Abstract:
Apparatus, including a sheath, consisting of a lumen having a sheath distal end which is configured to be inserted into a human patient. A magnetic structure is fixedly attached to the sheath distal end. The apparatus includes a probe, having a probe distal end which is configured to be inserted through the lumen into the human patient. The probe includes a magnetic transducer which is disposed in the probe distal end and which is configured to generate a signal in response to a magnetic field. The apparatus further includes a processor which is configured to sense a change in the signal due to proximity of the magnetic structure to the transducer.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to invasive medical procedures, and specifically to invasive insertion of a probe into a sheath guiding the probe. 
       BACKGROUND OF THE INVENTION 
       [0002]    In a number of medical procedures wherein a probe is inserted into a patient, the probe is inserted 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 undesired overlap of the sheath distal end relative to the probe distal end. 
         [0003]    Documents incorporated by reference in the present patent application are to be considered an integral part of the application except that to the extent any terms are defined in these incorporated documents in a manner that conflicts with the definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered. 
       SUMMARY OF THE INVENTION 
       [0004]    An embodiment of the present invention provides apparatus, including: 
         [0005]    a sheath, consisting of a lumen, having a sheath distal end which is configured to be inserted into a human patient; 
         [0006]    a magnetic structure fixedly attached to the sheath distal end; 
         [0007]    a probe, having a probe distal end which is configured to be inserted through the lumen into the human patient, the probe including a magnetic transducer which is disposed in the probe distal end and which is configured to generate a signal in response to a magnetic field; and 
         [0008]    a processor which is configured to sense a change in the signal due to proximity of the magnetic structure to the transducer. 
         [0009]    In a disclosed embodiment the magnetic structure includes a paramagnetic material. 
         [0010]    In an alternative disclosed embodiment the magnetic transducer includes a coil, and the change in the signal is in response to a change in inductance of the coil. The inductance may include a self-inductance of the coil, and the magnetic field may be generated by the coil. 
         [0011]    In some embodiments the magnetic transducer includes a further coil, and the inductance includes a mutual inductance between the coil and the further coil. The magnetic field may be generated by the further coil. 
         [0012]    In a further disclosed embodiment the apparatus includes a force sensor located in the probe distal end and configured to provide an indication of a force on the probe distal end, and the magnetic transducer is an operative component of the force sensor. 
         [0013]    In a yet further disclosed embodiment the apparatus includes a position sensor located in the probe distal end and configured to provide an indication of a position of the probe distal end, and the magnetic transducer is an operative component of the position sensor. 
         [0014]    Typically, the processor is configured to estimate a distance of the probe distal end from the sheath distal end in response to the change in the signal. 
         [0015]    In an alternative embodiment the magnetic structure includes a closed conductive coil. 
         [0016]    There is also provided, according to a further embodiment of the present invention, a method, including: 
         [0017]    providing a sheath, including a lumen, having a sheath distal end which is configured to be inserted into a human patient; 
         [0018]    fixedly attaching a magnetic structure to the sheath distal end; 
         [0019]    inserting a probe, having a probe distal end, through the lumen into the human patient, the probe including a magnetic transducer which is disposed in the probe distal end and which is configured to generate a signal in response to a magnetic field; and 
         [0020]    sensing a change in the signal due to proximity of the magnetic structure to the transducer. 
         [0021]    The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a schematic, pictorial illustration of a system for locating the termination of a probe sheath, according to an embodiment of the present invention; 
           [0023]      FIG. 2  is a schematic, pictorial view of a catheter in a sheath, according to an embodiment of the present invention; 
           [0024]      FIG. 3  is a schematic, sectional view of a distal end of the catheter, according to an embodiment of the present invention; 
           [0025]      FIG. 4  is a schematic, sectional view of the catheter distal end and of a sheath distal end, according to an embodiment of the present invention; and 
           [0026]      FIG. 5  is a flowchart describing steps for locating a sheath termination with respect to the catheter distal end, according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
       [0027]    An embodiment of the present invention provides a system for determining the location of the distal end of a probe in relation to the distal end of a sheath being used to guide the probe. The system is typically used in an invasive medical procedure, wherein the distal ends of the probe and the sheath are not accessible, and allows an operator of the system to accurately determine the protrusion of the probe distal end from the sheath distal end. Such a determination, for example, enables the operator to ensure that electrodes on the probe distal end are not obscured by the sheath. 
