Patent Publication Number: US-2022225891-A1

Title: Magnetic sensor for tracking the location of an object

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/210,469, filed 5 Dec. 2018, which claims the benefit of U.S. provisional application No. 62/594,942, filed 5 Dec. 2017, which are hereby incorporated by reference as though fully set forth herein. 
    
    
     BACKGROUND 
     a. Field 
     This disclosure relates to systems and apparatuses for tracking the location of an object. In particular, the instant disclosure relates to using a transducer and a sensor to track the location of a catheter relative to an introducer. 
     b. Background Art 
     Electrophysiology catheters are used in a variety of diagnostic and/or therapeutic medical procedures to correct conditions such as atrial arrhythmia, including for example, ectopic atrial tachycardia, atrial fibrillation, and atrial flutter. Arrhythmia can create a variety of dangerous conditions including irregular heart rates, loss of synchronous atrioventricular contractions and stasis of blood flow which can lead to a variety of ailments and even death. 
     Typically in a procedure, a catheter is manipulated through a patient&#39;s vasculature to, for example, a patient&#39;s heart, and carries one or more electrodes which may be used for mapping, ablation, diagnosis, or other treatments. Once at the intended site, treatment may include radio frequency (RF) ablation, cryoablation, lasers, chemicals, high-intensity focused ultrasound, etc. An ablation catheter imparts such ablative energy to cardiac tissue to create a lesion in the cardiac tissue. This lesion disrupts undesirable electrical pathways and thereby limits or prevents stray electrical signals that lead to arrhythmias. As readily apparent, such treatment requires precise control of the catheter during manipulation to and at the treatment site, which can invariably be a function of a user&#39;s skill level. 
     To position a catheter at a desired site within the body, some type of navigation may be used, such as using mechanical steering features incorporated into the catheter (or an introducer). In some examples, medical personnel may manually manipulate and/or operate the catheter using the mechanical steering features. 
     In order to facilitate the advancement of catheters through a patient&#39;s vasculature, a navigating system may be used. Such navigating systems may include, for example, electric-field-based positioning and navigating systems that are able to determine the position and orientation of the catheter (and similar devices) within the body. In such electric-field-based positioning and navigating systems, it can be important to know and/or determine the distance between electrodes and/or other sensors and when electrodes and/or sensors on the catheter are shielded inside of a sheath or introducer that is being used to position the catheter at a desired location. 
     The foregoing discussion is intended only to illustrate the present field and should not be taken as disavowal of claim scope. 
     BRIEF SUMMARY 
     The instant disclosure, in at least one embodiment, comprises an apparatus for emitting a field comprising a core, a conductive winding with a first end, a second end, and an intermediate portion, where the conductive winding surrounds a portion of the core and is wound about a winding axis, a protrusion for aligning the apparatus where the protrusion is parallel with the winding axis, and a conductive connector extending from the conductive winding, wherein the conductive connector is electrically coupled with the conductive winding at the intermediate portion. 
     In another embodiment, a system comprises a first transducer assembly with a first longitudinal axis, the first transducer assembly comprising a core, a conductive winding, and a conductive connector electrically coupled with the conductive winding, wherein the core is parallel with the first longitudinal axis, a catheter with a second longitudinal axis, wherein the catheter comprises a first sensor, and a plurality of electrodes, where a location of the second sensor is known in relation to each of the plurality of electrodes, where the first longitudinal axis of the transducer assembly is perpendicular with the second longitudinal axis of the catheter, and an electronic control unit (ECU) electrically coupled to the first transducer assembly and the first sensor and operable to do the following: (A) generate a magnetic field using the first transducer assembly; (B) measure a first signal from the first sensor, wherein the first signal varies based on a position of the first sensor along the second longitudinal axis in relation to the magnetic field generated by the first transducer assembly; (C) analyze the first signal to determine a relative position of the catheter along the second longitudinal axis based on the location of the first sensor; (D) generate a relative position information for the catheter using the analysis of the first signal. 
     In another embodiment, a method for detecting a position of a sensor on an elongate medical device comprises positioning a catheter at a first location along a first longitudinal axis, emitting a field from a first transducer with a second longitudinal axis, where the first longitudinal axis is perpendicular with the second longitudinal axis, moving the catheter to a second position along the longitudinal axis, measuring a signal, by an electronic control unit (ECU) comprising a processor, at a first sensor on the catheter while moving the catheter along the longitudinal axis from the first position to the second position within the field, analyzing the signal, by the ECU, from the first sensor, and determining, by the ECU, a position data of the first sensor based on the signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a system diagram showing a medical device and a medical positioning system, in accordance with embodiments of the present disclosure. 
         FIG. 2A  is a schematic diagram of a first transducer and a first sensor, where the first transducer can be used to determine the location of the first sensor, in accordance with embodiments of the present disclosure. 
         FIG. 2B  is a schematic diagram of an exemplary embodiment of the first transducer and the first sensor of  FIG. 2A , where the first transducer can be used to determine the location of the first sensor, in accordance with embodiments of the present disclosure. 
         FIG. 2C  is a schematic diagram of another exemplary embodiment of the first transducer and the first sensor of  FIG. 2A , where the first transducer can be used to determine the location of the first sensor, in accordance with embodiments of the present disclosure. 
         FIG. 3  is a schematic diagram of a transducer assembly that can be used for locating a position of a medical device, in accordance with embodiments of the present disclosure. 
         FIG. 4  is a schematic diagram of a transducer assembly, in accordance with embodiments of the present disclosure. 
         FIG. 5  is a side view of a transducer assembly that can be used for locating and/or tracking a position of a medical device, in accordance with embodiments of the present disclosure. 
         FIG. 6  is a schematic view of a transducer assembly that can be used for locating and/or tracking a position of a medical device, in accordance with embodiments of the present disclosure. 
         FIG. 7A   1  is a top partial cross-sectional view of an apparatus for measuring sensor spacing on an elongate medical device, where the apparatus includes a transducer that can be used for generating a field used for measuring the spacing on the elongate medical device, in accordance with embodiments of the present disclosure. 
         FIG. 7A   2  is a side view of the fixture of  FIG. 7A   1 , in accordance with embodiments of the present disclosure. 
         FIG. 7B  is a schematic view of the apparatus of  FIG. 7A  with an exemplary catheter that includes a sensor mounted on a catheter, where the catheter is movable relative to the transducer assembly, in accordance with embodiments of the present disclosure. 
         FIG. 7C  is a schematic view of the apparatus of  FIG. 7A  with the exemplary catheter of  FIG. 7B , where the catheter has moved relative to the transducer assembly, in accordance with embodiments of the present disclosure. 
         FIG. 7D  is a schematic view of the apparatus of  FIG. 7A  with the exemplary catheter of  FIGS. 7A-B , where the catheter has moved relative to the transducer assembly, in accordance with embodiments of the present disclosure. 
         FIG. 8  is a diagram of steps in an exemplary method for measuring sensor spacing on an elongate medical device, in accordance with embodiments of the present disclosure. 
