Patent Publication Number: US-2005137591-A1

Title: Apparatus and method for resecting and removing selected body tissue from a site inside a patient

Description:
RELATED APPLICATIONS  
      The present application is a continuation of U.S. application Ser. No. 10/146,444, filed May 14, 2002, the entire contents of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      The present invention is generally directed to an apparatus and method for resecting body tissue from a site within a patient and removing it from the patient. The present invention is more particularly directed to an electrosurgery apparatus for capturing, resecting, removing, and indexing selected body tissue, such as lymph nodes or other tissue.  
     BACKGROUND  
      One important step in the treatment of pulmonary cancer is resecting bronchial lymph nodes. Mediastinoscopy is one frequently employed procedure to resect lymph nodes in the thorax and bronchial system. An incision is made in the sternal or jugular notch at the base of the neck, and a tubular instrument called a mediastinoscope is inserted in the incision, placed between the trachea and the sternum, and directed into the chest cavity. The mediastinoscope permits direct visualization of the lymph nodes located in the paratracheal and parabronchial areas. The size of the instrument is large enough to allow the insertion of shearing and grabbing tools to resect regional lymph nodes. Once removed, the resected lymph nodes are sent to pathology for analysis.  
      The procedure for bronchial lymph node visualization and resection through the mediastinoscope has problems. For example, the procedure is limited to taking one lymph node at a time. Since a plurality of lymph nodes are generally required to be resected, the procedure can be relatively long. In addition, the instrumentation to grab and resect the lymph node may cause problems. For example, surgeons generally utilize long instruments such as ring forceps to suspend the target lymph node to be resected and removed. Such instruments can result in potential squeezing and damage to lymph nodes that may contain cancer cells, thus causing bleeding and releasing potentially hazardous byproducts such as tissue, blood, and cells into the chest cavity. Obviously, this is a condition to be avoided. Removing these hazardous byproducts from the patient is a problem. Another problem stems from the very narrow working channel and the close proximity of vulnerable structures such as arteries and the heart. Traditional biopsy and resection devices may impact the adjacent structures. The number of samples generally taken causes a storage problem and a cross-contamination problem. With present procedures and devices, each sample is individually removed and indexed according to its nodal station before being sent to pathology. This requires time and effort.  
      Hence, there is a need in the art for an improved apparatus and method for resecting a selected body tissue, such as bronchial and thoracic lymph nodes. More particularly, there is such a need for a device and method for quickly and efficiently capturing bronchial and thoracic lymph nodes, resecting them without causing bleeding or spilling byproducts into the patient, and indexing them. The present invention addresses that need.  
     SUMMARY OF THE INVENTION  
      The present invention provides a device for resecting selected body tissue from other body tissue at a site inside a patient and removing the selected body tissue. The device includes an electrosurgery device having an electrode that cuts through tissue to resect the selected body tissue from the other body tissue, a tubular member having a vacuum lumen that draws the selected body tissue into proximity with the electrode to permit the electrode to resect the selected body tissue from the other body tissue, and a collector that receives the resected body tissue. The device may further include a compliant port through which the selected body tissue is drawn, and that maintains a seal with the selected body tissue. The compliant port may include a flexible material carried on the tubular member and having an opening maintaining a vacuum against the other body tissue and resection byproducts when the selected body tissue has been resected. The collector may also receive resection byproducts. The collector may be in vacuum communication with the lumen. The vacuum lumen may draw the resected body tissue into the collector. The electrode may be an active electrode of an electrosurgery system. The electrode may form an RF blade, and may form an RF loop. The electrode may be a bipolar device. The electrode may be a monopolar device. The collector may include at least one collection chamber. The collector may comprise a plurality of collection chambers, each of which is selectively communicable with the vacuum lumen. The collection chamber may have reference markings. The collector may be disengaged from the device after receiving the resected body tissue. The collector may be disengaged from the device after receiving the resected body tissue and another collector may be engaged with the device to receive another resected body tissue. The tubular member may be curved at its distal tip. The distal tip may be shaped to accommodate a particular anatomy.  
