Patent Publication Number: US-2022211433-A1

Title: Microwave ablation device and system with impedance mismatch

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the filing date of provisional U.S. Patent Application No. 62/839,008, filed Apr. 26, 2019. 
    
    
     BACKGROUND 
     Practitioners use conventional microwave ablation systems during laparoscopic surgical procedures to treat target tissue, such as a tumor located within the abdomen or pelvis, by using a percutaneous microwave ablation device to deliver microwave energy to the target tissue. An energy source, such as a generator, is coupled to the microwave ablation device via a generator cable. The microwave ablation device typically includes an elongated tubular member that contains a coaxial cable and that has a needle at its distal portion. The tubular member serves as a conduit for the coaxial cable, which guides microwave energy from the generator through an incision in the patient and toward the distal portion, which radiates the microwave energy toward the target tissue. To minimize reflected power and thereby optimize microwave energy delivery from the generator to the distal portion, conventional microwave ablation systems typically include a generator output port, a generator cable, and a microwave ablation device coaxial cable that have respective impedances that are mutually matched to one another within an operational frequency range. 
     Recently, to reduce invasiveness and thereby reduce post-surgery healing time and the risk of bleeding or injury, practitioners have expressed a desire for a microwave ablation device having a tubular member that is reduced in outer diameter. One approach to reducing the diameter of the tubular member involves reducing respective outer diameters of a center conductor and a dielectric layer of the coaxial cable housed within the tubular member, to maintain the impedance of the coaxial cable despite its downsizing. This approach, however, results in excess loss in the coaxial cable and thereby decreases the efficiency of microwave energy delivery from the generator to the distal portion. In view of the foregoing, a need exists for a microwave ablation system including a microwave ablation device that has a tubular member that is reduced in diameter and that facilitates efficient delivery of microwave energy from the generator to the distal portion. 
     SUMMARY 
     In one aspect, this disclosure describes a microwave ablation system comprising a generator, a generator cable, and a microwave ablation device. The generator comprises a generator output port having a generator output port impedance. The generator cable has a generator cable impedance that is matched to the generator output port impedance within a degree of tolerance within a predetermined frequency range. The microwave ablation device comprises a coaxial cable that has a coaxial cable impedance and is coupled to the generator output port by way of the generator cable. Within the predetermined frequency range, the coaxial cable impedance is lower than at least one of the generator output port impedance or the generator cable impedance by at least a predetermined amount that exceeds the degree of tolerance. 
     In embodiments, the microwave ablation device further comprises a transition waveguide and an electrically conductive element. The transition waveguide comprises a transition waveguide input port, having a transition waveguide input port impedance, and a transition waveguide output port, having a transition waveguide output port impedance. The coaxial cable comprises a coaxial cable input port and a coaxial cable output port, and the coaxial cable input port is coupled to the transition waveguide output port, and the coaxial cable output port is coupled to the electrically conductive element. 
     In embodiments, at least one of the transition waveguide input port impedance or the transition waveguide output port impedance is matched to the generator output port impedance and the generator cable impedance within the degree of tolerance within the predetermined frequency range. 
     In embodiments, at least one of the transition waveguide input port impedance or the transition waveguide output port impedance is matched to the coaxial cable impedance within the degree of tolerance within the predetermined frequency range. 
     In embodiments, the transition waveguide input port impedance, the transition waveguide output port impedance, the generator output port impedance, and the generator cable impedance are in a range from approximately 48 ohms (Ω) to 52Ω, and the coaxial cable impedance is in a range from approximately 39Ω to 45Ω. 
     In embodiments, the coaxial cable comprises a center conductor and a dielectric layer that radially surrounds the center conductor, and an outer diameter of the center conductor is in a range from approximately 0.0070 inches to 0.0072 inches and an outer diameter of the dielectric layer is in a range from approximately 0.0189 inches to 0.0195 inches. 
     In embodiments, the degree of tolerance is approximately 5%. 
     In embodiments, the transition waveguide output port has a transition waveguide output port impedance that is matched to at least one of the transition waveguide input port impedance or the generator output port impedance within the degree of tolerance. 
     In embodiments, the microwave ablation system further comprises a balun comprising a balun insulator and a tubing member. The balun insulator has an inner diameter, and the tubing member radially surrounds the balun insulator and has a maximum outer diameter. The balun insulator inner diameter is approximately 0.023 inches, and the tubing member maximum outer diameter is approximately 0.041 inches. 
     In embodiments, the microwave ablation device further comprises an outer tubular member that houses the coaxial cable, and an outer diameter of the outer tubular member is approximately 15 gauge. 
     In embodiments, the predetermined frequency range includes at least one of 915 MHz, 2.45 GHz, or 5.8 GHz. 
     In another aspect, this disclosure describes a microwave ablation device comprising a transition waveguide, an electrically conductive element, and a coaxial cable. The transition waveguide comprises a transition waveguide input port, having a transition waveguide input port impedance, and a transition waveguide output port, having a transition waveguide output port impedance. The coaxial cable has a coaxial cable impedance and comprises a coaxial cable input port and a coaxial cable output port. The coaxial cable input port is coupled to the transition waveguide output port, and the coaxial cable output port is coupled to the electrically conductive element. Within a predetermined frequency range, the coaxial cable impedance is lower than at least one of the transition waveguide input port impedance or the transition waveguide output port impedance by at least a predetermined amount that exceeds a degree of tolerance. 
     In embodiments, the transition waveguide input port impedance and the transition waveguide output port impedance are in a range from approximately 48Ω to 52Ω, and the coaxial cable impedance is in a range from approximately 39Ω to 45Ω. 
     In embodiments, the coaxial cable comprises a center conductor and a dielectric layer that radially surrounds the center conductor, and an outer diameter of the center conductor is in a range from approximately 0.00070 inches to 0.00072 inches and an outer diameter of the dielectric layer is in a range from approximately 0.0189 inches to 0.0195 inches. 
     In embodiments, the microwave ablation device further comprises a balun comprising a balun insulator and a tubing member. The balun insulator has an inner diameter, and the tubing member radially surrounds the balun insulator and has a maximum outer diameter. The balun insulator inner diameter is approximately 0.023 inches, and the tubing member maximum outer diameter is approximately 0.041 inches. 
