Abstract:
An improved antenna for microwave ablation uses a triaxial design which reduces reflected energy allowing higher power ablation and/or a smaller diameter feeder line to the antenna.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
     The present invention relates to medical instruments for ablating tissue, and in particular to a microwave probe for ablation of tumors and the like. 
     Microwave ablation (MWA), like radio frequency ablation (RFA), uses localized heating to cause tissue necrosis. However, MWA can produce greater and more rapid heating and can easily support the use of multiple probes because current flow between the probes can be limited. The mode of heating in MWA also eliminates ground pads and charring concerns. 
     Unfortunately, current MFA equipment produces relatively small lesions because of practical limits in power and treatment time. Power is limited by the current carrying capacity of the small gauge feeder line as it passes through the patient to the site of the necrosis. Larger feeder lines are undesirable because they are not easily inserted percutaneously. Heating of the feeder line at high powers can also lead to burns around the insertion point of the MWA probe. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a triaxial microwave probe design for MWA where the outer conductor allows improved tuning of the antenna to reduce reflected energy through the feeder line. This improved tuning reduces heating of the feeder line allowing more power to be applied to the tissue and/or a smaller feed line to be used. Further, the outer conductor may slide with respect to the inner conductors to permit adjustment of the tuning in vivo to correct for effects of the tissue on the tuning. 
     Specifically, the present invention provides a probe for microwave ablation having a first conductor and a tubular second conductor coaxially around the first conductor but insulated therefrom. A tubular third conductor is fit coaxially around the first and second conductors. The first conductor may extend beyond the second conductor into tissue when a proximal end of the probe is inserted into a body for microwave ablation. The second conductor may extend beyond the third conductor into the tissue to provide improved tuning of the probe limiting power dissipated in the probe outside of the exposed portions of the first and second conductors. 
     Thus, it is one object of at least one embodiment of the invention to provide improved tuning of an MWA device to provide greater power to a lesion without risking damage to the feed line or burning of tissue about the feed line and/or to allow smaller feed lines in microwave ablation. 
     The third tubular conductor may be a needle for insertion into the body. The needle may have a sharpened tip and may use an introducer to help insert it. 
     Thus, it is another object of at least one embodiment of the invention to provide a MWA probe that may make use of normal needle insertion techniques for placement of the probe. 
     It is another object of at least one embodiment of the invention to provide a rigid outer conductor that may support a standard coaxial for direct insertion into the body. 
     The first and second conductors may fit slidably within the third conductor. 
     It is another object of at least one embodiment of the invention to provide a probe that facilitates tuning of the probe in tissue by sliding the first and second conductors inside of a separate introducer needle. 
     The probe may include a lock attached to the third conductor to adjustably lock a sliding location of the first and second conductors with respect to the third conductor. 
     It is thus another object of at least one embodiment of the invention to allow locking of the probe once tuning is complete. 
     The probe may include a stop attached to the first and second conductors to abut a second stop attached to the third conductor to set an amount the second conductor extends beyond the tubular third conductor into tissue. The stop may be adjustable. 
     Thus, it is another object of at least one embodiment of the invention to provide a method of rapidly setting the probe that allows for tuning after a coarse setting is obtained. 
     The second conductor may extend beyond the third conductor by an amount L 1  and the first conductor may extend beyond the second conductor by an amount L 2  and L 1  and L 2  may be multiples of a quarter wavelength of a microwave frequency received by the probe. 
     It is thus another object of at least one embodiment to promote a standing wave at an antenna portion of the probe. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a microwave power supply attached to a probe of the present invention for percutaneous delivery of microwave energy to a necrosis zone within an organ; 
         FIG. 2  is a perspective fragmentary view of the proximal end of the probe of  FIG. 1  showing exposed portions of a first and second conductor slideably received by a third conductor and showing a sharpened introducer used for placement of the third conductor; 
         FIG. 3  is a fragmentary cross sectional view of the probe of  FIG. 2  showing connection of the microwave power supply to the first and second conductors; and 
         FIG. 4  is a cross sectional view of an alternative embodiment of the probe showing a distal electric connector plus an adjustable stop thumb screw and lock for tuning the probe; 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a microwave ablation device  10  per the present invention includes a microwave power supply  12  having an output jack  16  connected to a flexible coaxial cable  18  of a type well known in the art. The cable  18  may in turn connect to a probe  20  via a connector  22  at a distal end  24  of the probe  20 . 
     The probe  20  provides a shaft  38  supporting at a proximal end  25  an antenna portion  26  which may be inserted percutaneously into a patient  28  to an ablation site  32  in an organ  30  such as the liver or the like. 
     The microwave power supply  12  may provide a standing wave or reflected power meter  14  or the like and in the preferred embodiment may provide as much as 100 watts of microwave power of a frequency of 2.45 GHz. Such microwave power supplies are available from a wide variety of commercial sources including as Cober-Muegge, LLC of Norwalk, Conn., USA. 
