Patent Publication Number: US-2020275972-A1

Title: Forming a lesion based on pre-determined amount of abaltive energy vs lesion size curve

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to radiofrequency (RF) ablation, and particularly to cardiac RF ablation. 
     BACKGROUND OF THE INVENTION 
     Various techniques for planning RF ablation were proposed in the patent literature. For example, U.S. Patent Application Publication 2011/0144524 describes a system for displaying characteristics of target tissue during an ablation procedure. The system includes an electronic control unit (ECU) configured to receive data regarding electrical properties of the target tissue for a time period. The ECU is also configured to determine values responsive to the data and 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 pop of the target tissue for the time period. The system further includes a display device configured to receive the values and display a visual representation the respective indicative parameters listed above. 
     As another example, U.S. Patent Application Publication 2014/0243813 describes 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 physical contact, as well as depth of a lesion being created. 
     U.S. Patent Application Publication 2014/0194869 describes a method and apparatus that utilizes a force-time integral for real time estimation of lesion size in catheter-based ablation systems. The apparatus measures the force exerted by a contact ablation probe on a target tissue and integrates the force over an energization time of the ablation probe. The force-time integral can be calculated and utilized to provide an estimated lesion size (depth, volume and/or area) in real time. The force-time integral may also account for variations in the power delivered to the target tissue in real time to provide an improved estimation of the lesion size. In one embodiment, the force metric can be used as feedback to establish a desired power level delivered to the probe to prevent steam popping. 
     U.S. Patent Application Publication 2017/014181 describes a method, consisting of ablating tissue for a time period, measuring a contact force applied during the time period, and measuring a power used during the time period. The method further includes ceasing ablating the tissue when a desired size of a lesion produced in the tissue, as estimated using an integral over the time period of a product of the contact force raised to a first non-unity exponent and the power raised to a second non-unity exponent, is reached. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention provides a method of ablation, including storing in a memory a pre-determined relation between lesion size and amount of ablative energy, for each of one or more selected temperatures. Using a processor, user input is received, that indicates a lesion size and a tissue temperature. Based on the relation, an amount of energy is determined, that matches the lesion size and the tissue temperature. An ablation probe is controlled to apply the amount of ablative energy that matches the selected lesion size. 
     In some embodiments, the selected tissue temperature includes a temperature of an ablative electrode that applies the ablative energy. 
     In some embodiments, the method further includes presenting to the user the indicated lesion size and tissue temperature, and the determined amount of energy. 
     In an embodiment, determining the amount of energy includes reading at least part of the relation from a lookup table. 
     There is additionally provided, in accordance with an embodiment of the present invention, a system for ablation, including a memory and a processor. The memory is configured to store a pre-determined relation between lesion size and amount of ablative energy, for each of one or more selected temperatures. The processor is configured to receive user input indicating a lesion size and a tissue temperature, to determine, based on the relation, an amount of energy that matches the lesion size and the tissue temperature, and to control an ablation probe to apply the amount of ablative energy that matches the selected lesion size. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, pictorial illustration of a system for cardiac radiofrequency (RF) ablation therapy, in accordance with an embodiment of the present invention; 
         FIG. 2  is a graph schematically showing a pre-determined relation between lesion size and RF ablative energy at different constant temperatures, in accordance with an embodiment of the present invention; and 
         FIG. 3  is a flow chart that schematically illustrates a method for planning RF ablation using the relation of  FIG. 2 , according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     A treatment of arrhythmia may include ablating a lesion in cardiac tissue using a source of thermal energy (e.g., by heating tissue). The clinical efficacy of the lesion largely depends on the depth of the lesion, which is governed by the amount of effective (e.g., useful) ablative energy deposited in tissue where the lesion is formed. 
