Abstract:
An electromagnetic surgical ablation probe having a tunable helical antenna element is disclosed. The probe includes a coaxial feedline having an inner conductor coaxially disposed within a dielectric, and an outer conductor coaxially disposed around the dielectric. The inner conductor and dielectric extend distally beyond a distal end of the outer conductor. A helical antenna element is operably coupled to a distal end of the inner conductor. During use, the antenna may be tuned by changing at least one dimension of the helical antenna element. Embodiments are presented wherein a dimensions of the helical antenna element is changed by state change of a shape memory alloy, by a change in temperature, by activation of a piston by fluidic pressure, by linear motion of a conical tip, and by a manual screw-type adjustment.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to systems and methods for providing energy to biological tissue and, more particularly, to a microwave ablation surgical antenna having a self-tuning or adjustable helical coil, and methods of use and manufacture thereof. 
     2. Background of Related Art 
     There are several types of microwave antenna assemblies in use, e.g., monopole, dipole and helical, which may be used in tissue ablation applications. In monopole and dipole antenna assemblies, microwave energy generally radiates perpendicularly away from the axis of the conductor. Monopole antenna assemblies typically include a single, elongated conductor. A typical dipole antenna assembly includes two elongated conductors, which are linearly aligned and positioned end-to-end relative to one another with an electrical insulator placed therebetween. Helical antenna assemblies include a helically-shaped conductor connected to a ground plane. Helical antenna assemblies can operate in a number of modes including normal mode (broadside), in which the field radiated by the helix is maximum in a perpendicular plane to the helix axis, and axial mode (end fire), in which maximum radiation is along the helix axis. The tuning of a helical antenna assembly may be determined, at least in part, by the physical characteristics of the helical antenna element, e.g., the helix diameter, the helix length, the pitch or distance between coils of the helix, and the position of the helix in relation to the probe assembly to which it is mounted. 
     The typical microwave antenna has a long, thin inner conductor that extends along the longitudinal axis of the probe and is surrounded by a dielectric material and is further surrounded by an outer conductor around the dielectric material such that the outer conductor also extends along the axis of the probe. In another variation of the probe that provides for effective outward radiation of energy or heating, a portion or portions of the outer conductor can be selectively removed. This type of construction is typically referred to as a “leaky waveguide” or “leaky coaxial” antenna. Another variation on the microwave probe involves having the tip formed in a uniform spiral pattern, such as a helix, to provide the necessary configuration for effective radiation. This variation can be used to direct energy in a particular direction, e.g., perpendicular to the axis, in a forward direction (i.e., towards the distal end of the antenna), or combinations thereof. 
     Invasive procedures and devices have been developed in which a microwave antenna probe may be either inserted directly into a point of treatment via a normal body orifice or percutaneously inserted. Such invasive procedures and devices potentially provide better temperature control of the tissue being treated. Because of the small difference between the temperature required for denaturing malignant cells and the temperature injurious to healthy cells, a known heating pattern and predictable temperature control is important so that heating is confined to the tissue to be treated. For instance, hyperthermia treatment at the threshold temperature of about 41.5° C. generally has little effect on most malignant growth of cells. However, at slightly elevated temperatures above the approximate range of 43° C. to 45° C., thermal damage to most types of normal cells is routinely observed. Accordingly, great care must be taken not to exceed these temperatures in healthy tissue. 
     In the case of tissue ablation, a high radio frequency electrical current in the range of about 300 MHz to about 10 GHz is applied to a targeted tissue site to create an ablation volume, which may have a particular size and shape. Ablation volume is correlated to antenna design, antenna tuning, antenna impedance and tissue impedance. Tissue impedance may change during an ablation procedure due to a number of factors, e.g., tissue denaturization or desiccation occurring from the absorption of microwave energy by tissue. Changes in tissue impedance may cause an impedance mismatch between the probe and tissue, which may affect delivery of microwave ablation energy to targeted tissue. 
     SUMMARY 
     The present disclosure is directed to a microwave ablation probe having a self-tuning, or adjustable, helical antenna element. The helical antenna element may be tuned dynamically and automatically during use, and/or may be tuned manually. 