     In embodiments, the microwave ablation device further comprises an outer tubular member that houses the coaxial cable, and an outer diameter of the outer tubular member is in a range from approximately 0.0715 inches to 0.0725 inches. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects and features of the present microwave ablation devices and systems are described herein below with references to the drawings, wherein: 
         FIG. 1  is a schematic diagram of a microwave ablation system, according to an embodiment of the present disclosure; 
         FIG. 2  shows an enlarged, cross-sectional view of the microwave ablation device shown in  FIG. 1  in accordance with an embodiment of the present disclosure; 
         FIGS. 3A, 3B, and 3C  show respective views of various portions of the microwave ablation device of  FIGS. 1 and 2 , and illustrate exemplary dimensions of components of the microwave ablation device, according to an embodiment of the present disclosure; 
         FIG. 4  is an enlarged, cross-sectional view of the indicated area of detail of  FIG. 2 , in accordance with an embodiment of the present disclosure; 
         FIG. 5A  is an enlarged, cross-sectional view of a portion of a coaxial cable of a microwave ablation device at a first stage during the assembly process, in accordance with an embodiment of the present disclosure; 
         FIG. 5B  is an enlarged, cross-sectional view of the portion of the coaxial cable shown in  FIG. 5A , at a second stage of the assembly process, in accordance with an embodiment of the present disclosure; 
         FIG. 5C  is an enlarged, cross-sectional view of the portion of the coaxial cable shown in  FIGS. 5A and 5B , at a third stage of the assembly process, in accordance with an embodiment of the present disclosure; and 
         FIG. 6  is a cross-sectional view of a portion of a probe assembly of the microwave ablation device, in accordance with an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is directed to a microwave ablation system including a microwave ablation device that has a tubular member that is reduced in outer diameter relative to tubular members of prior microwave ablation devices, and nonetheless facilitates efficient delivery, from the generator to the distal portion, of microwave energy suitable for effective treatment. The microwave ablation system and device of the present disclosure thus reduces invasiveness of microwave ablation procedures and thereby reduces post-surgery healing time and the risk of bleeding or injury. In general, as described in further detail below, the microwave ablation device of the present disclosure includes a coaxial cable (sometimes referred to as a feedline) that has an impedance lower than an impedance of one or more other components of the system, such as, for instance, the generator output port. 
     In one aspect, the impedance of the coaxial cable is reduced relative to the impedance of the generator output port by decreasing a ratio between an outer diameter of a dielectric layer of the coaxial cable to the outer diameter of the coaxial cable center conductor. In one embodiment, the decreased ratio, relative to a corresponding ratio for a coaxial cable that has an impedance that matches the generator output port impedance, is obtained by decreasing the outer diameter of the dielectric layer while keeping the center conductor outer diameter the same or greater. In another embodiment, the decreased ratio, relative to a corresponding ratio for a coaxial cable that has an impedance that matches the generator output port impedance, is obtained by increasing the center conductor outer diameter while keeping the outer diameter of the dielectric layer the same or lower. Despite the mismatched impedance between the coaxial cable of the present disclosure and the generator output port, the coaxial cable of the present disclosure includes a relatively large center conductor outer diameter, which has an increased thermal mass for absorbing heat and increased surface area for removal of the heat, and has a relatively thin insulating layer (e.g., PTFE), decreasing the amount that the insulating layer mitigates heat removal. The coaxial cable of the present disclosure thus provides performance advantages while minimizing the increase in heat buildup on the center conductor and reduces the risk of overheating the center conductor to the point of sudden and total failure (burnout). The relatively large center conductor also may cause less heat buildup on the center conductor and thereby reduce a risk of overheating the center conductor to the point of sudden and total failure or burnout. 
     Throughout this description, the term “proximal” refers to the portion of the device or component thereof that is closer to the clinician and the term “distal” refers to the portion of the device or component thereof that is farther from the clinician. The phrases “in an embodiment,” “in embodiments,” “in some embodiments,” or “in other embodiments” may each refer to one or more of the same or different embodiments in accordance with the present disclosure. A phrase in the form “A or B” means “(A), (B), or (A and B).” A phrase in the form “at least one of A, B, or C” means “(A); (B); (C); (A and B); (A and C); (B and C); or (A, B, and C).” 
     With reference to  FIG. 1 , a microwave ablation system  100  includes a display  110 , a table  120  including an electromagnetic (EM) field generator  121 , a microwave ablation device  130  including an EM sensor  131 , an ultrasound sensor  140  connected to an ultrasound workstation  150 , a peristaltic pump  160 , and a computing device  180  attached to or in operable communication with a microwave generator  170 . The computing device  180  may be, for example, a laptop computer, desktop computer, tablet computer, or other similar device. The computing device  180  may be configured to control the microwave generator  170 , the peristaltic pump  160 , a power supply (not shown in  FIG. 1 ), and/or any other accessories and peripheral devices relating to, or forming part of, the microwave ablation system  100 . The display  110  is configured to output instructions, images, and messages relating to the performance of the treatment procedure. 
     The table  120  may be, for example, an operating table or other table that is suitable for use during a treatment procedure and that includes the EM field generator  121 . The EM field generator  121  is used to generate an EM field during the treatment procedure and forms part of an EM tracking system that is used to track positions of surgical instruments within the body of a patient, such as by tracking a position of the EM sensor  131 . The EM field generator  121  may include various components, such as a specially designed pad to be placed under, or integrated into, an operating table or patient bed. An example of such an EM tracking system is the AURORA™ system sold by Northern Digital, Inc. 
     The microwave ablation device  130  is a surgical instrument for percutaneously accessing and treating a target location. The microwave ablation device  130  may include, or have attached to it, EM sensor  131  enabling the EM tracking system to track the location, position, and orientation (also known as the “pose”) of the microwave ablation device  130  inside the body of the patient. 
     The microwave generator  170  includes a generator output port  172  and a generator cable  174  couples the generator output port  172  to the microwave ablation device  130 . The microwave generator  170  delivers microwave energy from to the microwave ablation device  130  by way of the generator output port  172  and the generator cable  174 . The generator output port  172  has a corresponding generator output port impedance and the generator cable  174  has a corresponding generator cable impedance. In some embodiments, to minimize reflected power, the impedance of the generator cable  174  is matched, within a degree of tolerance, to the impedance of the generator output port  172  within a predetermined frequency range. 
     In general, the microwave ablation system  100  is designed to operate using signals within a predetermined frequency range (sometimes referred to as an operational frequency range). That is, various components of the microwave ablation system  100 , such as the microwave generator  170 , microwave generator output port  172 , generator cable  174 , and microwave ablation device  130 , are designed to generate, convey, radiate, or otherwise process signals (e.g., microwave signals) having a signal frequency that lies within the predetermined frequency range. The predetermined frequency range may be, for example, sufficiently wide to include a variety of operating frequencies, such as approximately 915 MHz, 2.45 GHz, and/or 5.8 GHz, or may be any other suitable frequency range. 
