Abstract:
Catheterization is carried out by inserting a sheath into a human patient and moving a catheter having an electrode through the sheath lumen. A variation between a first threshold value and a second threshold value in electrical current through the electrode is identified. Responsively to the variation, it is reported that a portion of the catheter has transitioned between an in-sheath condition and an out-of-sheath condition. The sheath is defined and identified by the historical data of the readings of the magnetic sensor during its movements.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to instruments for performing medical examinations of the interior of cavities. More particularly, this invention relates to instruments employing magnetic fields and impedance measurements for determining a position of a probe in the interior of a cavity. 
         [0003]    2. Description of the Related Art 
         [0004]    The meanings of certain acronyms and abbreviations used herein are given in Table 1. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Acronyms and Abbreviations 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 ACL 
                 Active Current Location 
               
               
                   
                 CPM 
                 Current Position Map 
               
               
                   
                   
               
             
          
         
       
     
         [0005]    In a number of medical procedures a probe is inserted into a patient through a sheath. Typically the sheath acts to guide the probe during its insertion, as well as to maintain the probe in a desired alignment. Once the probe and the sheath have been inserted into the patient, their distal ends are not visible, so that an operator performing the procedure may be unaware of the relationship of the sheath&#39;s distal end relative to the probe&#39;s distal end. It is possible to use fluoroscopy to locate the sheath, but at the cost of exposing, which exposes the subject to radiation. 
         [0006]    Impedance-based position sensing is one method used to guide the tip of the catheter to the target position in which the medical procedure is to be performed. Commonly assigned U.S. Patent Application Publication No. 20150126895, which is herein incorporated by reference, describes a sheath having an outer wall that surrounds an inner lumen and has multiple holes along the tube penetrating the outer wall. Multiple electrically conducting elements are inserted in the respective holes, so as to allow transmission of electrical current between the inner lumen and an exterior of the outer wall. 
       SUMMARY OF THE INVENTION 
       [0007]    In one cardiac catheterization technique, the operator first inserts a sheath having ACL electrodes percutaneously into the patient&#39;s body and guides the sheath&#39;s distal end to the target region. The operator then inserts a hybrid catheter having magnetic and ACL sensors into the lumen of the sheath and guides the tip of the catheter through the sheath. When the catheter has exited the sheath, a current position map is eventually built as part of the medical procedure. 
         [0008]    According to disclosed embodiments of the invention, data from magnetic sensors is used to trace the location of the catheter through the sheath. Once the distal portion of the catheter emerges from the sheath, the magnetic data is augmented by data from using the ACL sensors of the catheter. However the ACL data is not valid while the catheter tip is still within the sheath. By evaluating the currents in the ACL sensors of the catheter it is possible to detect the emergence of the catheter through the tip of the sheath. The combined magnetic and ACL data then locates the sheath tip prior to construction of the map. 
         [0009]    There is provided according to embodiments of the invention a method, which is carried out by inserting a sheath into a human patient and moving a catheter through the lumen of the sheath, wherein the catheter has at least one electrode disposed on its distal portion. The method is further carried out by measuring electrical current through the electrode with current-measuring circuitry, identifying a variation in the electrical current between a first threshold value and a second threshold value, and responsively to the variation, reporting that a portion of the catheter has transitioned between an in-sheath condition and an out-of-sheath condition. 
         [0010]    According to a further aspect of the method, the electrode and the current-measuring circuitry are components of an impedance measuring system and wherein the electrical current is a measure of an impedance of a heart. 
         [0011]    According to yet another aspect of the method, the sheath has a perforation formed through its outer wall, and identifying a variation occurs while the electrode is passing by the perforation. 
         [0012]    A plurality of electrodes may be disposed on the catheter. The method is further carried out by concurrently measuring respective electrical current in the plurality of electrodes, recognizing a transient decrease in current flow in one of the electrodes that does not fall below a lower threshold, and concluding that a position of the one electrode opposes the perforation during the transient decrease. 
         [0013]    Responsively to identifying a variation the method is further carried out by determining a location of the distal end of the catheter relative to the perforation. 