         [0028]    The probe distal end comprises at least one magnetic transducer, which may be an operative component of a force sensor or of a position sensor located in the probe distal end. The at least one magnetic transducer generates a signal in response to a magnetic field impinging on the transducer. 
         [0029]    A magnetic structure, typically formed of a paramagnetic material and/or in the shape of a closed conductive coil, is fixed to the sheath distal end. The magnetic structure alters the inductance of the transducers, i.e., the self-inductance of each transducer, as well as the mutual inductance between transducers, depending on the proximity of the sheath distal end to the probe distal end. 
         [0030]    The change in inductance causes the signal generated by the magnetic field acting on each transducer to change, and the change is measured by a processor. The processor may use the change to estimate the position of the probe distal end relative to the sheath distal end, and thus determine the protrusion of the probe distal end from the sheath distal end. 
       Detailed Description 
       [0031]      FIG. 1  is a schematic, pictorial illustration of a system  20  for locating the termination of a probe sheath, according to an embodiment of the present invention. System  20  may be based, for example, on the CARTO™ system, produced by Biosense Webster Inc. (Diamond Bar, Calif.). This system comprises an invasive probe in the form of a catheter  28  and a control console  34 . In the embodiment described hereinbelow, it is assumed that catheter  28  is used in ablating endocardial tissue using radiofrequency (RF) energy, as is known in the art. Alternatively, the catheter may be used, mutatis mutandis, for other therapeutic and/or diagnostic purposes in the heart or in other body organs. 
         [0032]    An operator  26 , such as a cardiologist, inserts catheter  28  through the vascular system of a patient  24  so that a probe distal end  30  of the catheter enters a chamber of the patient&#39;s heart  22 . In order to effectively strengthen the catheter, the cardiologist positions the catheter within a sheath  40 , which terminates in a sheath distal end  44 . Typically, and as assumed in the description herein, the sheath is inserted into the vascular system of the patient before insertion of the catheter into the sheath. Sheath  40  is typically constructed from a reinforced fluoroplastic having a low magnetic susceptibility, for example, the sheath may be formed from polytetrafluoroethylene (PTFE) braided with stainless steel. 
         [0033]    The operator advances the catheter so that the distal tip of the catheter engages endocardial tissue at a desired location or locations. Catheter  28  is typically connected by a suitable connector at its proximal end to console  34 . The console may comprise a radio frequency (RF) generator, which supplies high-frequency electrical energy via the catheter for ablating tissue in the heart at the locations engaged by the distal tip. Alternatively or additionally, the catheter and system may be configured to perform other therapeutic and diagnostic procedures that are known in the art. 
         [0034]    Console  34  may use magnetic position sensing to determine position coordinates of distal end  30  of catheter  28  inside heart  22 . For this purpose, a driver circuit  38  in console  34  drives field generators  32  to generate magnetic fields in the vicinity of the body of patient  24 . Typically, the field generators comprise coils, which are placed below the patient&#39;s torso at known positions external to the patient. These coils generate magnetic fields within the body in a predefined working volume that contains heart  22 . A magnetic field sensor within distal end  30  of catheter  28  (shown in  FIGS. 3 and 4 ) generates electrical signals in response to these magnetic fields. A signal processor  36  processes these signals in order to determine the position coordinates of the distal end, the position coordinates typically including both location and orientation coordinates. This method of position sensing is implemented in the above-mentioned CARTO system and is described in detail in U.S. Pat. Nos. 5,391,199, 6,690,963, 6,484,118, 6,239,724, 6,618,612 and 6,332,089, whose disclosures are all incorporated herein by reference. 