     Due to inconsistencies and/or imperfections in manufacturing, materials, variations in physical properties of components over signal frequency, and/or other factors, the various impedances of the microwave ablation system  100  and its components, such as the respective impedances of the generator output port  172  and the generator cable  174 , may vary from their nominal impedance values throughout the operational frequency range. The degree of tolerance, in some embodiments, is predetermined in recognition of this fact. The degree of tolerance, as used herein, generally refers to a permissible limit of variation in impedances that would not significantly affect the function of the microwave ablation system  100 . The degree of tolerance, for example, may be expressed as a variation in impedance amount relative to a nominal impedance value (such as a maximum number of Ω greater than or less than the nominal impedance value) or as a variation in impedance percentage relative to the nominal impedance value (such as, a maximum percentage greater than or less than the nominal impedance value). In various embodiments, the degree of tolerance may be, for example, approximately ±3Ω, ±5%, or any other suitable value. 
     In addition to outputting microwave energy, the microwave generator  170  is configured to control the peristaltic pump  160 , which is configured to pump fluid through the microwave ablation device  130 , cooling the microwave ablation device  130  during operation. While the present disclosure describes the use of the microwave ablation system  100  in a surgical environment, it is also envisioned that some or all of the components of the microwave ablation system  100  may be used in alternative settings, for example, an imaging laboratory and/or an office setting. 
     In addition to the EM tracking system, the surgical instruments may also be visualized by using ultrasound imaging. The ultrasound sensor  140 , such as an ultrasound wand, may be used to image the patient&#39;s body during the treatment procedure to visualize the location of the surgical instruments, such as the microwave ablation device  130 , inside the patient&#39;s body. The ultrasound sensor  140  may have an EM tracking sensor embedded within or attached to the ultrasound wand, for example, a clip-on sensor or a sticker sensor. As described further below, the ultrasound sensor  140  may be positioned in relation to the microwave ablation device  130  such that the microwave ablation device  130  is at an angle to the ultrasound image plane, thereby enabling the clinician to visualize the spatial relationship of the microwave ablation device  130  with the ultrasound image plane and with objects being imaged. Further, the EM tracking system may also track the location of the ultrasound sensor  140 . In some embodiments, one or more of the ultrasound sensors  140  may be placed inside the body of the patient. The EM tracking system may then track the location of such ultrasound sensors  140  and the microwave ablation device  130  inside the body of the patient. The ultrasound workstation  150  may be used to configure, operate, and view images captured by the ultrasound sensor  140 . Likewise, the computing device  180  may be used to configure, operate, and view images captured by the ultrasound sensor  140 , either directly or relayed via the ultrasound workstation  150 . 
     In embodiments, the microwave ablation device  130  is used to ablate a lesion or tumor (hereinafter referred to as a “target”) by using microwave energy to heat tissue in order to denature or kill cancerous cells. The construction and use of a system including such an ablation probe is more fully described in U.S. Patent Publication No. 2016/0058507, entitled MICROWAVE ABLATION SYSTEM, filed on Aug. 26, 2014, by Dickhans; U.S. Pat. No. 9,247,992, entitled MICROWAVE ABLATION CATHETER AND METHOD OF UTILIZING THE SAME, filed on Mar. 15, 2013, by Ladtkow et al.; and U.S. Pat. No. 9,119,650, entitled MICROWAVE ENERGY-DELIVERY DEVICE AND SYSTEM, filed on Mar. 15, 2013, by Brannan et al.; the entire contents of each of which are hereby incorporated by reference. 
     As noted above, the location of the microwave ablation device  130  within the body of the patient may be tracked during the treatment procedure. An example method of tracking the location of the microwave ablation device  130  is by using the EM tracking system, which tracks the location of the microwave ablation device  130  by tracking sensors, such as the EM sensor  131 , attached to or incorporated in the microwave ablation device  130 . Various types of sensors may be used, such as a printed sensor, the construction and use of which is more fully described in U.S. Patent Publication No. US 2016/0174873, entitled “MEDICAL INSTRUMENT WITH SENSOR FOR USE IN A SYSTEM AND METHOD FOR ELECTROMAGNETIC NAVIGATION”, filed on Oct. 22, 2015, by Greenburg et al., the entire contents of which are incorporated herein by reference. A percutaneous treatment system similar to the above-described microwave ablation system  100  is further described in U.S. Patent Application Publication No. 2016/0317224, entitled “MICROWAVE ABLATION PLANNING AND PROCEDURE SYSTEMS”, filed on Apr. 15, 2016, by Girotto et al., the entire contents of which are incorporated herein by reference. 
     While the above-described microwave ablation system  100  uses a microwave generator  170  to provide microwave energy to the microwave ablation device  130 , those skilled in the art will appreciate that various other types of generators and tools may be used without departing from the scope of the present disclosure. For example, radio frequency (RF) ablation tools receiving RF energy from RF generators may be substituted for the microwave generators and ablation tools described above. Further, while the above-described microwave ablation system  100  is designed for percutaneous access to tissue, those skilled in the art will appreciate that the methods described below may be used with systems and tools designed for endobronchial navigation to access treatment locations via the patient&#39;s airways and surrounding parenchyma without departing from the scope of the present disclosure. An example of such an endobronchial navigation system is described in U.S. Patent Application Publication No. 2016/0000302, entitled “SYSTEM AND METHOD FOR NAVIGATING WITHIN THE LUNG”, filed on Jun. 29, 2015, by Brown et al., the entire contents of which are incorporated herein by reference. 
       FIG. 2  shows a side cross-sectional view of a portion of an example embodiment of the microwave ablation device  130  shown in  FIG. 1 . Additional aspects of the microwave ablation device  130  shown in  FIG. 2  are described in U.S. Pat. No. 9,119,650, entitled “MICROWAVE ENERGY DELIVERY DEVICE AND SYSTEM,” filed on Mar. 15, 2013, by Brannan et al., the entire contents of which are hereby incorporated herein by reference. The microwave ablation device  130  includes an outer tubular member  222 , an inner tubular member  218 , a coaxial cable  230 , an antenna assembly (shown in  FIG. 6 ), and a tip  238 , which, when assembled, form a probe assembly  200 , or portions thereof. The microwave ablation device  130  generally includes two housing halves (not separately shown in  FIG. 2 ), which, when assembled, form a handle body  240 . The handle body  240  defines a handle-body chamber (not separately shown in  FIG. 2 ) therein. The microwave ablation device  130  includes a hub  202  (as well as other components described herein) disposed, at least in part, within the handle-body chamber. 
