Sparse calibration of the magnetic field generated by a coil in a metal-rich environment

Abstract

The present invention provides a calibration method, the method comprising receiving magnetic field values ​​generated by a plurality of real magnetic emitters (26) and measured at a plurality of locations on a grid in a region containing a magnetic field perturbing element. Receiving approximate positions of the real magnetic emitters. Using the approximate positions, characterizing a corresponding plurality of virtual magnetic sources (55) within the field perturbing element (40). Using the measured magnetic field values, the approximate positions, and the characterized virtual sources, iteratively calculating (i) actual positions of the real and virtual magnetic sources in the region, and (ii) modeled magnetic field values ​​that would be generated by the real and virtual magnetic sources at the actual positions. Using the calculated positions and the modeled magnetic field values ​​at the plurality of locations on the grid, a magnetic field calibration function for the region is derived.

Description

Technical Field

[0001] The present invention relates generally to sensing the position of an object placed within a living body, and particularly to compensating for magnetic interference affecting a position sensor. Background Art

[0002] Techniques for using magnetic fields to aid in tracking invasive probes within organ cavities have previously been proposed in the patent literature. For example, U.S. Patent Application Publication No. 2012 / 0165656 describes a method that includes generating a magnetic field in a region using a plurality of magnetic emitters and introducing a field-disturbing element into the region. The method includes characterizing a plurality of images of each magnetic emitter in the field-disturbing element and calculating a reactive magnetic field in the region based on the characterized images. The method also includes positioning a probe in the region and measuring a disturbed magnetic field at the probe, and determining a position of the probe based on the measured disturbed magnetic field and the calculated reactive magnetic field.

[0003] U.S. Patent Application Publication No. 2016 / 0011288 describes a medical device comprising: a magnetic resonance imaging system; a magnetic compensation coil for compensating for magnetic inhomogeneities within an imaging region; a gantry operable to rotate about the imaging region; a position sensor for measuring an angular position and an angular velocity of the gantry; at least one magnetic field distortion component located in the gantry, the at least one magnetic field distortion component being a memory storing machine-executable instructions and field correction data. The instructions cause a processor to: receive position and angular velocity data from the position sensor; determine coil control commands for controlling the magnetic compensation coil using the field correction data, the position data, and the angular velocity data; control the magnetic compensation coil using the coil control commands to compensate for magnetic inhomogeneities within the imaging region; and acquire magnetic resonance data. Summary of the Invention

[0004] Embodiments of the present invention provide a calibration method that includes receiving magnetic field values ​​generated by a plurality of real magnetic emitters and measured at a plurality of locations on a grid in a region containing a magnetic field perturbation element. Approximate positions of the real magnetic emitters are received. Using the approximate positions, a corresponding plurality of virtual magnetic sources within the field perturbation element is characterized. Using the measured magnetic field values, the approximate positions, and the characterized virtual sources, (i) the actual positions of the real and virtual magnetic sources in the region, and (ii) modeled magnetic field values ​​that would be generated by the real and virtual magnetic sources at the actual positions are iteratively calculated. Using the calculated positions and the modeled magnetic field values ​​at the plurality of locations on the grid, a magnetic field calibration function for the region is derived.

[0005] In some embodiments, receiving magnetic field values ​​includes positioning a probe in the region and measuring the magnetic field generated at the probe at the plurality of locations.

[0006] In some embodiments, characterizing the virtual magnetic sources includes estimating a position of each of the virtual magnetic sources within the perturbation element.

[0007] In one embodiment, the magnetic field calibration function is provided as a three-dimensional array of values ​​on a calibrated grid of locations that is denser than the grid used to measure the magnetic field values.

[0008] In another embodiment, deriving the magnetic field calibration function includes modeling the magnetic fields generated by the real and virtual magnetic sources as linear combinations of spherical harmonics and evaluating the modeled magnetic fields at the actual locations.