         [0014]    According to still another aspect of the method, the portion of the catheter is the distal end of the catheter. 
         [0015]    According to an additional aspect of the method, the portion of the catheter is another electrode. 
         [0016]    According to another aspect of the method, the at least one electrode includes a proximal electrode and a distal electrode. The method is further carried out by defining an upper threshold and a lower threshold, wherein moving the catheter includes iteratively advancing the catheter in the sheath while the electrical current through the distal electrode does not exceed the upper threshold, retracting the catheter in the sheath until the electrical current through the proximal electrode no longer exceeds the lower threshold, and thereafter reporting that the distal electrode is in the out-of-sheath condition and the proximal electrode is in the in-sheath condition. 
         [0017]    According to yet another aspect of the method, defining an upper threshold and a lower threshold includes accessing a database of sheath-catheter combinations. 
         [0018]    According to still another aspect of the method, a magnetic sensor is disposed in the catheter at a predetermined distance from the distal end, and reporting includes reading the magnetic sensor, and reporting a location of the distal end of the catheter from the reading. 
         [0019]    There is further provided according to embodiments of the invention an apparatus, including a plurality of body surface electrodes, which are adapted to be fixed to a surface of a body at respective locations and a controller having current-measuring circuitry. The controller is adapted to be coupled to a catheter having at least one catheter electrode disposed on its distal portion and to the body surface electrodes. The controller is operative for passing electrical current through the body between the at least one catheter electrode and the body surface electrodes, and for determining position coordinates of the catheter by measuring respective characteristics of the electrical current passing through the body surface electrodes. The apparatus further includes a memory and a processor with access to the memory, the processor being cooperative with the controller for measuring the electrical current through the at least one catheter electrode with the current-measuring circuitry, identifying a variation in the electrical current between a first threshold value and a second threshold value, and responsively to the variation, reporting that a portion of the catheter has transitioned between an in-sheath condition and an out-of-sheath condition. 
         [0020]    The apparatus may include a position processing system operative for reading signals from a magnetic sensor disposed in the catheter at a predetermined distance from the distal end of the catheter and reporting a location of the distal end of the catheter from the reading using the predetermined distance as an offset. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0021]    For a better understanding of the present invention, reference is made to the detailed description of the invention, by way of example, which is to be read in conjunction with the following drawings, wherein like elements are given like reference numerals, and wherein: 
           [0022]      FIG. 1  is a pictorial illustration of a system for performing catheterization procedures on a heart in accordance with a disclosed embodiment of the invention; 
           [0023]      FIG. 2  is a schematic pictorial view of catheter within a sheath in accordance with an embodiment of the invention; 
           [0024]      FIG. 3  is a schematic diagram illustrating a catheter having ACL electrodes and a sheath in accordance with an embodiment of the invention; 
           [0025]      FIG. 4  is a schematic diagram illustrating a catheter lying partially within a sheath in accordance with an embodiment of the invention; 
           [0026]      FIG. 5  is an electrical schematic of a circuit modeling the impedance and current of an ACL electrode in accordance with an embodiment of the invention; 
           [0027]      FIG. 6  is a plot of ACL electrode current obtained while an electrode transitions from an in-sheath to an out-of-sheath status accordance with an embodiment of the invention; 
           [0028]      FIG. 7  is a plot showing a distribution of out-of-sheath currents in accordance with an embodiment of the invention; 
           [0029]      FIG. 8  is a plot of ACL electrode current as a function of penetration into a sheath in accordance with an embodiment of the invention; 
           [0030]      FIG. 9  is a plot of ACL electrode current as a function of penetration into a sheath in accordance with an embodiment of the invention; 
           [0031]      FIG. 10  is a graph comparing the data of  FIG. 8  and  FIG. 9  in accordance with an embodiment of the invention; 
           [0032]      FIG. 11  displays two plots relating currents through a proximal electrode of a multi-electrode catheter to movement of the electrode into a perforated sheath in accordance with an embodiment of the invention; 
           [0033]      FIG. 12  is a schematic illustration of a test arrangement for determination of electrode currents in ACL electrodes in accordance with an embodiment of the invention; 
           [0034]      FIG. 13  is a schematic diagram of a lasso catheter, which has been inserted through a sheath in accordance with an embodiment of the invention; and 
           [0035]      FIG. 14  is a flow chart for determining an optimum location of a catheter being inserted through a sheath in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    In the following description, numerous specific details are set forth in order to provide a thorough understanding of the various principles of the present invention. It will be apparent to one skilled in the art, however, that not all these details are necessarily needed for practicing the present invention. In this instance, well-known circuits, control logic, and the details of computer program instructions for conventional algorithms and processes have not been shown in detail in order not to obscure the general concepts unnecessarily. 
