Hollow ferromagnetic core to boost air coil signal in electromagnetic navigation

Abstract

A method of forming an electromagnetic sensor for use with a navigation system including forming a hollow ferromagnetic core into a cylindrical shape having an axial length, wrapping a wire into coil layers around the hollow ferromagnetic core, and altering the axial length of the ferromagnetic core to obtain a desired gain for the electromagnetic sensor. The desired gain is associated with a magnetic field emitter of the navigation system.

Description

A0012994W001HOLLOW FERROMAGNETIC CORE TO BOOST AIR COIL SIGNAL IN ELECTROMAGNETIC NAVIGATION

[0001] This application claims the benefit of U.S. Provisional Patent Application Serial No. 63 / 748,697, filed January 23, 2025, the entire content of which is incorporated herein by reference.FIELD

[0002] The present technology is generally related to surgical procedures, and to the use of electromagnetic navigation to track a medical device during a surgical procedure.BACKGROUND

[0003] Electromagnetic navigation is commonly used in various minimally invasive medical procedures to track the location and orientation of a catheter (or a portion of a catheter). Electromagnetic navigation includes the use of a magnetic field emitter and a magnetic sensor. The magnetic field emitter lies under a patient, and emits a magnetic field through the patient (e.g., upwardly through the patient). When the magnetic sensor is placed in this field, a small current is induced in the magnetic sensor. This current may then be used to extract XYZ position and orientation values.SUMMARY

[0004] The techniques of this disclosure generally relate to an electromagnetic sensor that combines the signal-boosting properties of the ferromagnetic core of a micro coil sensor, and also the hollow benefits of an air coil sensor. The electromagnetic sensor includes a ferromagnetic core to provide a signal boost, but the ferromagnetic core is hollow in the center. The hollow center allows the electromagnetic sensor to be embedded around the shaft of a catheter, and allows materials to be passed through the center of the ferromagnetic core, similar to an air coil sensor. The use of the hollow ferromagnetic coreA0012994W001additionally allows the overall electromagnetic sensor to be made smaller than a typical air coil sensor, due to the signal-boosting properties of the hollow ferromagnetic core.

[0005] In one aspect, the disclosure provides a method of forming an electromagnetic sensor for use with a navigation system. The method includes forming a hollow ferromagnetic core into a cylindrical shape having an axial length, wrapping a wire into coil layers around the hollow ferromagnetic core, and altering the axial length of the ferromagnetic core to obtain a desired gain for the electromagnetic sensor. The desired gain is associated with a magnetic field emitter of the navigation system.

[0006] In another aspect, the disclosure provides an electromagnetic sensor for use with a navigation system. The electromagnetic sensor includes a hollow ferromagnetic core having a cylindrical shape with an axial length. The axial length is between 4 mm and 6 mm. The electromagnetic sensor also includes a wire wrapped in a plurality of coil layers around the hollow ferromagnetic core. The plurality of coil layers is between 4 coil layers and 6 coil layers. The hollow ferromagnetic core has an overall radial thickness between 0.130 inch and 0.190 inch.

[0007] The details of one or more aspects of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the techniques described in this disclosure will be apparent from the description and drawings, and from the claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. l is a schematic view of a navigation system according to one example, including a medical device and an electromagnetic sensor positioned on the medical device.

[0009] FIG. 2 is a schematic, cross-sectional view of the medical device at a location of the electromagnetic sensor.

[0010] FIG. 3 is a side view of the medical device, showing a hollow ferromagnetic core of the electromagnetic sensor.A0012994W001

[0011] FIG. 4 is a side view of the medical device, showing a coil of wire that has been wrapped around the hollow ferromagnetic core in multiple coil layers.

[0012] FIG. 5 is a chart illustrating testing that was performed on various versions of the electromagnetic sensor, demonstrating visibility of the electromagnetic sensor.

[0013] FIG. 6 is another chart illustrating testing that was performed on various versions of electromagnetic sensor, demonstrating the effects of altering an axial length of the hollow ferromagnetic core.