     The hub  202  includes a hub body  208  defining a hub-body chamber (not separately shown in  FIG. 2 ) therein. The microwave ablation device  130  includes a hub cap  204  and a hub divider  206 , which are configured to be receivable within the hub-body chamber in sealing engagement with the inner walls of the hub body. The outer tubular member  222 , the inner tubular member  218 , the hub  202 , and the components cooperative therewith (e.g., hub cap  204  and hub divider  206 ) are adapted to maintain fluid flow toward the electrically-conductive element  236 . The hub body  208  generally includes a first port  214  and a second port  216  to allow fluid communication with a coolant supply system (for example, the peristaltic pump  160  shown in  FIG. 1 ) via one or more coolant paths. The first port  214  and the second port  216  may be of any suitable shape, such as rectangular, cylindrical, and/or the like, and may include a groove adapted to receive an O-ring or other suitable sealing element. 
     In some embodiments, the hub body  208  may include one or more mechanical interfaces, such as a recess  242 , adapted to matingly engage with one or more corresponding mechanical interfaces associated with the handle body  240  to align the hub  202  within the handle body  240  and/or to fixedly secure the hub  202  within the handle-body chamber. Similarly, each of the housing halves may include a series of mechanical interfacing components, such as alignment pins  244 ,  246 , and  248 , configured to matingly engage with a corresponding series of mechanical interfaces (not shown in  FIG. 2 ) to align the two housing halves about the components and assemblies of the microwave ablation device  130 . 
     The hub divider  206  is configured and utilized to divide the hub-body chamber into a first chamber  210  disposed in fluid communication with the first port  214 , and a second chamber  212  disposed in fluid communication with the second port  216 . The first chamber  210  generally fluidly connects the first port  214  to the inner tubular member  218 . The second chamber  212  generally fluidly connects the second port  216  to the inner tubular member  222 . 
     In some embodiments, the inner walls of the hub body  208  may include a configuration of engagement portions adapted to provide sealing engagement with the hub cap  204  and/or the hub divider  206 . An O-ring  250  may be provided for engagement with the hub cap  204 . The O-ring  250  may provide sealing force that permits flexing and/or other slight movement of the hub cap  204  relative to the hub  202  under fluid-pressure conditions. 
     The outer tubular member  222  and the inner tubular member  218  may be formed of any suitable non-electrically-conductive material, such as, for example, polymeric or ceramic materials. In some embodiments, as shown in  FIG. 2 , the inner tubular member  218  is coaxially disposed around the coaxial cable  230  and defines a first lumen  220  therebetween, and the outer tubular member  222  is coaxially disposed around the inner tubular member  218  and defines a second lumen  224  therebetween. 
     The probe assembly  200  generally includes an antenna assembly  600  (shown in  FIG. 6  but not separately shown in  FIG. 2 ) having a first radiating portion (e.g., distal radiating section  602  shown in  FIG. 6 ) and a second radiating portion (e.g., proximal radiating section  604  shown in  FIG. 6 ). The antenna assembly  600 , which is described in more detail below, is operably coupled by the coaxial cable  230  to a transition waveguide  252  shown in  FIG. 2 , which is adapted to transmit microwave energy from the cable assembly  174  to the coaxial cable  230 . The transition waveguide  252  has a transition waveguide input port  254  having a transition waveguide input port impedance, and a transition waveguide output port  256  having a transition waveguide output port impedance. The coaxial cable  230  has a coaxial cable impedance and includes a coaxial cable input port  258 , and a coaxial cable output port  260 . The coaxial cable input port  258  is coupled to the transition waveguide output port  256 , and the coaxial cable output port  260  is coupled to the electrically-conductive element  236 . 
     The coaxial cable  230  may be any suitable transmission line, e.g., a coaxial cable. In some embodiments, as shown in  FIG. 4 , the coaxial cable  230  includes an inner conductor  402 , an outer conductor  406  coaxially disposed around the inner conductor  402 , and a dielectric material  404  disposed therebetween. The dielectric material  404  may be formed from any suitable dielectric material, e.g., polyethylene, polyethylene terephthalate, polyimide, or polytetrafluoroethylene (PTFE). The inner conductor  402  and the outer conductor  406  may be formed from any suitable electrically-conductive material. In some embodiments, the inner conductor  402  is formed from a first electrically-conductive material (e.g., stainless steel) and the outer conductor  406  is formed from a second electrically-conductive material (e.g., copper). Electrically-conductive materials used to form the coaxial cable  230  may be plated with other materials, e.g., other conductive materials, such as gold or silver, to improve their properties, e.g., to improve conductivity, decrease energy loss. The coaxial cable  230  may have any suitable length defined between its proximal and distal ends. In accordance with various embodiments of the present disclosure, the coaxial cable  230  is coupled at its proximal end to the transition waveguide  252  and coupled at its distal end to the antenna assembly  600 . The coaxial cable  230  is disposed at least in part within the inner tubular member  218 . 
     As shown in  FIG. 2 , the probe assembly  200  is disposed in part within the hub  202 , wherein the hub cap  204  and the hub divider  206  are disposed in sealing engagement with the inner walls of the hub body, and a proximal portion of the probe assembly  200  is disposed in association with the hub cap  204  and hub divider  206 . The hub divider  206  generally divides the hub-body chamber (not separately shown in  FIG. 2 ) into the first chamber  210  and the second chamber  212 . The first chamber  210  is disposed in fluid communication with the first port  214 . The second chamber  212  is disposed in fluid communication with the second port  216 . In some embodiments, as shown in  FIG. 2 , the proximal end of the inner tubular member  218  is disposed within the first chamber  210 , wherein the first lumen  220  is disposed in fluid communication with the first port  214 , and the proximal end of the outer tubular member  222  is disposed within the second chamber  212 , wherein the second lumen  224  is disposed in fluid communication with the second port  216 . 
     In some embodiments, as shown in  FIGS. 2 and 4 , the inner tubular member  218  is coaxially disposed around a coaxial cable  230  and defines the first lumen  220  therebetween, and the outer tubular member  222  is coaxially disposed around the inner tubular member  218  and defines the second lumen  224  therebetween. As shown in  FIG. 2 , the proximal end of the inner tubular member  218  is disposed within the first chamber  210 , the first lumen  220  is disposed in fluid communication with the first port  214 , the proximal end of the outer tubular member  222  is disposed within the second chamber  212 , and the second lumen  224  is disposed in fluid communication with the second port  216 . 
