US20080294155A1 - Radiation Applicator and Method of Radiating Tissue - Google Patents

Abstract

A dipole microwave applicator emits microwave radiation into tissue to be treated. The applicator is formed from a thin coax cable having an inner conductor surrounded by an insulator, which is surrounded by an outer conductor. A portion of the inner conductor extends beyond the insulator and the outer conductor. A ferrule at the end of the outer conductor has a step and a sleeve that surrounds a portion of the extended inner conductor. A tuning washer is attached to the end of the extended inner conductor. A dielectric tip encloses the tuning washer, the extended inner conductor, and the sleeve of the ferrule. The sleeve of the ferrule and the extended inner conductor operate as the two arms of the dipole microwave antenna. The tuning washer faces the step in the ferrule, and is sized and shaped to cooperate with the step in balancing and tuning the applicator.

Description

CROSS-REFERENCE TO RELATED APPLICATION
• The present application is related to commonly owned copending International patent application no. WO 2006/002943, concerning a Radiation Applicator and Method of Radiating Tissue and which is hereby incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• The present invention relates generally to medical technology, and more specifically to microwave radiation applicators and methods of thermal ablative treatment of tissue using radiated microwaves.
• 2. Background Information
• Thermal ablative therapies may be defined as techniques that intentionally decrease body tissue temperature (hypothermia) or intentionally increase body tissue temperature (hyperthermia) to temperatures required for cytotoxic effect, or to other therapeutic temperatures depending on the particular treatment. Microwave thermal ablation relies on the fact that microwaves form part of the electromagnetic spectrum causing heating due to the interaction between water molecules and the microwave radiation. The heat being used as the cytotoxic mechanism. Treatment typically involves the introduction of an applicator into tissue, such as tumors. Microwaves are released from the applicator forming a field around its tip. Heating of the water molecules occurs in the radiated microwave field produced around the applicator, rather than by conduction from the probe itself. Heating is therefore not reliant on conduction through tissues, and cytotoxic temperature levels are reached rapidly.
• Microwave thermal ablative techniques are useful in the treatment of tumors of the liver, brain, lung, bones, etc.
• U.S. Pat. No. 4,494,539 discloses a surgical operation method using microwaves, characterized in that microwaves are radiated to tissue from a monopole type electrode attached to the tip of a coaxial cable for transmitting microwaves. Coagulation, hemostasis or transaction is then performed on the tissue through the use of the thermal energy generated from the reaction of the microwaves on the tissue. In this way, the tissue can be operated in an easy, safe and bloodless manner. Therefore, the method can be utilized for an operation on a parenchymatous organ having a great blood content or for coagulation or transaction on a parenchymatous tumor. According to the method, there can be performed an operation on liver cancer, which has been conventionally regarded as very difficult. A microwave radiation applicator is also disclosed.
• U.S. Pat. No. 6,325,796 discloses a microwave ablation assembly and method, including a relatively thin, elongated probe having a proximal access end, and an opposite distal penetration end adapted to penetrate into tissue. The probe defines an insert passage extending therethrough from the access end to the penetration end thereof. An ablation catheter includes a coaxial transmission line with an antenna device coupled to a distal end of the transmission line for generating an electric field sufficiently strong enough to cause tissue ablation. The coaxial transmission line includes an inner conductor and an outer conductor separated by a dielectric material. A proximal end of the transmission line is coupled to a microwave energy source. The antenna device and the transmission line each have a transverse cross-sectional dimension adapted for sliding receipt through the insert passage while the elongated probe is positioned in the tissue. Such sliding advancement continues until the antenna device is moved to a position beyond the penetration end and further into direct contact with the tissue.
• However, a drawback with the existing techniques include the fact that they are not optimally mechanically configured for insertion into, and perforation of, the human skin, for delivery to a zone of soft tissue to be treated. Typically, known radiation applicator systems do not have the heightened physical rigidity that is desirable when employing such techniques.
• In addition, some radiation applicators made available heretofore do not have radiation emitting elements for creating a microwave field pattern optimized for the treatment of soft tissue tumors.
• Also, given the power levels employed in some applicators and treatments, there can be problems of unwanted burning of non-target, healthy tissue due to the very high temperatures reached by the applicator or the components attached thereto.
• Further, although small diameter applicators are known, and liquid cooling techniques have been used, there has been difficulty in designing a small diameter device with sufficient cooling in applications employing power levels required to deal with soft tissue tumors.