         [0037]    Documents incorporated by reference herein are to be considered an integral part of the application except that, to the extent that any terms are defined in these incorporated documents in a manner that conflicts with definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered. 
       System Overview. 
       [0038]    Turning now to the drawings, reference is initially made to  FIG. 1 , which is a pictorial illustration of a system  10  for evaluating electrical activity and performing ablative procedures on a heart  12  of a living subject, which is constructed and operative in accordance with a disclosed embodiment of the invention. The system comprises a catheter  14 , which is percutaneously inserted by an operator  16  through the patient&#39;s vascular system into a chamber or vascular structure of the heart  12 . The operator  16 , who is typically a physician, brings the catheter&#39;s distal tip  18  into contact with the heart wall, for example, at an ablation target site. Electrical activation maps may be prepared, according to the methods disclosed in U.S. Pat. Nos. 6,226,542, and 6,301,496, and in commonly assigned U.S. Pat. No. 6,892,091, whose disclosures are herein incorporated by reference. One commercial product embodying elements of the system  10  is available as the CARTO® 3 System, available from Biosense Webster, Inc., 3333 Diamond Canyon Road, Diamond Bar, Calif. 91765. This system may be modified by those skilled in the art to embody the principles of the invention described herein. 
         [0039]    Areas determined to be abnormal, for example by evaluation of the electrical activation maps, can be ablated by application of thermal energy, e.g., by passage of radiofrequency electrical current through wires in the catheter to one or more electrodes at the distal tip  18 , which apply the radiofrequency energy to the myocardium. The energy is absorbed in the tissue, heating it to a point (typically about 60° C.) at which it permanently loses its electrical excitability. When successful, this procedure creates non-conducting lesions in the cardiac tissue, which disrupt the abnormal electrical pathway causing the arrhythmia. The principles of the invention can be applied to different heart chambers to diagnose and treat many different cardiac arrhythmias. 
         [0040]    The catheter  14  typically comprises a handle  20 , having suitable controls on the handle to enable the operator  16  to steer, position and orient the distal end of the catheter as desired for the ablation. To aid the operator  16 , the distal portion of the catheter  14  contains position sensors (not shown) that provide signals to a processor  22 , located in a console  24 . The processor  22  may fulfill several processing functions as described below. 
         [0041]    Ablation energy and electrical signals can be conveyed to and from the heart  12  through one or more ablation electrodes  32  located at or near the distal tip  18  via cable  34  to the console  24 . Pacing signals and other control signals may be conveyed from the console  24  through the cable  34  and the electrodes  32  to the heart  12 . Sensing electrodes  33 , also connected to the console  24  are disposed between the ablation electrodes  32  and have connections to the cable  34 . 
         [0042]    Wire connections  35  link the console  24  with body surface electrodes  30  and other components of a positioning sub-system for measuring location and orientation coordinates of the catheter  14 . The processor  22  or another processor (not shown) may be an element of the positioning subsystem. The electrodes  32  and the body surface electrodes  30  may be used to measure tissue impedance at the ablation site as taught in U.S. Pat. No. 7,536,218, issued to Govari et al., which is herein incorporated by reference. A temperature sensor (not shown), typically a thermocouple or thermistor, may be mounted on or near each of the electrodes  32 . 
         [0043]    The console  24  typically contains one or more ablation power generators  25 . The catheter  14  may be adapted to conduct ablative energy to the heart using any known ablation technique, e.g., radiofrequency energy, ultrasound energy, and laser-produced light energy. Such methods are disclosed in commonly assigned U.S. Pat. Nos. 6,814,733, 6,997,924, and 7,156,816, which are herein incorporated by reference. 
