Device and method for real-time lesion estimation during ablation

Abstract

Disclosed herein are ablation systems and methods for providing feedback on lesion formation in real-time. The methods and systems assess absorptivity of tissue based on a degree of electric coupling or contact between an ablation electrode and the tissue. The absorptivity can then be used, along with other information, including, power levels and activation times, to provide real-time feedback on the lesions being created. Feedback may be provided, for example, in the form of estimated lesion volumes and other lesion characteristics. The methods and systems can provide estimated treatment times to achieve a desired lesion characteristic for a given degree of contact, as well as depth of a lesion being created. The degree of contact may be measured using different techniques, including the phase angle techniques and a coupling index.

Claims

1. An ablation control system, comprising: a measurement circuit to determine a contact state between an ablation electrode and a tissue to be ablated by measuring a degree of electrical coupling between the ablation electrode and the tissue to be ablated; a clock to measure a time period during which ablative energy is applied by the ablation electrode to the tissue to be ablated; a lesion analysis processor for estimating a volume of a lesion forming in the tissue to be ablated as a function of at least three interdependent variables, wherein the at least three interdependent variables are inversely related for a preset volume of the lesion forming in the tissue to be ablated, and wherein the at least three interdependent variables comprise: the contact state between the ablation electrode and the tissue to be ablated, as measured by the measurement circuit; a power level at which the ablation electrode applies ablative energy to the tissue to be ablated; the measured time period during which the ablation electrode applies ablative energy to the tissue to be ablated; and a controller coupled to the lesion analysis processor and configured to control an ablation power source to create a lesion of the preset volume based on information provided by the lesion analysis processor.

2. The ablation control system according to claim 1, wherein the measurement circuit determines the degree of electrical coupling between the ablation electrode and the tissue to be ablated using phase angles.

3. The ablation control system according to claim 2, wherein the measurement circuit comprises a phase detection circuit.

4. The ablation control system according to claim 2, wherein the measurement circuit comprises: a voltage measurement device for measuring a voltage signal being delivered to the tissue to be ablated; a current measurement device for measuring a current signal being delivered to the tissue to be ablated; and a phase measurement device to determine an amount of phase change between the measured voltage signal and the measured current signal.

5. The ablation control system according to claim 1, further comprising a tissue temperature sensor to measure a temperature of the tissue to be ablated, and wherein the controller is further coupled to the tissue temperature sensor such that, if the temperature of the tissue to be ablated exceeds a maximum desirable temperature, the controller sends a deactivation signal to the ablation power source.

6. The ablation control system according to claim 1, wherein: the lesion analysis processor utilizes a regression analysis of a plurality of previously collected data points, and each of the plurality of previously collected data points comprises information respectively for a plurality of lesions, including at least a degree of electrical coupling, a lesion volume, and an ablation power level.

7. A method of ablating tissue in need of treatment, comprising: delivering ablative power to the tissue in need of treatment via an ablation catheter having at least one electrode coupled to a programmable power source; using a lesion analysis processor, estimating a volume of a lesion forming in the tissue in need of treatment as a function of at least three interdependent variables, wherein the at least three interdependent variables are inversely related for a preset volume of the lesion forming in the tissue to be ablated, and wherein the at least three interdependent variables comprise: a contact state between the ablation catheter and the tissue in need of treatment, as determined by measuring a degree of electrical coupling between the ablation catheter and the tissue in need of treatment; a power level at which the programmable power source delivers ablative power to the tissue in need of treatment; and a measured time period over which ablative power is delivered to the tissue in need of treatment; and controlling the programmable power source based on information provided by the lesion analysis processor to a create a lesion having the preset volume.

8. The method according to claim 7, wherein the lesion analysis processor determines the degree of electrical coupling between the ablation catheter and the tissue in need of treatment using phase angles.

9. The method according to claim 8, wherein the lesion analysis processor determines an amount of phase change between a measurement of a voltage signal being delivered to the tissue in need of treatment and a measurement of a current signal being delivered to the tissue in need of treatment.

10. The method according to claim 7, wherein the lesion analysis processor utilizes a regression analysis of a plurality of previously collected data points, and each of the plurality of previously collected data points comprises information respectively for a plurality of lesions, including at least a degree of electrical coupling, a lesion volume, and an ablation power level.

11. The method according to claim 7, further comprising: measuring a temperature of the tissue in need of treatment while delivering ablative power thereto; and sending a signal from the controller to the programmable power unit to disable the programmable power unit if the measured temperature of the tissue in need of treatment exceeds a maximum desirable temperature.

12. An ablation control system, comprising: a measurement circuit to determine a contact condition between an ablation electrode and a tissue to be ablated; a clock to measure a time period during which ablative energy is applied by the ablation electrode to the tissue to be ablated; a lesion analysis processor for estimating a volume of a lesion forming in the tissue to be ablated as a function of at least three interdependent variables comprising: the determined contact condition between the ablation electrode and the tissue to be ablated; a power level at which the ablation electrode applies ablative energy to the tissue to be ablated; the measured time period during which the ablation electrode applies ablative energy to the tissue to be ablated; and a controller coupled to the lesion analysis processor and configured to control an ablation power source to form a lesion having a preset volume in the tissue to be ablated based on information provided by the lesion analysis processor, wherein the at least three interdependent variables are related for the preset volume according to an equation that approximates lesion volume as a product of the at least three interdependent variables.

13. The ablation control system according to claim 12, wherein the measurement circuit determines a degree of electrical coupling between the ablation electrode and the tissue to be ablated.

14. The ablation control system according to claim 13, wherein the measurement circuit determines the degree of electrical coupling between the ablation electrode and the tissue to be ablated using phase angles.

15. The ablation control system according to claim 14, wherein the measurement circuit comprises a phase detection circuit.

16. The ablation control system according to claim 14, wherein the measurement circuit comprises: a voltage measurement device for measuring a voltage signal being delivered to the tissue to be ablated; a current measurement device for measuring a current signal being delivered to the tissue to be ablated; and a phase measurement device to determine an amount of phase change between the measured voltage signal and the measured current signal.

