Patent Publication Number: US-10307205-B2

Title: System and method for detection of metal disturbance based on orthogonal field components

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to invasive medical devices capable of sensing displacement of a joint in a probe, such as a catheter that is applied to the body of a patient, and more specifically to a system and method for using such a catheter that is capable of detecting the presence metal objects located anywhere near the catheter as well as for accounting and/or correcting for the presence of such metal object. 
     BACKGROUND OF THE INVENTION 
     In some diagnostic and therapeutic techniques, a catheter is inserted into a chamber of the heart and brought into contact with the inner heart wall. In such procedures, it is generally important that the distal tip of the catheter engages the endocardium with sufficient pressure to ensure good contact. Excessive pressure, however, may cause undesired damage to the heart tissue and even perforation of the heart wall. 
     For example, in intracardiac radio-frequency (RF) ablation, a catheter having an electrode at its distal tip is inserted through the patient&#39;s vascular system into a chamber of the heart. The electrode is brought into contact with a site (or sites) on the endocardium, and RF energy is applied through the catheter to the electrode in order to ablate the heart tissue at the site. Proper contact between the electrode and the endocardium during ablation is necessary in order to achieve the desired therapeutic effect without excessive damage to the tissue. 
     A number of patent publications describe catheters with integrated pressure sensors for sensing tissue contact. As one example, U.S. Patent Application Publication 2007/0100332, whose disclosure is incorporated herein by reference, describes systems and methods for assessing electrode-tissue contact for tissue ablation. An electro-mechanical sensor within the catheter shaft generates electrical signals corresponding to the amount of movement of the electrode within a distal portion of the catheter shaft. An output device receives the electrical signals for assessing a level of contact between the electrode and a tissue. 
     To date, there have been no prior art systems and methods that are capable of determining accurate position information and force measurements for a medical device that is used within a body of a patient while being able to detect field distortion caused by metal interference from a metal object located near the device with the ability to account for and correct its force readings/measurements and/or its position information. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus for detecting metal disturbance during a medical procedure comprising a probe, which comprises an insertion tube, having a longitudinal axis and having a distal end and a distal tip, which is disposed at the distal end of the insertion tube and is configured to be brought into contact with tissue of the body. A joint, which couples the distal tip to the distal end of the insertion tube is used in conjunction with a joint sensor, contained within the probe, for sensing a position of the distal tip relative to the distal end of the insertion tube. The joint sensor comprises first and second subassemblies, which are disposed within the probe on opposite, respective sides of the joint and each comprise one or more magnetic transducers. 
     Apparatus also comprises a processor for determining a force measurement using the joint sensor, and having a threshold field value stored therein, and which is coupled to apply a current to one of the first and second subassemblies, thereby causing the one of the subassemblies to generate at least one magnetic field. The processor is coupled to receive and process one or more signals output by the other of the first and second subassemblies responsively to the at least one magnetic field so as to detect changes in a position of the distal tip relative to the distal end of the insertion tube, wherein the one or more signals output by the other of the first and second subassemblies define a sensed field value. 
     The processor compares the sensed field value to the threshold field value and identifies a presence of a metal object near a distal end of the probe when the sensed field value is below the threshold field value. In many instances, the sensed field value is based on a field in the axial direction. 
     The present invention is also directed to a method for detecting metal disturbance during a medical procedure performed on a body of a patient comprising the steps of providing a probe, which comprises an insertion tube, having a longitudinal axis and having a distal end, a distal tip, which is disposed at the distal end of the insertion tube and is configured to be brought into contact with tissue of the body, and a joint, which couples the distal tip to the distal end of the insertion tube, and a joint sensor, contained within the probe, for sensing a position of the distal tip relative to the distal end of the insertion tube. The joint sensor comprises first and second subassemblies, which are disposed within the probe on opposite, respective sides of the joint and each comprise one or more magnetic transducers. 
     A processor is used for determining a force measurement using the joint sensor and having a threshold field value stored therein. A current is applied to one of the first and second subassemblies, thereby causing the one of the subassemblies to generate at least one magnetic field wherein the at least one magnetic field at the other of the first and second subassemblies is received and one or more signals by the other of the first and second subassemblies is output responsively to the at least one magnetic field, wherein the one or more signals output by the other of the first and second subassemblies define a sensed field value. 
     Changes in a position of the distal tip relative to the distal end of the insertion tube are detected and the processor compares the sensed field value to the threshold field value and identifies a presence of a metal object near a distal end of the probe when the sensed field value is below the threshold field value. In many instances, the sensed field value is based on a field in the axial direction. 