     However, the effective ablative energy cannot be accurately estimated, as it depends, for example, on unknown tissue properties such as fat content. Therefore, for a given energy output of a generator, such as a radiofrequency (RF) generator, the resulting effective RF energy may be too low or too high. Low effective ablative energy may result in creating an insufficiently deep lesion, whereas an excessively high effective ablative energy may cause side effects such as steam-pops (e.g., due to very high tissue temperature), and side effects such as tissue perforation and collateral damage. 
     Tissue temperature during ablation is indeed considered indicative of the effective RF energy deposited, where a higher temperature predicts the formation of a deeper lesion. 
     Thus, predetermining a target tissue temperature to be maintained during ablation may assist in achieving both target lesion depth and avoiding side effects, such as those listed above. 
     Embodiments of the present invention that are described hereinafter provide a method for accurately predetermining and controlling both tissue temperature and lesion size (e.g., lesion depth) during thermal ablation, such as (RF) ablation. The disclosed method comprises planning the ablation in an energy mode, wherein, in order to meet both targets of lesion depth and tissue temperature, a processor selects, using a pre-determined relation between lesion depth and output energy at different constant temperatures, a corresponding amount of ablating RF energy to apply to tissue. 
     The disclosed pre-determined relation may be derived from a model and/or be based on calibration. For example, such a relation may be pre-measured in vitro (and/or using an animal model) and stored in a memory of the ablation system. Using the pre-determined relation, the disclosed method enables a processor to plan an ablation based only on two measured parameters (energy output of a generator and tissue temperature). 
     For example, in an embodiment, a physician selects (a) target lesion depth, and (b) tissue temperature during ablation to be, by example, 50° C., a low enough temperature at which it is known that side effects such as steam-pops do not occur in tissue. The processor that the physician operates then extracts the disclosed pre-determined relation between lesion depth and amount of ablative energy at the selected temperature, which can be in a form of a lookup table, and determines from the relation the amount of ablative energy required for achieving the selected lesion depth at the selected temperature. 
     In a subsequent ablation procedure, based on the selection, an ablation system applies an algorithm, as described below, in which the processor controls an ablation probe to apply the amount of ablative energy that matches the selected lesion size. In an embodiment, the processor may use one or two additional control parameters, such as irrigation flow rate, rather than attempting to control multiple parameters, which might include, for example, instantaneous RF power and force of contact. In an embodiment, the processor is configured to operate the algorithm to determine if the ablation is proceeding as planned, and to control the ablation based on feedback from readings of the two measured parameters (i.e., energy output of the generator and tissue temperature) In some embodiments, if the level of RF power applied is lowered by the processor, for example, in order to meet target tissue temperature, the processor is configured to extend the ablation time so that the selected amount of ablating RF energy is fully delivered, so as to achieve target lesion depth. 
     The described RF ablation planning technique, using the disclosed pre-determined relation, may enable achieving target lesion depth while maintaining tissue temperature, and therefore may improve the efficacy and safety of a catheter-based RF ablation procedure. 
     System Description 
       FIG. 1  is a schematic, pictorial illustration of a system  12  for cardiac radiofrequency (RF) ablation therapy, in accordance with an embodiment of the present invention. Typically, a memory  45  of system  20  stores numerous ablation protocols for different clinical scenarios, such as the protocol described in  FIG. 2 . 
     A physician  26  inserts a catheter  28  through a blood vessel into a chamber of a heart  24  of a subject  22 , and manipulates the catheter so that a distal end  32  of the catheter contacts the endocardium in an area that is to be treated. A tip electrode  51  of catheter  28 , seen in inset  25 , comprises one or more temperature sensors  50 . 
     After positioning distal end  32  at an ablation site, and ensuring that the tip is in contact with the endocardium, operator  26  actuates an RF energy generator  44  in a control console  42  to supply RF energy via a cable  38  to distal end  32 . Meanwhile, an irrigation pump  48  supplies a cooling fluid, such as normal saline solution, via a tube  40  and a lumen in catheter  28  to the distal end. Typically, both before and during the ablation, a display  46  displays those values of the ablation parameters, such as listed in Tables I-IV below, to physician  26 . 