17. The ablation control system according to claim 12, wherein the equation is of form v≈14.47[f.sub.2(Δφ)×P×t.sub.e], where v is the lesion volume, Δφ is representative of the determined contact condition, P is representative of the power level, t.sub.e is representative of the measured time period, and f.sub.2 is a frequency of the ablative energy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagrammatic illustration of an exemplary tissue ablation system which may be implemented to assess electrode-tissue contact during a tissue ablation procedure for a patient.

(2) FIG. 1a is a detailed illustration of the patient's heart in FIG. 1, showing the electrode catheter after it has been moved into the patient's heart.

(3) FIG. 2a illustrates exemplary levels of electrical contact or coupling between the electrode catheter and a target tissue.

(4) FIG. 2b illustrates exemplary levels of mechanical contact or coupling between the electrode catheter and a target tissue.

(5) FIG. 3 is a high-level functional block diagram showing the exemplary tissue ablation system of FIG. 1 in more detail.

(6) FIG. 4 is a model of the electrode catheter in contact with (or coupled to) target tissue.

(7) FIG. 4a is a simplified electrical circuit model corresponding to the model shown in FIG. 4.

(8) FIG. 5 is an exemplary high level functional block diagram for a phase detection circuit that may be implemented in the tissue ablation system for assessing electrode-tissue contact or coupling.

DETAILED DESCRIPTION OF THE INVENTION

(9) Exemplary embodiments of a tissue ablation system and methods of use to estimate characteristics of lesions being formed are depicted in the figures. As described further below, the tissue ablation system of the present invention provides a number of advantages, including, for example, the ability to estimate lesion characteristics during the ablation process and thereby reduce the likelihood of complications associated with applying excessive ablative energy to a target tissue. The invention also provides for enhanced lesion estimation and formation.

(10) FIG. 1 is a diagrammatic illustration of an exemplary electrode catheter system 10 which may be implemented to estimate lesion characteristics during a tissue ablation procedure for a patient 12. Catheter system 10 includes an electrode catheter 14, which may be inserted into the patient 12, e.g., for forming ablative lesions inside the patient's heart 16. During an exemplary ablation procedure, a user (e.g., the patient's physician or a technician) may insert the electrode catheter 14 into one of the patient's blood vessels 18, e.g., through the leg (as shown in FIG. 1) or the patient's neck. The user, guided by a real-time fluoroscopy imaging device (not shown) or other localization system, moves the electrode catheter 14 into the patient's heart 16 (as shown in more detail in FIG. 1A).

(11) In a preferred embodiment, the localization/mapping system is the EnSite NavX™ navigation and visualization system of St. Jude Medical, Atrial Fibrillation Division, Inc., which generates the electrical fields for locating a catheter. Other localization systems, however, may be used in connection with the present invention, including for example, the CARTO navigation and location system of Biosense Webster, Inc., or the AURORA® system of Northern Digital Inc., both of which utilize magnetic fields rather than electrical fields. The localization and mapping systems described in the following patents (all of which are hereby incorporated by reference in their entireties) can also be used with the present invention: U.S. Pat. Nos. 6,990,370; 6,978,168; 6,947,785; 6,939,309; 6,728,562; 6,640,119; 5,983,126; and 5,697,377.

(12) When the electrode catheter 14 reaches the patient's heart 16, electrodes 20 at the tip of the electrode catheter 14 may be implemented to electrically map the myocardium 22 (i.e., muscular tissue in the heart wall) and locate a target tissue 24. After locating the target tissue 24, the user must move the electrode catheter 14 into contact and electrically couple the catheter electrode 20 with the target tissue 24 before applying ablative energy to form an ablative lesion or lesions. For purposes of this application the term “contact” shall mean that the electrode catheter is sufficiently close to the target tissue such that it is electrically coupled to the target tissue and can transfer sufficient ablation energy to form a lesion; in this sense “contact” does not require, but may include, mechanical contact.

(13) The electrode-tissue contact refers to the condition when the catheter electrode 14 physically touches the target tissue 24 thereby causing a mechanical coupling between the catheter electrode 14 and the target tissue 24. Electrical coupling refers to the condition when a sufficient portion of electrical energy passes from the catheter electrode 14 to the target tissue 24 so as to allow efficient lesion creation during ablation. For target tissues with similar electrical and mechanical properties, electrical coupling includes mechanical contact. That is, mechanical contact is a subset of electrical coupling. Thus, the catheter electrode may be substantially electrically coupled with the target tissue without being in mechanical contact. In other words, if the catheter electrode is in mechanical contact, it is also electrically coupled. The range or sensitivity of electrical coupling, however, changes for tissues with different electrical properties. For example, the range of electrical coupling for electrically conductive myocardial tissue is different from the vessel walls. Likewise, the range or sensitivity of electrical coupling also changes for tissues with different mechanical properties, such as tissue compliance. For example, the range of electrical coupling for the relatively more compliant smooth atrial wall is different from the relatively less compliant pectinated myocardial tissue. The level of contact and electrical coupling are often critical to form sufficiently deep ablative lesions on the target tissue 24 without damaging surrounding tissue in the heart 16. The catheter system 10 may be implemented to assess the level of contact (illustrated by display 11) between the electrode catheter 14 and the target tissue 24, as described in more detail below.

(14) FIG. 2a illustrates exemplary levels of electrical contact or coupling between an electrode catheter 14 and a target tissue 24. FIG. 2b illustrates exemplary levels of mechanical contact or coupling between an electrode catheter 14 and a target tissue 24. Exemplary levels of contact or coupling may include “little or no contact” as illustrated by contact condition 30a, “light to medium contact” as illustrated by contact condition 30b, and relatively “hard contact” as illustrated by contact condition 30c. In an exemplary embodiment, the catheter system 10 may be implemented to display or otherwise output the contact condition for the user, e.g., as illustrated by light arrays 31 a-c corresponding to contact conditions 30a-c, respectively. While the light arrays 31 a-c depict three light levels, any number of light levels may be used to distinguish increasing degrees of contact.