     In another embodiment, the present invention is directed to apparatus for detecting metal disturbance during a medical procedure comprising a probe, which comprises an insertion tube, having a longitudinal axis and having a distal end and a distal tip, which is disposed at the distal end of the insertion tube and is configured to be brought into contact with tissue of the body. The probe also has a joint, which couples the distal tip to the distal end of the insertion tube; and a joint sensor, contained within the probe, for sensing a position of the distal tip relative to the distal end of the insertion tube. The joint sensor comprises first and second subassemblies, which are disposed within the probe on opposite, respective sides of the joint and each comprise one or more magnetic transducers. 
     A processor is used for determining a force measurement using the joint sensor, and having a pre-established baseline value stored therein, and which is coupled to apply a current to one of the first and second subassemblies, thereby causing the one of the subassemblies to generate at least one magnetic field. 
     The processor is coupled to receive and process one or more signals output by the other of the first and second subassemblies responsively to the at least one magnetic field so as to detect changes in a position of the distal tip relative to the distal end of the insertion tube. 
     The magnetic transducers comprise coils, and wherein the first subassembly comprises a first coil having a first coil axis parallel to the longitudinal axis of the insertion tube, and wherein the second subassembly comprises two or more other coils in different, respective radial locations within a section of the probe that is spaced apart axially from the first subassembly. 
     The processor applies current to one coil of the two or more other coils of the second subassembly and measures a signal output by a remainder of the two or more other coils of the second subassembly. The signal output by a remainder of the two or more other coils of the second subassembly defines a pickup value, wherein the processor compares the pickup value to the pre-established baseline value and identifies a presence of a metal object near a distal end of the probe when the pickup value is outside of a range of the pre-established baseline value. In many instances, the pickup value is based on mutual inductance measured from the remainder of the two or more other coils of the second subassembly. 
     The present invention is also directed to a method for detecting metal disturbance during a medical procedure performed on a body of a patient comprising the steps of providing a probe, which comprises an insertion tube, having a longitudinal axis and having a distal end, a distal tip, which is disposed at the distal end of the insertion tube and is configured to be brought into contact with tissue of the body, a joint, which couples the distal tip to the distal end of the insertion tube, and a joint sensor, contained within the probe, for sensing a position of the distal tip relative to the distal end of the insertion tube. 
     The joint sensor comprises first and second subassemblies, which are disposed within the probe on opposite, respective sides of the joint and each comprise one or more magnetic transducers, and a processor for determining a force measurement using the joint sensor. The processor has a pre-established baseline value stored therein, and which is coupled to apply a current to one of the first and second subassemblies, thereby causing the one of the subassemblies to generate at least one magnetic field, and which is coupled to receive and process one or more signals output by the other of the first and second subassemblies responsively to the at least one magnetic field so as to detect changes in a position of the distal tip relative to the distal end of the insertion tube. 
     The magnetic transducers comprise coils, and wherein the first subassembly comprises a first coil having a first coil axis parallel to the longitudinal axis of the insertion tube, and wherein the second subassembly comprises two or more other coils in different, respective radial locations within a section of the probe that is spaced apart axially from the first subassembly. 
     Current is then applied to one coil of the two or more other coils of the second subassembly a signal output is measured by a remainder of the two or more other coils of the second subassembly, wherein the signal output by a remainder of the two or more other coils of the second subassembly define a pickup value. The pickup value is then compared to the pre-established baseline value and a presence of a metal object near a distal end of the probe is identified when the pickup value is outside of a range of the pre-established baseline value. In many instances, mutual inductance is measured from the remainder of the two or more other coils of the second subassembly for determining the pickup value. 
     In yet another embodiment according to the present invention, the present invention is directed to apparatus for detecting metal disturbance during a medical procedure comprising a probe, which comprises an insertion tube, having a longitudinal axis and having a distal end and a distal tip, which is disposed at the distal end of the insertion tube and is configured to be brought into contact with tissue of the body. The probe also comprises a joint, which couples the distal tip to the distal end of the insertion tube; and a joint sensor, contained within the probe, for sensing a position of the distal tip relative to the distal end of the insertion tube. The joint sensor comprises first and second subassemblies, which are disposed within the probe on opposite, respective sides of the joint and each comprise one or more magnetic transducers. 