[0009] According to another embodiment of the present invention, there is also provided a device comprising a memory and a processor. The memory is configured to store magnetic field values ​​generated by a plurality of real magnetic emitters and measured at a plurality of locations on a grid in a region containing a magnetic field perturbing element, and the memory is configured to store approximate positions of the real magnetic emitters. The processor is configured to (a) characterize a corresponding plurality of virtual magnetic sources within the field perturbing element using the approximate positions of the real magnetic emitters, (b) iteratively calculate (i) actual positions of the real and virtual magnetic sources in the region using the measured magnetic field values, the approximate positions, and the characterized virtual sources, and (ii) modeled magnetic field values ​​that would be generated by the real and virtual magnetic sources at the actual positions, and (c) derive a magnetic field calibration function for the region using the calculated positions and the modeled magnetic field values ​​at the plurality of locations on the grid.

[0010] The present invention will be more fully understood through the following detailed description of embodiments of the present invention in conjunction with the accompanying drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 is a schematic illustration of an ear, nose and throat (ENT) system including a magnetic position tracking subsystem according to an embodiment of the present invention;

[0012] Figure 2 is installed in accordance with an embodiment of the present invention Figure 1 a schematic diagram front view of a magnetic radiator assembly on a metal base;

[0013] Figure 3 According to an embodiment of the present invention Figure 2 Schematic illustration of real magnetic sources and corresponding virtual magnetic sources in a metal base;

[0014] Figure 4 is a graph depicting convergence of iteratively calculated positions without and with a virtual magnetic source in accordance with an embodiment of the present invention;

[0015] Figure 5 is a graph of the convergence of the positions of real and virtual magnetic sources during iterative calculations according to an embodiment of the present invention; and

[0016] Figure 6 is a flow chart schematically illustrating a method for sparsely calibrating a magnetic field in the presence of a metal-rich object according to an embodiment of the present invention. DETAILED DESCRIPTION

[0017] Overview

[0018] Tracking a probe within a patient's organ using a magnetic position sensor mounted at the distal end of the probe requires calibration of the external magnetic tracking system. In some cases, the magnetic field generated by the system is disturbed by the presence of magnetic field-perturbing metal-rich elements, such as a piece of equipment near the system's magnetic field generator. This disturbance complicates calibration because it requires the use of a dense grid of calibration locations in space. Typically, this complex calibration can require a day-long measurement of tens of thousands of magnetic field values ​​in 3D space using the calibration probe.

[0019] Embodiments of the present invention provide a method for rapid calibration of a disturbed magnetic field near a static metal-rich element, such as a metal base that provides mechanical support for a set of coil magnetic field generators (also referred to as "real magnetic sources" or "coil transmitters" below) of a position tracking system.

[0020] To account for the presence of perturbation elements, such as the metal base for the coils, embodiments of the present invention provide a calibration model that assumes that each coil transmitter generates eddy currents in the perturbation element. These eddy currents can be modeled as one or more image (or virtual) magnetic sources. The calibration model also assumes that each virtual magnetic source generates a corresponding reaction field that causes a perturbation in the magnetic field based on linear superposition.

[0021] The disclosed rapid calibration method includes (a) receiving magnetic field values ​​generated by a plurality of real magnetic transmitters and measured at a plurality of locations on a grid in a region containing a magnetic field perturbing element, (b) receiving approximate locations of the real magnetic transmitters, (c) using the approximate locations to characterize a corresponding plurality of virtual magnetic sources within the field perturbing element, (d) using the measured magnetic field values, the approximate locations, and the characterized virtual sources, iteratively calculating (i) actual locations of the real and virtual magnetic sources in the region, and (ii) modeled magnetic field values ​​that would be generated by the real and virtual magnetic sources at the actual locations, and (e) deriving a magnetic field calibration function for the region using the calculated locations and the modeled magnetic field values ​​at the plurality of locations on the grid.

[0022] In some embodiments, each virtual magnetic source is characterized by a combination of magnetic multipoles, i.e., dipoles, quadrupoles, and / or higher-order multipoles. The characteristics of each virtual magnetic source also depend, among other things, on the real emission field that produces the virtual magnetic source. The calibration model calculates the reaction field from each multipole virtual magnetic source by assuming that the field can be represented by a spherical harmonic function expansion based on the characteristics of the virtual magnetic source. Spherical functions are particularly convenient for this application, but other expansions, such as wavelets, are also valid.