      The present invention further provides a device for resecting selected body tissue from other body tissue at a site inside a patient and removing the selected body tissue. The device resecting selected body tissue includes an electrosurgery device having an electrode that cuts through tissue to resect the selected body tissue from the other body tissue, a tubular member having a vacuum lumen that draws the selected body tissue into proximity with the electrode to permit the electrode to resect the selected body tissue from the other body tissue, a compliant port carried on the tubular member through which the selected body tissue is drawn, and that maintains a seal with the selected body tissue, and a collector having a plurality of collection chambers, each of which is selectively communicable with the vacuum lumen to receive the resected body tissue.  
      The present invention still further provides a method of resecting selected body tissue from other body tissue at a site inside a patient and removing the selected body tissue. The method includes the steps of disposing a tubular member having a lumen adjacent to the selected body tissue, creating a vacuum inside the lumen to draw the selected body tissue inside the lumen, drawing the selected body tissue inside the lumen with a vacuum, cutting through tissue to resect the selected body tissue from the other body tissue with an electrosurgery device, and collecting the resected body tissue in a collector. The method may include the further step of aspirating the selected body tissue from the patient out of another end of the lumen. The method may include the further step of drawing the selected body tissue with the vacuum through a compliant port that maintains a seal with the selected body tissue. The compliant port may include a flexible material carried on the tubular member and having an opening maintaining a vacuum against the other body tissue. The step of collecting the resected body tissue may include the further step of collecting resection byproducts.  
      The present invention further includes a device for resecting selected body tissue from other body tissue at a site inside a patient and removing the selected body tissue. The device includes electrosurgery means for resecting the selected body tissue from the other body tissue, vacuum directing means for drawing the selected body tissue into proximity with the electrosurgery means to permit the electrosurgery means to resect the selected body tissue from the other body tissue, and collection means for receiving the resected body tissue.  
      These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like referenced numerals identify identical elements, and wherein:  
       FIG. 1  illustrates a person&#39;s neck, collarbones, and respiratory system;  
       FIG. 2  illustrates several of the lymph nodes of the respiratory system;  
       FIG. 3  illustrates a monopolar electrosurgery system suitable for use with an electrosurgery device to resect body tissue at a site within a patient, in accordance with the present invention;  
       FIG. 4  is a perspective view of an electrosurgery device, in accordance with the present invention;  
       FIG. 5  is a partial longitudinal section view of the distal tip of the tubular member of the electrosurgery device of  FIG. 4 , according to an embodiment of the invention;  
       FIG. 6  illustrates an electrosurgery device inserted through an incision in the sternal notch and into a thorax, in accordance with the invention;  
       FIG. 7  illustrates a step in resecting a lymph node where the distal tip of the tubular member is adjacent to a lymph node for resection, in accordance with the present invention;  
       FIG. 8  illustrates an intermediate step where a lymph node has been drawn in proximity to the compliant port by the vacuum draw;  
       FIG. 9  illustrates an intermediate step where the lymph node has been drawn substantially through the compliant port and into the resection lumen;  
       FIG. 10  illustrates an intermediate step where the electrode has been partially deployed and moved centrally;  
       FIG. 11  illustrates an intermediate step where the electrode has cut through tissue to resect the lymph node from other body tissue;  
       FIG. 12  illustrates a step where the vacuum draws the resected lymph node into a lumen to aspirate it from the patient;  
       FIG. 13  is a side, partial longitudinal section view illustrating a distal tip of an electrical surgery device having a lasso-shaped active electrode, in accordance with an embodiment of the present invention;  
       FIG. 14  illustrates the lasso-shaped active electrode loop of  FIG. 13  cinched around a perimeter of the lymph node;  
       FIG. 15  is a side, partial longitudinal section view illustrating a distal tip of an electrical surgery device having a loop-shaped bipolar electrode, in accordance with an embodiment of the present invention;  
       FIG. 16  illustrates an oval distal tip according to an embodiment of the invention;  
       FIG. 17  illustrates a distal tip geometry designed to accommodate the trachea according to an embodiment of the invention;  
       FIG. 18  illustrates an asymmetrical distal tip geometry according to an embodiment of the invention; and  
       FIG. 19  illustrates a conical distal tip geometry according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings that form a part hereof. The detailed description and the drawings illustrate specific exemplary embodiments by which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is understood that other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the present invention. The following detailed description is therefore not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.  
      Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of “a”, “an”, and “the” include plural references. The meaning of “in” includes “in” and “on.” Referring to the drawings, like numbers indicated like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.  
      Briefly stated, an electrosurgery device according to an embodiment of the invention described captures a lymph nodule and resects it. The lymph node is captured with a vacuum and resected with an electrode, which minimizes bleeding and limits the potentially malignant node from coming into contact with surrounding tissue as it is resected and removed. This limits the potential for inadvertent cancer spread. An electrosurgery device according to an embodiment of the invention also allows several lymph nodes to be resected in a single procedure, each lymph node being easily indexed according to its nodal station and stored in a manner that limits the potential for cross-contamination. An electrosurgery device according to an embodiment of the invention further provides a collector for individually receiving resected lymph nodes. The collector may be easily detached and sent to pathology without interrupting resection of other lymph nodes.  
       FIG. 1  illustrates a person&#39;s neck, collarbones, and respiratory system. The respiratory system  20  resides within the thorax  22  that occupies a space defined by the chest wall  24  and the diaphragm  26 .  
      The respiratory system  20  includes the trachea  28 , the left mainstem bronchus  30 , the right mainstem bronchus  32 , which then further divides into bronchial branches and sub-branches. The respiratory system  20  further includes left lung lobes  52  and  54  and right lung lobes  56 ,  58 , and  60 . Each bronchial branch and sub-branch communicates with a respective different portion of a lung lobe, either the entire lung lobe or a portion thereof.  
      The right clavicle  21  and the left clavicle  23  join at the top of the sternum  25  (only the top portion of sternum  25  is shown for clarity) and lie anteriorly to the trachea  28 . A sternal or jugular notch  27  is formed in the sternum  25  and at the base of the neck.  
       FIG. 2  illustrates several of the lymph nodes of the respiratory system. The nodes have been classified into regional nodal stations for lung cancer staging, and the stations are used to index biopsied or resected lymph nodes.  FIG. 2  illustrates right and left upper paratracheal nodes  71   r  and  711  (indexed as stations  2 R and  2 L); right and left lower paratracheal nodes  73   r  and  731  (indexed as stations  4 R and  4 L); right and left tracheobronchial angle nodes  75   r  and  751  (indexed as stations  10 R and  10 L); right and left interlobar nodes  77   r  and  771  (indexed as stations  11 R and  11 L); and subcarinal node  79  (indexed as station  7 ). Typically, several lymph nodes may be located at one station. Additional nodes and stations were omitted from  FIG. 2  for clarity.  
       FIG. 3  illustrates a monopolar electrosurgery system suitable for use with an electrosurgery device to resect body tissue at a site within a patient, in accordance with the present invention. Electrosurgery system  80  includes an aspiration pump  81 , an electrosurgery RF generator  82 , a ground electrode  84 , a ground electrode coupler  85 , a foot control unit  86 , a foot control unit coupler  87 , an aspiration coupler  88 , an electrosurgery coupler  89 , and an electrosurgery device  90  including a tubular member  92 . The depiction of a monopolar electrosurgery system in  FIG. 3  is not intended to limit the practice of the present invention to only monopolar devices. In an alternative embodiment of the invention, a bipolar electrosurgery system may be used, eliminating the need for ground electrode  84  and ground electrode coupler  85 .  
      Electrosurgery RF generator  82  is coupled to ground electrode  84  by ground electrode coupler  85 , to foot control unit  86  by foot control unit coupler  87 , and to electrosurgery device  90  by electrosurgery coupler  89 . Electrosurgery device  90  includes an active electrode (not shown) and tubular member  92 . Aspiration pump  81  is coupled to electrosurgery device  90  and to tubular member  92  by aspiration coupler  88 .  
