A lesion is formed based on a curve of a pre-determined amount of ablation energy versus lesion size

Abstract

The present invention is titled "Forming a lesion based on a curve of a pre-determined amount of ablation energy and lesion size". The present invention provides an ablation method that includes storing in a memory a pre-determined relationship between lesion size and the amount of ablation energy for each of one or more selected temperatures. Using a processor, receive user input indicating lesion size and tissue temperature. Based on the relationship, determine the amount of energy that matches the lesion size and tissue temperature. Control an ablation probe to apply the amount of ablation energy that matches the selected lesion size.

Description

Technical Field

[0001] The present invention generally relates to radiofrequency (RF) ablation, and more particularly, to cardiac RF ablation. Background Art

[0002] Various techniques for planning RF ablation have been proposed in the patent literature. For example, U.S. Patent Application Publication 2011 / 0144524 describes a system for displaying characteristics of a target tissue during an ablation procedure. The system includes an electronic control unit (ECU) configured to receive data regarding the electrical properties of the target tissue over a period of time. The ECU is also configured to determine, in response to the data, values indicative of at least one of: a predicted depth of a lesion in the target tissue, a predicted temperature of the target tissue, and a likelihood of steam pops in the target tissue over the period of time. The system further includes a display device configured to receive the values and display a visual representation of the respective indicative parameters listed above.

[0003] As another example, U.S. Patent Application Publication 2014 / 0243813 describes ablation systems and methods for providing real-time feedback regarding lesion formation. These methods and systems evaluate the absorption rate of tissue based on the degree of electrical coupling or contact between an ablation electrode and the tissue. The absorption rate, along with other information including power level and activation time, can then be used to provide real-time feedback regarding the lesion being created. The feedback can be provided, for example, in the form of an estimated lesion volume and other lesion characteristics. These methods and systems can provide an estimated treatment time to achieve desired lesion characteristics for a given degree of physical contact and depth of the lesion created.

[0004] U.S. Patent Application Publication 2014 / 0194869 describes a method and apparatus that utilizes force-time integration to estimate lesion size in a catheter-based ablation system in real time. The apparatus measures the force applied to the target tissue by a contact ablation probe and integrates the force over the energization time of the ablation probe. The force-time integration can be calculated and utilized to provide a real-time estimate of the ablated lesion size (depth, volume, and / or area). The force-time integration can also account for changes in the power delivered to the target tissue in real time to provide an improved estimate of the lesion size. In one embodiment, the force measurement can be used as feedback to establish a desired power level delivered to the probe to prevent steam pops.

[0005] U.S. Patent Application Publication 2017 / 014181 describes a method that includes ablating tissue for a period of time, measuring the contact force applied during that period of time, and measuring the power used during that period of time. The method also includes stopping ablating the tissue when a desired size of a lesion generated in the tissue is reached, the desired size being as estimated by the integral over the period of time of the product of the contact force raised to a first non-uniform exponent and the power raised to a second non-uniform exponent. SUMMARY OF THE INVENTION

[0006] One embodiment of the present invention provides an ablation method that includes storing in a memory a pre-determined relationship between lesion size and amount of ablation energy for each of one or more selected temperatures. Using a processor, receiving user input indicating lesion size and tissue temperature. Based on the relationship, determining the amount of energy that matches the lesion size and tissue temperature. Controlling an ablation probe to apply the amount of ablation energy that matches the selected lesion size.

[0007] In some embodiments, the selected tissue temperature includes the temperature of an ablation electrode to which ablation energy is applied.

[0008] In some embodiments, the method further includes presenting to the user the indicated lesion size and tissue temperature and the determined amount of energy.

[0009] In one embodiment, determining the amount of energy includes reading at least a portion of the relationship from a look-up table.

[0010] According to an embodiment of the present invention, there is further provided herein a system for ablation that includes a memory and a processor. The memory is configured to store a pre-determined relationship between lesion size and amount of ablation energy for each of one or more selected temperatures. The processor is configured to receive user input indicating lesion size and tissue temperature, determine the amount of energy that matches the lesion size and tissue temperature based on the relationship, and control an ablation probe to apply the amount of ablation energy that matches the selected lesion size.

