Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B90/00—Instruments, implements or accessories specially adapted for surgery or diagnosis and not covered by any of the groups A61B1/00 - A61B50/00, e.g. for luxation treatment or for protecting wound edges
  • A61B90/36—Image-producing devices or illumination devices not otherwise provided for
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B34/00—Computer-aided surgery; Manipulators or robots specially adapted for use in surgery
  • A61B34/20—Surgical navigation systems; Devices for tracking or guiding surgical instruments, e.g. for frameless stereotaxis
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B34/00—Computer-aided surgery; Manipulators or robots specially adapted for use in surgery
  • A61B34/20—Surgical navigation systems; Devices for tracking or guiding surgical instruments, e.g. for frameless stereotaxis
  • A61B2034/2046—Tracking techniques
  • A61B2034/2051—Electromagnetic tracking systems
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B34/00—Computer-aided surgery; Manipulators or robots specially adapted for use in surgery
  • A61B34/20—Surgical navigation systems; Devices for tracking or guiding surgical instruments, e.g. for frameless stereotaxis
  • A61B2034/2072—Reference field transducer attached to an instrument or patient
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B90/00—Instruments, implements or accessories specially adapted for surgery or diagnosis and not covered by any of the groups A61B1/00 - A61B50/00, e.g. for luxation treatment or for protecting wound edges
  • A61B90/04—Protection of tissue around surgical sites against effects of non-mechanical surgery, e.g. laser surgery
  • A61B2090/0481—Protection of tissue around surgical sites against effects of non-mechanical surgery, e.g. laser surgery against EM radiation, e.g. microwave
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B34/00—Computer-aided surgery; Manipulators or robots specially adapted for use in surgery
  • A61B34/70—Manipulators specially adapted for use in surgery

Definitions

• the present invention relates to a navigation system for medical devices based on the use of magnetic fields. More particularly, this invention relates to a method and system for determining the position and orientation of a catheter probe being used during a surgical procedure in the presence of extraneous objects that may introduce extraneous magnetic fields.
• Systems and methods for determining the position and orientation of surgical probes based on the use of magnetic fields are known. See, for example, U.S. Patent No. 5,592,939, herein incorporated by reference. Such systems and methods generally rely on the ability to solve a known equation for a known field strength in order to obtain the unknown position and orientation variables.
• the position and orientation of a rigid catheter probe in three-dimensions is a function of 5 unknown variables.
• These variables can be expressed in a form that utilizes both Cartesian and spherical coordinates.
• the position of sensor coil 14 can be defined by three Cartesian coordinates x, y, and z as is shown in FIG. 2, and the orientation of sensor coil 14 along coil axis 21 is defined by the variables ⁇ and ⁇ , also as shown in FIG. 2.
• a conducting object within the area of influence of the known navigation field can develop what is known as an eddy current in response to the known navigation field.
• These eddy currents can, in turn, generate an extraneous magnetic field of unknown strength and orientation in the vicinity of the catheter. Depending upon the size of the object, this effect can be large.
• an object with a ferromagnetic core will act to focus magnetic field flux lines through the core and thus distort the known navigation field, again, in an unknown manner.
• objects with ferromagnetic and conductive cores are used in surgical settings, such as tools used to drill, ream and tap holes in the vertebrae of a patient.
• the invention in one aspect comprises a correction system for determining the effect of an interfering object on a field sensor within a navigational domain.
• the system includes a first transmitter configured to project, into the navigational domain, field energy in a first waveform sufficient to induce a first signal value in the field sensor, where the first signal value is influenced by the interfering object.
• the system further includes a second transmitter configured to project, into the navigational domain, field energy in a second waveform sufficient to induce a second signal value in the field sensor, where the second signal value is also influenced by the interfering object.
• the system further includes a signal processor configured to receive the first signal value and the second signal value, and to determine the influences of the interfering object on the field sensor, to thereby permit a substantially precise location of the field sensor to be determined despite the presence of the interfering object.
• the field energy is magnetic field energy.
• the interfering object is an electrically conductive object.
• the field sensor includes an electrically conductive sensing coil.
• the first waveform is a sinusoidal waveform at a first frequency
• the second waveform is a sinusoidal waveform at a second frequency
• the first transmitter and the second transmitter include three unidirectional coil sets, where each set is driven by a drive unit capable of driving the unidirectional coil set at the first frequency and at the second frequency.
• the first and second transmitters include six delta coil sets, where each the set is driven by a drive unit capable of driving the delta coil set at the first frequency and the second frequency.
• the three unidirectional coil sets and the six delta coils sets produce the field energy at the first and second frequencies.
• the three unidirectional coil sets include a first unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in an x direction, a second unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in a y direction, and a third unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in a z direction.
• the x, y and z directions are substantially mutually orthogonal.
• the first unidirectional coil set has a first coil pair including a first coil element and a second coil element, and a second coil pair including a third coil element and a fourth coil element.
• the first coil element and the third coil element are disposed in a major surface of a platform
• the second coil element is disposed in a first lateral wall of the platform
• the fourth coil element is disposed in a second lateral wall of the platform.
• the first lateral wall and the second lateral wall are substantially normal to the major surface and substantially parallel to one another.
• the second unidirectional coil set has a first coil element and a second coil element disposed within a platform.
• the coil elements are spaced apart and substantially parallel to one another.
• the third unidirectional coil set has a first coil pair including a first coil element and a second coil element, and a second coil pair including a third coil element and a fourth coil element.
• the first coil element and the third coil element are disposed in a major surface of a platform
• the second coil element is disposed in a first lateral wall of the platform
• the fourth coil element is disposed in a second lateral wall of the platform.
• the first lateral wall and the second lateral wall are substantially normal to the major surface, and substantially parallel to one another.
• the six delta coil sets include a first pair of coil elements, a second pair of coil elements, and a third pair of coil elements.
• the coil elements are disposed so as to be substantially mutually coplanar within a major surface of a platform.
• each of the pairs of coil elements includes a long coil and a short coil, and the pairs of coils are disposed at equal angles on a circle about an axis extending substantially perpendicular to the major surface.
• a radius of the circle extends perpendicular to a direction of elongation of the pair, proceeding from the long coil to the short coil.
• each of the pairs of coil elements further includes at least one compensation coil, constructed and arranged to modify at least one termination point of the coil elements, so as to provide relatively a high spatial field gradient along two orthogonal axes, and substantially zero field amplitude along a third orthogonal axis.
• the signal processor includes a first sequencer configured to sequentially activate each of the three unidirectional coil sets and the six delta coil sets at the first frequency, and to measure the first signal value corresponding to each of the unidirectional and delta coil sets at the first frequency.
• the first sequencer further sequentially activates each of the three unidirectional coil sets and the six delta coil sets at the second frequency, and measures the second signal value corresponding to each of the unidirectional and delta coil sets at the second frequency.
• the signal processor further includes a processor configured to calculate, for each of the unidirectional and delta coil sets, and adjusted signal value as a predetermined function of the first signal value and the second signal value, so as to produce nine adjusted signal values, each corresponding to field energy from one of the unidirectional coil sets and the delta coil sets.