     A processor is used for determining a force measurement using the joint sensor, and having a threshold field value stored therein. The processor is coupled to apply a current to one of the first and second subassemblies, thereby causing the one of the subassemblies to generate at least one magnetic field, and which is coupled to receive and process one or more signals output by the other of the first and second subassemblies responsively to the at least one magnetic field so as to detect changes in a position of the distal tip relative to the distal end of the insertion tube, wherein the one or more signals output by the other of the first and second subassemblies define a sensed field value. The processor compares the sensed field value to the threshold field value and identifies a presence of a metal object near a distal end of the probe when the sensed field value is greater than the threshold field value. In many cases, the sensed field value is based on a field in the radial or orthogonal direction. 
     The present invention is also directed to a method for detecting metal disturbance during a medical procedure performed on a body of a patient comprising the steps of providing a probe, which comprises an insertion tube, having a longitudinal axis and having a distal end, a distal tip, which is disposed at the distal end of the insertion tube and is configured to be brought into contact with tissue of the body, and a joint, which couples the distal tip to the distal end of the insertion tube, and a joint sensor, contained within the probe, for sensing a position of the distal tip relative to the distal end of the insertion tube. 
     The joint sensor comprises first and second subassemblies, which are disposed within the probe on opposite, respective sides of the joint and each comprise one or more magnetic transducers. A processor is used for determining a force measurement using the joint sensor and having a threshold field value stored therein. 
     Current is then applied to one of the first and second subassemblies, thereby causing the one of the subassemblies to generate at least one magnetic field. The at least one magnetic field is received at the other of the first and second subassemblies and one or more signals by the other of the first and second subassemblies is output responsively to the at least one magnetic field, wherein the one or more signals output by the other of the first and second subassemblies define a sensed field value. 
     Changes are detected in a position of the distal tip relative to the distal end of the insertion tube and the sensed field value is compared to the threshold field value; and a presence of a metal object near a distal end of the probe is identified when the sensed field value is greater than the threshold field value. In many cases, the sensed field value is based on a field in the radial or orthogonal direction. 
     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 
         FIG. 1  is a schematic, pictorial illustration of a catheter-based medical system, in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic detail view showing the distal tip of a catheter in contact with endocardial tissue, in accordance with an embodiment of the present invention; 
         FIG. 3  is a schematic, sectional view showing details of the distal end of a catheter, in accordance with an embodiment of the present invention; 
         FIG. 4  is a schematic, sectional view showing details of the distal end of a catheter, in accordance with another embodiment of the present invention, in the presence of a metal object; 
         FIG. 5  is a schematic, flow chart of a method for detecting the presence of the metal object of  FIG. 4  using the catheter-based medical system of  FIG. 1  and the catheter of  FIGS. 2, 3 and 4 , in accordance with an embodiment of the present invention; 
         FIG. 6  is a schematic, flow chart of an alternative embodiment of a method for detecting the presence of the metal object of  FIG. 4  using the catheter-based medical system of  FIG. 1  and the catheter of  FIGS. 2, 3 and 4 , in accordance with an alternative embodiment of the present invention; and 
         FIG. 7  is a schematic, flow chart of another alternative embodiment of a method for detecting the presence of the metal object of  FIG. 4  using the catheter-based medical system of  FIG. 1  and the catheter of  FIGS. 2, 3, and 4  in accordance with another alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This application uses the technical disclosure of commonly owned pending U.S. patent application Ser. No. 11/868,733, filed Oct. 8, 2007, and U.S. patent application Ser. No. 12/327,226, filed Dec. 3, 2008 which are assigned to the assignee of the present patent application and whose disclosure of both references is incorporated herein by reference. Accordingly, like or similar features are identified using the same reference numerals from U.S. patent application Ser. No. 12/327,226. 
     The above-mentioned U.S. patent application Ser. No. 11/868,733 describes a catheter whose distal tip is coupled to the distal end of the catheter insertion tube by a spring-loaded joint, which deforms in response to pressure exerted on the distal tip when it engages tissue. A magnetic position sensing assembly within the probe, comprising coils on opposite sides of the joint, senses the position of the distal tip relative to the distal end of the insertion tube. Changes in this relative position are indicative of deformation of the spring and thus give an indication of the pressure. 
     Embodiments of the present invention that are described herein below utilize the new design of the sensing assembly of U.S. patent application Ser. No. 12/327,226, which facilitates more precise measurement of tip movement and ultimately facilitates the detection of a metallic object located near the catheter. The configuration of the coils in this design permits precise sensing of very small deflections and compressions of the joint connecting the catheter tip to the insertion tube. Therefore, the pressure on the tip can be measured with enhanced accuracy, permitting the use a relatively stiffer spring in the catheter, which makes the catheter more reliable and easier to maneuver in the body. Moreover, operation of these coils on the catheter allow for the detection of metal objects located anywhere near catheter. 
