Patent Document

BACKGROUND 
     1. Technical Field 
     The present disclosure relates generally to microwave antennas used in tissue ablation procedures. More particularly, the present disclosure is directed to a microwave antenna having a coolant assembly for circulating a dielectric coolant fluid though the microwave antenna. 
     2. Background of Related Art 
     In the treatment of diseases such as cancer, certain types of cancer cells have been found to denature at elevated temperatures which are slightly lower than temperatures normally injurious to healthy cells. These types of treatments, known generally as hyperthermia therapy, typically utilize electromagnetic radiation to heat diseased cells to temperatures above 41° Celsius while maintaining adjacent healthy cells at lower temperatures where irreversible cell destruction will not occur. Other procedures utilizing electromagnetic radiation to heat tissue also include ablation and coagulation of the tissue. Such ablation procedures, e.g., such as those performed for menorrhagia, are typically done to ablate and coagulate the targeted tissue to denature or kill the tissue. Many procedures and types of devices utilizing electromagnetic radiation therapy are known in the art. Such therapy is typically used in the treatment of tissue and organs such as the prostate, heart, kidney, lung, brain, and liver. 
     Presently, there are several types of microwave probes in use, e.g., monopole, dipole, and helical which may be inserted into a patient for the treatment of tumors by heating the tissue for a period of time sufficient to cause cell death and necrosis in the tissue region of interest. Such microwave probes may be advanced into the patient, e.g., laparoscopically or percutaneously, and into or adjacent to the tumor to be treated. The probe is sometimes surrounded by a dielectric sleeve. 
     However, in transmitting the microwave energy into the tissue, the outer surface of the microwave antenna typically may heat up and unnecessarily desiccate, or even necrose, healthy tissue immediately adjacent the antenna outer surface. This creates a water or tissue phase transition (steam) that allows the creation of a significant additional heat transfer mechanism as the steam escapes from the local/active heating area and re-condenses further from the antenna. The condensation back to water deposits significant energy further from the antenna/active treatment site. This local tissue desiccation occurs rapidly resulting in an antenna impedance mismatch, which both limits power delivery to the antenna and effectively eliminates steam production/phase transition as a heat transfer mechanism for tissue ablation. 
     To prevent the charring of adjacent tissue, several different cooling methodologies are conventionally employed. For instance, some microwave antennas utilize balloons which are inflatable around selective portions of the antenna to cool the surrounding tissue. Thus, the complications associated with tissue damaged by the application of microwave radiation to the region are minimized. Typically, the cooling system and the tissue are maintained in contact to ensure adequate cooling of the tissue. 
     Other devices attempt to limit the heating of tissue adjacent the antenna by selectively blocking the propagation of the microwave field generated by the antenna. These cooling systems also protect surrounding healthy tissues by selectively absorbing microwave radiation and minimizing thermal damage to the tissue by absorbing heat energy. 
     SUMMARY 
     According to an embodiment of the present disclosure, a method of performing an ablation procedure includes the initial step of supplying a fluid to a cooling chamber defined within an antenna assembly. The method also includes the steps of decreasing the temperature of the fluid to form a solid material and insetting the antenna assembly into tissue. The method also includes the step of supplying energy to the antenna assembly to treat tissue. Residual heat from the antenna assembly transitions the solid material back to the fluid. The method also includes the step of circulating the fluid within the antenna assembly to dissipate heat emanating from the antenna assembly. 
     According to another embodiment of the present disclosure, a method of performing an ablation procedure includes the initial step of supplying fluid to a cooling chamber defined within a microwave antenna assembly. The method also includes the steps of decreasing the temperature of the fluid to from a solid material and inserting the antenna assembly into tissue. The method also includes the step of supplying microwave energy to the antenna assembly to treat tissue. Residual heat from the antenna assembly transitions the solid material back to the fluid. The method also includes the steps of circulating the fluid within the antenna assembly to dissipate heat emanating therefrom and withdrawing the fluid from the antenna assembly. 