     In one embodiment, a helical antenna element is formed from shape memory alloy (SMA). SMAs are a family of alloys having anthropomorphic qualities of memory and trainability. One of the most common SMAs is Nitinol which can retain shape memories for two different physical configurations and changes shape as a function of temperature. Recently, other SMAs have been developed based on copper, zinc and aluminum and have similar shape memory retaining features. 
     SMAs undergo a crystalline phase transition upon applied temperature and/or stress variations. A particularly useful attribute of SMAs is that after it is deformed by temperature/stress, it can completely recover its original shape on being returned to the original temperature. This transformation is referred to as a thermoelastic martenistic transformation. 
     Under normal conditions, the thermoelastic martenistic transformation occurs over a temperature range which varies with the composition of the alloy itself, and the type of thermal-mechanical processing by which it was manufactured. In other words, the temperature at which a shape is “memorized” by an SMA is a function of the temperature at which the martensite and austenite crystals form in that particular alloy. For example, nickel titanium alloys (NiTi), commonly known as Nitinol, can be fabricated so that the shape memory effect will occur over a wide range of temperatures, e.g., −2700° to +1000° Celsius. 
     A dimension of the helical coil, e.g., the coil span (e.g., the distance between helical turns) and/or the diameter of the helical antenna element may be configured to change upon transformation of the SMA material from an austenitic state to a martenistic state in response to temperature changes at the surgical site. In some embodiments, a dimension of the helical coil may be configured to change in response to mechanical actuation, such as without limitation, actuation of a piston, actuation member (e.g., the inner conductor), and/or an adjustment ring. A change in antenna tuning associated with higher temperatures is thus corrected by a corresponding dimensional change in the helical antenna element triggered by the higher temperature. The helical coil antenna may be continuously and/or infinitely adjustable. 
     In some embodiments, an ablation probe in accordance with the present disclosure includes an inner conductor, a dielectric coaxially disposed around the inner conductor, and an outer conductor coaxially disposed around the dielectric. The dielectric and the inner conductor extend distally from the outer conductor. A tunable helical antenna element is coaxially disposed about the distal extension of the dielectric and is operably joined at a distal end thereof to the inner conductor. The tunable helical antenna element has a first dimension corresponding to a first tuning and at least a second dimension corresponding to a second tuning. 
     In other embodiments, an ablation probe in accordance with the present disclosure includes a generally tubular inner conductor, a dielectric coaxially disposed around the inner conductor, and an outer conductor coaxially disposed around the dielectric. The dielectric and the inner conductor extend distally from the outer conductor. A helical slot is defined in at least one of the dielectric or inner conductor. A tunable helical antenna element is coaxially disposed about the distal extension of the dielectric and is operably joined at a distal end thereof to the inner conductor. The tunable helical antenna element has a first dimension corresponding to a first tuning and at least a second dimension corresponding to a second tuning. A piston is slidably disposed within the inner conductor, wherein a proximal end of the helical antenna element is operably coupled to a distal end of the piston through the helical slot. 
     In yet other embodiments, an ablation probe in accordance with the present disclosure includes a dielectric, and an inner conductor coaxially disposed within the dielectric and longitudinally movable with respect the dielectric. The inner conductor extends distally from the dielectric. An outer conductor is coaxially disposed around the dielectric, and the dielectric extends distally from the outer conductor. The disclosed probe includes a tip fixed to a distal end of the inner conductor, a biasing member configured to bias the tip distally, and a tunable helical antenna element coaxially disposed about the distal extension of the dielectric and operably joined at a distal end thereof to the inner conductor. The tunable helical antenna element has a first dimension corresponding to a first tuning, and at least a second dimension corresponding to a second tuning. 
     In still other embodiments, an ablation probe in accordance with the present disclosure includes an inner conductor, a dielectric coaxially disposed around the inner conductor, and an outer conductor coaxially disposed around the dielectric. The dielectric and the inner conductor extend distally from the outer conductor. A barrel is coaxially disposed about the outer conductor and is movable along a longitudinal axis thereof. The barrel includes an exterior threaded portion, an adjustment collar rotatable about a longitudinal axis thereof, and has an interior threaded portion adapted to cooperatively engage the exterior threaded portion of the barrel. A tunable helical antenna element is coaxially disposed about the distal extension of the dielectric and is operably joined at a distal end thereof to the inner conductor and operably joined at a proximal end thereof to the barrel. The tunable helical antenna element has a first dimension corresponding to a first tuning, and at least a second dimension corresponding to a second tuning. 
     A method for tuning an electromagnetic surgical ablation probe is also disclosed which includes the steps of providing an electromagnetic surgical ablation probe and changing at least one dimension of the helical antenna element provided therein. The provided electromagnetic surgical ablation probe includes an inner conductor, a dielectric coaxially disposed around the inner conductor, an outer conductor coaxially disposed around the dielectric, wherein the dielectric and the inner conductor extend distally from the outer conductor, and a helical antenna element coaxially disposed about the distal extension of the dielectric and operably joined at a distal end thereof to the inner conductor. 
     Also disclosed in an electromagnetic surgical ablation system that includes a source of ablation energy, and a tunable electromagnetic surgical ablation probe operably coupled to the source of ablation energy. The tunable electromagnetic surgical ablation probe includes an inner conductor, a dielectric coaxially disposed around the inner conductor, and an outer conductor coaxially disposed around the dielectric. The dielectric and the inner conductor extend distally from the outer conductor. The probe further includes a helical antenna element coaxially disposed about the distal extension of the dielectric that is operably joined at a distal end thereof to the inner conductor. The helical antenna element is tunable by changing at least one changeable dimension thereof, including without limitation gap distance, distance between turns, length, and diameter. 
    
    
     
       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 diagram of a microwave ablation system having a surgical ablation probe in accordance with an embodiment of the present disclosure; 
         FIG. 2A  is a cross sectional, side view of an embodiment of a surgical ablation probe having a helical antenna assembly in accordance with the present disclosure; 
         FIG. 2B  is a perspective view of the surgical ablation probe of  FIG. 2A ; 
         FIG. 3A  shows a side view of the surgical ablation probe of  FIG. 2A  wherein the helical antenna is in a first state; 
         FIG. 3B  shows a side view of the surgical ablation probe of  FIG. 2A  wherein the helical antenna is in a second state; 
         FIG. 4A  is a cross sectional side view of another embodiment of a surgical ablation probe having a helical antenna assembly in accordance with the present disclosure; 
         FIG. 4B  is a perspective view of the surgical ablation probe of  FIG. 4A ; 
         FIG. 4C  is a sectional view of the surgical ablation probe of  FIG. 4A ; 
         FIG. 5A  is a cross sectional side view of another embodiment of a surgical ablation probe having a helical antenna assembly in accordance with the present disclosure, wherein the helical antenna is in a first state; 
         FIG. 5B  is a cross sectional side view of the surgical ablation probe of  FIG. 5A  wherein the helical antenna is in a second state; 
         FIG. 5C  is a perspective view of the surgical ablation probe of  FIG. 5A ; 
         FIG. 6A  is a cross sectional side view of yet another embodiment of a surgical ablation probe having a helical antenna assembly in accordance with the present disclosure, wherein the helical antenna is in a first state; 
         FIG. 6B  is a cross sectional side view of the surgical ablation probe of  FIG. 6A  wherein the helical antenna is in a second state; 
         FIG. 6C  is a perspective view of the surgical ablation probe of  FIG. 6A ; 
         FIG. 7A  is a cross sectional side view of still another embodiment of a surgical ablation probe having a helical antenna assembly in accordance with the present disclosure, wherein the helical antenna is in a first state; 
         FIG. 7B  is a cross sectional side view of the surgical ablation probe of  FIG. 7A  wherein the helical antenna is in a second state; and 
         FIG. 7C  is a perspective view of the surgical ablation probe of  FIG. 7A . 
     
    
    
     DETAILED DESCRIPTION 
     Particular embodiments of the present disclosure will be described hereinbelow with reference to the accompanying drawings; however, the disclosed embodiments are merely examples of the disclosure, which may be embodied in various forms. Well-known or repetitive functions, constructions are not described in detail to avoid obscuring the present disclosure in unnecessary or redundant detail. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely 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. 
     In the drawings and in the descriptions that follow, the term “proximal,” as is traditional, shall refer to the end of the instrument that is closer to the user, while the term “distal” shall refer to the end that is farther from the user. 
       FIG. 1  shows an embodiment of a microwave ablation system  10  in accordance with the present disclosure. The microwave ablation system  10  includes an electromagnetic surgical ablation probe  5  connected by a cable  15  to connector  16 , which may further operably connect the antenna probe  10  to a generator assembly  20 . Probe  5  includes a distal radiating portion  11  having a helical antenna element  12 . Generator assembly  20  may be any suitable source of ablation energy, e.g., microwave or RF energy in the range of about 500 MHz to about 10 GHz. In some embodiments, generator assembly  20  may provide ablation energy in a range of about 915 MHz to about 2.45 GHz. Cable  15  may additionally or alternatively provide a conduit (not explicitly shown) configured to provide coolant from a coolant source  18  and/or a pressure source  14  to the electromagnetic surgical ablation probe  10 . Pressure source  14  may be configured to provide pneumatic pressure (e.g., compressed air or other gas), but it is envisioned any suitable pressurized media may be provided by pressure source  14 . 
     With reference to  FIGS. 2A and 2B , a microwave ablation probe  110  includes a shaft assembly  101  having an inner conductor  103 , a dielectric  104  coaxially disposed about the inner conductor  103 , and an outer conductor  105  coaxially disposed about the dielectric  104 . Inner conductor  103  and outer conductor  105  may be formed from any suitable heat-resistant electrically conductive material, including without limitation stainless steel. Inner conductor  103  and outer conductor  105  may be plated or clad with a biocompatible, electrically-conductive material, which may improve the electrical conductivity of the inner conductor  103  and outer conductor  105 . In some embodiments, inner conductor  103  and outer conductor  105  may be plated or clad with silver. Dielectric  104  may be formed from any suitable heat-resistant material having electrically insulative properties, e.g., ceramic, porcelain, or polymeric material. Inner conductor  103  and dielectric  104  extend distally beyond a distal end  108  of outer conductor  105 . A distal end  121  of inner conductor  103  is exposed at a distal end  109  of dielectric  104 . Helical antenna element  120  is disposed coaxially around a distal region of dielectric  104 . A distal end of antenna element  120  is electromechanically joined to inner conductor  103  at the exposed distal end  121  thereof by any suitable manner of joining, including without limitation laser welding, brazing, threaded coupler, and/or crimping. A proximal end of helical antenna element  120  may be detached (e.g., free-floating) to enable helical antenna element  120  to expand and/or contract as discussed in detail below. 
     Helical antenna element  120  may be formed from material that expands and/or contracts in response to changes in temperature, including without limitation, an SMA alloy such as nickel titanium (NiTi), commonly known as Nitinol. During manufacture, helical antenna element  120  may be formed from Nitinol wire by, e.g., winding the Nitinol wire stock around a form having a generally cylindrical shape; annealing the helical antenna element  120  to define the austenite shape and size thereof; and deforming (e.g., expanding or contracting) the helical antenna element  120  to define the martensite size and shape of helical antenna element  120 . In this manner, the desired hot (austenite) and cold (martensite) shapes of helical antenna element  120  may be imprinted into the crystalline structure of the Nitinol wire. 
     In use, it is believed that an increase in reflections that occur as a result of tissue desiccation and/or denaturization causes an increase in probe temperature. This, in turn, heats helical antenna element  120  and causes the size and/or shape thereof to change and, thus, adjusts and/or corrects the tuning of helical antenna element  120 . In particular, tuning may be affected by the gap distance “G” between a distal end of the outer conductor and a proximal end of the helical antenna element  120 , the distance “S” between turns of the helical antenna element  120 , the length “L” of the helical antenna element  120 , and/or the diameter “D” of the helical antenna element  120 . The probe  110 , shaft  101 , and/or distal end  109  may be coated with a lubricious material, such as without limitation, polytetrafluoroethylene (a.k.a. PTFE or Teflon®, manufactured by the E.I. du Pont de Nemours and Co. of Wilmington, Del., USA), polyethylene teraphthalate (PET), or the like. 
       FIGS. 3A and 3B  depict helical antenna element  120  in an austenite state and a martensite state, respectively. In use, heat generated during a microwave ablation surgical procedure causes an increase in temperature in the helical coil and/or associated components of the probe  110 , which, in turn, causes the helical antenna element  120  to transition between a martensite shape and size thereof, as best seen in  FIG. 3A , and an austenite size and shape thereof, as best seen in  FIG. 3B . As shown, helical antenna element  120  is configured such that an increase in temperature results in a decrease in coil length L due to the Nitinol phase transformation. It is also contemplated that helical antenna element  120  is configured such that an increase in temperature results in a decrease in coil diameter D. It is further contemplated that that helical antenna element  120  may be configured such that an increase in temperature results in an increase of length L and/or diameter D. In an embodiment this may be achieved by, e.g., annealing helical antenna element  120  during manufacture to imprint the desired (larger) austenite share thereupon. It is further contemplated that a multiple phase shape metal alloy, e.g., an SMA having more than two primary states may be used to construct helical antenna element  120 . 
     Turning to  FIGS. 4A and 4B , a microwave ablation probe  210  in accordance with another embodiment of the present disclosure is shown having a shaft  201  that includes a hollow inner conductor  203 , which may have a tubular or other suitable shape. The hollow interior of inner conductor  203  defines an inflow conduit  207  that is adapted to deliver a fluid generally to the shaft, and more specifically, to a tip  230 , and to an outflow conduit  202 . Any suitable fluid having a low dielectric constant may be utilized, including without limitation water, deionized water, saline, and/or biocompatible oils or gases. Shaft  201  also includes an outer dielectric  204  coaxially disposed about coolant outflow conduit  202 , an inner dielectric  206  axially disposed about inner conductor  203 , and an outer conductor  205  coaxially disposed about the outer dielectric  204 . Outflow conduit  202  is defined by the region between outer dielectric  204  and inner dielectric  206 . Tip  230  is fixed to a distal end of outer dielectric  204  and may include a fluid chamber  231  defined therein. 
     In use, according to one embodiment, coolant flows distally through inflow conduit  207  from coolant source  18 , into fluid chamber  231 , and flows proximally through outflow conduit  202 . Additionally or alternatively, coolant flow may be reversed, e.g., flowing distally though outflow conduit  202  and proximally through inflow conduit  207 . Probe  210  may include a sensor (not explicitly shown) that is operably coupled at least one of generator  20  or coolant source  18  and is adapted to sense a surgical parameter, such as without limitation probe temperature and/or tissue impedance. Generator  20  and/or coolant source  18  may be configured to receive a sensed surgical parameter and regulate the flow of ablation energy and/or coolant in response thereto. In this manner, the temperature of helical antenna element  220  may be regulated and, in turn, cause the size and/or shape of helical antenna element  220  to change, thus adjusting and/or correcting the tuning of helical antenna element  220 . 
     Referring to  FIGS. 5A ,  5 B, and  5 C, a microwave ablation probe  310  in accordance with yet another embodiment of the present disclosure is shown. The probe  310  includes a shaft  301  having therein a piston  312  that is slidably disposed longitudinally within a sleeve  307 . The shaft  301  includes a tubular outer conductor  305  that is coaxially disposed around a tubular inner conductor  303  having a tubular dielectric  304  disposed therebetween. In some embodiments, a diameter of sleeve  307  may be about the same as a diameter of inner conductor  303 . Dielectric  304  and inner conductor  303  extend distally beyond a distal end of outer conductor  305 . A tip  330 , which may be substantially conical in shape to improve ease of insertion of the probe into tissue, is fixed at a distal end  308  of dielectric  304 . Piston  312  is dimensioned to slide and/or rotate freely within sleeve  307  while maintaining a substantially gas-tight or liquid-tight seal therebetween. Piston  312  includes a support  313  that extends distally from a distal end of piston  312  and includes a coupling pin  315  that operably engages a proximal end of helical antenna element  320  through a helical slot  316  defined in dielectric  304  and inner conductor  303 . A distal end of helical antenna  320  is coupled to a distal end  321  of inner conductor  303 . Piston  312  may be actuated by media, e.g., gas and/or liquid, that is introduced into and/or withdrawn from plenum  302 . Any suitable media may be utilized, for example and without limitation, water, saline, air, oxygen, nitrogen, carbon dioxide, and/or biocompatible oil. 
     In use, media is introduced into, and/or withdrawn from, plenum  302 , driving piston  312  distally. As piston  312  traverses distally, coupling pin  315  rides within helical slot  316  and compresses helical antenna element  320  to adjust the tuning thereof. A sensor (not explicitly shown) may be included within probe  310  to sense a physical or surgical parameter related thereto, including without limitation plenum pressure, probe temperature, and/or tissue impedance. Generator  20  and/or pressure source  14  may be configured to receive a sensed surgical parameter and regulate ablation energy and/or plenum pressure in response thereto. In this manner, the tuning of helical antenna element  320  may be regulated and, in turn, cause the size and/or shape of helical antenna element  320  to change, thus adjusting and/or correcting the tuning of helical antenna element  320 . 