     In some embodiments, as shown in  FIG. 2 , the inner tubular member  218  includes a first portion having a first outer diameter, a second portion having a second outer diameter greater than the first outer diameter, and a neck portion  226  disposed therebetween. In some embodiments, the opening in the hub divider  206  is configured for sealing engagement with the second portion of the inner tubular member  218  having the second outer diameter. In some embodiments, located within the interior of the second portion of the inner tubular member  218  is a high hoop strength metal cylinder  228 . The metal cylinder  228  engages the inner diameter of the inner tubular member  218 . The hub divider  206  is formed of an elastomeric material and when forced into place within the hub  202 , as shown in  FIG. 2 , the elastomeric material of the hub divider  206  creates an improved water tight seal separating the first hub chamber  210  from the second hub chamber  212 . The metal cylinder  228  improves this seal by ensuring better contact between the elastomeric material of the hub divider  206  and the inner tubular member  218  upon application of lateral forces to the hub divider  206 . 
     The hub body  208  may be configured to sealingly engage the coolant supply lines forming coolant paths to fluid inlet port  214  and fluid outlet port  216 . The fluid inlet port  214  and the fluid outlet port  216  may have any suitable configuration, including without limitation nipple-type inlet fittings, compression fittings, and recesses, and may include an O-ring type elastomeric seal. 
       FIGS. 3A, 3B, and 3C , which show respective views of various portions of the microwave ablation device  130  of  FIGS. 1 and 2 , illustrate exemplary dimensions of components of the microwave ablation device  130 , in accordance with embodiments of the present disclosure. In particular,  FIG. 3A  shows an assembly including the coaxial cable  230  having the transition waveguide  252  coupled to its proximal end, having the electrically conductive element  236  coupled to its distal end, and having a balun  510  disposed adjacent to, and on a proximal side of, the electrically-conductive element  236 .  FIG. 3B  shows an enlarged view of a distal portion of the assembly shown in  FIG. 3A .  FIG. 3C  shows an enlarged cross-sectional view of a proximal portion of the balun  510  identified in  FIG. 3B . 
     As shown in  FIG. 3A , a tubular member  308 , which in some embodiments may be a stainless steel tube, is coaxially disposed around the coaxial cable  230  and is partially disposed within the transition waveguide  252 . A tubing member  512 , which, in some embodiments is a heat-shrink tubing member (described below in connection with  FIGS. 5A-5C ) is coaxially disposed around the coaxial cable  230  in a position that is distal to that of the tubular member  308 . The balun  510  is coaxially disposed around the coaxial cable  230  in a position that is distal to that of the heat-shrink tubing member  512 . And the electrically conductive element  236  is coupled to the coaxial cable  230  at its distal end. 
     In general, various dimensions of the microwave ablation device  130  may vary in various embodiments. The view in  FIG. 3A  illustrates various exemplary dimensions of the microwave ablation device  130  and corresponding degrees of tolerance for those dimensions. In particular, a dimension  302  from a proximal end portion of the center conductor  402  to a distal end portion of the center conductor  402  may be, for example, approximately 8.8 inches in one embodiment or approximately 6.833 inches in another embodiment. A dimension  304  from the proximal end portion of the center conductor  402  to a distal end portion of the balun  510  may be, for example, approximately 7.438 inches in one embodiment or approximately 5.471 inches in another embodiment. A dimension  306  from a proximal portion of the transition waveguide  252  to a distal end portion of the electrically-conductive element  236  may be, for example, approximately 9.016 inches in one embodiment or approximately 7.049 inches in another embodiment. An outer diameter  309  of the tubular member  308  may be, for example, approximately 0.0343 inches. An outer diameter  309  of the tubular member  308  may be, for example, approximately 0.0343 inches. A dimension  310  from the proximal portion of the transition waveguide  252  to a distal end portion of the balun  510  may be, for example, approximately 7.294 inches in one embodiment or approximately 5.327 inches in another embodiment. A dimension  312  from a distal end portion of the tubular member  308  to the proximal end portion of the balun  510  may be, for example, approximately 0.670 inches. A dimension  314  from a distal end portion of the heat-shrink tubing member  512  to a distal end portion of the heat-shrink tubing member  512  may be, for example, approximately 0.50 inches. An outer diameter  316  of a middle portion of the balun  510  may be, for example, approximately 0.041 inches. An outer diameter  318  of a distal portion of the balun  510 , more particularly, of a dielectric layer  504  (described below in connection with  FIGS. 5A-5C ) of the balun  510  exposed at its distal portion, may be, for example, approximately 0.0375 inches. An outer diameter  320  of the outer conductor  406  of the coaxial cable  230  may be, for example, approximately 0.0230 inches. An outer diameter  322  of exposed dielectric material  404  of the coaxial cable  230  may be, for example, approximately 0.0192 inches. 
     Referring now to  FIG. 3B , a dimension  324  from the proximal end portion of the balun  510  to a distal end portion of the center conductor  402  may be, for example, approximately 1.362 inches. A dimension  325  from the proximal end portion of the balun  510  to a distal end portion of a tubing member  514  of the balun  510  may be, for example, approximately 0.822 inches. In various embodiments, the tubing member  514  may be a heat-shrink tubing member, a metal (e.g., copper) tubing member crimped in place, and/or the like. A dimension  326  from the proximal end portion of the tubing member  514  of the balun  510  to the distal end portion of the tubing member  514  of the balun  510  may be, for example, approximately 0.811 inches. A dimension  328  from a proximal end portion of the dielectric layer  504  of the balun  510  to the distal end portion of the tubing member  514  of the balun  510  may be, for example, approximately 0.780 inches. A dimension  330  from a proximal end portion of the dielectric layer  504  of the balun  510  to a distal end portion of the dielectric layer  504  of the balun  510  may be, for example, approximately 0.880 inches. A dimension  332  from a proximal end of an exposed portion of the dielectric layer  504  to a distal end portion of the exposed portion of the dielectric layer  504  may be, for example, approximately 0.100 inches. A dimension  334  from the proximal end of the exposed portion of the dielectric layer  504  to a distal end portion of the outer conductor  406  may be, for example, approximately 0.400 inches. A dimension  336  from a distal end of the exposed portion of the dielectric layer  504  to the distal end portion of the outer conductor  406  may be, for example, approximately 0.300 inches. A dimension  338  from the distal end portion of the outer conductor  406  to a distal end portion of the exposed dielectric material  404  of the coaxial cable  230  may be, for example, approximately 0.100 inches. A dimension  340  from the distal end portion of the exposed dielectric material  404  of the coaxial cable  230  to the distal end portion of the center conductor  402  may be, for example, approximately 0.040 inches. A dimension  342  from the distal end portion of the center conductor  402  to the distal end portion of the electrically-conductive element  236  may be, for example, approximately 0.040 inches. 