     According to another embodiment of the present disclosure, a microwave ablation system includes an antenna assembly configured to deliver microwave energy from a power source to tissue. A coolant source is operably coupled to the power source and is configured to selectively provide fluid to a cooling chamber defined within the antenna assembly. The system also includes a cooling device configured to transition the fluid within the cooling chamber to a solid material. Residual heat from the antenna assembly transitions the solid material back to the fluid when energy is supplied to the antenna assembly to treat tissue. The fluid is configured to circulate within the cooling chamber to dissipate heat emanating from the antenna assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic diagram of the microwave ablation system according to an embodiment of the present disclosure; 
         FIG. 2  is a perspective, internal view of the microwave antenna assembly according to the present disclosure; 
         FIGS. 3 and 4  are enlarged, cross-sectional views showing areas of detail indicated in  FIG. 2  of a portion of the microwave antenna assembly of  FIG. 2 ; 
         FIG. 5  is a schematic, top view of a connection hub of the microwave antenna assembly of  FIG. 1  according to the present disclosure; and 
         FIG. 6  a cross-sectional view of a series of inflow tubes of the microwave antenna assembly of  FIG. 1  according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     Generally, the present disclosure is directed to a microwave antenna having a coolant assembly for circulating a dielectric coolant fluid through the microwave antenna. More particularly, the present disclosure is directed to solidifying a suitable material (e.g., freezing the dielectric coolant fluid) within the microwave antenna to temporarily stiffen the antenna during percutaneous insertion thereof. That is, a suitable material that is in a liquid phase is supplied to the microwave antenna and through various methods is caused to transition from the liquid phase to a solid phase, thereby stiffening the relatively flexible antenna. Once the antenna is inserted through tissue and placed within the desired surgical site, the resulting heat generated by the application of microwave energy from the antenna to tissue causes the material to transition from the solid phase back to the liquid phase such that the material may be circulated through the microwave antenna and subsequently withdrawn from the microwave antenna, as will be discussed in further detail below. 
       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 generator  14  is configured to provide microwave energy at an operational frequency from about 500 MHz to about 5000 MHz, although other suitable frequencies are also contemplated. 
     In the illustrated embodiment, the antenna assembly  12  includes a radiating portion  18  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  having an outlet fluid port  30  and an inlet fluid port  32  that are connected in fluid communication with a sheath  38 . The sheath  38  encloses radiating portion  18  and feedline  20  to form a chamber  89  ( FIG. 2 ) allowing a coolant fluid  37  to circulate from ports  30  and  32  around the antenna assembly  12 . The ports  30  and  32  are also coupled to a supply pump  34  that is, in turn, coupled to a supply tank  36  via supply line  86 . The supply pump  34  may be a peristaltic pump or any other suitable type. The supply tank  36  stores the coolant fluid  37  and, in one embodiment, may maintain the fluid at a predetermined temperature. More specifically, the supply tank  36  may include a coolant unit that cools the returning liquid from the antenna assembly  12 . In another embodiment, the coolant fluid  37  may be a gas and/or a mixture of fluid and gas. 
       FIG. 2  illustrates the radiating portion  18  of the antenna assembly  12  having a dipole antenna  40 . The dipole antenna  40  is coupled to the feedline  20  that electrically connects antenna assembly  12  to the generator  14 . As shown in  FIG. 3-4 , the feedline  20  includes an inner conductor  50  (e.g., wire) surrounded by an inner insulator  52  with suitable dielectric properties, which is surrounded by an outer conductor  56  (e.g., cylindrical conducting sheath). The inner and outer conductors  50  and  56  respectively, 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. 
     The dipole antenna  40  includes a proximal portion  42  and a distal portion  44  interconnected at a feed point  46 . The distal portion  44  and the proximal portion  42  may be either balanced (e.g., of equal lengths) or unbalanced (e.g., of unequal lengths). The proximal portion  42  is formed from the inner conductor  50  and the inner insulator  52  which are extended outside the outer conductor  56 , as shown best in  FIG. 3 . In one embodiment, in which the feedline  20  is formed from a coaxial cable, the outer conductor  56  is stripped to expose inner conductor  50 , as shown in  FIG. 3 . 