• Another embodiment of the invention further includes a third transmitter configured to project into the navigational domain a third waveform sufficient to induce a third signal value in the field coil, where the third signal value is influenced by the interfering object.
• the signal processor is further configured to receive the third signal value.
• Another embodiment of the invention further includes a fourth transmitter configured to project into the navigational domain a fourth waveform sufficient to induce a fourth signal value ins aid field coil, the fourth signal value being influenced by the interfering object.
• the signal processor is further configured to receive the fourth signal value.
• Another embodiment of the invention further includes N-4 transmitters configured to project into the navigational domain N waveforms sufficient to induce N signal values in the field coil.
• the N signal values are influenced by the interfering object.
• the signal processor is further configured to receive N signal values.
• the invention comprises a correction system for determining an effect of an interfering object on first and second field sensors in a navigational domain.
• the system includes a transmitter configured to project into the navigational domain, field energy sufficient to induce a first signal value in the first field sensor, and to induce a second signal value in the second field sensor.
• the system further includes a signal processor configured to receive the first signal value and the second signal value, and to determine the effect of the interfering object on the first field sensor, to thereby permit a substantially precise location of the first field sensor to be determined despite the presence of the interfering object.
• the field energy is magnetic field energy.
• the interfering object is a ferromagnetic and electrically conductive object.
• the field sensor includes an electrically conductive sensing coil.
• the transmitter includes three unidirectional coil sets. Each unidirectional coil set is driven by a unit capable of driving the unidirectional coil set at a first sinusoidal waveform at a first frequency.
• the transmitter further includes six delta coil sets, each of which is driven by a drive unit capable of driving the delta coil set at the first sinusoidal waveform at the first frequency, such that the three unidirectional coil sets and the six delta coil sets produce the field energy at the first frequency.
• the three unidirectional coil sets include a first unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in an x direction, a second unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in a y direction, and a third unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in a z direction, such that the x, y and z directions are substantially mutually orthogonal.
• the first unidirectional coil set has a first coil pair including a first coil element and a second coil element, and a second coil pair including a third coil element and a fourth coil element.
• the first coil element and the third coil element are disposed in a major surface of a platform, the second coil element is disposed in a first lateral wall of the platform, and the fourth coil element is disposed in a second lateral wall of the platform.
• the first lateral wall and the second lateral wall are substantially normal to the major surface and substantially parallel to one another.
• the second unidirectional coil set has a first coil element and a second coil element disposed within a platform, and the coil elements are spaced apart and substantially parallel to one another.
• the third unidirectional coil set has a first coil pair including a first coil element and a second coil element, and a second coil pair including a third coil element and a fourth coil element.
• the first coil element and the third coil element are disposed in a major surface of a platform
• the second coil element is disposed in a first lateral wall of the platform
• the fourth coil element is disposed in a second lateral wall of the platform.
• the first lateral wall and the second lateral wall are substantially normal to the major surface and substantially parallel to one another.
• the six delta coil sets include a first pair of coil elements, a second pair of coil elements, and a third pair of coil elements.
• the coil elements are disposed so as to be substantially mutually coplanar within a major surface of a platform.
• each of the pairs of coil elements includes a long coil and a short coil, and the pairs of coils are disposed at equal angles on a circle about an axis extending substantially perpendicular to the major surface. For each of the pairs of coils, a radius of the circle extends perpendicular to a direction of elongation of the pair, proceeding from the long coil to the short coil.
• each of the pairs of coil elements further includes at least one compensation coil, constructed and arranged to modify at least one termination point of the coil elements, so as to provide relatively a high spatial field gradient along two orthogonal axes, and substantially zero field amplitude along a third orthogonal axis.
• the signal processor further includes a first sequencer configured to sequentially activate each of three unidirectional coil sets and six delta coil sets at the first frequency, and to measure the first signal value and the second signal value corresponding to each of the unidirectional and delta coil sets at the first frequency.
• the signal processor also includes a processor configured to calculate, for each of the unidirectional and delta coil sets, an adjusted signal value as a predetermined function of the first signal value and the second signal value, so as to produce nine adjusted signal values, each corresponding to field energy from one of the unidirectional coil sets and the delta coil sets.
• the invention comprises a correction system for determining an effect of a field influencing shield device on a field sensor in a navigational domain.
• the correction system includes a transmitter configured to project into the navigational domain field energy sufficient to induce a signal value in the field sensor.
• the correction system further includes a storage device containing information corresponding to the field energy in the navigational domain at selected locations within the navigational domain.
• the information includes shield information incorporating the effect of the field influencing shield device at the selected locations.
• the correction system further includes a processor for accessing the storage device and the signal value to determine the effect of the shield device on the field sensor, to thereby permit a substantially precise location of the field sensor to be determined despite the presence of the field influencing shield device.
• the field energy is magnetic field energy.
• the field sensor includes an electrically conductive sensing coil.
• the transmitter in another embodiment, includes three unidirectional coil sets, each unidirectional coil set being driven by a unit capable of driving the unidirectional coil set at a first sinusoidal waveform at a first frequency.
• the transmitter further includes six delta coil sets, each the delta coil set being driven by a drive unit capable of driving the delta coil set at the first sinusoidal waveform at the first frequency, such that the three unidirectional coil sets and the six delta coil ' sets produce the field energy at the first frequency.
• the three unidirectional coil sets include a first unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in an x direction, a second unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in a y direction, and a third unidirectional coil set oriented so as to produce a substantially uniform amplitude field directed in a z direction, such that the x, y and z directions are substantially mutually orthogonal.
• the first unidirectional coil set has a first coil pair including a first coil element and a second coil element, and a second coil pair including a third coil element and a fourth coil element.
• the first coil element and the third coil element are disposed in a major surface of a platform
• the second coil element is disposed in a first lateral wall of the platform
• the fourth coil element is disposed in a second lateral wall of the platform, wherein the first lateral wall and the second lateral wall are substantially normal to the major surface and substantially parallel to one another.
• the second unidirectional coil set having a first coil element and a second coil element disposed within a platform, the coil elements being spaced apart and substantially parallel to one another.
• the third unidirectional coil set has a first coil pair including a first coil element and a second coil element, and a second coil pair including a third coil element and a fourth coil element.
• the first coil element and the third coil element are disposed in a major surface of a platform
• the second coil element is disposed in a first lateral wall of the platform
• the fourth coil element is disposed in a second lateral wall of the platform, wherein the first lateral wall and the second lateral wall are substantially normal to the major surface and substantially parallel to one another.
• the six delta coil sets include a first pair of coil elements, a second pair of coil elements, and a third pair of coil elements. The coil elements are disposed so as to be substantially mutually coplanar within a major surface of a platform.
• each of the pairs of coil elements includes a long coil and a short coil, and the pairs of coils are disposed at equal angles on a circle about an axis extending substantially perpendicular to the major surface. For each of the pairs of coils, a radius of the circle extends perpendicular to a direction of elongation of the pair, proceeding from the long coil to the short coil.