         [0044]    In one embodiment, the positioning subsystem comprises a magnetic position tracking arrangement that determines the position and orientation of the catheter  14  by generating magnetic fields in a predefined working volume and sensing these fields at the catheter, using field generating coils  28 . The positioning subsystem is described in U.S. Pat. No. 7,756,576, which is hereby incorporated by reference, and in the above-noted U.S. Pat. No. 7,536,218. 
         [0045]    As noted above, the catheter  14  is coupled to the console  24 , which enables the operator  16  to observe and regulate the functions of the catheter  14 . Console  24  includes a position processor  27 , preferably a computer with appropriate signal processing circuits. The processor is coupled to drive a monitor  29 . The signal processing circuits typically receive, amplify, filter and digitize signals from the catheter  14 , including signals generated by sensors such as electrical, temperature and contact force sensors, and a plurality of location sensing electrodes (not shown) located distally in the catheter  14 . The digitized signals are received and used by the console  24  and the positioning system to compute the position and orientation of the catheter  14 , and to analyze the electrical signals from the electrodes. 
         [0046]    In order to generate electroanatomic maps, the processor  22  typically comprises an electroanatomic map generator, an image registration program, an image or data analysis program and a graphical user interface configured to present graphical information on the monitor  29 . 
         [0047]    Typically, the system  10  includes other elements, which are not shown in the figures for the sake of simplicity. For example, the system  10  may include an electrocardiogram (ECG) monitor, coupled to receive signals from one or more body surface electrodes, in order to provide an ECG synchronization signal to the console  24 . As mentioned above, the system  10  typically also includes a reference position sensor, either on an externally-applied reference patch attached to the exterior of the subject&#39;s body, or on an internally-placed catheter, which is inserted into the heart  12  maintained in a fixed position relative to the heart  12 . Conventional pumps and lines for circulating liquids through the catheter  14  for cooling the ablation site are provided. The system  10  may receive image data from an external imaging modality, such as an MRI unit or the like and includes image processors that can be incorporated in or invoked by the processor  22  for generating and, displaying images. 
         [0048]    Reference is now made to  FIG. 2 , which is a schematic pictorial view of catheter  14  in sheath  40 , according to an embodiment of the invention. The catheter  14  is typically a hybrid type, having electrodes  31  for impedance-based position sensing and a tri-coil magnetic sensor  58 . Proximal end  46  of the catheter  14  and a proximal end  48  of the sheath  40  are both able to be manipulated by operator  16  ( FIG. 1 ). By manipulation of their proximal ends, operator  16  is able adjust the position of sheath tip  45  relative to distal end  52  of the catheter  14 , so that the sheath  40  encloses a portion of the catheter  14 . 
         [0049]    In one mode of operation, the operator first inserts sheath  40  percutaneously into the patient body and guides sheath tip  45  to the target region. The operator then inserts the catheter  14  into the lumen of the sheath  40  and guides distal end  52  through the sheath  40  to the target region (not shown). The sheath  40  may be provided with electrodes  42  for impedance-based position sensing. In such impedance-based systems, a processor in the console  24  generates and then measures a plurality of currents between at least of the electrodes  42  and a plurality of body surface electrodes, fixed at respective locations on a surface of the patient body. The processor then computes a plurality of impedances based on the measured currents and detects the position of the sheath using the computed impedances as taught in the above-noted U.S. Pat. No. 7,536,218. 
         [0050]    While the catheter  14  is en route through the sheath  40 , the sheath tip  45  may be conventionally located using imaging modalities, such as fluoroscopy, although such procedures may expose the patient to radiation. 
         [0051]    After navigating the distal end  52  of the catheter  14  through the sheath  40 , a current position map (CPM) is generated during the catheterization session during the catheterization session using impedance measurements obtained via the catheter electrodes  31  and readings from the magnetic sensor  58 . One method for generation of a CPM is described in commonly assigned U.S. Pat. No. 8,478,383 to Bar-Tal et al., which is herein incorporated by reference. 
