HK1133169B - Radio-frequency based catheter system and method for ablating biological tissues - Google Patents

Description

Radio frequency based catheter system and method for ablating biological tissue

Technical Field

The present invention relates to a radio frequency based catheter system that uses the electric field generated by an RF antenna to ablate tissue and occlusions, particularly tissue and occlusions in animal perfusate lumens such as the human heart, liver, arteries and vessels, and in particular to a control system and method for controlling RF energy to target biological tissue based on a preset threshold level.

Background

In recent years, medical devices have gained acceptance in the medical community as an important form of treatment for heart disease and other serious diseases that have traditionally been treated with drugs or surgery. Two fundamental trends in the treatment of cardiac diseases have emerged. The first is a shift from open heart surgery to less invasive and less expensive catheter-based therapies that are safer and consume less energy.

The second trend is to move from the use of antiarrhythmic drugs to minimal catheter intervention or other device-based therapies to alleviate incurable arrhythmias. For example, automatic cardioverter defibrillators are often implanted in patients with fatal ventricular arrhythmias to reduce the risk of sudden death. Radio frequency (sub-microwave) catheter ablation is therefore now employed in many patients suffering from cardiac arrhythmias.

Despite these technical advantages, atrial fibrillation ("AF") remains a significant challenge. AF, a rapid irregular rhythm in the atria or upper ventricles of the heart due to uneven electrical impulses, represents a significant cause of stroke and heart disease and a major health care burden. To date, the most effective surgical procedure for treating AF is the Maze procedure under open heart surgery. In the Maze procedure, incisions are made along predetermined lines outside the atrium, and then sutured. As healing progresses, a scar forms along the incision line, thereby forming a barrier to the conduction of an electronic pulse. By establishing such a barrier, AF is not maintained and normal heart rhythm is restored. However, Maze surgery has not been widely adopted due to complications and associated mortality from open-heart surgery, including opening the chest cavity and removing the sternum.

One new approach to mimic the Maze procedure is a catheter-based radio frequency ablation technique, in which, instead of a surgical incision, a catheter-electrode is applied to ablate or ablate the heart tissue located in the atrial chamber. As is customary in the medical community, the catheter-electrode is passed into the atrium via arterial access. Within the atrium, the tip of the catheter-electrode is typically placed by means of an x-ray or fluoroscopic device and brought into contact with the heart tissue at the desired location or location to be ablated. In this position, the tissue is ablated by resistive heating generated by the catheter electrode. Thereafter, the catheter-electrode is repositioned at the next ablation site. A series of site ablations is achieved, mimicking the linear lesions that oppose the conduction of the electrical pulse as achieved by the Maze procedure.

It can be appreciated that existing catheter-based ablation procedures have fewer interventions than open heart surgery. Furthermore, in ablation, cardiovascular damage is reduced. However, a successful catheter-based radio frequency ablation procedure requires that the tissue points ablated deviate in space or proximity between adjacent locations, typically by less than 2 mm, to prevent the passage of electrical pulses. In this regard, precise placement of the catheter electrode is a determining factor for successful surgery.

A major drawback of this prior procedure is that placing the catheter-electrode at the desired ablation site in the atrium is a time-consuming process when the ventricular muscle is beating. Movement of the atrial wall or myocardium often results in precise placement of the catheter-electrode being very difficult and slippage of the catheter-electrode causing damage to the portion of the atrium where ablation is not desired. Placement of catheter-based RF ablation is therefore not effectively achieved and the procedure time is extended, which can be expected to exceed 12 hours. Furthermore, during surgery, x-ray or other irradiation devices are often used to position and place the catheter electrodes, which requires the electrophysiologist to use heavy guide protection devices. Thus, this inconvenience is often amplified by the prolonged procedure time, which in turn makes the use of catheter-based electrodes ineffective as an effective means of tissue ablation.

To overcome these difficulties, for example, in U.S. Pat. No.5,741,249, a catheter-based microwave antenna is described in which the antenna includes a distal end for securing it to the atrial wall. However, while this design reduces the likelihood of antenna or catheter electrode slippage during each ablation procedure, it does not eliminate the time consuming task of precisely securing the antenna along the desired ablation path during each ablation step. Thus, as described above, after each ablation step, the antenna must be repositioned and accurately fixed at the next position located spatially or close to the offset of the ablation path.

Thus, effective treatment of atrial fibrillation with catheter ablation requires the creation of long or staggered straight or curved ablation lesions on the inner surface of the atrium. These lesions then act as a conduction barrier to the electrical impulses, thereby preventing atrial fibrillation.

It is also recognized that a strict requirement for catheter-based effective ablation of atrial fibrillation is the ability to secure the catheter and microwave antenna within the atrial chamber. The development of catheter-based medical procedures for minimal intervention in atrial fibrillation requires new catheter ablation systems, preferably capable of producing long or staggered straight or curved ablation lesions.

U.S. patent No.6,190,382, published at 20/2/2001 and U.S. patent application No.09/459,058, filed 11/2001, both disclose catheters based on radio frequency or microwave energy for ablating biological tissue within a body vessel of a patient. The catheter includes a proximal end, a distal end having a distal end, and a lumen extending from the proximal end to the distal end. The catheter includes an elongated catheter guide positioned within the catheter lumen with one end secured to the distal end of the catheter and the other end extending proximally into the catheter lumen for coupling to a positioning device. The catheter guide is extendable beyond the distal end of the catheter to form a loop to conform to the internal contours of the body vessel.