• each of the pairs of coil elements further includes at least one compensation coil, constructed and arranged to modify at least one termination point of the coil elements, so as to provide relatively a high spatial field gradient along two orthogonal axes, and substantially zero field amplitude along a third orthogonal axis.
• the processor further includes a first sequencer for sequentially activating each of the three unidirectional coils and the six delta coils at the first frequency, and measuring the signal value corresponding to each of the unidirectional and delta coils at the first frequency.
• the processor also includes a data manipulating device for manipulating, for each of the unidirectional and delta coils, the storage means as a predetermined function of the shield device, so as to produce nine sets of manipulated magnetic field values, each corresponding to navigational magnetic energy from one of the unidirectional coils and delta coils.
• Another aspect of the invention comprises a method of determining a substantially precise location of a field sensor within a navigational domain influenced by a field interfering object.
• the method includes inducing within the field sensor a first signal value at a first waveform, the first signal value being influenced by the field interfering object.
• the method further includes inducing within the field sensor a second signal value at a second waveform, the second signal value being influenced by the field interfering object.
• the method also includes determining a correction to the first signal value for the effects of the field interfering object. In another embodiment of the invention, determining a correction further includes calculating an adjusted signal value as a predetermined function of the first signal value and the second signal value.
• Another aspect of the invention comprises a method of determining a substantially precise location of a first field sensor within a navigational domain influenced by a field interfering object.
• the method includes inducing within the first field sensor a first signal value, the first signal value being influenced by the field interfering object.
• the method further includes inducing within a second field sensor a second signal value, the second signal value being influenced by the field interfering object.
• the method also includes determining a correction to the first signal value for the effects of the field interfering object.
• determining a correction further includes calculating an adjusted signal value as a predetermined function of the first signal value and the second signal value.
• Another aspect of the invention comprises a method of determining a substantially precise location of a field sensor within a navigational domain influenced by a field influencing shield device.
• the method includes inducing within the field sensor a first signal value, the first signal value being influenced by the field interfering object.
• the method further includes accessing information from a storage device, the information including shield information incorporating the effect of the field influencing shield device at selected locations.
• the method further includes determining a correction to the first signal value for the effects of the field influencing shield device.
• determining a correction further including manipulating the storage device as a predetermined function of the shield information, so as to produce a set of manipulated magnetic field values corresponding to the effects of the field influencing shield device.
• the invention comprises a method of determining a substantially precise location of a field sensor within a navigational domain influenced by a field interfering object. The method includes sequentially projecting into the navigational domain, via three unidirectional coils and six delta coils, navigational energy at a first frequency, and measuring a first signal value in the field sensor corresponding to each of the three unidirectional coils and the six delta coils, so as to produce nine of the first signal values.
• the method further includes sequentially projecting into the navigational domain, via three unidirectional coils and six delta coils, the navigational energy at a second frequency, and measuring a second signal value in the field sensor corresponding to each of the three unidirectional coils and the six delta coils, so as to produce nine of the second signal values.
• the method further includes calculating, for each of the unidirectional and delta coils, an adjusted signal value as a predetermined function of the first signal value and the second signal value, so as to produce nine adjusted signal values, each corresponding to navigational magnetic energy from one of the unidirectional coils and delta coils.
• the method also includes forming three independent equations including three adjusted signal values corresponding to the unidirectional coils, three predetermined field magnitude values due to each of the unidirectional coils and corresponding to the navigational energy at a last navigational point of the sensing coil, and unknown orientation variables, and simultaneously solving the independent equations to determine the orientation variables corresponding to the compensated orientation of the sensing coil.
• the method also includes generating three lines and determining an intersection of the three lines. The intersection corresponds to the compensated position of the sensing coil. Each of the lines is generated from adjusted signal values corresponding to a pair of the delta coils, and predetermined field magnitude values due to the pair of delta coils and corresponding to the navigational energy at the last navigational point of the sensing coil while oriented according to the compensated orientation.
• the invention comprises a method of determining a substantially precise location of a first field sensor within a navigational domain influenced by a field interfering object.
• the method includes sequentially projecting into the navigational domain, via three unidirectional coils and six delta coils, the navigational energy at a first frequency, measuring a first signal value in the field sensor corresponding to each of the three unidirectional coils and the six delta coils, so as to produce nine of the first signal values, and measuring a second signal value in a second field sensor corresponding to each of the three unidirectional coils and the six delta coils, so as to produce nine of the second signal values.
• the method further includes calculating, for each of the unidirectional and delta coils, an adjusted signal value as a predetermined function of the first signal value and the second signal value, so as to produce nine adjusted signal values, each corresponding to navigational magnetic energy from one of the unidirectional coils and delta coils.
• the method also includes forming three independent equations including three adjusted signal values corresponding to the unidirectional coils, three predetermined field magnitude values due to each of the unidirectional coils and corresponding to the navigational energy at a last navigational point of the sensing coil, and unknown orientation variables, and simultaneously solving the independent equations to determine the orientation variables corresponding to the compensated orientation of the sensing coil.
• the method also includes generating three lines and determining an intersection of the three lines, the intersection corresponding to the compensated position of the sensing coil.
• Each of the lines is generated from adjusted signal values corresponding to a pair of the delta coils, and predetermined field magnitude values due to the pair of delta coils and corresponding to the navigational energy at the last navigational point of the sensing coil while oriented according to the compensated orientation.
• Another aspect of the invention comprises a method of determining a substantially precise location of a field sensor within a navigational domain influenced by a field influencing shield device.
• the method includes sequentially projecting into the navigational domain, via three unidirectional coils and six delta coils, the navigational energy at a first frequency, and measuring a first signal value in the field sensor corresponding to each of the three unidirectional coils and the six delta coils, so as to produce nine of the first signal values.
• the method further includes forming three independent equations including three adjusted signal values corresponding to the unidirectional coils, three predetermined field magnitude values due to fields from each of the unidirectional coils and corresponding to the navigational energy at a last navigational point of the field sensor, the predetermined field magnitude values being manipulated so as to account for the shield device, and unknown orientation variables, and simultaneously solving the independent equations to determine the orientation variables corresponding to the compensated orientation of the sensing coil.
• the method also includes generating three lines and determining an intersection of the three lines, the intersection corresponding to the compensated position of the sensing coil.
• Each of the lines is generated from adjusted signal values corresponding to a pair of the delta coils, and predetermined field magnitude values due to the pair of delta coils and corresponding to the navigational energy at the last navigational point of the sensing coil while oriented according to the compensated orientation, the predetermined field magnitude values being manipulated so as to account for the effect of the shield device.