     Operation of the RF energy generator and the irrigation pump may be coordinated in order to give the appropriate volume of irrigation during ablation, so as to cool the tip of the catheter and the tissue without overloading the heart with irrigation fluid. Each temperature sensor inside temperature sensors  50  provides feedback to console  42  for use, for example, in controlling the RF power and/or irrigation flow rate to maintain a given tissue-temperature. 
     In order to operate system  12 , a processor  41  includes a number of modules used by the processor to operate the system. These modules comprise a temperature module  52 , a power control module  54 , and an irrigation module  55 , the functions of which are described below. In particular, processor  41  runs a dedicated algorithm as disclosed herein, included in  FIG. 3 , that enables processor  41  to perform the disclosed steps, as further described below. 
     Although the pictured embodiment relates specifically to the use of a tip ablation device for ablation of heart tissue, the methods described herein may alternatively be applied in ablation devices comprising multiple ablation electrodes, when the operation of each electrode is independently controlled by processor  41 . 
     Forming a Lesion Based on Pre-Determined Amount of Abaltive Energy Vs. Lesion Size Curve 
       FIG. 2  is a graph schematically showing a pre-determined relation  100  between lesion size and RF ablative energy at different constant temperatures, in accordance with an embodiment of the present invention. As seen, relation  100  comprises a set of curves  60 - 64 , where each curve gives an expected lesion depth as a function of generator  44  output RF energy, and at three different constant tissue temperatures T 1 &lt;T 2 &lt;T 3 , respectively. 
     For example, at an output energy level  75 , temperature T 1  corresponds to lesion depth  70 , T 2  corresponds to lesion depth  72 , and T 3  corresponds to lesion depth  74 . Thus, maintaining lower tissue temperature during ablation results in a shallower lesion. 
     As further seen in  FIG. 2 , not only different temperatures T 1 , T 2  and, T 3  correspond to tissue depths  70 ,  72  and  74 , respectively, but also overall different lesion sizes (e.g., volumes)  70   a ,  72   a , and  74   a , respectively. 
     In some cases, for example if the risk of steam-pops is less significant, a user may select achieving the same lesion depth from different temperature curves. Such a selection amounts to using a different amount of effective ablative energy with each temperature, based on the disclosed relation. This is seen by ablative energies  75 ,  77 , and  77 , which all produce the same lesion depth, indicated on curves  60 - 64  by points  70 ,  82 , and  84 , respectively. 
     An ablation method in an energy mode, which may vary RF power and irrigation flow rate (and, contrary to the herein disclosed technique allows also temperature to vary) is described in a U.S. patent application entitled “Energy-Guided Radiofrequency (RF) Ablation,” Attorney Docket Number BIO6070USNP1/1002-1904, which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference. 
     Finally, tissue temperature may be affected by the effective energy that electrode  51  passes through tissue surface  58 . The effectivity of electrode  51  in passing energy directly to tissue beneath it may depend on the contact force that electrode  51  exerts on tissue surface  58 . 
       FIG. 3  is a flow chart that schematically illustrates a method for planning RF ablation using the curves of  FIG. 2 , according to an embodiment of the present invention. The process begins by physician  26  uploading pre-determined relation  100  comprising lesion depth as a function of ablative energy, such as curves  60 - 64 , at a pre-determined relation uploading step  90 . Next, physician  26  selects lesion depth, at a lesion depth selection step  92 . Physician  26  further selects target tissue temperature at tissue temperature selection step  94 . Based on steps  90 - 94 , processor  41 , operated by physician  26 , extracts the required energy using the disclosed relation (e.g., stored lookup table), at an energy selection step  96 . The physician then sets the selections into system  20  using a user interface, as inputs for an ablation algorithm applied by system  20 , at a parameter setting step  98 . 
     In some embodiments, the processor presents the above selections in one of Tables I-IV, for example on a display of system  20 . Typically, the allowed range of power and irrigation flowrate are automatically set by the system. 
     Tables I-IV provide four different settings that may be used for optimizing lesion depth while minimizing collateral damage, depending on the clinical need:
         Table I— Low depth   Table II— Medium depth   Table III— High depth   Table IV— Extra high depth       

