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phase back to the liquid phase such that the material may be circulated through the microwave antenna and subsequently withdrawn from the microwave antenna , as will be discussed in further detail below . fig1 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 ( fig2 ) 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 . fig2 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 fig3 - 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 fig3 . 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 fig3 . fig3 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 fig3 , the gap “ g ” may be coated with a dielectric seal coating as discussed in more detail below . with reference to fig2 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 fig2 . 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 ( fig3 ) 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 fig3 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 fig5 . 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 fig5 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 . fig6 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 fig6 . 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 .
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 inserting 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 .