• FIG. 1 is a graphical view of prior art methods of identifying a general point in three dimensional space
• FIG. 2 is a graphical view of a prior art method of identifying the position and orientation of a sensing coil in three dimensional space;
• FIG. 3 is a perspective view of an examination deck used for implementing the location and compensation methods according to the present invention
• FIG. 4 shows a schematic representation of a unidirectional coil set for generating uniform x-directed fields in the navigational domain
• FIG. 5 shows a schematic representation of a unidirectional coil set for generating uniform y-directed fields in the navigational domain
• FIG. 6 shows a schematic representation of a unidirectional coil set for generating uniform z-directed fields in the navigational domain
• FIG. 7 shows a schematic representation of a coil configuration for generating vector gradient fields in the navigational domain
• FIG. 8 shows a schematic representation of the coil configuration of FIG. 7, including constant signal surfaces generated by those coils;
• FIG. 9 shows an upper plan schematic view of a delta coil group from FIG. 7 relative to an inner circular space representing the projection of navigational domain into the plane of the delta coils;
• FIG. 10 depicts an example of a tool coil attached to a surgical tool used to drill holes in the vertebrae of a patient, as well as an exemplary hysteresis graph of such a ferromagnetic tool.
• FIG. 11 depicts an example of the use of the surgical tool of FIG. 10 with an attached tool coil.
• FIG. 12 illustrates a perspective view of an examination deck suitable for implementing the location and compensation methods in the presence of a shield device, according to the present invention
• FIG. 13 shows a flow diagram for a method of eddy current compensation in accordance with the present invention
• FIG. 14 shows a flow diagram for a method of ferromagnetic and conductor compensation in accordance with the present invention
• FIG. 15 shows a flow diagram for a method of shield device compensation in accordance with the present invention
• FIG. 16 shows a more detailed flow diagram for the orientation calculation step of FIG. 13;
• FIG. 17 shows a more detailed flow diagram for the access step of FIG. 13;
• FIG. 18 shows a more detailed flow diagram for the orientation calculation step of FIG. 15;
• FIG. 19 shows a more detailed flow diagram for the position calculation step of FIG. 15;
• FIG. 20A shows a perspective view of the disposition of the nine coil sets of the examination deck
• FIG. 20B shows a schematic view of the coil sets of FIG. 20 A
• FIG. 21 A shows a perspective view of an exemplary disturbance which is in the form of a metallic ring, along with the equivalent circuit model;
• FIG. 2 IB shows a perspective view of an elliptical slab of metal including an off center circular hole, along with the equivalent circuit model;
• FIG. 21C shows a perspective view of a solid square metal plate, along with the equivalent circuit model.
• the present invention is directed to a system and method for determining the position and orientation of a catheter or other suitable probe inserted into a selected body cavity of a patient undergoing a surgical procedure in the presence of field- influencing objects.
• position and orientation data is determined from a series of measurements of voltage amplitudes induced within a sensing coil affixed to the distal end of the catheter probe as a result of the use of multiple waveforms. These voltage amplitudes, as a function of the waveforms, are induced in the sensing coil in response to known independent electromagnetic fields that are projected into the anatomical region of interest.
• the measurements provide information to compute the angular orientation and the positional coordinates of the sensing coil and account for the distortion of the known field by arbitrary conductors with field-induced eddy currents.
• position and orientation data is determined from a series of measurements of voltage amplitudes induced within a sensing coil and a tool coil.
• the sensing coil is affixed to the distal end of the catheter probe.
• the tool coil is affixed to a field-influencing object with a ferromagnetic and conducting core.
• the field-influencing object in the presence of a known electromagnetic field, distorts that field and influences the measurement of the sensing coil.
• the voltage amplitudes from both coils are stored and can be mathematically manipulated so as to isolate the effect of the field-influencing object on the sensing coil.
• the measurements of the induced voltage amplitudes on the sensing coil and on the tool coil provide information to account for the presence of the field-influencing object with a ferromagnetic and conductor core.
• the position and orientation data is determined from a series of measurements of voltage amplitudes induced within a sensing coil affixed to the distal end of the catheter probe in the presence of a shield device. These voltage amplitudes are induced in the sensing coil in response to two fields. One of the fields is the known independent electromagnetic field projected into the anatomical region of interest from field coils. The other field is that of the known field as reflected from the shield device.
• the measurements of the induced voltage amplitudes and the knowledge of the geometry and effect of the shield device provide sufficient information to compute the angular orientation and the positional coordinates of this sensing coil in the presence of the shield device.
• sensing coil refers to an electrically conductive, magnetically sensitive element that is responsive to time-dependent magnetic fields and generates an induced voltage as a function of and representative of the applied time-dependent magnetic field.
• the sensing coil is adaptable for secure engagement to the distal end of a catheter probe.
• tool coil refers to an electrically conductive, magnetically sensitive element that is responsive to time-dependent magnetic fields and generates an induced voltage as a function of and representative of the applied time-dependent magnetic field.
• the tool coil is adaptable for secure engagement to an object with a ferromagnetic and conducting core.
• navigational domain refers to a fully enclosed spatial region whose internal volume substantially encloses the complete prospective range of movement of the sensing coil.
• the navigational domain may be defined by any geometrical space, but preferably takes the form of a spherical volume. Under surgical operating conditions, the navigational domain will correspond to an anatomical region of the recumbent patient where surgical viewing or investigation is desired (e.g., a diseased area of tissue or an organ).
• peripheral domain refers to the spatial region outside of the navigational domain. Under surgical operating conditions, the peripheral domain may include the region that contains the operating table, or the region that encompasses other equipment within the operating room.
• last navigational point refers to the most recently determined location of the sensing coil before another iteration of the location algorithm is performed.
• uniform amplitude field refers to a magnetic field having a large magnetic field amplitude component in a specified direction and relatively smaller magnetic field amplitude components in the other directions.
• the uniform amplitude field is characterized by substantially uniform field amplitude values, throughout the navigational domain.
• the amplitudes of the induced voltage drops developed by such fields in the sensing coil are designated with superscripts V x , V ⁇ , and V z , respectively.
• waveform refers to the temporal shape of a magnetic field, illustrated graphically by a plot of the magnitude of a magnetic field as a function of time.
• a waveform in general can take on the characteristics of any form.
• a waveform can also be sawtooth in nature, or square in nature.
• unidirectional coils refer to a magnetic assembly that is operative to generate a uniform amplitude field (as defined above) within the navigational domain. A distinct magnetic assembly is employed for each uniform amplitude field.
• the unidirectional coils described herein are preferably implemented with a collection of appropriately designed magnetic coils, this implementation should not be construed as a limitation of the present invention.
• the unidirectional coils may be constructed from any magnetic configuration that is sufficient to generate the uniform amplitude fields.
• vector gradient field refers to a time-dependent magnetic field having nonzero vector gradient field components (i.e., magnetic field vector components with a high spatial gradient) in two of the three magnetic field components, and a substantially zero vector gradient field component in the remaining magnetic field component in an appropriately chosen coordinate system.
• the appropriately chosen coordinate system is an x-y-z coordinate system at a position R in the navigational domain
• the magnetic field amplitude H" (R) (a vector field) can be written as:
• H"(R),H"(R), and H"(R) represent the magnetic field
• amplitude strengths of the nth coil (designated by the superscript " «") in the x- direction, ⁇ -direction, and z-direction, respectively and are individually scalar quantities.