     Preferred embodiments according to the present invention as further described in detail later on in this disclosure are directed to a system and method for using such a catheter that is capable of detecting the presence metal objects located anywhere near the catheter as well as for accounting and/or correcting for the presence of such metal object. 
       FIG. 1  is a schematic, pictorial illustration of a system  20  for cardiac catheterization, in accordance with 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, 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. 
     An operator  26 , such as a cardiologist, inserts catheter  28  through the vascular system of a patient  24  so that a distal end  30  of the catheter enters a chamber of the patient&#39;s heart  22 . 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. 
     Console  34  uses 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  FIG. 3 ) 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, 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, in PCT Patent Publication WO 96/05768, and in U.S. Patent Application Publications 2002/0065455 A1, 2003/0120150 A1 and 2004/0068178 A1, whose disclosures are all incorporated herein by reference. 
     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 tangible media, such as optical, magnetic or electronic memory media. Alternatively, some or all of the functions of processor 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, as well as regarding displacement of the distal tip of the catheter, and status information and guidance regarding the procedure that is in progress. 
     Alternatively or additionally, system  20  may comprise an automated mechanism for maneuvering and operating catheter  28  within the body of patient  24 . Such mechanisms are typically capable of controlling both the longitudinal motion (advance/retract) of the catheter and transverse motion (deflection/steering) of the distal end of the catheter. Some mechanisms of this sort use DC magnetic fields for this purpose, for example. In such embodiments, processor  36  generates a control input for controlling the motion of the catheter based on the signals provided by the magnetic field sensor in the catheter. These signals are indicative of both the position of the distal end of the catheter and of force exerted on the distal end, as explained further hereinbelow. 
       FIG. 2  is a schematic sectional view of a chamber of a heart  22 , showing distal end  30  of catheter  28  inside the heart, in accordance with an embodiment of the present invention. 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. An electrode  56  on a distal tip  52  of the catheter engages endocardial tissue  58 . Pressure exerted by the distal tip against the endocardium deforms the endocardial tissue locally, so that electrode  56  contacts the tissue over a relatively large area. In the pictured example, the electrode engages the endocardium at an angle, rather than head-on. Distal tip  52  therefore bends at an elastic joint  54  relative to the distal end of insertion tube  50  of the catheter. The bend facilitates optimal contact between the electrode and the endocardial tissue. 
     Because of the elastic quality of joint  54 , the angle of bending and the axial displacement of the joint are proportional to the pressure exerted by tissue  58  on distal tip  52  (or equivalently, the pressure exerted by the distal tip on the tissue). Measurement of the bend angle and axial displacement thus gives an indication of this pressure. The pressure 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. 
       FIG. 3  is a schematic, sectional view of distal end  30  of catheter  28 , showing details of the structure of the catheter in accordance with an embodiment of the present invention. Insertion tube  50  is connected to distal tip  52  by joint  54 , as noted above. 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 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 , which is 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. The distal tip is typically relatively rigid, by comparison with the flexible insertion tube. 
     Joint  54  comprises a resilient coupling member  60 . In this 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 application Ser. No. 12/134,592, filed Jun. 6, 2008, which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference. Alternatively, the coupling member may comprise a coil spring or any other suitable sort of resilient component with the desired flexibility and strength characteristics. 
     The stiffness of coupling member  60  determines the range of relative movement between tip  52  and insertion tube  50  in response to forces exerted on the distal tip. Such forces are encountered when the distal tip is pressed against the endocardium during an ablation procedure. The desired pressure for good electrical contact between the distal tip and the endocardium during ablation is on the order of 20-30 grams. The coupling member is configured to permit axial displacement (i.e., lateral movement along the axis of catheter  28 ) and angular deflection of the distal tip in proportion to the pressure on the tip. Measurement of the displacement and deflection by processor  36  gives an indication of the pressure and thus helps to ensure that the correct pressure is applied during ablation. 
     A joint sensing assembly, 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. 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. 
     The coils in catheter  28  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  emit 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 . 
     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. 
     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 . 
     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 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. 
     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 that 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, so long as one of the subassemblies comprises at least two coils, in different radial positions, to allow differential measurement of joint deflection. 