• Accordingly, there is a need for methods of treatment of soft tissue tumors, and for radiation applicators that overcome any or all of the aforementioned problems of the prior art techniques, and provide improved efficacy.
SUMMARY OF THE INVENTION
• In accordance with one aspect of the present invention, there is provided a dipole microwave applicator for emitting microwave radiation into tissue, the assembly comprising: an outer conductor having an end; an inner conductor disposed within the outer conductor, and including a section that extends outwardly beyond the end of the outer conductor; a ferrule disposed at the end of the outer conductor, and having a sleeve portion that surrounds a portion of the outwardly extending section of the inner conductor; and a dielectric tip surrounding the sleeve portion of the ferrule and the outwardly extending section of the inner conductor, whereby the sleeve portion of the ferrule and at least a portion of the outwardly extending section of the inner conductor operate as corresponding arms of the dipole microwave applicator.
• Particular embodiments are set out in the dependent claims.
• Briefly, the present invention is directed to a microwave applicator for ablating tissue. The applicator is a dipole microwave antenna that transmits microwave radiation into the tissue being treated. The applicator is formed from a thin coaxial cable having an inner conductor surrounded by an insulator, which is surrounded by an outer conductor or shield. The end of the coaxial cable is trimmed so that a portion of the insulator and inner conductor extend beyond the outer conductor, and a portion of the inner conductor extends beyond the insulator. The applicator further includes a tubular ferrule defining an aperture therethrough. One end of the ferrule is attached to the outer conductor, while the other end, which forms a sleeve, extends out beyond the end of the insulator and around a portion of the extended inner conductor. A step is preferably formed on the outer surface of the ferrule between its two ends. A solid spacer having a central bore to receive the inner conductor abuts an end of the ferrule and surrounds the extended inner conductor. A tuning element is attached to the end of the extended inner conductor, and abuts an end of the spacer opposite the ferrule. The tuning element faces the step in the ferrule, and the step and the tuning element are both sized and shaped to cooperate in balancing and tuning the applicator. A hollow tip, formed from a dielectric material, has an open end and a closed end. The tip encloses the tuning element, the spacer, and the extended inner conductor. The tip also encloses the sleeve of the ferrule, thus defining outer surface of the ferrule that is surrounded by the dielectric tip. The open end of the tip preferably abuts the step in the ferrule. A rigid sleeve surrounds the coaxial cable and extends away from the ferrule opposite the tip. The sleeve, which abuts the step of the ferrule opposite the tip, has an inner diameter that is larger than the coaxial cable, thereby defining an annular space between the outside of the coaxial cable and the inner surface of the sleeve. The sleeve further includes one or more drainage holes, which permit fluid communication between the annular space around the coaxial cable and the outside of the applicator.
• In operation, microwave energy from a source is applied to the coaxial cable, and is conveyed to the tip. The portion of the inner conductor that extends beyond the end of the ferrule forms one arm of the dipole, and emits microwave radiation. In addition, the microwave energy flowing along the inner conductor of the coaxial cable and in the aperture of the ferrule induces a current to flow along the outer surface of the sleeve of the ferrule that is surrounded by the tip. This, in turn, causes microwave radiation to be emitted from the sleeve of the ferrule, which operates as the second arm of the dipole. In this way, microwave energy is emitted along a substantial length of the applicator, rather than being focused solely from the tip. By distributing the emission of microwave radiation along a length of the applicator, higher power levels may be employed.
• To keep the coaxial cable and the applicator from overheating, a cooling fluid is introduced from a source into the annular space defined by the outside of the coaxial cable and the inside of the sleeve. The cooling fluid flows along this annular space, and absorbs heat from the coaxial cable. The cooling fluid, after having absorbed heat from the coaxial cable, then exits the annular space through the one or more drainage holes in the sleeve, and perfuses adjacent tissue.
• The closed end of the tip is preferably formed into a blade or point so that the Microwave applicator may be inserted directly into the tissue being treated. The tip, ferrule, and rigid sleeve, moreover, provide strength and stiffness to the applicator, thereby facilitating its insertion into tissue.
• The present invention further provides a method of treating target tissue, such as a tumor, the tumor being formed of, and/or being embedded within, soft tissue. The method includes inserting the microwave applicator into the tumor, and supplying electromagnetic energy to the applicator, thereby radiating electromagnetic energy into the tumor.
BRIEF DESCRIPTION OF THE DRAWINGS
• Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:
• FIG. 1 is a schematic, partial cross-sectional view of a radiation applicator in accordance with one embodiment of the invention;
• FIG. 2A shows an axial cross-section, andFIG. 2B shows an end elevation of the radiating tip of the radiation applicator ofFIG. 1;
• FIG. 3 shows a partial transverse cross-section of the tube of the radiation applicator ofFIG. 1;
• FIG. 4A shows a transverse cross-section, andFIG. 4B shows an axial cross-section of the tuning washer of the radiation applicator ofFIG. 1;
• FIG. 5A shows an axial cross-section, andFIG. 5B shows an end elevation of the ferrule of the radiation applicator ofFIG. 1;
• FIG. 6A shows an axial cross-section, andFIG. 6B shows a transverse cross-section of a handle section that may be attached to the radiation applicator ofFIG. 1;
• FIG. 7 illustrates the portion of coaxial cable that passes through the tube of the radiation applicator ofFIG. 1;
• FIG. 8 is a plot of S₁₁against frequency for the radiation applicator ofFIG. 1;
• FIG. 9A illustrates the E-field distribution, andFIG. 9B illustrates the SAR values around the radiation applicator ofFIG. 1, in use;
• FIGS. 10A-E show a preferred sequential assembly of the radiation applicator ofFIG. 1;
• FIG. 11 schematically illustrates a treatment system employing the radiation applicator ofFIG. 1;
• FIG. 12 is an exploded, perspective view of another embodiment of the present invention;
• FIGS. 13-18 show a preferred sequential assembly of the radiation applicator ofFIG. 12; and
• FIG. 19 is a schematic, partial cross-sectional view of the radiation applicator ofFIG. 12.
DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
• In the following description, like references are used to denote like elements, and where dimensions are given, they are in millimeters (mm). Further, it will be appreciated by persons skilled in the art that the electronic systems employed, in accordance with the present invention, to generate, deliver and control the application of radiation to parts of the human body may be as described in the art heretofore. In particular, such systems as are described in commonly owned published international patent applications W095/04385, W099/56642 and WOOO/49957 may be employed (except with the modifications described hereinafter). Full details of these systems have been omitted from the following for the sake of brevity.
• FIG. 1 is a schematic, partial cross-sectional view of a radiation applicator in accordance with one embodiment of the invention. The radiation applicator, generally designated102, includes a distal end portion of acoaxial cable104 that is used to couple to a source (not shown) of microwaves, acopper ferrule106, atuning washer108 attached on theend110 of the insulator part of thecoaxial cable104, and atip112. Preferably, theapplicator102 further includes ametal tube114.Tube114 is rigidly attached to theferrule106. Anannular space116 is defined between theouter conductor118 of thecable104 and the inner surface of thetube114, enabling cooling fluid to enter (in the direction of arrows A), contact the heated parts of theapplicator102 and exit in the direction of arrows B throughradial holes120 in thetube114, thereby extracting heat energy from theradiation applicator102.
• In assembly of theapplicator102, thewasher108 is soldered to asmall length122 of thecentral conductor124 of thecable104 that extends beyond theend110 of theinsulator126 of thecable104. Theferrule106 is soldered to a smallcylindrical section128 of theouter conductor118 of thecable104. Then, thetube114, which is preferably stainless steel, but may be made of other suitable materials, such as titanium or any other medical grade material, is glued to theferrule106 by means of an adhesive, such as Loctite638 retaining compound, at the contacting surfaces thereof, indicated at130 and132. Thetip112 is also glued preferably, using the same adhesive, on the inner surfaces thereof, to corresponding outer surfaces of theferrule106 and theinsulation126.
• When assembled, theapplicator102 forms a unitary device that is rigid and stable along its length, which may be of the order of 250 or somillimeters including tube114, thereby making theapplicator102 suitable for insertion into various types of soft tissue. Thespace116 andholes120 enable cooling fluid to extract heat from theapplicator102 through contact with theferrule106, theouter conductor118 of thecable104 and the end of thetube114. Theferrule106 assists, among other things, in assuring the applicator's rigidity. Theexposed end section134 ofcable104 from which theouter conductor118 has been removed, in conjunction with thedielectric tip112, are fed by a source of radiation of predetermined frequency. Theexposed end section134 anddielectric tip112 operate as a radiating antenna for radiating microwaves into tissue for therapeutic treatment. Theapplicator102 operates as a dipole antenna, rather than a monopole device, resulting in an emitted radiation pattern that is highly beneficial for the treatment of certain tissues, such as malignant or tumorous tissue, due to its distributed, spherical directly heated area.
• FIG. 2A shows an axial cross-section, andFIG. 2B shows an end elevation of thetip112 of theradiation applicator102 ofFIG. 1. As can be seen, thetip112 has innercylindrical walls202,204, and abuttingwalls206,208, for receiving and abutting thewasher108 and theferrule106, respectively, during assembly. Suitably, thetip112 is made of zirconia ceramic alloy. More preferably, it is a partially stabilized zirconia (PSZ) having yttria as the stabilizing oxidizing agent. Even more preferably, thetip112 is made of Technox 2000, which is a PSZ commercially available from Dynamic Cerarnic Ltd. of Staffordshire, England, having a very fine uniform grain compared to other PSZs, and a dielectric constant (k) of 25. As understood by those skilled in the art, the choice of dielectric material plays a part in determining the properties of the radiated microwave energy.
• It will be noted that the transverse dimensions of theapplicator102 are relatively small. In particular, the diameter ofapplicator102 is preferably less than or equal to about 2.4 mm. Thetip112, moreover, is designed to have dimensions, and be formed of the specified material, so as to perform effective tissue ablation at the operating microwave frequency, which in this case is preferably 2.45 Gigahertz (GHz). Theapplicator102 of the present invention is thus well adapted for insertion into, and treatment of, cancerous and/or non-cancerous tissue of the liver, brain, lung, veins, bone, etc.
• Theend210 of thetip112 is formed by conventional grinding techniques performed in the manufacture of thetip112. Theend210 may be formed as a fine point, such as a needle or pin, or it may be formed with an end blade, like a chisel, i.e. having a transverse dimension of elongation. The latter configuration has the benefit of being well suited to forcing thetip112 into or through tissue, i.e. to perforate or puncture the surface of tissue, such as skin.
• In use, thetip112 is preferably coated with a non-stick layer such as silicone or paralene, to facilitate movement of thetip112 relative to tissue.
• FIG. 3 shows a partial transverse cross-section of thetube114. As mentioned above, thetube114 is preferably made of stainless steel. Specifically, thetube114 is preferably made from 13 gauge thin wall304 welded hard drawn (WHD) stainless steel. Thetube114 is also approximately 215 mm in length. As can be seen, two sets ofradial holes120,120′ are provided at 12 mm and 13 mm, respectively, from theend302 of thetube114. Theseradial holes120,120′, as mentioned, permit the exit of cooling fluid. Although two sets of holes are shown, one, three, four or more sets of holes may be provided, in variants of the illustrated embodiment. In addition, although two holes per set are shown, three, four, five, or more holes per set may be provided, so long as the structural rigidity of thetube114 is not compromised. In this embodiment, theholes120,120′ are of 0.5 mm diameter, but it will be appreciated that this diameter may be quite different, e.g. any thing in the range of approximately 0.1 to 0.6 mm, depending on the number of sets of holes and/or the number of holes per set, in order to provide an effective flow rate. Although the illustrated distance from theend302 is 12 or 13 mm, in alternative embodiments, this distance may range from 3 mm to 50 mm from theend302, in order to control the length of track that requires cauterization.
• Further, in an embodiment used in a different manner, thetube114 may be omitted. In this case the treatment may comprise delivering the applicator to the treatment location, e.g., to the tumorous tissue, by suitable surgical or other techniques. For example, in the case of a brain tumor, the applicator may be left in place inside the tumor, the access wound closed, and a sterile connector left at the skull surface for subsequent connection to the microwave source for follow-up treatment at a later date.
• FIG. 4A shows a transverse cross-section, andFIG. 4B shows an axial cross-section of thetuning washer108. Thewasher108 is preferably made of copper, although other metals may be used. Thewasher108 has an innercylindrical surface402 enabling it to be soldered to thecentral conductor124 of the cable104 (FIG. 1). Although the washer is small, its dimensions are critical. Thewasher108 tunes theapplicator102, which operates as a dipole radiator, i.e., radiating energy from two locations, so that more effective treatment, i.e., ablation, of tissue is effected.
• FIG. 5A shows an axial cross-section, andFIG. 5B shows an end elevation of theferrule106. Theferrule106 is preferably made of copper, and is preferably gold plated to protect against any corrosive effects of the cooling fluid. Theferrule106 may be produced by conventional machining techniques, such as CNC machining.
• FIG. 6A shows an axial cross-section, andFIG. 6B shows a transverse cross-section at line B-B of ahandle section602 that may be attached to thetube114 of theradiation applicator102. Thehandle section602 is preferably made from the same material as thetube114, i.e., stainless steel. Thehandle section602 includes aforward channel604 enabling insertion of thetube114, and arear channel606 enabling insertion of thecoaxial cable104 during assembly. A transverse port608 having an internal thread610 enables the connection, through a connector, to a source of cooling fluid, discussed later. The connector may be formed from plastic. Once assembled, the arrangement ofhandle section602 enables cooling fluid to pass in the direction of arrow C into the tube114 (not shown).
• FIG. 7 illustrates the portion ofcoaxial cable104 that passes through thetube114. Thecable104 suitably comprises a low-loss, coaxial cable such as SJS070LL-253-Strip cable. Aconnector702, preferably a SMA female type connector permits connection of thecable104 to a microwave source (not shown), or to an intermediate section of coaxial cable (not shown) that, in turn, connects to the microwave source.
• FIG. 8 is a plot of S11 against frequency for theradiation applicator102 ofFIG. 1. This illustrates the ratio of reflective microwave power from the interface of theapplicator102 and treated tissue to total input power to theapplicator102. As can be seen, the design of theapplicator102 causes the reflected power to be a minimum, and therefore the transmitted power into the tissue to be a maximum, at a frequency of 2.45 GHz of the delivered microwaves.
• FIG. 9A shows the E-field distribution around theradiation applicator102 ofFIG. 1, in use. Darker colors adjacent to theapplicator102 indicate points of higher electric field. InFIG. 9A, the position of thewasher108 is indicated at902, and the position of the tip-ferrule junction is indicated at904. Two limited, substantially cylindrical zones906,908, of highest electric field are formed around theapplicator102 at thepositions902 and904 respectively.
• FIG. 9B shows the specific absorption rate (SAR) value distribution around theradiation applicator102 ofFIG. 1, in use. Darker colors adjacent theapplicator102 indicate points of SAR. InFIG. 9B, the position of thewasher108 is indicated at902, the position of the tip-ferrule junction is indicated at904, and the position of the ferrule-tube junction is indicated at905. Two limited, substantially cylindrical zones910,912, of highest SAR are formed around theapplicator102 at thepositions902 and between904 and905, respectively.
• FIGS. 10A-E show a preferred sequential assembly of components forming theradiation applicator102 ofFIG. 1. InFIG. 10A, thecoaxial cable104 is shown with theouter conductor118 and theinner insulator126 trimmed back, as illustrated earlier inFIG. 7.
• As shown inFIG. 10B, thetube114 is then slid over thecable104. Next, theferrule106 is slid over the cable104 (FIG. 10C), and fixedly attached to thetube114 and to thecable104, as described earlier. Then, thewasher108 is attached to theinner conductor124 by soldering, as shown inFIG. 1D. Finally, thetip112 is slid over thecable104 and part of theferrule106, and affixed thereto, as described earlier. The completed applicator is shown inFIG. 10E. This results in a construction of great rigidity and mechanical stability.
• FIG. 11 schematically illustrates atreatment system1102 employing theradiation applicator102 ofFIG. 1.Microwave source1104 is couple to the input connector1106 onhandle602 bycoaxial cable1108. In this embodiment, the microwave power is supplied at up to 80 Watts. However this could be larger for larger size applicators, e.g., up to 200 Watts for 5 mm diameter radiation applicators.
• Syringe pump1110 operates asyringe1112 for supplyingcooling fluid1114 viaconduit1116 andconnector1118 attached to handle602, to the interior of thehandle section602. The fluid is not at great pressure, but is pumped so as to provide a flow rate of about 1.5 to 2.0 milliliter(ml)/minute through thepipe114 in the illustrated embodiment. However, in other embodiments, where theradiation applicator102 is operated at higher powers, higher flow rates may be employed, so as to provide appropriate cooling. The cooling fluid is preferably saline, although other liquids or gases may be used, such as ethanol. In certain embodiments, a cooling liquid having a secondary, e.g., cytotoxic, effect could be used, enhancing the tumor treatment. In the illustrative embodiment, the cooling fluid1114 exits thetube1114, as shown by arrows B inFIG. 1, at a temperature on the order of 10° C. higher than that at which it enters thetube114, as shown by arrows A inFIG. 1. Thus, substantial thermal energy is extracted from the coaxial cable. The cooling fluid1114 may, for example, enter thetube114 at room temperature. Alternatively, the cooling fluid1114 may be pre-cooled to a temperature below room temperature by any suitable technique.
• As shown, the cooling system is an open, perfusing cooling system that cools the coaxial cable connected to theradiation applicator102. That is, after absorbing heat from the coaxial cable, the cooling fluid perfuses the tissue near theradiation applicator102.
• The methodology for use of theradiation applicator102 of the present invention may be as conventionally employed in the treatment of various soft tissue tumors. In particular, theapplicator102 is inserted into the body, laparoscopically, percutaneously or surgically. It is then moved to the correct position by the user, assisted where necessary by positioning sensors and/or imaging tools, such as ultrasound, so that thetip112 is embedded in the tissue to be treated. The microwave power is switched on, and the tissue is thus ablated for a predetermined period of time under the control of the user. In most cases, theapplicator102 is stationary during treatment. However, in some instances, e.g., in the treatment veins, theapplicator102 may be moved, such as a gentle sliding motion relative to the target tissue, while the microwave radiation is being applied.
• As described above, and as shown inFIGS. 9A and 9B,radiation applicator102, is a dipole antenna. The portion of theinner conductor124 that extends beyond theferrule106 operates as one arm of the dipole antenna. In addition, the transmission of microwave energy along theinner conductor124 and in the aperture of the ferrule induces a current to flow on that portion of the outer surface of theferrule106 that is located underneath thetip112. This induced current causes this enclosed, outer surface of theferrule106 to emit microwave radiation, thereby forming a second arm of the dipole antenna. The bipolar configuration of the applicator effectively spreads the microwave radiation that is being transmitted by theapplicator102 along a greater transverse, i.e., axial, length of theantenna102, rather than focusing the radiation transmission solely from thetip112 of theapplicator102. As a result, theapplicator102 of the present invention may be operated at much higher power levels, e.g., up to approximately 80 Watts, than prior art designs.
• An alternative embodiment of the present invention is shown inFIGS. 12-19.FIG. 12 is an exploded, perspective view of analternative radiation applicator1202. As shown, theapplicator1202 includes acoaxial cable1204 having anouter conductor1206 that surrounds aninsulator1208 that, in turn, surrounds an inner orcentral conductor1210. Theapplicator1202 further includes aferrule1212. Theferrule1212 is generally tubular shaped so as to define an aperture therethrough, and has first andsecond ends1212a,1212b. Theferrule1212 also has three parts or sections. Afirst section1214 of theferrule1212 has an inner diameter sized to fit over theouter conductor1206 of thecoaxial cable1204. Asecond section1216 of theferrule1212 has an inner diameter that is sized to fit over theinsulator1208 of thecoaxial cable1204. Thesecond section1216 thus defines an annular surface or flange (not shown) around the inside theferrule1212. The outer diameter of thesecond section1216 is preferably larger than the outer diameter of thefirst section1214, thereby defining a step or flange around the outside of theferrule1212. Athird section1218 of theferrule1212 has an inner diameter also sized to fit around theinsulator1208 of thecoaxial cable1204. Thethird section1218 has an outside diameter that is less than the outside diameter of thesecond section1216. Thethird section1218 thus defines an outer, cylindrical surface or sleeve.
• Applicator1202 further includes aspacer1220. Thespacer1220 is preferably cylindrical in shape with acentral bore1222 sized to receive theinner conductor1210 of thecoaxial cable1204. The outer diameter of thespacer1220 preferably matches the outer diameter of thethird section1218 of theferrule1212.Applicator1202 also includes atuning element1224 and atip1226. Thetuning element1224, which be may be disk-shaped, has acentral hole1228 sized to fit around theinner conductor1210 of thecoaxial cable1204. Thetip1226 is a hollow, elongated member, having anopen end1230, and aclosed end1232. Theclosed end1232 may be formed into a cutting element, such as a trocar point or a blade, to cut or pierce tissue.Applicator1202 also includes arigid sleeve1234. Thesleeve1234 has an inner diameter that is slightly larger than outer diameter of thecoaxial cable1204. As described below, an annular space is thereby defined between the outer surface of thecoaxial cable1204 and the inner surface of thesleeve1234. Thesleeve1234 further includes one ormore drainage holes1236 that extend through the sleeve.
• FIGS. 13-18 illustrate a preferred assembly sequence of theapplicator1202. As shown inFIG. 13, thecoaxial cable1204 is trimmed so that there is a length “m” ofinsulator1208 that extends beyond anend1206aof theouter conductor1206, and a length “l” ofinner conductor1210 that extends beyond anend1208aof theinsulator1208. Theferrule1212 slides over the exposedinner conductor1210 and over the exposedinsulator1208 such that thefirst section1214 surrounds theouter conductor1206, and the second andthird sections1216,1218 surround the exposed portion of theinsulator1208. The inner surface or flange formed on thesecond section1216 of theferrule1212 abuts theend1206aof theouter conductor1206, thereby stopping theferrule1212 from sliding any further up thecoaxial cable1204. Theferrule1212 is preferably fixedly attached to thecoaxial cable1204, such as by soldering theferrule1212 to theouter conductor1206 of thecoaxial cable1204. In the preferred embodiment, thethird section1218 of theferrule1212 extends past theend1208aof the exposedinsulator1208 as shown by the dashed line inFIG. 14.
• Next, thespacer1220 is slid over the exposed portion of theinner conductor1210, and is brought into contact with the second end1212bof theferrule1212. In the preferred embodiment, thespacer1220 is not fixedly attached to theferrule1212 or theinner conductor1210. Thespacer1220 is sized so that asmall portion1210a(FIG. 15) of theinner conductor1210 remains exposed. Thetuning element1224 is then slid over this remaining exposedportion1210aof theinner conductor1210. Thetuning element1224 is preferably fixedly attached to theinner conductor1210, e.g., by soldering. Thetuning element1224, in cooperation with theferrule1212, thus hold thespacer1220 in place.
• With thetuning element1224 in place, the next step is to install thetip1226 as shown inFIG. 16. Theopen end1230 of thetip1226 is slid over thetuning element1224, thespacer1224 and thethird section1218 of theferrule1212. Theopen end1230 of thetip1226 abuts the second section orstep1216 of theferrule1212. Thetip1226 is preferably fixedly attached to theferrule1212, e.g., by bonding. With thetip1226 in place, the next step is to install the sleeve1234 (FIG. 17). Thesleeve1234 is slid over thecoaxial cable1234, and up over thefirst section1214 of theferrule1212. Thesleeve1234 abuts thestep1216 in theferrule1212 opposite thetip1226.
• Those skilled in the art will understand that theapplicator1202 may be assembled in different ways or in different orders.
• As illustrated inFIG. 18, upon assembly, thetip1226,second section1216 of theferrule1212, andsleeve1234 all preferably have the same outer diameter, thereby giving the applicator1202 a smooth outer surface.
• Preferably, thesleeve1234 is formed from stainless steel, and theferrule1212 is formed from gold-plated copper. Thetip1226 and thespacer1220 are formed from dielectric materials. In the illustrative embodiment, thetip1226 and thespacer1220 are formed from an itrium stabilized zirconia, such as the Technox brand of ceramic material commercially available from Dynamic Ceramic Ltd. of Stoke-on-Trent, Staffordshire, England, which has a dielectric constant of 25. Thetip1226 may be further provided with a composite coating, such as a polyimide undercoat layer, for adhesion, and a paralyne overcoat layer, for its non-stick properties. Alternatively, silicone or some other suitable material could be used in place of paralyne. The composite coating may also be applied to the ferrule and at least part of the stainless steel sleeve, in addition to being applied to the tip.
• Those skilled in the art will understand that alternative materials may be used in the construction of theradiation applicator1202.
• FIG. 19 is a schematic, partial cross-sectional view of theradiation applicator1202. As shown, at least part of thefirst section1214 of theferrule1212 overlies and is attached to theouter conductor1206. Theinsulator1208 extends partially through the inside of theferrule1212. In particular, theend1208aof theinsulator1208 is disposed a predetermined distance back from the second end1212bof theferrule1212. Theinner conductor1210 extends completely through and beyond theferrule1212. Thesleeve1234 slides over and is bonded to thefirst section1214 of theferrule1212. As shown, the inside diameter of thesleeve1234 is greater than the outside diameter of thecoaxial cable1204, thereby defining anannular space1238 between the outside of thecoaxial cable1204 and the inside of thesleeve1234. Cooling fluid, such as saline, is pumped through thisannular space1238, as shown by arrows A. The cooling fluid absorbs heat from the coaxial cable that feeds radiation toapplicator1202. The cooling fluid is then discharged throughholes1236 in thesleeve1234, as shown by arrows B.
• In the preferred embodiment, theholes1236 are placed far enough behind theclosed end1232 of thetip1226 such that the discharged cooling fluid does not enter that portion of the tissue that is being heated by theradiation applicator1202. Instead, the discharged cooling fluid preferably perfuses tissue outside of this heated region. Depending on the tissue to be treated, a suitable distance between theclosed end1232 of thetip1226 and theholes1236 may be approximately 30 mm.
• Afirst end1220aof thespacer1220 abuts the second end1212bof theferrule1212, while a second end1220bof thespacer1220 abuts thetuning element1224. Accordingly a space, designated generally1240, is defined within theferrule1212 between theend1208aof the insulator and the second end1212bof the ferrule. In the illustrative embodiment, thisspace1240 is filled with air. Those skilled in the art will understand that the space may be filled with other materials, such as a solid dielectric, or it may be evacuated to form a vacuum. The inside surface of thetip1226 preferably conforms to the shape of thetuning element1224, thespacer1220, and thethird section1218 of theferrule1212 so that there are no gaps formed along the inside surface of thetip1226.
• As indicated above, operation of theradiation applicator1202 causes a current to be induced on the outer surface of thethird section1218 of theferrule1212, which is enclosed within the dielectric material of thetip1226. This induced current results in microwave energy being radiated from this surface of theferrule1212, thereby forming one arm of the dipole. The section of theinner conductor1210 that extends beyond theferrule1212 is the other arm of the dipole. Both the length of theinner conductor1210 that extends beyond theferrule1212, and the length of thethird section1218 of theferrule1212, which together correspond to the two arms of the dipole, are chosen to be approximately ¼ of the wavelength in thedielectric tip1226, which in the illustrative embodiment is approximately 6 mm. Nonetheless, those skilled in the art will understand that other factors, such as tissue permittivity, the action of the tuning element, etc., will affect the ultimate lengths of the dipole arms. For example, in the illustrative embodiment, the two arms are approximately 5 mm in length.
• Thetuning element1224, moreover, cooperates with the second section orstep1216 of the ferrule to balance the radiation being emitted by the two arms of the dipole. In particular, the size and shape of thetuning element1224 and thestep1216 are selected such that the coherent sum of the microwave power reflected back toward the cable at the aperture of the ferrule is minimized. Techniques for performing such design optimizations are well-known to those skilled in the relevant art.
• In use, theradiation applicator1202 is attached to a source of microwave radiation in a similar manner as described above in connection with theapplicator102 ofFIG. 1. The coaxial cable is also attached to a source of cooling fluid in a similar manner as described above. With the present invention, it is the dielectric tip, ferrule and stainless steel sleeve that cooperate to provide the necessary stiffness and mechanical strength for the applicator to be used in treatment procedures. The applicator does not rely on the coaxial cable for any of its strength. Indeed, a flexible coaxial cable, having little or no rigidity, could be used with the radiation applicator of the present invention.
• The foregoing has been a detailed description of illustrative embodiments of the invention. Various modifications and additions can be made without departing from the spirit and scope thereof. For example, the materials described herein are not exhaustive, and any acceptable material can be employed for any component of the described system and method. In addition, modifications can be made to the shape of various components. Accordingly, this description is meant to be taken only by way of example, and not to otherwise limit the scope of the invention.

Claims (15)

1. A dipole microwave applicator for emitting microwave radiation into tissue, the assembly comprising:

an outer conductor having an end;

an inner conductor disposed within the outer conductor, and including a section that extends outwardly beyond the end of the outer conductor;

a ferrule disposed at the end of the outer conductor, and having a sleeve portion that surrounds a portion of the outwardly extending section of the inner conductor; and

a dielectric tip surrounding the sleeve portion of the ferrule and the outwardly extending section of the inner conductor,

whereby the sleeve portion of the ferrule and at least a portion of the outwardly extending section of the inner conductor operate as corresponding arms of the dipole microwave applicator.

2. The dipole microwave applicator ofclaim 1, further comprising a dielectric spacer disposed within the dielectric tip, the dielectric spacer surrounding at least a portion of the inner conductor that extends beyond the sleeve portion of the ferrule.

3. The dipole microwave applicator ofclaim 1, wherein the ferrule has a first end that is attached to the end of the outer conductor.

4. The dipole microwave applicator ofclaim 1, further comprising a tuning element disposed within the dielectric tip, and attached to an end of the inner conductor.

5. The dipole microwave applicator ofclaim 4, wherein

the ferrule further includes a step adjacent to the sleeve portion, and

the tuning element and step cooperate to balance the corresponding arms of the dipole microwave applicator.

6. The dipole microwave applicator ofclaim 5, wherein the tuning element is substantially disc shaped.

7. The dipole microwave applicator ofclaim 5, further comprising a rigid sleeve adjacent to the ferrule, and surrounding and spaced from at least a portion of the outer conductor so as to define a space between the outer conductor and the rigid sleeve.

8. The dipole microwave applicator ofclaim 7, wherein one or more holes extend through the rigid sleeve, the one or more holes providing a fluid communication path from the space within the rigid sleeve to an area outside of the rigid sleeve.

9. The dipole microwave applicator ofclaim 1, wherein the ferrule is formed from copper, and the tip is formed from itrium stabilized zirconia.

10. The dipole microwave applicator ofclaim 8, wherein the sleeve is formed from stainless steel, the ferrule is formed from copper, and the tip is formed from itrium stabilized zirconia.

11. The dipole microwave applicator ofclaim 1, wherein microwave energy at a frequency of approximately 2.45 Gigahertz (GHz) and a power level of up to 80 watts is applied to the applicator.

12. The dipole microwave applicator ofclaim 2, further comprising an insulator disposed between the outer conductor and the inner conductor, wherein the spacer abuts an end of the sleeve of the ferrule and the insulator terminates within the sleeve so as to define a gap within the sleeve of the ferrule around the inner conductor.

13. The dipole microwave applicator ofclaim 10, wherein the gap is filled with air.

14. The dipole microwave applicator ofclaim 5, wherein

the dielectric tip has open end that abuts the step in the ferrule and a closed end opposite the open end, and

the closed end is configured for one of cutting or piercing tissue.

15. The dipole microwave applicator ofclaim 1, wherein at least one of the dielectric tip and the ferrule is coated with an inner layer of polyimide and an outer layer of paralyne.

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