• the value of a vector gradient of such a magnetic field amplitude ⁇ " (R) where the magnetic field has a substantially zero vector gradient component in the z- direction can be written as the following vector gradient (or tensor) field:
• VH n (R) VH ⁇ (R), VH" (R),0 ) , where
• VH"(R) ⁇ 0 VH"(R) ⁇ 0
• a substantially zero vector gradient component is generated when the magnitude of the substantially zero vector gradient component value is small compared to the magnitude of the net vector resulting from the other two magnetic field components.
• fixed orientation with respect to a catheter probe refers to a catheter probe with constant values of orientation variables ⁇ and ⁇ , over a selected range of x, y, and z positional values.
• constant signal surface or “constant voltage surface” refers to a surface contour along which at every possible point of location for the sensing coil, the same induced voltage is developed in the sensing coil.
• the constant signal surface will be a small planar region located near the LNP with the sensing coil at a fixed orientation (as defined above).
• delta coil refers to a magnetic assembly for generating a vector gradient field (as defined above) within the navigational domain.
• the delta coil will typically be described in the context of delta coil pairs including a long delta coil and a short delta coil, each pair generating vector gradient fields with the substantially zero component in the same- axial dimension but whose magnetic field patterns in the remaining components are independent of each other.
• Each of the long and short delta coils may be considered to generate a family of constant signal or constant voltage surfaces for the sensing coil within the navigational domain.
• delta coils are preferably implemented with an array of appropriately designed magnetic coils (discussed below), this preferred implementation should not serve as a limitation of the present invention as it should be apparent to those skilled in the art that other magnetic configurations may be used to adequately generate the vector gradient fields.
• magnetic look-up-table refers to a database including the magnetic field amplitude values at every x-y- z coordinate position within the navigational domain for the unidirectional coils and for each delta coil used by the present invention. Accordingly, input data consisting of an x-y-z coordinate and a magnetic field amplitude identifier, which designates a selected magnetic coil assembly, is indexed within the database to a corresponding set of magnetic field amplitude values constituting the output data.
• the output data is represented by the magnetic field amplitude variables H n H n H n where the subscript x-y-z indicates the axial dimension along
• the database is created through a computational analysis of the magnetic field amplitude patterns generated by the magnetic coil configurations used herein.
• the mathematical model to develop the necessary formulae defining the field patterns may be developed, for example, from near field electromagnetic theory.
• An instructive text for facilitating such an analysis is "Field and Wave Electromagnetics" 2nd edition Addison Wesley (1989) by D. K. Cheng, herein incorporated by reference.
• the database may be stored in any type of facility including, inter alia, read-only memory, firmware, optical storage, or other types of computer storage. Additionally, the database information may be organized into any type of format such as a spreadsheet.
• the z-axis coincides with the longitudinal dimension extending from the patient's head to foot.
• the x-axis coincides with a lateral dimension across the patient's body, and the y-axis is perpendicular to the planar top of the pallet or examination deck. These dimensions are identified as the patient is disposed in the recumbent position on the pallet.
• FIG. 3 schematically illustrates a perspective view of examination deck 17 that facilitates implementation of the location and compensation methods in accordance with all preferred embodiments of the present invention.
• Examination deck 17 employs a magnetic coil assembly arranged in a flat configuration.
• Examination deck 17 includes planar top platform 10 suitable for accommodating a recumbent patient disposed lengthwise.
• Navigational domain is illustratively depicted as the spherical volume enclosing sensing coil 14.
• Tool coil 19 is located in peripheral domain 15.
• Sensing coil 14 is attached via connection means 16 to an external signal detection apparatus (not shown).
• Tool coil 19 is likewise attached to an external signal detection apparatus. Although sensing coil 14 functions optimally within navigational domain 12, tool coil 19 may lie within peripheral domain 15 or navigational domain 12.
• the coil sets embedded in platform 10 are activated by a signal drive unit (not shown) connected via line 18.
• Examination deck 17 is preferably constructed from a suitable magnetically- permeable material to facilitate magnetic coupling between the embedded coil sets and the overlying sensing coil.
• First conducting body 23 and second conducting body 31 are shown in FIG. 3 in peripheral domain 15. However, both first conducting body 23 and second conducting body 3 lean lie within navigational domain 12 as well in accordance with a preferred embodiment of the present invention.
• First conducting body 23 and second conducting body 31 respond to the fields generated by the field coils by developing eddy currents. These eddy currents, in turn, generate new fields that can influence the measured voltage across sensor coil 14 as is discussed in more detail below.
• Ferromagnetic body 29 is shown in peripheral domain 15 and is shown enveloped within tool coil 19. Again, ferromagnetic body 29 and tool coil 19 can lie within navigational domain 12 as well in accordance with a preferred embodiment of the present invention. Ferromagnetic body 29 responds to the fields generated by the field coils by both focusing the magnetic flux lines and by introducing a phase shifted field. The focusing and phase shifting effect of ferromagnetic body 29 can influence the voltage drop as measured across sensor coil 14 as is discussed in more detail below. Coil sets for generating x-directed. y-directed. and z-directed fields
• FIGS. 4-6 schematically illustrate unidirectional coil sets for generating substantially uniform amplitude x-directed, -directed and z-directed fields, respectively, throughout navigational domain 12.
• the uniform amplitude field in one embodiment of the present invention can be operated at multiple waveforms, for example, a sinusoidal waveform with angular frequency ⁇ . and a sinusoidal waveform with angular frequency -y» are two different waveforms.
• Unidirectional coil set 25 of FIG. 4 includes a first coil pair with elements 20 and 24 and a second coil pair with elements 22 and 26, where the current flow as supplied by drive unit 28 is indicated by the arrow symbol.
• Coil elements 20 and 22 are disposed in the major surface of platform 10, while elements 24 and 26 are disposed in the lateral walls of platform 10.
• Elements 24 and 26 are preferably used as compensation coils to substantially cancel undesirable field components generated by elements 20 and 22 in the y and z directions.
• the coils cumulatively generate a substantially uniform amplitude x-directed field as indicated by representative field line 27.
• Unidirectional coil set 35 of FIG. 5 schematically illustrates a coil set for generating a substantially uniform amplitude y-directed field throughout navigational domain 12 as indicated by representative field line 33.
• the coil set includes a coil pair with elements 30 and 32 disposed in spaced-apart and parallel relationship within platform 10, with the indicated current flow as supplied by drive unit 34.
• Unidirectional coil set 45 of FIG. 6 generates a substantially uniform amplitude z-directed field as indicated by representative field line 43.
• Coil set 45 includes a first coil pair with elements 38 and 42 and a second coil pair with elements 36 and 40, where the current flow as supplied by drive unit 44 is indicated by the arrow symbol.
• Coil elements 36 and 38 are disposed in the major surface of platform 10, while elements 40 and 42 are disposed in the lateral walls of platform 10.
• Elements 40 and 42 are preferably used as compensation coils to substantially cancel undesirable field components generated by elements 36 and 38 in the y direction.