[0023] The actual positions of the real coil transmitters and the modeled positions of the various virtual magnetic sources are left as parameters to be solved using iterative calculations. Thus, the disclosed method includes adjusting the predetermined positions of the real coil transmitters, which may be inaccurate due to, for example, mechanical non-repeatability in the production of the transmitter base assembly and may therefore vary from system to system.

[0024] In one embodiment, a position tracking system includes five three-axis magnetic transmitters, for a total of fifteen real coil transmitters, as each three-axis transmitter includes three mutually orthogonal coils. Each of the fifteen real coil transmitters creates a virtual coil transmitter in the metal base of a positioning pad of the position tracking system. Applying calibration measurements and assuming that the real transmitter fields and virtual transmitter fields are given by a spherical harmonic model, the calibration provides effective positions of the real transmitters and virtual transmitters, which are then used to find the magnetic field at any point in the workspace. The calibration process using a sparse grid requires at least one-tenth (×1 / 10) fewer measurement data points than a previous calibration process over the same area. For this system, the disclosed technique reduces the typical duration of the calibration process from approximately one day to approximately one hour.

[0025] In one embodiment, the processor is configured to provide the magnetic field calibration function as a three-dimensional array of values ​​on a calibrated grid of positions that is denser than the sparse grid used in the measurements.

[0026] Typically, the processor is programmed in software containing specific algorithms that enable the processor to perform each of the processor-related steps and functions described above.

[0027] Once the system has been calibrated, the processor is able to track the position of the medical probe within the magnetically disturbed region with high accuracy during a medical session.

[0028] System Description

[0029] Figure 1is a schematic illustration of an ear, nose and throat (ENT) system 20 including a magnetic position tracking subsystem 23 according to an embodiment of the present invention. In the following description, an ENT tool 21 is in the head of a patient 28 during a medical procedure. The tool 21 includes one or more magnetic sensors 34, typically single-axis coils or three-axis coils, which are tracked by the magnetic position tracking subsystem 23 during the procedure.

[0030] For tracking to be effective, in system 20, a CT (computed tomography) image of patient 28 is registered with the reference frame of magnetic position tracking subsystem 23. Before and during the medical procedure, the magnetic tracking system, consisting of magnetic radiator assembly 24, is positioned under the patient's head.

[0031] To use the magnetic field transmission system as described herein, the magnetic radiator assembly 24 must be mounted on a metal base 40. (The metal base 40 protects the transmitter from interfering metal that may be present in the bed or chair in which the patient 28 lies or sits.) However, the transmitter and base (which itself can introduce perturbations into the magnetic field) must be calibrated.

[0032] Assembly 24 includes five magnetic field three-coil transmitters 26 that are fixed in place and transmit an alternating magnetic field to a region 30 where the head of patient 28 is located. The potential generated by sensors 34 in response to the magnetic field in region 30 enables measurement of its position and orientation in the reference frame of the magnetic tracking system. The position can be measured in three linear dimensions (3D), while the orientation of the distal end of tool 21 can be determined using one of the coils of sensor 34, the axis of which is aligned with the longitudinal axis of the distal end.

[0033] By way of example, the five coil transmitter assemblies 26 of assembly 24 are arranged in a generally horseshoe shape around the head of patient 28. However, alternative configurations of the coil transmitters of assembly 24 may be used, and all such configurations are intended to be included within the scope of the present invention. Each coil transmitter 26 includes three mutually orthogonal coils that generate a magnetic field. Thus, system 20 has a total of fifteen (15) transmitter coils.

[0034] Prior to the procedure, registration of the magnetic tracking system's frame of reference with the CT image can be performed by positioning a magnetic sensor at a known location in the image, such as above the patient's head.