         FIG. 9  is a schematic view of an optic-magnetic registration plate (OMRP), consistent with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Referring now to the figures, in which like reference numerals refer to the same or similar features in the various views,  FIG. 1  illustrates one embodiment of a system  10  for navigating a medical device within a body  12 . In the illustrated embodiment, the medical device comprises a catheter  14  that is shown schematically entering a heart that has been exploded away from the body  12 . The catheter  14 , in this embodiment, is depicted as an irrigated radiofrequency (RF) ablation catheter for use in the treatment of cardiac tissue  16  in the body  12 . It should be understood, however, that the system  10  may find application in connection with a wide variety of medical devices used within the body  12  for diagnosis or treatment. For example, the system  10  may be used to navigate an electrophysiological mapping catheter, an intracardiac echocardiography (ICE) catheter, or an ablation catheter using a different type of ablation energy (e.g., cryoablation, ultrasound, etc.). Further, it should be understood that the system  10  may be used to navigate medical devices used in the diagnosis or treatment of portions of the body  12  other than cardiac tissue  16 . Further description of the systems and components are contained in U.S. patent application Ser. No. 13/839,963 filed on 15 Mar. 2013, which is hereby incorporated by reference in its entirety as though fully set forth herein. 
     Referring still to  FIG. 1 , the ablation catheter  14  is connected to a fluid source  18  for delivering a biocompatible irrigation fluid such as saline through a pump  20 , which may comprise, for example, a fixed rate roller pump or variable volume syringe pump with a gravity feed supply from fluid source  18  as shown. The catheter  14  is also electrically connected to an ablation generator  22  for delivery of RF energy. The catheter  14  may include a handle  24 ; a cable connector or interface  26  at a proximal end of the handle  24 ; and a shaft  28  having a proximal end  30 , a distal end  32 , and one or more electrodes  34 . The connector  26  provides mechanical, fluid, and electrical connections for conduits or cables extending from the pump  20  and the ablation generator  22 . The catheter  14  may also include other conventional components not illustrated herein such as a temperature sensor, additional electrodes, and corresponding conductors or leads. 
     The handle  24  provides a location for the physician to hold the catheter  14  and may further provide means for steering or guiding the shaft  28  within the body  12 . For example, the handle  24  may include means to change the length of one or more pull wires extending through the catheter  14  from the handle  24  to the distal end  32  of shaft  28 . The construction of the handle  24  may vary. 
     The shaft  28  may be made from conventional materials such as polyurethane and may define one or more lumens configured to house and/or transport electrical conductors, fluids, or surgical tools. The shaft  28  may be introduced into a blood vessel or other structure within the body  12  through a conventional introducer. The shaft  28  may then be steered or guided through the body  12  to a desired location such as the tissue  16  using guide wires or pull wires or other means known in the art including remote control guidance systems. The shaft  28  may also permit transport, delivery, and/or removal of fluids (including irrigation fluids and bodily fluids), medicines, and/or surgical tools or instruments. It should be noted that any number of methods can be used to introduce the shaft  28  to areas within the body  12 . This can include introducers, sheaths, guide sheaths, guide members, guide wires, or other similar devices. For ease of discussion, the term introducer will be used throughout. 
     The system  10  may include an electric-field-based positioning system  36 , a magnetic-field-based positioning system  38 , a display  40 , and an electronic control unit (ECU)  42  (e.g., a processor). Each of the exemplary system components is described further below. 
     The electric-field-based positioning system  36  and the magnetic-field-based positioning system  38  are provided to determine the position and orientation of the catheter  14  and similar devices within the body  12 . The position and orientation of the catheter  14  and similar devices within the body  12  can be determined by the system  36  and/or the system  38 . The system  36  may comprise, for example, the EnSite™ NavX™ system sold by St. Jude Medical, Inc. of St. Paul, Minn., and described in, for example, U.S. Pat. No. 7,263,397 titled “Method and Apparatus for Catheter Navigation and Location Mapping in the Heart,” the entire disclosure of which is hereby incorporated by reference as though fully set forth herein. The systems  36  and  38  may comprise, for example, the EnSite Precision™ system sold by St. Jude Medical, Inc., of St. Paul, Minn. The system  36  operates based upon the principle that when low amplitude electrical signals are passed through the thorax, the body  12  acts as a voltage divider (or potentiometer or rheostat) such that the electrical potential or field strength measured at one or more electrodes  34  on the catheter  14  may be used to determine the position of the electrodes, and, therefore, of the catheter  14 , relative to a pair of external patch electrodes using Ohm&#39;s law and the relative location of a reference electrode (e.g., in the coronary sinus). 
     In the configuration is shown in  FIG. 1 , the electric-field-based positioning system  36  further includes three pairs of patch electrodes  44 , which are provided to generate electrical signals used in determining the position of the catheter  14  within a three-dimensional coordinate system  46 . The electrodes  44  may also be used to generate EP data regarding the tissue  16 . To create axes-specific electric fields within body  12 , the patch electrodes are placed on opposed surfaces of the body  12  (e.g., chest and back, left and right sides of the thorax, and neck and leg) and form generally orthogonal x, y, and z axes. A reference electrode/patch (not shown) is typically placed near the stomach and provides a reference value and acts as the origin of the coordinate system  46  for the navigation system. 
     In accordance with this exemplary system  36  as depicted in  FIG. 1 , the patch electrodes include right side patch  44   X1 , left side patch  44   X2 , neck patch  44   Y1 , leg patch  44   Y2 , chest patch  44   Z1 , and back patch  44   Z2 ; and each patch electrode is connected to a switch  48  (e.g., a multiplex switch) and a signal generator  50 . The patch electrodes  44   X1 ,  44   X2  are placed along a first (x) axis; the patch electrodes  44   Y1 ,  44   Y2  are placed along a second (y) axis, and the patch electrodes  44   Z1 ,  44   Z2  are placed along a third (z) axis. Sinusoidal currents are driven through each pair of patch electrodes, and voltage measurements for one or more position sensors (e.g., ring electrodes  34  or a tip electrode located near the distal end  32  of catheter shaft  28 ) associated with the catheter  14  are obtained. The measured voltages are a function of the distance of the position sensors from the patch electrodes. The measured voltages are compared to the potential at the reference electrode and a position of the position sensors within the coordinate system  46  of the navigation system is determined. 
     The magnetic-field-based positioning system  38  in this exemplary embodiment employs magnetic fields to detect the position and orientation of the catheter  14  within the body  12 . The system  38  may include the GMPS system made available by MediGuide, Ltd. and generally shown and described in, for example, U.S. Pat. No. 7,386,339 titled “Medical Imaging and Navigation System,” the entire disclosure of which is hereby incorporated by reference as though fully set forth herein. In such a system, a magnetic field generator  52  may be employed having three orthogonally arranged coils (not shown) to create a magnetic field within the body  12  and to control the strength, orientation, and frequency of the field. The magnetic field generator  52  may be located above or below the patient (e.g., under a patient table) or in another appropriate location. Magnetic fields are generated by the coils and current or voltage measurements for one or more position sensors (not shown) associated with the catheter  14  are obtained. The measured currents or voltages are proportional to the distance of the sensors from the coils, thereby allowing determination of a position of the sensors within a coordinate system  54  of system  38 . 
     The display  40  is provided to convey information to a physician to assist in diagnosis and treatment. The display  40  may comprise one or more conventional computer monitors or other display devices. The display  40  may present a graphical user interface (GUI) to the physician. The GUI may include a variety of information including, for example, an image of the geometry of the tissue  16 , electrophysiology data associated with the tissue  16 , graphs illustrating voltage levels over time for various electrodes  34 , and images of the catheter  14  and other medical devices and related information indicative of the position of the catheter  14  and other devices relative to the tissue  16 . 