• Unidirectional coil sets 25, 35, and 45 are illustrative only and should not be construed as a limitation of the present invention.
• a first connection means couples sensing coil 14 to a signal measuring device
• a second connection means couples tool coil 19 to a signal measuring device.
• FIGS. 7 and 8 show a coil configuration that can be used to determine the positional coordinates of sensing coil 14 in accordance with all of the preferred embodiments of the present invention.
• the configuration includes six coils grouped into three pairs of long and short delta coils (50 and 52, 54 and 56, 58 and 60).
• the delta coils are mutually coplanar and are disposed in the planar top of the examination deck immediately beneath the recumbent patient.
• Interconnection means between delta coil group 51, delta coil group 55, delta coil group 59 and a signal drive unit (not shown) is indicated.
• the coils are preferably arranged in a circular orientation about the y-axis such that there is an axis perpendicular to the direction of elongation of the coils, proceeding from the long coil set to the short coil set, where that axis is at 0°, 120° and 240° to the z-axis.
• the x-axis is considered to be oriented at 270° with respect to the z-axis in the x-z plane.
• the magnetic field generated by delta coil group 51 is shown representatively by the field lines extending from the upper region of the coils.
• the resulting vector gradient field can be operated at multiple waveforms. For example, a sinusoidal waveform with angular frequency ⁇ .
• FIG. 9 shows an upper plan schematic view of delta coil group 51 relative to an inner circular space representing the projection of navigational domain 12 into the plane of the delta coils. This design creates a high spatial gradience in two of the axis dimensions and a substantially zero field value in the remaining axial dimension.
• each of the long coils and short coils is modified by representative compensation coils 80 and 82, 84 and 86, 88, 90 and 94, and 92 and 96 respectively, disposed at the indicated endpoints and center of the corresponding delta coil.
• the long coil and short coil configurations are shown schematically for only delta coil group 51 , but similar configurations likewise exist for delta coil group 55 and delta coil group 59, shown representatively as the indicated lines.
• Parameters related to the quality of the coils such as (i) the degree of uniformity of the uniform amplitude field coils and (ii) how close to zero the vector gradient field is in the non-gradient direction for the delta coils, determine the size of navigational domain 12 and peripheral domain 15.
• FIG. 10 indicates tool coil 19 affixed to surgical tool 108, where surgical tool
• Hysteresis graph 100 illustrates the nonlinear behavior of a magnetic field H associated with surgical tool 108 in response to an applied magnetic field B. Such a response is typical of ferromagnetic objects.
• FIG. 11 indicates how surgical tool 108 and tool coil 19 of FIG. 10 may be used in practice.
• Surgical tool 108 is used to drill holes into vertebrae 110.
• FIG. 12 schematically illustrates a perspective view of examination deck 17 that facilitates implementation of the location and compensation algorithms in accordance with shield device 120.
• Examination deck 17 includes planar top platform 10 suitable for accommodating a recumbent patient disposed lengthwise.
• Examination deck 17 rests on base 122.
• Navigational domain 12 is illustratively depicted as the spherical volume enclosing a sensing coil 14 attached via connection means 16 to an external signal detection apparatus (not shown).
• the coil sets embedded in platform 10 (and described in connection with FIGS. 4-7) are activated by a signal drive unit (not shown) connected via line 18.
• the examination deck is preferably constructed from a suitable magnetically permeable material to facilitate magnetic coupling between the embedded coil sets and the overlying sensing coil.
• Shield device 120 can be made from aluminum, copper or virtually any other conductive material. It is also possible to use materials other than a conductive sheet such as a mesh or strips of material. A further possibility is to use a plastic of polymer film with a conductive coating.
• FIG. 13 indicates a schematic of a method for eddy current compensation consistent with the present invention.
• Measuring steps 132 and 134 involve activating each of nine coil sets and measuring the field at sensing coil 14 at different waveforms. For example, a sinusoidal waveform with angular frequency ⁇ . and a sinusoidal waveform with angular frequency ⁇ correspond to two different waveforms.
• the nine coil sets correspond to three unidirectional coil sets and three delta coil groups, where each delta coil group contains a long coil set and a short coil set.
• the unidirectional coil sets -generate uniform amplitude fields in the x, y, and z- directions and are depicted in FIGS. 4, 5, and 6. From FIG.
• the angular designations associated with the delta coil groups indicate the angle with respect to the z-axis of the coil dimension that is perpendicular to the direction of elongation of the delta coils as in FIG. 7. Accordingly, the three delta coil groups are arranged pair- wise in a circular orientation about the y-axis at angles of 0°, 120°, and 240°.
• a series of fields are generated and measured in sensing coil 14 at a first waveform (measuring step 132) and at an mth waveform (measuring step 134). For example, considering two waveforms where the waveforms correspond to substantially uniform amplitude fields with sinusoidal waveforms and angular frequencies ⁇ . and -y ? and considering x-directed fields of coil set 25 of
• FIG. 4 with measured values of V x ⁇ j,V x > ⁇ J in sensing coil 14, the value of a constant ⁇ , , defined by:
• V x . is calculated using ⁇ , , .
• V x V x
• measurements are performed at four different waveforms (measuring steps 132 and 134 in FIG. 13). For example, considering a magnetic field waveform that is sinusoidal in nature with angular frequency ⁇ , measurements at four frequencies -y. , -y_ , -y and ⁇ . correspond to measuring steps 132 and 134 at four different waveforms.
• V" djml can be determined using singular value decomposition as is described in more detail below. Again, this corresponds to calculation step 138 in FIG. 13 in one embodiment of the present invention.
• step 140 it is determined whether all unidirectional field coils have been activated. In the example here, only the x-directed coils have been activated (coil set 25 of FIG. 4). Thus, a corresponding set of substantially uniform amplitude fields with different waveforms are generated and measured at sensing coil 14 in the ⁇ -direction by coil set 35 of FIG. 5, and then likewise in the z-direction by coil set 45 of FIG. 6, with the appropriate calculations.
• each delta coil set is activated in succession the induced voltage is measured in sensing coil 14 at different waveforms. Again as above, the measurement of the voltage drops at the two waveforms allow for the calculation of an adjusted voltage drop across sensing coil 14.
• V adjst is the signal that would have been picked up by sensor coil 14 if the conductive body disturbance had not been present.
• step 144 an orientation calculation is performed in step 144 to determine the values of the sensing coil 14 orientation variables ⁇ and ⁇ , independent of the unknown sensing coil 14 positional variables (x,y,z).
• orientation calculation 144 is shown in FIG. 16 and is discussed in more detail below.
• step 146 of FIG. 13 a position calculation is performed to obtain the positional variables (x,y,z) of sensing coil 14.
• FIG. 17 A more detailed breakdown of position calculation 146 is shown in FIG. 17.
• access step 176 indicates that orientation calculation 144 is preferably performed before positional calculation 146.