[0035] The components of system 20, including coil transmitter 26 and sensor 34, are under the overall control of system processor 41. Processor 41 may be mounted in a console 50, which includes operating controls 58, typically including a keypad and / or a pointing device, such as a mouse or trackball. Console 50 is connected to coil transmitter 26 and sensor 34 wirelessly and / or via one or more cables. Doctor 54 uses operating controls 58 to interact with processor 41 while performing an ENT procedure using system 20. While the procedure is being performed, the processor may present the results of the procedure on screen 56.

[0036] System 20 also includes memory 42. Processor 41 uses software stored in memory 42 to operate system 20. The software may be downloaded to processor 41 in electronic form, for example, over a network, or alternatively or in addition, the software may be provided and / or stored on non-transitory tangible media such as magnetic, optical, or electronic memory. Specifically, processor 41 executes the software disclosed herein including Figure 6 The dedicated algorithm in the embodiment enables the processor 41 to perform the steps disclosed in the present invention, as further described below.

[0037] In some embodiments, the memory 42 is also used to store measured magnetic field values ​​and the approximate position of the coil transmitter 36 as part of the disclosed calibration scheme, as will be explained below.

[0038] Sparse calibration of the magnetic field generated by a coil in a metal-rich environment

[0039] Figure 2 is installed in accordance with an embodiment of the present invention Figure 1 A front view of the magnetic radiator assembly 24 on the metal base 40 of FIG. It can also be seen that eddy currents generated in the metal base 40 by the coil transmitter 26 disturb the magnetic field generated by the coil transmitter 26.

[0040] Figure 3 According to another embodiment of the present invention Figure 2 Schematic diagram of a real magnetic source 26 (e.g., coil transmitter 26) and a corresponding virtual magnetic source 55 in a metal base 40. The oscillating magnetic field generated by the coil 26 induces eddy currents in the metal base 40. The eddy currents generate a reactive magnetic field, which is modeled by assuming a single virtual source 55. For calibration, the method uses the sparsely measured total magnetic field to iteratively find the exact real position of the coil 26 and the modeled position of the virtual source 55 in order to have a self-consistent calibration function.

[0041] The disclosed calibration model assumes that both the real source 26 and the virtual source 55 can be described as a linear combination of spherical harmonics that produce the magnetic field, given by:

[0042] Equation 1

[0043] For an array of N real sources 26, the model computes 2N different B(r,θ,φ) functions (one for each virtual source), where Figure 2 In the implementation, each real source 26 is a three-axis coil, each coil emitting a magnetic field oscillating at a unique frequency. Typically, for each frequency, there are 15 unknown spherical harmonic coefficients, which corresponds to taking 15 Y in Equation 1. l,m The magnetic field at a given frequency is described by the term (θ, φ), where l ranges from 1 to 3 and m ranges from 0 to 1. Each three-axis coil adds 3 position coordinate unknowns. The virtual source has the same number of unknowns as the real source. Therefore, there are 48 unknowns to find for each three-axis coil (i.e., 45 spherical harmonic coefficients + 3 position coefficients). Therefore, for Figure 2 For the configuration of , that is, N = 5, there are a total of 480 unknowns to be found.

[0044] The optimization procedure uses input from P calibration data points. Three field components are measured at each point in space. Therefore, there are a total of 3P equations per frequency (transmit coil) (where P is several hundred, meaning there are typically over 1000 data points). Because this is overdetermined, for example by a factor of 5 or more, the system of equations (e.g., with 480 unknowns) is solved by optimization.

[0045] The initial position of the virtual magnetic source 55 is estimated based on the mechanical drawing and iteratively calculated using the disclosed technique. Furthermore, using the mechanical drawing, the position of each real magnetic source 26 is only approximately known, for example, within a given tolerance that is too wide for sufficiently accurate tracking of the distal end of the tool 21 within the patient 28, but accurate enough as an initial condition.