       FIG. 3  illustrates the distal portion  44  attached to the proximal portion  42 . The distal portion  44  may be soldered to the inner conductor  50  of the proximal portion  42  to establish electromechanical contact therebetween. A portion of the distal end of the inner conductor  50  is inserted into the distal portion  44  such that a dipole feed gap “G” remains between the proximal and distal portions  42  and  44  at the feed point  46 . The gap “G” may be from about 1 mm to about 3 mm. In one embodiment, the gap “G” may be thereafter filled with a dielectric material at the feed point  46 . In another embodiment, the inner insulator  52  is extended into the feed point  46 . The dielectric material may be polytetrafluoroethylene (PTFE), such as Teflon® sold by DuPont of Willmington, Del. In another embodiment, as shown in  FIG. 3 , the gap “G” may be coated with a dielectric seal coating as discussed in more detail below. 
     With reference to  FIGS. 2 and 4 , the antenna assembly  12  also includes a choke  60 . The choke  60  is disposed around the feedline  20  and includes an inner dielectric layer  62  and an outer conductive layer  64 . The choke  60  may be a quarter-wavelength shorted choke and is shorted to the outer conductor  56  of the feedline  20  at the proximal end (not illustrated) of the choke  60  by soldering or other suitable methods. The dielectric layer  62  may include more than one layer and/or have a variable thickness depending on dielectric performance. Also, dielectric layer  62  may be formed of multiple materials. In one embodiment, the dielectric layer  62  is formed from a fluoropolymer, such as tetrafluorethylene, perfluorpropylene, and the like, and has a thickness of about 0.005 inches. The dielectric of dielectric layer  62  may extend past the choke conductor layer  64  toward the distal end of the assembly  12 , as shown in  FIG. 2 . 
     Since the radiating portion  18  and the feedline  20  are in direct contact with the coolant fluid  37  these components of the assembly  12  are scaled by a protective sleeve  63  ( FIG. 3 ) to prevent any fluid seeping therein. This may be accomplished by applying any type of melt-processible polymers using conventional injection molding and screw extrusion techniques. In one embodiment, a sleeve of fluorinated ethylene propylene (FEP) shrink wrap may be applied to the entire assembly  12 , namely the feedline  20  and the radiating portion  18 , as shown in  FIGS. 3 and 4 . The protective sleeve  63  is then heated to seal the feedline  20  and radiating portion  18 . The protective sleeve  63  prevents any coolant fluid  37  from penetrating into the assembly  12 . 
     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. The tip  48  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, and polyamide thermoplastic resins. 
     The assembly  12  also includes the connection hub  22 , as shown in more detail in  FIG. 5 . The connection hub  22  includes a cable connector  79  and fluid ports  30  and  32 . The connection hub  22  may include a three-branch luer type connector  72 , with a first branch  74  being used to house the cable connector  79  and the second and third branches  76  and  78  to house the outlet and inlet fluid ports  30  and  32 , respectively. In one embodiment, the connection hub  22  may include only the first branch  74  or two of the branches  74 ,  76 ,  78  and have the fluid ports  30  and  32  disposed directly on the first branch  74 . 
     The connection hub  22  also includes a base  81  disposed at a distal end of the first branch  74 . More than one inflow  86  and outflow  88  tube may be used. The outflow tube  88  is coupled to the second branch  76  and is in fluid communication with the bypass tube  80  through the second branch  76 . In one embodiment, the assembly  12  includes one or more inflow tubes  86   a  and  86   b  that are fed through the third branch  78  as shown in  FIGS. 5 and 6 . 
     In one embodiment, the second and third branches  76  and  78  may include various types of female and/or male luer connectors adapted to couple inflow and outflow tubes  86  and  88 , respectively, from the pump  34  to the assembly  12 .  FIG. 6  shows the assembly  12  including two inflow tubes  86   a  and  86   b . The inflow tubes  86   a  and  86   b  may be any type of flexible tube having an external diameter sufficient to fit inside the chamber  89  between the feedline  20  and the sheath  38 . The inflow tubes  86   a  and  86   b  are inserted though the inlet fluid port  32 . 
