Abstract:
A catheter for sensing electrical events about a selected annulus region of the heart and for treating tissue in the selected annulus region has a handle assembly, and a shaft having a proximal end coupled to the handle assembly. The catheter also has a mapping element provided adjacent its distal end, and an ablation element positioned spaced apart along the shaft from the mapping element. The mapping element is first positioned distally to the desired treatment location in the selected annulus region and the distal location is mapped. The expandable member enclosing the ablation element is inflated and contrast medium injected to determine the orientation of the ablation element with respect to the annulus region. After the target ablation site is determined and the PV potentials verified, the ablation element is activated for therapeutic energy delivery.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Related Cases  
         [0002]     This is a continuation-in-part of co-pending Ser. No. 10/744,354, entitled “System and Method for Mapping and Ablating Body Tissue of the Interior Region of the Heart”, filed Dec. 22, 2003, which is in turn a continuation of Ser. No. 09/975,269, filed Oct. 11, 2001, now U.S. Pat. No. 6,671,533, whose disclosures are incorporated by this reference as though fully set forth herein.  
         [0003]     2. Field of the Invention  
         [0004]     The present invention is directed to systems and methods for mapping and ablating body tissue of the interior regions of the heart for treating cardiac arrrhythmias.  
         [0005]     3. Description of the Prior Art  
         [0006]     Atrial fibrillation (AF) is a common cardiac arrhythmia associated with significant morbidity and mortality. A number of clinical conditions may arise from irregular cardiac functions and the resulting hemodynamic abnormalities associated with AF, including stroke, heart failure and other thromboembolic events. AF is a significant cause of cerebral stroke, wherein the fibrillating motion in the left atrium induces the formation of thrombus. A thromboembolism is subsequently dislodged into the left ventricle and enters the cerebral circulation where stroke may result.  
         [0007]     For many years, the only curative treatment for AF has been surgical, with extensive atrial incisions used to compartmentalize the atrial mass below that critical for perpetuating AF. Recently, transcatheter linear radiofrequency ablation in the right or left atrium has been used to replicate surgical procedures in patients with paroxysmal or chronic AF. Such ablation is carried out by a catheter system that performs both mapping and ablation. With current techniques, there is still uncertainty regarding the number of lesions, the optimum ablation site, and the need for continuous lines. As a result, focal ablation has been proposed as an alternative approach, due to the belief that ectopic beats originating within or at the ostium of the pulmonary veins (PV) may be the source of paroxysmal and even persistent AF. Although successful, the technical feasibility of this technique is restricted by the difficulty in mapping the focus if the patient is in AF or has no consistent firing, the frequent existence of multiple foci causing high recurrence rates, and a high incidence of PV stenosis.  
         [0008]     However, there are a number of drawbacks associated with the catheter-based mapping and ablation systems that are currently known in the art. One serious drawback lies in the unstable positioning of the catheter inside the atrium of the heart. When a catheter is not properly stabilized, the mapping becomes difficult and inaccurate.  
         [0009]     Another drawback is associated with certain catheter-based systems that utilize an expandable balloon that is inflated to conform to the pulmonary vein ostium. After the balloon is inflated and the catheter positioned, it becomes difficult to map or record the distal PV potentials without removing this catheter and placing another mapping catheter inside the PV. Moreover, inflation of the balloon to conform to the pulmonary vein ostium blocks blood flow to the left atrium, and such prolonged blockage can have adverse effects to the patient. Blockage of blood flow from the PV deprives the patient from receiving oxygenated blood. In addition, the blockage may be a potential source for stenosis.  
         [0010]     Thus, there still remains a need for a catheter-based system and method that can effectively map and ablate potentials (also known as spikes) inside PVs which can induce paroxysmal AF, while avoiding the drawbacks set forth above.  
       SUMMARY OF THE DISCLOSURE  
       [0011]     It is an objective of the present invention to provide a system and method that effectively maps or records distal PV potentials and ablates the PV ostium.  
         [0012]     It is another objective of the present invention to provide a system and method that effectively maps and ablates potentials without blocking blood flow.  
         [0013]     In order to accomplish the objects of the present invention, there is provided a catheter for sensing electrical events about a selected annulus region of the heart and for treating tissue in the selected annulus region. The catheter has a handle assembly, and a shaft having a proximal end coupled to the handle assembly, a mapping element provided adjacent its distal end, and an ablation element positioned spaced apart along the shaft from the mapping element. The mapping element is first positioned distally to the desired treatment location in the selected annulus region and the distal location is mapped. The expandable balloon enclosing the ablation element is inflated and contrast medium injected to determine the orientation of the ablation element with respect to the annulus region. After the target ablation site is determined and the PV potentials verified, the ablation element is activated for therapeutic energy delivery. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  illustrates a mapping and ablation system according to one embodiment of the present invention.  
         [0015]      FIG. 2  is a perspective view of the catheter of the system of  FIG. 1 .  
         [0016]      FIG. 3  is an enlarged view of the distal tip section of the catheter of  FIGS. 1 and 2 .  
         [0017]      FIG. 4  is a cross-sectional view of the distal tip section of  FIG. 3  taken along lines A-A thereof.  
         [0018]      FIG. 5  is a cross-sectional view of the distal tip section of  FIG. 3  taken along lines B-B thereof.  
         [0019]      FIG. 6  illustrates how the catheter of  FIGS. 1 and 2  is deployed for use inside the heart of a patient.  
         [0020]      FIG. 7  is a cross-sectional view illustrating the catheter of  FIGS. 1 and 2  in use in a pulmonary vein during the mapping and ablation steps.  
         [0021]      FIG. 8  illustrates the steering mechanism of the catheter of  FIGS. 1 and 2 .  
         [0022]      FIG. 9  illustrates a mapping and ablation system according to another embodiment of the present invention.  
         [0023]      FIG. 10  is a perspective view of the catheter of the system of  FIG. 9 .  
         [0024]      FIG. 11  is an enlarged view of the distal tip section of the catheter of  FIGS. 9 and 10 .  
         [0025]      FIG. 12  is a cross-sectional view of the distal tip section of  FIG. 11  taken along lines A-A thereof.  
         [0026]      FIG. 13  is a cross-sectional view of the distal tip section of  FIG. 11  taken along lines B-B thereof.  
         [0027]      FIG. 14  is an enlarged persepective view of the distal tip section of the catheter of  FIGS. 9 and 10 .  
         [0028]      FIG. 15  illustrates a mapping and ablation system according to another embodiment of the present invention.  
         [0029]      FIG. 16  is an enlarged persepective view of the distal tip section of the catheter of  FIG. 15 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]     The following detailed description is of the best presently contemplated modes of carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating general principles of embodiments of the invention. The scope of the invention is best defined by the appended claims. In certain instances, detailed descriptions of well-known devices, compositions, components, mechanisms and methods are omitted so as to not obscure the description of the present invention with unnecessary detail.  
         [0031]     The present invention provides a catheter system that has two separate elements for performing the mapping and ablation operations. A first element that includes ring electrodes is provided along a distal ring and functions to map the region of the heart that is to be treated. After the mapping has been completed, a second element that includes a transducer mounted inside a balloon is positioned at the location where ablation is to be performed, and is used to ablate the selected tissue. During the ablation, the distal ring functions to anchor the position of the balloon, while the balloon is inflated to a diameter that is less than the diameter of the distal ring and the annulus where the treatment is taking place. As a result, blood can still flow unimpeded through the annulus.  
         [0032]     Even though the present invention will be described hereinafter in connection with treating AF, it is understood that the principles of the present invention are not so limited, but can be used in other applications (e.g., treatment of accessory pathways, atrial flutter, ventricular tachycardia), and in other body pathways (e.g., right atrium, superior vena cava, right ventricle, left ventricle).  
         [0033]      FIGS. 1-8  illustrate a catheter system  20  according to one embodiment of the present invention. The catheter system  20  has a tubular shaft  22  having a distal tip section  24 , a distal end  26 , a proximal end  28 , and at least one lumen  30  extending through the shaft  22 . A handle assembly  32  is attached to the proximal end  28  of the shaft  22  using techniques that are well-known in the catheter art.  
         [0034]     The distal tip section  24  includes an expandable balloon  38  and a distal ring  80  that makes up the distal-most end of the shaft  22 . A transducer  60  (e.g., piezoelectric or ultrasound) is housed inside the balloon  38 . The balloon  38  can be made from any conventional material (such as but not limited to silicone, polyurethane, latex, polyamide and polyethylene), and heat bonded or otherwise attached to the shaft  22  using techniques that are well-known in the catheter art.  
         [0035]     The distal ring  80  can be preformed into a generally curved or circular shape, resembling an open loop. The shape of the distal ring  80  corresponds to the circumferential geometry of a selected annulus (e.g., the PV) in the heart. In fact, the preformed shape of the distal ring  80  can be provided in a variety of curved geometries to overlie the anatomical geometry of the selected annulus. The distal ring  80  includes a transition section  82  that extends distally at an angle from the longitudinal axis of the shaft  22 , and has a generally open-looped circular section  84  that extends from the transition section  82 . As best seen from  FIG. 3 , the circular section  84  is oriented at an approximately perpendicular orientation from the longitudinal orientation of the shaft  22 . The distal ring  80  can be made from the same material as the shaft  22 . Such a material can be an electrically nonconductive, biocompatible, resilient plastic material which retains its shape and which does not soften significantly at human body temperature (e.g., Pebax™, polyethylene or polyester). As a non-limiting example, the geometry of the distal ring  80  can be created by thermoforming it into the desired shape.  
         [0036]     A plurality of thermocouple wires  54  can have their distal tips secured to the interior surface of the balloon  38  (see  FIG. 3 ), and are used to detect the temperature at the treatment site.  
         [0037]     A plurality of ring electrodes  58  are provided in spaced-apart manner about the circular section  84  of the distal ring  80 . The ring electrodes  58  can be made of a solid, electrically conducting material, like platinum or gold, that is attached about the circular section  84 . Alternatively, the ring electrodes  58  can be formed by coating the exterior surface of the circular section  84  with an electrically conducting material, such as platinum or gold. The coating can be applied by sputtering, ion beam deposition or similar known techniques. The number of ring electrodes  58  can vary depending on the particular geometry of the region of use and the functionality desired.  
         [0038]     As will be explained in greater detail below, the ring electrodes  58  function to map the region of the heart that is to be treated. After the mapping has been completed, the balloon  38  is positioned at the location where ablation is to be performed, and the distal ring  80  functions to anchor the position of the balloon  38 . The balloon  38  is expanded, but even the greatest expanded diameter of the balloon  38  will be provided to be less than the diameter of the distal ring  80  when the distal ring  80  is fully deployed (see  FIGS. 2, 3  and  7 ). The ablation is then carried out by energy that is emitted from the ultrasound transducer  60  through the inflation media (e.g., fluid, saline, contrast media or mixture) inside the balloon  38 , and the balloon  38  itself.  
         [0039]     A standard Luer fitting  34  is connected to the proximal end  36  of the handle assembly  32  using techniques that are well-known in the catheter art. The Luer fitting  34  provides a fluid line for inflation media to be introduced to inflate the balloon  38  at the distal tip section  24  of the shaft  22 . The inflation media is delivered via an inflation lumen  76  that extends from the handle assembly  32  (and coupled to the line  78  of the Luer fitting  34 ), and terminates at the balloon  38 .  
         [0040]     A connector assembly  40  is also connected to the proximal end  36  of the handle assembly  32  using techniques that are well-known in the catheter art. The connector assembly  40  has a proximal connector  42  that couples the handle assembly  32  to the connector  44  of a control line  46  that leads to an ultrasound generator  52 . An EKG monitor  50  is coupled to the ultrasound generator  52  via another line  48 . The EKG monitor  50  can be a conventional EKG monitor which receives (via the ultrasound generator  52 ) electrical signals detected by the ring electrodes  58  at the distal tip section  24 , and processes and displays these electrical signals to assist the physician in locating the site of potentials in a PV. The ultrasound generator  52  can be a conventional ultrasound generator that creates and transmits ablating energy to the ultrasound transducer  60  that is positioned inside the balloon  38 . The ultrasound transducer  60  will emit the energy to ablate the tissue that extends radially from the position of the balloon  38 .  
         [0041]     Electrical wires (not shown) extend from the ultrasound generator  52  along the lines  46  and  48 , and conductor wires  62  and ultrasound wires  63  extend through the connector assembly  40 , the handle assembly  32  and the lumen  30  of the shaft  22  to the distal tip section  24  of the shaft  22  to couple the ring electrodes  58  and the transducer  60 , respectively. In addition, the thermocouple wires  54  can extend from the balloon  38  through the lumen  30  of the shaft  22  and the handle assembly  32  to the proximal connector  42 , where they can be electrically coupled by the wires in the line  46  to the ultrasound generator  52  where the temperature can be displayed.  
         [0042]     The handle assembly  32  also includes a steering mechanism  70  that functions to deflect the distal tip section  24  of the shaft  22  for maneuvering and positioning the distal tip section  24  at the desired location in the heart. Referring to  FIGS. 1, 5  and  8 , the steering mechanism  70  includes a steering wire  72  that extends in the main lumen  30  of the shaft  22  from its proximal end at the handle assembly  32  to its distal end which terminates in the distal tip section  24  before the location of the balloon  38 . The proximal end of the steering wire  72  is wound around or secured to an anchor  77  that is fixedly positioned inside the handle assembly  32 . The steering mechanism  70  also includes a flat wire  75  that extends in the lumen  30  from the anchor  77  to its distal end at a location slightly proximal to the balloon  38  (as shown in  FIG. 5 ). The flat wire  75  is attached to the steering wire  72  at the distal ends of the flat wire  75  and the steering wire  72  so as to be controlled by the steering wire  72 . Specifically, by pushing the steering mechanism  70  forward in a distal direction, the steering mechanism  70  will pull the steering wire  72  in a proximal direction, causing the distal tip section  24  to deflect to one direction (see in phantom in  FIG. 8 ). By pulling back the steering mechanism  70  in a proximal direction, the steering wire  72  is deactivated and the distal tip section  24  returns to its neutral position or deflects to the opposite direction.  
         [0043]     The distal ring  80  can be preformed to a fixed size (i.e., diameter) and shape that cannot be changed. Alternatively, the diameter of the distal ring  80  can be adjusted using techniques and incorporating mechanisms that are well-known in the catheter art.  
         [0044]      FIGS. 6 and 7  illustrate how the catheter system  20  is used. First, a guide sheath  88  is provided to deliver the shaft  22  and distal ring  80  to the desired location (e.g., the left atrium) in the heart. The shaft  22  is slid into the hollow lumen of the guide sheath  88 , and the guide sheath  88  can slide forward and backward along the longitudinal axis of the shaft  22 . When the guide sheath  88  is slid forwardly towards the distal ring  80 , the distal ring  40  is progressively straightened out and drawn into the lumen of the guide sheath  88 . Thus, when confined with the guide sheath  88 , the distal ring  80  assumes the generally linear low profile shape of the guide sheath  88 , which allows a physician to employ conventional percutaneous access techniques to introduce the catheter  20  into a selected region of the heart through a vein or artery. When the guide sheath  88  is slid rearwardly away from the distal ring  80 , the distal ring  80  is uncovered and its resilient memory will cause the distal ring  80  to re-assume its preformed generally circular shape.  
         [0045]     To introduce and deploy the distal tip section  24  within the heart, the physician uses a conventional introducer to establish access to a selected artery or vein. With the guide sheath  88  confining the distal ring  80 , and with the balloon  38  deflated, the physician introduces the shaft  22  and the guide sheath  88  through a conventional hemostatic valve on the introducer and progressively advances the guide sheath  88  through the access vein or artery into the desired atrium, such as the left atrium as shown in  FIG. 6 . The physician observes the progress of the guide sheath  88  using fluoroscopic or ultrasound imaging. The guide sheath  88  can include a radio-opaque compound, such as barium, for this purpose. Alternatively, radio-opaque markers can be placed at the distal end of the guide sheath  88 .  
         [0046]     The shaft  22  and the guide sheath  88  can be maneuvered to the left atrium by the steering mechanism  70 . Once located in the left atrium, the physician slides the guide sheath  88  back to free the distal ring  80  which resiliently returns to its preformed shape. The distal ring  80  is then maneuvered into contact with the selected annulus (e.g., the ostium) with the aid of fluoroscopy. Good contact is established when the ring electrodes  58  contact the selected annulus, and at this time, the physician operates a control located on the ultrasound generator  52  to effectuate the mapping of the selected annulus by the ring electrodes  58 . The results of the mapping operation are processed and displayed at the EKG monitor  50 . A differential input amplifier (not shown) in the EKG monitor  50  processes the electrical signals received from the ring electrodes  58  via the wires  62 , and converts them to graphic images that can be displayed. The thermocouple wires  54  can also function to monitor the temperature of the surrounding tissue, and provide temperature information to the ultrasound generator  52 . Throughout this mapping operation, the balloon  38  remains deflated.  
         [0047]     Once the mapping operation has been completed and the desired position of the balloon  38  has been confirmed, the physician can then inflate the balloon  38  using inflation media. The balloon  38  is preferably manufactured using known techniques to a predetermined diameter so that its diameter at its maximum expansion will be less than the diameter of the distal ring  80  and the annulus or vessel (e.g., the PV in  FIG. 7 ) where the ablation is to take place. The physician then controls the ultrasound generator  52  to generate ultrasound energy that is propagated through the wires  63  to the ultrasound transducer  60  that is positioned inside the balloon  38 . The energy radiates in a radial manner from the transducer  60 , propagates through the inflation media (which acts as an energy transmitting medium) inside the balloon  38 , exits the balloon  38  and then reaches the selected tissue (typically in a waveform) to ablate the tissue. See the arrows E in  FIG. 7  which illustrate the radiation of the energy from the transducer  60 .  
         [0048]     During the ablation, the distal ring  80  functions to anchor the distal tip section  24  inside the PV at the desired location so that the ablation can be performed accurately. In contrast to known catheter systems where the same element is used to anchor and ablate, by providing a separate element (i.e., the distal ring  80 ) to anchor the distal tip section  24 , the function of the ablation element (i.e., the balloon  38  and transducer  60 ) will not be affected by the anchoring device, thereby ensuring that the ablation is performed accurately and effectively. In addition, since the maximum diameter of the balloon  38  is always smaller than the smallest diameter of the distal ring  80 , blood will be able flow through the distal ring  80  and around the surfaces of the balloon  38 .  
         [0049]     When the ablation has been completed, the balloon  38  is deflated and the distal tip section  24  withdrawn from the heart.  
         [0050]      FIGS. 9-14  illustrate modifications made to the catheter system  20  of  FIGS. 1-5  to allow contrast medium to be introduced while the catheter is located within the vessel ostium and the balloon  38  inflated. The catheter system  20   a  in  FIGS. 9-14  essentially provides an additional tubing and lumen to facilitate the injection of the contrast medium. The catheter system  20  in  FIGS. 1-5  did not provide an additional lumen, so the contrast medium for vessel geometry and catheter location could not be readily verified. Hence, the catheter system  20   a  makes it easier to verify vessel geometry and catheter location since the blood flow from within the vessel will not wash out when the contrast medium is injected due to balloon inflation.  
         [0051]     Since the catheter system  20   a  merely includes modifications to the catheter system  20 , the descriptions relating to the same elements and their functions will not be repeated herein. Instead, the same numerals used to designate elements in  FIGS. 1-5  will be used to designate the same elements in  FIGS. 9-14 , except that an “a” will be added to the designations in  FIGS. 9-14 .  
         [0052]     The catheter system  20   a  provides an additional tubing  100  that extends from the handle assembly  32   a  (see  FIGS. 9-10 ). This tubing  100  is connected to a lumen  102  that extends through the shaft  22   a,  the transducer  60   a  inside the balloon  38   a,  and exits at the distal-most end of the shaft  22   a.  See  FIGS. 11 and 14 . The contrast medium can be injected via the tubing  100  and the lumen  102  by a syringe (not shown), and exits the catheter into the blood vessel at the location of the distal ring  80   a  to provide visibility of the location of the distal ring  80   a  and the balloon  38   a.  A guidewire (not shown) can be inserted into this lumen  102  to increase the mobility of the shaft  22   a  into branches of the main vessel.  
         [0053]     In addition, the flat wire  75   a  extends in the lumen  30   a  from the distal section of the shaft  22   a  (not shown in  FIGS. 9-14 ).  
         [0054]      FIGS. 15-16  illustrate yet another modification that can be made to the system  20  in  FIGS. 1-5 . The catheter system  20   b  in  FIGS. 15-16  is comprised of two separate catheters, a first catheter  120  that carries the balloon  38   b  and the transducer  60   b,  and a second catheter  122  that carries the distal ring  80   b.    
         [0055]     Since the catheter system  20   b  merely includes modifications to the catheter system  20   a,  the descriptions relating to the same elements and their functions will not be repeated herein. Instead, the same numerals used to designate elements in  FIGS. 9-14  will be used to designate the same elements in  FIGS. 15-16 , except that a “b” or a “c” will be added to the designations in  FIGS. 15-16 . The only notable differences are (i) the catheter  120  has the same structure as the catheter  20   a  with the exception of the distal ring  80   a,  and (ii) the catheter  122  has the same structure as the catheter  120  except for the balloon  38   a,  the transducer  60   a,  and the thermocouples.  
         [0056]     The distal ring  80   b  and the shaft  22   c  of the catheter  122  can be inserted through the lumen  102   b  of the catheter  120 . In this regard, the distal ring  80   b  can progressively straightened out and drawn into the lumen  102   b  of the catheter  120 . Thus, when confined with the catheter  120 , the distal ring  80   b  assumes the generally linear low profile shape of the catheter  120 . When the distal ring  80   b  exits the distal-most end  124  of the catheter  120  (see  FIG. 16 ), the distal ring  80   b  is uncovered and its shape memory (e.g., Nitinol) will cause the distal ring  80   b  to re-assume its preformed generally circular shape.  
         [0057]     The catheter  122  can also be steered so that the diameter of the distal ring  80   b  can be varied. This can be accomplished by providing a pulling wire (not shown, but can be the same as  72  or  72   a ), and then pulling the pulling wire. The catheter  120  can also be steered so that the distal end  124  can be deflected. The steering of the catheters  120 ,  122  can be accomplished using steering mechanisms  70   b,    70   c  that can be the same as the steering mechanism  70  described in  FIGS. 1-5 .  
         [0058]     The main lumen  30   b  of the catheter  120  can be used to accomodate a guidewire (not shown), and can also be used for delivering contrast medium. Therefore, the catheter system  20   b  does not require an additional tubing (such as  100 ) or lumen (such as  102 ) as in the catheter system  20   a,  although it is also possible to provide an additional tubing (such as  100 ) or lumen (such as  102 ) if such is desired.  
         [0059]     The following illustrates one example of a possible use of the catheter system  20   b.  A transseptal sheath (with a dilator in the sheath lumen) is typically inserted into the patient&#39;s femoral vein and placed into the right atrium. Using a transseptal (Brockenbrough) needle, a puncture is produced in the fossa ovalis in the septal wall to provide access from the right atrium to the left atrium. The sheath is then brought inside the left atrium, the needle removed, and a guidewire is inserted through the lumen of the dilator to the target pulmonary vein or its branches. The distal opening of the dilator inside the sheath follows the guidewire to the pulmonary vein. When catheter  20   a  is used, the dilator and the guidewire are removed and the catheter inserted into the transseptal sheath into the pulmonary vein. When catheter  120  is used, only the dilator is removed and the lumen  102   b  of the distal of the catheter follows the path of the guidewire and into the target PV. Once the catheter  20   a  or  120  is situated in the pulmonary vein ostium, the balloon  38   a  or  38   b  is inflated until it engages the ostial wall. Contrast media is injected into the lumen  102  or  102   b  to visually verify the location of the transducer  60   a  with respect to the pulmonary vein anatomy.  
         [0060]     For the catheter  20   a,  the location of the transducer  60   a  can be verified via contrast medium injection while the distal ring  80   a  records the PV potentials. This has not been possible with the conventional systems.  
         [0061]     For the catheter system  20   b,  the catheter  122  is inserted through the tubing  100   b  and the distal ring  80   b  exits from the lumen  102   b.  The diameter of the distal ring  80   b  can be adjusted to fit the different sizes of the pulmonary vein. The electrodes  58   b  are again used to pick up the PV potentials. Once the potentials (or intracardiac signals) are recorded, the catheter  122  can be removed, and if needed, contrast medium can be injected for locating the transducer. Energy can then be delivered to perform the ablation, as described above.  
         [0062]     While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention.