal data, that is either input by the user or stored in memory, to generate an error signal. The error signal is used as input to the process controller that generates correcting influence on the feedback network circuit. The correcting influence can be used to affect the RF pulse train illustrated in  FIG. 1 . In the case of temperature feedback, the amount of energy delivered to the probe can be varied in order to maintain a temperature set point. In the cases of temperature, impedance and current, power can be ramped down to a predetermined level during a coagulation procedure when there is an indication that tissue contact is insufficient. 
   As discussed previously, the voltage output to the electrodes may be decreased from a higher level to a lower level upon a loss of tissue-electrode contact, as determined by the controller based on an error signal associated with measured temperature or impedance. The rate of RF voltage increase may also be limited upon resumption of coagulation to avoid tissue damage caused by overheating due to transient overshoot of high voltages. For example, voltage may be reduced to 5-20V if the measured temperature is more than 10° C. higher than the set temperature to avoid overheating or when there has been a loss of tissue-electrode contact. Once the overheating has been eliminated or contact has been reestablished, the power may be ramped back up to the level necessary to achieve the set temperature over a period of about 5 seconds. 
   V. Variable Temperature Set Point 
   In accordance with one of the present inventions, a variable temperature set point may be employed in a coagulation procedure. In other words, instead of immediately ramping up to the input desired maximum tissue temperature and maintaining that temperature, the controller first ramps tissue temperature up to at least one other temperature which is lower than the set maximum temperature. The controller then maintains that temperature for a predetermined period prior to increasing tissue temperature to the desired maximum temperature. This may be accomplished by, for example, using a PID temperature control algorithm with a temperature setpoint that varies over time, also referred to as a variable temperature set point curve. 
   The temperature at which tissue is maintained prior to ramping up to the input maximum temperature should be at least sufficient to desiccate tissue and is preferably sufficient to create a transmural lesion in a relatively thin tissue structure. The maximum temperature is preferably a temperature that will create a transmural lesion in a relatively thick structure. Thus, in those instances where the tissue structure turns out to be relatively thin, a transmural lesion may be completed (and power delivery stopped) before the temperature reaches the maximum temperature. The determination of lesion completion may be made through visual inspection or through the use of the techniques described above. Should the tissue structure turn out to be relatively thick, temperature will continue to ramp up to the maximum temperature. 
   As shown by way of example in  FIG. 13 , one embodiment of the invention includes a temperature control algorithm wherein temperature is maintained at two different temperatures prior to reaching the set temperature T SET . In the illustrated embodiment, the set point curve is based on a coagulation procedure where T SET  is 70° C. and the total energy application time is 60 seconds. Both of these variables may be input by the physician. The controller initially sets the temperature setpoint at a first temperature T 1  for about 10 to 15 seconds. Temperature T 1  is about 10° C. less than T SET . The controller then increases the temperature setpoint to a second temperature T 2  for about 10 seconds. Temperature T 2  is about 5° C. less than T SET . Finally, the temperature setpoint is increased to the input temperature setpoint T SET  for the remainder of the coagulation procedure. 
   The exemplary temperature set point curve described in the preceding paragraph is especially useful when creating a transmural lesion on the epicardial surface. Of course, other curves can be used in other situations. In endocardial applications where a transmural lesion is desired, for example, a temperature setpoint T SET  of 70° C. may be necessary. The temperature set point curve used here preferably starts at a first temperature T 1  (about 60° C.) for about 10 to 15 seconds. Thereafter, the temperature setpoint increases at a rate of between about 1 and 2° C. per second until it reaches the input setpoint of 70° C. Other useful applications of the variable temperature setpoint include coagulation procedures where expandable (or “balloon”) electrodes and other relatively large electrodes are used. Here, a setpoint curve wherein a first temperature setpoint T 1  (about 60° C.) was maintained for about 10 seconds and then increased at a rate of between about 1 and 2° C. per second until reaching an input setpoint of about 85° C. was found to be useful. 
   A variety of variable temperature setpoint curves that are specifically designed for a variety of procedures may be stored in the controller memory. Alternatively, or in addition, the controller may be provided with a program that generates a variable temperature setpoint curve based on input parameters such as maximum temperature, total energy application time, and type of procedure. 
   VI. User Interface System 
   One advantageous feature of the present user interface is that it will provide an indication that a therapeutic lesion has been successfully formed on the intended type of tissue. For example, some lesions will be formed based on information input by the physician via the user interface. The information may include lesion type, tissue type and type of probe (including electrode configuration). Probe type may also be automatically input by providing a device on the probe which is indicative of probe type in, for example, the manner disclosed in U.S. Pat. No. 5,743,903, which is incorporated herein by reference. The system controller will use these parameters to, for example, select a time variable set curve, the appropriate power level and a total energy application time. Of course, time, temperature, power level and other parameters can also input manually. 