     The ECU  42  provides a means for controlling the operation of various components of the system  10 , including the catheter  14 , the ablation generator  22 , and magnetic generator  52  of the magnetic-field-based positioning system  38 . The ECU  42  may also provide a means for determining the geometry of the tissue  16 , electrophysiology characteristics of the tissue  16 , and the position and orientation of the catheter  14  relative to tissue  16  and the body  12 . The ECU  42  also provides a means for generating display signals used to control the display  40 . 
     As the catheter  14  moves within the body  12 , and within the electric field generated by the electric-field-based positioning system  36 , the voltage readings from the electrodes  34  change, thereby indicating the location of catheter  14  within the electric field and within the coordinate system  46  established by the system  36 . The ring electrodes  34  communicate position signals to ECU  42  through a conventional interface (not shown). In order to avoid introducing undesirable shift or drift into the determined catheter position and orientation based upon readings obtained by the electric-field based positioning system  36 , it can be important to know when the catheter electrodes  34  are inside the introducer. In particular, if the catheter electrodes  34  are located inside the introducer, the data coming off of those shielded electrodes may be degraded/compromised. 
       FIG. 2A  is a schematic diagram of a first transducer (e.g., an emitter)  56  and a first sensor  58 , where the first transducer  56  can be used to determine the location of the first sensor  58 , in accordance with embodiments of the present disclosure. The first transducer  56  can have a first longitudinal axis represented by a line A-A. The first sensor  58  can have a second longitudinal axis represented by a line B-B. The first transducer  56  can be positioned with the first longitudinal axis at an angle with respect to a second longitudinal axis represented by a line B-B of the first sensor  58 . For example, the angle between the lines A-A and B-B can be 90°. 
     The first transducer  56  can be used to generate, for example, a magnetic field  60 A-B (e.g., the first transducer  56  can be a coil or a magnetic field generator similar to magnetic generator  52  (e.g., a “drive coil”)). Other embodiments of the transducer can emit other electro-magnetic fields. The magnetic field  60 A-B can be used to determine a position of the first sensor  58 . The magnetic field  60 A-B generated can have various field lines as shown in  FIG. 2A . The magnetic field  60 A-B can generate zero (0) voltage, as measured across the first sensor  58 , when the first sensor  58  is in a specific position (e.g., the center (longitudinally) of the first sensor  58  is aligned with a longitudinal axis of the first transducer  56 ). 
     In some embodiments, the first transducer  56  and the first sensor  58  can be the same physical structure (e.g., same size, same components/elements) where the difference is how the two are used. In that embodiment, the first transducer  56  can be used to generate a field (e.g., a magnetic field) and the first sensor  58  can be used to sense the field (e.g., measure a voltage across the first sensor  58 ). In another embodiment, the functions could be reversed (e.g., the first sensor  58  could be used to generate a field (e.g., a magnetic field) and the first transducer  56  could be used to sense the field (e.g., measure a voltage across the first transducer  56 ). 
     For example, the magnetic field  60 A-B can be positioned such that a voltage can be measured across the first sensor  58 , and that voltage can convey the location of the first sensor  58  within the magnetic field and along the longitudinal axis represented by the line B-B. For example, the voltage can be analyzed to determine the location of the second sensor. When the center of the first sensor  58  is to the left of the longitudinal axis in magnetic field  60 A, represented by the line A-A, of the first sensor, the voltage measured can be a positive voltage value (see  FIG. 2B  and related discussion). When the first sensor  58  is centered in the field as shown in  FIG. 2A , the voltage measured across the first sensor  58  can be zero or close to zero. When the center of the first sensor  58  is to the right of the longitudinal axis in magnetic field  60 B, represented by the line A-A, of the first transducer  56 , the voltage measured across the first sensor  58  can be a positive voltage value measured across the first sensor  58  (see  FIG. 2C  and related discussion). Electrical current can flow either way through the first transducer  56  and a positive voltage will still be measured across the first sensor  58 . A change in current direction will produce a phase shift and a polarity shift. 
     A corresponding graph  62 A in  FIG. 2A  shows the relationship of distance represented as x-axis (e.g., a position) to voltage represented as y-axis, as measured across the first sensor  58 , for first transducer  56  with respect to the first sensor  58  (as measured by movement/change of position by the first sensor  58  along the longitudinal axis B-B). As shown in  FIG. 2A  and the graph  62 A, when the first sensor  58  is centered on the first transducer  56 , the voltage can be zero. Moving the first sensor  58  in relation to the first transducer  56  (e.g., left or right) can cause a change in the measured voltage for the first sensor  58 . For example, moving the first sensor  58  to the left in magnetic field  60 A can cause the voltage to increase (e.g., a greater positive voltage value as you move left) for the first sensor  58  (e.g., see  FIG. 2B ). Moving the second sensor to the right in magnetic field  60 B can cause the voltage to increase (a larger positive voltage value as you move right) for the first sensor  58  (e.g., see  FIG. 2C ). The voltage measured across the first sensor  58  can vary when the first sensor  58  is moved along the longitudinal axis B-B. The voltage can be positive for movement to the left and the right of the centerline of graph  62 A because of a phase shift in the voltage. 
     Information about the position and/or movement of the first sensor  58  with respect to the first transducer  56  can be used by, for example, a position detection module that is part of the ECU to determine a position and/or movement of, for example, the distal end  32  ( FIG. 1 ) of the catheter  14  ( FIG. 1 ) in relation to a distal end of an introducer because of a known position of the first sensor  58  with relation to the distal end  32  of the catheter  14 . In some embodiments, the position detection module can be provided as part of the electric-field-based positioning system  36  ( FIG. 1 ) and/or the magnetic-field-based positioning system  38  (e.g., as part of the ECU  42  of  FIG. 1 ). See  FIGS. 7A-D  and related discussion below for more information. 
     The position detection module can include, for example, a processor and a memory storing non-transitory computer-readable instructions, as discussed herein (e.g., as part of the ECU  42 , or a separate processor and a separate memory, or a combination of the two). The ECU  42  may be programmed with a computer program (e.g., software) encoded on a computer-readable storage medium for assessing the distances between an electrode (e.g., electrode  34  of  FIG. 1 ) and one or more sensors (e.g., the first transducer  56  and the first sensor  58 ). The instructions can be executable to compute, for example, the amount of movement and/or position(s) of the distal end  32  (e.g., an electrode  34  of  FIG. 1 , as a distance between the first sensor  58  and the electrode  34  can be known/determined) of the shaft  28  ( FIG. 1 ) of the catheter  14  and the distal end of the introducer, the rate of change in the distances/movement, or other characteristics related to the position of the distal end  32  of the shaft  28  of the catheter  14  in relation to the distal end of the introducer. The distance between various electrodes  34  ( FIG. 1 ) can be known from previous measurements (e.g., determined during/after manufacturing of the catheter and included with the catheter) or the distance between various electrodes and/or sensors can be determined and described herein. The program can include code for calculating a value responsive to magnitudes of, for example, a voltage between the first transducer  56  and the first sensor  58 . 