[0014] FIG. 7 is another chart illustrating testing that was performed on various versions of the electromagnetic sensor, demonstrating the effects of altering the number of coil layers of the coil wire.DETAILED DESCRIPTION

[0015] Electromagnetic sensors in many designs are either a micro coil sensor or an air coil sensor. Micro coil sensors are typically comprised of a copper or gold plated wire, wrapped in a coil form, with a solid ferromagnetic core that extends through the center of the coil. The ferromagnetic core boosts the signal of the micro coil sensor, allowing for a very small coil to be used and embedded into the catheter with accurate tracking. Though small coils are generally desired in many designs, there may be use cases in which it is desired or necessary to use a larger coil, with the ability to pass items through the coil.

[0016] Air coil sensors are also typically comprised of copper or gold plated wire, wrapped in a coil form around a hollow core, permitting materials to pass through. Since the air coil sensor does not have a ferromagnetic core, however, it is generally weaker than respective coil sensors with ferromagnetic cores, and has lower strength signals which can lead to less accurate position and orientation tracking as compared to a micro coil sensor. Thus, the wrapped coil of the air coil sensor must be made larger as compared to the micro coil sensor to obtain an accuracy similar to that of the micro coil sensor.

[0017] FIG. 1 illustrates a navigation system 110 for use during a medical procedure. The navigation system 110 includes a magnetic field emitter 114 positionedA0012994W001under a patient 118, and a medical device 122 having at least one electromagnetic sensor 126 positioned on the medical device 122. In some examples, the medical device 122 is a catheter. In other examples, the medical device 122 is an implant or any other type of medical device 122 that may benefit from tracking and navigation. Overall, the navigation system 110 tracks the location of the medical device 122 through the use of the electromagnetic sensor(s) 126 and / or any other sensors located on the medical device 122.

[0018] With continued reference to FIG. 1, in some examples the navigation system 110 additionally includes a navigation control unit 130 (e.g., processor) in communication with the electromagnetic sensor(s) 126 and / or the magnetic field emitter 114. The navigation control unit 130 may send or receive signals to and from the magnetic field emitter 114 (e.g., to control the magnetic field emitter 114), and / or to and from the electromagnetic sensor 126 (e.g., to receive information from the electromagnetic sensor 126), and / or provide other general control over the navigation system 110.

[0019] With continued reference to FIG. 1, in some examples the navigation system 10 additionally interfaces with, or integrally includes, an imaging system 134. The imaging system 134 may acquire pre-operative, intra-operative, post-operative, or realtime image data of the patient 118, and in some examples may be coupled to the navigation control unit 130. The imaging system 134 may use magnetic resonance imaging (MRI), positron emission tomography imaging (PET), and / or various other types of imaging to obtain images of the patient 118. The navigation system 110 may use image data from the imaging system 134, (and for example any information from the tracking of the electromagnetic sensor 126), to illustrate various locations of the tracked medical device 122 during the medical procedure.

[0020] With reference to FIGS. 1-4, in the illustrated example the medical device 122 is a catheter that includes a hollow main body 138 (e.g., outer catheter sheath) extending along an axis Al (e.g., a longitudinal axis). The main body 138 includes a main body proximal end 142, a main body distal end 146, and a central lumen 150 (FIG. 2) that extends axially through the main body 138 along the axis Al. The main body 138 and the central lumen 150 are sized and shaped such that additional medical device componentsA0012994W001(e.g., other catheter bodies, or other medical tools or devices for use during the medical procedure) may be passed axially through the central lumen 150.

[0021] With reference to FIG. 2, in the illustrated example the electromagnetic sensor 126 is positioned (e.g., partially or completely embedded) in a wall 154 at or near the main body distal end 146 (e.g., near a tip jacket). The wall 154 defines the central lumen 150. The wall 154 and other components illustrated in FIG. 2 (and FIG. 1) are purely schematic in nature. Accordingly, the size and / or ratio of size of the components may vary and be different in different examples.

