Patent Publication Number: US-2009240247-A1

Title: Systems and methods for performing simultaneous ablation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of co-pending U.S. application Ser. No. 10/713,357, filed on Nov. 14, 2003, the disclosures of which is hereby incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of the invention relates to medical devices, and more particularly, to systems and methods for ablating or otherwise treating tissue using electrical energy. 
     2. Background of the Invention 
     Tissue may be destroyed, ablated, or otherwise treated using thermal energy during various therapeutic procedures. Many forms of thermal energy may be imparted to tissue, such as radio frequency electrical energy, microwave electromagnetic energy, laser energy, acoustic energy, or thermal conduction. 
     In particular, radio frequency ablation (RFA) may be used to treat patients with tissue anomalies, such as liver anomalies and many primary cancers, such as cancers of the stomach, bowel, pancreas, kidney and lung. RFA treatment involves the destroying undesirable cells by generating heat through agitation caused by the application of alternating electrical current (radio frequency energy) through the tissue. 
     Various RF ablation devices have been suggested for this purpose. For example, U.S. Pat. No. 5,855,576 describes an ablation apparatus that includes a plurality of wire electrodes deployable from a cannula or catheter. Each of the wires includes a proximal end that is coupled to a generator, and a distal end that may project from a distal end of the cannula. The wires are arranged in an array with the distal ends located generally radially and uniformly spaced apart from the catheter distal end. The wires may be energized in a monopolar or bipolar configuration to heat and necrose tissue within a precisely defined volumetric region of target tissue. The current may flow between closely spaced wire electrodes (bipolar mode) or between one or more wire electrodes and a larger, common electrode (monopolar mode) located remotely from the tissue to be heated. To assure that the target tissue is adequately treated and/or to limit damaging adjacent healthy tissues, the array of wires may be arranged uniformly, e.g., substantially evenly and symmetrically spaced-apart so that heat is generated uniformly within the desired target tissue volume. Such devices may be used either in open surgical settings, in laparoscopic procedures, and/or in percutaneous interventions. 
     During tissue ablation, the maximum heating often occurs in the tissue immediately adjacent the emitting electrodes. In general, the level of tissue heating is proportional to the square of the electrical current density, and the electrical current density in tissue generally falls rapidly with increasing distance from the electrode. The decrease of a current density depends upon a geometry of the electrode. For example, if the electrode has a spherical shape, the current density will generally fall as the second power of distance from the electrode. On the other hand, if the electrode has an elongate shape (e.g., a wire), the current density will generally fall with distance from the electrode, and the associated power will fall as the second power of distance from the electrode. For the case of spherical electrode, the heating in tissue generally falls as the fourth power of distance from the electrode, and the resulting tissue temperature therefore decreases rapidly as the distance from the electrode increases. This causes a lesion to form first around the electrodes, and then to expand into tissue disposed further away from the electrodes. 
     Due to physical changes within the tissue during the ablation process, the size of the lesion created may be limited. For example, the concentration of heat adjacent to wires often causes the local tissue to desiccate, thereby reducing its electrical conductivity. As the tissue conductivity decreases, the impedance to current passing from the electrode to the tissue increases so that more voltage must be supplied to the electrodes to affect the surrounding, more distant tissue. The tissue temperature proximate to the electrode may approach 100° C., so that water within the tissue boils to become water vapor. As this desiccation and/or vaporization process continues, the impedance of the local tissue may rise to the point where a therapeutic level of current can no longer pass through the local tissue into the surrounding tissue. 
     Thus, the rapid fall-off in current density may limit the volume of tissue that can be treated by the wire electrodes. As such, depending upon the rate of heating and the size of the wire electrodes, existing ablation devices may not be able to create lesions that are relatively large in size. Longer wire electrodes and/or larger arrays have been suggested for creating larger lesions. The effectiveness of such devices, however, may be limited by the desiccation and/or vaporization process discussed previously. While wire electrodes can be deployed, activated, retracted, and repositioned sequentially to treat multiple locations within a tissue region, such an approach may increase the length of time of a procedure, and precise positioning to ensure that an entire tissue region is treated may be difficult to accomplish. 
     Accordingly, improved systems and methods for tissue ablation would be useful. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to systems and methods for delivering energy to tissue, and more particularly to systems and methods for delivering energy substantially simultaneously to multiple electrode arrays to increase a volume of tissue being treated. 
