Patent Description:
Lower back injuries and chronic joint pain are major health problems resulting not only in debilitating conditions for the patient, but also in the consumption of a large proportion of funds allocated for health care, social assistance and disability programs. In the lower back, disc abnormalities and pain may result from trauma, repetitive use in the workplace, metabolic disorders, inherited proclivity, and/or aging. The existence of adjacent nerve structures and innervation of the disc are very important issues in respect to patient treatment for back pain. In joints, osteoarthritis is the most common form of arthritis pain and occurs when the protective cartilage on the ends of bones wears down over time.

The treatment of pain using high-frequency electrical current has been applied successfully to various regions of patients' bodies suspected of contributing to chronic pain sensations. For example, with respect to back pain, which affects millions of individuals every year, high-frequency electrical treatment has been applied to several tissues, including intervertebral discs, facet joints, sacroiliac joints as well as the vertebrae themselves (in a process known as intraosseous denervation). In addition to creating lesions in neural structures, application of radiofrequency energy has also been used to treat tumors throughout the body. Further, with respect to knee pain, which also affects millions of individuals every year, high-frequency electrical treatment has been applied to several tissues, including, for example, the ligaments, muscles, tendons, and menisci.

Radiofrequency ablation (RFA) is a minimally invasive therapy for treating chronic pain, cardiac arrhythmias, and tumors in many patients. RFA systems operate based on temperature feedback control, where RF power is modulated in order to reach a set point temperature. For instance, if the set point temperature is <NUM> and the current temperature is <NUM>, power is applied until the temperature reaches <NUM>. The underlying temperature-power control is based on proportional-integral-derivative (PID) control theories.

<CIT> discloses an ablation system and method for detecting and addressing distortion caused by a variety of factors. A method includes measuring a temperature curve at target tissue; applying ablation energy to the target tissue; determining a peak temperature on the temperature curve; if the peak temperature is greater than the predetermined peak temperature, determining a time at which the temperature curve crosses to a lower temperature; and if the determined time is greater than a predetermined time, generating a message indicating that the target tissue was successfully ablated. Another method includes determining a distance between a remote temperature probe and an ablation probe, applying ablation energy to target tissue, measuring temperature at the remote temperature probe, estimating ablation size based on the determined distance and the temperature measured by the remote temperature probe, and determining whether the target tissue is successfully ablated based on the estimated ablation size.

Thus, the art is continuously seeking new and improved systems and methods that continuously improve upon RFA systems. Accordingly, the present disclosure is directed to a system and method for power-/energy-based control of an RFA procedure that accounts for heat transfer due to local tissue perfusion.

Objects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.

Described herein but not claimed is a method for performing a radiofrequency (RF) ablation procedure with a cooled RF probe. The method includes measuring one or more local perfusion characteristics at an ablation site within a patient. The method also includes determining a heat transfer due to local perfusion at the ablation site based on the one or more local perfusion characteristics. Further, the method includes determining an operating threshold for the cooled RF probe based, at least in part, on the heat transfer. Moreover, the method includes controlling the cooled RF probe based on the operating threshold to create a lesion at the ablation site within the patient.

In one unclaimed method, the local perfusion characteristic(s) may include, for example, a steady state temperature within the cooled RF probe, a tissue temperature outside of the ablation site, a lesion temperature, a change in temperature, or an amount of perfusion.

In another method, the lesion temperature is dependent on the steady state temperature and the tissue temperature. Such methods may include determining the heat transfer due to local perfusion at the ablation site as a function of the lesion temperature.

Further methods may include measuring the local perfusion characteristic(s) at the ablation site via one or more sensors. More specifically, the sensor(s) may include, at least, a thermocouple positioned at a distal end of the cooled RF probe. In such methods, the local perfusion at the ablation site actively transfers heat between the thermocouple and tissue outside of the ablation site.

In additional methods, determining the heat transfer due to local perfusion at the ablation site may include activating cooling flow within the cooled RF probe and generating a temperature response profile for the ablation site. In certain methods, if the local perfusion is below a predetermined threshold, the ablation site equilibrates to a first temperature closer to the steady state temperature within the cooled RF probe, and if the local perfusion is at or above the predetermined threshold, the ablation site equilibrates to a second temperature closer to the tissue temperature outside of the ablation site. As such, the method may also include determining the heat transfer due to local perfusion at the ablation site based on a slope of the temperature response profile prior to achieving equilibrium.