[0011] The present invention will be more fully understood in conjunction with the accompanying drawings, through the following detailed description of embodiments of the present invention, wherein: BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a schematic illustration of a system for cardiac radiofrequency (RF) ablation therapy according to an embodiment of the present invention;

[0013] Figure 2 is a graph schematically showing a pre-determined relationship between lesion size and RF ablation energy at different constant temperatures; and

[0014] Figure 3 is a flowchart schematically showing a method of planning RF ablation using the relationship of Figure 2 . DETAILED DESCRIPTION

[0015] Overview

[0016] Treatment of arrhythmias can include ablating lesions in cardiac tissue using a thermal energy source (e.g., by heating the tissue). The clinical efficacy of the lesion depends to a large extent on the depth of the lesion, which is determined by the amount of effective (e.g., useful) ablation energy deposited at the location where the lesion is formed in the tissue.

[0017] However, the effective ablation energy cannot be accurately estimated because it depends on, for example, unknown tissue properties such as fat content. Thus, for a given energy output of a generator such as a radio frequency (RF) generator, the resulting effective RF energy may be too low or too high. Low effective ablation energy may result in an insufficiently deep lesion, while too high effective ablation energy may cause side effects such as steam pops (e.g., due to very high tissue temperatures), as well as side effects such as tissue perforation and collateral damage.

[0018] The tissue temperature during ablation is actually considered an indication of the effective RF energy deposited, where a higher temperature indicates the formation of a deeper lesion. Thus, pre-determining the target tissue temperature to be maintained during ablation can help to simultaneously achieve the target lesion depth and avoid side effects (such as those listed above).

[0019] The embodiments of the present invention described below provide a method for accurately pre-determining and controlling both tissue temperature and lesion size (e.g., lesion depth) during thermal ablation such as (RF) ablation. The disclosed method includes planning ablation in an energy mode, where, to meet the two goals of lesion depth and tissue temperature, the processor uses a pre-determined relationship between lesion depth and output energy at different constant temperatures to select the corresponding amount of ablation RF energy to be applied to the tissue.

[0020] The disclosed pre-determined relationship can be derived from a model and / or based on calibration. For example, such relationships can be pre-measured in vitro (and / or using an animal model) and stored in the memory of the ablation system. Using the pre-determined relationship, the disclosed method enables the processor to plan ablation based on only two measured parameters (the energy output of the generator and the tissue temperature).

[0021] For example, in one embodiment, the physician selects (a) the depth of the target lesion, and (b) the tissue temperature during ablation (e.g., 50 °C), which is a temperature low enough such that it is known that side effects such as steam pops do not occur in tissue at that temperature. The processor, operated by the physician, then extracts the pre - determined relationship between the lesion depth and the amount of ablation energy at the selected temperature, which can be in the form of a look - up table, and determines the amount of ablation energy required to achieve the selected lesion depth at the selected temperature based on that relationship.

[0022] In a subsequent ablation procedure, based on that selection, the ablation system applies the algorithm as described below, where the processor controls the ablation probe to apply an amount of ablation energy that matches the selected lesion size. In one embodiment, instead of attempting to control multiple parameters, which can include, for example, instantaneous RF power and contact force, the processor can use one or two additional control parameters, such as the flush flow rate. In one embodiment, the processor is configured to operate an algorithm to determine whether the ablation is proceeding as planned and to control the ablation based on feedback from readings of two measured parameters, namely the energy output of the generator and the tissue temperature.

[0023] In some embodiments, if the applied RF power level is reduced by the processor, e.g., to meet the target tissue temperature, the processor is configured to extend the ablation time such that a selected amount of ablation RF energy is fully delivered, thereby achieving the target lesion depth.

[0024] Using the disclosed pre - determined relationship, the described RF ablation planning technique can achieve the target lesion depth while maintaining tissue temperature and can thus improve the efficacy and safety of catheter - based RF ablation procedures.