• access step 178 the LUT is accessed to obtain the magnetic field values at the LNP for a long delta coil set and a short delta coil set. These magnetic field values and the as-computed values for the orientation angles ⁇ and ⁇ are substituted into the appropriate induced voltage equations to calculate for each delta coil the value of the voltage amplitude signal induced in the sensing coil at the LNP.
• step 180 Based on the difference between the measured and the LNP values for the induced voltage signals, a calculation is performed in step 180 that permits identification of a line 3" on which sensing coil 14 lies, as described in more detail below.
• query step 182 it is determined whether such a line 3" has been identified for each delta coil group.
• step 184 is performed to determine where in space the lines 3 1 ,32 ,33 intersect. That position in space is the location (x,y,z) of sensing coil 14.
• FIG. 14 indicates a schematic for ferromagnetic and conductor compensation consistent with the present invention.
• Measuring step 152 is similar to measuring step 132 as described above for FIG. 13.
• Measuring step 154 is different, however, in that a measurement is performed across tool coil 19.
• the nine coil sets correspond to three unidirectional coil sets and three delta coil groups, where each delta coil group contains a long coil set and a short coil set.
• the unidirectional coil sets generate uniform amplitude fields in the x, y, and z-directions and are depicted in FIGS. 4, 5, and 6.
• the delta coil sets are depicted in FIG. 7 and described above.
• sensing coil 14 measuring step 152
• tool coil 19 measuring step 154
• each delta coil is activated in succession and a corresponding induced voltage is measured in sensing coil 14 (measuring step 152) and tool coil 19 (measuring step 154).
• the measured voltage drops allow for the calculation of an adjusted voltage drop across sensing coil 14, the voltage that would be present if there were no disturbance.
• V n the value of V n ,. is calculated using K
• step 160 of FIG. 14 This is step 160 of FIG. 14 and is described in more detail below.
• FIG. 15 indicates a schematic of a method for shield device compensation consistent with the present invention.
• a magnetic assembly of nine individual coil sets and shield device 120 are used to generate magnetic fields sufficient to develop a corresponding set of nine induced voltage signals at sensing coil 14.
• Measurement step 162 indicates such a measurement.
• the major difference associated with shield device compensation is observed in calculation steps 164 and 166.
• orientation calculation step 164 of FIG. 15 is shown in more detail in FIG. 18.
• position calculation step 166 of FIG. 15 is shown in more detail in FIG. 19.
• FIG. 19 In FIG.
• an additional manipulation step 188 is incorporated as compared to the schematic of FIG. 16.
• an additional manipulation step 192 is incorporated, as compared to the schematic of FIG. 17.
• the additional manipulation step associated with the shield device compensation method involves the manipulation of the values of the LUT. Specifically, knowledge of the geometry of the coil sets and shield device 120 is sufficient to allow manipulation of the LUT in order to account for the effect of shield device 120 within navigational domain 12. The effects of arbitrary field-influencing objects that lie in peripheral domain 15 and anterior to shield device 120 are thereby cancelled.
• the input data for the LUT consists of the x-y-z coordinates and a designation of which coil set is being used to generate the magnetic fields (the superscript " «").
• the LUT supplies the magnetic field amplitude values and H"(R) at the selected x-y-z coordinates for the designated coil set. Note that in the previously discussed preferred embodiments of the present invention, the LUT can only be successfully utilized after compensation for field-influencing objects has occurred. However, in the preferred embodiment of the present invention, compensation for shield device 120 may be incorporated into the LUT.
• the LUT is present to speed up the operational sequence of the location algorithm. Otherwise, an undesirable computational delay exists if the required magnetic fields from the nine coil sets must be individually calculated during each iteration of the algorithm.
• the location algorithm need only access the LUT to retrieve the appropriate field value without endeavoring into any complex field analysis. This is especially true when shield device compensation is an issue.
• an interpolation procedure is employed to calculate the field value. Detailed description of method for determining unknown position coordinates by sampling a known navigation field
• the system and method used herein is directed to the development of a series of measurements and calculations able to account for the effects of a field-influencing object that would otherwise introduce error into a position and orientation determination.
• the relationships defined by these systems and methods are such that the unknown effects of the field-influencing object are separable.
• the angular orientation of sensor coil 14 is represented by an angle ⁇ corresponding to the angle of departure from the z-axis and an angle ⁇ corresponding to the angle between the x-axis and the projection onto the x-y plane of the vector coincident with the longitudinal axis of sensing coil 14 as shown in FIG. 2.
• the unidirectional coils are activated in succession, each generating a substantially uniform amplitude field that projects into navigational domain 12 and induces a corresponding voltage signal in sensing coil 14.
• the LUT is then accessed three times to acquire the magnetic field values at the LNP for each of the three unidirectional coils.
• these values and the measured voltage signals are then substituted into the appropriate equations set forth below to solve for the unknown variables -?and ⁇ that define the orientation of sensing coil 14.
• the time-dependent magnetic fields projected into the navigational domain induce voltages in sensor coil 14 that are representative of the orientation of coil axis a 21 relative to the lines of magnetic flux.
• the development of an induced voltage in sensing coil 14 in response to a changing magnetic field is defined by Faraday's law.
• V(t) the induced voltage as a function of time V(t) around this path is equal to the negative time rate of change of the total magnetic flux through the closed path (one turn).
• the magnetic flux ⁇ through S is given by, where ⁇ is the magnetic permeability of free space and is a constant, the superscript
• V n (t) - N ⁇ ⁇ . ⁇ da
• V n (t) - N - ⁇ N ⁇ r 2 ⁇ Q H n cos(p) exp(- i ⁇ - i ⁇ /2)
• V n (t) -N ⁇ r 2 ⁇ Q H n cos( ) exp(-/ ⁇ >t - i ⁇ /2)
• V n (t) -N ⁇ r 2 ⁇ Q H n cos(p)
• the induced voltage amplitude V n in sensing coil 14 will vary with changes in the angular orientation between the coil axis and the direction of the magnetic field lines
• P- A useful reference frame for spatially conceptualizing the interaction between sensing coil 14 and the magnetic fields in navigational domain 12 is the Cartesian coordinate system defined by mutually perpendicular axes x-y-z. As above, coil axis
• the angles , ⁇ , ⁇ that the coil axis 21 makes with the unit coordinate vectors x, y, and z respectively, are called the direction angles of coil axis 21 ; the trigonometric terms cos( ⁇ ),cos( ?)and cos(/)represent direction cosine values.
• V ⁇ ⁇ H sin( ⁇ )cos( ⁇ )+ ⁇ Hy sm( ⁇ )sin( ⁇ )+ ⁇ Hz cos ), and
• V z ⁇ -yH sin( ⁇ .)cos(-?)+ ⁇ - ⁇ H y sinf ⁇ )sin(#)+ ⁇ H z cos )
• the LNP is the center of the viewing field.
• AV n is known, the variables K and ⁇ are known, as is coil axis a 21.
• the gradient V ⁇ H ? • ⁇ j can be calculated from the LUT using standard techniques from numerical methods.
