Abstract:
A dual slot microwave probe for tissue ablation provides axially spaced slots producing an improved heating pattern with reduced axial extent. Degradation in this heating pattern caused by the addition of ceramic support elements and/or fluid cooling is realized through a feeding structure delivering separate sources of microwave energy to the different slots of the probe aligned with the slots of the probe.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under CA142737 awarded by NIH. The government has certain rights in the invention. 
    
    
     CROSS REFERENCE TO RELATED APPLICATION 
     N/A 
     BACKGROUND OF THE INVENTION 
     The present invention relates to microwave probes for tissue ablation and in particular to a microwave antenna providing improved localization of tissue heating. 
     Microwave and radio frequency ablation may be used to treat tumors, for example in the liver, in patients who are not eligible for surgical removal of the tumor. In microwave ablation, electrical energy with a frequency in the megahertz to gigahertz range is directed into the tumor using a specially designed antenna (ablation probe). The microwave energy received by the tumor and surrounding tissue is converted to heat which destroys tumor cells. Microwave ablation does not require a separate ground pad attached to the patient, and thus may be distinguished from ablation at lower frequencies often termed radiofrequency ablation. 
     The heat energy deposited into the tissue for a given microwave power and duration may be characterized by the Specific Absorption Rate (SAR) of tissue in the vicinity of the probe. The SAR pattern for a microwave probe may therefore be used to characterize a size and shape of the ablation region. In many applications, the ideal SAR pattern of the microwave probe will be concentrated at the tip of the probe (the portion located in the tumor) and not along the shaft of the probe such as may affect healthy tissue or preclude the use of thermal ablation as a treatment option. Such problems may be reduced, but not eliminated by cooling or insulating the shaft of the probe to decrease thermal conduction between the tissue and the shaft, the latter which may be heated by resistive losses in the transmission of microwave energy. Such thermal conduction provides tissue heating in addition to that produced by radiated microwave energy. 
     A “dual slot antenna” described in C. Brace, Dual-Slot Antennas for Microwave Tissue Heating Parametric Design Analysis and Experimentation Validation, Med. Phys. 38(7) 4232-4240 (2011), provides an experimental design for a microwave probe providing a distally concentrated SAR at the boundary of the ablation region. Modifying this design by providing thermal cooling of the probe shaft and robust high temperature insulating materials, such as a ceramic antenna support structure, significantly degrades this desirable SAR pattern. 
     SUMMARY OF THE INVENTION 
     The present invention provides a dual slot antenna design for a microwave ablation probe having a feeding structure generating the microwaves that are directed through the dual antenna slots. This feeding structure significantly improves the SAR iso-contour of the dual slot antenna when necessary thermal and structural changes are implemented for practical clinical use. In one embodiment, a feeding structure having a localized emission point is used to balance the energy emitted from the dual slots of the antenna. 
     Specifically, the present invention provides a probe for microwave ablation having a generally elongate shaft extending along a shaft axis and sized for percutaneous insertion into a patient along the axis. The shaft includes an antenna shell having first and second antenna openings for the passage of microwave energy, the first opening being at the distal end of the shaft and the second opening displaced proximally along the axis. A feeding structure is positioned within and spaced from the antenna shell, the feeding structure providing a center conductor connectable to a source of microwave power and a conductive feeding shell surrounding the center conductor and spaced therefrom, the feeding shell having a feeding opening providing a gap between axially displaced sections of the conductive feeding shell for radial passage of microwave energy therethrough. 
     It is thus a feature of at least one embodiment of the invention to improve the distal concentration of microwave energy in a dual slot microwave antenna offsetting any degradation caused by high temperature ceramic materials and coolant necessary for a practical clinical device. 
     The feeding opening maybe substantially aligned in a radial direction with the second antenna opening. 
     It is thus a feature of at least one embodiment of the invention to use the location of the feeding slot to boost the emissions from the proximal antenna slot. 
