Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 12/556,238, filed Sep. 9, 2009 and entitled “Method for Constructing a Dipole Antenna,” the contents of which are hereby incorporated by reference herein in their entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates generally to microwave applicators used in tissue ablation procedures. More particularly, the present disclosure is directed to a microwave applicator having either a liquid or solid loaded tip dipole antenna. 
         [0004]    2. Background of Related Art 
         [0005]    Treatment of certain diseases requires destruction of malignant tissue growths (e.g., tumors). It is known that tumor cells denature at elevated temperatures that are slightly lower than temperatures injurious to surrounding healthy cells. Therefore, known treatment methods, such as hyperthermia therapy, heat tumor cells to temperatures above 41° C., while maintaining adjacent healthy cells at lower temperatures to avoid irreversible cell damage. Such methods involve applying electromagnetic radiation to heat tissue and include ablation and coagulation of tissue. In particular, microwave energy is used to coagulate and/or ablate tissue to denature or kill the cancerous cells. 
         [0006]    Microwave energy is applied via microwave ablation antennas that penetrate tissue to reach tumors. There are several types of microwave antennas, such as monopole and dipole. In monopole and dipole antennas, microwave energy radiates perpendicularly from the axis of the conductor. A monopole antenna includes a single, elongated microwave conductor. Dipole antennas may have a coaxial construction including an inner conductor and an outer conductor separated by a dielectric portion. More specifically, dipole microwave antennas may have a long, thin inner conductor that extends along a longitudinal axis of the antenna and is surrounded by an outer conductor. In certain variations, a portion or portions of the outer conductor may be selectively removed to provide for more effective outward radiation of energy. This type of microwave antenna construction is typically referred to as a “leaky waveguide” or “leaky coaxial” antenna. 
         [0007]    Conventional microwave antennas typically has a long, thin inner conductor which extends along the axis of the probe and is surrounded by a dielectric material and is further surrounded by an outer conductor around the dielectric material such that the outer conductor also extends along the axis of the probe. In another variation of the probe which provides for effective outward radiation of energy or heating, a portion or portions of the outer conductor can be selectively removed. This type of construction is typically referred to as a “leaky waveguide” or “leaky coaxial” antenna. Another variation on the microwave probe involves having the tip formed in a uniform spiral pattern, such as a helix, to provide the necessary configuration for effective radiation. This variation can be used to direct energy in a particular direction, e.g., perpendicular to the axis, in a forward direction (i.e., towards the distal end of the antenna), or a combination thereof. 
         [0008]    Invasive procedures and devices have been developed in which a microwave antenna probe may be either inserted directly into a point of treatment via a normal body orifice or percutaneously inserted. Such invasive procedures and devices potentially provide better temperature control of the tissue being treated. Because of the small difference between the temperature required for denaturing malignant cells and the temperature injurious to healthy cells, a known heating pattern and predictable temperature control is important so that heating is confined to the tissue to be treated. For instance, hyperthermia treatment at the threshold temperature of about 41.5° C. generally has little effect on most malignant growths of cells. However, at slightly elevated temperatures above the approximate range of 43° C. to 45° C., thermal damage to most types of normal cells is routinely observed; accordingly, great care must be taken not to exceed these temperatures in healthy tissue. 
         [0009]    However, many types of malignancies are difficult to reach and treat using non-invasive techniques or by using invasive antenna probes designed to be inserted into a normal body orifice, i.e., a body opening which is easily accessible. These types of conventional probes may be more flexible and may also avoid the need to separately sterilize the probe; however, they are structurally weak and typically require the use of an introducer or catheter to gain access to within the body. Further, the manufacturing techniques for the conventional probe tend to be cumbersome, time consuming, and prohibitively expensive. Moreover, the addition of introducers and catheters necessarily increase the diameter of the incision or access opening into the body thereby making the use of such probes more invasive and further increasing the probability of any complications that may arise. 
       SUMMARY 
       [0010]    A method of fabricating a microwave antenna assembly, which is structurally robust enough for unaided direct insertion into tissue is described herein. The microwave antenna assembly is generally comprised of a radiating portion which may be connected to a feedline (or shaft) which in turn may be connected by a cable to a power generating source such as a generator. The microwave assembly may be a monopole microwave antenna assembly but is preferably a dipole assembly. The distal portion of the radiating portion preferably has a tapered end which terminates at a tip to allow for the direct insertion into tissue with minimal resistance. The proximal portion is located proximally of the distal portion. 