[0022] While a single electromagnetic sensor 126 is illustrated at or near the main body distal end 146, the electromagnetic sensor 126 may alternatively be placed at a different location along the medical device 122 (e.g., at or near the main body proximal end 142, or at any location between the main body proximal end 142 and the main body distal end 146), to aid in navigation and tracking of the medical device 122. In some examples, the medical device 122 includes a plurality of electromagnetic sensors 126 placed at various locations along the catheter main body 138 to aid in navigation and tracking. Each electromagnetic sensor 126 may be identical. In other examples, one electromagnetic sensor 126 may be positioned at or near the main body distal end 146, whereas another (and different) sensor may be positioned at the main body proximal end 142.

[0023] The electromagnetic sensor 126 may be used to track a position and / or orientation of the main body distal end 146 to aid a surgeon, surgical robot, or other medical personnel with understanding whether the main body distal end 146 is located properly relative to the patient 118, and to aid in navigation of the medical device 122.

[0024] While the illustrated medical device 122 is shown having a straight configuration, it is understood that one or more portions of the medical device 122 may have a curvature, and / or may be flexible so as to achieve a curvature during use.Additionally, while not illustrated, the medical device 122 may further have one or more other components at least partially embedded within, and / or in contact with, the wall 154. For example, the medical device 122 may include one or more pull wires, or pull wire rings, that are partially or entirely embedded within the wall 154, and / or may include oneA0012994W001or more liners, jackets, and / or other structure that form part of the wall 154. In some examples, the electromagnetic sensor 126 may be in contact with, or in close proximity to, one or more of these other components, and / or may extend around or underneath one or more of these other components.

[0025] In some examples, and as illustrated in FIG. 2, the wall 154 may include one or more lumens 156 (e.g., channels or other structures) that facilitate wiring for coupling the electromagnetic sensor 126 to a controller or processor (e.g., the navigation control unit 130), and / or may include one or more lumens (e.g., the same lumens 156 or other lumens) for a pull ring wire or wires, or other components. In some examples, the lumen or lumens 156 extend axially, or radially, or both axially and radially through the wall 154.

[0026] With continued reference to FIG. 2, the wall 154 has an inner diameter DI and an outer diameter D2. In the illustrated example, the inner diameter DI is 0.134 inch and the outer diameter D2 is 0.182 inch. Other examples include other values and ranges of values. In some examples, the wall 154 has an inner diameter DI of less 0.150 inch, or less than 0.140 inch, or less than 0.130 inch. In some examples, the wall 154 has an outer diameter D2 of less than 0.200 inch, less than 0.190 inch, or less than 0.180 inch.

[0027] With reference to FIGS. 2-4, the electromagnetic sensor 126 includes a hollow ferromagnetic core 158 embedded at least partially in the wall 154. The hollow ferromagnetic core 158 may be formed for example from a sheet of ferromagnetic material (e.g., Mu-metal or other suitable ferromagnetic material) that is wrapped into a cylindrical shape. In the illustrated example, the hollow ferromagnetic core 158 is formed from Mumetal (80), having 80% Ni, 5% Mo, 15% Fe, and less than 0.5% Mn and Si. The sheet may then be joined along its edges, for example via resistance welding or other suitable joining processes. In other examples, the hollow ferromagnetic core 158 may be formed and / or joined together in a different manner.

[0028] Once formed, and as illustrated in FIG. 3, the hollow ferromagnetic core 158 has an axial length LI (e.g., as measured along the axis Al). The axial length LI may also represent the overall axial length of the electromagnetic sensors 126. In the illustrated example, the hollow ferromagnetic core 158 is formed from Mu-metal, and has an axialA0012994W001length LI as measured between a first axial end 162 (FIG. 3) and an opposite, second axial end 166 (FIG. 3) of the hollow ferromagnetic core 158 along the axis Al. In some examples, the axial length LI is least 3.5 mm, at least 4.5 mm, at least 5.5mm, between 3.5 mm and 5.5 mm, between 4 mm and 5mm, or between 4 mm and 6 mm, or other values and ranges of values.

[0029] With reference to FIG. 2, once formed, the hollow ferromagnetic core 158 may have an inner diameter D3 and an outer diameter D4. In the illustrated example, the inner diameter D3 is 0.138 inch, and the outer diameter D4 is 0.140 inch. Other examples include other values and ranges of values. In some examples, the inner diameter D3 is less than 0.150 inch, less than 0.140 inch, or less than 0.130 inch. In some examples, the outer diameter D4 is less than 0.150 inch, or less than 0.140 inch, or less than 0.130 inch.