         [0052]    In catheters having a magnetic sensor and multiple ACL electrodes, with known inter-electrode distances and offsets from the sensor, the location of all the catheter ACL electrodes can be evaluated at any time during insertion of the catheter. However, as to the sheath, until the CPM is available the impedance information provided by the sheath ACL electrodes  42  is insufficient to define the location of the sheath tip  45  with respect to physical landmarks in the heart with the precision required by a typical medical procedure. However, stability of distal portion  60  of the sheath can be assessed by monitoring readings of the sheath ACL electrodes. 
         [0053]    It is helpful to know the precise position of sheath tip  45  in the heart. As noted above, an out-of-sheath status is required for construction of a CPM. Conventionally, however, without the CPM, it is necessary to resort to fluoroscopy or other imaging in order to obtain the position and conform the out-of-sheath status. It is desirable for the operator to have an indication of the position of catheter tip  56  relative to sheath tip  45  in order to avoid evaluating possibly invalid ACL data while the catheter is still inside the sheath as well as to acquire valid CPM data once the operator is assured that the catheter has exited the sheath. 
         [0054]    Reference is now made to  FIG. 3 , which is a schematic diagram illustrating a catheter  62  having electrodes extending through a sheath  64  in accordance with an embodiment of the invention. The catheter  62  in this example has three ACL electrodes  66 ,  68 ,  70  and a tip electrode  72  that is used for ablation. The catheter  62  may be provided with additional ACL electrodes (not shown). The tip electrode  72  and the electrodes  66 ,  68 ,  70  have all been extended beyond sheath tip  74 , and in this configuration a CPM may be validly constructed. 
         [0055]    Reference is now made to  FIG. 4 , which is a schematic diagram illustrating catheter  62  lying partially within sheath  64  and indicating the position of catheter tip  76  relative to sheath tip  74  in accordance with an embodiment of the invention. During ablation, the physical stiffness of the sheath  64  is needed for better catheter maneuverability. In such a situation the operator may keep one or two of the most proximal ACL electrodes  66 ,  68  in the sheath, while the distal ACL electrode  70  and any other distal ACL electrodes (not shown) are out of the sheath. The position of the distal end of the catheter, here occupied by tip electrode  72 , can be computed based on the known offsets from any of the electrodes  66 ,  68 ,  70 . The presence of a short portion  78  of the catheter  62  out of the sheath enhances catheter maneuverability and stability because the sheath  64  is more rigid than the catheter  62 . At the same time maintain the portion  78  outside the sheath  64  guarantees that the tip electrode  72  is also out of the sheath  64  and does not transfer energy into the sheath  64 . When manipulating the catheter  62 , the operator may view an indication on the monitor  29  ( FIG. 1 ) in order to achieve the desired configuration. 
         [0056]    The inventors have found that values of the catheter ACL electrodes currents provide discrimination between an in-sheath and an out-of-sheath status of the catheter. Several parameters influence the values:
       Electrical Impedance of human heart;   Impedance measuring circuitry, e.g., the ACL electrical circuitry of the CARTO 3 system;   Sheath material (including its insulative properties); and   Sheath structure: Diameter; presence or absence of perforations; placement of perforations relative to the sheath end.       
 
         [0061]    The heart may be treated as an ambient with resistance of 100-150 ohms. Under such resistance the ACL circuits yield a nominal current of 500 microamperes. When the electrode experiences a higher resistance while in a known position within the sheath, it may be assumed that some of the current is leaking via other pathways. 
         [0062]    Without being bound by a particular theory, it would seem that when the electrode is inserted into the sheath, it is surrounded by insulative material, i.e., the sheath, and by conductive fluid but with relatively little blood. The conductive fluid can be considered to form a cylinder around the catheter. An electrical description of the model includes parasitic impedance from the sheath and from the electrical circuitry of the system. As the catheter is taken deeper into the sheath, the proportion of blood relative to the conductive fluid increases, and the parasitic impedance begins to approach the impedance of the ambient blood. Thus, some current leaks via the parasitic impedance path of the ACL electrical circuitry. Finally, as the catheter is taken even farther into the sheath, the current reading from the catheter ACL electrodes does not change as all the current follows the parasitic impedance path. 