         [0035]    Alternatively or additionally, console  34  may use other methods known in the art to determine the position coordinates of distal end  30 , such methods including, for example, measurements of impedances between the distal end and electrodes on the skin of patient  24 . U.S. patent applications Ser. No. 11/182,272 filed Jul. 15, 2005 (now issued U.S. Pat. No. 7,536,218) and Ser. No. 12/556,639 filed Sep. 10, 2009 (U.S. Patent Publication No. 2010/0079158), whose disclosures are incorporated herein by reference, describe methods of position sensing using both magnetic field generators and impedance measurements. 
         [0036]    Processor  36  typically comprises a general-purpose computer, with suitable front end and interface circuits for receiving signals from catheter  28  and controlling the other components of console  34 . The processor may be programmed in software to carry out the functions that are described herein. The software may be downloaded to console  34  in electronic form, over a network, for example, or it may be provided on non-transitory tangible media, such as optical, magnetic or electronic memory media. Alternatively, some or all of the functions of processor  36  may be carried out by dedicated or programmable digital hardware components. Based on the signals received from the catheter and other components of system  20 , processor  36  drives a display  42  to give operator  26  visual feedback regarding the position of distal end  30  in the patient&#39;s body, and the relation of probe distal end  30  to sheath distal end  44 . Display  42  may also provide status information and guidance regarding the procedure that is in progress. 
         [0037]      FIG. 2  is a schematic, pictorial view of catheter  28  in sheath  40 , according to an embodiment of the present invention. A proximal end  46  of the catheter and a proximal end  48  of the sheath are both able to be manipulated by operator  26 , by virtue of being outside the body of patient  24 . One or more generally similar ring-like electrodes  31  are mounted on and encircle distal end  30  of the catheter. Herein by way of example there are assumed to be two electrodes  31 . In addition, an electrode  56  is formed on a distal tip  52  of catheter  28 . A magnetic structure  45 , described in more detail below with reference to  FIG. 4 , is mounted on sheath distal end  44 , in proximity to the sheath distal end termination. As illustrated in  FIG. 2 , and as explained in more detail with respect to  FIGS. 3 ,  4 , and  5 , by manipulation of their proximal ends, operator  26  is able adjust the position of sheath distal end  44  relative to probe distal end  30  so that the sheath encloses a portion of the probe, so as to strengthen catheter  28  while not covering electrodes  31  and  56 . 
         [0038]      FIG. 3  is a schematic, sectional view of distal end  30  of catheter  28 , showing details of the structure of a force sensor  65  at the distal end, according to an embodiment of the present invention. For clarity in the following description, distal end  30  is assumed to be generally cylindrical, and is assumed to define an orthogonal set of axes, with the cylindrical axis being parallel to the z-axis. The catheter comprises an insertion tube  50 , which is typically inserted into the heart percutaneously through a blood vessel, such as the vena cava or the aorta. Typically, electrode  56  on distal tip  52  of the catheter engages endocardial tissue (for simplicity the tissue is not shown in the diagram). Alternatively or additionally, at least some of electrodes  31  may contact the endocardial tissue. 
         [0039]    Insertion tube  50  is connected to distal tip  52  by an elastic joint  54  which comprises a resilient coupling member  60 . In an exemplary embodiment, the coupling member has the form of a tubular piece of an elastic material, with a helical cut along a portion of its length. For example, the coupling member may comprise a superelastic alloy, such as nickel titanium (Nitinol). The helical cut causes the tubular piece to behave like a spring in response to forces exerted on distal tip  52 . Further details regarding the fabrication and characteristics of this sort of coupling member are presented in U.S. patent applications Ser. No. 12/134,592 filed Jun. 6, 2008 (will issue on May 15, 2012 as U.S. Pat. No. 8,180,431) and Ser. No. 12/327,226 filed on Dec. 3, 2008 (U.S. Publication No. 2009/0138007) whose disclosures are incorporated herein by reference. 
         [0040]    Alternatively, the coupling member has a plurality of helical cuts in a portion of its length, such as is described in U.S. patent application Ser. No. 12/627,327 filed on Nov. 30, 2009 (U.S. Publication No. 2011/0130648), whose disclosure is incorporated herein by reference. Further alternatively, the coupling member may comprise a coil spring or any other suitable sort of resilient component with the desired flexibility and strength characteristics. 
         [0041]    The insertion tube is covered by a flexible, insulating material  62 , such as Celcon®, Teflon®, or heat-resistant polyurethane, for example. The area of joint  54  is covered, as well, by a flexible, insulating material, which may be the same as material  62  or may be specially adapted to permit unimpeded bending and compression of the joint. (This material is shown cut away in  FIG. 3  in order to expose the internal structure of the catheter.) Distal tip  52  may be covered, at least in part, by electrode  56 , and material  62  may be encircled by electrodes  31 . Electrodes  31  and  56  are typically made of a conductive material, such as a platinum/iridium alloy. Alternatively, other suitable materials may be used, as will be apparent to those skilled in the art. Further alternatively, for some applications, the distal tip may be made without a covering electrode, and/or electrodes  31  may not be present. The distal tip is typically relatively rigid, by comparison with the flexible insertion tube. 
         [0042]    Coupling member  60  is configured to permit axial displacement (i.e., lateral movement along the z-axis) and angular deflection of the distal tip in x and/or y directions, in proportion to the force on the tip. Measurement of the displacement and deflection by processor  36  thus gives an indication of the force on the tip. The force indication may be used by the operator of catheter  20  is ensuring that the distal tip is pressing against the endocardium firmly enough to give the desired therapeutic or diagnostic result, but not so hard as to cause undesired tissue damage. 
         [0043]    A joint sensing assembly  63 , comprising coils  64 ,  66 ,  68  and  70  within catheter  28 , provides accurate reading of the position of distal tip  52  relative to the distal end of insertion tube  50 , including axial displacement and angular deflection. These coils are one type of magnetic transducer that may be used in embodiments of the present invention, and act as operative components of force sensor  65 , as explained below. A “magnetic transducer,” in the context of the present patent application and in the claims, means a device that generates a magnetic field in response to an applied electrical current and/or outputs an electrical signal in response to an applied magnetic field. Although the embodiments described herein use coils as magnetic transducers, other types of magnetic transducers may be used in alternative embodiments, as will be apparent to those skilled in the art. 
         [0044]    The coils in assembly  63  are divided between two subassemblies on opposite sides of joint  54 : One subassembly comprises coil  64 , which is driven by a current via a cable  74  from console  34  to generate a magnetic field. This field is received by a second subassembly, comprising coils  66 ,  68  and  70 , which are located in a section of the catheter that is spaced axially apart from coil  64 . (The term “axial,” as used in the context of the present patent application and in the claims, refers to the direction of the longitudinal axis of distal end  30  of catheter  28 , which is identified as the z-direction in  FIG. 3 . An axial plane is a plane perpendicular to this longitudinal axis, and an axial section is a portion of the catheter contained between two axial planes.) Coils  66 ,  68  and  70  generate electrical signals in response to the magnetic field generated by coil  64 . These signals are conveyed by cable  74  to processor  36 , which processes the signals in order to measure the axial displacement and angular deflection of joint  54 . 
         [0045]    Coils  66 ,  68  and  70  are fixed in catheter  28  at different radial locations. (The term “radial” refers to coordinates relative to the catheter axis, i.e., coordinates in an X-Y plane in  FIG. 3 .) Specifically, in this embodiment, coils  66 ,  68  and  70  are all located in the same axial plane at different azimuthal angles about the catheter axis. For example, the three coils may be spaced azimuthally 120° apart at the same radial distance from the axis. 
