Abstract:
A handheld microwave waveguide provides a planar microwave beam for rapid coagulation of a strip of tissue in an organ providing a barrier against blood loss during resection operations.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with United States government support awarded by the following agency:
       NIH DK058839       
 
         [0003]    The United States government has certain rights in this invention. 
     
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0000]    
       
         
           
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       BACKGROUND OF THE INVENTION 
       [0005]    The present invention relates to surgical resection of tissue and in particular to an apparatus using microwave coagulation to control bleeding during the resection of a portion of an organ. 
         [0006]    The liver is a common site for both primary and metastatic cancer. Surgical resection (hepatectomy) is currently the preferred treatment for liver cancer. During resection, the surgeon typically removes a lobe of the liver, a time consuming procedure where the surgeon must cut through tissue while avoiding or closing large blood vessels. Blood loss during this procedure can adversely affect patient survival, increase hospital stay, and increase complication rates. 
         [0007]    Some studies have investigated the use of radio frequency (RF) ablation or microwave (MW) ablation to coagulate tissue before resection. In RF ablation, one or more electrodes are inserted into the tissue and current passing from the electrodes through the patient to a large area ground pad on the patient&#39;s skin coagulates the tissue near the electrode through resistive heating, sealing it against blood flow. In order to ablate the necessary area of tissue, the electrode is removed and reapplied at a series of locations along the tissue slice. The time required for this procedure is generally too long for clinical practice. 
         [0008]    Pending US patent application 2006/0064084 published Mar. 13, 2006 and entitled: “Electrode Array for Tissue Ablation” describes a system for RF ablation using multiple electrodes positioned within a holder for insertion into the tissue along an ablation line. The use of multiple electrodes greatly reduces the time required for this procedure. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention provides a microwave horn antenna that may be used in lieu of insertable electrodes for creating a coagulated zone for bloodless resection. The horn antenna is compact for easy manipulation by a surgeon and produces a fan-shaped microwave radiation pattern leading to heating in a region conforming to a resection plane and minimizing the generation of high temperature water vapor such as makes the coagulation process less controllable. A dielectric within the horn allows the use of lower frequencies which propagate more deeply into the tissue. A quarter wavelength stub provides improved shaping of the emission pattern and an insulating coating allows handheld operation with inevitable heating of the horn at the necessary power. 
         [0010]    Specifically, the present invention provides a method for bloodless resection of an organ by applying microwave energy along a resection line through a microwave horn antenna emitting a generally fan-shaped microwave beam having an elongated cross-section whose longest dimension is aligned with the resection line. After the resection line is coagulated, a cut is made along the resection line within the coagulated tissue to resect the organ. 
         [0011]    It is thus one object of the invention to provide a rapid method of coagulating a zone within an organ for bloodless resection without the need for the placement of electrode arrays. 
         [0012]    The coagulation and cutting steps may be repeated to continue resection of the organ at an increasing depth along a plane aligned with the resection line. 
         [0013]    It is thus an object of the invention to provide a method that accommodates limited microwave penetration and that provides a flexible and incremental procedure that accommodates variances and uncertainty in tissue depth. 
         [0014]    The longest cross-sectional dimension of the microwave horn may be more than twice the height of the cross-section perpendicular to the longest dimension. 
         [0015]    It is thus an object of the invention to provide a microwave emission pattern that reduces internal heat concentrations in the tissue such as generate migrating high temperature water vapor. 
         [0016]    The microwave horn may be filled with a solid dielectric material such as Macor® and the microwave energy may be substantially at a frequency of less than 4 GHz. 
         [0017]    It is thus an object of the invention to permit the microwave horn to be used with lower frequency microwave energy for greater penetration while maintaining a compact size that may be inserted between partially resected tissue walls for movement along the resection line by the surgeon. 
         [0018]    The horn is sized to be held within a surgeon&#39;s hand and has an outer surface tactility indicating the orientation of the longest dimension and the direction of microwave emission. 
         [0019]    It is thus an object of the invention to provide a system amenable to use by a single individual who may wield both the microwave horn and a scalpel. 