In yet another method, the operating threshold may include a power threshold and/or a deposited or total energy threshold.

According to the present invention, there is provided a radiofrequency (RF) ablation system for performing an RF ablation procedure according to clam <NUM>. It should also be understood that the RF ablation system may further include any of the additional features as described herein.

Reference will now be made in detail to one or more embodiments of the invention, examples of the invention, examples of which are illustrated in the drawings. Each example and embodiment is provided by way of explanation of the invention, and is not meant as a limitation of the invention. For example, features illustrated or described as part of one embodiment may be used with another embodiment to yield still a further embodiment. It is intended that the invention include these and other modifications and variations as coming within the scope of the invention.

For the purposes of this invention, a lesion refers to the region of tissue that has been irreversibly damaged as a result of the application of thermal energy, and the invention is not intended to be limited in this regard. Furthermore, for the purposes of this description, proximal generally indicates that portion of a device or system next to or nearer to a user (when the device is in use), while the term distal generally indicates a portion further away from the user (when the device is in use).

Referring now to the drawings, <FIG> illustrates a schematic diagram of one embodiment of a radiofrequency (RF) ablation system <NUM> for performing an RF ablation procedure according to the present invention. As shown, the ablation system <NUM> includes an energy source <NUM> for delivering energy to a patient's body, a plurality of probe assemblies <NUM> (only one of which is shown) electrically coupled to the energy source <NUM> via one or more cables <NUM>, a dispersive return pad <NUM> electrically coupled to the energy source <NUM>, one or more cooling devices <NUM>, a pump cable <NUM>, one or more proximal cooling supply tubes <NUM> and one or more proximal cooling return tubes <NUM>.

As shown in the illustrated embodiment, the energy source <NUM> is a radiofrequency (RF) generator, but may optionally be any power source that may deliver other forms of energy, including but not limited to microwave energy, thermal energy, ultrasound and optical energy. Further, the energy source <NUM> may include a display incorporated therein. The display may be operable to display various aspects of a treatment procedure, including but not limited to any parameters that are relevant an ablation procedure, such as temperature, impedance, etc. and errors or warnings related to a treatment procedure. If no display is incorporated into the energy source <NUM>, the energy source <NUM> may include means of transmitting a signal to an external display. In one embodiment, the energy source <NUM> is operable to communicate with one more devices, for example with one or more of the probe assemblies <NUM> and/or the one or more cooling devices <NUM>. Such communication may be unidirectional or bidirectional depending on the devices used and the procedure performed.

In addition, as shown, a distal region <NUM> of the cable <NUM> may include a splitter <NUM> that divides the cable <NUM> into two or more distal ends <NUM> such that the probe assemblies <NUM> can be connected thereto. A proximal end <NUM> of the cable <NUM> is connected to the energy source <NUM>. This connection can be permanent, whereby, for example, the proximal end <NUM> of the cable <NUM> is embedded within the energy source <NUM>, or temporary, whereby, for example, the proximal end <NUM> of cable <NUM> is connected to energy source <NUM> via an electrical connector. The two or more distal ends <NUM> of the cable <NUM> terminate in connectors <NUM> operable to couple to the probe assemblies <NUM> and establish an electrical connection between the probe assemblies <NUM> and the energy source <NUM>. In alternate embodiments, the system <NUM> may include a separate cable for each probe assembly <NUM> being used to couple the probe assemblies <NUM> to the energy source <NUM>. Alternatively, the splitter <NUM> may include more than two distal ends. Such a connector is useful in embodiments having more than two devices connected to the energy source <NUM>, for example, if more than two probe assemblies are being used.

The cooling device(s) <NUM> may include any means of reducing a temperature of material located at and proximate to one or more of the probe assemblies <NUM>. For example, the cooling devices <NUM> may include a pump assembly having one or more peristaltic pumps operable to circulate a fluid from the cooling devices <NUM> through one or more proximal cooling supply tubes <NUM>, the probe assemblies <NUM>, one or more proximal cooling return tubes <NUM> and back to the one or more cooling devices <NUM>.

Still referring to <FIG>, the proximal cooling supply tubes <NUM> may include proximal supply tube connectors <NUM> at the distal ends of the one or more proximal cooling supply tubes <NUM>. Additionally, the proximal cooling return tubes <NUM> may include proximal return tube connectors <NUM> at the distal ends of the one or more proximal cooling return tubes <NUM>. In one embodiment, the proximal supply tube connectors <NUM> are female luer-lock type connectors and the proximal return tube connectors <NUM> are male luer-lock type connectors although other connector types are intended to be within the scope of the present invention.