(15) Contact condition 30a (“little or no contact”) may be experienced before the electrode catheter 14 comes into contact with the target tissue 24. Insufficient contact may inhibit or even prevent adequate lesions from being formed when the electrode catheter 14 is operated to apply ablative energy. However, contact condition 30c (“hard contact”) may result in the formation of lesions which are too deep (e.g., causing perforations in the myocardium 22) and/or the destruction of tissue surrounding the target tissue 24. Accordingly, the user may desire contact condition 30b (“light to medium contact”).

(16) It is noted that the exemplary contact or coupling conditions 30a-c in FIG. 2a-b are shown for purposes of illustration and are not intended to be limiting. Other contact or coupling conditions (e.g., finer granularity between contact conditions) may also exist and/or be desired by the user. The definition of such contact conditions may depend at least to some extent on operating conditions, such as, the type of target tissue, desired depth of the ablation lesion, and operating frequency of the RF radiation, to name only a few examples.

(17) FIG. 3 is a high-level functional block diagram showing the catheter system 10 in more detail as it may be implemented to assess contact or coupling conditions for the electrode catheter 14. It is noted that some of the components typical of conventional tissue ablation systems are shown in simplified form and/or not shown at all in FIG. 1 for purposes of brevity. Such components may nevertheless also be provided as part of, or for use with the catheter system 10. For example, electrode catheter 14 may include a handle portion, a fluoroscopy imaging device, and/or various other controls, to name only a few examples. Such components are well understood in the medical devices arts and therefore further discussion herein is not necessary for a complete understanding of the invention.

(18) Exemplary catheter system 10 may include a generator 40, such as, e.g., a radio frequency (RF) generator, and a measurement circuit 42 electrically connected to the electrode catheter 14 (as illustrated by wires 44 to the electrode catheter). The electrode catheter 14 may also be electrically grounded, e.g., through grounding patch 46 affixed to the patient's arm or chest (as shown in FIG. 1).

(19) Generator 40 may be operated to emit electrical energy (e.g., RF current) near the tip of the electrode catheter 14. It is noted that although the invention is described herein with reference to RF current, other types of electrical energy may also be used for assessing contact conditions.

(20) In an exemplary embodiment, generator 40 emits a so-called “pinging” frequency (e.g., low power) as the electrode catheter 14 approaches the target tissue 24. The “pinging” frequency may be emitted by the same electrode catheter that is used to apply ablative energy for lesion formation. Alternatively, a separate electrode catheter may be used for applying the “pinging” frequency. In such an embodiment, the separate electrode may be in close contact with (or affixed to) the electrode for applying ablative energy so that a contact or coupling condition can be determined for the electrode which will be applying the ablative energy.

(21) The system can be used to assess the electrode-tissue contact as described in greater detail in the following applications: U.S. Application No. 60/748,234; International Application No. PCT/US06/46565; and U.S. application Ser. No. 12/096,070; each of the foregoing applications is hereby incorporated by reference in its entirety, as if fully set forth herein. The applications explain in detail how the contact may be assessed using the phase angle component of impedance measurements, or more simply by measuring the phase change between the pinging voltage signal and the pinging current signal.

(22) The present invention can also be utilized with U.S. application Ser. No. 12/253,637, which application is hereby incorporated by reference in its entirety, as if fully set forth herein. This application describes methods used to determine a coupling index indicative of a degree of contact between an electrode and a tissue wherein the coupling index is calculated from the components of complex impedance, such as resistance, reactance, impedance magnitude, and impedance phase angle. One of ordinary skill will appreciate that the coupling index may be utilized with the teachings of the present invention to estimate lesion characteristics during lesion formation.

(23) After the user has successfully guided the electrode catheter 14 into the desired contact or coupling condition with the target tissue 24, a generator, such as generator 40 or a second generator, may be operated to generate ablative (e.g., high power) energy for forming an ablative lesion or lesions on the target tissue 24. In an exemplary embodiment, the same generator 40 may be used to generate electrical energy at various frequencies both for the impedance measurements (e.g., “pinging” frequencies) and for forming the ablative lesion. In alternative embodiments, however, separate generators or generating units may also be implemented without departing from the scope of the invention.

(24) In an exemplary embodiment, measurement circuit 42 may be operatively associated with a processor 50 and memory 52 to analyze the measured impedance. By way of example, processor 50 may determine a phase angle, and based on the phase angle, the processor 50 may determine a corresponding contact or coupling condition for the electrode catheter 14. In an exemplary embodiment, contact or coupling conditions corresponding to various phase angles may be predetermined, e.g., during testing for any of a wide range of tissue types and at various frequencies. The contact or coupling conditions may be stored in memory 52, e.g., as tables or other suitable data structures. The processor 50 may then access the tables in memory 52 and determine a contact or coupling condition corresponding to impedance measurements based on the reactance component and/or phase angle. The contact or coupling condition may be output for the user, e.g., at display device 54. Processor 50 may comprise a conventional general purpose computer, a special purpose computer, a distributed computer, or any other type of computer. Processor 50 may comprise one or more processors, such as a single central-processing unit, or a plurality of processing units, commonly referred to as a parallel processing environment.

(25) Although impedance measurements may be used to determine the phase angle, in an alternative embodiment, the measurement circuit 42 may be implemented as a phase detection circuit to directly determine the phase angle. An exemplary phase detection circuit 80 is shown in FIG. 5 in a high-level functional block diagram. Phase detection circuit 80 is shown and described with reference to its functional components. It is noted that a particular hardware configuration is not necessary for a full understanding of the invention. Implementation of the phase detection circuit 80 in digital and/or analog hardware and/or software will be readily apparent to those having ordinary skill in the electronics art after becoming familiar with the teachings herein.

(26) Exemplary phase detection circuit 80 may include a current sensor 82 and voltage sensor 84 for measuring current and voltage at the electrode-tissue interface. The current and voltage measurements may be input to a phase comparator 86. Phase comparator 86 provides a direct current (DC) output voltage proportional to the difference in phase between the voltage and current measurements.

(27) Optionally, current measurements may be phase shifted by phase shift circuit 88 to facilitate operation of the phase comparator 86 by “correcting” phase lag between the measured current and the measured voltage. Also optionally, output from the phase comparator 86 may be “corrected” by phase adjustment circuit 90 to compensate for external factors, such as the type of grounding patch 46 being used. A signal scaling circuit 92 may also be provided to amplify the output (e.g., from milli-volts to volts) for use by various devices (e.g., the processor 50 and display device 54 in FIG. 3).