         [0020]    The method may move the microwave horn along the resection line and apply sequential microwave applications over a distance larger than the longest dimension. 
         [0021]    It is thus an object of the invention to provide a system that can work for arbitrary organ sizes while maintaining a compact form factor. 
         [0022]    The microwave horn may include a flexible coaxial cable removably connected to the horn to provide microwave power thereto in an amount greater than 50 W. 
         [0023]    It is thus an object of the invention to provide a high power microwave device that allows relatively quick coagulation of tissue. 
         [0024]    The dielectric of the microwave horn may further include metal inserts within the dielectric for field shaping. 
         [0025]    It is thus an object of the invention to permit cross-sections of high aspect ratio enabling faster coagulation of longer sections. 
         [0026]    The horn may have its outer surfaces covered by a thermally insulating coating. 
         [0027]    It is thus an object of the invention to manage transmission inefficiencies while preserving handheld operation. 
         [0028]    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 
         [0029]      FIG. 1  is a perspective view of the microwave device of the present invention as can be held in one hand showing generally a coagulation region from microwaves projected downward along an axis; 
           [0030]      FIGS. 2   a - 2   c  are perspective views of the device of  FIG. 1  being used to resect an organ through successive coagulation and cutting operations; 
           [0031]      FIG. 3  is a side elevational cross-section of the device of  FIG. 1  taken along lines  3 - 3  showing an internal dielectric material and coupling of microwave power to the dielectric material by means of a quarter wavelength stub; 
           [0032]      FIG. 4  is a cross-section taken along lines  4 - 4  of a rectangular microwave pattern that reduces high center temperatures; 
           [0033]      FIG. 5  is a figure similar to that of  FIG. 4  showing the emission cross-section of a typical microwave cylindrical horn such as produces high center temperatures causing migration of high temperature water vapor disrupting the coagulation pattern; and 
           [0034]      FIG. 6  is a front elevational cross-section and bottom plan view of an alternative embodiment of the  FIG. 1  having metal inserts increasing the aspect ratio of the microwave cross-section. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0035]    Referring now to  FIG. 1 , a microwave applicator  10 , in a first embodiment of the present invention, provides a generally rectangular housing  11  having outer dimensions of approximately 4×4.5×2 cm. As depicted in  FIG. 1 , and in use, the longest dimension is oriented vertically and aligns with a microwave propagation axis  12  extending downward from a lower face  15  of the microwave applicator  10 . The width of 4 cm is along the housing&#39;s transverse axis  17  allowing it to be comfortably grasped in a surgeon&#39;s hand  14  between the thumb and fingers thereby to be readily manipulated in orientation. 
         [0036]    A standard N-type flanged microwave connector  16  is attached to a rear vertical wall of the housing  11  to releaseably connect the microwave applicator  10  to a flexible coaxial microwave cable  18 . This attachment allows the cable  18  to pass over the hand  14  and be supported and guided thereby. The outer dimensions of the housing  11  of the microwave applicator  10  and its orthogonal walls provide tactile feedback indicating the orientation of the housing  11 . 
         [0037]    A generally fan shaped microwave beam is emitted from the microwave applicator  10  providing a coagulation region  20  approximately 1.8 cm long in a direction aligned with the transverse axis  17  and approximately 1.1 cm in depth along the axis  12 , when the microwave applicator  10  is placed with its lower face  15  against tissue. 
         [0038]    Referring now to  FIG. 2   a,  the microwave applicator  10  may be placed with its lower face  15  against an upper surface of an organ  22  that will have a portion resected. The transverse axis  17  of the microwave applicator  10  is oriented to be parallel to the resection line  24  and the microwave applicator  10  is moved transversely, as indicated by arrow  26 , to successive locations along the resection line  24  while following the upper surface of the organ  22 . At each successive location, microwave power of up to 90 W may be applied for approximately 30 seconds to provide a set of rectangular coagulation zones together forming a continuous coagulation strip  25  along the resection line  24 . 