         [0063]    Reference is now made to  FIG. 5 , which is an electrical schematic of a circuit  80  modeling the impedance and current of an ACL electrode in accordance with an embodiment of the invention. The circuit  80  comprises an alternating current generator  82  that produces a voltage V. The load of the generator  82  is a capacitor  84  having an impedance Z p  in parallel with a resistor  86  having an impedance Z e . 
         [0064]    The impedance of the in-sheath electrode is given by: 
         [0000]    
       
         
           
             
               
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         [0000]    where 
         [0065]    R 1  is the sheath radius; 
         [0066]    R 2  is the catheter radius; 
         [0067]    ρ is the specific impedance of the heart; 
         [0068]    l is the distance of the electrode from the sheath end; 
         [0069]    l 0  is the distance from the electrode to the most distal electrode; 
         [0070]    Z is the total system impedance; 
         [0071]    Z e  is the ACL electrode impedance; and 
         [0072]    Z p  is the parasitic impedance of the system. 
       EXAMPLE 1 
       [0073]    Experiments were performed by measuring the in-sheath and out-of-sheath currents. An aquarium was used to as a patient-simulant. The tests employed Navistar® and Thermocool® SF (7.5 F) catheters in Agilis™ (8.5 F) and MobiCath™ (8 F) sheaths. The catheters were moved in and out of the sheath in a controlled manner and the respective currents were recorded. 
         [0074]    Reference is now made to  FIG. 6 , which is a plot of ACL electrode current obtained while the electrode transitions from an in-sheath to an out-of-sheath status accordance with an embodiment of the invention. The plot shows three zones of interest: (1) an in-sheath condition; (2) a transient position, in which the electrode is at the end of the sheath or partially out of the sheath; and (3) an out-of-sheath condition, in which the electrode is fully out of the sheath. Thus, the sheath is defined and identified by the historical data of the readings of the magnetic sensor during its movements. 
         [0075]    A threshold value of 400 microamperes differentiates an in-sheath form an out-of-sheath condition, based on an accumulated database of recordings. Reference is now made to  FIG. 7 , which is a plot showing a distribution of out-of-sheath currents, in accordance with an embodiment of the invention. Each bar stands for the lowest value for all catheter electrodes while the electrodes are out-of-sheath. The current distribution can be fitted to a half Gaussian form (the nominal value is around 500 microamperes.). There is a non-zero probability in the Gaussian tail (not shown in  FIG. 7 ) that currents as low as 400 microamperes and somewhat lower still represent out of sheath currents. 
         [0076]    In the following examples a perforation or hole in the wall of the sheath serves as a zero point, as the hole, being a break in the insulation of the sheath, provides a highly conductive path for the electrode current. One function of the hole is to maintain the sheath clear of obstruction. Passage of an electrode by the hole produces current readings almost as high as those in an out-of-sheath condition. Nevertheless, because the distance between the hole and the sheath tip, and the offsets of the catheter tip from the electrodes are all known, the changes in current readings when the catheter electrode is abeam of the hole can be exploited to localize the catheter tip with respect to the sheath tip. 
         [0077]    Reference is now made to  FIG. 8 , which is a plot of ACL electrode current as a function of retraction of the catheter from an out-of-sheath condition to an in-sheath condition into a sheath in accordance with an embodiment of the invention. In this experiment a Thermocool SF (7.5 F) catheter was navigated through two different Agilis (8.5 F) sheaths. The two sets of data that were acquired are shown as open and closed triangles. 
         [0078]    Reference is now made to  FIG. 9 , which is a plot similar to  FIG. 8 , in accordance with an embodiment of the invention. In this example a Thermocool SF (7.5 F) catheter was navigated through a MobiCath (8 F) sheath. Two sets of data were acquired. It is evident that the observations are highly reproducible. 
