Patent Description:
Various techniques for planning RF ablation were proposed in the patent literature. For example, <CIT> 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, <CIT> 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.

<CIT> 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.

<CIT> 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.

<CIT> discloses systems and method to control tissue eating or ablation with porous electrode structures, in which a controller includes a processing element <NUM> that retains a function that correlates an observed relationship among lesion boundary depth, ablation power level, ablation time, actual sub-surface tissue temperature, and electrode temperature.

<CIT> discloses systems and methods for obtaining desired lesion characteristics while ablating body tissue. The system includes a master controller coupled to the RF generator and the temperature sensor. The master controller includes in memory a matrix of operating conditions. The master controller includes an input device. The physician uses the controller input device to set a desired lesion depth, to set a desired maximum RF power level PMAX, a desired time t; and a desired maximum tissue temperature TMAX.

<CIT> discloses Cardial ablation systems using temperature monitoring, including feedback loops.

<NPL> discloses a finite element method (FEM) analysis method to analyze the lesion formation during temperature-controlled radiofrequency (RF) cardiac ablation.

The methods mentioned below are not part of the claimed invention.

Disclosed herein is 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 examples, the selected tissue temperature includes a temperature of an ablative electrode that applies the ablative energy.

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

In an example, 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.

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.

Described hereinafter is 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, a physician selects (a) target lesion depth, and (b) tissue temperature during ablation to be, by example, <NUM>, 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.

<FIG> is a schematic, pictorial illustration of a system <NUM> for cardiac radiofrequency (RF) ablation therapy, in accordance with an embodiment of the present invention. Typically, a memory <NUM> of system <NUM> stores numerous ablation protocols for different clinical scenarios, such as the protocol described in <FIG>.

A physician <NUM> inserts a catheter <NUM> through a blood vessel into a chamber of a heart <NUM> of a subject <NUM>, and manipulates the catheter so that a distal end <NUM> of the catheter contacts the endocardium in an area that is to be treated. A tip electrode <NUM> of catheter <NUM>, seen in inset <NUM>, comprises one or more temperature sensors <NUM>.

After positioning distal end <NUM> at an ablation site, and ensuring that the tip is in contact with the endocardium, operator <NUM> actuates an RF energy generator <NUM> in a control console <NUM> to supply RF energy via a cable <NUM> to distal end <NUM>. Meanwhile, an irrigation pump <NUM> supplies a cooling fluid, such as normal saline solution, via a tube <NUM> and a lumen in catheter <NUM> to the distal end. Typically, both before and during the ablation, a display <NUM> displays those values of the ablation parameters, such as listed in Tables I-IV below, to physician <NUM>.

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 <NUM> provides feedback to console <NUM> 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 <NUM>, a processor <NUM> includes a number of modules used by the processor to operate the system. These modules comprise a temperature module <NUM>, a power control module <NUM>, and an irrigation module <NUM>, the functions of which are described below. In particular, processor <NUM> runs a dedicated algorithm as disclosed herein, included in <FIG>, that enables processor <NUM> 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 <NUM>.

<FIG> is a graph schematically showing a pre-determined relation <NUM> between lesion size and RF ablative energy at different constant temperatures, in accordance with an embodiment of the present invention. As seen, relation <NUM> comprises a set of curves <NUM>-<NUM>, where each curve gives an expected lesion depth as a function of generator <NUM> output RF energy, and at three different constant tissue temperatures T<NUM><T<NUM><T<NUM>, respectively.

For example, at an output energy level <NUM>, temperature T<NUM> corresponds to lesion depth <NUM>, T<NUM> corresponds to lesion depth <NUM>, and T<NUM> corresponds to lesion depth <NUM>. Thus, maintaining lower tissue temperature during ablation results in a shallower lesion.

As further seen in <FIG>, not only different temperatures T<NUM>, T<NUM> and, T<NUM> correspond to tissue depths <NUM>, <NUM> and <NUM>, respectively, but also overall different lesion sizes (e.g., volumes) 70a, 72a, and 74a, 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 <NUM>, <NUM>, and <NUM>, which all produce the same lesion depth, indicated on curves <NUM>-<NUM> by points <NUM>, <NUM>, and <NUM>, 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. Patent Application entitled "Energy-Guided Radiofrequency (RF) Ablation," Attorney Docket Number BIO6070USNP1/<NUM>-<NUM>, which is assigned to the assignee of the present patent application.

Finally, tissue temperature may be affected by the effective energy that electrode <NUM> passes through tissue surface <NUM>. The effectivity of electrode <NUM> in passing energy directly to tissue beneath it may depend on the contact force that electrode <NUM> exerts on tissue surface <NUM>.

<FIG> is a flow chart that schematically illustrates a method for planning RF ablation using the curves of <FIG>. The process begins by physician <NUM> uploading pre-determined relation <NUM> comprising lesion depth as a function of ablative energy, such as curves <NUM>-<NUM>, at a pre-determined relation uploading step <NUM>. Next, physician <NUM> selects lesion depth, at a lesion depth selection step <NUM>. Physician <NUM> further selects target tissue temperature at tissue temperature selection step <NUM>. Based on steps <NUM>-<NUM>, processor <NUM>, operated by physician <NUM>, extracts the required energy using the disclosed relation (e.g., stored lookup table), at an energy selection step <NUM>. The physician then sets the selections into system <NUM> using a user interface, as inputs for an ablation algorithm applied by system <NUM>, at a parameter setting step <NUM>.

In some embodiments, the processor presents the above selections in one of Tables I-IV, for example on a display of system <NUM>. 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:.

Relation uploading step <NUM> is implemented before physician <NUM> performs an ablation.

At a subsequent ablation session <NUM>, system <NUM> uses the selected parameters based on the disclosed relation (e.g., curves <NUM>-<NUM> that may be provided as a lookup table) to achieve the required lesion depth while maintaining target tissue temperature.

A display of system <NUM> may be further configured to display to physician <NUM>, by methods which are known in the art, the progress of the RF delivery to the electrode.

The example flow chart shown in <FIG> 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 <NUM> 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.

Claim 1:
A system (<NUM>) for ablation, the system comprising:
a memory (<NUM>), which is configured to store a pre-determined relation (<NUM>) between lesion size (70a, 72a, 74a) and amount of ablative energy, for each of one or more selected temperatures (T<NUM>, T<NUM>, T<NUM>); and
a processor (<NUM>) which is configured to:
receive user input indicating a selected lesion size and a tissue temperature;
determine, based on the relation, an amount of energy that matches the selected lesion size and the tissue temperature;
receive measurements of energy output from a generator; and
control an ablation probe (<NUM>) to apply from the generator the amount of ablative energy that matches the selected lesion size, wherein controlling the ablation probe to apply the amount of ablative energy comprises:
monitoring actual tissue temperature;
controlling ablation power to maintain the tissue temperature within a given tolerance; and
controlling ablation time so that the amount of ablative energy is fully delivered.