The catheter guide carries a catheter having a radio frequency or microwave energy based antenna at the distal end of the catheter. The antenna includes a helical coil for guiding the catheter therethrough. The radio frequency antenna is adapted to receive and radiate radio frequency energy in the microwave range, typically above 300 megahertz (MHz) of electromagnetic spectrum frequencies, to ablate biological tissue along a biological ablation pathway.

Further improvements to catheters having the above-described radio frequency or microwave energy-based characteristics are incorporated in U.S. patent application No.10/306,757, filed 2002, 11/27, which is incorporated herein by reference in its entirety, and which includes the same inventors as the present application, describes advanced deflectable and shapeable catheter structural features and particularly antenna portions. These features substantially enhance the electrophysiologist's ability to adapt to the configuration and shape of the catheter and antenna that is consistent with the contours of the ablation site, as well as the ability to accurately prescribe an ablation path.

Disclosure of Invention

The catheter of the present invention provides further improvements and features over U.S. patent nos. 6,190,382, 6,663,625, and 7,004,938, and U.S. patent application No. 10/637,325, filed on 3.8.2003. Among these improvements and features are a radio frequency ("RF") generator to selectively generate high frequency RF energy of variable power output that is delivered to an RF antenna. The RF antenna includes a helical coil and has an axial passage to accommodate a steering control line.

In accordance with one embodiment of the present invention, an improved radio frequency based catheter system is provided for ablating biological tissue of a body vessel, including an atrium of a patient. The system includes an RF generator in the microwave frequency range for RF energy delivery with a catheter insertable into a body vessel and includes a flexible antenna guide positioned within a lumen of the catheter. The catheter includes an RF transmission line and an RF antenna at the distal end of the catheter to receive and transmit radio frequency energy for tissue ablation. After the RF antenna is placed in a body vessel, the RF generator is activated to energize the antenna. In one embodiment, a controller connected to the RF generator will monitor and minimize the reflected to forward power ratio of the antenna and antenna-tissue interface by adjusting the microwave frequency for effective tissue ablation. In another embodiment of the invention, a temperature sensing system is combined with an RF antenna and the temperature is monitored and controlled by adjusting the power setting. In a representative embodiment of the invention, the reflected to forward power ratio and temperature may be monitored and controlled within preset limits, although in alternate embodiments only one of these parameters may be controlled.

The temperature change measured by the temperature sensor may be related to the combination of the RF energy effect (ablation) of the biological tissue and the antenna system as a whole. By establishing a set point for the measured temperature and adjusting the RF frequency and energy delivered to the target tissue within the preset temperature set point, an effective method for tissue ablation is provided.

In one exemplary embodiment of the invention, the antenna guide includes an elongated portion that is secured to a control slide for positioning, placement, and bend control. Alignment of the antenna with the desired tissue ablation path is made easier in one embodiment by the use of radio-opacity (radio-opaque) markers and/or radio-opaque antenna elements.

After the RF antenna is placed in proximity to body tissue within a body vessel, the RF generator is activated to energize the antenna. The RF generator monitors and minimizes the reflected to forward power ratio of the antenna and antenna-tissue interface by adjusting the microwave frequency for effective tissue ablation.

In one embodiment of the invention, a sensor is provided to sense the amount of reflected RF energy from the antenna. If the reflected energy is too high, the RF generator will automatically adjust to scale down the frequency of the synthesized waveform to maximize the tissue-wide energy release.

In another embodiment, the catheter is configured with an RF antenna integrated with a temperature sensing system and the RF energy delivered to the target biological tissue is optimized by controlling the reflected power (reverse power) and the detected temperature.

These and other aspects of the invention will become apparent by consideration of the following detailed description and accompanying drawings, which illustrate features of the invention.

Drawings

The present invention will be more readily understood from the detailed description of certain representative embodiments of the invention taken in conjunction with the accompanying drawings, in which like reference numerals designate like parts, and in which:

fig. 1A and 1B are representative side views of an RF ablation catheter in one embodiment of the invention.

Fig. 2A and 2B are representative side views of an RF ablation guide in another embodiment in which the handle includes an improved steering device.

Fig. 3A and 3B are distal cross-sectional views of the radio frequency based catheter of fig. 1 or 2.

Fig. 4 is a functional block diagram of a control system for controlling the delivery of RF energy to the RF ablation catheter of fig. 1-3 in accordance with an exemplary embodiment of the present invention.

Fig. 5 is a flow chart illustrating the establishment and control of microwave frequencies that minimize the reflected to forward power ratio in the system of fig. 1-4.

Fig. 6A and 6B are cross-sectional views similar to fig. 3A and 3B, but depicting an improved RF ablation catheter including a temperature sensor in accordance with another exemplary embodiment of the present invention.

Fig. 7 is a functional block diagram of a power and temperature control system including the RF ablation catheter of fig. 6, in accordance with a representative embodiment of the present invention.

FIG. 8 is a schematic flow chart diagram illustrating a method for controlling reflected power and tissue temperature in the system of FIG. 7.

Detailed Description

Embodiments disclosed herein provide Radio Frequency (RF) based catheter systems and methods for biological tissue ablation, and more particularly, to a system and method for controlling RF energy to target biological tissue. For example, one method and system disclosed herein allows for controlling the delivery of RF energy by controlling the reflected to forward power ratio, and another method and system disclosed herein includes a temperature sensor for monitoring and controlling the reflected temperature in addition to controlling the reflected/forward power ratio.