         [0011]    The adequate rigidity necessary for unaided direct insertion of the antenna assembly into tissue, e.g., percutaneously, preferably comes in part by a variety of different methods. A method of fabricating an antenna includes providing a proximal portion having an inner conductor and an outer conductor, the inner conductor extending at least partially therein. The method further includes providing a distal portion disposed distally of the proximal portion, with the inner conductor extending at least partially therein. A high strength polyimide material may be injected from an inflow slot to an outflow slot of the distal portion such that the polyimide material is disposed in-between the inner conductor and a ceramic layer. The polyimide material bonds the distal portion and the ceramic layer to the proximal portion while providing mechanical strength to the distal portion. 
         [0012]    To further aid in strengthening the antenna assemblies the inner conductor may be affixed within the distal radiating portion in a variety of ways, for instance, welding, brazing, soldering, or through the use of adhesives. Forcing the inner conductor into a tensile condition helps to force the outer diameter of the antenna into a compressive state. This bi-directional stress state in turn aids in rigidizing the antenna assembly. 
         [0013]    To enable a compressive state to exist near the outer diameter of the distal portion, a ceramic layer may be bonded to the polyimide material. Materials such as ceramic generally have mechanical properties where fracturing or cracking in the material is more likely to occur under tensile loading conditions. Accordingly, placing the distal portion under pre-stressed conditions, may aid in preventing mechanical failure of the distal portion if the antenna were to incur bending moments during insertion into tissue which could subject the distal portion under tensile loads. The ceramic layer and a coolant jacket also act as a dielectric buffer for aiding in keeping the efficiency of the antenna constant event though tissue is changing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the following detailed description when taken in conjunction with the accompanying drawings in which: 
           [0015]      FIG. 1  is a schematic diagram of a microwave ablation system according to an embodiment of the present disclosure; 
           [0016]      FIG. 2  is an isometric view of a microwave antenna assembly according to the present disclosure; 
           [0017]      FIG. 3  is an enlarged, cross-sectional view of a portion of the microwave antenna assembly of  FIG. 2 ; 
           [0018]      FIG. 4  is an enlarged, cross-sectional view of a portion of the microwave antenna assembly of  FIG. 2 ; 
           [0019]      FIG. 5  is a side view of a distal portion of a feedline of the microwave antenna assembly of  FIG. 2 ; 
           [0020]      FIG. 6  is an exploded view of the microwave antenna assembly according to the present disclosure; 
           [0021]      FIGS. 7A-7C  are enlarged cross-sectional views of sections A-A, B-B, and C-C of the microwave antenna assembly of  FIG. 4 ; 
           [0022]      FIG. 8  is a schematic diagram of a mold according to the present disclosure; and 
           [0023]      FIG. 9  is a side view of another tip of the microwave assembly of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Particular embodiments of the present disclosure will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail. 
         [0025]      FIG. 1  shows a microwave ablation system  10  that includes a microwave antenna assembly  12  coupled to a microwave generator  14  via a flexible coaxial cable  16 . The microwave antenna assembly  12  may be a dipole antenna of 1.6 cm in length. In order to ablate small tumors, microwave antenna assembly  12  has a Short Radiating Section (SRS). The microwave antenna assembly  12  is capable of reducing the antenna length to one-quarter of the wavelength length required, effectively using half the length of the half wave length dipole antenna. The generator  14  is configured to provide microwave energy at an operational frequency from about 500 MHz to about 5000 MHz. 
         [0026]    Antenna assembly  12  is generally comprised of radiating portion  18 , which may be connected by feedline  20  (or shaft) to the cable  16 . More specifically, the antenna assembly  12  is coupled to the cable  16  through a connection hub  22 . The connection hub  22  also includes an outlet fluid port  30  and an inlet fluid port  32  defined therein that are in fluid communication with a coolant jacket  38  and flow channel  13  (see  FIG. 4 ). The coolant jacket  38  encloses a proximal portion  42  and the feedline  20  allowing coolant fluid from the ports  30  and  32  to be supplied and circulated around a portion of the antenna assembly  12 . The ports  30  and  32  also include inner lumens defined therein (not shown) that are in fluid communication with the flow channel  13 . The ports  30  and  32  are coupled to a supply pump  34  that is, in turn, coupled to a supply tank  36 . The supply tank  36  stores the coolant fluid and maintains the fluid at a predetermined temperature. In one embodiment, the supply tank  36  may include a coolant unit which cools the returning liquid from the antenna assembly  12 . In another embodiment, the coolant fluid may be a gas and/or a mixture of fluid and gas. 
         [0027]    Assembly  12  also includes a tip  48  having a tapered end  24  that terminates, in one embodiment, at a pointed end  26  to allow for insertion into tissue with minimal resistance at a distal end of the radiating portion  18 . In those cases where the radiating portion  18  is inserted into a pre-existing opening, tip  48  may be rounded or flat. 
         [0028]      FIG. 2  illustrates the radiating portion  18  of the antenna assembly  12  having an unbalanced dipole antenna  40 . The dipole antenna  40  includes a proximal portion  42  and a distal portion  44  interconnected by an injection molded seal  46 . The distal portion  44  and the proximal portion  42  are of different, unequal lengths so that the dipole antenna  40  is unbalanced. In one embodiment, the distal portion  44  may be longer than the proximal portion  42 . In one embodiment, in which the feedline  20  is formed from a coaxial cable, the outer conductor  56  and the inner insulator  52  may be sliced off to reveal the inner conductor  50 , as shown in  FIG. 5 . 
         [0029]    The dipole antenna  40  is coupled to the feedline  20  that electrically connects antenna assembly  12  to the generator  14  ( FIG. 1 ). The assembly  12  includes a coolant jacket  38  coupled to a fluid seal  8  (see  FIG. 4 ), which in turn is coupled to an injection molded seal  46 . The coolant jacket  38  may be formed from a medical grade metal. The injection molded seal  46  may be made of a high strength polyimide resin. The polyimide resin may be VESPEL® sold by DuPont of Wilmington, Del. 
         [0030]    In one embodiment, the injection molded seal  46  is fabricated by injecting a polyimide material into an inflow slot  9 A to an outflow slot  9 B of the distal portion  44 . As shown in  FIG. 3 , the injection molded seal  46  is disposed in-between a distal radiating section  44  and the ceramic layer  2 . The ceramic layer may be made of alumina ceramic. The injection molded seal  46  bonds the distal portion  44  and the ceramic layer  2  to the proximal portion  42  while providing mechanical strength to the distal portion  44 . As shown in  FIGS. 3-4 , the feedline  20  includes an inner conductor  50  (e.g., wire) surrounded by an inner insulator  52 , which is then surrounded by an outer conductor  56  (e.g., cylindrical conducting sheath). The inner and outer conductors  50 ,  56  may be constructed of copper, gold, stainless steel or other conductive metals with similar conductivity values. The metals may be plated with other materials, e.g., other conductive materials, to improve their properties, e.g., to improve conductivity or decrease energy loss, etc. In one embodiment, the inner insulator layer  52  is formed from a fluoropolymer, such as tetrafluorethylene, perfluorpropylene, and the like, and has a thickness of about 0.011-0.013 inches. 
         [0031]    In one embodiment, the feedline  20  may be formed from a coaxial semi-rigid or flexible cable having a wire with a 0.047″ outer diameter rated for 50 Ohms. The inner insulator  52  may have a dielectric constant from about 1 to 10. 
         [0032]    Overlaying the outer conductor  56  is a flow channel  13  that cools the majority of the proximal portion  42 . The flow channel  13  is in fluid communication with fluid ports  30 ,  32 . A polyimide inflow sleeve  15  is disposed in the flow channel  13  to create an inflow channel  17   a  and an outflow channel I  7   b  for the coolant. The abundance of cooling fluid from the concentric in-flow design of the polyimide inflow sleeve  15  in the flow channel  13  acts as a lossy material to absorb the microwave energy as well as to cool the feedline  20  for percutaneous use. 
         [0033]    In another embodiment, the fluid seal  8  may also be formed from solid wire machined component or a cylindrical conductor filled with solder. The fluid seal  8  is thereafter coupled to the outer conductor  56  (joint E), as shown in  FIGS. 3-4 . This may be accomplished by soldering the fluid seal  8  to the outer conductor  56 , such as by melting the solder of the fluid seal  8  and inserting the outer conductor  56  therein. 
         [0034]    The distal portion  44  includes a conductive member  45  that may be formed from any type of conductive material, such as metals (e.g., copper, stainless steel, tin, and various alloys thereof). The distal portion  44  may have a solid structure and may be formed from solid wire (e.g., 10 AWG). In another embodiment, the distal portion  44  may be formed from a hollow sleeve of an outer conductor of coaxial cable or another cylindrical conductor. The cylindrical conductor may then be filled with solder to convert the cylinder into a solid shaft. More specifically, the solder may be heated to a temperature sufficient to liquefy the solder within the cylindrical conductor (e.g., 500° F.), thereby creating a solid shaft. 
         [0035]    As shown in  FIGS. 2 and 3 , the distal portion  44  is coupled to the tip  48 , which may be formed from a variety of heat-resistant materials suitable for penetrating tissue, such as metals (e.g., stainless steel) and various thermoplastic materials, such as poletherimide, polyimide thermoplastic resins, an example of which is Ultem® sold by General Electric Co. of Fairfield, Conn. The tip  48  may be machined from various stock rods to obtain a desired shape. The tip  48  may be attached to the distal portion  44  using various adhesives, such as epoxy seal. If the tip  48  is metal, the tip  48  may be soldered to the distal portion  44  or may be machined as one continuous component. 
         [0036]      FIG. 6  is an exploded view of the microwave antenna assembly  12 . The microwave antenna assembly  12  includes a proximal portion  42  and a distal portion  44 . The proximal portion  42  may include an inner conductor  50 , an inner insulator layer  52 , and an outer conductor  56 . The proximal portion  42  may also include a flow channel  13  defined therein (not shown) that includes an inflow channel  17   a  and an outflow channel  17   b  that are separated by a polyimide inflow tube  15 . The polyimide inflow tube  15  (not shown) may be inserted into a pocket of a fluid seal  8  to cool the proximal portion  42 . The distal portion  44  includes an inner conductor  50 , a distal radiating section  51 , an injection molded seal  46 , and a ceramic layer  2 . 
         [0037]      FIGS. 7A-7C  are enlarged cross-sectional views of sections A-A, B-B, and C-C of the microwave antenna assembly of  FIG. 4 .  FIG. 7A  illustrates a cross section at section A-A. Section A-A illustrates from the inside towards the outer surface, an inner conductor  50 , insulator  52 , outer conductor  56 , flow channel  13  (specifically inflow channel  17   a ), polyimide inflow tube  15 , flow channel  13  (specifically outflow channel  17   b ), and coolant jacket  38 . 
         [0038]      FIG. 7B  illustrates a cross section at section B-B. Section B-B illustrates from the inside towards the outer surface, an inner conductor  50 , insulator  52 , outer conductor  56 , flow channel  13  (specifically inflow channel  17   a ), polyimide inflow tube  15 , flow channel  13  (specifically outflow channel  17   b ), fluid seal  8 , and coolant jacket  38 .  FIG. 7C  illustrates a cross section taken at section C-C of the distal portion. Section C-C illustrates from the inside towards the outer surface, an inner conductor  50 , insulator  52 , an injection molded seal  46 , and a ceramic layer  2 . 
         [0039]    Referring back to  FIGS. 2-4 , the microwave antenna assembly  12  may be manufactured in various steps. A coaxial cable that includes the inner conductor  50 , insulator layer  52 , and outer conductor  56  may be manufactured and assembled as one component. The outer conductor  56  may be soldered to the fluid seal  8 , for example at joint E, to provide the electrical joint, if needed. The coolant jacket  38  may be bonded, threaded, laser welded, soldered or crimped to the fluid seal  8  at joint D. The coolant jacket  38  and the fluid seal  8  may be assembled as one component. The polyimide inflow tube  15  may be configurable to slide into a pocket of the fluid seal  8 . 
         [0040]    The inner conductor  50  is configured to slide inside a hole of the distal portion  44 . The distal portion  44  is affixed to a distal end of the inner conductor  50  by laser welding, soldering or crimping at joint F. The coaxial cable, coolant jacket  38 , proximal portion  42 , ceramic layer  2 , and distal portion  44  are placed into an injection mold cavity. 
         [0041]      FIG. 8  is a schematic diagram of a mold according to the present disclosure. The mold  53  is used to inject a high strength polyimide material in-between the inner conductor  50  and the ceramic layer  2 . The mold  53  includes mold halves  111   a  and  111   b.  Mold halves  111   a  and  111   b  include portions/cavities to receive coaxial cable, coolant jacket  38 , fluid seal  8 , ceramic layer  2 , and trocar tip  48 . Mold halves  111   a  and  111   b  also include an inflow slot  9 A and an outflow slot  9 B. The mold halves  111   a  and  111   b  are clamped tightly together and heated polyimide is injected into the inflow slot  9 A until the heated polyimide fills into outflow slot  9 B defined therein. The polyimide flows into a cavity to form a uniform layer of polyimide layer along the distal portion  44 . The polyimide material bonds the distal portion  44  and the ceramic layer to the proximal portion while providing mechanical strength to the distal portion. 
         [0042]    In another embodiment, the mold  53  does not include a cavity for the trocar tip  48 . In such an embodiment, when the injection molding process is complete, the antenna assembly  12  is finished by installing the trocar tip  48 .  FIG. 9  illustrates various shapes and forms of a trocar tip  48  installed onto a sheath  38 , namely a stainless steel tip  48   a  and a dielectric tip  48   b.  Both tips  48   a  and  48   b  include insertion bases  51   a  and  51   b  having an external diameter that is smaller than diameter of the tips  48   a  and  49  allowing for easier insertion into a sheath  38 . The configuration also provides for a better seal between the tip  48  and the sheath  38 . In another embodiment, the sheath  38  and tip  48   c  maybe threaded so as to attach to each other. Therefore, the tip  48   c  may be tightly screwed into the sheath  38 . 
         [0043]    The described embodiments of the present disclosure are intended to be illustrative rather than restrictive, and are not intended to represent every embodiment of the present disclosure. Various modifications and variations can be made without departing from the spirit or scope of the disclosure as set forth in the following claims both literally and in equivalents recognized in law.

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