      In operation, electrosurgery RF generator  82  supplies a source of electrical current, typically in the radio frequency range, to the active electrode of electrosurgery device  90  and ground electrode  84  (which is sometimes known as a dispersive electrode). The current forms an electrical arc ahead of the active electrode and volatizes the tissues, separating them as if they were cut. Ground electrode  84  provides a large surface for patient electrical contact, and is placed in electrical contact with the patient. The active electrode directs the current flow into the tissue of the patient, and ground electrode  84  directs current flow from the patient to electrosurgery generator. The current waveform supplied by electrosurgery RF generator  82  may vary in strength and frequency, and it may be pulsed. The RF energy may be modulated in a sinusoidal or square waveform. It may also be mixed mode or combination thereof. The particular electrosurgery current waveform is selected to accomplish the objectives of the procedure being performed. The surgeon may use foot control unit  86  to control electrosurgery RF generator  82  or a hand controlled switch on electrosurgery device  90 .  
      Tubular member  92  has a vacuum lumen for drawing selected body tissue into proximity to the active electrode, and for aspirating the resected tissue from the patient. Tubular member  92  carries the active electrode. Electrosurgery device  90  is configured to be carried in a conduit or catheter of an endoscope, such as a mediastinoscope, or alternatively may be used separately in conjunction with an endoscope or other viewing device.  
       FIG. 4  is a perspective view of electrosurgery device  90 , in accordance with the present invention.  FIG. 5  is a partial longitudinal section view of the distal tip  99  of the tubular member  92  of the electrosurgery device  90  of  FIG. 4 , in accordance with the present invention. Electrosurgery device  90  includes a body  98 , a tubular member  92 , a handle  93 , an activation device  94 , and a collector  96  having chambers  97   a - 97   f . Tubular member  92  includes a distal tip  99 , an electrode  100 , an electrode lumen  102 , a primary vacuum lumen  104 , a secondary vacuum lumen  106 , a compliant port  110 , and a resection lumen  115 .  
      Body  98  carries tubular member  92 , handle  93 , activation device  94 , and collector  96 . Aspiration coupler  88  and electrosurgery coupler  89  are coupled to electrosurgery device  90  at handle  93  in a preferred embodiment. Aspiration coupler  88  is in vacuum communication with collector  96 , and lumens  102  and  104 . Collector  96  is in vacuum communication with lumens  102  and  104 . Collector  96  can be disengaged from body  98 , and another collector  96  can be engaged with body  98  without electrosurgery device  90  being removed from a patient. Collector  96  may have a plurality of selectable chambers, each chamber being selectively communicable with vacuum lumens  102  and  104 . In a preferred embodiment, collector  96  has six chambers,  97   a - 97   f , and is arranged much like a cylinder on a six-shot revolver, with collector  96  being rotatable to select a chamber, and removable and replaceable. Chambers  97   a - 97   f  are indexed or reference marked corresponding to the lymph node stations likely to be encountered. For example, collector  96  is reference marked for the upper and lower paratracheal nodes, stations  2 R,  2 L,  4 R and  4 L, in the embodiment illustrated.  
      Electrode  100  is carried in lumen  102  of tubular member  92 , is exposed to resection lumen  115 , and is coupled to the electrosurgery generator by electrode coupler  89 . In a preferred embodiment, electrode  100  is an active electrode in the shape of a blade, and consisting of radio frequency surgical materials suitable for cutting through tissue, including resecting selected tissue such as lymph nodes from other body tissue. In an alternative embodiment, electrode  100  may be sharpened. The electrode may be maneuverable. Electrode  100  may be carried substantially within lumen  102  until needed, and then extended for use.  
      Activation device  94  is a trigger-like device used to extend electrode  100  for cutting and to retract it.  FIGS. 10 and 11  illustrate extension of electrode  100  for cutting.  
      Tubular member  92  may be made of any material suitable for surgical use. When the electrosurgery device  90  is a bi-polar device, the tubular member  90  may be any material suitable for surgical use and for forming a return electrode. One end is coupled to body  98 , and the other end including distal tip  99  is arranged for insertion into an incision in the sternal notch and down into the thorax. While illustrated as round in  FIG. 5 , distal tip  99  can be any cross-sectional shape suitable for the purpose of electrosurgery device  90 .  FIGS. 16-19  illustrate alternative shaped embodiments of the distal tip  99 . In an alternative embodiment, the distal tip  99  may be arranged for insertion through an incision in another part of the body. For example, lymph nodes at stations  7 ,  8 , and  9  of  FIG. 2  can only be reached by entry between certain ribs. While distal tip  99  is illustrated as lying in a single plane perpendicular to the longitudinal axis of tubular member  92 , it may be formed at another angle and/or in multiple planes to facilitate access to lymph nodes. In an alternative embodiment, distal tip  99  may have a bend or curve to facilitate access to lymph nodes. Tubular member  92  includes lumen  102  for delivery of electrode  100 . Tubular member  92  also includes primary vacuum lumen  104  and secondary vacuum lumen  106  providing vacuum draws  105  and  107  in the resection lumen  115 . In an alternative embodiment, a plurality of secondary vacuum lumens  106  may be provided. In another alternative embodiment, electrosurgery system  80  includes a system providing a saline solution to electrosurgery device  90 , and tubular member  99  includes a lumen that provides the saline solution to assist in RF cautery and for cooling.  
      The compliant port  110  includes the opening  112  and is carried on the distal tip  99 . The complaint port  110  may be made from a flexible material such as silicone. The opening  112  is flexible to allow a wide range of lymph node sizes to be drawn through, while maintaining a vacuum seal against the lymph node. The vacuum draws  105  and  107  provide the vacuum draw  113  through the opening  112 .  
       FIG. 6  illustrates an electrosurgery device inserted through an incision in the sternal notch and into a thorax, in accordance with the present invention.  FIG. 6  depicts the tubular member  92  of electrosurgery device  90  inserted through an incision at the sternal notch  27  just above the sternum  25 . The procedure for placing the tubular member  92  involves anesthetizing the patent, making an incision at the sternal notch  27  just above the sternum  25 , and inserting tubular member  92  through the incision and between the trachea  28  and the top of the sternum  25 . Tubular member  92  is advanced to where its distal tip  99  is adjacent to lymph node  711  or any other selected lymph node. Electrosurgery device  90  may be incorporated into another device that provides viewing of the selected lymph nodes, or may be accompanied in use by a viewing device.  
       FIGS. 7-12  illustrate several steps in resecting a lymph node, in accordance with the present invention.  FIG. 7  illustrates the distal tip  99  of tubular member  92  adjacent to lymph node  711  for resection. Vacuum draw  113  creates a vacuum drawing lymph node  711  toward opening  112  in compliant port  110 . Vacuum draw  113  may be used to gently tease the lymph node  711  out from adjacent tissues, and move it toward opening  112 .  
       FIG. 8  illustrates lymph node  711  having been drawn in proximity to compliant port  110  by vacuum draw  113 .  FIG. 9  illustrates lymph node  711  having been drawn substantially through compliant port  110  by vacuum draw  113  (not shown) and by vacuum draws  105  and  107 , into resection lumen  115 . In this position, lymph node  711  is in proximity with electrode  100 , permitting it to be resected. Compliant port  110  and opening  112  maintain a seal with lymph node  711 , causing a vacuum in resection lumen  115 .  FIG. 10  illustrates electrode  100  having been partially deployed toward lymph node  711  and moved centrally. Electrode  100  may be deployed by squeezing the activation device  94  of  FIG. 4 .  FIG. 11  illustrates electrode  100  having been fully deployed and activated to cut through tissue at a location near opening  112  to resect lymph node  711  from other body tissue  118 . Activation of the electrode may be by further squeezing the activation device or by using foot control unit  86  of  FIG. 2 . Vacuum draws  105  and  107  continue to maintain a vacuum in resection lumen  115  against the seal with lymph node  711  provided by compliant port  110  and opening  112 . Using an electrosurgery device such as electrode  100  minimizes bleeding and release potentially hazardous tissue, blood, and cells (hereafter referred to as “resection byproducts  117 ”). If any resection byproducts  117  are created, the vacuum maintained in resection lumen  115  retains the byproducts  117 , and limits any escape into the chest cavity. Until vacuum draws  105  and  107  are reduced to release other body tissue  118  from opening  112 , the vacuum is maintained in resection lumen  115 .  
       FIG. 12  illustrates vacuum draw  105  pulling resected lymph node  711  into lumen  104  to aspirate resected lymph node  711  from the patient. After lymph node  711  is resected as illustrated in  FIG. 11 , vacuum draw  105  pulls resected lymph node  711  into lumen  104  and aspirates it from the patient and into the collector. Vacuum draw  107  maintains the vacuum in resection lumen  115  providing a backup vacuum draw in case resected lymph node  711  obstructs or reduces vacuum draw  105  while it is being aspirated. Vacuum draw  107  may also aspirate any resection byproducts  117  from the resection lumen  115 . Lumens  104  and  102  are both in vacuum communication with the collector  96  of  FIG. 4 . The resected lymph node  711  is drawn through lumen  104  by vacuum draw  105  into the collector. Any resection byproducts  117  are also drawn into the collector through lumens  104  and  106 . In an embodiment where collector  96  includes a plurality of collection chambers, resected lymph node  711  and any resection byproducts  117  are drawn into a selected collection chamber. In the embodiment where the collection chambers are indexed, the chamber with reference mark  2 L would be selected. The collector may be quickly indexed by rotation to another chamber. As described in conjunction with  FIG. 4 , the collector may be disengaged from the electrosurgery device after receiving the resected lymph node  711 , and another collector may be engaged to receive other resected body tissue without removing the electrosurgery device from the patient. The electrosurgery device according to an embodiment of the invention allows a lymph node to be captured and to be resected with a single device.  
       FIGS. 13 and 14  are side, partial longitudinal section views illustrating a distal tip  99  of an electrical surgery device having a lasso-shaped active electrode  120 , in accordance with an embodiment of the present invention. A lasso-shaped active electrode  120  may be used in an alternative embodiment of the present invention to resect a lymph node.  FIG. 13  illustrates lasso-shaped active electrode  120  arranged to form a loop  122  that may be cinched around a perimeter of lymph node  711 . Lymph node  711  is drawn into resection lumen  115  in the manner described in  FIGS. 7-9 .  FIG. 14  illustrates loop  122  having been cinched around a perimeter of lymph node  711  by retraction of electrode  120 . The retraction may be controlled by squeezing the activation device  94  of  FIG. 4 . Electrode  120  is activated to cut through tissue to resect lymph node  711  from other body tissue. Activation of the electrode may be by further squeezing the activation device, or by foot control unit  86  of  FIG. 2 . Lymph node  711  is aspirated from the patent in the manner described in  FIG. 12 . In an alternative embodiment, active electrode  120  may be a rigid loop that is retracted against a portion perimeter of lymph node  711  and activated to cut through the lymph node  711 .  
       FIG. 15  is a side, partial longitudinal section view illustrating a distal tip  99  of an electrical surgery device having a loop-shaped bipolar electrode  130 , in accordance with an embodiment of the present invention. Bipolar electrode  130  includes insulation  132  and electrode  134 . Insulation  132  electrically isolates bipolar electrode  130  from the electrosurgery device. Electrode  134  is formed into a loop  122  to snare lymph node  711  and perform the cutting. The bipolar electrode  130  is operated by an electrosurgery system substantially similar to the system illustrated in  FIG. 3 , except no ground electrode is required. The operation of loop-shaped bipolar electrode  130  is similar to the operation of electrode  120  described in conjunction with  FIGS. 13 and 14 .  
       FIGS. 16-19  are perspective views illustrating alternative shapes of the distal tip  99  according to an embodiment of the invention. The distal tip  99  can be any shape, can lie in a single or in multiple planes, and can be at any angle to the longitudinal member  92  suitable for the intended use of the electrosurgery device.  FIG. 16  illustrates an oval distal tip  99   a .  FIG. 17  illustrates a distal tip  99   b  having a geometry designed to accommodate the trachea.  FIG. 18  illustrates a distal tip  99   c  having an asymmetrical geometry.  FIG. 19  illustrates distal tip  99   d  having a conical geometry.  
      Although the present invention has been described in considerable detail with reference to certain preferred embodiments, other embodiments are possible. Therefore, the spirit or scope of the appended claims should not be limited to the description of the embodiments contained herein. It is intended that the invention resides in the claims hereinafter appended.