     In accordance with a first aspect of the present invention, a system for treating tissue within a tissue region is provided that includes a source of energy, a first ablation device including a plurality of wires coupled to the source of energy, and a second ablation device including a plurality of wires coupled to the source of energy in parallel with the first ablation device, whereby the first and second ablation devices can substantially simultaneously create first and second lesions, respectively, within a tissue region. 
     In a preferred embodiment, the wires of the first and second ablation devices are electrodes and the source of energy is a source of electrical energy, e.g., a radio frequency (RF) generator. Preferably, the first and second ablation devices include an array of wires deployable from a cannula. 
     The source of electrical energy may include first and second terminals coupled in parallel to one another. The first ablation device may be coupled to the first terminal and the second ablation device may be coupled to the second terminal. Alternatively, the source of electrical energy may include a terminal, and a “Y” cable or other connector may be coupled between the first and second ablation devices and the terminal to couple the first and second ablation devices in parallel. Optionally, a ground electrode may be coupled to the source of energy opposite the first and second ablation devices, e.g., to provide a return path for electrical energy delivered to the tissue from the electrodes. 
     In accordance with another aspect of the present invention, a method is provided for creating a lesion within a tissue region, e.g., a benign or cancerous tumor within a liver or other tissue structure. A first array of electrodes may be inserted into a first site within the tissue region, and a second array of electrodes may be inserted into a second site within the tissue region. Preferably, the second array of electrodes is coupled in parallel with the first array of electrodes, e.g., to a RF generator or other source of energy. 
     In one embodiment, the first and second arrays of electrodes may be introduced into the first and second sites from first and second cannulas, respectively. Preferably, the first and second cannulas are introduced into the tissue region until distal ends of the first and second cannulas are disposed adjacent the first and second sites, respectively. The first and second arrays of electrodes may then be deployed from the distal ends of the first and second cannulas into the first and second sites, respectively. 
     Energy may be substantially simultaneously delivered to the first and second arrays of electrodes to generate lesions at the first and second sites within the tissue region. Preferably, the first and second sites are disposed adjacent to one another within the tissue region such that the first and second lesions at least partially overlap. Optionally, at least one or both of the first and second arrays of electrodes may be removed from the tissue region and introduced into a third (and fourth) site within the tissue region, and activated to increase the size of the lesion created. In other embodiments, the first and second arrays of electrodes can be placed at different sites, each of which is associated with a treatment region. In such arrangement, separate tissues at different treatment sites can be ablated simultaneously. 
     Other aspects and features of the invention will be evident from reading the following detailed description of the preferred embodiments, which are intended to illustrate, not limit, the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate the design and utility of preferred embodiments of the present invention, in which similar elements are referred to by common reference numerals. In order to better appreciate how advantages and objects of the present inventions are obtained, a more particular description of the present inventions briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated in the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings. 
         FIG. 1  illustrates a system for delivering electrical energy to tissue, in accordance with a preferred embodiment of the present invention. 
         FIG. 2  illustrates a variation of the ablation system of  FIG. 1 , showing the power supply having a plurality of output terminals. 
         FIG. 3  is a cross-sectional side view of an embodiment of an ablation device, showing electrode wires constrained within a cannula. 
         FIG. 4  is a cross-sectional side view of the ablation device of  FIG. 3 , showing the wires deployed from the cannula. 
         FIGS. 5A-5D  are cross-sectional views, showing a method for treating tissue, in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, in which similar or corresponding parts are identified with the same reference numeral,  FIG. 1  shows a preferred embodiment of an ablation system  10 , in accordance with the present invention. The ablation system  10  includes a source of energy  12 , e.g., a radio frequency (RF) generator, having an output terminal  14 , a connector  16 , a first ablation device  18 , and a second ablation device  20 . One or both of the first and the second ablation devices  18 ,  20  may be capable of being coupled to the generator  12 . 