It will be clear to those skilled in the art, after reading this description, how to implement the invention in various alternative embodiments and alternative applications. However, while various embodiments of the present invention have been described herein, it should be understood that they have been presented by way of example only, and not limitation. As such, these detailed description of various alternative embodiments should not be construed to limit the scope or breadth of the present invention as set forth in the appended claims.

Embodiments of the present invention may be used for ablation of biological tissue. Advantageously, these embodiments enable ablation of tissue located in a living mammalian internal blood vessel by using a Radio Frequency (RF) antenna as part of the catheter. The catheter can be inserted into the lumen or body vessel of these mammals and the RF antenna is located proximate to the tissue to be ablated where RF energy is provided to effect tissue ablation.

Embodiments of the present invention also provide an apparatus for generating a series of pulses of RF energy, particularly in the microwave frequency range, which are transmitted via an electrical transmission line to an RF antenna. The frequency of the RF energy pulses can be selectively varied depending on the electrical characteristics of the electrical transmission line and the load impedance associated with tissue ablation.

In one embodiment, an apparatus is provided for sensing forward and reflected power associated with a microwave frequency energy pulse to determine a reflected to forward power ratio. The output frequency of the energy pulses of the RF generator may then be adjusted to minimize this reflected to forward power ratio. This fine-tunes the impedance of the system energy output to substantially match the ablation load and deliver the ablation energy to the desired location. Thus, not only is a means of generating and delivering RF energy to the RF antenna for ablation of tissue provided, but a means of increasing the efficiency of operation of the RF antenna to reduce the risk of overheating of the electrical transmission line is also provided.

In one embodiment of the present invention, as shown in FIGS. 1A and 1B, a radio frequency ("RF") ablation catheter 100 includes a shapeable antenna apparatus 110 adapted for insertion into a body vessel of a patient, and the shapeable antenna apparatus 110 includes a radio frequency antenna for delivering electromagnetic energy to a treatment site. Before describing the shapeable antenna apparatus 110 of the present invention, the catheter 100 will be described.

The catheter 100 has a flexible elongate tubular body 120 including a proximal portion 130 and a distal portion 140. One or more lumens 150 (fig. 3A, 3B) extend from the proximal portion 130 to the distal portion 140 of the catheter 100. A handle disk 160 is located at the proximal end portion 130 of the catheter 100 to provide the necessary steering and positioning controls, which will be described in detail below. The proximal end 160 of the catheter 100 has a coupling component 170 to connect the catheter 100 to one or more electronic devices, such as the RF generator and control system described in fig. 4 for supporting the ablation process.

The catheter 100 is sized as needed to suit a particular medical procedure, as is well known in the medical community. In one embodiment, the catheter 100 is used to ablate myocardial tissue; however, in alternative embodiments, the catheter 100 may be used to ablate other types of tissue. Typically, the catheter is tubularBody 120 may be constructed from a polymeric material that is biocompatible in the body's vascular environment. Without being limiting, examples of such materials include those having varying degrees of radiopacity, hardness, and elasticity, available from Autochem CermanyPolyethylene, polyurethane, polyester, polyimide and polyamide.

The catheter 100 may be composed of multiple segments using one or more of the aforementioned materials to cause the catheter body 120 to become progressively flexible toward its distal end. The segments are joined together by thermal bonding, butt-joining, or gluing. Braided reinforcement may also be added to the circumferential surface of the tubular body 120 to achieve the desired level of stiffness and torsional strength for the catheter 100. This allows the catheter 100 to be advanced and passed through the body vessel of the patient and allows torque to be transmitted along the length of the catheter from the proximal portion to the distal portion.

Further, with reference to fig. 3A and 3B, the distal portion 140 of the tubular body 120 will be described in greater detail. This portion may comprise a softer polymer than the proximal portion 130, with little or no braiding, to provide the desired flexibility to accommodate distal bending (deflection) and shaping of the shapeable antenna apparatus 110. The bending and shaping of the shapeable antenna apparatus 110 may be achieved by using a pre-shaped bending element 180 and/or a bending adjustment element 190. The pre-shaped bending element 180 and/or the bend adjustment element 190 may extend from the handle chassis 160 to the distal portion 140 of the tubular body 140 via the tubular body. The distal portion 140 of the tubular body includes an RF antenna 250 having a flexible, helically coiled radiating antenna element 255 for ablation of body vessels.

In the exemplary embodiment, RF antenna 250 includes a conductive material or wire strip that is helically wound to form a flexible, helical coil winding. The selection of the appropriate coil winding diameter, pitch and length, as well as the conductive material or wire strip, may vary according to the particular process and flexibility requirements. In the depicted embodiment, the RF antenna is connected to one or more conductors 260 that extend along the length of the catheter and are connected to the power control system of fig. 4 via coupling 170. Although conductors 260 are shown in the depicted embodiment as being flexible mesh or braided wire structures, it is understood that these conductors may have alternative structures in other embodiments, such as thin film conductive material, or extended, coaxial, circumferentially aligned inner or outer conductors, etc.

The shapeable catheter apparatus of fig. 1,2, 3A and 3B may carry one or more Electrocardiogram (ECG) electrodes (not shown) to allow the physician to obtain optimal tissue approximation and electrical conductivity activity before and after tissue ablation. The electrodes may be fixed in position along the length of the catheter.