         [0046]    The axes of coils  64 ,  66 ,  68  and  70  are parallel to the catheter axis (and thus to one another, as long as joint  54  is undeflected). Consequently, coils  66 ,  68  and  70  will output strong signals in response to the field generated by coil  64 , and the signals will vary strongly with the distances of coils  66 ,  68  and  70  from coil  64 . (Alternatively, the axis of coil  64  and/or coils  66 ,  68  and  70  may be angled relative to the catheter axis, as long as the coil axes have a sufficient parallel component in order to give substantial signals.) Angular deflection of tip  52  will give rise to a differential change in the signals output by coils  66 ,  68  and  70 , depending on the direction and magnitude of deflection, since one or two of these coils will move relatively closer to coil  64 . Compressive displacement of the tip will give rise to an increase in the signals from all of coils  66 ,  68  and  70 . 
         [0047]    Processor  36  analyzes the signals output by coils  66 ,  68  and  70  in order to measure the deflection and displacement of joint  54 . The sum of the changes in the signals gives a measure of the compression, while the difference of the changes gives a measure of the deflection. The vector direction of the difference gives an indication of the bend direction. A suitable calibration procedure may be used to measure the precise dependence of the signals on deflection and displacement of the joint. 
         [0048]    Various other configurations of the coils in the sensing subassemblies may also be used, in addition to the configuration shown and described above. For example, the positions of the subassemblies may be reversed, so that the field generator coil is on the proximal side of joint  54 , and the sensor coils are in the distal tip. As another alternative, coils  66 ,  68  and  70  may be driven as field generators (using time- and/or frequency-multiplexing to distinguish the fields), while coil  64  serves as the sensor. The sizes and numbers of the coils in  FIG. 3  are shown only by way of example, and larger or smaller numbers of coils may similarly be used, in various different positions. Typically, one of the subassemblies comprises at least two coils, in different radial positions, to allow differential measurement of joint deflection. However, in some embodiments measuring joint compression, each subassembly comprises only one coil. 
         [0049]    Joint sensing assembly  63 , together with elastic joint  54 , acts as force sensor  65 , being able to measure both the direction and the magnitude of the force acting on tip  52 . Force sensor  65  may also act as a pressure sensor, assuming that an area of tip  52  to which the force is applied is known or can be estimated. 
         [0050]    As described below, in a force sensor part of a prior calibration of the force sensor, processor  36  determines a relation between the force on tip  52  and the coil signals of the assembly, the coil signals indicating movement of joint  54 . Another part of the calibration, relating the position of sheath  40  to the sensing assembly is also described below, with reference to  FIG. 4 . 
         [0051]    One or more of coils  64 ,  66 ,  68  and  70  may also be used to output signals in response to the magnetic fields generated by field generators  32 , and thus serve as position sensing coils. Processor  36  processes these signals in order to determine the coordinates (position and orientation) of distal end  30  in the external frame of reference that is defined by the field generators. Additionally or alternatively, one or more further coils (or other magnetic sensors) acting as operative components of a position sensor  76  may be deployed in the distal end of the catheter for this purpose. The position sensing coils in distal end  30  of catheter  28  enable console  34  to output both the location and orientation of the catheter in the body and the displacement and deflection of tip  52 , as well as the force on the tip. 
         [0052]      FIG. 4  is a schematic, sectional view of probe distal end  30  and of sheath distal end  44 , according to an embodiment of the present invention. Sheath distal end  44  of sheath  40  terminates at a sheath termination  80 , and magnetic structure  45  is mounted on the sheath distal end in proximity to the termination, typically at the termination. In one embodiment structure  45  is in the form of a closed conductive coil or ring that encircles the sheath at its distal end, so that the structure interacts with a magnetic field generated by or coupled to coils in the probe distal end, such as coils  66 ,  68 , and  70 . Alternatively, structure  45  may comprise one or more elements mounted on sheath distal end  44  that are galvanically insulated from each other. Typically, structure  45  is substantially symmetrical with respect to an axis  82  of the sheath. 
