Patent Application
20110201973
The present invention relates generally to ablation devices and, more specifically, to an ultrasound compatible radiofrequency (RF) ablation electrode.
Catheters are flexible, tubular devices that are widely used by physicians performing medical procedures to gain access into interior regions of the body. For example, ablation catheters are sometimes used to perform ablation procedures to treat certain conditions of a patient. A patient experiencing arrhythmia, for example, may benefit from ablation to prevent irregular heart beats caused by arrhythmogenic electrical signals generated in cardiac tissues. By ablating or altering cardiac tissues that generate such unintended electrical signals the irregular heart beats may be stopped. Ablation catheters are known, and may include one or more ablation electrodes supplying RF (radiofrequency) energy to targeted tissue. With the aid of sensing and mapping tools that are also known, an electrophysiologist can determine a region of tissue in the body, such as cardiac tissue, that may benefit from ablation. One technique utilizes ultrasound imaging guidance for RF ablation therapy. See, e.g., U.S. Patent Application Publication No. 2007/0021744.
Embodiments of the present invention are directed to an ultrasound compatible ablation electrode for use in ultrasound imaging guidance of ablation therapy using RF or the like. The ablation electrode incorporates a plastic body coated with a thin metal film that provides electrical contact for RF ablation or the like and at the same time allows ultrasound to penetrate easily therethrough without substantial artifacts in the resulting image. The unique structure of the ablation electrode facilitates the proper functioning of two otherwise incompatible modalities that are RF tissue ablation and ultrasound imaging. As a result, an operator can use the ablation electrode to ultrasonically visualize the tissue to ablate and to ablate the tissue simultaneously in real time. Some of the advantages of this approach include a more precise placement of the catheter in or on the tissue to be ablated, improved ultrasound visualization of the ablation process including clot formation and tissue changes during and after ablation, and better decision making on the movement of the catheter if a linear or pattern ablation is to be made. The ablation electrode desirably has a dome or curved shape and includes a fluid cavity with one or more cooling fluid entry ports and one or more cooling fluid exit ports for an irrigated catheter.
In accordance with an aspect of the present invention, an ultrasound compatible ablation catheter comprises a catheter body having a distal end and an ultrasonic transducer directing ultrasonic beams for imaging a target; and an ablation electrode connected to the catheter body, the ablation electrode having a plastic shell and a metallic coating on the plastic shell which are disposed in a path of the ultrasonic beams of the ultrasonic transducer between the ultrasonic transducer and the target, the metallic coating of the ablation electrode to be energized for ablation.
In specific embodiments, the plastic shell has an acoustic impedance magnitude which is in a range of 1500×103 to 1750×103 Rayls (kg/m2s) at a temperature of 37° C. The metallic coating is substantially thinner than the plastic shell. The thickness of the plastic shell is preferably at least about 10 times the thickness of the metallic coating. The plastic shell has a thickness of at most about 500 microns and the metallic coating has a thickness of at most about 20 microns. The plastic shell comprises TPX® (polymethylpentene). The ablation region further comprises an electrical barrier layer on an exterior surface of the metallic coating, the barrier layer being substantially thinner than the metallic coating. The catheter body and the ablation electrode form a fluid cavity to store a fluid through which the ultrasonic beams of the ultrasonic transducer are transmitted across the ablation electrode to the target. The plastic shell has an acoustic impedance which is substantially equal to an acoustic impedance of the fluid. The catheter further comprises at least one fluid entry port for the fluid cavity, and at least one fluid exit port for the fluid cavity.
In specific embodiments, the ablation electrode comprises an ablation tip disposed near the distal end. The ablation tip is dome-shaped to provide a rounded ablation surface on the metallic coating. The ablation electrode has one of an uneven surface or a faceted surface to scatter reflective energy of the ultrasonic beams passing therethrough between the ultrasonic transducer and the target. The ultrasonic transducer is disposed on the distal end of the catheter body and comprises an array for forward looking imaging.