     The generator  12  is preferably capable of operating with a fixed or controlled voltage so that power and current diminish as impedance of the tissue being ablated increases. Exemplary generators are described in U.S. Pat. No. 6,080,149, the disclosure of which is expressly incorporated by reference herein. The preferred generator  12  may operate at relatively low fixed voltages, typically below one hundred fifty volts (150 V) peak-to-peak, and preferably between about fifty and one hundred volts (50-100 V). Such radio frequency generators are available from Boston Scientific Corporation, assignee of the present application, as well as from other commercial suppliers. It should be noted that the generator  12  is not limited to those that operate at the range of voltages discussed previously, and that generators capable of operating at other ranges of voltages may also be used. 
     The connector  16  includes an input terminal  22 , a first output terminal  24 , and a second output terminal  26  that is connected in parallel with the first output terminal  24 . The first and second output terminals  24  and  26  of the connector  16  are configured for coupling to the first and second ablation devices  18 ,  20 , respectively, while the input terminal  22  of the connector  16  is configured for coupling to the output terminal  14  of the generator  12 . Optionally, the ablation system  10  may include one or more cables  28 , e.g., extension cables or cables that extend from the first and second ablation devices  18 ,  20 . If cables  28  are not provided, the first and second ablation devices  18 ,  20  may be coupled directly to the output terminals  24  and  26 , respectively, of the connector  16 . In the illustrated embodiment, the connector  16  may deliver power from the generator  12  simultaneously to the first and second ablation devices  18 ,  20 . If it is desired to deliver power to more than two ablation devices, the connector  16  may have more than two output terminals connected in parallel to one another (not shown). 
     Alternatively, as shown in  FIG. 2 , instead of the “Y” connector  16 , a generator  12 ′ may be provided that includes two (or optionally more) output terminals  14 ′ coupled in parallel with one another. In this case, first and second ablation devices  18 ,′  20 ′ may be coupled to separate output terminals  14 ′ of the generator  12 ′ without requiring a connector  16  (not shown, see  FIG. 1 ). However, if the generator  12 ′ does not provide an adequate number of output terminals  14  for the number of ablation devices desired, one or more connectors  16  (not shown) may be used to couple two or more ablation devices to a single output terminal of the generator  12 .′ 
     The output terminals  14 ′ of the generator  12 ′ may be coupled to common control circuits (not shown) within the generator  12 .′ Alternatively, the generator  12 ′ may include separate control circuits coupled to each of the output terminals  14 .′ The control circuits may be connected in parallel with one another, yet may include separate impedance feedback to control energy delivery to the respective output terminals  14 .′ Thus, the output terminals  14 ′ may be connected in parallel to an active terminal of the generator  12 ′ such that the ablation devices  18 ,′  20 ′ deliver energy to a common ground pad electrode (not shown) in a monopolar mode. Alternatively, the output terminals  14 ′ may be connected to opposite terminals of the generator  12 ′ for delivering energy between the ablation devices  18 ,′  20 ′ in a bipolar mode. 
     Turning to  FIGS. 3 and 4 , in a preferred embodiment, each of the ablation devices  18 ,  20  of  FIG. 1  (or alternatively, the ablation devices  18 ,′  20 ′ of  FIG. 2 ) may be a probe assembly  50 . The probe assembly  50  may include a cannula  52  having a lumen  54 , a shaft  56  having a proximal end  58  and a distal end  60 , and a plurality of electrode wires  62  secured to the distal end  60  of the shaft  56 . The proximal end  58  of the shaft  56  may include a connector  63  for coupling to the generator  12 . For example, the connector  62  may be used to connect the probe assembly  50  to a cable  66 , which may be part of the connector  16  (not shown, see  FIG. 1 ), an extension cable, or a cable that extends from the output terminal  14  of the generator  12 . Alternatively, the probe assembly  50  may itself include a cable (not shown) on the proximal end  58  of the shaft  56 , and a connector may be provided on the proximal end of the cable (not shown). 
     The cannula  52  may have a length between about five and thirty centimeters (5-30 cm), and/or an outer diameter or cross sectional dimension between about one and five millimeters (1-5 mm). However, the cannula  52  may also have other lengths and outer cross sectional dimensions, depending upon the application. The cannula  52  may be formed from metal, plastic, and the like, and/or may be electrically active or inactive within the probe assembly  50 , depending upon the manner in which electrical energy is to be applied. 