[0025] System Description

[0026] Figure 1 FIG. 16 is a schematic illustration of a system 12 for cardiac radiofrequency (RF) ablation therapy according to an embodiment of the present invention. Generally, the memory 45 of the system 20 stores multiple ablation protocols for different clinical scenarios, such as Figure 2 the protocols described in

[0027] The physician 26 inserts the catheter 28 through a blood vessel into a chamber of the heart 24 of the subject 22 and manipulates the catheter to bring the distal end 32 of the catheter into contact with the endocardial area to be treated. The tip electrode 51 of the catheter 28, seen in inset 25, includes one or more temperature sensors 50.

[0028] After positioning the distal end 32 at the ablation site and ensuring contact of the tip with the endocardium, the operator 26 activates the RF energy generator 44 in the console 42 to supply RF energy to the distal end 32 via the cable 38. At the same time, the flush pump 48 supplies a cooling fluid such as a saline solution to the distal end through the lumen in the tube 40 and the catheter 28. Typically, before and during ablation, the display 46 shows the values of ablation parameters to the physician 26, such as those listed in Tables I - IV below.

[0029] The operation of the adjustable RF energy generator and the flush pump can be adjusted to provide an appropriate volume of flush fluid during ablation to cool the catheter tip and the tissue, without adding too much flush fluid to the heart. Each temperature sensor inside the temperature sensor 50 provides feedback to the console 42 for, for example, controlling the RF power and / or the flush flow rate to maintain a given tissue temperature.

[0030] To operate the system 12, the processor 41 includes a plurality of modules that the processor uses to operate the system. These modules include a temperature module 52, a power control module 54, and a flush module 55, and the functions of these modules are described below. Specifically, the processor 41 runs a dedicated algorithm as disclosed herein and included in Figure 3 which enables the processor 41 to perform the disclosed steps, as further described below.

[0031] Although the illustrated embodiments specifically relate to using a tip ablation device to ablate heart tissue, the methods described herein can alternatively be applied to ablation devices including multiple ablation electrodes when the operation of each electrode is independently controlled by the processor 41.

[0032] Form a lesion based on a curve of a pre-determined amount of ablation energy versus lesion size

[0033] Figure 2 is a graph schematically showing a predetermined relationship 100 between lesion size and RF ablation energy at different constant temperatures according to an embodiment of the present invention. As shown, the relationship 100 includes a set of curves 60 to 64, where each curve gives the expected lesion depth as a function of the output RF energy of the generator 44 and is at three different constant tissue temperatures T1 < T2 < T3.

[0034] For example, at the output energy level 75, temperature T1 corresponds to a lesion depth 70, T2 corresponds to a lesion depth 72, and T3 corresponds to a lesion depth 74. Thus, maintaining a lower tissue temperature during ablation results in a shallower lesion.

[0035] As Figure 2As further seen, not only do different temperatures T1, T2, and T3 respectively correspond to tissue depths 70, 72, and 74, but overall different lesion sizes (e.g., volumes) 70a, 72a, and 74a respectively correspond to said tissue depths.

[0036] In some cases, e.g., if the risk of steam burst is less significant, the user can choose to achieve the same lesion depth from different temperature curves. Based on the disclosed relationships, such a choice is equivalent to using different amounts of effective ablation energy at each temperature. This can be seen from ablation energies 75, 77, and 77, which all produce the same lesion depth and are represented by points 70, 82, and 84 on curves 60 to 64 respectively.

[0037] A method of ablation in an energy mode, which can vary RF power and flush flow rate (and, contrary to the technology disclosed herein, also allows temperature variation), is described in a U.S. patent application titled “Energy-Guided Radiofrequency (RF) Ablation” (Attorney Docket No.: BIO6070USNP1 / 1002-1904), which is assigned to the assignee of the present patent application and the disclosure of which is incorporated herein by reference.

[0038] Finally, the tissue temperature can be affected by the effective energy transferred by electrode 51 through tissue surface 58. The effectiveness of electrode 51 in directly transferring energy to the tissue below it can depend on the contact force applied by electrode 51 on tissue surface 58.