         [0039]    Referring to  FIG. 2   b,  the coagulation strip  25  may then be resected using a scalpel  28 , again moving transversely as indicated by arrow  26  cutting through the coagulation strip to prevent bleeding. Generally, this cut may not completely sever the organ  22 , such as the liver, whose thickness exceeds the limited depth of the coagulation region  20 . 
         [0040]    In this case, and referring to  FIG. 2   c,  the microwave applicator  10  may be inserted into the cut made by the scalpel  28  and the process of coagulation per  FIG. 2   a  above repeated with the lower face  15  of the microwave applicator  10  adjacent to tissue in the bottom of the cut. Successive coagulation and cutting permits organs of irregular dimensions to be readily resected by multiple passes of the microwave applicator  10 , possibly changing the transverse distance moved by the microwave applicator  10  at the various levels through the organ  22  as the transverse width of the organ  22  changes with depth. The present inventors recognize that this process may be repeated for an arbitrary number of times of successive cutting and coagulation per  FIGS. 2   b  and  2   c  to handle organs thicker than the depth of the coagulation region  20  available from the microwave applicator  10  provided the microwave applicator  10  were compact and manipulable. 
         [0041]    Referring now to  FIG. 3 , the outer surface of the housing  11  may present a layer of electrically and thermally insulating material  30 , for example, epoxy covering an inner conductive metallic shell  32 . The metallic shell forms walls of a rectangular box opened at the lower face  15  and thereby defines a generally rectangular inner volume contained between the walls of the conductive metallic shell  32 . A rear wall  35  of the metallic shell  32  and its insulating material  30  may be pierced by the microwave connector  16  at its upper end to permit the introduction of a quarter wavelength stub  36  conductor from the co-axial cable  18  into the inner volume. 
         [0042]    The stub  36  may be received within a solid dielectric material  38  filling the inner volume, the dielectric material preferably being Macor®, a machinable glass ceramic material available from Corning Inc. having offices worldwide. Macor® generally provides a dielectric constant greater than 5.5, a dielectric strength of approximately 9.4 kV/mm and a coefficient of expansion of approximately 93×10 −7 /° C. 
         [0043]    The metallic shell  32  connects electrically to the outer conductor of the waveguide formed by the microwave coaxial cable  18  and together with the quarter wavelength stub  36 , creates a tuned cavity controlling the coagulation region  20 . The solid dielectric material  38  permits the waveguide to operate at the lower frequency microwaves of 2.45 GHz while retaining a compact handheld form factor. 
         [0044]    Referring now to  FIGS. 4 and 5 , generally the coagulation region  20  has an aspect ratio (the ratio of a dimension along the transverse axis  17  to a dimension perpendicular to the transverse axis  17  in a cross-sectional plane perpendicular to the radiation axis  12 ) of greater than one to promote coagulation in a thin strip of tissue  39  of the greatest possible length for a given power. In the first embodiment, a coagulation region having cross-sectional dimensions of 3×2.5 cm and an aspect ratio of 1.2 is obtained. A high aspect ratio as provided by the present invention provides an improved, relatively uniform heating within the coagulation region  20 . In contrast a generally circular coagulation region  40  provided by standard microwave horns promote the development of an over-temperature region  42  resulting from the relatively thick layer of elevated temperature of the surrounding tissue preventing the escape of heat. It is believed that such over-temperature regions  43  create high temperature water vapor that migrates through the tissue changing the coagulation region unpredictably. 
         [0045]    Referring now to  FIG. 6 , an alternative embodiment of the applicator  10 ′ can provide even a higher aspect ratio in the cross-section of the coagulation region  20  by using metallic shell  32  having an internal volume with an inverted T-shaped elevational cross-section so that the lower face  15  of the housing  11  flares along the transverse axis  17 . Metallic inserts  50  placed within the dielectric material  38  at the lower face  15  provide field shaping to extend the coagulation region  20  into this higher aspect ratio. Simulations of this design have indicated that a cross-section of the coagulation region  20  having dimensions of approximately 5.5×1.75 cm and thus an aspect ratio of greater than 3 may be obtained to produce a coagulation region  20  with a tissue depth of 1.25 cm. 
         [0046]    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.