         [0053]    The material of structure  45  may be selected so that its magnetic properties cause it to interact with a magnetic field such as that described above. Typically the material is an inert, bio-compatible material that is paramagnetic, with a relatively large magnetic susceptibility. The material may comprise an element or a compound. In one embodiment, structure  45  is formed from platinum. 
         [0054]    The magnetic interaction between magnetic structure and the coils in the probe distal end changes the self-inductance of each coil, as well as the mutual inductance between the coils. For a given coil the change in self-inductance is a function of the distance between the structure and the coil, as well as of the relative orientation of the structure and the coil. For a given pair of coils, the change in mutual induction is a function of the distances between the structure and each of the coils, and of the relative orientations of the structure with each of the coils. Typically, the change in self-inductance or mutual inductance is relatively large if the axis of structure  45  is parallel to the axis of the coil or coils, and is relatively small if the axis of the structure is orthogonal to the axis of the coils. 
         [0055]    Thus, introduction of the distal sheath end, with its attached magnetic structure  45 , into the vicinity of the joint sensing assembly alters the self-inductance of each of the coils in the assembly, as well as the mutual inductance between each of the coils. The changes in the self-inductance and the mutual inductance are a function of the proximity of the magnetic structure to the coil or coils being considered. 
         [0056]    For coils such as coils  66 ,  68 , and  70  that are fixed in relation to each other in the distal end, a change in mutual inductance between any two of these coils may be used to measure proximity of structure  45 . Alternatively or additionally, a change in the self inductance of any of the coils in the distal end may be used to measure the proximity. The proximity may be quantified as a distance Δz of sheath termination  80  from an arbitrary point on the distal end. By way of example, and as illustrated in  FIG. 4 , distance Δz is assumed to be measured to the distal end of distal tip  52 . 
         [0057]    In a sheath location part of the calibration procedure referred to above, the change of inductance, self and/or mutual, for one or more of the coils at distal end  30  of the probe is measured for different values of distance Δz. The self-inductance for a given coil may be measured by injecting a signal of known amplitude and frequency into the coil, and determining an amplitude and/or a phase of the current generated. From the generated current and injected amplitude, and allowing for a DC resistance of the coil, the self-inductance may be determined. The mutual inductance for a given pair of coils may be found in a similar manner, by injecting a signal of known amplitude and frequency into a first coil of the pair, and determining the current produced in the second coil. 
         [0058]      FIG. 5  is a flowchart  100  describing steps for locating sheath termination  80  with respect to distal end  30 , according to an embodiment of the present invention. The description assumes the presence of a force sensor, with magnetic transducers, corresponding to joint sensing assembly  63 , wherein coil  64  acts as a magnetic field transmitter. Those having ordinary skill in the art will be able to adapt the description, mutatis mutandis, for the presence of magnetic transducers other than those of a force sensor, such as coordinate sensing coil  72 . 
         [0059]    In a first calibration step  102 , processor  36  implements the sheath location part of the calibration procedure, which by way of example, is assumed to use changes of mutual inductance. Probe  28  is inserted into sheath  40 , generally as illustrated in  FIG. 2 , but with distal ends  44  and  30  outside the body of patient  24  so that distance Δz may be independently measured. A sheath calibration signal of known amplitude and frequency, herein termed the sheath location frequency, is injected into coil  66 , and the processor measures the signals generated by coils  68  and  70 , caused respectively by the mutual inductance between the pair of coils  66  and  68 , and the pair of coils  66  and  70 . The processor records the changes in signals in coils  68  and  70  for different values of distance Δz of the sheath termination, and forms a sheath location calibration relationship, typically using interpolation, between the signal changes and distances Δz. The processor stores the calibration relationship for use during a procedure involving sheath and probe  28 . The changes in signals are typically calculated as differences from the signals in coils  68  and  70  when the sheath termination is not close to joint sensing assembly  63 , so that it does not influence the signals generated in coils  68  and  70 . During step  102 , coil  64  is typically inoperative. 