[0039] Figure 3 is a flowchart of a method for planning RF ablation using Figure 2 curves according to an embodiment of the present invention. The process begins with physician 26 uploading a predetermined relationship 100 at a predetermined relationship upload step 90, the predetermined relationship including lesion depth as a function of ablation energy, such as curves 60 to 64. Next, physician 26 selects a lesion depth at a lesion depth selection step 92. Physician 26 further selects a target tissue temperature at a tissue temperature selection step 94. Based on steps 90 to 94, at an energy selection step 96, a processor 41 operated by physician 26 uses the disclosed relationships (e.g., a stored look-up table) to extract the required energy. Then, at a parameter setting step 98, the physician uses the user interface to set the selections into system 20 as inputs to an ablation algorithm applied by system 20.

[0040] In some embodiments, the processor presents the above selections in one of Tables I-IV, e.g., on a display of system 20. Generally, the ranges of allowable power and flush flow rate are automatically set by the system.

[0041] Tables I-IV provide four different settings that can be used to optimize lesion depth while minimizing collateral damage, depending on the clinical need:

[0042] Table I - Low Depth

[0043] Table II - Medium Depth

[0044] Table III - High Depth

[0045] Table IV - Ultra-High Depth

[0046] Low depth parameters:

[0047]

[0048] Table I

[0049] Medium depth parameters:

[0050]

[0051] Table II

[0052] High depth parameters:

[0053]

[0054] Table III

[0055] Ultra-high depth parameters:

[0056]

[0057] Table IV

[0058] The relationship upload step 90 is implemented before the physician 26 performs ablation.

[0059] At a subsequent ablation session 100, the system 20 uses the selected parameters based on the disclosed relationships (e.g., curves 60 to 64 that can be provided as a look-up table) to achieve the desired lesion depth while maintaining the target tissue temperature.

[0060] The display of the system 20 can be further configured to display the progress of RF delivery to the electrode to the physician 26 by methods known in the art.

[0061] Figure 3The exemplary flowcharts shown are chosen solely for clarity of concept. This implementation also includes additional steps of the algorithm, such as checking the level of contact force of the electrode 51 with the tissue. In one implementation, during a subsequent ablation procedure, the processor applying the disclosed planning method is configured to monitor the actual tissue temperature to maintain the temperature within a given tolerance. During ablation, both the flush flow rate and the RF power output level can be automatically adjusted by the processor to keep the tissue temperature within a given tolerance.

[0062] Although the implementations described herein mainly discuss cardiac applications, the methods and systems described herein can also be used, for example, to plan ablation of other organs of the body.

[0063] Accordingly, it should be understood that the implementations described above are cited by way of example, and the present invention is not limited to what is specifically shown and described above. Instead, the scope of the present invention includes combinations and sub - combinations of the various features described above, as well as their variations and modifications, which would occur to those skilled in the art upon reading the above description and which are not disclosed in the prior art. Documents incorporated by reference into this patent application are considered an integral part of this application, except that if any terms defined in these incorporated documents conflict with the definitions expressly or implicitly given in this specification, only the definitions in this specification shall be considered.

Claims

1. A system for ablation, the system comprising: a memory configured to store a pre-determined relationship between lesion size and amount of ablation energy for each of one or more selected temperatures; and a processor configured to: receive user input indicative of lesion size and tissue temperature; determine, based on the relationship, the amount of energy that matches the lesion size and the tissue temperature; and control an ablation probe to apply the amount of ablation energy that matches the selected lesion size; wherein controlling the ablation probe to apply the amount of ablation energy comprises: monitoring the actual tissue temperature; controlling the ablation power to maintain the tissue temperature within a given tolerance; and controlling the ablation time such that the amount of ablation energy is fully delivered.

2. The system of claim 1, wherein the selected tissue temperature comprises the temperature of an ablation electrode at which the ablation energy is applied.

3. The system of claim 1, wherein the processor is further configured to present to the user the indicated lesion size and tissue temperature and the determined amount of energy.

4. The system of claim 1, wherein the pre-determined relationship is stored in a look-up table.

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