WO1997040882A2

WO1997040882A2 - Systems and methods for optimizing the delivery of radio frequency energy for lesion formation within human tissue

Info

Publication number: WO1997040882A2
Application number: PCT/US1997/007311
Authority: WO
Inventors: John N. Hendrick; Laurent P. Bonnaure; Donald P. Kannenberg; Bruno Strul
Original assignee: Vidamed Incorporated
Priority date: 1996-05-01
Filing date: 1997-04-30
Publication date: 1997-11-06
Also published as: WO1997040882A3; AU3116897A; TW351673B

Abstract

This invention is improved systems and methods for effecting lesion formation within human tissue via radio frequency (RF) ablation. An RF generator system (17) is provided which automatically regulates an amount of RF energy delivered to an electrode (43) deployed within area of tissue to be ablated. The RF energy is delivered in such a manner that the temperature of the tissue in the area to be ablated is increased in a controlled manner to a target value over a first selected period of time, and thereafter, maintained at the target value for a second period of time. Maximum lesion formation may be achieved through the use of systems and methods in accordance with the present invention.

Description

DESCRIPTION

Systems And Methods For Optimizing The

Delivery Of Radio Frequency Energy For

Lesion Formation Within Human Tissue

Field of the Invention

This invention relates generally to systems and methods for delivering radio frequency (RF) energy to an area of tissue within an organ for lesion formation and, more particularly, to improved systems and methods for optimizing the delivery of RF energy for lesion formation within human tissue.

Background of the Invention

Radio frequency (RF) ablation has heretofore been used particularly in cardiology for the treatment of cardiac arrhythmias and in neurosurgery for the relief of pain and other disorders. The use of RF energy for the treatment of benign prostate hyperplasia (BPH) in transurethral needle ablation procedures is disclosed in co-pending Application Serial No. 08/368,936 filed January 5, 1995 and Application Serial No. 08/588,452 filed January 18, 1996, both of which are hereby incorporated by reference as if fully set forth herein.

Although such procedures have been proven to be reliable, predictable and efficient, there still exists a need to develop improved operating parameters for use within such procedures to achieve optimum lesion formation within an area of tissue. Similarly, there exists a need to develop improved systems for delivering RF energy to an area of tissue, such that maximum lesion formation will be achieved within that area.

One reason for this is that, until now, it was not recognized or understood that, to form a lesion of maximum size within an area of tissue, it is far more important to deliver RF energy to the area in a controlled and effi- cient manner than it is to deliver a large amount of RF energy to the area. Indeed, prior to the invention described herein it was believed that the bulk of lesion formation was achieved through conduction, i.e., the transfer of heat from an area of tissue surrounding an electrode to adjacent tissues. However, as will be explained more fully below, this is not the case. Through substantial testing it has been found by the inventors herein that, to achieve maximum lesion size within an area of tissue, it is necessary to achieve substantial heat transfer within the tissue area through convection, i.e., the movement of heated material from an area near the electrode(s) to an area more remote from the electrode (s) . It follows that, if an area of tissue is heated too quickly the transfer of cellular ions and other materials within the area will be reduced and, thus, convection will be impeded; and if an area of tissue is heated too slowly, insufficient heat will be transferred to the tissue thereby minimizing lesion formation. Thus, when it is desired to create a lesion of maximum size within an area of tissue by RF ablation, the critical factor which must be addressed is not the amount of RF energy provided to the area of tissue but, rather, the manner in which the RF energy is provided to the area.

Summary of the Invention

The present invention is directed to improved systems and methods for delivering RF energy to an area of tissue within the human body, such that one or more lesions of a predetermined and, in some instances, maximum size may be produced in that area.

In one innovative aspect, the systems and methods of the present invention provide for the formation of a lesion of maximum size within an organ or other selected area of tissue, such as the male prostate. In such an embodiment, RF energy is delivered to one or more elec¬ trodes disposed within an area of tissue such that the temperature of the tissue surrounding the electrode (s) may be elevated in a controlled manner over a first period of time to a selected target value and, thereafter, main¬ tained at substantially the selected target value for a second period of time. In this manner, heat transfer through convection may be maximized and maximum lesion formation may be obtained.

In another innovative aspect, the systems and methods of the present invention provide a second means for regulating the amount of RF energy applied to an electrode deployed within an area of tissue to insure that a sub¬ stantial amount of heat transfer within the area will be achieved via convection, thus allowing for maximum lesion formation. In this embodiment, the impedance of the tissue surrounding the electrode is monitored and, if it is determined that the rate of change of the impedance exceeds a predetermined value, the amount of RF energy delivered to the electrode(s) is reduced by a selected amount. In still another innovative aspect, the present invention is directed to an improved microprocessor controlled RF generator system, which may be operated in either an automatic or manual mode, to effect optimal lesion formation within a selected organ or area of tissue. In a preferred form, the microprocessor may be configured to automatically control the amount of RF energy delivered to one or more electrodes deployed within a selected area of tissue in response to feedback signals indicative of the temperature and/or impedance of the tissue surrounding electrode(s) , such that maximum lesion formation is achieved within the area.

Accordingly, it is one object of the present invention to provide improved systems and methods for delivering RF energy to an area of tissue within a body to effect lesion formation within that area.

It is another object of the invention to provide improved systems and methods of the above character which are particularly suitable for ablation of tissue in the prostate.

It is still another object of the present invention to provide an improved RF generator system for use in medical applications.

It is still another object of the present invention to provide improved algorithms for regulating power delivery in RF generator systems.

These and other objects, features and advantages will be more clearly understood from the following detailed description when read in conjunction with the accompanying drawings.

Brief Description of the Drawings

Fig. 1 is an isometric view of an apparatus utilized in effecting lesion formation within a human prostate in accordance with one embodiment of the present invention.

Fig. 2 is a graph illustrating a tissue heating profile in accordance with one form of the present inven tion. Fig. 3 is a block diagram illustrating the elements comprising an RF generator in accordance with one form of the present invention.

Fig. 4 is a flow chart illustrating a series of software control functions which may be executed by the microprocessor of a RF generator system in accordance with the present invention.

Fig. 5 is a flow chart illustrating a series of software control functions which may be executed by the microprocessor of a RF generator system in accordance with a second preferred form of the present invention.

Description of Preferred Embodiments

Turning now to the drawings, the following discussion will first describe an exemplary method in accordance with the present invention for effecting lesion formation within an area of human tissue, e.g., the male prostate. Then, the structure, function and operation of an RF generator in accordance with a preferred form of the present will be described additional detail.

As explained above, in one aspect the present inven- tion is directed to an improved method for effecting lesion formation, via RF ablation, within a selected area or target volume of tissue. While the following discus¬ sion focuses primarily on improved methods for effecting lesion formation within the male prostate, it should be understood that the described methods are not limited to use within prostatic tissue but, on the contrary, may have application in a wide variety of tissues and/or organs.

The human male has a bladder with a base and a penis with a urethra therein formed by a urethral wall extending into the base of the bladder along a longitudinal axis with the tissue of the prostate surrounding the urethra near the base of the bladder. Thus, a method of trans- urethral needle ablation may be performed by the use of an elongate probe member having proximal and distal extremi ties, wherein the probe member is sized so that it can be introduced into the urethra and has a length so that when the distal extremity of the elongate probe member is disposed in the vicinity of the prostate, the proximal extremity extends out of the urethra. A control handle is coupled to the proximal extremity of the elongate probe member. At least one, but more preferably first and second stylets are slidably carried by the elongate probe member. The control handle includes control means coupled to the first and second stylets for moving the first and second stylets through the urethral wall into the target volume of tissue in the prostate. Each of the stylets has an electrode and an insulating sleeve surrounding the electrode, each of the electrodes being coupled to an RF energy power supply. Once the stylets are deployed in a selected area of prostate tissue, RF energy from the RF power supply may be provided to the electrodes to heat the tissue surrounding the electrodes in a controlled fashion to a final temper¬ ature of, for example, between 50°C and 60°C. The RF energy is supplied over a first period of time (e.g., a rise time) ranging, for example, from 2 to 8 minutes to cause the temperature of the tissue in the prostate to rise, preferably in a substantially linear fashion, from an initial temperature to approximately the final temper¬ ature. Thereafter, RF energy is supplied to maintain the prostatic tissue being ablated at or near the final temperature for a second period of time (e.g., a dwell time) of, for example, from 1 to 4 minutes. Thereafter the application of RF energy is terminated.

An exemplary apparatus and system in accordance with the present invention for effecting lesion formation within the male prostate is shown in Fig. 1. As shown, the system may comprise a transurethral needle ablation probe or catheter 12 which has mounted therein an optical device 13. The probe or catheter 12 is connected by a cable 16 to a radio frequency (RF) generator 17. A return electrode 18 is provided which is connected by a cable 19 to the RF generator 17. A foot pedal 21 is provided which is connected by a cable 22 to the RF generator 17 for controlling the operation of the RF generator 17 as hereinafter described. An optional rectal temperature monitoring device 26 is provided which is adapted to be connected by a cable 27 to the RF generator 17.

The transurethral needle ablation probe or catheter 12 can be of any suitable type such as those disclosed in earlier co-pending applications which are referenced in U.S. Patent Application Serial No. 08/368,936 filed on January 5, 1995 or, alternatively, it can be of the type disclosed in U.S. Patent Application Serial No. 08/588,452 filed on January 18, 1996.

A transurethral needle ablation probe or catheter 12, such as that shown in Fig. 1, may comprise a bridge assembly 31 which has removably mounted thereon an elon¬ gate rigid sheath 32 formed of a suitable material such as stainless steel and having a suitable size as for example 22 French. The sheath 32 is provided with an oval-shaped bore 33 which extends from the bridge assembly 31 to a distal extremity 34 of the sheath 32 where the bore opens into an upwardly facing opening 36 provided in the sheath 32. First and second stylets 41 and 42 extend through the bore 33 and through the upwardly facing opening 36 for the formation of lesions as hereinafter described. A scale 38 is provided on the exterior of the sheath 32 making it possible to ascertain the distance the sheath 32 has been inserted into the urethra.

The first and second stylets 41 and 42 are of the type described in co-pending U.S. Patent Application Serial No. 08/588,452 filed January 18, 1996 and include a flexible electrode 43 covered by a movable or, if desired, stationary insulating sleeve 44 so that the electrode can be introduced into the prostate through the urethral wall with the insulating sleeve extending through the urethral wall and protecting the urethral wall during RF ablation. The first and second stylets 41 and 42 extend through the bore 33 in the sheath 32 and into the bridge assembly 31. The bridge assembly 31 includes a depending control handle 46 which carries a knob mechanism 47 coupled to the stylets 41 and 42 for controlling the deployment of the first and second stylets 41 and 42, either one at a time or in unison from the sheath 32. Another knob 48 is provided for controlling the relative deployment between the electrode 43 and the insulating sleeve 44 of each stylet 41 and 42. The optical device 13 is removably mounted in the bridge assembly 13 and includes rod lens optics (not shown) and has a distal extremity forming an eyepiece 57 as shown in Fig. 1 which is disposed adjacent the opening 36 to permit viewing of the first and second stylets 41 and 42 as they are deployed. The optical device 13 also includes an eyepiece 57 to permit viewing by the human eye during the time that the transurethral needle ablation probe or catheter 12 is being utilized. The optical device 13 is connected by an optical cable 58 to a light source 59.

The other portions of the apparatus, namely the return electrode 18, the foot pedal 21 and the rectal probe 26 are conventional and therefore will not be described in detail.

The RF generator 17 may comprise a metal case 66 which is provided with an inclined front wall 67 that is labelled front panel 68. The front panel 68 has a plurality of display areas 71 having associated indicia 72 as hereinafter described. The RF generator 17 is provided with a power cord (not shown) which is connected through an on-off switch 76 mounted on a vertical panel 77 ad- joining the front inclined wall 67. This front wall 67 carries a plurality of receptacles as shown for connection to the various cables hereinbefore described and as shown.

The RF generator 17 also includes electronics of the type disclosed in U.S. Patent No. 5,484,400 and in U.S. Patent Application Serial No. 08/314,190 filed on Septem ber 28, 1994, both of which are hereby incorporated by reference as if fully set forth herein. However, it should be understood that, as explained more fully below, the microprocessor of an RF generator in accordance with the present invention is configured or, stated different¬ ly, programmed to implement the methods described and claimed herein. As indicated in the above described patent and application, the RF generator 17 has the capability of supplying dual RF signals which are applied to the target prostate tissue through the first and second stylets 41 and 42.

In accordance with the present invention and as described in said co-pending application, the RF genera¬ tor 17 includes means for monitoring the temperature in the urethra at the proximal extremity of the insulating sleeve 44, as for example by a temperature sensor carried by the proximal extremity of the sleeve 44. The RF generator 17 also includes means for monitoring the temperature of tissue in the prostate by temperature sensors carried by the distal extremity of the insulating sleeve for each of the first and second stylets 41 and 42, and means for monitoring an impedance of the tissue surrounding the first and second stylets 41 and 42.

In addition, although it is not necessary, tempera¬ tures can be monitored in the rectal area in the vicinity of the prostate, as for example as many as three spaced- apart electrodes can be carried by the rectal probe 26.

Utilizing the apparatus hereinbefore described, the methods of the present invention may be employed to perform a transurethral needle ablation procedure as described below. After appropriate surgical preparations, the rigid sheath is introduced into the urethra so that the distal extremity is disposed in the vicinity of the prostate with the proximal extremity remaining extending out of the urethra by the physician's use of the control handle. Assuming appropriate adjustment of the knob 48, one or more of the stylets can be deployed through the urethral wall by operation of the knob assembly 47 so that the needle electrodes protrude through the urethral wall in a direction which is at substantially right angles to the longitudinal axis of the urethra and with the insu- lating sleeve of each stylet following the electrode so that the electrode is disposed in the tissue with the insulating sleeve still being disposed in the urethral wall to protect the urethral wall. The amount of exten¬ sion of the electrode beyond the insulating sleeve is determined by the setting of the knob 48. As first and second stylets 41 and 42 are being disposed in the manner hereinbefore described, their positioning can be observed by the physician through the eyepiece 57.