         [0060]    In a second calibration step  104 , processor  36  implements the force sensor part of the calibration procedure described above. I.e., the processor injects a force calibration signal of known amplitude and frequency, herein termed the force sensor frequency, into coil  64 . The processor then measures signals from coils  66 ,  68 , and  70  for known forces and deflections of distal tip  52 , and compiles relations between the signals and the forces and deflections. The procedure for step  104  is repeated for different positions of sheath termination  80 , i.e., for different values of distance Δz. One of the sets of relations is for the case when magnetic structure  45  does not affect the signals in coils  66 ,  68 , and  70 , i.e., for a value of Δz that is large. 
         [0061]    For each different value of distance Δz there is a set of relations between the coil signals and the forces and deflections of the distal tip. Typically, the processor uses interpolation in order to generate a force sensor calibration relationship between sets of coil signals, the forces and deflections on distal tip  52 , and values of distance Δz. 
         [0062]    Steps  102  and  104  constitute a calibration section of flowchart  100 . Typically, the sheath location frequency and the force sensor frequency are different from each other, and also from other frequencies, such as the ablating frequency and the field generator frequencies, used during the procedure. Using different frequencies enables each coil in distal end  30  to perform multiple functions simultaneously, as well as reducing interference, such as may occur during tissue ablation. 
         [0063]    In an initiate procedure step  106 , sheath  40  is inserted into patient  24  so that the distal end of the sheath is in the general area of the endocardial tissue to be ablated. 
         [0064]    In a probe insertion step  108 , probe  28  is inserted into sheath  40 , until distal tip  52  contacts the endocardial tissue to be ablated. During the insertion, processor  36  injects the sheath calibration signal into coil  66 , and measures the resulting signals generated in coils  68  and  70 . The processor compares the measured signals with the sheath location calibration relationship determined in step  102 . From the comparison, processor  36  estimates the position of sheath termination  80 , corresponding to measuring the value of distance Δz. The processor may present the distance, in a graphical and/or text format, on display  42 . Typically, using the information on the display, operator  26  may manipulate the proximal ends of the sheath and probe to achieve a desired protrusion of probe distal end  44  from sheath termination  80 . 
         [0065]    In a force determination step  110 , the processor injects the force calibration signal into coil  64 , and measures the resulting signals generated in coils  66 ,  68 , and  70 . From the signals, and knowing the position of sheath termination  80  determined in step  108 , the processor uses the force sensor calibration relationship to determine the force and deflection of distal tip  52 . 
         [0066]    Consideration of flowchart  100  illustrates that not only is system  20  able to achieve a desired protrusion of distal end  44  from termination  80 , but the system is also able to allow for any changes in signals generated by the coils of assembly  63  due to the proximity of magnetic structure  45 . 
         [0067]    It will be understood that the description of flowchart  100 , assuming the presence of magnetic transducers in a force sensor in distal end  30 , is by way of example. In an alternative embodiment, rather than using transducers which are part of a force sensor, other transducers, such as coils in distal end  30  which are used to determine the coordinates of the distal end, may be used. It will also be understood that using signal changes due to the proximity of structure  45  changing the mutual inductance between magnetic transducers in distal end  30  is by way of example, and that signal changes due to the change in self-inductance in one or more magnetic transducers in the distal end may be used in place of, or in addition to, the changes caused by the change in mutual inductance. 
         [0068]    It will thus be appreciated that the embodiments described above are cited by way of example, and 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 subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.