     Prior calibration of the relation between pressure on tip  52  and movement of joint  54  may be used by processor  36  in translating the coil signals into terms of pressure. By virtue of the combined sensing of displacement and deflection, this pressure sensing system reads the pressure correctly regardless of whether the electrode engages the endocardium head-on or at an angle. The pressure reading is insensitive to temperature variations and free of drift, unlike piezoelectric sensors, for example. Because of the high sensitivity to joint motion that is afforded by the arrangement of coils  64 ,  66 ,  68  and  70  that is shown in  FIG. 3 , processor  36  can measure small displacements and deflections with high precision. Therefore, coupling member  60  can be made relatively stiff, and processor  36  will still be able to sense and measure accurately the pressure on tip  52 . The stiffness of the coupling member makes it easier for the operator to maneuver and control the catheter. 
     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  72  and  73  (or other magnetic sensors) may be deployed in the distal end of the catheter for this purpose as best illustrated in  FIG. 4 . 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 pressure on the tip. 
     Although the operation of a magnetic position sensing assembly and its use in sensing pressure are described above in the context of catheter-based ablation, the principles of the present invention may similarly be applied in other applications that require accurate sensing of the movement of a joint, and particularly in therapeutic and diagnostic applications that use invasive probes, both in the heart and in other organs of the body. As one example, the devices and techniques for position and pressure sensing that are implemented in system  20  may be applied, mutatis mutandis, in guiding and controlling the use of a catheter insertion sheath. If the position of the sheath is not properly controlled and excessive force is used in its insertion, the sheath may perforate the heart wall or vascular tissue. This eventuality can be avoided by sensing the position of and pressure on the distal tip of the sheath. In this regard, the term “distal tip” as used herein should be understood to include any sort of structure at the distal end of a probe that may be bent and/or displaced relative to the main body of the probe. 
     One drawback for the technology described above is the presence of ferromagnetic material (metallic material)  80  located close to distal end and coils  64 ,  66 ,  68 ,  70 ,  72  of catheter  28  as best illustrated in  FIG. 4 . Such material may cause a distortion in the magnetic field and therefore may change the force readings. This distortion cannot be identified by looking at the force raw data alone, since this data is valid and acts as legitimate force readings, i.e. any three readings from the contact force sensors (coils) in joint  54  (even distorted) are directly mapped to a relevant force. The interference is noticeable when another metal containing device  80  comes in close proximity to distal tip  52  and joint  54  as show in  FIG. 4 . 
     This phenomenon is referred to as shaft proximity interference (SPI). The present invention is directed to novel apparatus and methods that provide a mechanism which enables the ability to distinguish between legitimate force readings and a force reading caused by a SPI. 
     As best illustrated in  FIG. 4 , the arrangement of coils  64 ,  66 ,  68 ,  70 ,  72  and  73  in catheter tip  52  enables sensitive measurement of the catheter bend angle and contact force experienced by catheter tip  52 . This arrangement serves as the joint sensing assembly, comprising coils  64 ,  66 ,  68 ,  70 ,  72  and  73  which provides accurate reading of the position/location (in the form of position and orientation coordinate information in X,Y, and Z axis directions and yaw, pitch and roll orientations) of distal tip  52  relative to the distal end of insertion tube  50 . Transmit coil  64  is driven by current via cable  74  from console  34  to generate a magnetic field in the axial direction (Z-axis according to  FIGS. 3 and 4 ). This field is received by coils  66 ,  68  and  70 , which are fixed at different radial locations such that these coils emit electrical signals in response to the magnetic field generated by coil  64 . The signals are conveyed by cable  74  to the processor  36  ( FIG. 1 ), which uses them to measure the axial displacement and angular deflection of joint  54 . 
     One or more of the above coils, as well as additional coils (such as coils  72  and  73 ) oriented in the X and Y directions, are typically also used to output signals in response to magnetic fields generated by external field generators, and thus serve as position sensing coils for the catheter tip. 
     With this type of magnetic field position and force measurement system  20 , it is important to detect the field distortion caused by metal objects located within the magnetic fields especially those near the distal end of catheter  30  in order to avoid coils  66 ,  68 ,  70 ,  72  and  73  outputting incorrect signals which ultimately lead to erroneous measurements/readings of force information or SPI. This problem of SPI is addressed by all of the embodiments of the present invention detailed throughout this disclosure, for example, those embodiments of  FIG. 4 ,  FIG. 5 ,  FIG. 6  and  FIG. 7 . 
     Accordingly, when a metal object  80  is brought into proximity with distal tip  52 , it will distort the magnetic field generated by transmit coil  64  and may, as a result, introduce error into the measurements of displacement, and deflection. This situation of SPI, in which metal object  80  (in this example, another catheter  80 ) comes close to tip  52 , is illustrated in  FIG. 4 . It is important to detect the field distortion in order to avoid outputting incorrect readings of force. 
     The parasitic effect of metal object  80  near tip  52  causes the axial field sensed by coils  66 ,  68  and  70  to decrease sharply. This decrease has a very strong dependence on the distance of the metal object from the tip. When the metal object is very close to the tip, the signals received from coils  66 ,  68  and  70  will be indicative of a “negative force,” i.e., they will be lower than the signals received from the coils in the rest position of joint  54 , as though tip  52  were being pulled away from insertion tube  50 . This situation is unlikely to be encountered in reality. And thus, constitutes an accurate predictor of the presence of metal object  80  near tip  52  of the catheter  30 . 