The probe assembly <NUM> may also include a proximal region <NUM>, a handle <NUM>, a hollow elongate shaft <NUM>, and a distal tip region <NUM> that includes the one or more energy delivery devices <NUM>. Further, as shown, the proximal region <NUM> includes a distal cooling supply tube <NUM>, a distal supply tube connector <NUM>, a distal cooling return tube <NUM>, a distal return tube connector <NUM>, a probe assembly cable <NUM>, and a probe cable connector <NUM>. In such embodiments, the distal cooling supply tube <NUM> and distal cooling return tube <NUM> are flexible to allow for greater maneuverability of the probe assemblies <NUM>, but alternate embodiments with rigid tubes are possible.

The probe cable connector <NUM> may be located at a proximal end of the probe assembly cable <NUM> and may be operable to reversibly couple to one of the connectors <NUM>, thus establishing an electrical connection between the energy source <NUM> and the probe assembly <NUM>. The probe assembly cable <NUM> may include one or more conductors depending on the specific configuration of the probe assembly <NUM>. For example, in one embodiment, the probe assembly cable <NUM> may include five conductors allowing probe assembly cable <NUM> to transmit RF current from the energy source <NUM> to the one or more energy delivery devices <NUM> as well as to connect multiple temperature sensing elements to the energy source <NUM> as discussed below.

The energy delivery devices <NUM> may include any means of delivering radiofrequency energy to a region of tissue adjacent to the distal tip region <NUM>. For example, the energy delivery devices <NUM> may include an electrode or any other radiofrequency energy delivery means and the invention is not limited in this regard. In one embodiment, the energy delivery devices <NUM> may include an electrode. The active region of the electrode may be <NUM> to <NUM> millimeters (mm) in length and energy delivered by the electrode is electrical energy in the form of current in the RF range. The size of the active region of the electrode can be optimized for placement within an intervertebral disc; however, different sizes of active regions, all of which are within the scope of the present invention, may be used depending on the specific procedure being performed. In some embodiments, feedback from the energy source <NUM> may automatically adjust the exposed area of the energy delivery device <NUM> in response to a given measurement such as impedance or temperature.

Still referring to <FIG>, the ablation system <NUM> includes a controller <NUM> for facilitating communication between the energy source <NUM>, the dispersive return pad <NUM>, and/or the cooling devices <NUM>. In this way, feedback control is established between the cooling devices <NUM> and the energy source <NUM>. The feedback control may include the energy source <NUM>, the probe assemblies <NUM>, the dispersive return pad <NUM>, and/or the cooling devices <NUM>, although any feedback between any two devices is within the scope of the present invention. The feedback control may be implemented, for example, in a control module which may be a component of the energy source <NUM>. In such embodiments, the energy source <NUM> is operable to communicate bi-directionally with the probe assemblies <NUM> as well as with the dispersive return pad <NUM> and/or the cooling devices <NUM>. In the context of this invention, bi-directional communication refers to the capability of a device to both receive a signal from and send a signal to another device.

Referring now to <FIG>, the energy delivery devices <NUM> may also include a temperature sensing element <NUM> that protrudes beyond a distal end thereof. More specifically, as shown, the temperature sensing element <NUM> may have a length <NUM> of less than about <NUM> millimeter (mm) that extends from a distal end <NUM> of the energy delivery device <NUM>. Accordingly, the temperature sensing elements <NUM> are configured to control and optimize the size of the lesion for different anatomical locations, for instance creating smaller lesions in regions adjacent to critical structures such as arteries and motor nerves.

In addition, the temperature sensing element <NUM> is configured to increase (or decrease) a power demand of the energy delivery device <NUM>. Further, as shown, the temperature sensing element <NUM> may include a stainless steel hypotube <NUM> that is electrically conductive and may be electrically coupled to the energy delivery device <NUM>. Thus, in such an embodiment, whereby energy may be conducted to the protrusion and delivered from the protrusion to surrounding tissue, the protrusion may be understood to be a component of both temperature sensing element <NUM> as well as the one or more energy delivery devices <NUM>. Placing the temperature sensing elements <NUM> at this location, rather than within a lumen <NUM> defined by the energy delivery device <NUM>, is beneficial because it allows the temperature sensing element <NUM> to provide a more accurate indication of the temperature of tissue proximate to the energy delivery device <NUM>. This is due to the fact that, when extended beyond the energy delivery device <NUM>, the temperature sensing element <NUM> will not be as affected by the cooling fluid flowing within the lumen <NUM> as it would be were it located within lumen <NUM>. Thus, in such embodiments, the probe assembly <NUM> includes a protrusion protruding from the distal region of the probe assembly, whereby the protrusion is a component of the temperature sensing element <NUM>.