     The second antenna and feeding openings have different axial lengths. 
     It is thus a feature of at least one embodiment of the invention to provide an additional dimension of adjustment for controlling the shape of the radiated field by adjusting the size of the feeding and antenna slots as well as their relative location to the antenna openings. 
     The first and second antenna openings may be axially flanked by conductive material. 
     It is thus a feature of at least one embodiment of the invention to control forward projection of the microwave energy by adopting a slot structure at the distal end of the probe. 
     The probe may include a conductive tip on the distal end of the shaft spaced from the antenna shell and the feeding shell. 
     It is thus a feature of at least one embodiment of the invention to provide a simple mechanical structure for producing a slot wall for the distal openings. 
     The distal end of the center conductor may be spaced from the conductive tip. 
     It is thus a feature of at least one embodiment of the invention to eliminate problems incident to temperature induced stresses that may form in any connection between the center conductor and a metallic tip as caused by high temperature operation of the probe. 
     The conductive tip may be a sharpened point extending axially. 
     It is thus a feature of at least one embodiment of the invention to combine the functions of a metallic antenna structure with a forward cutting element facilitating insertion of the probe into tissue. 
     The antenna shell may be spaced coaxially from the feeding shell to provide a space between portions thereof and a blocking wall may be positioned to extend radially from an outer surface of the feeding shell to an inner surface of the antenna shell, the blocking wall being on a proximal side of the second antenna and feeding openings and defining a compartment sealed at a distal end and open at the proximal end of the shaft for receiving and circulating a cooling fluid. 
     It is thus a feature of at least one embodiment of the invention to provide active shaft cooling to prevent conductive as well as microwave heating along the shaft extent proximal to the tip of the probe. 
     The probe may include a coolant tube fitting within the sealed compartment for conducting cooling fluid from the proximal end of the shaft to a point proximate to the blocking wall. 
     It is thus a feature of at least one embodiment of the invention to provide a simple and compact method of circulating fluid in the full-length of the shaft before the tip of the probe. 
     The probe may include a cooling fluid of water within the chamber. 
     It is thus a feature of at least one embodiment of the invention to provide a microwave conductive cooling fluid joining the antenna shell and feeding shell proximal to the second openings. 
     The probe may include a dielectric material extending radially between portions of the antenna shell and the feeding shell distal to the second antenna and feeding openings. 
     It is thus a feature of at least one embodiment of the invention to provide a dual slot microwave antenna that is sufficiently rugged for insertion through tissue. 
     The dielectric material may be a ceramic. 
     It is thus a feature of at least one embodiment of the invention to provide a dimensionally stable and high temperature material for supporting the antenna shell. 
     A portion of the dielectric material may extend within the antenna shell proximal to the second antenna opening to provide a blocking impedance to microwave conduction along the antenna shell proximal to the second opening in the antenna shell. 
     It is thus a feature of at least one embodiment of the invention to employ the supporting dielectric material to block standing wave formation on the shaft of the probe such as may produce resistive heating away from the distal end of the probe. 
     These particular features 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 simplified block diagram of a microwave ablation procedure using the probe of the present invention and showing an external microwave source and cooling system with the probe inserted percutaneously into a tumor region; 
         FIG. 2  is a side elevational view of the probe of  FIG. 1  showing specific absorption rate iso-contours reflecting a heating pattern provided by the probe; 
         FIG. 3  is an exploded perspective view of the components of the probe showing the formation of two slots in an antenna structure and a central feeding structure; 
         FIG. 4  is a fragmentary side elevational cross-section of the probe of  FIG. 3  in an assembled form; and 
         FIG. 5  is a detailed fragment of the cross-section of  FIG. 4  showing the formation of a microwave standing wave blocking element from a dielectric spacer supporting a portion of the antenna shell. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a microwave ablation system  10  suitable for use with the probe of the present invention may provide a microwave source  12  generating a microwave electrical signal in the microwave region (typically from 1 to 3 GHz), for example at substantially 2.45 GHz for the embodiment described below. A microwave signal from the microwave source  12  may be conducted along a flexible coaxial cable  14  to a connector  16  on a proximal end  18  of a microwave ablation probe  20 . 