[0030] With reference to FIGS. 2 and 4, the electromagnetic sensor 126 additionally includes a wire that 170 that is wrapped onto, and around, a portion (e.g., substantial portion) of the underlying hollow ferromagnetic core 158 in at least one coil layer. In the illustrated example, and as seen in FIG. 2, the wire 170 is wrapped in four coil layers (i.e., a first coil layer 174a, a second coil layer 174b, a third coil layer 174c, and a fourth coil layer 174d). In other examples, the wire 170 forms two coil layers that extend around the hollow ferromagnetic core 158, or six coil layers, or eight coil layers, or ten coil layers, or other numbers of coil layers that extend around the hollow ferromagnetic core 158. In some examples, the wire 170 has no more than 6 coil layers, or no more than 8 coil layers, to maintain a limited overall size and profile. Each of the coil layers may extend, for example, from the first axial end 162 of the hollow ferromagnetic core 158 toward the opposite, second axial end 166, and as described above may cover a substantial portion (e.g., 80%, 85%, 90%, 95%, etc.) of the hollow ferromagnetic core 158. In some examples, and as illustrated in FIG. 4, a portion of the hollow ferromagnetic core 158 (e.g., at the second axial end 166) is left uncovered by the wire 170.

[0031] The wire 170 overall may be formed for example from CS-95(R) beryllium copper alloy, or any other suitable material, and may be silver plated, gold plated, plated with a different material, or have no plating. Other examples include other types of wireA0012994W001

[0032] With reference to FIG. 2, the coil layers of the wire 170 together form an overall coil having an inner diameter (equivalent to the outer diameter D4 of the hollow ferromagnetic core 158) and an outer diameter D5. In the illustrated example, the inner diameter of the overall coil is 0.142 inch, and the outer diameter D5 is 0.162 inch. Other examples include other values and ranges of values. In some examples, the inner diameter is less than 0.150 inch, less than 0.140 inch, or less than 0.130 inch. In some examples, the outer diameter D5 is less than 0.170 inch, or less than 0.160 inch, or less than 0.150 inch, or less than 0.140 inch.

[0033] With reference to FIG. 2, and as described above, the electromagnetic sensor 126 is at least partially embedded in the wall 154. The wall 154 may have a radial thickness, defined as a distance between the inner diameter DI and the outer diameter D2. Accordingly, an overall radial thickness of the electromagnetic sensor 126 (defined as the combination of the radial thicknesses of the hollow ferromagnetic core 158 and each of the layers of wire 170) may be less than the radial thickness of the wall 154, such that the entire electromagnetic sensor 126 is fully embedded within the wall 154. In some examples, the overall radial thickness of the electromagnetic sensor 126 is between 0.130 inch and 0.190 inch, or between 0.138 inch and 0.182 inch, or between 0.138 inch and 0.162 inch, or less than 0.200 inch, or less than 0.190 inch, or less than 0.180 inch, or less than 0.170 inch, or less than 0.160 inch, or less than 0.150 inch, or less than 0.140 inch, or less than 0.130 inch, or other values and ranges of values.

[0034] During use of the navigation system 110, the magnetic field emitter 114 emits a magnetic field. The magnetic field may induce a current within the coils of wire 170. If sufficiently strong, this current (or currents) may be detected and / or processed by the navigation control unit 130, thereby allowing the navigation system 110 to determine and track the location of the electromagnetic sensor 126 and the overall catheter (or other medical device 122).