The pre-shaped bending element 180 and/or the bend adjustment element 190 may be proximally fixed to the bend control device 220 (fig. 2A, 2B) or the slider 200 (fig. 1A, 1B). In one embodiment, the slider 200 is slidably inserted into an axial slot of the handle disk 160. Axial movement of the slider 200 along the axial slot allows the physician to shape or bend the shapeable antenna apparatus 110 between a straight configuration (fig. 1A), a curved shaped configuration (fig. 1B), or any configuration. The slider 200 may include friction capture means (not shown) to maintain a secure position in the axial slot. Many such devices are commercially available. Such means include, for example, setting release means, pressure switches or self-locking means.

Fig. 2A and 2B depict an RF ablation catheter 210 similar to the RF ablation catheter 100 described above, but which includes an alternative embodiment of a bend control device 220 to shape or bend the shapeable antenna apparatus 110. The bend control apparatus 220 may include a rotatable ring 230 that circumferentially surrounds and is rotatably coupled to a handle shaft 240 of the handle hub 160 to control axial movement of the pre-shaped bending member 180 and/or the bend adjustment member 190. The handle chassis 160 may house a translation device to translate rotational movement of the collar 230 into axial movement of the pre-shaped bending element 180 and/or the bend adjustment element 190. The rotational movement of the ring 230, in comparison to the handle shaft 240, allows the physician to shape or bend the shapeable antenna apparatus 110 between a straight configuration (fig. 2A), a curved shaped configuration (fig. 2B), or any configuration.

Fig. 4 is a functional block diagram of a control system 300, the control system 300 controlling RF output signals of the ablation catheter of fig. 1-3 in accordance with an embodiment of the present invention. Fig. 4 depicts the electrical and signal components of the system. Catheter system 300 includes a power switch 308, a power supply system 310, a micro-control system 320, an RF signal generator or oscillator 330, an RF amplifier 334 including a preamplifier 331, an RF bi-directional coupler 336, a control input 350, a display device 360, and an alarm output 370. The bi-directional coupler 336 is connected to the distal end of an RF transmission line 342 and the proximal end of the transmission line is connected to an RF antenna 343. The transmission line 342 and antenna 343 are disposed on the steerable ablation catheter 430. In one embodiment, the ablation catheter 340 may be identical to the ablation catheter 100 of fig. 1-3, and the antenna 343 may be the helical RF antenna 250 of fig. 3A and 3B, while the transmission line 342 includes the conductor 260.

The RF-based catheter system 300 is powered by a common ac power source, and it may also be powered by a suitable dc power source. The power switch 300 connects the power source and the power supply system 310. The power supply system provides the primary patient safety isolation and the various dc voltages required for the device to perform effective tissue ablation.

The microprocessor-based microcontroller 320 provides for user input, display of inputs and outputs, and sets system alarm conditions. The microcontroller 320 also monitors and controls the RF power synthesis and communication with the RF antenna 343 and the ablation tissue. As shown in fig. 4, the microcontroller 320 monitors and controls an RF signal oscillator 330, which receives power from the power supply system 310. The RF signal oscillator generates a continuous RF frequency wave signal 332 at a power level and frequency determined and controlled by the microcontroller 320.

In an embodiment of the present invention, the RF signal oscillator 330 is electrically coupled to the power amplifier 334. The power amplifier 334 includes a pre-amplifier 331, the pre-amplifier 331 initially amplifies the signal waves 332 from the RF generator and generates a first sequence of relatively low energy pulses. After amplification by the RF amplifier 334, the energy pulses are transmitted via transmission line 342 to an RF antenna 343 located near the tissue to be ablated.

As shown in fig. 4, bi-directional coupler 336 is electrically interposed between amplifier 334 and transmission line 342. The coupler samples the relatively low energy forward pulses along the transmission line as well as the energy pulses reflected by the target ablated tissue and uses the sampled signal as feedback to the microcontroller 320. The feedback mechanism provided by sampling the signal at coupler 336 is useful for scaling down the amount of reflected energy. Too many signal reflections can potentially damage sensitive system 300 components or cause patient injury.

The microcontroller 320 monitors the forward and reflected energy pulses by being in electrical communication with the bi-directional coupler 336. The microcontroller 320 then determines the ratio of reflected to forward energy pulses. In one embodiment, the ratio comprises a Voltage Standing Wave Ratio (VSWR) calculated as follows:

wherein gamma is₀Representing the load reflection coefficient calculated using appropriate boundary conditions along RF transmission line 342.

A low ratio will indicate that most of the energy generated by the system is being used for the ablation load, representing an impedance match between the device and the ablation load. On the other hand, a high ratio would indicate that an appreciable amount of the energy produced by the system is reflected, representing a high return loss or leakage due to poor impedance matching.

For the impedance of RF transmission line 342 to be affected by the frequency of pulse 332, one embodiment provides a means to enable the frequency of the system power output to be varied, thereby matching the line impedance to the load impedance. The means for sensing (e.g., a bi-directional coupler in one embodiment) and the means for adjusting include means for adjusting the RF signal source 330 and an RF amplifier 334 responsive to the control means (e.g., microcontroller 320) to match the transmission line impedance to the load impedance. For example, if the ratio indicates that too much energy is being reflected (e.g., a high VSWR), the microcontroller 320 adjusts the frequency of the RF signal 332 generated by the oscillator 330 to achieve a reduced ratio of reflected and forward energy pulses. This reduction in power ratio achieves impedance matching between the transmission line and the ablation load. The amount of echo loss that is acceptable depends on the actual situation. But since it is not possible to achieve a perfect impedance match, the microcontroller 320 may allow the user to adjust the frequency so that the ratio falls below some threshold. The threshold may be below 1.4:1, in one embodiment 0.4: 1.