(28) It is noted that phase detection circuit 80 shown in FIG. 5 is provided as one example, and is not intended to be limiting. Other implementations may also be readily provided by those having ordinary skill in the electronics arts after becoming familiar with the teachings herein without departing from the scope of the invention.

(29) In another exemplary embodiment, measurement circuit 42 as depicted in FIG. 3 includes circuitry to measure voltage and current for different types of power supplies that are used to assess contact between the catheter and tissues being treated.

(30) Measurement circuit 42 also includes temperature measurement circuits that are used to measure and monitor the temperature of the tissue being treated. Measurement circuit 42 utilizes previously described circuitry and/or additional circuitry to measure and monitor power that is delivered by generator 40 and/or by ablation catheter 20.

(31) In an exemplary embodiment, measurement circuit 42 is operatively associated with a processor 50 and memory 52 to analyze degrees of contact or coupling, temperature changes, power and energy. In an exemplary embodiment, processor 50 is programmed to perform regression analysis as described below. Display device 54 may be used to provide feedback to the operator in graphical or numerical form. For example, processor 50 may calculate and display on display device 54 the volume of a lesion as it is being formed. Display device may also display a target lesion volume so that the target may be compared to the size of the lesion being formed. Preferably, processor 50 is programmed to display tissue temperature and to deactivate the ablation power source when the tissue temperature reaches a desired temperature for lesion formation (e.g., about 50-55° C.).

(32) In tissue ablation, a thermal lesion is created when the tissue undergoes coagulation necrosis at temperatures above about 55° C. In radiofrequency (RF) ablation, lesion formation depends upon a number of parameters, such as: (i) electrode-tissue electrical coupling; (ii) the amount of power applied to the electrode from the RF generator; (iii) the duration of power being applied to the electrode by the RF generator; and (iv) the thermal properties and effects of aspects such as (a) the electrode; (b) the tissue; and (c) the fluid around the electrode.

(33) For purposes of this application, “lesion size” is intended to refer to the volume of the lesion being created, and more particularly, the volume of the tissue that underwent coagulation necrosis. Thus, the term “lesion size” is used interchangeably with “lesion volume.”

(34) The electrical coupling between the electrode and the tissue refers to the fraction of the electrical energy that flows from the electrode into the tissue. Electrical coupling is effected by: (i) the electrode proximity to or mechanical contact with the tissue; (ii) the size (e.g., surface area) of the electrode; and (iii) the electrical properties of: (a) the electrode; (b) the tissue; and (c) the fluid around the electrode.

(35) Creation of thermally mediated lesion using RF current in tissue is generally described by the bio-heat transfer equation, which in its simplified form may be written as,

(36) $\rho c\frac{\partial T}{\partial t}=\nabla .Math.k\nabla T+\mathrm{JE}-Q_h$
where: JE is the specific absorption rate (“SAR”) of energy applied to the tissue; ∇.Math.k∇T is the thermal diffusion effect due to thermal conductivity of the tissue; Q.sub.h is the rate of thermal energy transfer due the convective effects, such as perfusion, blood flow, irrigated cooling; ρc ∂T/∂t is rate of energy accumulation in the tissue, which causes the tissue temperature to increase; T is the tissue temperature; ρ is the tissue density; c is the specific heat capacity of the tissue; ∂T/∂t is the rate of change in tissue temperature; J is the electric current density within the tissue; E is the electric field intensity within the tissue.

(37) The current density J is defined as
J=i/A,
where: i is the RF current; A is the area normal to the flow of the current, and is the effective area of electrical coupling between the electrode and tissue.

(38) J is related to the electric field E as,
J=−σ∇V=σE,
where: σ is the electrical conductivity of the medium; V is the electrical potential; and ∇V is the gradient of electrical potential.

(39) Alternately, SAR may be rewritten as

(40) $\begin{array}{c}JE={.Math.J.Math.}^2/\sigma \\ =i^2/\left[A^2\sigma \right].\end{array}$

(41) Thus an estimation of SAR depends upon the RF current i, the effective area A, and electrical conductivity σ. In energy dosimetry applications of RF ablation, the electrical conductivity σ is a tissue property that is generally known, the RF current i is readily measurable, but A is not readily measurable. The difficulty in measuring A in catheter-based cardiac ablation application is further compounded by: the compliance of the tissue when the catheter is in physical contact with the heart wall; and the contour of the tissue, such as trabeculated tissue, when the electrode may not necessarily be in physical contact with the heart wall.

(42) The phase angle method of measuring electrical coupling provides the reactance X.sub.R of the tissue to the application of RF energy:
X.sub.R=1/(2πfC)
where: f is frequency C is capacitance, which for parallel plate capacitors is given as
C=εA/d
where: ε is permittivity of the medium between the plates A is the cross sectional area between the plates d is the distance between the plates.

(43) When the electrode is in contact with or close to the tissue, ε may be taken as the permittivity of the tissue, d may be approximated as the depth in tissue that yields enough strength of the RF field so as to create a lesion. In that respect, A may be taken as the effective area of electrical coupling.

(44) The contact or coupling area is an important parameter in RF ablation. It determines: (i) the current density and energy density at the electrode-tissue interface; and (ii) the portion of RF energy absorbed in the tissue during ablation. High current density and energy density at the electrode-tissue interface increase the susceptibility to coagulum and thrombus formation. Excessive energy absorption leads to tissue charring and increases the risk of perforation.

(45) A measure of electrode contact or coupling area, therefore, has prime clinical significance in RF ablation. Such information will allow customizing ablation parameters at individual sites by titrating power levels and ablation time necessary to create safe lesions.

(46) A technical advantage of the present invention is to provide lesion size feedback to physicians based on the degree of electrode-tissue contact or coupling, the RF power levels being delivered to the tissue, and the duration of the RF ablation treatment.

(47) Another technical advantage of the present invention is to provide lesion feedback on lesion characteristics to physicians based on electro-physiological changes in the tissue as measured during the RF ablation treatment.