In some embodiments, the ultrasonic transducer and the ablation electrode are disposed on the catheter body, and the ultrasonic transducer comprises an array for side looking imaging. The ultrasonic transducer and the ablation electrode are disposed on opposite sides with respect to a longitudinal axis of the catheter body.
In specific embodiments, the catheter further comprises a control unit which controls an ultrasound generator to supply ultrasound energy to the ultrasonic transducer, an ultrasound receiver to accept echo signals, and an RF energy source to supply RF energy to the metallic coating of the ablation electrode, for ultrasound imaging and RF ablation simultaneously. The ablation electrode is constructed of materials and thicknesses to produce an absorption loss of less than about 50% of ultrasonic beam energy of the ultrasonic beams of the ultrasonic transducer for imaging the target.
In accordance with another aspect of the invention, an ultrasound compatible ablation catheter comprises a catheter body having a distal end and an ultrasonic transducer directing ultrasonic beams for imaging a target; and an ablation electrode connected to the catheter body, the ablation electrode having a plastic shell and a metallic coating on the plastic shell which are disposed in a path of the ultrasonic beams of the ultrasonic transducer between the ultrasonic transducer and the target, the metallic coating of the ablation electrode to be energized for ablation. The catheter body and the ablation electrode form a fluid cavity to contain a fluid through which the ultrasonic beams of the ultrasonic transducer are transmitted across the ablation electrode to the target. The plastic shell has an acoustic impedance magnitude which is in a range of 1500×103 to 1750×103 Rayls (kg/m2s) at a temperature of 37° C. The metallic coating is substantially thinner than the plastic shell.
In accordance with another aspect of the invention, an ultrasound compatible ablation catheter comprises a catheter body having a distal end and an ultrasonic transducer directing ultrasonic beams for imaging a target; and an ablation electrode connected to the catheter body, the ablation electrode having a plastic shell and a metallic coating on the plastic shell which are disposed in a path of the ultrasonic beams of the ultrasonic transducer between the ultrasonic transducer and the target, the metallic coating of the ablation electrode to be energized for ablation. The catheter body and the ablation electrode form a fluid cavity to contain a fluid through which the ultrasonic beams of the ultrasonic transducer are transmitted across the ablation electrode to the target. The plastic shell has an acoustic impedance which is substantially equal to an acoustic impedance of the fluid. The metallic coating is substantially thinner than the plastic shell.
These and other features and advantages of the present invention will become apparent to those of ordinary skill in the art in view of the following detailed description of the specific embodiments.
a is a schematic illustration of an ultrasound transducer in the form of a microlinear array disposed in the body of the catheter tip and parallel with the longitudinal dimension of the catheter.
b is a schematic illustration of an ultrasound transducer in the form of a microlinear array disposed in the body of the catheter tip and perpendicular with the longitudinal dimension of the catheter.
c is a schematic illustration of an ultrasound transducer in the form of a microlinear array disposed in the body of the catheter tip with both parallel and perpendicular features with respect to the longitudinal dimension of the catheter.
a and 14b show different arrangements of hole shapes and positions in the ultrasound compatible ablation member to permit irrigation fluid flow.
In the following detailed description of the invention, reference is made to the accompanying drawings which form a part of the disclosure, and in which are shown by way of illustration, and not of limitation, exemplary embodiments by which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. Further, it should be noted that while the detailed description provides various exemplary embodiments, as described below and as illustrated in the drawings, the present invention is not limited to the embodiments described and illustrated herein, but can extend to other embodiments, as would be known or as would become known to those skilled in the art. Reference in the specification to “one embodiment,” “this embodiment,” or “these embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention, and the appearances of these phrases in various places in the specification are not necessarily all referring to the same embodiment. Additionally, in the following detailed description, numerous specific details are set forth in order to produce a thorough understanding of the present invention. However, it will be apparent to one of ordinary skill in the art that these specific details may not all be needed to practice the present invention. In other circumstances, well-known structures, materials, circuits, processes and interfaces have not been described in detail, and/or may be illustrated in block diagram form, so as to not unnecessarily obscure the present invention.