Referring now to <FIG>, in one embodiment, the first and second probe assemblies <NUM> may be operated in a bipolar mode. For example, as shown, <FIG> illustrates one embodiment of two probe assemblies <NUM>, wherein the distal tip regions <NUM> thereof are located within an intervertebral disc <NUM>. In such embodiments, electrical energy is delivered to the first and second probe assemblies <NUM> and this energy is preferentially concentrated therebetween through a region of tissue to be treated (i.e. an area of the intervertebral disc <NUM>). The region of tissue to be treated is thus heated by the energy concentrated between first and second probe assemblies <NUM>. In other embodiments, the first and second probe assemblies <NUM> may be operated in a monopolar mode, in which case an additional grounding pad is required on the surface of a body of a patient, as is known in the art. Any combination of bipolar and monopolar procedures may also be used. It should also be understood that the system may include more than two probe assemblies. For example, in some embodiments, three probe assemblies may be used and the probe assemblies may be operated in a triphasic mode, whereby the phase of the current being supplied differs for each probe assembly.

Referring now to <FIG>, a schematic diagram of one embodiment of the probe assembly <NUM> having the energy delivery device <NUM> is illustrated. More specifically, as shown, the diagram illustrates the heat transfer between the probe assembly <NUM>, the lesion <NUM>, and the surrounding tissue <NUM> at an ablation site. Thus, as shown and described herein, the energy delivery device <NUM> is internally cooled by the circulation of a cooling fluid delivered therethrough (as indicated by arrows <NUM>). Prior to the application of RF energy, the cooling fluid is at an equilibrium temperature denoted by T_1. At the distal tip <NUM> of the energy delivery device <NUM>, the temperature sensing element <NUM> measures the local lesion temperature denoted by T_2. The local lesion temperature T_2 is dependent upon the steady state temperature T_1 of the cooling fluid within the probe assembly <NUM> and the physiologic tissue temperature T_3, as the temperature sensing element <NUM> is thermally conductive to the upstream cooling fluid circulation and the tissue. Blood perfusion within the lesion zone <NUM> actively transfers heat (denoted as Q) between the active distal tip <NUM> of the energy delivery device <NUM> and the tissue <NUM>. The amount of perfusion can vary widely depending on the local vascularization within the patient.

As such, the present invention is directed to a system for performing an RF ablation procedure with the probe assembly <NUM> described herein that accounts for local perfusion. More specifically, as shown in <FIG>, a flow diagram of one method <NUM> is illustrated. In general, the method <NUM> will be described herein with reference to the probe assembly <NUM> shown in <FIG>. However, it should be appreciated that the disclosed method <NUM> may be implemented with probe assemblies having any other suitable configurations. In addition, although <FIG> depicts steps performed in a particular order for purposes of illustration and discussion, the methods discussed herein are not limited to any particular order or arrangement. One skilled in the art, using the disclosures provided herein, will appreciate that various steps of the methods disclosed herein can be omitted, rearranged, combined, and/or adapted in various ways.

As shown at (<NUM>), the method <NUM> includes measuring one or more local perfusion characteristics at an ablation site within the patient. For example, in one method, the local perfusion characteristic(s) may include, for example, a steady state temperature within the probe assembly <NUM> (e.g. T_1), a tissue temperature outside of the ablation site (e.g. T_3), a lesion temperature (e.g. T_2), a change in temperature, or an amount of perfusion. As such, certain methods <NUM> may include measuring the local perfusion characteristic(s) at the ablation site via one or more sensors. More specifically, the sensor(s) may include, at least, the temperature sensing element <NUM> described herein.

As shown at (<NUM>), the method <NUM> also includes determining a heat transfer due to local perfusion at the ablation site based on the local perfusion characteristic(s). For example, the local perfusion at the ablation site actively transfers heat between the temperature sensing element <NUM> and tissue outside of the ablation site. As such, the lesion temperature may be dependent on the steady state temperature and the tissue temperature. Thus, the method <NUM> may include determining the heat transfer due to local perfusion at the ablation site as a function of the lesion temperature.