     Referring now to  FIG. 3C , an outer diameter  344  of the balun at its proximal end portion may be, for example, approximately 0.0410 inches. A dimension  346  from a proximal end portion of the balun short  502  to the proximal end portion of the tubing member  514  may be, for example, approximately 0.011 inches. A dimension  348  from the proximal end portion of the balun short  502  to a distal end portion of balun short  502  may be, for example, approximately 0.042 inches. 
     As mentioned above, in some embodiments, the microwave ablation device  130  has an outer tubular member  222  (not shown in  FIGS. 3A-3B ) that is reduced in outer diameter relative to outer tubular members of prior microwave ablation devices. In some examples, for instance, the outer tubular member  222  is approximately 15 gauge in diameter, or has an outer diameter in a range from approximately 0.074 inches+/−0.0035 inches or from approximately 0.074 inches+/−0.002 inches. The microwave ablation system  100  and device  130  thus reduces invasiveness of microwave ablation procedures and thereby reduces post-surgery healing time and the risk of bleeding or injury. Despite the reduction in size of the outer tubular member  222 , however, the microwave ablation system  100  and microwave ablation device  130  facilitate the efficient delivery of microwave energy from the generator  170  to the antenna assembly  600  for effective treatment. In particular, the microwave ablation system  100  and device  130  are designed such that the coaxial cable  230  includes a relatively large outer diameter  350  of the center conductor  402 , even though maintaining a relatively large outer diameter  350  of the center conductor  402  while downsizing the outer tubular member  222 , in at least some instances, gives rise to an impedance mismatch along a signal path from the generator  170  to the coaxial cable  230 . Nonetheless, even with an impedance mismatch located at a particular portion along the signal path from the generator  170  to the coaxial cable  230 , the microwave ablation system  100  and device  130  of the present disclosure yields performance advantages while minimizing the increase in energy loss to the antenna assembly  600  relative to that that would result by using a coaxial cable that has a impedance that is matched to the impedance of the generator output port  172  but that has a reduced outer diameter of its center conductor, in part because the coaxial cable  230  of the present disclosure maintains a relatively large outer diameter  350  of the center conductor  402 . 
     As noted above, the impedance mismatch may be located between the coaxial cable  230  and one or more other components of the microwave ablation system  100 . For example, the impedance mismatch may be located (1) at a junction where the coaxial cable  230  is coupled to the transition waveguide  252 , (2) at a portion internal to the transition waveguide  252 , or (3) at a junction where the transition waveguide  252  is coupled to the generator cable  174 . In this manner, the impedance of the coaxial cable  230  may be mismatched with (for example, by being at least a predetermined amount lower than) at least one of the impedances of the generator  170 , the generator output port  172 , the transition waveguide input port  254 , and/or the transition waveguide output port  256 . 
     In some embodiments, for example, the impedance mismatch is located at a junction where the coaxial cable  230  is coupled to the transition waveguide  252 , or, more particularly, where the input port  258  of the coaxial cable  230  is coupled to the output port  256  of the transition waveguide  252 . For instance, the respective impedances of the transition waveguide output port  256 , the transition waveguide input port  254 , the generator cable  174 , and the generator output port  172  may be matched to one another within the degree of tolerance and within the predetermined frequency range, and the impedance of the coaxial cable  230  may be lower than the respective impedances of the transition waveguide output port  256 , the transition waveguide input port  254 , the generator cable  174 , and the generator output port  172  by at least a predetermined amount, which exceeds the degree of tolerance. In particular, in one embodiment, the impedance of the coaxial cable  230  is in a range from approximately 39Ω to 45Ω, and the respective impedances of the transition waveguide output port  256 , the transition waveguide input port  254 , the generator output port  172 , and the generator cable  174  are in a range from approximately 48Ω to 52Ω, although each of these ranges may be smaller or larger in other embodiments. 
     In another embodiment, the impedance mismatch is located at a portion of the transition waveguide  252  between the transition waveguide input port  254  and the transition waveguide output port  256 . For instance, within the predetermined frequency range, the impedance of the coaxial cable  230  may be matched to the impedance of the transition waveguide output port  256 , and the respective impedances of the transition waveguide input port  254 , the generator cable  174 , and the generator output port  172  may be matched to one another within the degree of tolerance. In particular, in one illustrative embodiment, the respective impedances of the coaxial cable  230  and the transition waveguide output port  256  may be in a range from approximately 39Ω to 45Ω, and the respective impedances of the transition waveguide input port  254 , the generator cable  174 , and the generator output port  172  may be in a range from approximately 48Ω to 52Ω. 
     In yet another embodiment, the impedance mismatch is located at a junction where the transition waveguide  252  is coupled to the generator cable  174 , or, more particularly, where the transition waveguide input port  254  is coupled to the generator cable  174 . For instance, within the predetermined frequency range, the impedance of the coaxial cable  230  may be matched to the respective impedances of the transition waveguide output port  256  and the transition waveguide input port  254 , and the respective impedances of the generator cable  174  and the generator output port  172  may be matched to one another within the degree of tolerance. In particular, in one embodiment, the respective impedances of the coaxial cable  230 , the transition waveguide output port  256 , and the transition waveguide input port  254  may be in a range from approximately 39Ω to 45Ω, and the respective impedances of the generator cable  174  and the generator output port  172  may be in a range from approximately 48Ω to 52Ω. 
     Referring now to the coaxial cable  230 , which may be solid conductor or a hollow conductor, the approximate characteristic impedance Z 0  of the coaxial cable  230  can be computed according to equation (1), shown below, where OD center conductor  represents the outer diameter  352  of the center conductor  402  of the coaxial cable  230 , OD dielectric  represents the outer diameter  350  of the dielectric layer  404  of the coaxial cable  230 , and ε r  represents the dielectric constant of the dielectric layer  404 . 
     
       
         
           
             
               
                 
                   
                     
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     In some embodiments, the impedance of the coaxial cable  230  is reduced relative to one or more other components of the microwave ablation system  100  by decreasing the ratio of the outer diameter  350  of the dielectric layer  404  to the outer diameter of the center conductor  402  of the coaxial cable  230 . For example, the decreased ratio (relative to a corresponding ratio for a coaxial cable that has an impedance that matches the impedance of the generator output port  172 ) is obtained by decreasing the outer diameter  350  of the dielectric layer  404  while (1) keeping the outer diameter  352  of the center conductor  402  constant or (2) increasing the outer diameter  352  of the center conductor  402 . In another embodiment, the decreased ratio is obtained by increasing the outer diameter  352  of the center conductor  402  while (1) keeping the outer diameter  350  of the dielectric layer  404  constant or (2) decreasing the outer diameter  350  of the dielectric layer  404 . In some embodiments, the outer diameter  352  of the center conductor  402  is in a range from approximately 0.00070 inches to 0.00072 inches, the outer diameter  350  of the dielectric layer  404  is in a range from approximately 0.0189 inches to 0.0195 inches, and the dielectric constant ε r  of the dielectric layer  404  is 2.01, thereby causing the coaxial cable  230  to have an impedance in a range from approximately 39Ω to 45Ω. 
     Referring now to  FIG. 4 , in some embodiments, the first lumen  220  is utilized as a fluid inflow conduit and the second lumen  224  is utilized as a fluid outflow conduit. In other embodiments, the first lumen  220  may serve as a fluid outflow conduit and the second lumen  224  may serve as a fluid inflow conduit. The outer tubular member  222  and/or the inner tubular member  218  may be adapted to circulate coolant fluid therethrough, and may include baffles, multiple lumens, flow restricting devices, or other structures that may redirect, concentrate, or disperse flow depending on their shape. The size and shape of the inner tubular member  218 , the outer tubular member  222 , the first lumen  220 , and the second lumen  224  may be varied from the configuration depicted in  FIGS. 2 and 4 . 
       FIG. 4  shows a portion of the probe assembly  200  of the microwave ablation device  130  of  FIG. 2  including the first lumen  220 , shown disposed between the outer tubular member  222  and the inner tubular member  218 , the second lumen  224 , shown disposed between the inner tubular member  218  and the coaxial cable  230 , and a transmission line  232  extending longitudinally within the second lumen  224 . The coaxial cable  230  is a coaxial cable that includes a center conductor  402 . As indicated by the direction of the arrow-headed lines in  FIG. 4 , the first lumen  220  serves as an inflow conduit for coolant fluid “F” and the second lumen  224  serves as an outflow conduit for coolant fluid “F,” however as noted above these could be reversed without departing from the scope of the present disclosure. In general, the impedance of the coaxial cable  230  may be determined by the inner diameter of the outer conductor  406 , the outer diameter of the center conductor  402 , and the dielectric constant of the dielectric material  404 , and reducing the inner diameter of the outer conductor  406  causes the impedance of the coaxial cable  230  to decrease. 
       FIG. 5A  shows a portion of the coaxial cable  230  including the inner conductor  402 , the outer conductor  406  coaxially disposed around the inner conductor  402 , and the dielectric material  404  disposed therebetween, shown with a balun short  502  coaxially disposed around a portion of the outer conductor  406 . During assembly, the balun short  502  is coupled, deposited or otherwise formed onto, or joined to, the outer conductor  406 . The balun short  502  may be formed as a single structure and electrically coupled to the outer conductor  406 , for example, by solder or other suitable electrical connection. The balun short  502  may be formed of any suitable electrically-conductive materials, such as copper, gold, silver, or other conductive metals or metal alloys. In some embodiments, the balun short  502  has a generally ring-like or truncated tubular shape. The balun short  502  is electrically coupled to the outer conductor  406  of the coaxial cable  230  by any suitable manner of electrical connection, e.g., soldering, welding, or laser welding. The size and shape of the balun short  502  may be varied from the configuration depicted in  FIG. 5A . 
       FIG. 5A  further depicts a dielectric layer  504  (also referred to herein as a balun insulator) coaxially disposed around the outer conductor  406  and coupled thereto. The balun insulator  504  may be formed of any suitable insulative material, including, but not limited to, ceramics, water, mica, polyethylene, polyethylene terephthalate, polyimide, polytetrafluoroethylene (PTFE) (e.g., Teflon®, manufactured by E. I. du Pont de Nemours and Company of Wilmington, Del., United States), glass, metal oxides or other suitable insulator, and may be formed in any suitable manner. In some embodiments, as shown in  FIG. 5A , the balun insulator  504  is a dielectric sleeve. The balun insulator  504  may be grown, deposited or formed by any other suitable technique. In some embodiments, the balun insulator  504  is formed from a material with a dielectric constant in the range of about 1.7 to about 10. 
       FIG. 5A  further depicts a temperature sensor  506  disposed in contact with a proximal end of the balun short  502 . The temperature sensor  506  is coupled to the transmission line  232  extending generally along a longitudinal axis of the coaxial cable  230 . In some embodiments, the temperature sensor  506  is a thermocouple and the transmission line  232  is a thermocouple wire. The thermocouple wire may be a two lead wire thermocouple wire, for example it may be comprised of an insulated (anodized) side-by-side constantine wire and a copper wire. The balun short  502  may include an engagement element  508  adapted to engage with the temperature sensor  506 , for example, to facilitate electrical and mechanical coupling of the temperature sensor  506  and the balun short  502 . In some embodiments, the engagement element  508  may be a groove, slot, or recess cut into the balun short  502 . Alternatively, the temperature sensor  506  may be soldered to balun short  502 . Placement of the thermocouple  506  directly against the balun short  502  improves the sensitivity and thermo-profiling characteristics of the microwave ablation device  130 , particularly as compared to traditional thermocouples in microwave ablation devices, which measure the temperature of the cooling fluid. As will be appreciated by those of skill in the art, the temperature of the coolant will lag the temperature of the balun itself, and thus provide only approximate indications of the temperature of the elements which are heated during operation. As a result, in instances where little or no coolant is flowing, the temperature of the balun  510  and coaxial cable  230  associated therewith can increase faster than that of the coolant and result in damage to the microwave ablation device  130  even before triggering a shut-off of the microwave ablation system  100  based on the temperature of the coolant. Accordingly, improved safety and performance can be achieved by direct sensing of temperature of the balun  510 . 
     Still further,  FIG. 5A  depicts a heat-shrink tubing member  512  disposed in a first configuration around the outer conductor  406 . During assembly, the heat-shrink tubing member  512  is utilized to secure a portion of the transmission line  232  to the coaxial cable  230 . The heat-shrink tubing member  512  may be any suitable tubing material with the capability to respond to heat and bind around an object, and may have any suitable length. In some embodiments, the heat-shrink tubing member  512  may be a thermoplastic. 