     The inflow tube  86   a  is inserted into the distal end of the distal portion  44  and the inflow tube  86   b  is inserted at a point proximate the midpoint of the assembly  12  (e.g., the feed point  46 ), as shown in  FIG. 6 . The inflow tubes  86   a  and  86   b  are then secured to the radiating portion  18  (e.g., using epoxy, glue, etc.). The inflow tubes  86   a  and  86   b  are positioned in this configuration to provide optimal coolant flow through chamber  89 . The fluid flow from the inflow tube  86   a  is directed into the tip  48  and reflected in the proximal direction. The fluid flow from the inflow tube  86   b  provides the coolant fluid  37  along the radiating portion  18 . During operation, the pump  34  supplies fluid to the assembly  12  through the inflow tubes  86   a  and  86   b , thereby circulating the coolant fluid  37  through the entire length of the assembly  12  including the connection hub  22 . The coolant fluid  37  is then withdrawn from the first branch  74  and the second branch  76  through the outlet fluid port  30 . 
     In some embodiments, the assembly  12  is substantially formed of flexible material (e.g., polymer) and, thus, is susceptible to bending upon application of pressure thereto. More particularly, attempting percutaneous insertion of assembly  12  may cause the assembly  12  to bend upon contact of tip  48  with tissue due to the flexible makeup of the material from which the assembly  12  is constructed. In this scenario, the antenna  12  may be stiffened prior to insertion into tissue to prevent bending. With this purpose in mind, a suitable material (e.g., the coolant fluid  37  circulated within the assembly  12 ) may be circulated within chamber  89  and, prior to percutaneous insertion, solidified (e.g., frozen) therein to stiffen the antenna assembly  12 . For example, coolant fluid  37  may be frozen to form ice within chamber  89 . Any suitable cooling device (not shown) may be utilized to solidify and/or freeze the selected material, such as without limitation, a freezer, a liquid nitrogen spray, a liquid nitrogen bath, thermoelectric cooling (e.g., a Peltier device), dry ice, bio-compatible crystals, and the like. 
     Upon stiffening, assembly  12  is inserted into the desired surgical site. Heat or residual heat generated by the antenna assembly  12  upon delivery of energy from the radiating portion  18  to tissue increases the temperature of the solidified material or coolant fluid  37  within chamber  89 , thereby returning the selected material from a solid state to a liquid state. The liquid state material is circulated throughout the antenna assembly  12 , as discussed hereinabove with respect to operation of antenna assembly  12 . Any suitable bio-compatible material (e.g., saline, deionized water, etc.) that may be frozen and/or solidified at a threshold temperature may be utilized in this scenario. For example, a suitable material may be substantially solid at or below ambient temperature and substantially liquid at temperatures above ambient temperature. In some embodiments, suitable bio-compatible crystals or waxes may be incorporated within the coolant fluid  37  such that coolant fluid  37  transitions between solid and liquid states at particular temperatures or within particular temperature ranges. In embodiments in which materials other than water are introduced into chamber  89 , water or other suitable fluids may be introduced into chamber  89  to flush chamber  89  and facilitate withdrawing of materials from chamber  89  through outlet fluid port  30 . 
     The above-discussed coolant system provides for circulation of dielectric coolant fluid  37  (e.g., saline, deionized water, etc.) through the entire length of the antenna assembly  12 . The dielectric coolant fluid  37  removes the heat generated by the assembly  12 . By keeping the antenna assembly  12  and/or the ablation zone cooled, there is significantly less sticking of tissue to the antenna assembly  12 . In addition, the dielectric coolant fluid  37  acts as a buffer for the assembly  12  and prevents near field dielectric properties of the assembly  12  from changing due to varying tissue dielectric properties. For example, as microwave energy is applied during ablation, desiccation of the tissue around the radiating portion  18  results in a drop in tissue complex permittivity by a considerable factor (e.g., about 10 times). The dielectric constant (er′) drop increases the wavelength of microwave energy in the tissue, which affects the impedance of un-buffered microwave antenna assemblies, thereby mismatching the antenna assemblies from the system impedance (e.g., impedance of the cable  16  and the generator  14 ). The increase in wavelength also results in a power dissipation zone which is much longer in length along the assembly  12  than in cross sectional diameter. The decrease in tissue conductivity (er″) also affects the real part of the impedance of the assembly  12 . The fluid dielectric buffering according to the present disclosure also moderates the increase in wavelength of the delivered energy and drop in conductivity of the near field, thereby reducing the change in impedance of the assembly  12 , allowing for a more consistent antenna-to-system impedance match and spherical power dissipation zone despite tissue behavior. 
     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. Embodiments of the present disclosure may also be implemented in a microwave monopolar antenna or other electrosurgical devices. 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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