     It should be understood that the system  10  (of  FIG. 1 ), particularly ECU  42 , as described above may include conventional processing apparatus known in the art, capable of executing pre-programmed instructions stored in an associated memory, all performing in accordance with the functionality described herein. It is contemplated that the methods described herein, including without limitation the method steps of embodiments of the disclosure, will be programmed in a preferred embodiment, with the resulting software being stored in an associated memory and where so described, may also constitute the means for performing such methods. Implementation of the embodiments, in software, in view of the foregoing enabling description, would require no more than routine application of programming skills by one of ordinary skill in the art. Such a system may further be of the type having both ROM, RAM, a combination of non-volatile and volatile (modifiable) memory so that the software can be stored and yet allow storage and processing of dynamically produced data and/or signals. 
       FIG. 2B  is a schematic diagram of an exemplary embodiment of the first transducer  56  and the first sensor  58  of  FIG. 2A , where the first transducer  56  can be used to determine the location of the first sensor  58 , in accordance with embodiments of the present disclosure. A corresponding graph  62 B in  FIG. 2B  shows the relationship of distance (e.g., a position) to voltage for the first transducer  56  and the first sensor  58 . As shown in  FIG. 2B  and the graph  62 B, when the center of the first sensor  58  is to the left of the center on the first transducer  56  (in magnetic field  60 A), the voltage, as measured across the first sensor  58 , can be a positive voltage value. (e.g., a larger positive voltage value as the first sensor  58  is moved left in  FIG. 2B ) for the second sensor. As the magnetic sensor  58  is moved left (or sensor  56  is moved right, this is essentially the same thing), more of the flux lines  60 A are able to move through the core—thereby creating more voltage. Any time there are flux lines  60 A and  60 B that can intercept the core at the same time, they work to cancel each other out, thereby reducing the voltage from its maximum potential. 
       FIG. 2C  is a schematic diagram of another exemplary embodiment of the first transducer  56  and the first sensor  58  of  FIG. 2A , where the first transducer  56  can be used to determine the location of the first sensor  58 , in accordance with embodiments of the present disclosure. A corresponding graph  62 C in  FIG. 2C  shows the relationship of distance (e.g., a position) to voltage for the first transducer  56  and the first sensor  58 . As shown in  FIG. 2C  and the graph  62 C, when the first sensor  58  is to the right of the center on the first transducer  56  (in magnetic field  60 B), the voltage, as measured across the first sensor  58 , can be positive (a larger positive voltage value as the first sensor  58  is moved right in  FIG. 2C ) for the first sensor  58  (see above discussion for additional detail). 
       FIG. 3  is a schematic diagram of a transducer assembly  64  that can be used for locating a position of a medical device, in accordance with embodiments of the present disclosure. The transducer assembly  64  can include a conductive winding (e.g., a coil)  66 , a core  68 , a tube  70 , a coating  72  disposed on and/or surrounding the core  68 , a conductive connector (e.g., a pair of wires)  74 , and a tube end  76 . In some embodiments, the transducer assembly  64  can be considered a “drive coil” (e.g., used to create a magnetic field; similar to the first transducer  56  in  FIGS. 2A-C ) for a another sensor (e.g., a “sensing” sensor; similar to the first sensor  58  in  FIGS. 2A-C ). The coating  72  on and/or surrounding the core  68  can be any suitable material including for example, an adhesive, a polymer (e.g., polyimide, fluorinated ethylene propylene, heat shrink (e.g., nylon, polyolefin), a metal, etc. The coating  72  can be used to create the size and/or shape of the sensor assembly. For example, the thicker the layer of the coating  72  (e.g., as measured radially from the core  68 ) the larger the diameter of the core/coil/coating portion of the transducer assembly  64 . The coating  72  can cover all of the coil  66  or a portion of the coil  66 . The coating  72  can be the same thickness as shown in  FIG. 3  (e.g., as measured radially from the core  68 ) or it can have varying thickness at different portions along the core  68  (e.g., see  FIG. 5  and related discussion). The pair of wires  74  can be, in some embodiments, a twisted pair of wires as shown in  FIG. 3 . The pair of wires  74  can be used to electrically couple the transducer assembly  64  to a power supply and/or to an electronic control unit (e.g., ECU  42 ). 
     The transducer assembly  64  can be used to generate a field (e.g., a magnetic field similar to the field shown in  FIGS. 2A-C  and described above). The field can be used for tracking the location of another sensor. Placement of the transducer assembly  64  is one factor that can affect the accuracy of the tracking of the location of the other sensor. As described above, aligning the transducer assembly  64  so that a longitudinal axis of the first transducer  56  is perpendicular to the path of movement that the first sensor  58  travels can provide more accurate information about the position of the first sensor  58  that is moving/changing position relative to the first transducer  56  (the drive coil). 
     Misalignment between a longitudinal axis (e.g., line C-C) of the sensor (e.g., the coil  62 A and the core  64 A) and the longitudinal axis (e.g., line D-D) of the sensor assembly (e.g., the tube) (e.g., the sensor is not “straight” inside the sensor assembly; see  FIG. 4  and below for more information) can introduce variations in the information between the actual location of the second sensor and the estimated location as the second sensor changes position/moves. For example, manufacturing tolerances can cause the tube  70  can be large enough to cause up to 0.5° angulation between the longitudinal axis of the tube  70  and the longitudinal axis of the coil  66  and the core  68 . This angulation can lead to issues (e.g., errors) in accurately detecting the position of sensors (e.g., inside catheters or other locations). For example, a misalignment of the coil and core of the first transducer  56  ( FIG. 2A ) can lead to an erroneous voltage (graph  62 A) for a given position of the first sensor  58 . See additional discussion below related to  FIG. 4  for additional information. 
       FIG. 4  is a schematic diagram of a transducer assembly  64 A, in accordance with embodiments of the present disclosure. A transducer assembly  64 A can include a coil  66 A, a core  68 A and a tube  70 A where the coil  66 A and the core  64 A can be coupled internally with a tube  70 A similar to  FIG. 3 . The coil  66 A and the core  68 A can be coupled with the tube  70 A through any suitable method, including a friction fit and/or adhesive. A diameter  74  of the tube  74 A can be sized to be slightly larger than a diameter of the coil  66 A and the core  68 A. The coil  66 A can be a winding (e.g., of wire) wrapped around the core  68 A. 
     In some instances, it is desirable to have a longitudinal axis of the coil  66 A and the core  68 A, represented by the line C-C align with a longitudinal axis of the tube  70 A, represented by the line D-D. The alignment of the two longitudinal axes C-C and D-D can allow for more accurate evaluation of the position of second sensor (e.g., first sensor  58  of  FIG. 2A -) with respect to the transducer assembly  64 A (e.g., first transducer  56  of  FIGS. 2A-C ). However, because of, for example, variations in the sizes of the coil  66 A, the core  68 A (e.g., a diameter  80  of the coil and core together), and a diameter  78  of the tube  70 A, it is possible for the longitudinal axis of the coil  66 A and the core  68 A (represented by the line C-C) to be offset by an angle θ from the longitudinal axis of the tube  70 A (represented by the line D-D) as shown in  FIG. 4 . As discussed above, the offset represented by the angle θ can cause up to 0.5° of angulation between the longitudinal axis of the tube  70 A and the longitudinal axis of the coil  66 A and the core  68 A which can lead to errors in determining a location/position of a second sensor (e.g., transducer assembly  64 A) with respect to a first transducer (a drive coil). 
       FIG. 5  is a side view of an transducer assembly  64 C that can be used for locating and/or tracking a position of a medical device, in accordance with embodiments of the present disclosure. The transducer assembly  64 C can include a coil  66 C, a core  68 C, a coating  72 C, and a pair of wires  74 C. The coil  66 C can have a first end  66 C 1 , a second end  66 C 3 , and an intermediate portion  66 C 2  and and can be a wire wound around the a portion of the core  68 C (directly or indirectly) in a generally concentric pattern (although any suitable pattern can be used, which could differ from the exemplary arrangement shown in  FIG. 5 ). The pair of wires  74 C can connect to the coil  66 C at the intermediate portion of the coil  66 C allowing for easier alignment/placement of the coil  66 C. The pair of wires  74 C can be integral to the coil  66 C (e.g., each wire of the pair of wires  74 C is separately wound around the core  68 C making the pair of wires  74 C and the coil  66 C the same element) or the pair of wires  74 C can be separate and coupled with the coil  66 C (e.g., soldered or otherwise attached, see  FIG. 6  as an exemplary embodiment). The pair of wires  74 C can be a twisted pair in some embodiments. The pair of wires  74 C can be generally perpendicular to a winding axis (e.g., a longitudinal axis) of the core  68 C (e.g., a “T-sensor” or “T-shaped sensor”). In some embodiments, the pair of wires  74 C can be configured at an angle not perpendicular to the longitudinal axis of the core  68 C. 
     The transducer assembly  64 C can also include a pair of protrusions  82 . The pair of protrusions  82  can be any portion of the transducer assembly  64 C that protrude from the transducer assembly  64 C allowing the transducer assembly  64 C to be coupled with, for example, an opening or slot (e.g., a receptacle) in a fixture or other structure (e.g., a transducer assembly receptacle  92  in  FIGS. 7A-D ). The opening/slot can allow the transducer assembly  64 C to be aligned in a specific manner as the pair of protrusions  82  can couple with portions of the the opening/slot (see  FIGS. 7A-D  and related discussion below). The alignment of the transducer assembly  64 C can be precise due to the construction of the transducer assembly  64 C and the opening/slot in the fixture, including the coupling of the protrusions  82  with the opening/slot in the fixture, which allows the direction of vectors of fields (e.g., magnetic fields) associated with the transducer assembly  64 C. While  FIG. 5  shows a pair of protrusions, some embodiments could have a single protrusion or more than two protrusions to be used for coupling and/or aligning the transducer assembly  64 C with a receptacle. Because the pair of wires  74 C can be coupled with the coil  66 C at the intermediate portion and not the first end and/or the second end, the pair of wires  74 C does not interfere with the protrusions and couple of the transducer assembly  64  with, for example, the opening/slot in the fixture, which allows for precise alignment of the transducer. 
     The coating  72 C on and/or surrounding the core  68 C can be any suitable material including for example, an adhesive, a polymer, (e.g., polyimide, fluorinated ethylene propylene, heat shrink (e.g., nylon, polyolefin), a metal, etc. The coating  72 C can be used to create the size and/or shape of the sensor assembly. For example, the thicker the layer of the coating  72 C (e.g., as measured radially from the core  68 C) the larger the diameter of the core/coil/coating portion of the transducer assembly  64 C. The coating  72 C can cover all of the coil  66  or a portion of the coil  66 . The coating  72 C can be the same thickness (e.g., as measured radially from the core  68 C) or it can have varying thickness at different portions along the core  68 C (e.g., see  FIG. 6  and related discussion below). 
     As described above, the position detection module can include, for example, a processor and a memory storing non-transitory computer-readable instructions, as discussed herein (e.g., as part of the ECU  42  in  FIG. 1 , or a separate processor and a separate memory, or a combination of the two). The ECU  42  may be programmed with a computer program (e.g., software) encoded on a computer-readable storage medium for assessing the distances between, for example, an electrode and one or more sensors. The instructions can be executable to compute, for example, movement and/or position(s) of the distal end (e.g., a tip electrode and/or one or more of the electrodes  34  of  FIG. 1 ) of a catheter (e.g., the catheter  14  of  FIG. 1 ) and a distal end of an introducer, the rate of change in the distances, or other characteristics related to the position of the distal end of the catheter in relation to the distal end of the introducer. The program can include code for calculating a value responsive to magnitudes of, for example, a voltage across a sensor (e.g., a sensor located on the catheter) and a drive coil (e.g., a transducer used to generate/emit a field and/or a signal). The magnitude of the voltage can be used to determine, for example, a distance between electrodes on a catheter, a position of a sensor on a catheter with respect to a another sensor (e.g., a sensor on a test fixture, a sensor on an introducer, etc.). 
       FIG. 6  is a schematic view of a transducer assembly that can be used for locating and/or tracking a position of a medical device, in accordance with embodiments of the present disclosure. The transducer assembly  64 D can include a coil  66 D, a core  68 D, a coating  72 D, and a pair of wires  74 D. The coil  66 D can have varying diameters in relation to a longitudinal axis of the core  64 D, represented by the line E-E, as shown in  FIG. 6 . In some embodiments, the diameter (e.g., as measured radially from the core  62 D) of the coil  66 D can be variable as depicted. In other embodiments, the diameter of the coil  66 D can be the same (e.g., see  FIGS. 3 and 5  and related discussion). 
     The coating  72 D on and/or surrounding the core  68 C can be any suitable material including for example, an adhesive, a polymer, (e.g., polyimide, fluorinated ethylene propylene, heat shrink (e.g., nylon, polyolefin), a metal, etc. The coating  72 D can be used to create the size and/or shape of the sensor assembly. For example, the thicker the layer of the coating  72 D (e.g., as measured radially from the core  68 D) the larger the diameter of the core/coil/coating portion of the transducer assembly  64 D. The coating  72 D can cover all of the coil  66 D or a portion of the coil  66 D. The coating  72 C can be the same thickness (e.g., as measured radially from the core  68 ) or it can have varying thickness at different portions along the core  68 D (e.g., see  FIG. 6  and related discussion below). 
     In some embodiment, the transducer assembly  64 D can be coupled with an elongate medical device (e.g., an introducer or a catheter sheath). When a distal end of a shaft (e.g., distal end  32 ) of a catheter (e.g., catheter  14  of  FIG. 1 ) that includes one or more electrodes (e.g., electrodes  34 ) that are known distances from a sensor (e.g., the second sensor in  FIGS. 2A-C ) coupled with the catheter, the transducer assembly  64 D can be used to determine a location of the catheter sensor (e.g., the second sensor in  FIGS. 2A-C ) and therefore the electrodes on the catheter with respect to the sensor on the introducer (e.g., a drive coil/the second sensor in  FIGS. 2A-C ). As the distance between the drive coil (e.g., transducer assembly  64 D) and the distal end of the introducer can be known (e.g., predetermined or measured), the position of the sensor (e.g., the second sensor in  FIGS. 2A-C ) on the catheter and, therefore, the position of the electrodes can be determined by, for example, an electronic control unit (e.g., ECU  42  in  FIG. 1 ). This arrangement can be used to determine a position for electrodes and/or sensors on a catheter when portions of the catheter are concealed inside the introducer (e.g., where the introducer shields/inhibits the electrodes and/or other sensors (e.g., magnetic coils) from providing information about the location/position of the catheter with respect to the introducer. 
     The embodiment of the transducer assembly  64 D shown in  FIG. 6  omits the tube  70 A included in the sensor assembly  62 A of  FIG. 4 . The lack of the tube eliminates the issue described above related to offset/and or misalignment that can cause up to 0.5° of angulation between the longitudinal axis of the tube  70 A and the longitudinal axis of the coil  66 A and the core  68 A which can lead to errors in determining a location/position of a second sensor (e.g., transducer assembly  64 A) with respect to a first transducer (a drive coil). The lack of the tube allows for easier (e.g., more accurate) placement and/or alignment of the transducer assembly  64 D to align the longitudinal axis of the transducer assembly  64 D (represented by line E-E) with a path of travel for another sensor (e.g., the first sensor  58  of  FIGS. 2A-C ). 
       FIG. 7A   1  is a top and partial cross-sectional view of an apparatus  84  for measuring sensor spacing on an elongate medical device, where the apparatus includes a transducer assembly  64 E that can be used for generating a field used for measuring the spacing on the elongate medical device, in accordance with embodiments of the present disclosure. The apparatus (or fixture)  84  can include the transducer assembly  64 E and a fixture  84 . 
     The fixture  84  can be configured to temporarily couple with a portion of a catheter (e.g., catheter  14  of  FIG. 1 ). In one embodiment, the fixture  84  can include a tube  86 . The tube  86  can be arranged so the longitudinal axis of the transducer assembly  64 E, represented by the line F-F, is perpendicular to a longitudinal axis of the tube, represented by the line G-G. The fixture can also include a base plate  88 . The tube  86  can be removably coupled with the base plate  88 . The base plate  88  can include a mounting hole  90  and a transducer assembly receptacle  92 . The mounting hole  90  can be used to secure the base plate  88  (e.g., to a surface or another apparatus). 
     The transducer assembly receptacle  92  can be configured to couple with a transducer assembly (e.g., the transducer assemblies  64 A-E). The transducer assembly receptacle  92  can include portions (e.g., slots, notches, recesses, openings, etc.) that accept the protrusions (e.g., the protrusions  82 ) of the transducer assembly (e.g., the transducer assemblies  64 A-E) to provide alignment of the transducer assembly (e.g., perpendicular to tube  86 ). The protrusions  82  can be, for example, portions of the core (e.g., the core  68 C/ 68 D of  FIG. 5 / FIG. 6 . The transducer assembly receptacle  92  can have portions that are closely sized to match portions of the transducer assembly  64 . For example, the the transducer assembly receptacle  92  could have portions that are close in size to the protrusions  82  (e.g., close tolerance to provide a precise fit, such as an interference fit) to allow for precise alignment of the protrusions  82 , which in turn, allows for precise alignment of the transducer assembly  64  with respect to the fixture  84 . In some embodiments, the transducer assembly receptacle can be adjustable (e.g., via set screws or other features that allow for fine adjustment of the alignment of the transducer assembly). 
     The transducer assembly  64 E can be a sensor similar to those described herein, where the transducer assembly  64 E can generate a signal and/or field (e.g., a magnetic field) that can be used to, for example, determine the position and/or location of a sensor on a catheter. The transducer assembly  64 E can be coupled with the fixture  80 . As discussed herein, the transducer assembly  64 E can be mounted to the fixture  80  so that the longitudinal axis (line F-F) of the transducer assembly  64 E is perpendicular to the longitudinal axis of a tube  86 , represented by the line G-G. 
     The tube  86  can have an open end  94  and a closed end  96 . The tube  86  can have a diameter  98  to accommodate the insertion of a portion of a catheter (e.g., catheter  14  of  FIG. 1 ) into the open end  94  of the tube  86 . The tube  86  can include, in some embodiments, portions of the tube wall  100  can be modified (e.g., cut out for windows, translucent sections of material, etc.) to facilitate viewing of the catheter while inserted into the tube  86  (not shown). The tube  86  can be removably coupled with the base plate  88  to allow for tubes of different diameter to be coupled with the base plate to accommodate catheters with different diameters. The tube  86  may be sized to allow insertion of the catheter (e.g., catheter  14  of  FIG. 1 ) into the tube  86 , but to limit excess movement of the catheter in the tube (e.g., to limit and/or minimize misalignment of the catheter within the tube with respect to the sensor assembly). 
     In some embodiments, the closed end  96  can be a conductive material (e.g., a metal). The catheter (e.g., catheter  14  of  FIG. 1 ) can be inserted until a distal top of the catheter is in contact with the closed end  96 . With a conductive surface on the catheter distal end (e.g., a tip electrode) in contact with the conductive material of the closed end  96 , contact of the catheter and the closed end  96  can be verified using, for example, a multimeter or similar tool, or a circuit can be used with an audible alert. Once contact with the catheter and the closed end  96  is confirmed, the catheter can be moved back (e.g., pulled out of the tube  86 ) until the sensor assembly (e.g., sensor assembly  63 E) is aligned with the center of the sensor on the catheter (e.g., sensor  58 A as shown in  FIG. 7C  below). The distance the catheter is moved back is the distance from the tip of the catheter to the center of the sensor (e.g., sensor  58 A as shown in  FIG. 7C   
     Because the tube  86  can be arranged with the longitudinal axis of the transducer assembly  64 E (line F-F) aligned (e.g., perpendicular) with respect to a longitudinal axis of the tube  86  (represented by the line G-G), a magnetic field generated by the transducer assembly  64 E can be used to determine a location and/or spacing of sensors (e.g., electrodes, magnetic coils, etc.) on a catheter. The catheter can be inserted into the open end  94  of the tube  86  and moved through the tube  86 . Longitudinal movement of the catheter (e.g., catheter  14  of  FIG. 1 ) can generate a signal (e.g., a voltage measured across a sensor coupled with the catheter) and the signal can indicate a longitudinal position of the sensor coupled with the catheter (e.g., the distance of the sensor from a tip, a distance between a first sensor and a second sensor, etc.). 
     Rotational movement of the catheter (e.g., rotating clockwise or counter-clockwise about the longitudinal axis of the catheter) can cause variations in the signal. The variations in the signal can be used to determine an orientation of the sensor on the catheter, including, for example, locations of multiple sensors the same distance from the tip, but spaced about a circumference of the catheter. For example, if two magnetic sensors (e.g., a first sensor and a second sensor) are located on the catheter, where the first sensor generates a higher voltage, the first sensor is closer to the drive coil and the second sensor is further from the drive coil. If the catheter is rotated 180°, then the second sensor will generate a higher voltage and the first sensor will generate a lower voltage. This concept can be used to determine the rotational location/position of sensors with respect to the drive coil. 
     The fixture  84  can be used during the manufacturing process (e.g., to check, calibrate, identify, etc.) of the catheter (e.g., catheter  14  of  FIG. 1 ) to assist with desired placement of sensors on the catheter. The fixture can also be used to identify the spacing and/or location of sensors to, for example, provide information to be used in medical procedures (e.g., catheters that are purchased from other sources and used after manufacturing). In some embodiments, the fixture  84  can be used to determine distances between various electrodes and/or sensors of unknown spacing on a catheter. In another embodiment, the fixture  84  can be used to confirm distances between various sensors and electrodes on a catheter. For example, once the spacing and/or location of sensors is determined using the information described herein, data regarding the sensor locations can be input and/or used by a system (e.g., ECU  42  of  FIG. 1 , etc.) to be used during a medical procedure that incorporates the catheter that was measured. This distance/location information can be useful for situations where portions of a catheter (e.g., catheter  14  of  FIG. 1 ) are shielded by an introducer during a procedure, preventing the use of other positioning determinations as described above. 
       FIG. 7A   2  is a side view of the fixture  84  of  FIG. 7A   1 , in accordance with embodiments of the present disclosure. As described above, the fixture  84  can include the tube  86  coupled with the base plate  88 . The tube  86  can have the open end  94  and the closed end  96 . The base plate can include a mounting hole  90  and a transducer assembly receptacle  92 . As shown in  FIG. 7A   2  the transducer assembly receptacle  92  can be an integral part of the base plate  88 . In some embodiments, the transducer assembly receptacle  92  can be a separate element that is coupled with the base plate  88  (e.g., coupled with a surface of the base plate  88  instead of a cavity formed in the base plate  88 ). 
       FIG. 7B  is a schematic view of the fixture  84  of  FIGS. 7A   1  and  7 A 2  with an exemplary catheter  14 A that includes a sensor  58 A mounted on the catheter  14 A, where the catheter  14 A is movable relative to the transducer assembly  64 E, in accordance with embodiments of the present disclosure. Similar to the discussion above related to  FIGS. 7A   1  and  7 A 2 , the fixture  84  can include a transducer assembly  64 E (e.g., a first transducer or a drive coil), a tube  86 , where the tube  86  is coupled with the base plate  88  and the base plate  88  can include a mounting hole  90 . The tube  86  can include the open end  94  and the closed end  96  and the diameter  98  of the tube  86  can be sized to allow the catheter  14 A to be inserted into the tube  86 . The tube  86  can be removably coupled with the base plate as discussed above. 
     The catheter  14 A can be inserted into an open end  94  of a tube  86  so that a first sensor  58 A (e.g., a magnetic coil) is in a first position (e.g., a position that places the first sensor  58  to the right (as shown in  FIG. 7B ) in the tube with respect to the transducer assembly  64 E (represented by the line F-F)). Similar to the discussion related to  FIG. 2C  above, a voltage, as measured across the first sensor  58 A, can be positive for the first sensor  58 A (e.g., a larger positive voltage value as you move the first sensor  58 A further to the right of the line F-F). See the discussion above related to  FIG. 2C  for more information. 
     As described herein, a position detection module can compute, for example, movement and/or position(s) of the catheter  14 A based on, for example, changes in voltage measured across the sensor  58 A. The distances between the sensor  58 A and an electrode  34 A (e.g., a tip electrode) and electrodes  34 B and  34 C of a catheter  14 A can be known (e.g., measured prior to and/or during use). With the distances between the sensor  58 A and the electrodes  34 A-C known, the current position, past positions, distance between positions, the rate of change in the distances, or other characteristics related to the position of the distal end of the catheter in relation to the distal end of the introducer can be tracked. The program (e.g., instructions) can include code (instructions) for calculating a value responsive to magnitudes of, for example, a voltage across a sensor (e.g., a sensor located on the catheter) and a drive coil (e.g., a transducer used to generate a field and/or a signal). The value can be used with information related to distances between various parts of the catheter  14 A (e.g., the electrodes  34 A-C, additional sensors, etc.) to determine a position of the catheter  14 A with respect to the transducer assembly  64 E. The position of the catheter  14 A can then be output to a display (e.g., display  40 ). The position information (e.g., a position data) can be in the form of data (e.g., coordinates, distances, etc.) and/or in a graphical representation of the catheter  14 A as it moves. 
     Changes in voltage can be a characteristic of the relationship of changes in distance between a first sensor (e.g., transducer assembly  64 E) and a second sensor (e.g., the sensor  58 A). Other characteristics that can be used include, for example, measuring changes in any one or more of a current, a magnetic field strength, a magnetic flux density, and/or other properties related to sensors and/or fields generated by the first sensor as described herein. 
       FIG. 7C  is a schematic view of the apparatus  84  of  FIGS. 7A   1  and  7 A 2  with the exemplary catheter  14 A of  FIG. 7B , where the catheter  14 A has moved relative to the transducer assembly  64 E, in accordance with embodiments of the present disclosure. In  FIG. 7C , the catheter  14 A is moved longitudinally in the tube  86  to a second position (e.g., from the open end  94  towards the closed end  96 ), where the first sensor  58 A (e.g., a magnetic coil) is centered (as shown in  FIG. 7C ) on the longitudinal axis of the transducer assembly  64 E (represented by the line F-F). Similar to the discussion related to  FIG. 2B  above, a voltage, as measured across the first sensor  58 A, can be zero for the first sensor  58 A when the first sensor  58 A is in the second position. See the discussion above related to  FIG. 2B  for more information. 
       FIG. 7D  is a schematic view of the fixture  84  of  FIGS. 7A   1  and  7 A 2  with the exemplary catheter  14 A of  FIGS. 7B-C , where the catheter  14 A has moved relative to the transducer assembly  64 E, in accordance with embodiments of the present disclosure. The catheter  14 A can be inserted further into the tube  86  (e.g., from the open end  94  towards the closed end  96 ) so that a first sensor  58 A (e.g., a magnetic coil) is in a third position of (e.g., a position that places the first sensor  58  to the left (as shown in  FIG. 7D ) of the longitudinal axis of the transducer assembly  64 E (represented by the line F-F)). Similar to the discussion related to  FIG. 2A  above, a voltage, as measured across the first sensor  58 A, can be positive for the first sensor  58 A (e.g., a larger positive voltage value as you move the first sensor  58 A further to the left of the line F-F). See the discussion above related to  FIG. 2A  for more information. 
     Another embodiment (not shown) similar to  FIGS. 7B-D  can include a sensor assembly (e.g., transducer assembly  64 E) coupled with an introducer. Similar to the arrangement shown in  FIGS. 7B-D , movement of a catheter (e.g., catheter  14 A) within the introducer would allow for a position detection module to track the location/position of the catheter as it moves within the introducer. For example, the transducer assembly  64 E of  FIGS. 7B-D  could be coupled with an introducer, where a longitudinal axis of the sensor assembly (e.g., longitudinal axis represented by line F-F in  FIGS. 7B-D ) is perpendicular to the longitudinal axis of the introducer, which would be the path of travel for a catheter in the introducer (e.g., similar to the catheter  14 A traveling through the tube  86  as shown in  FIGS. 7B-D ). The position of electrodes and/or sensors on the catheter could be determined as described herein. 
       FIG. 8  shows an exemplary method for detecting the position of a sensor on an elongate medical device, consistent with embodiments of the present disclosure. A method  110  can include positioning a catheter at an open end of a tube on a fixture at a block  112 , inserting the catheter into the open end of the tube at a block  114 , emitting a field from a first transducer at a block  116 , measuring a signal at a first sensor on the catheter while moving the catheter in a longitudinal direction at a block  118 , analyzing the signal, by an ECU comprising a processor, from the first sensor at a block  120 , determining, by the ECU, a position of the first sensor based on the signal at a block  122 , and outputting, by the ECU, the position to a display at a block  124 , in accordance with embodiments of the present disclosure. 
     Portions of the method can be repeated for catheters that have more than one sensor. For example, after the first sensor&#39;s position is detected as the catheter is moved in the longitudinal direction, the method can repeat by measuring the signal at the second sensor on the catheter while moving the catheter in the longitudinal direction; analyzing the signal, by an ECU comprising a processor, from the second sensor, determining, by the ECU, the position of the second sensor based on the signal, and outputting, by the ECU, the position to the display. 
       FIG. 9  is a schematic view of an optic-magnetic registration plate (OMRP), consistent with embodiments of the present disclosure. An OMRP  130  can include a plate  132  coupled with a plurality of objects  134  and a plurality of transducer assemblies  136  and a plurality of transducer assembly receptacles  138 . Each of the plurality of objects  134  can comprise, for example, one or more radiopaque fiducial markers. The plurality of objects  134  can either be located at a known position or the positions can be determined (e.g., by the system  10  of  FIG. 1 ). The plurality of objects  134  can be any suitable shape (round, square, oval, triangular, etc.) and made from any suitable material to be visible in the system being used (e.g., radiopaque for systems using x-rays, etc.). The plurality of objects  134  can be arranged in any suitable pattern. Each of the plurality of objects  134  can be the same size or the sizes can vary. 
     The plurality of transducer assemblies  136  can be located at any suitable known location on the OMRP  130  with respect to the plurality of objects  134 . In one embodiment, the plurality of transducer assemblies  136  can include three transducer assemblies,  136   A ,  136   B ,  136   C , that can be arranged so that a longitudinal axis of each of the transducer assemblies form an angle of, for example, 0°, 45° and 90° with respect to the longitudinal axis of the OMRP  130 . The orientations of the three of transducer assemblies  136   A ,  136   B ,  136   C  can be any three angles with respect to the longitudinal axis of the OMRP  130  (e.g., 5°, 50° and 95°; 10°, 45° and 90°; 0°, 60° and 90°; 30°, 60° and 100°, etc.) as provided by the available locations of the transducer assembly receptacle  138  locations on the OMRP  130 . The OMRP  130  can have multiple transducer assembly receptacle  138  locations (e.g., more than three) to allow for the transducer assemblies  136  to be placed in various locations on the OMRP  130  to allow for different configurations of the transducer assemblies  136 . In some embodiments the transducer assemblies  136  can be coupled with the OMRP via the transducer assembly receptacle (e.g., emitter assembly  136   A  as shown in  FIG. 9 ; also similar to the transducer assembly receptacle  92  shown in  FIGS. 7B-D  and described above) or the transducer assemblies can be coupled with the OMRP at locations without transducer assembly receptacles (e.g., transducer assemblies  136   B  and  136   C ) using other suitable techniques (e.g., adhesive, etc.). For simplicity, a conductive connector (e.g., a pair of wires  74 ) is not shown connected to each of the transducer assemblies  136   A ,  136   B , and  136   C  (see above for related discussion). 
     A system (e.g., the system  10  of  FIG. 1 ) can be used to detect a location of the plurality of objects  134  (e.g., using x-ray images, fluoroscopy, ultrasound, or any other suitable method) and a location of magnetic sensors (e.g., the transducer assemblies  136   A ,  136   B ,  136   C , first sensor  58  ( FIG. 2A ), etc.). An ECU (e.g., ECU  42 ) can use known relationships between detected items to co-register two coordinate systems. For example a first coordinate system can be relative to the system  10  (of  FIG. 1 ) and a second coordinate system can be relative to a body (e.g., the body  12  of  FIG. 1 ). The known relationships can either be measured by the system  10  or previously determined such as, for example, a set distance between detected items, such as a fixed feature on a plate (e.g., the fixed distances between the plurality of objects  134  on the OMRP  130 ). Additional information can be found in U.S. application no. 2013/0272592 (application Ser. No. 13/977,438), which is hereby incorporated by reference as if set forth fully herein. 
     Although at least one embodiment of an apparatus for detecting catheters to introducers has been described above with a certain degree of particularity, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this disclosure. All directional references (e.g., upper, lower, upward, downward, left, right, leftward, rightward, top, bottom, above, below, vertical, horizontal, clockwise, and counterclockwise) are only used for identification purposes to aid the reader&#39;s understanding of the present disclosure, and do not create limitations, particularly as to the position, orientation, or use of the disclosure. Joinder references (e.g., attached, coupled, connected, and the like) are to be construed broadly and can include intermediate members between a connection of elements and relative movement between elements and can also include elements that are part of a mixture or similar configuration. As such, joinder references do not necessarily infer that two elements are directly connected and in fixed relation to each other. It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not limiting. Changes in detail or structure can be made without departing from the spirit of the disclosure as defined in the appended claims. 
     Various embodiments are described herein to various apparatuses, systems, and/or methods. Numerous specific details are set forth to provide a thorough understanding of the overall structure, function, manufacture, and use of the embodiments as described in the specification and illustrated in the accompanying drawings. It will be understood by those skilled in the art, however, that the embodiments may be practiced without such specific details. In other instances, well-known operations, components, and elements have not been described in detail so as not to obscure the embodiments described in the specification. Those of ordinary skill in the art will understand that the embodiments described and illustrated herein are non-limiting examples, and thus it can be appreciated that the specific structural and functional details disclosed herein may be representative and do not necessarily limit the scope of the embodiments, the scope of which is defined solely by the appended claims. 
     Reference throughout the specification to “various embodiments,” “some embodiments,” “one embodiment,” or “an embodiment”, or the like, means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, appearances of the phrases “in various embodiments,” “in some embodiments,” “in one embodiment,” or “in an embodiment”, or the like, in places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. Thus, the particular features, structures, or characteristics illustrated or described in connection with one embodiment may be combined, in whole or in part, with the features structures, or characteristics of one or more other embodiments without limitation given that such combination is not illogical or non-functional. 
     It will be appreciated that the terms “proximal” and “distal” may be used throughout the specification with reference to a clinician manipulating one end of an instrument used to treat a patient. The term “proximal” refers to the portion of the instrument closest to the clinician and the term “distal” refers to the portion located furthest from the clinician. It will be further appreciated that for conciseness and clarity, spatial terms such as “vertical,” “horizontal,” “up,” and “down” may be used herein with respect to the illustrated embodiments. However, surgical instruments may be used in many orientations and positions, and these terms are not intended to be limiting and absolute. 
     Any patent, publication, or other disclosure material, in whole or in part, that is said to be incorporated by reference herein is incorporated herein only to the extent that the incorporated materials does not conflict with existing definitions, statements, or other disclosure material set forth in this disclosure. As such, and to the extent necessary, the disclosure as explicitly set forth herein supersedes any conflicting material incorporated herein by reference. Any material, or portion thereof, that is said to be incorporated by reference herein, but which conflicts with existing definitions, statements, or other disclosure material set forth herein will only be incorporated to the extent that no conflict arises between that incorporated material and the existing disclosure material.