       FIG. 5B  shows the coaxial cable  230  of  FIG. 5A  following application of heat to the heat-shrink tubing member  512 . During assembly, securing a portion of the transmission line  232  to the coaxial cable  230 , as shown in  FIG. 5C  keeps the transmission line stable and helps to maintain the electrical and mechanical coupling of the temperature sensor  506  and the balun short  502  during subsequent assembly operations.  FIG. 5B  further shows a second tubing member  514  disposed in a first configuration. 
     The tubing member  514  includes an inner layer of an electrically-conductive material  516 . The electrically-conductive layer  516  may be formed of any suitable electrically-conductive material, such as metallic material. In one embodiment the metallic material of electrically-conductive layer  516  is formed of a silver ink deposited or layered on an interior surface of the tubing member  514 . The tubing member  514  may have a length from approximately 1 to 3 inches in length. However, the shape and size of the tubing member  514  and the balun insulator  504  may be varied from the configuration depicted in  FIG. 5B  without departing from the scope of the present disclosure. Indeed, though described as one embodiment, the orientation and implementation of the feed line  230  as well as other aspects of the present disclosure are not so limited. For example, the feed line  230  may incorporate one or more aspects of the ablation system described in U.S. Pat. No. 9,247,992, filed on Mar. 15, 2013, entitled “MICROWAVE ABLATION CATHETER AND METHOD OF UTILIZING THE SAME,” the entire contents of which are incorporated herein by reference. 
       FIG. 5C  shows the balun  510  after the application of thermal energy or crimping, as the case may be, to the tubing member  514  and the resultant shrinkage or deformation. As shown  FIG. 5C , the electrically-conductive material  516  is disposed in intimate contact with the balun short  502  and a portion of the balun insulator  504 . In some embodiments, as shown in  FIG. 5C , a portion of the balun insulator  504  may extend distally beyond the distal end of the tubing member  514  and electrically conductive layer  516 , to create a gap  518 . The gap  518  improves the microwave performance of the probe assembly  200  and can assist in achieving a desired ablation pattern. More specifically, the gap  518  ensures adequate coupling of microwave energy from a proximal radiating section  604  ( FIG. 6 ) into the balun  510 , improving the performance of the balun  510  over a wide range of tissue dielectric conditions. Further,  FIG. 5C  shows the tubing member  514  securing the portion of the transmission line  232  between the heat-shrink tubing member  512  and the balun short  502  to the coaxial cable  230  preventing its movement and substantially preventing the temperature sensor  506  from being removed from physical contact with the balun short  502 . 
     In some embodiments, the balun insulator  504  has an inner diameter, and the tubing member  514 , which radially surrounds the balun insulator  504 , has a maximum outer diameter  344 , which is its diameter at its largest portion. For example, the balun insulator  504  inner diameter may be approximately 0.041 inches, and the maximum outer diameter of the tubing member  514  may match the maximum outer diameter  344  of the tubing member  514  and may be approximately 0.041 inches. In this manner, despite having an outer diameter of the tubing member  514  that is relatively small compared to prior baluns of microwave ablation devices, the balun  510  is able to have a relatively large thickness, because the thickness of the outer diameter of the coaxial cable  230  has been decreased. This relatively thick size of the balun  510  facilitates efficient and concentrated delivery of energy to the antenna assembly  600  of the microwave ablation device  130 , despite the downsizing of the microwave ablation device  130  relative to prior devices. 
       FIG. 6  is a cross-sectional view of a portion of the probe assembly  200  illustrating the balun  510  of  FIG. 5C  connected to the antenna assembly  600 , in accordance with an embodiment of the present disclosure. In operation, microwave energy having a wavelength, lambda (λ), is transmitted through the antenna assembly  600  and radiated into the surrounding medium, e.g., tissue. The length of the antenna assembly  600  for efficient radiation may be dependent on the effective wavelength, λ eff , which is dependent upon the dielectric properties of the treated medium. The antenna assembly  600  through which microwave energy is transmitted at a wavelength λ, may have differing effective wavelengths, λ eff , depending upon the surrounding medium, e.g., liver tissue as opposed to breast tissue, lung tissue, kidney tissue, etc. 
     The antenna assembly  600 , according to the embodiment shown in  FIG. 6 , includes a proximal radiating section  604  having a length “L 1 ”, a distal radiating section  602  including an electrically-conductive element  236  having a length “L 2 ”, and a feed point  606  disposed therebetween. In some embodiments, the proximal radiating section  604  may have a length “L 1 ” in a range from approximately 0.05 inches to about 0.80 inches. The electrically-conductive element  236  may be formed of any suitable electrically-conductive material, e.g., metal such as stainless steel, aluminum, titanium, copper, or the like. In some embodiments, the electrically-conductive element  236  may have a length “L 2 ” in a range from about 0.15 inches to about 1.0 inches. 
     As shown in  FIG. 6 , the electrically-conductive element  236  has a stepped configuration, such that the outer diameter of the distal portion  608  is less than the outer diameter of the proximal portion  310 . Further, the inner conductor  402  of the coaxial cable  230  is arranged such that it extends past the distal end of the insulator  404  and into the proximal portion  310  of the electrically-conductive element  236 . A hole  610  formed in the proximal portion  310  approximately at 90 degrees to the inner conductor  402  allows for solder, a set screw, or other securing mechanisms to physically secure the electrically-conductive element  236  to the inner conductor  402  and therewith the coaxial cable  230  of the microwave ablation device  130 . 
     The embodiments disclosed herein are examples of the disclosure and may be embodied in various forms. For instance, although certain embodiments herein are described as separate embodiments, each of the embodiments herein may be combined with one or more of the other embodiments herein. Specific structural and functional details disclosed herein are not to be interpreted as limiting, but as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present disclosure in virtually any appropriately detailed structure. Like reference numerals may refer to similar or identical elements throughout the description of the figures. 
     The foregoing description is only illustrative of the present microwave ablation systems and devices. Various alternatives and modifications can be devised by those skilled in the art without departing from the disclosure. Accordingly, the present disclosure is intended to embrace all such alternatives, modifications and variances. The embodiments described with reference to the attached drawing figures are presented only to demonstrate certain examples of the disclosure. Other elements, steps, methods, and techniques that are insubstantially different from those described above and/or in the appended claims are also intended to be within the scope of the disclosure.