As soon as it has been determined that the stylets have been properly positioned within the appropriate lobe of the prostate, RF energy may be supplied from the RF generator 17 in the manner hereinafter described either singly or in parallel and either in a monopolar mode in which the return path is through the electrode 18 and a bipolar mode in which the return path is through one of the stylets 41 and 42. After the ablation has been performed in one lobe, the stylets 41 and 42 can be retracted and the sheath 32 rotated to another angular position and the stylets 41 and 42 can be introduced into another lobe in a similar manner and RF energy supplied to the electrodes to ablate tissue in the other lobe of the prostate. A more detailed description of the method utilized in performing the procedure of the present invention is set forth below.

When RF power is supplied from the RF generator 17 to at least one and preferably both of the first and second stylets 41 and 42, RF lesions are created in the prostatic tissue. These lesions can be characterized in a number of ways, for example by their electrical characteristics, their thermal pathological characteristics and their thermal physiological characteristics. In the consider- ation of an RF electrical lesion which is made in the monopolar mode, the active electrode surface area of the needle electrode is much smaller than that of a dispersive electrode and therefore the current density is very high in the vicinity of the needle electrode as shown by Equation 1 below.

J=^ 2πrTl ⁽1⁾

where: J = current density (A/m²) I = intensity (A) r = distance (radius) from the needle (m) 1 = needle length (m) . In a typical TUNA procedure 1 = 0.022 m - 0.006 m = 0.016 m.

RF high frequency current is delivered from the electrode into the tissue to heat the tissue because of the resistance encountered. This typically only happens where the current density is high, i.e., up to 3 mm away from the needle electrode. The mechanism of this resis¬ tive RF heating occurs because of the frequency of the current. Ions in the tissue around the needle electrode try to follow changes in the current direction of the alternating current. This agitation of the ions develops frictional heat and conductive heating. The tissue being subjected to ablation by the needle electrode has a typical rugby-ball shape surrounding the needle. The cells in this rugby-ball shape are penetrated by the alternating currents produced by the RF energy. This agitates the ions inside the cells causing boiling of the intracellular water and vaporization the proteins and lipids which are also present. The cellular membrane then breaks and the cell is destroyed. The cellular ions are liberated to cause a lowering of impedance during the initiation of a transurethral needle ablation procedure. As the temperature increases in the rugby-ball shape surrounding the needle electrode to a temperature in the range of 60°C to 100°C, the tissue around the needle electrode is desiccated and the blood is coagulated. At temperatures higher than 100°C, the tissue carbonizes which causes electrical impedance to increase dramatically because of the fact that the charred tissue is no longer able to conduct electricity. In connection with a trans urethral needle ablation procedure, it is undesirable to have tissue carbonize because this causes the electrode to "impede out" .

Frequencies ranging from 300 kHz to 1 MHZ have been used for RF ablation of tissue. In accordance with the present invention, it has been found preferable to utilize frequencies ranging from 400 KHz to 550 kHz with 460 kHz being an optimum RF. The effect of such RF currents in creating ablations in the human prostate can be calculated utilizing the geometry of the stylets in the transurethral needle ablation probe 12. Utilizing simple algebra the effect of RF currents can be characterized as follows: where: P = power (W) dP I² (2) dV σ(2πrl)²

V = volume (m³) o = tissue electrical conductivity (S/m)

Equation 2 establishes that the heat generated varies as L/R⁴ which assumes cylindrical integration versus spherical integration, although spherical integration is more appropriate for the transurethral needle ablation geometry.

Combining heat equation 2 with the continuity equation 1 gives:

dV t where: ΔT = temperature rise during the lesion (K) p = tissue density (kg/m³) C = thermal capacity (J.kg^.K^-1) t = lesion time(s) Combining Equations 2 and 3:

Δ=^ il _t = -^J² t (4) pσC (2πri)² pσC Equation 4 is not a steady state equation and is valid when secondary effects such as heat transfer to surrounding tissues or physiologic endothermic effects are negligible.

In a typical transurethral needle ablation configuration: we have 1 = 0.016 m, a = .5 S/m (average of blood, muscle and kidney conductivities at 100 kHz) , p = 1,000 kg/m³ (standard), C=4200 J.kg^.K^"1 (value for vessel tissue), I = 0.2A. This leads to:

Equation 5 establishes that in each part in the lesion, the temperature is growing linearly with time. However at 1 mm away from the needle electrode where the major heating effect is known to be purely RF electrical, the temperature should then vary as 0.897 t. After a three-minute lesion, the temperature should therefore be in the 200°C range, which however does not occur. Audible pops do occur when the tissue is overheated and boils, after which it vaporizes or eventually chars. This indicates that other important thermal phenomena occur in the overall lesion formation to balance the effect of RF electrical direct heating.

Utilizing Equation 5, the following can be stated:

T ^ electiode —T stait _ r i I K \

^Tr~^Tst&zt ^τe_lectzode

where: T_eleotrode = temperature at the interface between the electrode and the tissue at a time t

T_sCart = temperature at the beginning of the le¬ sion, i.e., 37 deg C

T_r = temperature of the tissue at a distance r_r from the center of the needle at a time t r_r = distance from the center of the needle ^r _ei_ectrode = radius of the electrode (0.018") Typically in a transurethral needle ablation proce¬ dure, a needle/tissue interface is at a temperature of approximately 90°C. Experiments have been carried out utilizing the values obtained from Equation 6 with in vi tro experiments and have demonstrated that the creation of the overall lesion is primarily due to other RF induced effects rather than RF electrical injury. Equation 5 also shows that the control parameters for the RF electrical energy are not only the lesion time and the current density (or power) , but also by the contact area between the needle electrode and the tissue. Therefore, a good contact between the needle electrode and tissue is necessary. Improved results are obtained when the contact between the needle electrode and the prostatic tissue is constant.

The other RF induced effects will now be discussed, including the thermal, physiological and pathological effects. The small portion of tissue heated by RF energy around the needle electrode acts as a heater to the remaining surrounding prostatic tissue and, as will be more fully described below, preferably causes that sur¬ rounding tissue to be heated in substantial part by convection. This heating effect can be maintained by supplying low RF power to the needle electrode as during a dwell portion of a transurethral needle ablation proce dure as hereinafter described. From a macroscopic stand point, two thermal lesion zones, in addition to the RF electrical lesion hereinbefore discussed, are found in a transurethral needle ablation. The pathological lesion is created by typical coagulation necrosis, and the physio¬ logical lesion surrounds the pathological zone or lesion with edema, hemorrhage and acute inflammation forming a gelatinous layer around the pathological zone. This can be explained by considering that when one specific part of the tissue is progressively heated, it first develops into a physiological lesion at 45°C to 50°C. At this tempera ture, the protein chains change, the amino-acids crosslink and the electrophysiology properties change from a pro- gressive reversible depolarization of the cell to total irreversible loss of the cellular excitability and tissue injury as temperature increases. This physiological injury is accompanied with edema, inflammation and loss of the water and of the ions in the tissue. The cell plasma is liberated and creates a gelatinized layer which tries to protect the rest of the tissue against further heat damage. When the tissue reaches temperatures higher than 50°C, the tissue is reliably destroyed and the physiolog¬ ical lesion turns into a pathological lesion.

It has been found by the inventors listed herein that the main heat transfer in RF-induced thermal injuries is substantially solely convective. The blood circulating in the prostate therefore has two opposite effects. One, the blood vessels dissipate heat which limits the lesion size and two, the blood enhances convection which favors thermal lesion formation. The blood flow in large vessels is not affected by the RF ablation. However, when the temperature exceeds 40°C to 46°C, the microcirculation ceases and RF-induced thermal damage desiccates the capillary blood vessels in the prostate and seals them. Such blood would be constricted inside the lesion area and would be available to convect heat within the lesion and thus would lead to tissue necrosis. Lesions in the prostate usually have a high plasma/water content. Because of the gradient of ion concentration within the overall lesion, diffusion and convection of ions (Na+, K+ and Ca++ mainly) occurs in accordance with Pick's law.

The pathophysiology of the RF-induced thermal lesion occurs principally because of convective heat transfer which explains the fairly large overall lesion size (up to 20 mm away from the needle electrode, whereas the RF electrical injury is only within 2 mm from the needle electrode) .

After a transurethral needle ablation procedure in accordance with the present invention, the physiological lesion area will progress or regress over time depending on the degree of injury in the lesion. Typically the region of acute physiological injury is more extensive than the acute pathological lesion which explains the critical late effects of a transurethral needle ablation in accordance with the present invention in which urinary flow may keep increasing even 12 months after the proce¬ dure. Thus it can be seen that the RF-induced thermal, physiological and pathological lesions are created by convection mechanism in which gelatinization, as herein¬ before described, creates a lag in the creation of the lesion.

With this information at hand, optimum parameters for performing transurethral needle ablation procedures have been developed using the physics of RF lesion creation in which the overall lesion size is affected by the RF electrical injury and the two RF induced thermal injuries. Since these three effects are linked, it is undesirable to emphasize one over the other. Applying RF energy too fast to reach a high final temperature as fast as possible to favor the thermal injuries will cause charring. Applying RF energy too slowly will make a small lesion and defeats the objective of having a short procedure.

Several important parameters for creating RF lesions in accordance with the present invention are the final temperature, the power delivered to the tissue of the prostate, the total amount of energy delivered to the tissue of the prostate and, perhaps most importantly, the temperature profile during the procedure. Some of these parameters can be linked as follows:

/lesion P ( t) dt (7)

0

and dP _ Δ- Tρ C_βquivalent ^ g j dV~ t where C_equivalβnt = equivalent thermal capacity of the whole lesion (J.kg^.K^'1)

It is believed that a slowly ascending curve provides a better control of the growth of the lesion than a rapidly ascending curve. The prostate tends to thermo- regulate itself. Thus it is important to establish a plateau in the temperature profile in order to properly cook the tissue being treated in the prostatic gland. Because of Equation 8, it is desirable that the tempera¬ ture rise in the tissue of the prostate be as steady as possible.

In order to achieve optimization in RF lesion forma- tion, three parameters were and are used which are as follows: (1) final temperature, (2) temperature rise time and (3) temperature dwell time. Using this criteria, the optimum settings for use on the RF generator 17 are as follows: 2 to 7 minutes and preferably 4 minutes rise time (see Fig. 2) .

0.5 to 3 minutes and preferably 1.5 minutes dwell time (see Fig. 2) .

Final temperature of 50°C and 60°C and preferably 55°C. It has been found that the dwell time is the most important factor simply because of the latent gelatiniza tion heat effect.

In arriving at such optimum settings, it was found that the lesion volume was highly correlated with the energy delivered to the prostatic tissue except where the lesion volume was the greatest, even though the energy delivered may actually be very low. This phenomena can be explained in that the tissue being treated may be in a thermal resonance mode with the three different types of heat transfer hereinbefore discussed in different areas of the tissue interacting, i.e., the conductive resistive heat for the RF electrical lesion, the convective heat for the RF-induced thermal physiological and pathological lesions and the latent gelatinization heat. Thermal resonance modes can be described with thermal equations of the same nature as mechanical equations for mechanical resonance. At the optimum parameters set forth above, the RF energy is applied to tissue in the prostate serving as a heater so that the remainder of the tissue of the prostate being ablated will heat in phase (or with a 180° out-of-phase) and enter a resonance mode with the remain der of the tissue of the prostate. The term "resonance" as used herein means that the energy is delivered to the prostatic tissue in a manner that matches the tissue characteristics.

Using this information, it is believed that the optimum control strategy for the RF generator 17 is to perform an RF ablation with a 4.0 minute temperature rise time and to hold this temperature for 1.5 minutes. The targeted temperature is 60°C, but in case the tissue impedance starts to rise too soon, the temperature objec tive is lowered to 55°C by an adaptive control algorithm in the microprocessor of the RF generator 17.

In determining the overall RF lesion size, it has been found that there is a linear correlation between the lesion size and the temperature at the interface between the tissue and the needle electrode. There is also believed to be a strong correlation between lesion size and the energy delivered in Joules. However, fundamental to obtaining a large lesion in ablation in accordance with the present transurethral needle ablation procedure, it is far more important to deliver the energy to the tissue in an appropriate manner (i.e., to obtained a desired tem¬ perature profile) , than it is to deliver a large amount of energy or heat to the tissue.

In accordance with the present invention it can be seen that the optimum settings for the RF generator 17 as set forth above can be easily programmed into the RF generator. In utilizing these optimized settings for the RF generator, it is desirable to consider the specific characteristics of each patient (size, wetness, impedance, vascularization, conductivity, thermal capacity, electri cal polarization) to further consider the optimized parameters and to ascertain whether or not they should be varied for a particular patient to prevent particular patients from impeding out or alternatively from obtaining a prostatic lesion of too small a size.

The RF generator 17 in operating with these optimized settings will supply the necessary power to reach the desired temperature under the control of an appropriate algorithm in the computer in the RF generator 17. Uti¬ lizing the temperature measurements that are provided to it from thermocouples carried by the distal extremities of the insulating sleeves 44, the algorithm requires that the power delivered by the RF generator 17 be adequate to arrive at the desired temperature within a predetermined period of time. For example, as shown in Fig. 2, it may be required to reach the desired temperature of 55°C within a period of two minutes. By way of example, energy delivered by the RF generator 17 can be in the vicinity of 2 to 2.3 watts per electrode, and an electrode with 6 millimeters or less being exposed in the tissue of the insulating sleeve 44, depending upon the size of the prostate with the higher wattage being utilized for the larger prostates. Under the control of the algorithm, the RF energy delivered is progressively increased to reach the desired temperature without causing an undue rise in the temperature at the needle which could result in desiccation of the tissue around the needle, causing significant resistance to the flow of current and inter¬ fering with the formation of the optimal thermal lesion in accordance with the present invention. Thus, by way of example, the algorithm can cause automatic increase of the RF energy delivered by increasing the wattage 0.5 watt at each 30-second interval with the increases continuing throughout the entire rise time. If the temperature does not increase as rapidly as required by the algorithm, the computer will cause additional RF energy to be delivered to the electrode. Thus the power supplied by the RF generator 17 typically can vary from 2 to 9 watts per needle, with the supply of power being controlled by the computer and the RF generator 17 to insure that the final temperature is reached within a predetermined rise time without creating undesired desiccation of tissue, which could cause the needle to impede out. Thereafter, under the control of the RF generator 17, RF power is continued to be supplied to the tissue in the prostate during the dwell time by keeping the tissue at the desired final temperature, also without the desiccation of prostatic tissue. It should be appreciated that the foregoing steps which have been set forth in the method have been carried out automatically under the control of the computer in the RF generator 17. After the desired rise and dwell times have elapsed, the application of RF power is automatically terminated.

Although the foregoing steps have been described as being carried out automatically by the RF generator 17, the physician at any time can assume control if desired by touching an appropriate switch on the control panel after which the physician can control the remaining steps manually. Also, at any time the physician can again return to automatic control by touching the appropriate control legend on the control panel of the RF generator 17. In accordance with the present invention, the opti¬ mized settings discussed above need only be utilized during the first 30 seconds of a transurethral needle ablation procedure, after which an adaptive or LQR control system can be utilized to automate the entire transure- thral needle ablation procedure and to have as its objec tive an optimal 55°C final temperature. Utilizing such a control algorithm, the controller in the RF generator 17 adapts itself based on the values of the stated parameters and of the control parameters recorded following the beginning of the transurethral needle ablation procedure on the patient. Such a control algorithm will not force the prostatic tissue to impede out when trying to reach 55°C and will not permit a minimal lesion to be created with a 55°C final temperature. The foregoing considerations have primarily been set forth in connection with operation of the first and second stylets in a monopolar mode in which the RF energy travels between the stylets and the return electrode 18 rather than in a bipolar mode in which the current is concentrat¬ ed between the two needle electrodes of the first and second stylets 41 and 42. The current density is dis- tributed equally between the two needle electrodes and the current concentration around them in comparison to current concentration in the monopolar mode is limited. This limits charring of the "heater" portion of the tissue being ablated. , the power required is minimal and the RF lesion will be much larger. Also, the shape of the lesion will be more evenly distributed, and in particular between the two needle electrodes.

From the foregoing it can be seen that a definitive analysis has been made of the treatment of tissue of the human prostate by the use of RF apparatus to ascertain the optimum settings for optimizing the delivery of RF energy for lesion formation in the human prostate. By assuring proper lesion formation, the efficacy of the procedure is improved, eliminating the necessity for an additional treatment. This also assures that the patient undergoing the transurethral needle ablation procedure is not sub¬ jected to an unduly long treatment.

Turning now to Fig. 3, in a preferred form, an RF generator system 100 in accordance with the present invention may comprise an RF energy power source 110, a microprocessor 112, a memory 114 (for example, RAM, ROM, EPROM or EEPROM), a temperature feedback circuit 116, and an impedance feedback circuit 118. The specific imple¬ mentation of the aforementioned components is well know in the art, and an exemplary implementation is set forth in U.S. Patent 5,484,400, which has been incorporated herein by reference. However, it must be understood that the microprocessor 112 of the RF generator 100 of the present invention is configured in a manner which is markedly different from that illustrated in U.S. Patent No. 5,484,400. Similarly, the programming stored in the memory 114 of the RF generator 100 differs markedly from that utilized by the system described in U.S. Patent No. 5,484,400. In accordance with the present invention, the microprocessor 112 may be configured to automatically regulate the amount of RF energy provided to the elec- trodes of the stylets 41 and 42 in response to signals received from the temperature and impedance feedback circuits 116 and 118, respectively.

Turning now also to Fig. 4, when configured such that the RF generator system 100 operates in an "automatic" mode, the microprocessor 112, in one embodiment, may cause the RF power source to deliver power to the electrode of a given channel in the following manner. Upon initiation of the therapy cycle, the microprocessor 112 will prompt the user to input a rise time value (step 200) , a dwell time value (step 201) and a target temperature value (step 202) . The microprocessor 112 will then set the power to be delivered (P_x) to a value appropriate for the selected rise time and target temperature settings (step 203) . Generally, the initial power P_x will be set to 1.5 watts. Following this step, the microprocessor 112 will cause the RF power source to provide, for example, 3.0 watts of power at a frequency of 460 kHz to the electrode (step 204) . The microprocessor 112 will then monitor the temperature and impedance of the tissue surrounding the electrode, via the temperature and impedance feedback circuits 116 and 118, respectively. So long as the temperature of the tissue surrounding the electrode rises at a rate of, for example, less than or equal to 5°C per 30 second interval (step 209) , and so long as the imped- ance of the tissue surrounding the electrode rises at a rate of, for example, no more than 25 ohms per 0.1 second interval (step 210) , the microprocessor 112 will cause the RF power source 110 to increase the amount of power supplied to the electrode by 0.5 watts (step 211) , at each 30 second interval, until the temperature of the tissue surrounding the electrode reaches the selected target temperature of, for example, between 45°C and 70°C with the default value being 55°C (step 206) . However, if at any 30 second interval the microprocessor 112 observes that the temperature of the tissue surrounding the elec¬ trode is increasing too quickly, the microprocessor 112 will not step up the power delivered by the RF power source 110 at the next interval. Further, if the micro¬ processor 112 determines that the impedance of the tissue surrounding the electrode is rising at a rate of more than 25 ohms per 0.1 second interval (step 210), the micropro cessor 112 will cause the RF power source 110 to reduce the amount of power delivered to the electrode by, for example, 0.5 watts (step 212). While the time provided for raising the temperature of the tissue surrounding the electrode to the target temperature is programmable, it is presently preferred that this "rise time" period have a duration of between 2.0 and 7.0 minutes with 4.0 minutes being the default value.

Once the target temperature is achieved, the micro¬ processor will maintain the amount of power delivered by the RF power source 110 at a constant level for a program- mably selectable period of time (step 207) . This period is referred to as the "dwell time" or "hold time" period and, it is presently preferred that the dwell time range from 1.0 to 4.0 minutes with 1.5 minutes being the default value.

Turning now to Fig. 5, in a presently preferred embodiment, the microprocessor 112 may regulate the power delivered to a selected electrode in the following manner. As in the case previously discussed, the microprocessor 112 will first prompt the operator of the system to input a rise time value (step 300) , a dwell (or hold) time value

(step 301) and a target temperature value (step 302) . The microprocessor 112 will then establish a temperature profile (step 303) to be followed during the procedure. In the presently preferred embodiment, the temperature profile for a given channel is established by (1) deter¬ mining the temperature of the tissue adjacent an eiec- trode, e.g., via the temperature sensor located at the distal end of the insulating sleeve of the electrode; (2) setting the profile start temperature (T₀) to a value 2°C higher than the tissue temperature; (3) plotting a curve between the start temperature value T₀ and the target temperature (or final temperature) value T_f over the entered rise time period; and (4) setting the temperature for the dwell time period to the entered target tempera¬ ture value. Once the temperature profile has been estab lished (step 303) , the microprocessor will set the power to be delivered (P_:) to an initial value, for example, 1.5 watts (step 304) , and the microprocessor will cause the RF power source 110 to commence the delivery of power to the electrode (step 305) . The microprocessor 112, thereafter, will continuously adjust the amount of power provided by the RF power source 110 to the electrode, such that the temperature of the tissue in the vicinity of the electrode is elevated and maintained substantially as defined by the temperature profile (step 306) . In a preferred embodi- ment, the following methodology is utilized to maintain the tissue temperature elevation in accordance with the temperature profile. If the desired profile temperature T_p is more than 3°C greater than the tissue temperature (i.e., T_p-T_tia > 3°C) , the power delivered by the RF power supply will be increased linearly at a rate of 0.5 watts per minute. If the profile temperature T_P exceeds the tissue temperature T_tis by 3°C or less (i.e., T_p-T_tis ≥ 3°C) , the power delivered by the RF power supply will be in¬ creased at a rate of (T_p-T_tis/3) (0.5) watts per second. If the tissue temperature T_tls exceeds the desired profile temperature T_p by 3°C or less (i.e., T_tia-T_P ≥ 3°C) , the power delivered by the RF power source will be decreased at a rate of (T_tis-T_p/3) (0.2) watts per second, and if the tissue temperature T_tis exceeds the desired profile temper- ature T_p by more than 3°C (i.e., T_tis-T_p > 3°C) , the power delivered by the RF power supply will be reduced by M, and held at that level for 6.0 seconds. Thereafter, the power regulation will continue as described above.

Finally, once the dwell time portion of the procedure has been completed, the microprocessor 112 will terminate the procedure (step 307) .

If at any time during the procedure the microprocessor 112 determines that the impedance of the tissue in the area of the electrode is increasing too quickly, for example, at a rate in excess of 25 ohms per 0.1 second or, alternatively, if the microprocessor 112 determines that the impedance of the tissue is more than, for example, 25 ohms greater than an average running impedance of the tissue (step 310) , the microprocessor 112 will cause the RF power source 110 to reduce the amount of power delivered to the electrode by a selected amount, for example, to the minimum operating power (step 308) . The power will be held at that level for a selected period, for example, 0.5 seconds, and thereafter, at selected intervals, e.g., 1.0, 2.0 or 5.0 second intervals, the microprocessor will cause the power delivery to be restored to 25% of the power delivered at the impedance rise point (P_m) , 50% of the power delivered at the impedance rise point P_m, and 75% of the power delivered at the impedance rise point, if that is possible without exceeding the impedance rate limit and without exceeding the desired temperature value, as determined from the temperature profile (step 309) . The power delivery will then be continuously adjusted to maintain the temperature of the tissue in accordance with the temperature profile (step 306), as described above. If at any point within the power stepping up procedure the impedance again rises too quickly, the procedure described above will be repeated.

As has been explained above, in a preferred form the RF power may be delivered via two separate and distinct channels and in either a bi-polar or mono-polar mode, as described in U.S. Patent No. 5,484,400. In such an embodiment, the power delivered to each channel may be independently controlled by the microprocessor 112 in the manner set forth above.

It will also be noted that, if at any time during a procedure the temperature of the patient's urethra exceeds a predetermined temperature, for example, a temperature exceeding 47°C, the procedure will be terminated, and the delivery power to the electrode(s) will be terminated. Further, if at any time during a procedure the impedance of the tissue surrounding an electrode exceeds, for example, 540 ohms, the delivery of RF energy to that electrode will be terminated.

Finally, when configured in a "manual" mode, provision may be made for the operator of the RF generator system 100 to control both the amount of power delivered by the RF power source to the electrode(s) and the duration of the delivery, provided that the impedance of the tissue surrounding the electrode(s) does not increase at a rate in excess of 25 ohms per 0.1 second interval. Moreover, if the impedance of the tissue surrounding the electrode(s) increases at a rate in excess of the pre¬ scribed limit, the microprocessor 112 will cause the amount of power delivered by the RF power source 110 to be decreased by a selected amount, for example, 0.5 watts, until the rise in tissue impedance is brought under control.

Alternatively, it may be desirable in a manual mode to forgo monitoring the rate of change of the tissue impedance and to, simply, terminate the delivery of power in the event that the tissue impedance reaches a predeter¬ mined value, such as 540 ohms. In such an embodiment, an ablation procedure might be reinitiated after the power termination, provided that substantial tissue charring did not result from the initial procedure. While the invention of the subject application is susceptible to various modifications and alternative forms, specific examples thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the invention is not to be limited to the particular forms or methods disclosed, but to the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims.

Claims

1. A method for optimizing the delivery of RF energy for lesion formation in a target volume of tissue of a prostate in a human male having a bladder with a base and a penis with a urethra therein formed by a urethral wall extending into the base of the bladder along a longitudi nal axis with the tissue of the prostate surrounding the urethra near the base of the bladder by the use of an elongate probe member having proximal and distal extremi ties and being sized so that it can be introduced into the urethra and having a length so that when the distal extremity of the elongate probe member is disposed in the vicinity of the prostate the proximal extremity extends out of the urethra, a control handle coupled to the proximal extremity of the elongate probe member, at least one stylet slidably carried by the elongate probe member, control means carried by the control handle and coupled to the at least one stylet for moving the at least one stylet through the urethral wall into the target volume of tissue in the prostate, said at least one stylet having an electrode and an insulating sleeve surrounding the elec¬ trode and RF power supply means, the method comprising the steps of : supplying RF energy from the RF power supply means to said electrodes of said at least one stylet when said at least one electrode is disposed in the tissue of the prostate so that the final temperature in the tissue of the prostate is in the range of 50°C to 60°C, supplying the RF energy over time ranging from 3 to 6 minutes to cause the temperature of the tissue in the prostate to rise from an initial temperature to approximately said final temperature, continuing to supply the RF energy to maintain the prostatic tissue being ablated at or near the final temperature for a dwell time ranging from ^ minute to 3 minutes, and terminating the application of RF energy to the electrode in the prostate thereafter.

2. A method as in Claim 1 wherein said rise time is approximately 5 minutes.

3. A method as in Claim 1 wherein said dwell time is approximately 2 minutes.

4. A method as in Claim 1 wherein said final temper¬ ature is 55°C.

5. A method as in Claim 1 wherein the lesion formed is comprised of an RF electrical lesion, an RF induced thermal pathological lesion, and an RF-induced thermal physiological lesion.

6. A method as in Claim 1 for use with a probe member wherein said at least one stylet is comprised of first and second stylets of the same construction and slidably carried by the elongate probe member, further comprising the steps of delivering RF energy to the electrodes of the first and second stylets at the same time with substantially the same rise times and dwell times.

7. A method as in Claim 1 for use with an electrode mounted on the body of the human male and wherein said ablation is carried out in a monopolar mode.

8. A method as in Claim 6 wherein said ablation is carried out in a bipolar mode with current flow being concentrated between the electrodes of the first and second stylets.

9. A method as in Claim 1 in which said RF energy is delivered to the tissue of the prostate so that it matches the characteristics of said tissue to create a resonance in the tissue.

10. A method as in Claim 1 in which the steps set forth are performed automatically by the RF generator.

11. A method of delivering RF energy to an area of tissue within an organ to effect lesion formation within a target volume of said tissue, said method comprising the steps of : deploying an electrode at a location within said target volume of said tissue; delivering a selected and controllable amount of RF energy to said electrode; and automatically controlling said amount of RF energy delivered to said electrode based upon a tissue temperature and a tissue impedance measured at said location of said electrode, such that said tissue within said target volume may be heated to a selected temperature in a controlled fashion and thereafter maintained at substantially said selected temperature for a selected period of time.

12. The method of claim 11, wherein said organ comprises a male prostate, said selected temperature is between 50°C and 60°C, and said selected period of time ranges from 30 seconds to 3 minutes.

13. The method of claim 12, wherein said selected temperature is 55°C and said selected period of time is substantially 2 minutes.

14. A radio frequency (RF) generator for use in medical applications, said RF generator comprising: an RF power source for providing RF energy to an electrode; a temperature feedback circuit; an impedance feedback circuit; and a microprocessor, said microprocessor being coupled to said RF power source, said temperature feedback circuit and said impedance feedback circuit, and being configured to automatically control an amount of RF energy provided by said RF power source to said electrode in response to signals received from said temperature feedback and impedance feedback circuits such that a target volume of tissue surrounding said electrode may be heated to a selected temperature in a controlled fashion and, thereafter, may be maintained at substantially said selected temperature for a selected period of time.

15. The RF generator of claim 14, wherein said medical applications include ablation of tissue within a male prostate, wherein said microprocessor is programmed to cause said RF power source to provide at selected intervals automatically increasing amounts of RF energy to said electrode until a signal is received from said temperature feedback circuit indicating that an area of tissue located adjacent said electrode has reached said selected temperature, a signal is received from said impedance feedback circuit indicating that an impedance of said tissue in said area adjacent said electrode has increased at a rate in excess of a predetermined value, or a signal is received from said temperature feedback circuit indicating that said temperature of said tissue in said area adjacent said electrode has increased at a rate in excess of a predetermined value, and wherein said microprocessor is programmed to cause said RF power source to deliver a constant amount of

RF energy to said electrode for said selected period of time once said temperature of said tissue in said area adjacent said electrode reaches said selected temperature.

16. A radio frequency (RF) generator for use in medical applications, said RF generator comprising: an RF power source for providing RF energy to an electrode; a temperature feedback circuit; and a microprocessor, said microprocessor being coupled to said RF power source and said temperature feedback circuit; said microprocessor being configured to cause said RF power source to deliver increasing amounts of RF energy to said electrode over a first selected period of time, such that a temperature of an area of tissue adjacent said electrode increases in a substantially linear fashion until said temperature reaches a selected target temperature within said first selected period of time; and said microprocessor being configured to cause said RF power source to deliver a constant amount of energy to said electrode for a second selected period of time after said temperature reaches said selected target temperature.

17. The RF generator of claim 16 further comprising an impedance feedback circuit for providing an indication of an impedance of said area of tissue adjacent said electrode to said microprocessor.

18. The RF generator of claim 17, wherein said microprocessor is further configured to cause said RF power source to deliver a decreased amount of energy to said electrode when said impedance of said area of tissue adjacent said electrode increases at a rate in excess of a predetermined value.

19. The RF generator of claim 16, wherein said selected target temperature is 55°C, said first selected period of time is between 2 and 7 minutes, and said second selected period of time is between 1 and 4 minutes.

20. The RF generator of claim 18, wherein said predetermined value is substantially 25 ohms per 0.1 second interval.

21. The RF generator of claim 18, wherein said rate in excess of a predetermined value corresponds to a detected impedance more than 25 ohms greater than a running average impedance.

22. A method of radio frequency tissue ablation comprising the steps of: delivering a selected amount of RF energy to an electrode deployed in an area of tissue to be ablated; and reducing said amount of RF energy delivered to said electrode when an impedance of said tissue in said area to be ablated increases at a rate in excess of a predetermined rate; whereby a lesion in said area may be produced in substantial part via thermal convection.

23. The method of claim 22, wherein said predeter¬ mined rate is 25 ohms per 0.1 second interval.

24. The method of claim 22, wherein said rate in excess of a predetermined value corresponds to a detected impedance more than 25 ohms greater than a running average impedance.

25. The method of claim 23, wherein said predeter¬ mined rate is a programmably selectable rate.

26. A method of radio frequency tissue ablation comprising the steps of: delivering over a first selected period of time and over selected intervals within said first selected period of time increasing amounts of RF energy to an electrode deployed within an area of tissue to be ablated to raise a temperature of said tissue in a controlled and substantially linear fashion to a selected target value; and delivering over a second selected period of time following said first selected period of time a constant amount of RF energy to said electrode.

27. The method of claim 26, wherein said first selected period of time, said second selected period of time, and said selected target value are programmably selectable and, thus, may be tailored to provide varying therapy regimens.

28. The method of claim 27, further comprising the steps of: monitoring an impedance of said tissue; and reducing said amount of RF energy delivered to said electrode by a selected amount, when said impedance increases at a rate in excess of a predetermined rate.

29. The method of claim 28, wherein said predeter¬ mined rate is programmably selectable.

30. The method of claim 29, wherein said first period is substantially 4.0 minutes, said second period is substantially 1.5 minutes, and said target value is substantially 55°C.

31. A method of radio frequency tissue ablation comprising the steps of: forming an RF electrical lesion within an area of tissue to be ablated by delivering RF energy to an electrode deployed within said area; allowing an RF-induced thermal pathological lesion to form within said area of tissue to be ablated; allowing an RF-induced thermal physiological lesion to form within said area of tissue to be ablated; and regulating said delivery of RF energy to said electrode to promote maximum thermal convection within said area of tissue to be ablated.

32. A memory circuit for use in a radio frequency (RF) generator system, said memory circuit comprising: a plurality of memory elements, at least some of said memory elements defining a program to be executed by a microprocessor control circuit in a RF generator system; said program comprising a series of instructions for enabling said microprocessor to control an amount of energy delivered by an RF power source to an electrode deployed in an area of tissue to be ablated; and said series of instructions, upon initiation of an ablation procedure, enabling said microprocessor to (1) cause said RF power source to deliver to said electrode over a first programmably selectable period of time and over selected intervals within said first programmably selectable period of time increasing amounts of RF energy to said electrode; and (2) cause said RF power source to deliver to said electrode over a second programmably selectable period of time following said first programmably selectable period of time a constant amount of RF energy to said electrode.

33. The memory circuit of claim 32, wherein said series of instructions further enable said microprocessor to cause said RF power source to reduce said RF energy delivered to said electrode by a selected amount, when an impedance of said tissue within said area increases at a rate in excess of a predetermined rate.

34. The memory circuit of claim 32, wherein said memory elements comprise a read only memory circuit, such as a ROM, EPROM or EEPROM.

35. The memory circuit of claim 33, wherein a pause period separates said first programmably selectable period of time and said second programmably selectable period of time.

36. A radio frequency (RF) generator for use in medical applications, said RF generator comprising: an RF power source for providing RF energy to an electrode; a temperature feedback circuit; and a microprocessor, said microprocessor being coupled to said RF power source and said temperature feedback circuit; said microprocessor being configured to cause said RF power source to deliver increasing amounts of RF energy to said electrode over a first programmably selectable period of time, such that a temperature of an area of tissue adjacent said electrode increases in a substantially linear fashion until said temperature reaches a programmably selectable target temperature within said first programmably selectable period of time; and said microprocessor being configured to cause said RF power source to deliver a constant amount of energy to said electrode for a second programmably selectable period of time after said temperature reaches said programmably selectable target temperature.

37. The RF generator of claim 36 further comprising an impedance feedback circuit for providing an indication of an impedance of said area of tissue adjacent said electrode to said microprocessor.