In the following description, relative orientation and placement terminology, such as the terms horizontal, vertical, left, right, top and bottom, is used. It will be appreciated that these terms refer to relative directions and placement in a two dimensional layout with respect to a given orientation of the layout. For a different orientation of the layout, different relative orientation and placement terms may be used to describe the same objects or operations.
Exemplary embodiments of the invention, as will be described in greater detail below, provide apparatuses and methods for ultrasound imaging guidance of RF ablation therapy using an ultrasound compatible RF ablation electrode.
The ablation electrode 10 has a dome shape which may be generally spherical or elliptical, and is made of a plastic shell coated with a thin electrically conductive metal layer 30 on the outer surface. The metal layer 30 provides a rounded or dome-shaped ablation surface to provide a smooth atraumatic exterior and as a means of reflecting undesirable ultrasound echoes (see “A” and “B” in
The ultrasound transducer 20 may be configured in the form of an array.
Generally speaking, the ultrasound imaging pathway must be free of obstructions to permit ultrasound beam energy to penetrate to tissue depths so that the echo reflections can be visualized. Previous RF ablation tips are made of relatively thick metal with relatively broad contact areas to permit good distribution of RF energy into the tissue to be ablated, but they also obstruct ultrasound beam energy. In contrast, ultrasound, even at the higher common frequencies around 10 to 30 MHz, can penetrate easily a thin plastic such as TPX® (polymethylpentene, TPX is a trademark of Mitsui Chemicals, Inc.) without many artifacts in the resulting image. The addition of a thin metal film on the thin plastic ablation electrode is permissible, as long as the metal film is not too thick. Heretofore in RF ablation methodologies, a thin conductive metal layer would generally not be used due to the potential for heating at the ablation electrode and for reasons of mechanical strength. The present ablation electrode 10 allows for both cooling with irrigation fluid and strength in the use of a dome-like contact. In this design, a reasonable compromise can be struck between the needs of both ultrasound and ablation modalities.
The materials and dimensions of the ablation electrode 10 are chosen such that ultrasound from the transducer 20 to be used for image guidance and procedural feedback can penetrate the ablation electrode 10 without substantial distortion, so as to permit reasonably good imaging results of the tissue just beyond the ablation electrode 10. For instance, the ultrasound reflected intensity ratio with respect to the transmitted intensity is preferably less than about 0.01, more preferably less than about 0.0001, and most preferably less than about 10−6. The dome-like structure of the ablation electrode 10 provides a desirable broad surface at the electrode of a therapeutic ablation catheter which is metallized for the purpose of supplying a high power radiofrequency electrical signal to tissues of the body intended for thermal ablative therapy. The catheter includes irrigation fluid cooling of the ablation electrode 10 which is desirable, both as a way of avoiding tissue surface contact “charring” from the ablation and as a way of cooling the thin metal surface from heating beyond the a temperature suitable for the plastic material upon which the metal is supported. The moving water also helps prevent any air bubbles from forming and adhering to the inside of the tip.
The plastic shell of rounded, or faceted, shape is relatively thin and the metal coating is even thinner. The metallic coating is substantially thinner than the plastic shell (i.e., at least several times thinner). For example, the thickness of the plastic shell is preferably at least about 10 times the thickness of the metallic coating. In specific embodiments, the plastic shell has a thickness of equal to or less than about 500 microns (e.g., in the range of about 20 to 500 microns) and the metallic coating has a thickness of equal to or less than about 20 microns (e.g., in the range of several microns to possibly slightly more than 20 microns). The thicknesses discussed here are generally to be designed in an inverse relation with the frequency of ultrasound used. To make use of 10 MHz ultrasound imaging, for example, the plastic shell could be about 75 microns and the metallic coating a total thickness of about 3 microns.