     Accordingly, as schematically illustrated in  FIG. 5 , the method according to the present invention is to establish a threshold field value for system  20  ( FIG. 1 ) while catheter  30  is in its rest position (without axial displacement or angular deflection at its distal end  52  and joint  54 ) based on the strong signals output by coils  66 ,  68  and  70  as described previously above. The threshold field value or strength is set as step  100 . This threshold field value step  100  is conducted as part of the calibration of the system  20 , i.e. prior to use of the catheter  30  on patient  24 , wherein the threshold field value  100  is stored in the software of processor  36  of system  20 . 
     Once the threshold field value is set  100 , system  20  is ready for use and a magnetic field is generated by magnetic field generator coil  64  ( FIGS. 3 and 4 ), as described above, in step  105 . This generated magnetic field  105  is sensed by coils  66 ,  68  and  70  wherein each respective coil provides a signal output in step  110  based on the strength of the magnetic field from coil  64 . 
     In step  115 , processor  36  ( FIG. 1 ) compares the magnetic field  105  sensed by coils  66 ,  68  and  70  to the threshold field value  100  (based on the signals output by coils  66 ,  68  and  70 ). The logic program in software of processor  36  compares the sensed magnetic field value  110  to the threshold field value  100 . If the sensed magnetic field value  110  to the threshold field value  100  is at or above threshold field value  100 , system  20  continues to operate under normal operating conditions thereby continuing normal cycle of operations with continue generation of magnetic field  105  at magnetic field generator coil  64  with all steps  105 ,  110 ,  115  repeated in cycle. 
     However, in accordance with the present invention, in step  115 , if the sensed magnetic field value  110  is below the threshold field value  100 , due to coils  66 ,  68  and  70  providing output signals that are below the pre-established threshold field value  100 , the processor  36  identifies SPI, i.e. the force measurement as being influenced by the presence of metal  80  ( FIG. 4 ) in step  120 . 
     Processor  36  then determines that the there is indeed SPI present and that the force measurement readings are invalid in step  125  (due to the disturbance of the magnetic field from generator coil  64 ). When this sort of field disturbance is detected due to SPI, the processor  36  marks the position coordinate information as suspicious on the display  42 ). Step  125  optionally includes both a visual indication or indicia depicted in real time on an electro-anatomical map, for example, on display  42  as well as an audible warning or alarm and/or distinct tactile feedback to the operator  26  and steps  105 ,  110 ,  115  are repeated in cycle. 
     As mentioned above, this threshold field value is typically chosen to correspond to zero force on the catheter tip  52  (established in step  100  as part of the calibration and/or start-up procedure), so that sub-threshold readings in step  115  correspond to negative force and automatically identified as metal interference due to presence of metal  80  near tip  52 . 
     Moreover, although coil  64  normally generates only axial field components (along the Z-axis) in the near field, the parasitic effect of a metal object  80  ( FIG. 4 ) near the distal end and/or tip  52  will typically generate radial (X and/or Y axis) components at the driving frequency of coil  64 . The amplitude of these parasitic orthogonal field components (X-axis and Y-axis components in this example) has a strong dependence on the distance of the metal object  80  from the distal end of catheter  30 . Thus, when the metal object is close to the distal end, the signals received from coils  72 ,  73  at the driving frequency of coil  64  will increase resulting in SPI. 
     Therefore, when a metal object  80  is present near distal end of catheter  30 , another alternative embodiment of the present invention, as best illustrated in  FIG. 6 , is also used to detect the presence of the metal object  80  and account for SPI and correct readings and measurements of force that may be skewed or erroneous due to the SPI. 
     In accordance with a further embodiment of the present invention as best illustrated in  FIG. 4  and  FIG. 6 , in order to distinguish between authentic force measurements and distorted ones due to SPI, the present invention utilizes X and Y coils, (the coils  72  and  73  respectively in  FIG. 4 ) Although the X and Y coils ( 72  and  73 ) are used for the magnetic position/location information determination, they also receive the force signal although with less sensitivity than the coils  66 ,  68  and  70  (also referred to as coils S 1 , S 2  and S 3  for purposes of the algorithm and calculations below). 
     All five measurements (from coils S 1 -S 2 -S 3  and X-Y) relate linearly to the force vector. Thus, there is linear relation between the S 1 -S 2 -S 3  and the X-Y readings from coils  66 - 68 - 70  and coils  72 - 73  respectively. Because of the different orientation between the X-Y coils ( 72 ,  73 ) and the S 1 -S 2 -S 3  coils ( 66 ,  68 ,  70 ), the distortion in the magnetic field caused by a ferromagnetic material  80 , causes a different distortion in the X-Y coils ( 72 ,  73 ) as opposed to S 1 -S 2 -S 3  coils ( 66 ,  68 ,  70 ), and therefore the relation between the S 1 -S 2 -S 3  and X-Y signal readings is altered in a measurable manner. This effect enables the present invention to distinguish between a valid force measurement and a distorted force measurement. 
     The regular correlation between S 1 -S 2 -S 3  readings (from coils  66 ,  68 ,  70 ) to X-Y readings ( 72 ,  73 ), which is related to valid forces, can be expressed by the following linear relation: 
     
       
         
           
             
               [ 
               
                 
                   
                     
                       
                         X 
                         ^ 
                       
                       Force 
                     
                   
                 
                 
                   
                     
                       
                         Y 
                         ^ 
                       
                       Force 
                     
                   
                 
               
               ] 
             
             = 
             
               
                 T 
                 Matrix 
               
               · 
               
                 [ 
                 
                   
                     
                       
                         S 
                         1 
                       
                     
                   
                   
                     
                       
                         S 
                         2 
                       
                     
                   
                   
                     
                       
                         S 
                         3 
                       
                     
                   
                 
                 ] 
               
             
           
         
       
     
     The SPI-Force Distortion Measure parameter is based on the difference between the expected X-Y measurement and the measured X-Y according to the algorithm of the present invention:
 
SPI=√{square root over (( {circumflex over (X)}   Force   −X   Force ) 2 +( Ŷ   Force   −Y   Force ) 2 )}
 
     The method according to this alternative embodiment of the present invention as best illustrated in  FIG. 6  for addressing SPI is to establish a threshold field value  100   a  for system  20  ( FIG. 1 ) while catheter  30  is in its rest position (without axial displacement or angular deflection at its distal end (includes tip  52  and joint  54 ) based on the strong signals output by coils  66 ,  68  and  70  (as well as base line signal from coils  72 ,  73 ) as described previously above. The threshold field value or strength is set for system  20  in step  100   a . This threshold field value step  100   a  is conducted as part of the calibration of the system  20 , i.e. prior to use of the catheter  30  on patient  24 , wherein the threshold field value  100   a  is stored in the software of processor  36  of system  20 . 
     Once the threshold field value  100   a  is set, system  20  is ready for use and a magnetic field is generated by magnetic field generator coil  64  ( FIGS. 3 and 4 ), as described above, in step  105 . This generated magnetic field  105  is sensed by coils  66 ,  68  and  70  and coils  72 ,  73  wherein each respective coil provides a signal output in step  110  based on the strength of the magnetic field from transmit coil  64 . 
     In step  115   a , processor  36  ( FIG. 1 ) compares the magnetic field  105  sensed by coils  66 ,  68  and  70  to the threshold field value  100   a  (based on the signals output by coils  66 ,  68  and  70 ). The logic program in software of processor  36  compares the sensed magnetic field value  110  to the threshold field value  100   a  using the algorithm address previously above. If the sensed magnetic field value  110  is at or below the threshold field value  100   a , system  20  continues to operate under normal operating conditions thereby continuing normal cycle of operations with continue generation of magnetic field  105  at magnetic field generator coil  64  with all steps  105 ,  110 ,  115   a  repeated in cycle. 
     However, in step  115   a , when coil  72  or coil  73 , i.e. the corresponding Y-direction coil and/or X-direction coil, gives an output at the driving frequency of coil  64  that is above, i.e. greater than, the given threshold  100   a , the processor  36  determines and identifies that magnetic interference is present—which is likely being caused by another device having metal components such as device  80  ( FIG. 4 ) located near distal end of catheter  30 . 
     Since coil  64  normally generates only axial field components along the Z-axis in the near field, the parasitic effect of metal object  80  near distal end of catheter  30  will typically generate radial field components along the X-axis and/or Y-axis at the driving frequency of coil  64 . The amplitude of these parasitic orthogonal or radial field components has a strong dependence on the distance of the metal object  80  from the distal end of catheter  30 . Thus, when the metal object  80  is close to the catheter  30 , the signals received from X-axis coils and Y-axis coils (for example coils  68  and  70  and coil  72 , designated as the position sensing coil in this example), as well as coils  72  and  73 , at the driving frequency of transmit coil  64  will increase in a non-consistent way (relative to the expected change received from the calibration process). 
     Thus, in accordance with the present invention, in step  115   a , if the sensed magnetic field value  110  is greater than the threshold field value  100   a , due to coils  66 ,  68 ,  70  and  72 ,  73  providing output signals that are above the pre-established threshold field value  100   a , in step  120 , the processor  36  automatically identifies that metal  80  is present and that the present position information (the six dimensional position and orientation coordinate information determined based on position signals received from the coil(s) designated as magnetic position sensors, such as coil  72  in this example, as being a position coordinate influenced by the presence of metal  80  ( FIG. 4 ) in step  125 . 
     In this situation, processor  36  determines that the present force readings are invalid in step  125  (due to the disturbance of the magnetic field from generator coil  64 ). When this sort of field disturbance is detected, the processor  36  optionally invalidates the position coordinate reading/information (or at least marks the position coordinate information as suspicious on the display  42 ). Step  125  optionally includes both a visual indication or indicia depicted in real time on an electro-anatomical map, for example, on display  42  as well as an audible warning or alarm and/or distinct tactile feedback to the operator  26  and steps  105 ,  110 ,  115   a  are repeated in cycle until the metal object is cleared from the magnetic field or accounted for with adjustment calculations to the force measurement readings. 
     As mentioned above, this threshold field value  100   a  is typically chosen to correspond to zero force on the catheter tip  52  (established in step  100   a  as part of the calibration and/or start-up procedure), so that above-threshold readings from axial component signals in the X-axis direction and Y-axis direction in step  115   a  are automatically identified as metal interference due to presence of metal  80  near distal end of catheter  30 . 
     Alternatively or additionally, when the interference from presence of metal  80  is not too extreme, i.e. signal outputs slightly above threshold values  100   a , the processor  36  can attempt to correct the force readings to compensate for the metal interference (SPI). 
     In another method according to yet another alternative embodiment of the present invention for addressing SPI, the parasitic effect of a metal object  80  near distal end of catheter  30 , such as near the tip  52 , causes a change in the mutual inductance between coils  66 ,  68  and  70  in the magnetic field generation position tracking system  20  ( FIGS. 1-4 ). This change in mutual inductance has a strong dependence on the distance of the metal object  80  from the distal end of catheter  30 . 
     Accordingly, in this alternative embodiment of the present invention, as best illustrated in  FIG. 7 , the system  20  ( FIG. 1 ) provides for a method ( FIG. 7 ) wherein an initial baseline value for the mutual inductance detected between coils  66 ,  68  and  70  of catheter  30  is pre-determined or pre-established at time of manufacture or just prior to use on a patient  24  and stored in the logic of processor  36 . This mutual inductance baseline value is an expected range (including an acceptable deviation factor +/−) of inductance measured amongst the coils  66 ,  68  and  70  when one of the coils has current applied directly thereto without any metal being present near the catheter  30 . Thus, this is a calibration step  200  for determining the mutual inductance baseline value of the coils  66 ,  68  and  70  in the absence of any metal object. 
     The mutual inductance coils  66 ,  68  and  70  is measured, for example, by applying or injecting a current at a certain, known frequency through one of these coils, for example coil  66 , and measuring the signals that are induced in the other remainder coils, for example adjacent coils  68  and  70 , as a result. Under normal circumstances, i.e. in the calibration stage  200 , these pickup signals should be invariant. A change in these pickup signals indicates that the mutual inductance has changed, presumably due to a metal object  80  located near the distal end of catheter  30 . 
     Therefore, in this embodiment of the present invention, a current at a predetermined frequency is periodically applied or injected into one or more of coils  66 ,  68  and  70  in step  205 , and the resulting pickup signal in the other remainder coil(s), i.e. the non-injected coils, are measured  210 . As mentioned above, the baseline pickup value is measured under conditions without metal interference in step  200  and pre-stored in processor  36  which allows processor  36  in step  215  to compare the pickup value (based on the measured pickup signals from the non-injected coils) to the baseline value range (which includes the acceptable deviation factor expected in an environment without metal being present) and if the pickup signal value subsequently deviates from the baseline value by more than the permitted amount (the acceptable range), the processor  36  in step  220  makes a determination and identifies SPI, i.e. that the magnetic field has been disturbed by a nearby metal object  80 . 
     When this sort of field disturbance is detected, the processor  36  in step  225  typically invalidates (or at least marks as suspicious) the force measurements made by coils  66 ,  68  and  70 . The user  26  then has the option to seek out and remove the source of the metal object interference and/or take evasive measures to account for the presence of such a metal object  80 . 
     In this situation, processor  36  determines that the present force measurement readings are invalid in step  225  (due to the mutual inductance measurements being out-of-bounds, i.e. outside baseline value  200 ) and when this sort of field disturbance is detected, the processor  36  invalidates the Force reading/information. Step  225  optionally includes both a visual indication or indicia depicted in real time on an electro-anatomical map, for example, on display  42  as well as an audible warning or alarm and/or distinct tactile feedback to the operator  26  and steps  205 ,  210 ,  215  are repeated in cycle until the metal object is cleared from the magnetic field or accounted for with adjustment calculations to the force measurement readings. 
     In step  215 , if the pickup signal value is within the established baseline value  200 , the process continues in cycle, i.e. steps  205 ,  210  and  215  are repeated in cycle and magnetic field position sensing system continues to operate in this manner providing accurate six-dimensional position and orientation coordinate information along with the accurate force measurement capability of catheter  30  thereby eliminating the effects from SPI. 
     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.