[0035] With reference to FIGS. 1-4, the electromagnetic sensor 126 has an associated gain, defined generally as the amplification of the magnetic field flowing through the electromagnetic sensor 126. The illustrated electromagnetic sensor 126, however, has a larger, or boosted gain, as compared to an air coil of similar size (e.g.,A0012994W001diameter), due to the added hollow ferromagnetic core 158 located under the coils of wire 170. Adding the hollow ferromagnetic core 158 under the coiled wire 170 boosts the gain (and thus the signal), without needing to increase the size (e.g., outer diameter) of the coil of wire 170. The hollow nature of the hollow ferromagnetic core 158 additionally allows the electromagnetic sensor 126 to be embedded partially or entirely within the wall 154, and thereby allows the additional components (e.g., other catheter bodies, or other medical tools or other devices for use during the medical procedure) to be passed axially through the central lumen 150. In some examples, the boost in gain from use of the hollow ferromagnetic core 158 (e.g., a Mu-metal core) has additionally been found to further increase the accuracy of the overall electromagnetic sensor 126.

[0036] With reference to FIGS. 1-4, the axial length LI of the underlying hollow ferromagnetic core 158 may be altered, to adjust the gain of the electromagnetic sensor 126 and / or obtain a desired gain. For example, increasing or decreasing the axial length LI of the underlying hollow ferromagnetic core 158 significantly affects the overall gain associated with the electromagnetic sensor 126, and that this alteration may be made to form an electromagnetic sensor 126 for use with a particular application and / or with a particular (e.g., already existing) navigation system.

[0037] For example, some navigation systems (such as the magnetic field emitter designed by Northern Digital Inc. (NDI)) are designed to identify (e.g., with a read only memory (ROM) file) and work only with a particular electromagnetic sensor (e.g., a micro coil sensor such as the NDI 610157 micro coil sensor). In some examples, the navigation system identifies a sensor based on gain. Accordingly, an electromagnetic sensor may only be used with one of these systems if the electromagnetic sensor has a particular gain, or falls within a particular range of gain.

[0038] As is discussed with relation to FIG. 5, studies confirmed that altering the axial length LI of the hollow ferromagnetic core 158, and wrapping the hollow ferromagnetic core in the wire 170, creates an electromagnetic sensor 126 that may be recognized by, and used by, a particular navigation system (e.g., the magnetic field emitter designed by Northern Digital Inc. (NDI)). Further, studies confirmed that ] the electromagnetic sensor 126 may be used with the navigation system even if theA0012994W001electromagnetic sensor 126 is not a micro coil sensor, or other electromagnetic sensor commonly used by the navigation system. Further, the electromagnetic sensor 126 may be used on a medical device 122, or at a particular region of the medical device 122 (e.g., at a distal end), even if the electromagnetic sensor commonly associated with the navigation system is unable to be used on such a medical device 122 or at the particular region of the medical device 122.

[0039] Testing of the electromagnetic sensor 126 included, for example, using an air coil winder to wind the wire 170 (and coil layers of wire 170) onto the hollow ferromagnetic core 158, and then testing various electromagnetic sensors 126 (e.g., for gain and performance). During testing, the internal diameter of the electromagnetic sensor 126 (i.e., the inner diameter D3 of the hollow ferromagnetic core 158) was kept constant. The axial lengths LI of the hollow ferromagnetic core 158 and the number of coil layers of wire 170 wrapped around the hollow ferromagnetic core 158 were both altered in the testing / study. During testing, the gain of each electromagnetic sensor 126 was measured as the voltage (mV) produced by the coil of wire 170 divided by the voltage (mV) produced by a Helmholtz coil used in the testing facility.

[0040] With reference to FIG. 5, the gain of each electromagnetic sensor 126 was analyzed and plotted. As illustrated in FIG. 5, gains close to that of the NDI 610157 micro coil sensor (associated with the magnetic field emitter designed by NDI) permitted visualization with the desired ROM file. Three of the electromagnetic sensors 126 had gains similar to that of the NDI 610157 sensor (thus mimicking the NDI 610157 sensor), and were thus able to be visualized, whereas ten of the electromagnetic sensors 126 were not able to be visualized. Each of the gains similar to that of the NDI 610157 sensor was between 0.0006 and 0.0008.