For efficient transmission of the ultrasonic beams through the ablation electrode 10, the plastic shell has an acoustic impedance which preferably is substantially equal to an acoustic impedance of the fluid inside the fluid cavity formed by the distal end 16 of the catheter body 14 and the ablation electrode 10. The fluid is typically water or saline which produces low absorption loss of high frequency ultrasound in the range of about 5-30 MHz used for imaging. In use, the catheter 12 is typically inside a blood vessel with blood flowing therethrough. The acoustic impedance of blood is reasonably close to that of water or saline. The acoustic impedance magnitude of blood at 37° C. is the product of its acoustic velocity (1590 m/s) and density (1.06 g/cm3) (see, e.g., F. A. Duck, The Physical Properties of Tissue: A Comprehensive Reference Book, San Diego, Calif., Academic Press, Inc., 1990) or 1680×103 Rayls (kg/m2s). The acoustic impedance magnitude of cardiac tissue is very close to this impedance as well. In specific embodiments, the plastic shell has an acoustic impedance magnitude which is substantially equal to about 1680×103 Rayls (kg/m2s) at a temperature of about 37° C. The metallic coating 30 has a different acoustic impedance magnitude, but its effect on the absorption loss is kept relatively insignificant due to its small thickness.
The ablation electrode 10 is based upon several design elements that work well together. These design elements include the use of a plastic material (e.g., TPX) which has a low absorption loss at even high frequency ultrasound (e.g., about 10 MHz), the use of a high conductivity metal layers (e.g., platinum-iridium, or chrome/gold, or titanium, nickel, gold, etc.) which provides a good, large surface area for a low resistive contact to body tissues, the use of a dome-like electrode shape for the ablation electrode 10 which is strong (to prevent crushing upon contact) and provides a good contact surface that is not necessarily position dependent (for ease in contact with tissues), and the use of open irrigation fluid flow which is supplied by the catheter fluid channels to both cool the ablation electrode and help maintain its general dome-like shape via fluid flow pressure. The absorption loss depends on the acoustic impedance values of the ablation electrode as well as the material properties and thicknesses of the plastic shell and metallic coating of the ablation electrode. The ablation electrode 10 is constructed of materials and thicknesses to produce an absorption loss of preferably less than about 10% of the ultrasonic beam energy of the ultrasonic beams of the ultrasonic transducer 20 for imaging the target, more preferably less than about 1%, and most preferably less than about 0.1%.
In alternative embodiments, various aspects of the ablation electrode can be adjusted to optimize the design for specific operating conditions or environments. One example is the construction of the plastic shell, which may include TPX variants for the material, the thickness of the shell, the ultrasound characteristics, etc. The shell may be machined from a block or injection molded. Another example is the construction of the metal coating (material, thickness, ultrasound characteristics, etc.). Platinum-iridium is a standard material used in RF ablation devices, and this metal can potentially be sputtered onto the surface of the plastic shell. Gold can be sputtered as well. In any sputtering process in which a plastic is used as the substrate material, a “seed layer” metal is typically used which promotes good adhesion to the plastic substrate. Yet another example is the shape of the dome-like structure for imaging purposes and for mechanical strength reasons.
The surface features of the ablation electrode may be machined or molded. Another method of creating the surface features is by heat treating the plastic shell of the ablation electrode to a temperature slightly below melting so that the plastic shell starts to distort into an irregular shape. The metal layer is formed on the plastic shell with the surface irregularities after the heat treating process. Yet another way to create the surface features is to provide an ablation electrode that is sufficiently flexible such that when the ablation electrode is pressed against tissue to be ablated, the ablation electrode undergoes sufficient flexure or deformation so as to reduce or avoid undesired specular reflections of the imaging ultrasonic beams passing through the ablation electrode.