(48) In one embodiment, the present invention comprises an ablation catheter system that permits electrical contact sensing. The catheter system includes: (i) Circuit to measure RF current signal; (ii) Circuit to measure RF voltage signal; and (iii) Circuit to measure phase angle between current and voltage.

(49) The present invention determines the degree of contact sensing based on the measurement of phase angle Δφ between the RF voltage signal and the RF current signal or the complex impedance as measured using the measured voltage and current. The phase angle Δφ changes with the level of electrode-tissue contact. Preferably, the phase angle Δφ between the RF voltage signal and the RF current signal is measured before or shortly after the RF energy source is activated, though the phase angle Δφ can be measured at any point in time and be used to assess varying degrees of contact. The phase angle Δφ also changes with tissue temperature, and so, as more ablation energy is delivered to the tissue, reliance upon changes in phase angle Δφ must take into consideration the effect of the increased tissue temperature. Generally speaking, the magnitude of phase angle Δφ increases as the degree of tissue-electrode contact increases.

(50) The RF voltage signal being used to measure the phase angle may be supplied by the same RF power source that provides the ablation energy or may be provided by a separate independent supply. The frequency of the RF energy used to assess the electrical coupling is preferably between about 1 KHz and about 1000 KHz, and more preferably between about 50 KHz and about 500 KHz, and most preferably between about 400 KHz and about 500 KHz. The power of the RF signal needed to assess electrical coupling is much lower than the power levels required for ablation, and preferably is less than about 1 W, and more preferably less than about 0.001 W. The current amperage is also significantly less than is necessary for effective ablation and is preferably less than 10 mA, and more preferably less than about 1 mA.

(51) An alternative approach to the device describe above that determines phase angle by comparing the voltage signal to the current is to use a circuit that measures the complex impedance, which is the complex sum of resistance and reactance. The phase angle of the complex impedance is the same phase angle that would be measured by comparing the voltage signal to the current signal.

(52) Yet another alternative approach to the device described above that determines phase angle by comparing the voltage signal to the current is to use a circuit that measures a coupling index which is calculated from the measured components of complex impedance, such as resistance, reactance, impedance magnitude, and impedance phase angle.

(53) It is contemplated that mechanical contact sensing and/or electromechanical sensing may be utilized in conjunction with the electrical contact sensors described above, including for example, piezoelectric sensors, PVDF (polyvinylidene fluoride) film based sensors, and fiber optic-based opto-mechanical contact sensors. In the case of an electrode that senses contact with a beating heart wall, the electrode sensor generates a dynamic voltage signal. The dynamic voltage signal obtained from these contact sensors exhibit waveform characteristics similar to the EKG signals that cause myocardial contractions. It is also contemplated that piezoresistive sensors and Quantum Tunneling Composite (QTC) based sensors may be used, where the resistance of the sensor changes with the contact pressure of the electrode with the tissue.

(54) Contact sensing is important to the ablation process because delivery of ablation energy is proportional to improved electrode-tissue contact. In other words, electrode-tissue contact dictates the energy absorption in tissue, and it is energy absorption in tissue that results in an increase in tissue temperature. As discussed above, a goal of ablation treatment is to increase the tissue temperature to a point that creates irreversible electro-physiological changes in the tissue in the formation of a lesion. Lesion formation in tissues by thermo-therapeutic means is achieved by maintaining the tissue temperature above about 50° C.

(55) The total energy ξ.sub.a in Joules applied by the RF generator at power P Watts for a duration of t.sub.e seconds is
ξ.sub.a=P×t.sub.e  (1)

(56) When this energy is applied to an electrode in an endocardial application, part of this energy, ξ.sub.t, is absorbed into the tissue to create the lesion at the target site, while the remainder of the energy, ξ.sub.b, is lost to the surrounding blood, tissue, electrolyte, and/or the electrode:
ξ.sub.a=ξ.sub.t+ε.sub.b  (2)

(57) Alternatively, the portion of applied energy ξ.sub.a absorbed in the tissue ξ.sub.t may be expressed as
ξ.sub.t=αξ.sub.a  (3)
where α is an absorptivity factor with a numerical value between 0 and 1, such that α=0 when all the applied energy is lost without any absorption in the tissue; α=1 when all the energy is absorbed in the tissue.

(58) As the energy ξ.sub.t is absorbed into the tissue, the temperature of the tissue increases. The energy required to create a lesion of volume v.sub.l may be estimated from

(59) $\begin{array}{cc}\begin{array}{c}{\xi }_{t=}{\rho }_l\times v_l\times c_l\times \Delta T\\ \approx {\rho }_l\times v_l\times c_l\times \left(T_l-37\right)\end{array}& \left(4\right)\end{array}$
where ρ.sub.l is the density of the tissue; c.sub.l is the tissue specific heat at constant pressure; ΔT is the temperature rise required to cause tissue ablation; T.sub.l is the temperature for coagulation necrosis and is also the tissue temperature at the lesion boundary. The patient's body temperature is 37° C. prior to application of ablation energy.

(60) Expressed in terms of the lesion volume v.sub.l, Eq. (4) becomes

(61) $\begin{array}{cc}\begin{array}{c}{v}_l=\frac{{\xi }_t}{{\rho }_l\times c_l\times \Delta T}\\ \approx \frac{{\xi }_t}{{\rho }_l\times c_l\times \left(T_l-37\right)}\end{array}& \left(5\right)\end{array}$

(62) Substituting ξ.sub.t from Eq. (3) into Eq. (5)

(63) $\begin{array}{cc}\begin{array}{c}{v}_l=\frac{{\mathrm{\alpha \xi }}_a}{{\rho }_l\times c_l\times \Delta T}\\ \approx \frac{{\mathrm{\alpha \xi }}_a}{{\rho }_l\times c_l\times \left(T_l-37\right)}\end{array}& \left(6\right)\end{array}$

(64) Substituting ξ.sub.a from Eq. (1) into Eq. (6)

(65) $\begin{array}{cc}\begin{array}{c}{v}_l=\frac{\alpha \left(P\times t_e\right)}{{\rho }_l\times c_l\times \Delta T}\\ \approx \frac{\alpha \left(P\times t_e\right)}{{\rho }_l\times c_l\times \left(T_l-37\right)}\end{array}& \left(7\right)\end{array}$