     Turning to  FIGS. 6A ,  6 B, and  6 C, a microwave ablation probe in accordance with still another embodiment of the present disclosure is shown wherein a probe  410  includes a spring-loaded tip  430 . The probe  410  includes a tubular outer conductor  405  that is coaxially disposed around an inner conductor  403  having a dielectric  404  disposed therebetween. Inner conductor  403  is slidably disposed within dielectric  404  and may be operably coupled at a proximal end thereof to an actuator (not explicitly shown) that imparts longitudinal motion to inner conductor  403 . For example, and without limitation, an actuator may include a lever, a handle, a threaded adjustment device (e.g., a thumbscrew), or an electromechanical actuator, such as a solenoid, servo, and/or a stepper motor. Inner conductor  403  extends distally beyond a distal end  419  of dielectric  404 , and is coupled at a distal end  418  thereof to tip  430  by any suitable manner of attachment, including without limitation, welding, brazing, crimping, clamping, adhesive, and threaded attachment. A biasing member  416  is disposed between tip  430  and distal end  419  of dielectric  404  and is configured to bias tip  430  away from distal end  419  of dielectric  404 , e.g., distally therefrom. 
     Probe  410  includes a helical antenna element  420  that is operably coupled at a distal end thereof to a distal end  418  of inner conductor  403 . Additionally or alternatively, helical antenna element  420  may be coupled to inner conductor  403  via lead wire  415  which extends from inner conductor  403  to a surface of tip  430 , where lead wire  415  may be joined to helical antenna element  420  at junction  421 . A proximal end of helical antenna element  420  may be fixed to dielectric  404  at an outer surface thereof by any suitable manner of attachment. Probe  410  may include a positive stop (not explicitly shown) that is configured to retain the combination of tip  430 , biasing member  416 , and/or inner conductor  403  to dielectric  404  such that full extension of spring  416  does not cause separation of tip  430 , spring  416 , and/or inner conductor  403  from dielectric  404 . In an embodiment, the actuator (not explicitly shown) may limit distal movement of inner conductor  403  to prevent separation of tip  430 , biasing member  416 , and/or inner conductor  403  from dielectric  404 . During use, helical antenna element  430  may be tuned by causing inner conductor  403  to move longitudinally, e.g., by using an aforesaid actuator to cause inner conductor  403  to move proximally and/or distally, until a desired tuning is achieved. 
     With reference now to  FIGS. 7A ,  7 B, and  7 C, disclosed is a microwave ablation probe  510  having a manually-adjustable helical antenna element  520  disposed at a distal end thereof. Probe  510  includes a shaft  501  having an outer conductor  505  that is coaxially disposed around an inner conductor  503  with a dielectric  504  disposed therebetween. Dielectric  504  and inner conductor  503  extend distally beyond a distal end of outer conductor  505 . Probe  510  includes an outer barrel  512  that is coaxially disposed around at least outer conductor  505  and includes an exterior threaded portion  513  at a proximal end of outer barrel  512 . Exterior threaded portion  513  is configured to operably engage adjustment collar  516  which has an interior threaded portion  517 . Adjustment collar  516  may include one or more ergonomic and/or friction enhancing elements  518  to facilitate handling, e.g., scallops, protuberances, knurling, elastomeric material, etc. 
     A distal end of helical antenna element  520  is operably coupled to a distal end  521  of inner conductor  503 . A proximal end of helical antenna element  520  is fixed to a distal end  519  of outer barrel  512 . During use, a user (e.g., a surgeon) may adjust the tuning of helical antenna element  520  by rotating adjustment collar  516  to cause outer barrel  512  to move distally and/or proximally to achieve a desired tuning. One or more stop members (not explicitly shown) may be included to maintain outer barrel  512  and/or adjustment collar  516  in cooperative orientation, e.g., to ensure translation of rotational motion of adjustment collar  516  into the desired linear longitudinal motion of outer barrel  512 . 
     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. Further variations of the above-disclosed embodiments and other features and functions, or alternatives thereof, may be made or desirably combined into many other different systems or applications 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.