[0046] The disclosed iterative calculations can be performed using library functions of commercial software such as MATLAB. For example, the precise location of the magnetic source can be found using, for example, MATLAB's f_min_search function, where the solution of the above equations can be accomplished using, for example, the pinv (Moore-Penrose pseudo-inverse) function. A typical cost function for the error after the Mth iteration is the magnetic field calculated at the Mth iteration. With the measured magnetic field B Meas The sum of the square roots of the differences between , summed over a sparse grid of locations:

[0047] Equation 2

[0048] Where (i, j, k) is the 3D grid position index. Equation 2 represents three equations, one for each field component measured.

[0049] In one embodiment, using Equation 1, the processor is configured to provide the magnetic field calibration function as a three-dimensional array of values ​​on a calibrated grid of positions that is denser than the sparse grid used in the measurements.

[0050] Figure 4 7 is a graph depicting the convergence of iteratively calculated positions without and with the use of a virtual magnetic source 55 (72) according to another embodiment of the present invention. As shown, by using a virtual magnetic source and doubling the number of equations solved in each iteration, multiple iterations are performed until the calibration error is reduced below a predetermined acceptance value, the former being ten times the latter (e.g., M=8K iterations versus less than about M≈0.8K iterations, where K is an integer).

[0051] Figure 5 is a graph illustrating the convergence of the positions of real and virtual magnetic sources during iterative calculations according to an embodiment of the present invention. Figure 5 The results shown are applied to Figure 1 The results of the disclosed calibration process for the system 20 are shown in FIG. The open circles are intermediate values ​​of position. The solid circle 126 is the final position of the real emitter 26 of the system 20, while the solid circle 150 is the final position of each virtual source 55 used in the calibration model.

[0052] Figure 6 is a flow chart schematically illustrating a method for sparsely calibrating a magnetic field in the presence of a metal-rich object according to another embodiment of the present invention.

[0053] The algorithm according to this embodiment performs a process that first uploads the disclosed model incorporating a virtual magnetic emitter in a field perturbation element to the processor at a model upload step 70. Next, at a model initialization step 72, the initial position of the real magnetic source is input into the model in the processor, for example by someone running a calibration algorithm based on the disclosed model. The initial position can be obtained, for example, from a technical drawing that gives the approximate position of the real emitter and the dimensions of the perturbation element. The initial position is typically stored in memory 42.

[0054] Typically, the algorithm automatically generates a corresponding set of positions of virtual magnetic sources, for example, as mirror reflections of real sources about the axis of symmetry, as described above.

[0055] Steps 70 and 72 are generally common to all systems of a given model and can be completed in advance without requiring hardware-dependent data.

[0056] At a system calibration initiation step 74, calibration data from the particular system being calibrated is uploaded to a processor, for example, by the person performing the calibration. This data typically includes uploading a file storing magnetic field readings in the form of voltages at a sparse grid of locations in space, where the data in the file was measured independently of the steps associated with running the calibration model, for example, by performing measurements at the system's manufacturing level, as described below in steps 90-94.

[0057] In steps 76-80, the processor uses the calibration data to iteratively calculate the calibration function required for the particular system.

[0058] In a first iteration step 76, the processor calculates the positions of the real magnetic sources (26) and virtual magnetic sources (55) in the region and the corresponding magnetic fields they produce. Next, in an error calculation step 78, the processor calculates the error between the calculated magnetic field and the measured magnetic field using the cost function described above.

[0059] At a check error step 80, the processor compares the error to a predetermined value. If the error is greater than the predetermined value, the processor loops back to step 76 to refine the calculation in the next iteration. If the error is less than the predetermined value, at a calibration step 82, the processor uses the calculated positions of the real and virtual magnetic sources to derive a magnetic field calibration function for the system being calibrated. Finally, at a calibration storage step 84, the processor stores the derived magnetic field calibration function in memory 42.

[0060] As described above, the disclosed model is used to calibrate a particular system.In preliminary step 90, the system is operated to generate a magnetic field, for example by real transmitter 26, in a region (eg, region 30 of system 20) containing a perturbation element (eg, plate 40).