[0041] With reference to FIG. 6, the performance attributes of each electromagnetic sensor 126 were also analyzed, comparing hollow ferromagnetic cores 158 having different axial lengths LI. In each test, the number of coil layers remained constant (e.g., 2 coil layers). As illustrated in FIG. 6, significant changes in gain were achieved by altering the axial lengths LI of the hollow ferromagnetic cores 158, such that when the axial length LI was set at 5.5mm, the electromagnetic sensor 126 was able to beA0012994W001visualized via the ROM file on the NDI emitter. The testing confirmed that even slight changes in the axial length LI may be made to alter the tested gain of the electromagnetic sensor 126, and to thus achieve an output gain that is sufficient to enable the electromagnetic sensor 126 to be used with a system such as the NDI emitter.

[0042] With reference to FIG. 7, the performance attributes of each electromagnetic sensor 126 were also analyzed, comparing electromagnetic sensors 126 having different numbers of coil layers. As illustrated in FIG. 7, the 2-layer, 4-layer, 6-layer, and 8-layer electromagnetic sensors 126 had similar accuracy as compared to the NDI micro coil sensor (e.g., the NDI 610157 micro coil sensor). Adding the Mu-Metal hollow ferromagnetic core 158 retained the accuracy, while decreasing the amount of space required for the electromagnetic sensor.

[0043] It should be understood that various aspects disclosed herein may be combined in different combinations than the combinations specifically presented in the description and accompanying drawings. It should also be understood that, depending on the example, certain acts or events of any of the processes or methods described herein may be performed in a different sequence, may be added, merged, or left out altogether (e.g., all described acts or events may not be necessary to carry out the techniques). In addition, while certain aspects of this disclosure are described as being performed by a single module or unit for purposes of clarity, it should be understood that the techniques of this disclosure may be performed by a combination of units or modules associated with, for example, a medical device.

[0044] The invention may be further described by reference to the following numbered examples:

[0045] Example 1. A method of forming an electromagnetic sensor for use with a navigation system, the method comprising: forming a hollow ferromagnetic core into a cylindrical shape having an axial length; wrapping a wire into coil layers around the hollow ferromagnetic core; and altering the axial length of the ferromagnetic core to obtain a desired gain for the electromagnetic sensor, wherein the desired gain is associated with a magnetic field emitter of the navigation system.A0012994W001

[0046] Example 2. The method of Example 1, wherein the step of altering the axial length includes altering the axial length until the axial length is between 4 mm and 6 mm.

[0047] Example s. The method of Example 2, wherein the step of altering the axial length includes altering the axial length until the axial length is 5.5 mm.

[0048] Example 4. The method of Example 2 or according to Example 1, wherein the step of wrapping a wire into coil layers includes forming between 2 and 8 coil layers.

[0049] Example s. The method of Example 4, wherein the step of wrapping a wire into coil layers includes forming between 4 and 6 coil layers.

[0050] Example 6. The method of any of the preceding Examples, wherein the desired gain is between 0.0006 and 0.0008.

[0051] Example 7. The method of Example 1, wherein the step of altering the axial length includes altering the axial length until the axial length is 5.5 mm, and wherein the step of wrapping a wire into coil layers includes forming between 4 and 6 coil layers.

[0052] Example 8. The method of any of the preceding Examples, wherein the step of forming the hollow ferromagnetic core includes forming the hollow ferromagnetic core from Mu-metal.

[0053] Example 9. The method of any of the preceding Examples, wherein the electromagnetic sensor has an overall radial thickness of between 0.130 inch and 0.190 inch.

[0054] Example 10. The method of any of the preceding Examples, wherein the method further includes altering a number of the coil layers to obtain the desired gain.

[0055] Example 11. An electromagnetic sensor for use with a navigation system, the electromagnetic sensor comprising: a hollow ferromagnetic core having a cylindrical shape with an axial length, wherein the axial length is between 4 mm and 6 mm; and a wire wrapped in a plurality of coil layers around the hollow ferromagnetic core, whereinA0012994W001the plurality of coil layers is between 4 coil layers and 6 coil layers; wherein the hollow ferromagnetic core has an overall radial thickness between 0.130 inch and 0.190 inch.

[0056] Example 12. The electromagnetic sensor of Example 11, wherein the axial length is 5.5 mm.