         [0079]    Reference is now made to  FIG. 10 , which is a plot comparing the results of the experiments using the Agilis (8.5 F) sheath ( FIG. 8 ) and the MobiCath (8 F) ( FIG. 9 ) in accordance with an embodiment of the invention. In all cases there is a relatively rapid decline (≧50%) in the first 10 mm. The decline is more rapid in the narrower MobiCath (8 F) sheath. The measurements showed good agreement with the model represented by the impedance equations given above. The different sheath-catheter combinations produce characteristic curves. Analysis of the curves by known discrimination techniques define thresholds that optimally discriminate among in-sheath and out-of-sheath conditions and the transition zone noted above. A database of such sheath-catheter combinations may be accumulated and referenced at patient sessions. 
         [0080]    Reference is now made to  FIG. 11 , which are two plots taken from a multi-electrode catheter as the proximal electrode is retracted into a sheath having a perforation 5 mm proximal to its distal end. The tracings show a decline at the 2 mm mark. Allowing for the width of the proximal electrode, the edge of the electrode is at 4 mm, at which point the effect of the perforation begins to be seen. Beyond the 5 mm mark the current declines. By 6 mm the electrode has passed the hole and the current has declined more than 100 μa from the baseline. At 6 mm a pre-defined threshold (400 μa) will have been crossed, after which there is a true in-sheath indication. The current decline at 2 mm can be differentiated from a true transition between an in-sheath and an out-of-sheath condition by (1) its transient nature and (2) by the level of the minimum, which exceeds the threshold value. 
       EXAMPLE 2 
       [0081]    Reference is now made to  FIG. 12 , which is a schematic illustration of a test arrangement for determination of electrode currents in ACL electrodes in accordance with an embodiment of the invention. In an initial position, tip  90  of catheter  92  lies at distal end  94  of sheath  96 . A hole  98  in the sheath  96  is placed 4 mm from its distal end  94 . The catheter  92  has four electrodes  100 ,  102 ,  104 ,  106 , which are 1 mm wide. 
         [0082]    The hole  98  enables conductive fluid, blood or salt water in the case of the test arrangement, to be in contact with the electrodes  100 ,  102 ,  104 ,  106  when they are within the sheath and generally opposite the hole  98 . The electrodes  100 ,  102 ,  104 ,  106  are positioned such that in the tests, a true in-sheath indication occurs only when the distal edge  108  of electrode  100 , i.e., the most proximal of the electrodes  100 ,  102 ,  104 ,  106 , is at least 1 mm from the hole  98  in the proximal direction. As that point the current in the electrode  100  begins to rise above or is falling below the in-sheath threshold, according to whether the electrode  100  is approaching or receding from the hole  98 , respectively. The electrodes  100 ,  102 ,  104 ,  106  are positioned on the catheter  92  such that, the most proximal electrode, electrode  100 , functions as a guard: During retraction of the catheter  92 , when electrode  100  passes by the hole  98  and its current begins to decline, an out-of-sheath condition cannot no longer be confirmed. Similarly, when advancing the catheter  92 , a transient decline in current in the distal electrode  106  indicates that an in-sheath condition is no longer certain, as the offset between the electrode  106  and the tip  90  is such that the tip  90  is beginning to protrude through the distal end  94  of the sheath  96 . At this time the location of the tip  90  and hence the location of the distal end  94  of the sheath  96  can be determined using the information from a magnetic sensor (e.g., magnetic sensor  58  as shown in  FIG. 2 ) of the catheter  92 . 
         [0083]    In a perforated sheath, during movement of a catheter electrode into the sheath, a definitive drop in electrode current from a baseline (an out-of-sheath indication) occurs only after the electrode has crossed the hole. 
         [0084]    In summary, in a perforated sheath, where the location of the hole is known, two points on the sheath can be determined for each catheter electrode:
       the sheath end where the current rises to the steady out-of-sheath state and remains steady regardless of further distal movement of the electrode; and   the location of the hole when retracting the catheter when the electrode crosses the hole, and current has declined below the in-sheath threshold.       
 
         [0087]    Further inside the sheath, the electrode location can be calculated based on the above-described reference points. In the example of  FIG. 12 , there is a 4 mm interval of relative uncertainty between the sheath end and the hole regarding the location of a catheter electrode. However, such an electrode can still be located based on information obtained from other electrodes. 