[0061] Next, at a measurement step 92, the magnetic field is measured at a plurality of locations on a grid in the area using, for example, a calibration probe equipped with a three-axis magnetic sensor, such as the magnetic sensor 34 of the ENT tool 21. Finally, at a data storage step 94, the measured calibration data is stored in the memory 42 for subsequent use in the calibration steps 74-84.

[0062] Once the above-described calibration process is complete, the processor 41 of the system 20 applies the calibration data to accurately track the distal end of the tool 21 during the medical procedure.

[0063] Although the embodiments described herein are primarily directed to ENT medical navigation systems, the methods and systems described herein can also be used in other applications such as cardiac, pulmonary, and digestive system procedures, as well as any magnetic tracker with static conductors in the environment.

[0064] It should therefore be understood that the embodiments described above are cited by way of example, and that the present invention is not limited to what is specifically shown and described above. On the contrary, the scope of the present invention includes combinations and subcombinations of the various features described above, as well as variations and modifications thereof, which will occur to those skilled in the art upon reading the above description and which are not disclosed in the prior art. The documents incorporated by reference into this patent application are considered an integral part of this application, except that if any term defined in these incorporated documents conflicts with a definition explicitly or implicitly given in this specification, only the definition in this specification shall be considered.

Claims

1. A calibration method comprising: receiving magnetic field values ​​generated by a plurality of real magnetic transmitters and measured at a plurality of locations on a grid in an area containing the magnetic field perturbing element; receiving an approximate position of the real magnetic transmitter; characterizing a corresponding plurality of virtual magnetic sources within the field perturbing element using the approximate positions; using the measured magnetic field values, the approximate positions, and the characterized virtual sources, iteratively calculating (i) actual positions of real and virtual magnetic sources in the region, and (ii) modeled magnetic field values ​​that would be generated by the real and virtual magnetic sources at the actual positions; as well as A magnetic field calibration function for the region is derived using the calculated positions and the modeled magnetic field values ​​at the plurality of positions on the grid.

2. The calibration method according to claim 1, wherein: Receiving the magnetic field values ​​includes positioning a probe in the region and measuring the magnetic field generated at the probe at the plurality of locations.

3. The calibration method according to claim 1, wherein: Characterizing the virtual magnetic sources includes estimating a position of each of the virtual magnetic sources within the perturbation element.

4. The calibration method according to claim 1, wherein: The magnetic field calibration function is provided as a three-dimensional array of values ​​on a calibrated grid of locations that is denser than the grid used to measure the magnetic field values.

5. The calibration method according to claim 1, wherein: Deriving the magnetic field calibration function includes modeling the magnetic field generated by the real magnetic source and the virtual magnetic source as a linear combination of spherical harmonics and evaluating the modeled magnetic field at the actual location.

6. A device comprising: a memory for storing magnetic field values ​​generated by a plurality of real magnetic emitters and measured at a plurality of locations on a grid in an area containing a magnetic field perturbing element, and for storing approximate locations of the real magnetic emitters; and a processor configured to: characterizing a corresponding plurality of virtual magnetic sources within the field perturbing element using the approximate positions of the real magnetic emitters; using the measured magnetic field values, the approximate positions, and the characterized virtual sources, iteratively calculating (i) actual positions of real and virtual magnetic sources in the region, and (ii) modeled magnetic field values ​​that would be generated by the real and virtual magnetic sources at the actual positions; as well as A magnetic field calibration function for the region is derived using the calculated positions and the modeled magnetic field values ​​at the plurality of positions on the grid.

7. The apparatus according to claim 6, wherein The processor is configured to characterize the virtual sources by estimating a position of each of the virtual magnetic sources within the perturbation element.

8. The apparatus according to claim 6, wherein The processor is configured to provide the magnetic field calibration function as a three-dimensional array of values ​​on a calibrated grid of positions that is denser than the grid used to measure the magnetic field values.

9. The apparatus according to claim 6, wherein The processor is configured to derive the magnetic field calibration function by modeling the magnetic field generated by the real magnetic source and the virtual magnetic source as a linear combination of spherical harmonics and evaluating the modeled magnetic field at the actual location.

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