a is a schematic illustration of an ultrasound transducer in the form of a microlinear array disposed in the body of the catheter tip and parallel with the longitudinal dimension of the catheter. A transducer array 203a (one-dimensional array) has elements aligned in parallel with the longitudinal dimension of the catheter which permits an acoustic image plane 210a to exist as a plane at right angle with respect to the catheter shaft. The catheter 201 may be equipped with EP (electrophysiological) electrodes 209 which may be arranged on either side of the imaging array 203a and special metal coated ablation portion arranged as a cylindrical shell portion 202. Water irrigation inflow 207 may be produced which is used both for an ultrasound conduction medium in the region between the array 203a and the metal coated ablation portion 202, and as a coolant for the ablation surface which is the outer metallized surface of the ablation portion 202 in a manner similar to that described for the distal ablation tip of
b is a schematic illustration of an ultrasound transducer in the form of a microlinear array disposed in the body of the catheter tip and perpendicular with the longitudinal dimension of the catheter. The difference in
c is a schematic illustration of an ultrasound transducer in the form of a microlinear array 203c (two-dimensional array) disposed in the body of the catheter tip with both parallel and perpendicular features with respect to the longitudinal dimension of the catheter. The microlinear array 203c produces a volumetric beam 210c.
The array 303 can be located anywhere in the interior volume 305, but may preferentially be located below the center line (longitudinal axis) of the catheter to permit the ultrasound beam 304a to focus at a point 304b which is close to the catheter. This location of the array 303 well below the center line helps to avoid the undesirable coherently added echo reflections from the inside surface of the ultrasound compatible ablation member 302. A hole 302a in the ultrasound compatible ablation member 302 may be made to allow for a great proportion of the acoustic energy to be transmitted through this hole with very little degradation in the effectiveness of the metal electrode on the ablation member 302 as an EP ablation electrode.
The irrigation fluid path escape holes may be placed at any number of positions, but the position with the best effectiveness may be a hole 302a as shown. As discussed earlier, the irrigation fluid inflow 307 is brought to the interior volume or chamber 305 by a lumen 306. The walls of this chamber 305 are angled as shown to avoid undesired ultrasound echoes from the walls of this chamber. Similarly as described with the ultrasound compatible ablation member mounted at the tip of the catheter, the shape of the surface of the ultrasound compatible ablation member 302 may be either smooth or in plates or faceted to permit good ultrasound performance (i.e., good echo transmission through the ablation member but with few coherent echoes from the surface thereof). Since this chamber 305 for the ultrasound array 303 is located near the tip of the catheter, there should be plenty of room for the steering wire assemblies needed to steer the catheter. These anchor points can be made in the catheter region to the “left” of the chamber 305 region.
An interior region (312 in
The saline escape hole(s) 302a in the ultrasound compatible ablation member 302 may be made to allow for a great proportion of the acoustic energy to be transmitted through this hole with very little degradation in the effectiveness of the ablation member metal electrode as an EP ablation electrode. The water path escape holes may be placed at any number of positions, but the position with the best effectiveness is likely a hole 302a as shown. The hole(s) 302a in the ablation member 302 may be made with a rounded-rim feature 302b to enhance safety by assuring an atraumatic ablation member surface. The walls of this chamber 305 are preferably angled as shown to avoid undesired ultrasound echoes from the walls of this chamber.
a and 14b show different arrangements of hole shapes and positions of the holes 302a and gaps 308 in the ultrasound compatible ablation member 302 to permit irrigation fluid flow 307a. These variants shown are not mutually exclusive; many variations with hole positions, shapes, and numbers may be used to tailor a specific design requirement.
In the description, numerous details are set forth for purposes of explanation in order to produce a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that not all of these specific details are required in order to practice the present invention. Additionally, while specific embodiments have been illustrated and described in this specification, those of ordinary skill in the art appreciate that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments disclosed. This disclosure is intended to cover any and all adaptations or variations of the present invention, and it is to be understood that the terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with the established doctrines of claim interpretation, along with the full range of equivalents to which such claims are entitled.
This application claims the benefit of U.S. Provisional Patent Application No. 61/305,693, filed Feb. 18, 2010, the entire disclosure of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61305693 | Feb 2010 | US |