(66) For myocardial tissue ρ.sub.l=1200 kg.Math.m.sup.−3, c.sub.l=3200 J.Math.kg.sup.−1.Math.°C..sup.−1. Taking the T.sub.l=55° C., the temperature rise required to cause tissue ablation is ΔT≈(55−37)° C., or simply, 18° C. Eq. (7) then reduces to:

(67) $\begin{array}{cc}\begin{array}{c}{v}_l=\frac{\alpha \left(P\times t_e\right)}{1200\times 3200\times 18}{m}^3\\ \approx \frac{\alpha \left(P\times t_e\right)}{69.12\times {10}^6}{m}^3\\ \approx 14.47\left(\alpha \times P\times t_e\right){\mathrm{mm}}^3\end{array}& \left(8\right)\end{array}$

(68) Eq. (8) shows that the main variables determining lesion volume are (i) power P, (ii) duration t.sub.e, and (iii) absorptivity factor α.

(69) The absorptivity factor α depends upon a number of factors that mainly include (i) the degree of electrode-tissue contact ζ, (ii) tissue characteristics χ, and (iii) ambient flow conditions ℑ at the site of the contact:

(70) $\begin{array}{cc}\begin{array}{c}\alpha =f\left\{\begin{array}{c}\mathrm{Electrode}\text{-}\mathrm{tissue}\mathrm{contact}),\left(\mathrm{Tissue}\mathrm{characteristics}\right),\\ \left(\mathrm{Flow}\mathrm{conditions}\right),\mathrm{.Math.}\end{array}\right\}\\ =f\left\{\begin{array}{cccc}\zeta ,& \chi ,& ℱ,& \mathrm{.Math.}\end{array}\right\}\end{array}& \left(9\right)\end{array}$

(71) In the absorptivity factor, tissue characteristics and ambient flow conditions at the site of the electrode-tissue contact are physiological parameters. On the other hand, level of contact between the electrode and the tissue is established by the physician, and is, therefore, operator dependent.

(72) Limiting the dependency of the lesion volume to operator dependent variables, Eq. (8) can be rewritten to include the aspects of contact sensing as:
v.sub.l≈14.47[f.sub.1(ζ)×P×t.sub.e]mm.sup.3  (10)
where 0≦f.sub.1(ζ)≦1.

(73) For RF energy applications, phase change Δφ between the voltage and current is directly related to the level of electrode-tissue contact
Δφζ  (11)

(74) The lesion volume can then be related to phase change Δφ due to electrode-tissue contact as
v.sub.l≈14.47[f.sub.2(Δφ)×P×t.sub.e]mm.sup.3  (12)
where 0≦f.sub.2(Δφ)≦1.

(75) Eq. (12) shows that the lesion volume v.sub.l is a function of three variables: (i) phase change Δφ due to electrode-tissue contact ζ; (ii) power P; and (iii) duration t.sub.e.

(76) To determine the functional dependence of any one variable on lesion volume v.sub.l will require multiple data points as part of a validation study. Preferably, the validation study is created using a plurality of lesions in which information may be gained to permit estimation a priori. For ease of concept, a plurality of lesions can be made using various different levels of contact as follows: Preset power P, i.e. in the power control mode; Preset duration t.sub.e; and Different levels of contact, such as Δφ.sub.1, Δφ.sub.2, . . . Δφ.sub.n.

(77) As part of the validation study, the ablation system may be operated in power control mode, in which the ablation energy is delivered at a fixed power level. Each of the plurality of different levels of contact is determined by measuring the phase angle Δφ between the RF voltage signal and the RF current signal at a point in time before or shortly after the RF energy source is activated. If the phase angle is measured before the commencement of ablation energy, the changes in phase angle can be more easily compared across the plurality of lesions because each is measured at the same tissue temperature. While the phase measurement may be made shortly after the ablation energy is activated, the measurement is preferably made before any appreciable increase in temperature. Because the phase angle Δφ varies with changes in tissue temperature, once the ablation energy begins to cause an increase in tissue temperature, the impact on the phase angle as a measure of electric coupling will require adjustments.

(78) Generally speaking, in this set-up, only one variable is changing over the creation of the plurality of different lesions, namely the degree of contact (which causes a variation in the phase angle). At the end of each ablation process, for each of the plurality of lesions: Lesion volume v.sub.l is measured; Values of v.sub.l, P, and t.sub.e are substituted in Eq. (12) to obtain the value of f.sub.2(Δφ); and Values of f.sub.2(Δφ) are plotted against Δφ and a regression curve f.sub.2R(Δφ) is computed.

(79) The volume v.sub.l is measured using known volumetric measurements. For example, in the case of a hypothetical rectangular lesion, volume may be determined by multiplying width by length by depth. Depending on the shape, other formulas and/or measurements may be made using known volumetric techniques. Typically, the length and width of the lesion are in proportion to the electrode size, and accordingly, a lesion depth may be estimated using lesion length and lesion width.

(80) The collection of data points for the validation study is preferably generated, preferably, using a plurality of similar tissue samples. Of course, the tissue samples may be from the same human, multiple humans and/or non-humans. In addition, while the validation study may be completed in advance, the validation study (as well as any regression analysis of the underlying data) may continually be updated using additional data points as they are collected. As one example, the validation study may utilize several hundred data points performed in a pre-clinical animal study, upon which a regression analysis may be performed; additional data points may be added to the study as the additional data points are collected, and the associated regression analysis may be continually updated.

(81) A graph or lookup table for f.sub.2(Δφ) can be generated using a known regression analysis techniques that can be run on the plurality of data points previously collected; the resulting graph of lookup table is referred to as regression curve f.sub.2R(Δφ) or, more simply, f.sub.2R(Δφ). The f.sub.2R(Δφ) data can then be used to provide lesion size feedback in real-time for various power settings such as power control mode operations for various levels of contact or coupling.