More specifically, in one embodiment, the controller <NUM> is configured to determine the heat transfer due to local perfusion at the ablation site by activating cooling flow within the probe assembly <NUM> and generating a temperature response profile <NUM> (see, e.g. <FIG>) for the ablation site. For example, as shown in <FIG>, the graphs illustrate the process used to assess local perfusion characteristics at the ablation site. More specifically, <FIG> illustrates a graph of temperature (y-axis) versus time (x-axis). The curve <NUM> illustrates the temperature-time profile for the low perfusion ablation zone, whereas curve <NUM> illustrates the temperature-time profile for the high perfusion ablation zone. As shown at the start, both profiles <NUM>, <NUM> share similar steady state temperatures, which equilibrate near physiological tissue temperature T_3. At time <NUM>, the pump of the probe assembly <NUM> is activated and cooling fluid is transferred to the active tip <NUM> of the energy delivery device <NUM>. Thus, as shown, the ablation zone with the low perfusion <NUM> (e.g. local perfusion below a predetermined threshold) equilibrates to a first temperature closer to the cooling flow temperature T_1. In contrast, as shown, the ablation zone with the high perfusion <NUM> (e.g. local perfusion above the predetermined threshold) equilibrates to a second temperature closer to physiological tissue temperature T_3. Referring particularly to <FIG>, the slopes <NUM>, <NUM> of the temperature response profile <NUM> prior to achieving equilibrium is indicative of the local heat transfer capacity. Thus, higher local perfusion <NUM> results in a shallower equilibration slope than lower local perfusion <NUM>.

Referring back to <FIG>, as shown at (<NUM>), the method <NUM> further includes determining an operating threshold for the probe assembly <NUM> based, at least in part, on the heat transfer capacity due to the local perfusion. For example, the operating threshold may include a power threshold and/or a deposited or total energy threshold. In certain circumstances, the power threshold or total deposited energy can be a better predictor of the lesion creation process, thereby resulting in more consistent lesion size if the ablation procedure is also power- or total-energy-threshold controlled. By considering the local perfusion surrounding the ablation site, the power/energy-based control is more accurate.

Accordingly, as shown at (<NUM>), the method <NUM> includes controlling the probe assembly <NUM> based on the operating threshold to create a lesion at the ablation site within the patient. For example, in one embodiment, the slope value can be used by the controller <NUM> to compensate for heat loss due to the perfusion during power/total energy feedback control routines of the probe assembly <NUM>.

Referring now to <FIG>, a graph <NUM> of the normalized temperature drop (y-axis) versus time (x-axis) to depict experimental results of the method described herein is illustrated. As shown, the temperature-time profile was generated using the probe assembly <NUM> described herein in a water bath that was heated to <NUM>. In the low perfusion experimental case (as denoted by <NUM>), the water bath was not stirred. In the high perfusion experimental case (as denoted by <NUM>), the water bath was stirred to mimic blood perfusion at the ablation site. The results illustrate that for high perfusion <NUM>, the temperature equilibrates more rapidly to a higher equilibrium temperature than the low perfusion case <NUM>.

Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

Claim 1:
A radiofrequency, or RF, ablation system (<NUM>) for performing an RF ablation procedure, the RF ablation system comprising:
an energy source (<NUM>) for delivering energy to a patient's body;
one or more energy delivery devices (<NUM>, <NUM>) electrically coupled to the energy source, wherein the one or more energy delivery devices each comprise a cooled RF probe (<NUM>) comprising a proximal end and a distal end, the distal end comprising an active distal tip (<NUM>);
one or more sensors (<NUM>) for measuring local perfusion characteristics at an ablation site within the patient, the local perfusion characteristics including i) a steady-state temperature within the cooled RF probe, ii) a temperature of a tissue outside of the ablation site, and iii) a lesion temperature at the ablation site; and
at least one processor (<NUM>) configured to perform a plurality of operations, the plurality of operations comprising:
generating a temperature response profile by activating a flow of cooling fluid within the cooled RF probe and subsequently monitoring the local perfusion characteristics over a time period, wherein a slope of the temperature response profile defines a heat transfer due to local perfusion at the ablation site;
determining an operating threshold for the one or more energy delivery devices based on the temperature response profile, wherein the operating threshold compensates for heat loss due to the local perfusion based on the slope of the temperature response profile; and
controlling the one or more energy delivery devices to create a lesion at the ablation site within the patient by applying RF energy at the operating threshold.