     The cannula  52  may coaxially surround the shaft  56  such that the shaft  56  may be advanced axially from or retracted axially into the lumen  54  of the cannula  52 . Optionally, a handle  64  may be provided on the proximal end  58  of the shaft  56  to facilitate manipulating the shaft  56 . The wires  62  may be compressed into a low profile when disposed within the lumen  54  of the cannula  52 , as shown in  FIG. 3 . As shown in  FIG. 4 , the proximal end  58  of the shaft  56  or the handle  64  (if one is provided) may be advanced to deploy the wires from the lumen  54  of the cannula  52 . When the wires  62  are unconfined outside the lumen  54  of the cannula  52 , they may assume a relaxed expanded configuration.  FIG. 4  shows an exemplary two-wire array including wires  62  biased towards a generally “U” shape and substantially uniformly separated from one another about a longitudinal axis of the shaft  56 . Alternatively, each wire  62  may have other shapes, such as a “J” shape, and/or the array may have one wire  62  or more than two wires  62 . The array may also have non-uniform spacing to produce an asymmetrical lesion. The wires  62  are preferably formed from spring wire, superelastic material, or other material, such as Nitinol, that may retain a shape memory. During use of the probe assembly  50 , the wires  62  may be deployed into a target tissue region to deliver energy to the tissue to create a lesion. 
     Optionally, a marker (not shown) may be placed on the handle  64  and/or on the proximal end  58  of the shaft  56  for indicating a rotational orientation of the shaft  56  during use. The probe assembly  50  may also carry one or more radio-opaque markers (not shown) to assist positioning the probe assembly  50  during a procedure, as is known in the art. Optionally, the probe assembly  50  may also include a sensor, e.g., a temperature sensor and/or an impedance sensor (not shown), carried by the distal end of the shaft  56  and/or one or more of the wires  62 . 
     Exemplary ablation devices having a spreading array of wires have been described in U.S. Pat. No. 5,855,576, the disclosure of which is expressly incorporated by reference herein. 
     It should be noted that the ablation devices  18 ,  20  are not necessarily limited to the probe assembly  50  shown in  FIGS. 3 and 4 , and that either or both of the ablation devices  18 ,  20  may be selected from a variety of devices that are capable of delivering ablation energy. For example, medical devices may also be used that are configured for delivering ultrasound energy, microwave energy, and/or other forms of energy for the purpose of ablation, which are well known in the art. Furthermore, the first and second ablation devices  18 ,  20  are not necessarily limited to the same type of devices. For example, the first ablation device  18  may deliver ultrasound energy while the second ablation device  20  may deliver radio-frequency energy. Also, the first and second ablation devices  18 ,  20  may have different sizes of arrays of wires  62 , and/or different types or numbers of electrodes. For example, either of the first and second ablation devices  18 ,  20  may be an elongate member carrying a single electrode tip. 
     Referring now to  FIGS. 5A-5D , the ablation system  10  may be used to treat a treatment region TR within tissue located beneath skin or an organ surface S of a patient. The tissue TR before treatment is shown in  FIG. 5A . As shown in  FIG. 5B , the cannulas  52  of the first and second ablation devices  18 ,  20  may be introduced into the treatment region TR, so that the respective distal ends of the cannulas  52  of the first and second ablation devices  18 ,  20  are located at first and second target sites TS 1 , TS 2 . This may be accomplished using any of a variety of techniques. In some cases, the cannulas  52  and shafts  56  of the respective ablation devices  18 ,  20  may be introduced into the target site TS percutaneously, i.e., directly through the patient&#39;s skin, or through an open surgical incision. In this case, the cannulas  52  may have a sharpened tip, e.g., a beveled or pointed tip, to facilitate introduction into the treatment region. In such cases, it is desirable that the cannulas  52  be sufficiently rigid, i.e., have sufficient column strength, so that the cannulas  52  may be accurately advanced through tissue. 
     In an alternative embodiment, the cannulas  52  may be introduced without the shafts  56  using internal stylets (not shown). Once the cannulas  52  are positioned as desired, the stylets may be exchanged for the shafts  56  that carry the wires  62 . In this case, each of the cannulas  52  may be substantially flexible or semi-rigid, since the initial column strength of the apparatus  10  may be provided by the stylets. Various methods known in the art may be utilized to position the probe  50  before deploying the wires. 
     In a further alternative, one or more components or elements may be provided for introducing each of the cannulas  52  to the treatment region. For example, a conventional sheath and sharpened obturator (stylet) assembly (not shown) may be used to access the target site(s). The assembly may be positioned using ultrasonic or other conventional imaging. Once properly positioned, the obturator/stylet may be removed, providing an access lumen through the sheath. The cannula  52  and shaft  56  of each of the ablation devices  18 ,  20  may then be introduced through the respective sheath lumens so that the distal ends of the cannulas  52  of the first and second ablation devices  18 ,  20  advance from the sheaths into the target sites TS 1 , TS 2 . 
     Turning to  FIG. 5C , after the cannulas  52  of the ablation devices  18 ,  20  are properly placed, the shafts  56  of the respective ablation devices  18 ,  20  may be advanced distally, thereby deploying the arrays of wires  62  from the distal ends of the respective cannulas  52  into the target sites TS 1 , TS 2 . Preferably, the wires  62  are biased to curve radially outwardly as they are deployed from the cannulas  52 . The shaft  56  of each of the ablation devices  18 ,  20  may be advanced sufficiently such that the wires  62  fully deploy to circumscribe substantially tissue within the target sites TS 1 , TS 2  of the treatment region TR, as shown in  FIG. 5D . Alternatively, the wires  62  may be only partially deployed or deployed incrementally in stages during a procedure. 
     If the generator  12  of the ablation system  10  includes only one output terminal  14 , one or more connectors  16 , described previously, may be used to couple the ablation devices  18 ,  20  to the output terminal  14 . If the generator  12  includes more than one output terminals  14 , the ablation devices  18 ,  20  may be coupled directly to the generator  12  without using the connector  16 . Extension cables  28  may also be used to couple the ablation devices  18 ,  20  to the connector  16  or to the generator  12 . The ablation devices  18 ,  20  may be coupled to the generator  12  in parallel with one another after the wires  62  of the respective ablation devices  18 ,  20  have been deployed. Alternatively, the wires  62  may be coupled to the generator  12  before the cannulas  52  are introduced to the treatment region, or at any time before the tissue is ablated. A neutral or ground electrode, e.g., an external electrode pad, may be coupled to the opposite terminal (not shown) of the generator  12  and coupled to the patient, e.g., the patient&#39;s skin, in a conventional manner. 
     Next, energy, preferably RF electrical energy, may be delivered from the generator  12  to the wires  62  of the respective ablation devices  18 ,  20 , thereby substantially simultaneously creating lesions at the first and second target sites TS 1 , TS 2  of the treatment region TR, respectively. Because the ablation devices  18 ,  20  are connected in parallel to the generator  12 , as the impedance of tissue at one of the target sites TS 1 , TS 2  increases, e.g., as the tissue is desiccated or otherwise treated, current may continue to flow to the other target site(s) to complete treatment of both target sites. 
     Simultaneously creating two or more lesions within a treatment region may substantially reduce the duration of an ablation procedure. In addition, using only a single generator  12  (or fewer generators than deployed ablation devices) may reduce the cost of equipment necessary to complete a procedure. When desired lesions at the first and second target sites TS 1 , TS 2  of the treatment region TR have been created, the wires  62  of each of the ablation devices  18 ,  20  may be retracted into the respective lumens  54  of the cannulas  52 , and the ablation devices  18 ,  20  may be removed from the treatment region TR. In many cases, two ablation devices  18 ,  20  may be sufficient to create a desired lesion. However, if it is desired to perform further ablation to increase the lesion size or to create lesions at different site(s) within the treatment region TR or elsewhere, the wires  62  of either or both of the ablation devices  18 ,  20  may be introduced and deployed at different target site(s), and the same steps discussed previously may be repeated. 
     Although an embodiment has been described with reference to placing ablation devices at different sites that are within a treatment region, the scope of the invention should not be so limited. In alternative embodiments, the ablation devices  18 ,  20  are disposed at different sites, each of which is associated with a treatment region. In such arrangement, separate tissues at different sites can be ablated simultaneously. In addition, it should be noted that the scope of the invention should not be limited to the ablation system  10  having two ablation devices. In alternative embodiments, the ablation system  10  can have more than two ablation devices. 
     Thus, although several preferred embodiments have been shown and described, it would be apparent to those skilled in the art that many changes and modifications may be made thereunto without the departing from the scope of the invention, which is defined by the following claims and their equivalents.