     Low Depth Parameters: 
     
       
         
           
               
               
             
               
                 TABLE I 
               
               
                   
               
               
                 Parameter 
                 Range 
               
               
                   
               
             
            
               
                 Preset ablative energy 
                 270 J 
               
               
                 Maximum power level 
                 90 W 
               
               
                 Power range 
                 0-90 W 
               
               
                 Allowable temperature range 
                 45-65° C. (typically 50° C.) 
               
               
                 Allowable irrigation flow 
                 4-25 ml/min (this is mainly 
               
               
                 rate 
                 depending on the catheter design. 
               
               
                   
                 (Typically, 4-15 ml/min) 
               
               
                 Maximal ablation time 
                 3-6 Sec (Typically 4 sec) 
               
               
                   
               
            
           
         
       
     
     Medium Depth Parameters 
     
       
         
           
               
               
               
             
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 Parameter 
                 Range 
               
               
                   
                   
               
             
            
               
                   
                 Preset ablative energy 
                 360 J 
               
               
                   
                 Maximum power level 
                 90 W 
               
               
                   
                 Power range 
                 0-90 W 
               
               
                   
                 Allowable temperature range 
                 45-65° C. (typically 50° C.) 
               
               
                   
                 Allowable irrigation flow 
                 4-25 ml/min (mainly depending 
               
               
                   
                 rate 
                 on the catheter design. 
               
               
                   
                   
                 (Typically, 4-15 ml/min) 
               
               
                   
                 Maximal ablation time 
                 4-8 Sec (Typically 6 sec) 
               
               
                   
                   
               
            
           
         
       
     
     High Depth Parameters: 
     
       
         
           
               
               
               
             
               
                   
                 TABLE III 
               
               
                   
                   
               
               
                   
                 Parameter 
                 Range 
               
               
                   
                   
               
             
            
               
                   
                 Preset ablative energy 
                 560 J 
               
               
                   
                 Maximum power level 
                 70-90 W (Typically 70 W) 
               
               
                   
                 Power range 
                 0-90 W 
               
               
                   
                 Allowable temperature range 
                 45-65° C. (typically 50° C.) 
               
               
                   
                 Allowable irrigation flow 
                 4-25 ml/min (mainly depending 
               
               
                   
                 rate 
                 on the catheter design. 
               
               
                   
                   
                 (Typically, 4-15 ml/min) 
               
               
                   
                 Maximal ablation time 
                 6-12 Sec (Typically 10 sec) 
               
               
                   
                   
               
            
           
         
       
     
     Extra High Depth Parameters: 
     
       
         
           
               
               
               
             
               
                   
                 TABLE IV 
               
               
                   
                   
               
               
                   
                 Parameter 
                 Range 
               
               
                   
                   
               
             
            
               
                   
                 Preset ablative energy 
                 1500-3000 J 
               
               
                   
                 Maximum power level 
                 50 W 
               
               
                   
                 Power range 
                 0-50 W 
               
               
                   
                 Allowable temperature range 
                 40-55° C. (typically 45° C.) 
               
               
                   
                 Allowable irrigation flow 
                 4-25 ml/min (mainly depending 
               
               
                   
                 rate 
                 on the catheter design. 
               
               
                   
                   
                 Typically, 4-15 ml/min) 
               
               
                   
                 Maximal ablation time 
                 30-90 Sec (Typically 60 sec) 
               
               
                   
                   
               
            
           
         
       
     
     Relation uploading step  90  is implemented before physician  26  performs an ablation. 
     At a subsequent ablation session  100 , system  20  uses the selected parameters based on the disclosed relation (e.g., curves  60 - 64  that may be provided as a lookup table) to achieve the required lesion depth while maintaining target tissue temperature. 
     A display of system  20  may be further configured to display to physician  26 , by methods which are known in the art, the progress of the RF delivery to the electrode. 
     The example flow chart shown in  FIG. 3  is chosen purely for the sake of conceptual clarity. The present embodiment also comprises additional steps of the algorithm, such as checking a level of contact force of electrode  51  with tissue. In an embodiment, during the subsequent ablation procedure, a processor that applies the disclosed planning method is configured to monitor actual tissue temperature to maintain the temperature within a given tolerance. During the ablation, both irrigation flow rate and the level of RF power output may be automatically adjusted by the processor in order to maintain tissue temperature within the given tolerance. 
     Although the embodiments described herein mainly address cardiac applications, the methods and systems described herein can also be used, for example, in planning an ablation of other organs of the body. 
     It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art. Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered.