Abstract:
Systems and methods for measuring electrical potentials and other data associated with body tissue and generating electrograms of the tissue based on the data. In one embodiment, a device for measuring parameters of human tissue includes a multielectrode catheter for taking multiple measurements of the electrical characteristics of the human tissue, a concentric tube catheter located inside the multielectrode catheter, for providing structural support to the multi-electrode catheter and for serving as a conduit for advancing or withdrawing the multielectrode catheter over its surface; and an imaging catheter located inside the concentric tube catheter for taking multiple measurements of anatomical characteristics of the human tissue.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. patent application Ser. No. 10/256,188, filed Sep. 26, 2002, which claims the benefit of U.S. Provisional Patent Application 60/325,707, filed Sep. 28, 2001, each of which is incorporated by reference as if set forth herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates generally to medical devices and methods, and more particularly to catheters that can be used to measure electrical potentials and other data associated with body tissue, wherein the data can then be used to generate electrograms of the tissue. 
         [0004]    2. Related Art 
         [0005]    Heart rhythm disorders (atrial and ventricular arrhythmias) result in significant morbidity and mortality. Unfortunately, current pharmacological therapy for managing cardiac arrhythmias is often ineffective and, at times, can cause arrhythmias, thereby shifting emphasis to nonpharmacological therapy (such as ablation, pacing, and defibrillation). Due to limitations in present mapping techniques, brief, chaotic, or complex arrhythmias (such as atrial fibrillation and ventricular tachycardia) cannot be mapped adequately during catheterization, resulting in unsuccessful elimination of the arrhythmia. In addition, localizing abnormal beats and delivering and quantifying the effects of therapy such as ablation are very time consuming during catheterization. Selecting appropriate pharmacological therapies and advancing nonpharmacological methods to manage cardiac arrhythmias are contingent on developing mapping techniques that identify mechanisms of arrhythmias, localize their sites of origin with respect to underlying cardiac anatomy, and elucidate effects of therapy. Therefore, to successfully manage cardiac arrhythmias, electrical-anatomical imaging on a beat-by-beat basis, simultaneously, and at multiple sites is required. 
         [0006]    Electrical mapping of the heartbeat, whereby multielectrode arrays are placed on the exterior surface of the heart (epicardium) to directly record the electrical activity, has been applied extensively in both animals and humans. Although epicardial mapping provides detailed information on sites of origin and mechanisms of abnormal heart rhythms (arrhythmias), its clinical application has great limitation: it is performed at the expense of open-chest surgery. In addition, epicardial mapping does not provide access to interior heart structures that play critical roles in the initiation and maintenance of abnormal heartbeats. 
         [0007]    Many heart rhythm abnormalities (arrhythmias) originate from interior heart tissues (endocardium). Further, because the endocardium is more safely accessible (without surgery) than the epicardium, most electrical mapping techniques and delivery of nonpharmacological therapies (e.g. pacing and catheter ablation) have focused on endocardial approaches by catheterization. However, current endocardial mapping techniques have certain limitations. Traditional electrode-catheter mapping performed during electrophysiology catheterization procedures is confined to a limited number of recording sites, is time consuming, and is carried out over several heartbeats without accounting for possible beat-to-beat variability in activation. While other catheter-mapping approaches provide important three-dimensional positions of a roving electrode-catheter through the use of “special” sensors, mapping is still performed over several heartbeats. On the other hand, although multielectrode basket-catheters measure endocardial electrical activities at multiple sites simultaneously by expanding the basket inside the heart so that the electrodes are in direct contact with the endocardium, the basket is limited to a fixed number of recording sites, may not be in contact with the entire endocardium, and may result in irritation of the myocardium. 
         [0008]    An alternative mapping approach utilizes a noncontact, multielectrode cavitary probe that measures electrical activities (electrograms) from inside the blood-filled heart cavity from multiple directions simultaneously. The probe electrodes are not necessarily in direct contact with the endocardium; consequently, noncontact sensing results in a smoothed electrical potential pattern. Nonsurgical insertion of a noncontact, multielectrode balloon-catheter, that does not occlude the blood-filled cavity, has been reported in humans. 
         [0009]    Present mapping systems cannot provide true images of endocardial anatomy during catheterization. Present systems often delineate anatomical features based on ( 1 ) extensive use of fluoroscopy; ( 2 ) deployment of multiple catheters, or roving the catheters, at multiple locations; and ( 3 ) assumptions about properties of recorded electrograms in relation to underlying anatomy (e.g. electrograms facing a valve are low in amplitude). However, direct correlation between endocardial activation and cardiac anatomy is important in order to clearly identify the anatomical sources of abnormal heartbeats, to understand the mechanisms of cardiac arrhythmias and their sequences of activation within or around complex anatomical structures, and to deliver appropriate therapy. 
         [0010]    Early applications of the “inverse problem” of electrocardiography sought to noninvasively reconstruct (compute) epicardial surface potentials (electrograms) and activation sequences of the heartbeat based on noncontact potentials measured at multiple sites on the body surface. The computed epicardial potentials were in turn used to delineate information on cardiac sources within the underlying myocardium. To solve the “inverse problem”, numeric techniques have been repeatedly tested on computer, animal, and human models. Similarly, computing endocardial surface electrical potentials (electrograms) based on noncontact potentials (electrograms) measured with the use of a multielectrode cavitary probe constitutes a form of endocardial electrocardiographic “inverse problem.” 
         [0011]    The objective of the endocardial electrocardiographic “inverse problem” is to compute virtual endocardial surface electrograms based on noncontact cavitary electrograms measured by multielectrode probes. Methods for acquisition of cavitary electrograms and computation of endocardial electrograms in the beating heart have been established and their accuracy globally confirmed. Determining the probe-endocardium geometrical relationship (i.e. probe position and orientation with respect to the endocardial surface) is required to solve the “inverse problem” and a prerequisite for accurate noncontact electrical-anatomical imaging. In previous studies, fluoroscopic imaging provided a means for beat-by-beat global validation of computed endocardial activation in the intact, beating heart. Furthermore, epicardial echocardiography was used to determine the probe-cavity geometrical model. However, complex geometry, such as that of the atrium, may not be easily characterized by transthoracic or epicardial echocardiography. 
         [0012]    Accurate three-dimensional positioning of electrode-catheters at abnormal electrogram or ablation sites on the endocardium and repositioning of the catheters at specific sites are important for the success of ablation. The disadvantages of routine fluoroscopy during catheterization include radiation effects and limited three-dimensional localization of the catheter. New catheter-systems achieve better three-dimensional positioning by (1) using a specialized magnetic sensor at the tip of the catheter that determines its location with respect to an externally applied magnetic field, (2) calculating the distances between a roving intracardiac catheter and a reference catheter, each carrying multiple ultrasonic transducers, (3) measuring the field strength at the catheter tip-electrode, while applying three orthogonal currents through the patient&#39;s body to locate the catheter; and (4) emitting a low-current locator signal from the catheter tip and determining its distance from a multielectrode cavitary probe. With these mapping techniques true three-dimensional imaging of important endocardial anatomical structures is not readily integrated (only semi-realistic geometric approximations of the endocardial surface), and assumptions must often be made about properties of recorded electrograms in relation to underlying anatomy (e.g. electrograms facing the tricuspid and mitral annuli are low in amplitude). 
       SUMMARY OF THE INVENTION 
       [0013]    This disclosure is directed to systems and methods for use in measuring electrical potentials and other data associated with body tissue, and using the data to generate electrograms of the tissue, wherein one or more of the problems discussed above are solved. 
         [0014]    For example, systems and methods are described that make possible the combined use of (1) a lumen-catheter carrying a plurality of sensing electrodes (multielectrode catheter-probe) for taking multiple noncontact and contact measurements, from different directions, of the electrical characteristics of interior tissue such as the heart (endocardium) and (2) an internal coaxial catheter carrying one or more imaging elements for visualizing the anatomical characteristics of the tissue. A middle coaxial lumen-catheter (sheath) provides structural support and serves as a conduit for advancing or withdrawing the multielectrode catheter over its surface, or inserting the anatomical imaging catheter through its lumen. The imaging catheter is inserted inside the multielectrode catheter-probe (or the supporting lumen-catheter when in use) and is moved to detect the tissue from inside the lumen using different modalities such as ultrasound, infrared, and magnetic resonance. Both the electrical and anatomical measurements are sent to a data acquisition system that in turn provides combined electrical and anatomical graphical or numerical displays to the operator. 
         [0015]    Another feature of one embodiment is that the catheter imaging system simultaneously maps multiple interior heart surface electrical activities (endocardial electrograms) on a beat-by-beat basis and combines three-dimensional activation-recovery sequences with endocardial anatomy. Electrical-anatomical imaging of the heart, based on (1) cavitary electrograms that are measured with a noncontact, multielectrode probe and (2) three-dimensional endocardial anatomy that is determined with an integrated anatomical imaging modality (such as intracardiac echocardiography), provides an effective and efficient means to diagnose abnormal heartbeats and deliver therapy. 
         [0016]    Another feature of one embodiment is that the integrated electrical-anatomical imaging catheter system contains both a multielectrode probe and an anatomical imaging catheter, which can be percutaneously introduced into the heart in ways similar to standard catheters used in routine procedures. This “noncontact” imaging approach reconstructs endocardial surface electrograms from measured probe electrograms, provides three-dimensional images of cardiac anatomy, and integrates the electrical and anatomical images to produce three-dimensional isopotential and isochronal images. 
         [0017]    Another feature of one embodiment is that the method improves the understanding of the mechanisms of initiation, maintenance, and termination of abnormal heartbeats, which could lead to selecting or developing better pharmacological or nonpharmacological therapies. Mapping is conducted with little use of fluoroscopy on a beat-by-beat basis, and allows the study of brief, rare, or even chaotic rhythm disorders that are difficult to manage with existing techniques. 
         [0018]    Another feature of one embodiment is that there is a means to navigate standard diagnostic-therapeutic catheters, and accurately guide them to regions of interest within an anatomically-realistic model of the heart that is derived from ultrasound, infrared, or magnetic resonance. The various embodiments of the present invention may provide considerable advantages in guiding clinical, interventional electrophysiology procedures, such as imaging anatomical structures, confirming electrode-tissue contact, monitoring ablation lesions, and providing hemodynamic assessment. 
         [0019]    Another feature of one embodiment is that some of the sensing electrodes on the surface of the multielectrode catheter-probe are brought in direct contact with the interior surface of the tissue. The multielectrode catheter simultaneously measures contact and noncontact potentials resulting from electrical activity from multiple locations in the tissue. 
         [0020]    Another feature of one embodiment is that the multielectrode catheter-probe is navigated inside a blood-filled cavity and placed at different locations. Meanwhile, the multielectrode catheter continuously measures contact and noncontact potentials resulting from electrical activity from multiple locations in the tissue. 
         [0021]    Numerous other embodiments are also possible. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Other objects and advantages of the invention may become apparent upon reading the following detailed description and upon reference to the accompanying drawings. 
           [0023]      FIG. 1  illustrates a system in accordance with one embodiment of the present invention in use with a human patient. 
           [0024]      FIG. 2  illustrates a lumen sheath with a pig-tail at its distal end and a guide wire inside its lumen. 
           [0025]      FIG. 3A  illustrates a multielectrode catheter-probe with a lumen inside its shaft. 
           [0026]      FIG. 3B  illustrates an alternative embodiment of a multielectrode lumen catheter-probe whereby a grid of electrodes can be expanded. 
           [0027]      FIG. 3C  illustrates an alternative embodiment of a multielectrode lumen catheter-probe with a pig-tail at its distal end for structural support. 
           [0028]      FIG. 4  illustrates an anatomical imaging catheter such as intracardiac echocardiography catheter. 
           [0029]      FIG. 5A  illustrates a configuration that combines the sheath (of  FIG. 2 ) with the multielectrode catheter-probe (of  FIG. 3A ) over its surface at the proximal end and the anatomical imaging catheter (of  FIG. 4 ) advanced inside the lumen at the distal end. 
           [0030]      FIG. 5B  illustrates an alternative embodiment that combines the sheath (of  FIG. 2 ) with the multielectrode catheter-probe (of  FIG. 3B ) advanced over its surface to the distal end and the anatomical imaging catheter (of  FIG. 4 ) inside the lumen at the proximal end. 
           [0031]      FIG. 6  illustrates an alternative embodiment that combines the multielectrode catheter-probe (of  FIG. 3C ) with the anatomical imaging catheter (of  FIG. 4 ) inside its lumen. 
       
    
    
       [0032]    While the invention is subject to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and the accompanying detailed description. It should be understood, however, that the drawings and detailed description are not intended to limit the invention to the particular embodiment which is described. This disclosure is instead intended to cover all modifications, equivalents and alternatives falling within the scope of the present invention as defined by the appended claims. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0033]    One or more embodiments of the invention are described below. It should be noted that these and any other embodiments described below are exemplary and are intended to be illustrative of the invention rather than limiting. 
         [0034]      FIG. 1  illustrates an electrical-anatomical imaging catheter-system  10  in use in a human patient. The catheter is percutaneously inserted through a blood vessel (vein or artery) and advanced into the heart cavity. The catheter detects both electrical and anatomical properties of interior heart tissue (endocardium). Measured electrical properties are in the form of contact and noncontact potentials detected by electrodes (sensors)  24  (illustrated in  FIG. 3A ). Measured anatomical properties are in the form of tissue geometry, structure, and texture features detected by an anatomical imaging catheter  18  (illustrated in  FIG. 4 ). 
         [0035]    Referring now to  FIG. 2 , the electrical-anatomical imaging catheter system  10  includes a lumen sheath  12  (about  3  mm in diameter) which has a pig tail distal end  14  to minimize motion artifacts inside the heart cavity. A guide wire  15  is advanced to a tip  13  to guide the sheath  12 . The sheath  12  provides structural support for a coaxial multielectrode catheter-probe  16  (illustrated in  FIG. 3A  and  FIG. 3B ) that slides over the surface of the sheath  12 , and records noncontact cavitary electrical signals (electrograms) from multiple directions and at several locations along the sheath. The sheath  12  also functions as a conduit for inserting an anatomical imaging catheter  18  (illustrated in  FIG. 4 ) such as a standard intracardiac echocardiography (ICE) catheter that records continuous echocardiographic images of the heart interior. With this approach, the sheath  12  maintains the same imaging axis and direction over several deployments inside the heart cavity of both the probe  16  and the anatomical imaging catheter  18 . Radiopaque and sonopaque ring marker  20  at the distal end of the sheath  12  and radiopaque and sonopaque ring marker  22  at the proximal end of the sheath  12  aid in verifying the probe  16  and the anatomical imaging catheter  18  locations. 
         [0036]    Referring now to  FIG. 3A , the electrical-anatomical imaging catheter system  10  includes a lumen catheter which carries a plurality of sensing electrodes  24  on its surface that make up the multielectrode probe  16 . The electrodes  24  are arranged in columns. The diameter of the probe  16  is similar to that of shaft  23  of the probe  16 (on the order of  3  mm). The sheath  12  and the anatomical imaging catheter  18  both coaxially fit inside the lumen of the probe  16 . The catheter-probe  16  has a straight distal end  45  that permits sliding the probe  16  over the coaxial lumen sheath  12 . In this state the probe  16  is easily inserted percutaneously by the operator through a blood vessel and advanced into the heart cavity. By sliding the catheter-probe  16  over the central sheath  12 , it is possible to place the probe  16  at multiple locations over the sheath and along the axis of the cavity. The shaft  23  of the probe  16  is shorter than the central sheath  12  so that it slides easily over the sheath  12  in and out of the heart cavity. 
         [0037]      FIG. 3B  illustrates another embodiment of part of the electrical anatomical imaging catheter-system  10  of the present system, in which for the probe  16 , the electrodes  24  are laid on a central balloon  26  that is inflated to a fixed diameter without the electrodes  24  necessarily touching the interior surface of the heart. The balloon is similar to angioplasty catheters used in routine catheterization procedures. The balloon  26  is inflated inside the heart cavity to enlarge the probe  16 . The sheath  12  and the anatomical imaging catheter  18  (illustrated in  FIG. 4 ) fit inside the lumen  50 . The probe  16  has a straight distal end  45  that permits sliding the probe  16  over the coaxial lumen sheath  12 . By sliding the probe  16  over the central sheath  12 , it is possible to place the probe  16  at multiple locations over the sheath and along the axis of the heart cavity. In its collapsed state the size of the probe  16  is similar to that of the sheath  12 . Thus, the operator is able to insert the probe percutaneously and inflate it inside the heart without occluding the cavity. The shaft  23  of the probe  16  is shorter than the central sheath  12  so that the probe  16  slides easily over the sheath  12  in and out of the cavity. 
         [0038]    In another embodiment of the electrical-anatomical imaging catheter system  10 ,  FIG. 3C  illustrates the probe  16  with a pig-tail  46  at its distal end to minimize motion artifacts of the probe  16 . In this embodiment, the probe  16  is used independently of the lumen sheath  12 . The anatomical imaging catheter  18  (illustrated in  FIG. 4 ) fits inside the lumen of the probe  16 . 
         [0039]    Referring now to  FIG. 4 , the anatomical imaging catheter  18  is used to image interior structures of the heart. In the preferred embodiment, the catheter  1   8  is a 9-MHz intracardiac echocardiography catheter (Model Ultra ICE, manufactured by Boston Scientific/EPT, located in San Jose, Calif.). To acquire echocardiographic images, the catheter  18  connects to an imaging console (Model ClearView, manufactured by Boston Scientific/EPT, located in San Jose, Calif.). The catheter  18  has a distal imaging window  30  and a rotatable imaging core  32  with a distal transducer  34  that emits and receives ultrasound energy. Continuous rotation of the transducer provides tomographic sections of the heart cavity. The design of the present system allows for integrating other anatomical imaging catheters presently under development such as echocardiography catheters carrying multiple phased-array transducers, infrared, and magnetic resonance imaging catheters. While the anatomical imaging catheter  18  is in use, the three-dimensional anatomical reconstruction assumes that the catheter  18  is straight and thus straightens the image of the heart cavity. If the catheter  18  curves, the image is distorted, or, if the catheter  18  rotates during pullback, the image is twisted. Therefore, in the preferred embodiment, a position and orientation sensor  40  is added to the catheter  18 . 
         [0040]    Referring now to  FIG. 5A , an integrated, noncontact, electrical anatomical imaging catheter-system  10  is illustrated that combines the sheath  12  with the multielectrode catheter-probe  16  over its surface at the proximal end, and the anatomical imaging catheter  18  inside the lumen at the distal end. In operation, the probe  16  is preloaded over the central sheath  12 , thereby enabling the probe  16  to move in and out of the heart cavity in small increments at several locations over a fixed axis. The guide wire  15  is placed inside the central sheath  12  to ensure the pig-tail end  14  remains straight during insertion through a blood vessel. With the probe  16  loaded on the sheath  12  and pulled back, the sheath  12  is advanced through a blood vessel and placed inside the heart cavity under the guidance of fluoroscopy, and the guide wire  15  is then removed. The anatomical imaging catheter  18  is then inserted through the lumen of the central sheath  12 , replacing the guide wire  15 , and advanced until a tip  19  of the catheter  18  is situated at the pre-determined radiopaque and sonopaque distal marker  20  on the sheath  12 . The catheter  18  is pulled back from the distal marker  20  to the proximal marker  22  on the sheath  12  at fixed intervals, and noncontact anatomical images are continuously acquired at each interval. 
         [0041]    Referring now to  FIG. 5B , under the guidance of fluoroscopy, the probe  16  is advanced over the central sheath  12  until a tip  17  is at the distal marker  20 , and the balloon  26  (if used) is inflated to unfold the probe  16 . The probe  16  then simultaneously acquires noncontact cavitary electrograms. 
         [0042]    Referring now to  FIG. 6 , an alternate embodiment of the integrated electrical-anatomical imaging catheter system  10  is illustrated, labeled as an integrated electrical-anatomical imaging catheter system  11 , in which the lumen sheath  12  is eliminated. A multielectrode lumen catheter-probe  16  with a pig-tail  46  at its distal end is inserted inside the heart cavity and is used to acquire noncontact electrograms. In operation, the multielectrode catheter-probe is navigated inside the cavity and placed at different locations. The anatomical imaging catheter  18  is inserted inside the lumen of the catheter-probe  16 , and imaging is performed from inside the probe  16 . 
         [0043]    Unipolar cavitary electrograms sensed by the noncontact multielectrode probe  16  with respect to an external reference electrode  55  (shown in  FIG. 1 ) along with body surface electrocardiogram signals, are simultaneously acquired with a computer-based multichannel data acquisition mapping system, which, in the preferred embodiment, is the one built by Prucka Engineering-GE Medical Systems, located in Milwaukee, Wis. In operation, the multielectrode catheter-probe  16  senses both noncontact potentials (electrograms) by electrodes  24  not in contact with the tissue interior, and contact potentials (electrograms) by electrodes  24  in direct contact with the tissue interior. The mapping system amplifies and displays the signals at a  1  ms sampling interval per channel. The mapping system displays graphical isopotential and isochronal maps that enable evaluation of the quality of the data acquired during the procedure and interaction with the study conditions. The multiple anatomical images (such as ICE) are digitized, and the interior heart borders automatically delineated. The cavity three-dimensional geometry is rendered in a virtual reality environment, as this advances diagnostic and therapeutic procedures. 
         [0044]    To reconstruct the electrical activities (electrical potentials, V) on the interior heart surface (endocardium) based on noncontact electrical potentials measured by the cavitary multielectrode probe  16  and anatomical information derived from the anatomical imaging catheter  18 , Laplace&#39;s equation (F 2V=0) is numerically solved in the blood-filled cavity between the probe  16  and the endocardium. The boundary element method is employed in computing the electrical potentials at the tissue surface in a three-dimensional geometry on the basis of noncontact cavitary potentials sensed by electrodes  24 . A numeric regularization technique (filtering) based on the commonly used Tikhonov method is employed to find the electrical potentials on the endocardium. Here, with the probe  16  positioned at one location inside the cavity, the electrical potentials are then uniquely reconstructed on the real endocardial anatomy derived from the anatomical imaging catheter  18 . 
         [0045]    Due to the irregular shape of the tissue and its continuous dynamic motion throughout the cardiac cycle, some of electrodes  24  may be in contact with the tissue. At other times, some of electrodes  24  may be intentionally placed in contact with the tissue when positioning the multielectrode probe  16  in complex regions of the cavity. Select electrodes  24  on the surface of the probe  16  that are in contact with the tissue, as identified by the anatomical imaging catheter  18 , record contact electrical potentials. Meanwhile, the remainder of electrodes  24  on the surface of probe  16  measure noncontact potentials. Values of tissue contact potentials may be used as boundary conditions when numerically solving Laplace&#39;s equation (i.e. V=Vcontact at the interior tissue boundary). By applying the boundary element method and numeric regularization, the resulting solution is a set of electrical potentials at multiple locations throughout the tissue surface. 
         [0046]    In cases of complex cavity geometry, the multielectrode probe  16  may be navigated to different locations inside the cavity. Meanwhile, electrodes  24  may record noncontact electrical potentials at multiple locations of probe  16 , thereby providing a large number of spatial samples pf noncontact cavitary potentials that improve the accuracy of potentials computed at the interior tissue surface. The noncontact potentials recorded at multiple locations of probe  16  may be combined into one large set of data to simultaneously reconstruct the potentials at the tissue surface. Alternatively, potentials at the tissue surface may be repeatedly reconstructed on the basis of each individual location of probe  16  inside the cavity, with final tissue potentials computed as the average for all probe locations. In either approach, the potentials at the tissue surface continue to be reconstructed by numerically solving Laplace&#39;s equation and applying the boundary element method and numeric regularization. 
         [0047]    Nonfluoroscopic three-dimensional positioning and visualization of standard navigational electrode-catheters is clinically necessary for (1) detailed and localized point-by-point mapping at select interior heart regions, (2) delivering nonpharmacological therapy such as pacing or ablation, (3) repositioning the catheters at specific sites, and (4) reducing the radiation effects of fluoroscopy during catheterization. To guide three-dimensional positioning and navigation of standard electrode-catheters, a low-amplitude location electrical signal is emitted between the catheter tip-electrode and the external reference electrode  55 , and sensed by multiple electrodes  24  on the surface of the probe  16 . The catheter tip is localized by finding the x, y, and z coordinates of a location point p. The location of the emitting electrode is determined by minimizing [F(p)−V(p)]T[F(p)−V(p)] with respect to p, where V(p) are the electrical potentials measured on the probe  16 , and F(p) are the electrical potentials computed on the probe  16  using an analytical (known) function and assuming an infinite, homogeneous conducting medium. This process also constructs the shape of the catheter within the cavity by determining the locations of all catheter electrodes. Alternatively, the location and shape of the roving electrode-catheter is determined with respect to the underlying real anatomy by direct visualization with the anatomical imaging catheter  18 . 
         [0048]    The present method senses the location signal by multiple probe electrodes  24  simultaneously, thereby localizing the roving catheter more accurately than prior art methods. Furthermore, the method reconstructs the shape of the roving catheter during navigation by emitting a location signal from each of the catheter electrodes and determining their locations within the cavity. With this approach, online navigation of standard electrode-catheters is performed and displayed within an anatomically-correct geometry derived from ultrasound, infrared, or magnetic resonance, and without extensive use of fluoroscopy. 
         [0049]    The benefits and advantages which may be provided by the present invention have been described above with regard to specific embodiments. These benefits and advantages, and any elements or limitations that may cause them to occur or to become more pronounced are not to be construed as critical, required, or essential features of any or all of the claims. As used herein, the terms “comprises,” “comprising,” or any other variations thereof, are intended to be interpreted as non-exclusively including the elements or limitations which follow those terms. Accordingly, a system, method, or other embodiment that comprises a set of elements is not limited to only those elements, and may include other elements not expressly listed or inherent to the claimed embodiment. 
         [0050]    While the present invention has been described with reference to particular embodiments, it should be understood that the embodiments are illustrative and that the scope of the invention is not limited to these embodiments. Many variations, modifications, additions and improvements to the embodiments described above are possible. It is contemplated that these variations, modifications, additions and improvements fall within the scope of the invention as detailed within the following claims.