         [0088]    Reference is now made to  FIG. 13 , which is a schematic diagram of a lasso catheter  110 , which has been inserted through a sheath  112  in accordance with an embodiment of the invention. The lasso catheter  110  is provided with mapping electrodes  114  for building the CPM. A proximal guard electrode  116  and a distal guard electrode  118  are located on the stem of the lasso catheter  110 . If it is desired only to build a CPM. It is only necessary to advance the catheter such that both the proximal and distal electrodes are positioned outside the sheath by reference only to a threshold current value. The catheter is advanced until an out-of-sheath condition is signified by the current levels in both guard electrodes  116 ,  118 . 
         [0089]    It will be recalled that mechanical stability of the catheter is important when ablation is to be performed. The following procedure may be applied, mutatis mutandis, to perforated and non-perforated sheaths by appropriate positioning of the ACL electrodes. Reference is now made to  FIG. 14 , which is a flow chart for determining an optimum location of a catheter being inserted through a sheath in accordance with an embodiment of the invention. In this embodiment the guard electrode  118  is in an out-of-sheath condition, while the guard electrode  116  remains within the shield. Once positioned, the catheter is to be used for ablation of a target. The method is presented with reference to the example of  FIG. 13 . 
         [0090]    At initial step  120  the sheath is placed conventionally into the heart. Upper and lower threshold current values are chosen according to data previously accumulated for the particular sheath and catheter being used. For example, the upper and lower threshold current values could be 400 and 250 microamperes, respectively. These values discriminate between definite in-sheath and out-of-sheath conditions and define a transitional zone therebetween. 
         [0091]    During the following procedure, when current in a suitably positioned ACL electrode exceeds the upper threshold, the electrode is determined to be outside the sheath. Otherwise the electrode may be inside the sheath or in a transitional zone, as explained above. When the ACL electrode current is below the lower threshold, the electrode is determined to be inside the sheath. In this and subsequent flow charts some steps may be performed in different orders and not all illustrated process steps may be required to implement the method. 
         [0092]    Next, at step  122  the catheter is advanced within the sheath. Typically, this is done in small increments when the catheter approaches the distal end of the sheath. 
         [0093]    At decision step  124 , it is determined if the current in the distal electrode (guard electrode  118 ;  FIG. 13 ) exceeds the upper threshold. If the determination is negative, then the distal electrode has not yet exited the sheath. Control returns to step  122 . 
         [0094]    If the determination at decision step  124  is affirmative, then at decision step  126 , it is determined if the current in the proximal electrode (guard electrode  116 ;  FIG. 13 ) has risen above the lower threshold. If the determination is affirmative, then the catheter was advanced too far in the last performance of step  122 . Control then proceeds to step  128 . The catheter is retracted somewhat, typically less than its last advancement in step  122 . Control returns to decision step  124  to redetermine its position. 
         [0095]    If the determination at decision step  126  is negative, then the proximal electrode remains within the sheath while the distal electrode is outside the sheath. This configuration is appropriate for performing ablation. Control proceeds to final step  130 , and the procedure ends. 
         [0096]    The method of  FIG. 14  may be modified for application to a multielectrode catheter when determining an optimum location of a catheter tip with respect to the sheath tip. For some ablation procedures the tolerances must be tightly controlled. For example when using a SmartTouch™ catheter, which has a contact force sensor, ACL electrodes are appropriately located with respect to the sensor in order to adjust the catheter&#39;s position such that the spring of the sensor is fully out of the sheath, while the proximal electrode is at a marginal position that still gives an in-sheath indication. For such applications a value of the upper threshold, such as 400 microamperes, may be chosen. 
         [0097]    The procedure is performed by measuring the currents in all the electrodes concurrently, and adjusting the position of the catheter such that the current in a selected electrode, having a known offset to the tip of the catheter, is within the dynamic segment of the curve, while the current in more distal electrodes provide an out-of-sheath indication and more proximal electrodes provide an in-sheath indication. For example, referring to  FIG. 8 , the current value in the selected electrode should be between 250 and 400 μa in order to guarantee that the tip of the catheter protrudes through the sheath, but not more than a specified distance. 
         [0098]    It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.