     The probe  20  provides a substantially rigid elongate shaft  22  whose distal end  24  may be inserted percutaneously to the skin of the patient  26  so that the distal end  24  lies within a tumor  28 . It will be appreciated that the structure of the probe  20  may also be used in open surgery without percutaneous insertion. 
     An external cooling system  30  may connect with the probe  20  and provide for a pump  32  and heat exchanger  34  communicating via a flexible hose  36  with the connector  16  providing a cooling fluid (such as chilled water or gas) to the probe  20  to cool the shaft  22  of the probe  20  as will be described. A second hose  38  also communicates with the connector  16  to collect exhausted (heated) cooling fluid from the probe  20  for return to the pump  32  to the heat exchanger  34 . 
     Referring now to  FIG. 2 , a distal end  24  of the probe  20  provides a sharpened tip  40  directed along an axis  42  of the shaft  22  permitting insertion of the shaft  22  through tissue. The sharpened tip  40  may be part of a conductive antenna shell  44  extending in a radially symmetric manner about the axis  42  of the probe  20 , the conductive antenna shell  44  having a first antenna opening  46  proximate to the sharpened tip  40  and a second antenna opening  48  axially displaced away from the first antenna opening  46  and the sharpened tip  40 . Both the first antenna opening  46  and second antenna opening  48  provide circumferential slots in the conductive antenna shell  44  separating the conductive material of the conductive antenna shell  44  into resistively isolated sharpened tip  40 , spacer conductive shell  50  (displaced distally and spaced from the sharpened tip  40  by the first antenna opening  46 ), and shaft shell  52  (displaced distally and spaced from the spacer shell  50  by the second antenna opening  48 ). Each of the sharpened tip  40 , spacer shell  50  and shaft shell  52  may, for example, be constructed of a conductive stainless steel material providing biocompatibility and electrical conductivity. 
     Microwaves emanating through the first antenna opening  46  and second antenna opening  48  interfere to provide an axially concentrated outer SAR iso-contour  54  defining an ablation region that is axially compressed encompassing a region positioned at the distal end  24 . Ideally, a spherical SAR is desired at applicator tip, while axially narrow SAR is desired in preference to an axially elongate SAR for the rest of shaft shell  52 , the latter which risks body burning and which does not comport well to typical tumor dimensions. 
     In one embodiment, an axial length of the first antenna opening  46  may be made approximately 4 mm in axial length, the spacer shell  50  approximately 8 mm in axial length, and the second antenna opening  48  approximately 1 mm in axial length. The diameter of the shaft  22  may, for example, match that of a 17 gauge steel catheter. These dimensions will vary according to the desired shape of the ablation region, the frequency of the microwaves, and other factors including the permittivity of the tissue and are intended simply as guidance and not as a limitation to the invention. Generally antenna opening widths from 1 to 10 mm separated by 1 to 20 mm may be considered and evaluated experimentally or by simulation. Notably, the two antenna openings may be of different axial lengths. Selection and calculation of the proper dimensions for the slots may be determined with reference to the Brace paper cited above and hereby incorporated by reference. 
     Referring now to  FIGS. 3 and 4 , the sharpened tip  40 , spacer shell  50  and shaft shell  52 , together providing the antenna shell  44 , form an outermost electrically active component of the probe  20 . The spacer shell  50  may be supported on tubular dielectric support  60  receiving at a distal end a cylindrical boss  62  extending from the base of the conical sharpened tip  40  and fitting into the inner diameter of the dielectric support  60  to be retained axially therein. The first antenna opening  46  may be formed by an exposed portion of the dielectric support  60  extending distally beyond the spacer shell  50 . Likewise the second antenna opening  48  may be formed by a portion of the dielectric support  60  extending proximally beyond the spacer shell  50  on the opposite side of the spacer shell  50 . 
     A proximal end of the dielectric support  60  may provide a reduced diameter section  64  fitting into the inner diameter of the shaft shell  52  which may, for example, be a standard steel catheter tube having an inner diameter of 0.059 inches. The dielectric support  60  will generally be electrically insulating high temperature material such as a ceramic. 
     A feeding structure  70  fits within the cylindrical volume defined by the inner wall of the dielectric support  60 . The feeding structure  70  generally is formed from a 020-C semirigid coaxial cable having a central center conductor  72  surrounded by an insulating dielectric layer  74  (typically polytetrafluoroethylene (PTFE)) with an outer surrounding coaxial conductive spacer shell  75 . In a preferred embodiment, the center conductor  72  is spaced from the sharpened tip  40  (for example by a millimeter) providing improved field shaping and eliminating the need for a robust mechanical connection between the conductor  72  and the sharpened tip  40  that might experience high stresses produced by material expansion with heating of the sharpened tip  40  of the probe. 
     The portion of the feeding structure  70  beneath the spacer shell  50  provides the standard coaxial construction of the center conductor  72  surrounding dielectric layer  74  and outer conductive spacer shell  75 . 
     A feeding opening  79  providing a microwave emission region of the feeding structure  70  is located in a portion of the feeding structure  70  aligned with the second antenna opening  48 . The feeding opening  79  is provided by removing the outer conductor of the coaxial cable of the feeding structure in the region beneath the second antenna opening  48  to allow direct broadcasting of microwave energy from the center conductor  72  radially through the feeding opening  79  and out of the second antenna opening  48  and first antenna openings  46 . 
     The remaining length of the feeding structure  70  extending proximally away from the second antenna opening  48  again assumes the standard coaxial cable topology with the center conductor  72 , dielectric layer  74 , and outer shell  81  providing the coaxial conductive shield. 
     A blocking wall  80  extends radially from the outer shell  81  of the feeding structure on a proximal side of the second feeding opening  79  to the inner wall of the shaft shell  52  proximal to the opening  48  to define a compartment  82  proximal to the wall  80  that may be filled with a cooling fluid  84  from the cooling system  30  shown in  FIG. 1 . For this purpose, a small supply tube  86  may be threaded into the compartment  82  to discharge cooling fluid or gas near to the wall  80  which may then flow backward through the compartment  82  to the proximal end of the shaft  22  for extraction therefrom. The cooling fluid  84  may be, for example, water providing an effective conductive short at microwave frequencies between the shell  81  and the shaft shell  52 . The blocking wall  80  may, for example, be formed of an insulating material such as epoxy or a conductive material. 
     Cooling of the shaft shell  52  outside of the ablation region further focuses the ablation toward the distal end of the probe  20  reducing body burns and skin burns. As noted, the feeding structure  70  overcomes the degradation of the ablation pattern when cooling structure and ceramic materials are added to the probe  20 . 
     Referring now to  FIGS. 3 ,  4  and  5 , the reduced diameter section  64  of the dielectric support  60  fitting within the shaft shell  52  may provide an effective high impedance to standing waves forming on the shaft shell  52  which may otherwise create hotspots if not suppressed. Generally, the axial length of the reduced diameter section  64  fitting under the shaft shell  52  may be adjusted to provide a relative phase shift between microwave energy passing directly through the shaft shell  52  from the tip and microwave energy passing through the reduced diameter section  64  from the tip, to provide for destructive cancellation at the desired microwave frequency (for example producing a one half wavelength phase lag). The result is a reduction of standing waves on the shaft shell  52  and thus resistive heating of the shaft shell  52 . 
     The outer surfaces of the probe  20  may optionally be covered by a conformal coating or lubricant material. 
     Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
     When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to 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. All of the publications described herein, including patents and non-patent publications are hereby incorporated herein by reference in their entireties.