Because the load impedance may vary widely between tissue types, and may vary depending on the quality and quantity of fluid (e.g., a congested lumen or chamber) surrounding the tissue, the control device supports a wide range of frequency adjustment settings to flexibly configure the system 300 in the art.

After impedance matching is achieved, the microcontroller 320 adjusts the power amplifier 334 to generate a series of relatively high energy pulses that will be delivered to the RF antenna via the transmission line to effect tissue ablation. In one embodiment of the present invention, the power level generated for the ablation process is approximately 60 watts.

In addition to providing monitoring and adjustment functions through the RF pulse frequency, the microcontroller 320 also communicates various signals and instructions to the user (e.g., to an electrophysiologist). The system supports manual override (override) in terms of RF frequency, output power, and set ablation duration. In a typical configuration, the control input 350 of the present invention may be equipped with a multi-line display, a set of up and down keys to adjust output power levels and ablation periods, an ablation on/off key to activate an ablation procedure, a mode/set key to change display modes and/or configure I/O ports.

The output power level of the RF amplifier 334 is continuously monitored during the ablation process. The RF bi-directional coupler 336 provides the function of sampling the forward and reverse power levels at the attenuation level and is connected to the microcontroller assembly. The microcontroller component compares the two signals and adjusts the two signal sources and the preamplifier/power amplifier gain to achieve the lowest reverse to forward power ratio.

The RF-based catheter system 300 monitors and controls the microwave frequency and power output to minimize the reflected to forward power ratio over a typical range of 900MHz to 930 MHz. The RF antenna 343 is typically an antenna that has been fabricated and tuned to 915MHz in saline proximate to the biological tissue and fluid filled animal body vessel to be ablated. When entering a body vessel and coming into contact with biological tissue for ablation, the electrical dimensions of the RF antenna 343 may be slightly temporarily altered to increase the reflected power. The increased reflected power reduces the total power available for radiation and thus reduces the effective tissue ablation. If the reflected power is unconstrained and grows too much, local heating of the RF antenna 343 can occur and cause undesirable ablation effects.

Fig. 5 is a flow chart of a method for biological tissue ablation in accordance with an embodiment of the present invention. The method may be used to program the instruction set of microcontroller 320 to perform the ablation procedure described herein.

The process begins after the user provides power to the system, typically by turning on the power switch 301. At step 401, the system typically runs a series of initialization routines to establish system integrity. Self-checking may include, for example, displaying a logo on a display device and checking the system ROM for appropriate hardware.

At conditional block 403, if the power-on self-test fails, the process branches to a system error. In one embodiment, if the power-on self test fails, alarm output 370 will sound.

If the self-test passes in conditional block 403, ablation parameters are set automatically in step 405 or manually by the electrophysiologist. In one embodiment, the ablation parameters are ablation power and ablation time period. The preset ablation parameters may be changed in steps 407 and 408. Once the ablation parameters are set, the operator may select whether to begin the ablation process (step 406). During the initial ablation procedure, ablation is performed under constant monitoring conditions (step 409) so that the frequency of the oscillator 330 can be appropriately adjusted, as in the case of measurement of excessive reflected and forward power. Several parameters may be monitored in real time to ensure that critical system thresholds are not exceeded. For example, in step 409, the power output and reflected/forward power ratio may be monitored to ensure that a prescribed ablation exposure is provided. Too much irradiation will lead to undesirable results, such as ablation of benign tissue.

If the reflected/forward power ratio is detected to be above a preset limit (step 410), the RF amplifier is turned off and a system alarm at alarm output 370 in FIG. 4 will sound (step 411) and the ablation process is stopped (412). If the ratio is below the preset limit but the power output is above the preset limit (step 413), the RF amplifier will again be turned off and an alarm sounded (step 412). However, as long as the power ratio and power output are within preset limits, the ablation process will continue until the ablation cutoff time is reached (step 414), after which the system will return to step 405 and wait for ablation parameters for a subsequent ablation process to be entered. The user will set the ablation period as one parameter to enter step 405. Any suitable alert output may be provided, including voice, view, or both.

Fig. 6A and 6B depict the proximal end of an ablation catheter 500 in accordance with another embodiment of the invention. The ablation catheter 500 is similar to the catheter 100 of fig. 1 to 3, but further includes a temperature sensor 510. Catheter 500 is otherwise identical to catheter 100 of fig. 1-3, and like reference numerals refer to like elements. The temperature sensor 510 may be a thermistor, a thermocouple, or the like. The temperature sensor 510 has a sensing tip or thermocouple junction 520 near the distal end of the catheter 500 and a pair of conductors 530, 532 extending from the junction 520 to the proximal end of the catheter via the catheter lumen 270 where they are connected to control circuitry as will be described in detail below with reference to fig. 7 and 8. Although in the embodiment of fig. 6A and 6B the temperature sensor 510 is mounted inside the conduit, it will be appreciated that in alternative embodiments it may be fixed along the outside of the conduit, or mounted on the wall of the conduit.

Fig. 7 and 8 depict a control system 600 and associated method for monitoring and controlling reflected/forward power ratio, power output, and temperature in an RF ablation catheter (similar to catheter 500 in fig. 6A and 6B) that includes a temperature sensor. The system 600 includes a power switch 602, and a power system 604, which may be the same as the power system 310 in FIG. 3, the power system 604 being used to power the various elements of the system shown in FIG. 7. The system 600 includes a microcontroller 610 that controls the operation of the system in accordance with programmed instructions and operator inputs at control inputs 612. A display module 614 and an output alarm module 615 are connected to appropriate outputs of the microcontroller 610.

The microcontroller 610 is also connected to an RF signal generator or oscillator 616, which may be a phase-locked loop (PLL) oscillator. The RF signal oscillator 616 is connected to a power amplifier 618, the power amplifier 618 including a preamplifier that first amplifies the output signal from the RF oscillator 616, and a second RF amplifier that last amplifies the signal. After amplification by the RF amplifier 618, the pulsed RF signal is transmitted via the bi-directional coupler 620 to the RF transmission line 560 of the ablation device 500. Similar to the control circuitry of the previous embodiment (fig. 4), the bi-directional coupler 620 samples the relatively low energy forward pulses transmitted along the transmission line to the RF antenna 550 and samples the energy pulses reflected back from the target ablation tissue and provides the forward and reflected pulse samples as feedback to the microcontroller 610. A temperature signal detection and conditioning module 622 is connected to temperature signal conductors 530, 532 (see fig. 6A, 6B) at the end of RF transmission line 560, and module 622 is also connected to microcontroller 610.

The microcontroller is programmed to monitor the power output and the forward and reflected energy pulses as in the previous embodiment, and to calculate the ratio between the forward and reflected energy pulses, or the previously defined voltage standing wave ratio VSWR, in relation to the control system in the embodiment of fig. 4 and 5 above. In addition, in this embodiment, the microcontroller is also programmed to monitor the temperature sensed by the temperature sensor 520, which is very close to the temperature of the tissue resulting from the ablation process, since the temperature sensor 520 is close to the ablation point. It will be readily appreciated that in alternative embodiments, the temperature sensor 520 may be disposed on the outer surface of the catheter or at the distal end of the catheter.

In the embodiment of fig. 7 and 8, as in the previous embodiments, microcontroller 610 is programmed to adjust the frequency to achieve a minimum reflected to forward power ratio and to adjust the RF power level to achieve a selected temperature setting. The temperature setting may be a temperature set point plus or minus a few degrees or may be a selected temperature range as described in detail in connection with the flow chart of fig. 8. This reflected power is proportional to the combined impedance of the biological tissue and the antenna system in common, and therefore minimizing the reflected power is the same as the system impedance match, maximizing the transmission of forward power delivered to the tissue to be ablated. At the same time, the temperature change measured by the temperature sensor may be related to the combination of the RF energy effect (ablation) of the biological tissue and the antenna system. By establishing a temperature set point for the measured temperature and adjusting the RF frequency and power delivered to the target tissue in the preset temperature set point, an efficient tissue ablation method can be provided. While the present embodiment uses the sensed temperature and reverse to forward power ratio as control parameters in adjusting the RF signal parameters to achieve a temperature and power ratio close to the user or default settings, alternative embodiments may use temperature alone as the control parameter. The microcontroller may also monitor output power and temperature to ensure that they do not exceed the maximum limits for safe operation.

In the system of fig. 7 and 8, the microcontroller adjusts the RF frequency, and thus the reflected/forward power ratio, by controlling oscillator 330. The transmitted RF power can be adjusted in magnitude by controlling the amplifier 618 to adjust the reflected temperature. Fig. 8 is a flow chart depicting steps in an ablation procedure in accordance with an embodiment of the invention using the control system of fig. 7 and an RF ablation catheter incorporating a temperature sensor as shown in fig. 6. Power is first turned on by the switch 602 (step 650), and then a power-on reset, initialization, and self-test process is performed (step 655). As described above in connection with step 401 of fig. 5, in this step the system runs a series of initialization routines to establish system integrity. If the self-test fails (condition block 660), the RF amplifier will be turned off and the ablation process will stop (step 665), and an alarm will be displayed and sounded (step 670). If the self-test is successful, default or pre-user selected parameters will be displayed on the display module 614 and the system will wait for the user to turn on the ablation switch before the ablation process begins (step 680).

Ablation parameters may be adjusted or set by the operator at the input module 612 at the beginning of the ablation procedure (step 685). The parameters that may be changed by the operator may be temperature set points, power levels, frequency, and ablation time periods. The desired parameters will vary depending on the target biological tissue and other factors. The system includes default initial values for frequency and power level, and both may be adjusted as necessary to achieve the lowest possible reverse/forward power ratio, and temperature detected by the process or sensor 520 near the temperature set point. In addition to the preset or operator selected operating power level, temperature level, frequency and ablation period, the system also has fixed maximum limits of power ratio, power and temperature set for safe operation independent of the control loop. The maximum power ratio, power level and temperature are indicative of the maximum limit that the catheter can withstand in safe operation of the system.

As described above, the operator may change the set point temperature or temperature range, power level, frequency, and ablation period in step 685 by changing the settings of control input 612. The operator-set temperature setting input may be a specific temperature or temperature range. If the input is a particular temperature, the system will control the RF signal pulses so that the detected temperature is equal to the particular temperature selected by the operator plus or minus a few degrees. If the input is a temperature range, the system controls the RF signal pulses so that the detected temperature is within a selected range. The selected set point temperature or temperature range may be in the range of 45 degrees celsius to 125 degrees celsius, and the exact temperature setting will depend on the target ablation tissue. For example, in the heart, the temperature setting or set point may be between 50 and 90 degrees celsius. In non-intracardiac tissue, such as the liver, the outer surface of the heart, or other non-intracardiac tissue regions, the temperature setting or set point may be, for example, between 60 and 120 degrees celsius. The system has recommended temperature levels or ranges for different types of biological tissue for operator reference purposes. At step 695, the microcontroller will change the RF frequency to achieve and maintain the lowest reverse/forward power ratio possible and adjust the RF power level above and below the set point to achieve a temperature at or near the set point or within a range of set points (where the temperature setting is a range rather than a specific temperature). At the same time, the power output, temperature and timer will all be monitored and compared to the set values and the maximum limits of system output power and process temperature.

As described above in connection with the previous embodiments, it is desirable for tissue ablation to be able to match the impedance of the transmission line as closely as possible to the load impedance. If the reverse to forward power ratio or the input power is too high, indicating that too much energy is reflected, e.g., not absorbed by the tissue, the signal frequency is adjusted to produce a reduced power ratio. Since in practice a perfect impedance match cannot be achieved, the microcontroller adjusts the frequency and power level in step 695 to achieve the lowest possible level limited within the selected ablation temperature set point. As shown in the previous embodiment, a threshold value for the ratio may be set, for example 0.4:1, and then the controller may adjust the frequency until the ratio falls below the threshold value. In an alternative embodiment, as described above, rather than using a power ratio, the system controls the frequency and power level of the RF signal to maintain the selected temperature setting.

In the described embodiment, the RF frequency and power level are varied to achieve a temperature as close as possible to the selected temperature set point while maintaining the desired reflected to forward power ratio. The temperature detected by sensor 510 will be indicative of the combined RF energy effects on the biological tissue. Controlling the temperature at or near the set point will improve or optimize tissue ablation.

As described above, the system has maximum set limits for power ratio, power level, temperature level, and if any of these limits are exceeded, the ablation process stops (step 700). If one of the maximum limits is exceeded, the RF amplifier is turned off and the ablation process stops (step 665), and the system will display and sound an alarm 615 in step 670. As long as the power ratio, power level, and temperature are within the maximum limits, the ablation process will continue for a set period of time or until the ablation switch is turned off by the operator (step 710). When the ablation period is over or the operator turns the ablation switch off, the system will return to step 680 displaying default or pre-user determined ablation parameters and awaiting further input by the user or operator.

In this embodiment, the ablation device or catheter is equipped with an RF antenna that integrates a temperature sensing system for more precise control and also to reduce the risk of excessive temperatures or the like. RF energy delivery to the target biological tissue is optimized by controlling the reflected/forward power ratio and the reflected tissue temperature, by monitoring the output of the temperature sensor and varying the power level to achieve a temperature at or near a selected set point. It is necessary to adjust the RF frequency and signal power level to achieve the desired biological tissue effect, so the reflected/forward power ratio and the detected temperature are interdependent in the ablation process control. The reflected power reflects the combined impedance of the biological tissue and the antenna system as a whole, so minimizing the ratio of reflected power to forward power equates to a system impedance match that maximizes the forward power transfer delivered to the biological tissue to be ablated. The temperature changes detected by the temperature sensor can be correlated to the combined RF energy effect (ablation) of the biological tissue and the antenna system as a whole, so that the temperature can be controlled individually to obtain the desired ablation effect, and the steps of monitoring and controlling the reflected/forward power ratio can be omitted, if necessary. By establishing a measured temperature set point within the preset temperature set points and adjusting the RF frequency and power delivered to the target tissue, the tissue ablation process will be improved. In the representative embodiments depicted in fig. 6 through 8, the combination of power ratio control and temperature control will increase the effectiveness of the ablation system.

Fig. 7 and 8 depict a control system and method that continuously monitors forward power, reflected power, and temperature, and adjusts the frequency and power level in conjunction with a near set point temperature to achieve and maintain the lowest reflected/forward power ratio possible. The frequency and power level are set by the microcontroller, firmware adjustment of the RF oscillator frequency and the output level fed to the preamplifier module 618. The ablation procedure begins with default values for frequency and power level, both of which may be adjusted as necessary to achieve the lowest reflected/forward power ratio, while the temperature of the procedure is near the temperature set point. The system also has maximum limits of power ratio, power level and temperature independent of the control loop and if these values are monitored to be exceeded, the process is stopped and the user is alerted.

The radio frequency-based catheter system and method for ablating biological tissue can be used in a variety of medical applications. The description and drawings contained herein describe certain representative embodiments of the invention and are, therefore, representative of the subject matter which is broadly contemplated by the present invention. The scope of the invention fully encompasses other embodiments that may become obvious to those skilled in the art and that the scope of the invention is accordingly limited by nothing other than the appended claims.

Claims (26)

1. A system for biological tissue ablation, comprising:

an RF antenna located near a biological tissue site;

a transmission line having a first end connected to the RF antenna and a second end;

an RF signal generator for generating a series of RF pulses within the transmission line for transmission to the RF antenna;

a temperature sensor coupled to the RF antenna;

a controller connected to the RF signal generator and the temperature sensor, the controller having:

a temperature control module configured to compare a detected temperature of the temperature sensor with a preset temperature setting, an

A control module configured to adjust the RF signal until the detected temperature is as close as possible to the preset temperature setting; and

an RF signal detector coupled to the transmission line to detect a reflected signal and a forward signal of the RF pulse,

the controller comprises a processing module configured to calculate a ratio of a detected reverse signal to a detected forward signal of the RF pulses,

the control module is further configured to control the RF signal generator to vary the frequency of the RF pulses such that the calculated ratio varies until the calculated ratio is below a predetermined threshold such that the transmission line impedance substantially matches a combined load impedance common to the RF antenna and the biological tissue.

2. The system of claim 1, wherein the control module is configured to control the RF signal generator to vary the frequency and power level of the RF energy pulses to maintain the calculated ratio at least close to the selected ratio while maintaining the detected temperature at least close to the preset temperature setting.

3. The system of claim 2, wherein the selected ratio is the lowest ratio achievable.

4. The system of claim 2, wherein the selected ratio is a preset threshold.

5. The system of claim 1, wherein the temperature setting is a temperature range.

6. The system of claim 1, wherein the temperature setting is a temperature set point plus or minus a predetermined amount.

7. The system of claim 1, further comprising an alarm device coupled to the controller, the controller further comprising an alarm module configured to compare a detected temperature to a preset maximum temperature and activate the alarm device to generate an alarm when the detected temperature is above the preset maximum temperature.

8. The system of claim 7, wherein the controller is configured to turn off the RF signal generator when the detected temperature is above the preset maximum temperature.

9. The system of claim 7, wherein the controller is configured to compare a detected power level to a preset maximum power level and activate the alarm device to generate an alarm when the detected power level is above the preset maximum power level, the alarm module further configured to turn off the RF signal generator when the power level exceeds the preset maximum power level.

10. The system of claim 1, further comprising a user input module coupled to the controller, the user input module configured to input a user selected parameter by a user.

11. The system of claim 10, wherein the user selected parameters include a temperature setting and a RF signal power level, the controller further configured to maintain the RF signal as close as possible to the user selected power level while maintaining the selected temperature setting.

12. The system of claim 11, wherein the user-selected parameter further comprises an ablation period.

13. The system of claim 1, wherein the RF antenna is flexible.

14. The system of claim 1, wherein the RF antenna is shapeable.

15. The system of claim 1, further comprising a catheter to place the RF antenna to the biological tissue load.

16. A system for biological tissue ablation, comprising:

an RF signal module configured to generate a series of RF energy pulses that are transmitted within the transmission line to an RF antenna disposed proximate to the biological tissue load;

an RF sensor module configured to sense reflected and forward signals of the pulse of RF energy when the RF antenna is placed in proximity to the biological tissue to be ablated;

a temperature sensor module configured to detect a temperature when the RF antenna is placed in proximity to the biological tissue to be ablated; and

a control module connected to the RF signal module, RF sensor module, and temperature sensor module, the control module configured to adjust the series of RF energy pulses to maintain the ratio of reflected power to forward signal power at least near a predetermined ratio and to maintain the detected temperature at least near a preselected temperature setting.

17. The system of claim 16, wherein the control module is configured to determine a ratio of reflected signal power to forward signal power in the transmission line, and the predetermined ratio is the lowest achievable ratio of reflected signal power to forward signal power while maintaining the detected temperature at least close to the selected temperature setting.

18. The system of claim 16, wherein the control module is configured to measure the vswr and the predetermined ratio is the vswr.

19. The system of claim 16, wherein the RF signal module includes an RF oscillator module that generates a series of RF pulses, and an amplifier module connected to an output of the oscillator module, the amplifier module for amplifying the series of RF energy pulses, and the control module is configured to control the oscillator module and the amplifier module to vary a frequency and a power level of the pulses.

20. The system of claim 16, wherein the RF sensor module comprises a bidirectional signal detection module that samples the forward signal and the reflected signal.

21. The system of claim 16, further comprising an elongated catheter inserted into a body vessel, the catheter having a proximal end coupled to the RF signal module, a distal end portion containing the RF antenna located near a selected ablated tissue site, a transmission line extending along the catheter from the proximal end to the RF antenna, the temperature sensor module including a temperature sensor located at the distal end portion of the catheter and a connection line extending along the catheter from the temperature sensor to the proximal end, the connection line connected to a temperature sensor input of the control module.

22. The system of claim 16, further comprising a user input module connected to the control module to receive user control input from a user.

23. The system of claim 16, further comprising an alarm module that provides an alarm output when the output of the sensor is detected to exceed a predetermined maximum limit.

24. A system for biological tissue ablation, comprising:

an RF antenna located near a biological tissue site;

a transmission line having a first end connected to the RF antenna and a second end;

an RF signal generator for generating a series of RF pulses, the RF signal generator having an output coupled to the second end of the transmission line;

an RF signal detector coupled to the transmission line to detect a reflected signal and a forward signal of the RF pulse; and

a controller coupled to the RF signal generator and the RF signal detector and having a processing module configured to calculate a voltage standing wave ratio of the detected reflected signal to the detected forward signal and having a control module for controlling the RF signal generator to vary the frequency of the RF pulses until the calculated voltage standing wave ratio substantially corresponds to a selected voltage standing wave ratio that affects a substantial matching of transmission line impedance to tissue load impedance.

25. The system of claim 24, further comprising a temperature sensor coupled to the RF antenna located near the biological tissue site, the controller having a temperature control module configured to compare a detected temperature of the temperature sensor to a preset temperature setting, the control module further configured to adjust the RF signal until the detected temperature is as close as possible to the preset temperature setting while maintaining the calculated ratio as close as possible to the selected ratio.

26. The system of claim 24, wherein the RF antenna is a shapeable antenna to conform to a contour of a body vessel near a biological tissue load.