[0057] Example 13. The electromagnetic sensor of Example 11 or Example 12, wherein the electromagnetic sensor is configured to produce a gain of between 0.0006 and 0.0008.

[0058] Example 14. The electromagnetic sensor of any of Examples 11-13, wherein the electromagnetic sensor is configured to produce a gain that mimics a micro coil sensor associated with the navigation system.

[0059] Example 15. The electromagnetic sensor of any of Examples 11-14, wherein the hollow ferromagnetic core comprises Mu-metal.

[0060] Example 16. The electromagnetic sensor of any of Examples 11-15, wherein the wire is a plated wire.

[0061] Example 17. A medical device comprising: a main body having a proximal end and a distal end; and the electromagnetic sensor of any of paragraphs 11-16 coupled to the main body.

[0062] Example 18. The medical device of Example 17, wherein the main body includes a wall defining a central lumen.

[0063] Example 19. The medical device of Example 18, wherein the electromagnetic sensor is embedded completely within the wall.

[0064] Example 20. The medical device of Example 19 or according to Example 18, wherein the medical device includes a further lumen within the wall to facilitate wiring for the electromagnetic sensor.

[0065] Although various aspects and examples have been described in detail with reference to certain examples illustrated in the drawings, variations and modificationsA0012994W001exist within the scope and spirit of one or more independent aspects described and illustrated.

Claims

A0012994W001CLAIMSWhat is claimed is:

1. A method of forming an electromagnetic sensor (126) for use with a navigation system (110), the method comprising:forming a hollow ferromagnetic core (158) into a cylindrical shape having an axial length;wrapping a wire (170) into coil layers around the hollow ferromagnetic core (158); andaltering the axial length of the ferromagnetic core to obtain a desired gain for the electromagnetic sensor (126), wherein the desired gain is associated with a magnetic field emitter (114) of the navigation system (110).

2. The method of claim 1, wherein the step of altering the axial length includes altering the axial length until the axial length is between 4 mm and 6 mm.

3. The method of claim 2, wherein the step of altering the axial length includes altering the axial length until the axial length is 5.5 mm.

4. The method of claim 2, wherein the step of wrapping a wire (170) into coil layers includes forming between 2 and 8 coil layers.

5. The method of claim 4, wherein the step of wrapping a wire (170) into coil layers includes forming between 4 and 6 coil layers.

6. The method of any of the preceding claims, wherein the desired gain is between 0.0006 and 0.0008.

7. The method of claim 1, wherein the step of altering the axial length includes altering the axial length until the axial length is 5.5 mm, and wherein the step of wrapping a wire (170) into coil layers includes forming between 4 and 6 coil layers.A0012994W0018. The method of any of the preceding claims, wherein the step of forming the hollow ferromagnetic core (158) includes forming the hollow ferromagnetic core (158) from Mumetal.

9. The method of any of the preceding claims, wherein the electromagnetic sensor (126) has an overall radial thickness of between 0.130 inch and 0.190 inch.

10. The method of any of the preceding claims, wherein the method further includes altering a number of the coil layers to obtain the desired gain.

11. An electromagnetic sensor (126) for use with a navigation system (110), the electromagnetic sensor (126) comprising:a hollow ferromagnetic core (158) having a cylindrical shape with an axial length, wherein the axial length is between 4 mm and 6 mm; anda wire (170) wrapped in a plurality of coil layers around the hollow ferromagnetic core (158), wherein the plurality of coil layers is between 4 coil layers and 6 coil layers;wherein the hollow ferromagnetic core (158) has an overall radial thickness between 0.130 inch and 0.190 inch.

12. The electromagnetic sensor (126) of claim 11, wherein the axial length is 5.5 mm.

13. The electromagnetic sensor (126) of claim 11 or claim 12, wherein the electromagnetic sensor (126) is configured to produce a gain of between 0.0006 and 0.0008.

14. The electromagnetic sensor (126) of any of claims 11-13, wherein the electromagnetic sensor (126) is configured to produce a gain that mimics a micro coil sensor associated with the navigation system (110).

15. The electromagnetic sensor (126) of any of claims 11-14, wherein the hollow ferromagnetic core (158) comprises Mu-metal.

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