     Referring now to  FIGS. 1 and 2 , generally a shaft  38  of the probe  20  includes an electrically conductive tubular needle  40  being, for example, an 18-gauge needle of suitable length to penetrate the patient  28  to the ablation site  32  maintaining a distal end  24  outside of the patient  28  for manipulation. 
     Either an introducer  42  or a coaxial conductor  46  may fit within the needle  40 . The introducer  42  may be a sharpened rod of a type well known in the art that plugs the opening of the needle  40  and provides a point  44  facilitating the insertion of the probe  20  through tissue to the ablation site  32 . The needle  40  and introducer  42  are of rigid material, for example, stainless steel, providing strength and allowing easy imaging using ultrasound or the like. 
     The coaxial conductor  46  providing a central first conductor  50  surrounded by an insulating dielectric layer  52  in turn surrounded by a second outer coaxial shield  54 . This outer shield  54  may be surrounded by an outer insulating dielectric not shown in  FIG. 2  or may be received directly into the needle  40  with only an insulating air gap between the two. The coaxial conductor  46  may, for example, be a low loss 0.86-millimeter coaxial cable. 
     Referring still to  FIG. 2 , the central conductor  50  with or without the dielectric layer  52 , extends a distance L 2  out from the conductor of the shield  54  whereas the shield  54  extends a distance L 1  out from the conductor of the needle  40 . L 1  is adjusted to be an odd multiple of one quarter of the wavelength of the frequency of the microwave energy from the power supply  12 . Thus the central conductor  50  in the region of L 2  provides a resonant monopole antenna having a peak electrical field at its proximal end and a minimal electric field at the end of the shield  54  as indicated by  56 . 
     At 2.45 GHz, the length L 2  could be as little as 4.66 millimeters. Preferably, however, a higher multiple is used, for example, three times the quarter wavelength of the microwave power making L 2  approximately fourteen millimeters in length. This length may be further increased by multiple half wavelengths, if needed. 
     Referring to  FIG. 3 , the length L 1  is also selected to be an odd multiple of one quarter of the wavelength of the frequency of the microwave energy from the power supply  12 . When needle  40  has a sharpened or bevel cut tip, distance L 1  is the average distance along the axis of the needle  40  of the tip of needle  40 . 
     The purpose of L 1  is to enforce a zero electrical field boundary condition at line  56  and to match the feeder line  56  being a continuation of coaxial conductor  46  within the needle  40  to that of the antenna portion  26 . This significantly reduces reflected energy from the antenna portion  26  into the feeder line  56  preventing the formation of standing waves which can create hot spots of high current. In the preferred embodiment, L 1  equals L 2  which is approximately fourteen millimeters. 
     The inventors have determined that the needle  40  need not be electrically connected to the power supply  12  or to the shield  54  other than by capacitive or inductive coupling. On the other hand, small amounts of ohmic contact between shield  54  and needle  40  may be tolerated. 
     Referring now to  FIGS. 1 ,  2  and  4 , during use, the combination of the needle  40  and introducer  42  are inserted into the patient  28 , and then the introducer  42  is withdrawn and replaced by a the coaxial conductor  46  so that the distance L 2  is roughly established. L 2  has been previously empirically for typical tissue by trimming the conductor  50  as necessary. 
     The distal end  24  of needle  40  may include a tuning mechanism  60  attached to the needle  40  and providing an inner channel  64  aligned with the lumen of the needle  40 . The tuning mechanism provides at its distal end, a thumbwheel  72  having a threaded portion received by corresponding threads in a housing of the tuning mechanism and an outer knurled surface  74 . A distal face of the thumbwheel provides a stop that may abut a second stop  70  being clamped to the coaxial conductor  46  thread through the tuning mechanism  60  and needle  40 . When the stops  70  and on thumbwheel  72  abut each other, the coaxial conductor  46  will be approximately at the right location to provide for extension L 1 . Rotation of the thumbwheel  72  allows further retraction of the coaxial conductor  46  to bring the probe  20  into tuning by adjusting L 1 . The tuning may be assessed by observing the reflected power meter  14  of  FIG. 1  and tuning for reduced reflected energy. 
     The tuning mechanism  60  further provides a cam  62  adjacent to the inner channel  64  through which the coaxial conductor  46  may pass so that the cam  62  may press and hold the coaxial conductor  46  against the inner surface of the channel  64  when a cam lever  66  is pressed downwards  68 . Thus, once L 1  is properly tuned, the coaxial conductor  46  may be locked in position with respect to needle  40 . 
     The distal end of the coaxial conductor  46  may be attached to an electrical connector  76  allowing the cable  18  to be removably attached to disposable probes  20 . 
     The present invention provides as much as a ten-decibel decrease in reflected energy over a simple coaxial monopole in simulation experiments and can create a region of necrosis at the ablation site  32  greater than two centimeters in diameter. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.