(82) For example, once the regression analysis has been completed, lesion size feedback can be provided on future ablation treatment. An ablation catheter may be placed in contact with tissue to be treated, and the degree of electrical coupling can be determined by measuring the phase change Δφ between the voltage and current curves as described above. Using the f.sub.2R(Δφ) data embodied in the regression curve, the measured Δφ can be used to determine a value of f.sub.2(Δφ). The determined value f.sub.2(Δφ) can then be substituted into Eq. (12), along with the value of power P, to estimate the size of lesion development v.sub.l as a function of time. This methodology is useful for providing lesion size feedback throughout the ablation process.

(83) The characteristics of a lesion formed during ablation depends upon the electro-physiological changes that occur in the tissue during the ablation process, including for example, the temperature attained by the tissue. The combination of lesion size feedback and tissue temperature is most useful to increasing the efficacy of an ablation treatment, and the present invention provides real-time feedback on the characteristics of the lesion being formed as the surgeon performs the ablation process.

(84) In an exemplary embodiment of the present invention, an ablation catheter of the ablation system is placed in contact with tissue to be treated at a particular level of contact. The degree of contact is determined by applying a first power signal and then measuring the phase angle between the voltage signal and the current signal; alternatively the degree of contact may be determined by measuring the complex impedance using the applied first power signal, and in particular, using the phase angle associated with the complex impedance. Ablation energy is then delivered at the same degree of contact to create a lesion, and the system estimates on a real-time basis the size and other characteristics of the lesion being created by using at least the following: i) information obtained through regression analysis; ii) information related to the degree of contact; iii) information related to the power of ablation energy being delivered; and iv) the amount of time for which the ablation energy has been activated. Optionally, the lesion volume of the lesion being created may be estimated and the treatment time period (for which ablation energy is to be applied) is adjusted based on the estimated lesion volume in an effort to achieve particular lesion characteristics. For example, adjustments may be made to the ablation power source in order to slow the delivery of ablation energy to help reduce the risk of tissue pop.

(85) While the degree of contact is preferably measured before any significant increase in temperature from the delivery of ablation energy (for example, by measuring before the ablation energy is activated), it is contemplated that the degree of contact may be measured throughout the ablation process. Of course, to compare degrees of contact will require adjustments based on changes in tissue temperature. Generally speaking, the phase angle associated with a particular degree of contact will reduce in magnitude as the temperature of the tissue increases.

(86) Accounting For Changes In Tissue Temperature.

(87) Preliminary in vitro studies show that at any contact level, phase angle is also affected by changes in tissue temperature. In other words, phase angle change is a function of tissue temperature as well as electrical coupling—the latter of which was discussed in detail above:
Δφ=Δφ(ζ,ΔT).  (12)

(88) Including such effects, Eq. (7) can be recast in a form indicative of electro-physiologic changes in the lesion volume v.sub.l obtained by subjecting the tissue to a temperature increase ΔT; thus

(89) $\begin{array}{cc}{v}_l=\frac{f_2\left\{\Delta \phi \left(\zeta ,\Delta T\right)\right\}\times P\times t}{{\rho }_l\times c_l\times \Delta T}& \left(14\right)\end{array}$

(90) In Eq. (14) the lesion volume v.sub.l is volume of the tissue that underwent coagulation necrosis; P is the applied power; t is the duration; ρ.sub.t is the density of the tissue; c.sub.l is the tissue specific heat at constant pressure; ΔT is the rise in tissue temperature obtained during ablation.

(91) While the previous discussion utilized a fixed power P (such as will occur in a power-controlled mode of ablation), another embodiment of the present invention may be utilized with varying levels of power, such as may occur in a temperature-controlled mode of ablation. In this embodiment, the equations may be modified to account for variations in power as a function of time (which will be adjusted by using the function P(t)). This introduces another variable into the system of equations, which variable requires adjustments in the equations.

(92) Under temperature control mode, the applied power P is generally not constant but varies with time when the preset maximum power is higher than needed to attain the tissue temperature. In that case, the total energy ξ.sub.a applied by the RF generator cannot be expressed as P×t.sub.e. Instead, ξ.sub.a is obtained from the time integral of P as

(93) 0$\begin{array}{cc}{\xi }_a={\int }_0^{t_e}P\left(t\right)dt& \left(15\right)\end{array}$

(94) Rewriting Eq. (14) for temperature control mode operation

(95) $\begin{array}{cc}\begin{array}{c}{v}_l=\frac{f_2\left\{\mathrm{\Delta \phi }\left(\xi ,\Delta T\right)\right\}\times {\xi }_a}{{\rho }_l\times c_l\times \Delta T}\\ =\frac{f_2\left\{\mathrm{\Delta \phi }\left(\xi ,\Delta T\right)\right\}\times {\int }_0^{t_e}P\left(t\right)dt}{3.84\times {10}^6\times \Delta T}{m}^3\\ =\frac{260.4\times f_2\left\{\mathrm{\Delta \phi }\left(\xi ,\Delta T\right)\right\}\times {\int }_0^{t_e}P\left(t\right)dt}{\Delta T}{\mathrm{mm}}^3\end{array}& \left(16\right)\end{array}$
In Eq. (16), ΔT=(T.sub.set−37)° C., where T.sub.set is the preset temperature in the temperature control mode; t.sub.e is the duration at the instant when the recorded temperature T first attains T.sub.set.

(96) Eq. (16) shows that the lesion volume v.sub.l is a function of four variables: (i) temperature rise ΔT (ii) phase change Δφ due to electrode-tissue contact ζ (iii) phase change Δφ due to temperature rise ΔT (iv) total applied energy ξ.sub.a

(97) To determine the functional dependence of any one variable on lesion volume v.sub.l will require multiple data points as part of a validation study. Preferably, the validation study is created using a plurality of lesions in which information may be gained to permit estimation a priori. For ease of concept, a plurality of lesions can be made using various different levels of contact as follows: Preset cutoff temperature T.sub.set, i.e. in the temperature control mode Preset maximum power P.sub.max Different levels of contact, such as Δφ.sub.1, Δφ.sub.2, . . . Δφ.sub.n

(98) At the end of the creation of each of the plurality of lesions: At t=t.sub.e, phase change Δφ (ΔT)=Δφ|.sub.t=te due to temperature rise ΔT=(T.sub.set−37° C. is recorded Lesion volume v.sub.l is measured Values of v.sub.l, P(t), t.sub.e, and ΔT are substituted in Eq. (16) to obtain the value of f.sub.2(Δφ) Values of f.sub.2(Δφ) are plotted against Δφ(ζ)=Δφ|.sub.t=0 and ΔT to compute the regression curve f.sub.2R[Δφ(ζ, ΔT)]

(99) The volume v.sub.l is measured using known volumetric measurements. For example, in the case of a hypothetical rectangular lesion, volume may be determined by multiplying width by length by depth. Depending on the shape, other formulas and/or measurements may be made using known volumetric techniques. Typically, the length and width of the lesion are in proportion to the electrode size, and accordingly, a lesion depth may be estimated using lesion length and lesion width.

(100) The collection of data points for the validation study is preferably generated, using a plurality of similar tissue samples. Of course, the tissue samples may be from the same human, multiple humans and/or non-humans. In addition, while the validation study may be completed in advance, the validation study (as well as any regression analysis of the underlying data) may continually updated using additional data points as they are collected. As one example, the validation study may utilize several hundred data points performed in a pre-clinical animal study.

(101) A graph or lookup table for f.sub.2[Δφ(ζ, ΔT)] can be generated using a known regression analysis techniques that can be run on the plurality of data points previously collected; the resulting graph of lookup table is referred to as regression curve f.sub.2R[Δφ(ζ, ΔT)] or, more simply, f.sub.2R[Δφ(ζ, ΔT)]. The f.sub.2R[Δφ(ζ, ΔT)] data can then be used to provide lesion size feedback in temperature control mode operations for various levels of contact or coupling.

(102) For example, once the regression analysis has been completed, lesion size feedback can be provided on future ablation treatment. An ablation catheter may be placed in contact with tissue to be treated, and the degree of electrical coupling can be determined by measuring the phase change Δφ between the voltage and current curves as described above. Using the f.sub.2R[Δφ(ζ, ΔT)] data embodied in the regression curve, the measured Δφ can be used to determine a value of f.sub.2[Δφ(ζ, ΔT)]. The determined value f.sub.2[Δφ(ζ, ΔT)] can then be substituted into Eq. (16), along with the monitored information on P(t) and ΔT to estimate the size of lesion development v.sub.l as a function of time. This methodology is useful for providing lesion size feedback throughout the ablation process.

(103) In an exemplary embodiment of the present invention, an ablation catheter of the ablation system is placed in contact with tissue to be treated at a particular level of contact. The degree of contact is determined by applying a first power signal and then measuring the phase angle between the voltage signal and the current signal; alternatively the degree of contact may be determined by measuring the complex impedance using the applied first power signal, and in particular, using the phase angle associated with the complex impedance. Ablation energy is then delivered at the same degree of contact to create a lesion using a mode of operation in which the ablation power level may vary over time. The system estimates on a real-time basis the size and other characteristics of the lesion being created by using at least the following: i) information obtained through regression analysis; ii) information related to the degree of contact; iii) information related to the total amount of ablation energy being delivered over time, including variations of power over time; and iv) the amount of time for which the ablation energy has been activated. Optionally, the estimation may be based, in part, on changes in tissue temperature, and changes in phase angle due to changes in tissue temperature. The lesion volume of the lesion being created may be estimated and the treatment time period (for which ablation energy is to be applied) may be adjusted based on the estimated lesion volume in an effort to achieve particular lesion characteristics. For example, adjustments may be made to the ablation power source in order to slow the delivery of ablation energy to help reduce the risk of tissue pop.

(104) In an exemplary embodiment of the present invention, a system may be used to estimate treatment times to achieve a lesion having certain characteristics, for example, a lesion with a particular depth.

(105) In another exemplary embodiment of the present invention, a system may be used to estimate lesion characteristics for a given treatment time period. For example, lesion depth may be estimated in real time using the time for which an ablation treatment has been on-going. As discussed above, lesion volume may be estimated as a function of time (namely, the time for which an ablation treatment has been active). Also as discussed above, the length and width of the lesion are in proportion to the electrode size, and can be estimated for any given electrode shape. Once the width and length has been estimated for an electrode (for example, but using a regression analysis on detailed volumetric measurements which include, width, length, depth and a shape factor), lesion depth may be estimated on a real-time basis.

(106) While the degree of contact is preferably measured before any significant increase in temperature from the delivery of ablation energy (for example, by measuring before the ablation energy is activated), it is contemplated that the degree of contact may be measured throughout the ablation process.

(107) One of ordinary skill will appreciate that the coupling index described in U.S. application Ser. No. 12/253,637, referenced and incorporated above, may also be utilized with the teachings of the present invention to estimate absorptivity factor (α). The coupling index can be calculated from the components of complex impedance, such as resistance, reactance, impedance magnitude, and impedance phase angle, and can be used to estimate absorptivity as described above. Once an absorptivity factor (α) has been estimated, the absorptivity can be used to estimate lesion characteristics, including for example, lesion volume (including width, length, and depth) as well as to estimate time needed to complete treatment in order to achieve lesions having certain characteristics.

(108) While the power source used for ablation is described in certain embodiments above as being an RF power source, the principles of the present invention are applicable to other power sources.

(109) While the power source used for assessing tissue-electrode contact is described in certain embodiments above as being an RF power source, the principles of the present invention can be used with any alternating current power source, including very low frequency, low frequency and high frequency power sources. It is anticipated that the circuits describe above may be modified to accommodate the changes in frequency and to filter out noise that may otherwise be present.

(110) Although several embodiments of this invention have been described above with a certain degree of particularity, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention.

(111) All directional references (e.g., upper, lower, upward, downward, left, right, leftward, rightward, top, bottom, above, below, vertical, horizontal, clockwise, and counterclockwise) are only used for identification purposes to aid the reader's understanding of the present invention, and do not create limitations, particularly as to the position, orientation, or use of the invention. Joinder references (e.g., attached, coupled, connected, and the like) are to be construed broadly and may include intermediate members between a connection of elements and relative movement between elements. As such, joinder references do not necessarily infer that two elements are directly connected and in fixed relation to each other. It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not limiting. Changes in detail or structure may be made without departing from the spirit of the invention as defined in the appended claims.