38. The RF generator of claim 37, wherein said microprocessor is further configured to cause said RF power source to deliver a decreased amount of energy to said electrode when said impedance of said area of tissue adjacent said electrode increases at a rate in excess of a programmably selectable rate of change.

39. The RF generator of claim 36, wherein said programmably selectable target temperature is 55°C, said first programmably selectable period of time is between 2 and 7 minutes, and said second programmably selectable period of time is between 1 and 4 minutes.

40. The RF generator of claim 39, wherein said programmably selectable rate of change is substantially 25 ohms per 0.1 second interval.

41. The RF generator of claim 39, wherein said programmably selectable rate of change corresponds to a detected impedance more than 25 ohms greater than a running average impedance.

42. A method for creating a lesion in human tissue via RF energy ablation, said method comprising the steps of: delivering RF energy to an area of tissue such that a temperature of said tissue will be elevated to a programmably selected target temperature in a substantially linear fashion within a first programmably selectable period of time; and reducing said programmably selected target temperature by a predetermined amount when an impedance of said tissue rises at a rate in excess of a predetermined limit.

43. The method of claim 42, further comprising the step of: maintaining said tissue at a final target temperature for a second programmably selected period of time following said first programmably selected period of time.

44. A method for creating a lesion in human tissue via RF energy ablation, said method comprising the steps of: delivering RF energy to an area of tissue such that a temperature of said tissue will be elevated to a programmably selectable target temperature in a substantially linear fashion within a first programmably selectable period of time; and maintaining said tissue at said target temperature for a second programmably selectable period of time following said first programmably selectable period of time.

45. The method of claim 44, wherein said step of delivering RF energy to said area of tissue such that a temperature of said area of tissue will be elevated to a programmably selectable target temperature further com¬ prises the followings steps: determining an initial temperature of said tissue; determining a temperature profile based upon said initial temperature, said programmably selectable target temperature and said first programmably selectable period of time; and automatically controlling an amount of RF energy delivered to said area of tissue, such that said tempera ture of said area of tissue is elevated in a manner substantially tracking said temperature profile.

46. The method of claim 45, wherein said RF energy delivered to said area of tissue is varied continuously.

47. The method of claim 45, wherein said RF energy delivered to said area of tissue is varied incrementally.

48. The method of claim 46, wherein said step of automatically controlling said RF energy delivery includes the steps of: increasing said amount of energy delivered at a rate of 0.5 watts per minute, if a desired profile temperature T_p is more than 3°C greater than said tissue temperature (i.e., T_p-T_tis > 3°C) ; increasing said amount of energy delivered at a rate of (T_p-T_tis/3) (0.5) watts per second, if said desired profile temperature T_p exceeds said tissue temperature T_tls by 3°C or less (i.e., T_p-T_tis ≥ 3°C) ; decreasing said amount of energy delivered at a rate of (T_cis-T_p/3) (0.2) watts per second, if said tissue temperature T_cis exceeds said desired profile temperature T_p by 3°C or less (i.e., T_tis-T_P ≥ 3°C) ; and reducing by % said amount of energy delivered and holding said amount of energy delivered at the reduced level for a period 6.0 seconds, if said tissue temperature

T_tis exceeds said desired profile temperature T_P by more than 3°C (i.e., T_ti3-T_p > 3°C) .

49. The method of claim 48, wherein said target temperature is 55°C, said first programmably selectable period of time is substantially 4.0 minutes and said second programmably selectable period of time is substan tially 1.5 minutes.

50. A radio frequency (RF) generator for use in medical applications, said RF generator comprising: an RF power source for providing RF energy to an electrode; a temperature feedback circuit; and a microprocessor, said microprocessor being coupled to said RF power source and said temperature feedback circuit; said microprocessor being configured to cause said RF power source to deliver a continuously variable amount of RF energy to said electrode over a first programmably selectable period of time, such that a temperature of an area of tissue adjacent said electrode increases in a substantially linear fashion until said temperature reaches a programmably selectable target temperature within said first programmably selectable period of time; and said microprocessor being configured to cause said RF power source to deliver a continuously variable amount of energy to said electrode for a second programmably selectable period of time after said first programmably selectable period of time, such that said temperature is maintained at substantially said programmably selectable target temperature for said second programmably selectable period of time.

51. The RF generator of claim 50 further comprising an impedance feedback circuit for providing an indication of an impedance of said area of tissue adjacent said electrode to said microprocessor.

52. The RF generator of claim 51, wherein said microprocessor is further configured to cause said RF power source to deliver a decreased amount of energy to said electrode when said impedance of said area of tissue adjacent said electrode increases at a rate in excess of a programmably selectable rate of change.

53. The RF generator of claim 50, wherein said programmably selectable target temperature is 55°C, said first programmably selectable period of time is between 2 and 7 minutes, and said second programmably selectable period of time is between 1 and 4 minutes.

54. The RF generator of claim 52, wherein said programmably selectable rate of change is substantially 25 ohms per 0.1 second interval.

55. The RF generator of claim 52, wherein said programmably selectable rate of change corresponds to a detected impedance more than 25 ohms greater than a running average impedance.

56. A memory circuit for use in a radio frequency (RF) generator system, said memory circuit comprising: a plurality of memory elements, at least some of said memory elements defining a program to be executed by a microprocessor control circuit in a RF generator system; said program comprising a series of instructions for enabling said microprocessor control circuit to regulate an amount of energy delivered by an RF power source to an electrode deployed in an area of tissue to be ablated; and said series of instructions, upon initiation of a RF ablation procedure, enabling said microprocessor control circuit to

(1) cause said RF power source to deliver to said electrode over a first programmably selectable period of time a continuously variable amount of energy such that a temperature of said tissue is elevated from an initial value to a target value in a substantially linear fashion over a first programmably selectable period of time; and

(2) cause said RF power source to deliver to said electrode over a second programmably selectable period of time following said first programmably selectable period of time a continuously variable amount of energy such that said temperature of said tissue is maintained at substantially said target value for said second programmably selectable period of time.

57. The memory circuit of claim 56, wherein said series of instructions enables said microprocessor during said first programmably selectable period of time to reduce said RF energy delivered to said electrode by a selected amount, when an impedance of said tissue within said area increases at a rate in excess of a predetermined rate.

58. The memory circuit of claim 57, wherein said plurality of memory elements comprise a read only memory (ROM) .

59. The memory circuit of claim 57, wherein said plurality of memory elements comprise an electrically programmable read only memory (EPROM) .

60. The memory circuit of claim 57, wherein said plurality of memory elements comprise an electrically erasable programmable read only memory (EEPROM) .

61. A radio frequency (RF) generator system for use in medical applications, such as transurethral needle ablation procedures, said RF generator comprising: power supply means for providing RF energy to an electrode deployed in a selected area of tissue to be ablated; and power regulation means for regulating an amount of RF energy provided to said electrode such that a temperature of said tissue is elevated to a programmably selectable target temperature in a substantially linear fashion over a first programmably selectable period of time, and such that thereafter said temperature is maintained at substantially said programmably selectable target temperature for a second programmably selectable period of time; whereby a lesion may be formed within said selected area of tissue substantially by means of thermal convection.