   At the end of the input or automatically selected time period, the user interface will either instruct the operator to discontinue power application or simply indicate that power has been discontinued. The same operations may be performed in those instances where the controller determines that a therapeutic lesion has been formed based on, for example, tissue impedance measurements. Should the physician reach the point where there is an instruction to discontinue power (or power is automatically discontinued) without an error signal (such as loss of contact) he or she will know that a therapeutic lesion has been formed. Alternatively, an audible or visual indication may be provided when a therapeutic lesion has been successfully formed. 
   In accordance with one embodiment of the invention, software drives the processor  102  to operate the coagulation processes described above. Such software will also drive operation of a user interface, such as the console  103  illustrated in  FIGS. 1 and 2 , which provides information obtained before and during the coagulation procedure. 
   A flow chart illustrating the steps performed by the exemplary software is illustrated in  FIG. 12 . The software may be written in any standard industry software language, such as the C or C++ languages. It is contemplated that the software is executable code residing in RAM, or may be part of a dedicated processor existing as firmware, ROM, EPROM or the like. The steps listed in the program correspond to, inter alia, function calls, modules, subroutines, classes, and/or lines of executable code in a machine readable program operating a general purpose microprocessor and, therefore, constitute apparatus for executing such steps in addition to methods of operation. 
   In step  501 , the software program performs a power-on self test. Here, the program checks to see whether the supply voltages are within the specified range, whether the front panel console and remote LEDs are functional, etc. In step  503 , the initial selected values are displayed by the console. Such values may include a set tissue temperature with, for example, an initial default value of 70° C. and measured values from the sensors. Step  505  is the start of the switch checking process. In step  507 , the application of RF energy is started if the RF energy ON switch is depressed. Otherwise, the power up/down switch is checked (step  509 ), and if that switch is depressed, the upper power limit allowed for the power supply is adjusted as necessary in step  511 . If the power up/down switch is not depressed, the temperature up/down switch is checked (step  513 ) and the temperature setpoint is adjusted as necessary in step  515 . If the temperature up/down switch is not depressed, the process loops back to step  505 . 
   In step  517 , which follows a RF energy ON finding in step  507 , the ablation process begins. Here, an “RF energy ON” signal is provided by the console and an audible tone emitted. The global power ON/OFF switch(es) are checked in step  519 , such as a foot pedal switch or switch on a remote control device. In step  521 , the processor checks for electrode contact verification between the electrodes and tissue and, if there is contact (step  523 ), proceeds to step  525 . Otherwise, the coagulation process is either stopped by ramping down power to a predetermined level, disabling a selected electrode or, if the process has yet to begin, simply not started. Audio and/or visual signals concerning the lack of suitable contact are then activated (step  527 ) and the software returns to step  503 . 
   If the RF power ON switch is still activated in step  525 , the power increment UP/DOWN switch is checked (step  529 ). Otherwise, the program returns to step  527 . If the power increment UP/DOWN switch is depressed, step  531  is pursued to adjust the power limit, while continuing the coagulation process, and the return path  533  is taken to step  519 . If the power increment UP/DOWN switch has not been depressed, the temperature UP/DOWN switch is checked (step  535 ). The temperature setpoint is adjusted in step  537 , while continuing the coagulation process, and then returning to step  519 . 
   Additional and optional steps may be added to the software after step  535 . [Note steps  540  and  545 .] Such additional steps may, for example, include determining whether or not a probe has been attached to the power supply and control apparatus, determining the elapsed time of coagulation, determining the power outputted. Other additional steps include providing an audible tone at a predetermined interval, such as every 30 seconds, and displaying the probe type. Also, at each and every step, the processor may refresh the console display with relevant data that is displayed. 
   This specification discloses multiple electrode structures in the context of cardiac tissue coagulation because the structures are well suited for use in the field of cardiac treatment. Nevertheless, it should be appreciated that the disclosed structures are applicable for use in other applications. For example, various aspects of the invention have applications and procedures concerning other regions of the body such as the prostate, brain, gall bladder and uterus. 
   Although the present invention has been described in terms of the preferred embodiments above, numerous modifications and/or additions to the above-described preferred embodiments would be readily apparent to one skilled in the art. It is intended that the scope of the present invention extends to all such modifications and/or additions and that the scope of the present invention is limited solely by the claims set forth below.