• the delta coil pairs are constructed so that the substantially zero gradient component of both the short coil and the long coil field are in the same direction. Denoting such a direction by c n , this implies by definition that
• three coils are desirable because of the possibility that the orientation of the sensing coil 14 a may be such that
• FIG. 20A shows the disposition of the nine coil sets of the examination deck 17; the coil sets are also shown in FIG. 20B where they are schematically represented as 1-9. Also shown in 20B is first conducting object 23, it is subjected to a field
• H 1 (R ,) which is the field produced by coil set #1 at the location of first conducting object 23.
• This field will induce eddy currents ID that flow in first conducting object 23 and these currents will produce disturbing magnetic fields of their own, these are designated H D l (x,y,z).
• H D l x,y,z
• the field present at X,Y,Z, the location of sensing coil 14 has two parts, ⁇ '(X,Y,Z) the desired field upon which navigation is calculated and
• the signal produced in sensing coil 14 also has two components corresponding to the two fields. In order to isolate and separate these, one must understand the characteristics of eddy currents produced in first conducting object 23 of arbitrary shape and characteristics.
• FIG. 21 A depicts an exemplary disturbance in the form of a metallic ring.
• This ring will have an inductance L and a resistance R that depend on the dimensions and materials of the ring.
• FIG. 21B shows an elliptical slab of metal with an off center circular hole cut in it. The eddy current performance of this shape is largely described by three current loops or modes of excitation.
• each of these modes has its' own L, R and r and its own degree of interaction to an incoming field, the eddy current equivalent is therefore a summation of three circuits each similar to that for the case shown in FIG. 21 A.
• FIG. 21C which corresponds to a solid square plate.
• a dominant mode (#1) which corresponds to the current circulation around the largest dimensions of the disturbing shape, this gives the largest interaction with the incoming field and also produce the largest field generation.
• any shape disturbance can always be described as an infinite summation of simple LR circuits.
• the first objective in the following description is to show how the dominant mode of any disturbance 23 can be eliminated in its effect of the signal V (t) and therefore eliminate its effect on navigation.
• the second objective is to show how higher order modes can be eliminated.
• the induced voltage at the sensing coil 14 has two components, the direct coupling from the transmitter coils and the indirect coupling from the first conducting object 23, which gives ⁇ exp(- i ⁇ t - i ⁇ ) ⁇ e"ddyL £ eddy
• V n (t) ⁇ exp(- i ⁇ t - i ⁇ /2) ⁇ + ⁇
• V n (t) V n exp(- i ⁇ t - i ⁇ /2), the following result is obtained:
• ⁇ n n _ ⁇ l _ ⁇ eddy_ eddy_ eddy ' eddy
• first conducting object 23 has altered the magnitude of the measured voltage amplitude due to the source coils, at the expected phase shifted point exp(- i ⁇ t-i ⁇ /2 ), by an amount AV y equal to
• AV n-eddy edd y ⁇ + ⁇ 2 ⁇ e 2 ddy n AV ⁇ y contains the two unknowns x , , and ⁇ y .
• the measured voltage drop V n can be rewritten as: ⁇ VH
• V n the orientation and position calculations discussed previously can be applied using the LUT in order to obtain the proper orientation and position of sensing coil 14.
• V ,_ The value of the amplitude V ,_ can be obtained directly by measurement, since it lags the expected voltage drop signal by the additional phase of ⁇ /2 radians, or 90°. Thus, only r , , need be determined in order to compensate for an extraneous eddy current.
• a second linearly independent equation is obtained by taking a voltage amplitude measurement using a second magnetic field waveform, for example, taking a voltage amplitude measurement at a second angular frequency.
• first conducting object 23 and second conducting object 31 each with a different value of x. and ⁇ . , respectively, and with different coupling
• V n concerned n _ ⁇ _ n n ⁇ i. ⁇ . n + ⁇ 0 ⁇ + ⁇ ] l ⁇ 1 + ⁇ 2 r .2 1 , 1 + ⁇ 2 x .2
• the general result is that resolving m values of x , requires m measurements of real and imaginary parts of the potential drop across sensing coil 14, using m different waveforms. For example, measurements of a sinusoidal waveform at the set of frequencies ( ⁇ . , ⁇ , . . . ⁇ )
• the quantity V" is measured at sensing coil 14 at a frequency ⁇ . while the quantity
• ⁇ can be determined from measurements at sensing coil 14 at any two of d ⁇ ⁇ ⁇ ) three frequencies ⁇ . , ⁇ , and ⁇ . , or using all three frequencies ⁇ . , ⁇ _ , and -yford .
• the adjusted value for the potential drop across sensing coil 14 can be determined according to:
• Another embodiment of the present invention utilizes the relationship derived above for the case of first conducting object 23 only:
• Imi " j is also determined from a measurement across sensing coil 14, ⁇ . is
• Vetterling This, again, corresponds to calculation step 138 of FIG. 13.
• This section describes in more detail the ferromagnetic and conductive object compensation method as indicated by the schematic of FIG. 14.
• a pure magnetic core is a source of magnetic flux.
• the primary quality of a pure magnetic core is that it can enhance, or focus, magnetic field flux lines along a preferred direction.
• V n ⁇ t ⁇ ⁇ ⁇ • a da S dx
• the quantity on the right is proportional to the time rate of change of magnetic flux
• V magn( t ) - T , ⁇ ' '' ""mmaaggnn "a”magn
• T is a multiplication factor that represents the enhancement of the magnetic flux through tool coil 19.
• Surgical tool 108 of FIG. 10 has some qualities of a pure magnetic object, as well as additional properties. Like a pure magnetic object, it can act to enhance the detection of flux lines through the center of tool coil 19 as above. However, it also responds to an applied field in a nonlinear manner, as is indicated on hysteresis graph 100. Furthermore, the additional presence of conductive elements introduce additional fields that are out of phase by ⁇ /2 with any enhanced field in the vicinity. Thus, a general form for the voltage drop across tool coil 19 affixed around surgical tool 108:
• V f n f erro (t) ' ⁇ exO(- i ⁇ t - i ⁇ / ' 2i ⁇ O R a e - iT / j m V r f"erro
• V j"erro ⁇ ' -( ⁇ ⁇ Re - iT I ⁇ m )' ⁇ ⁇ ⁇ Q T ⁇ . a j f erro da j,erro ferro
• V n (t) ⁇ e ⁇ p(- i ⁇ t - i ⁇ /2) ⁇ + ⁇ exp(- i ⁇ t -i ⁇ /2f ⁇ Re - ⁇ lv ⁇ l f n erro
• V n ⁇ '%0 ⁇ T f n ,erro - iT Im' )
• disturbances can include the operating table, a head-holder or other any number of other metallic items.
• any field-influencing effect of an operating room table has the potential to create a larger than typical distortion in the fields located in the navigational domain.
• shield device 120 is provided to restrict the propagation of magnetic fields through it, as, for example, a sheet of conductive material. It may be arranged in any suitable geometry that has a fixed relationship to the transmission coils. There are many possible materials that the plate can be made from, such as aluminum, copper or virtually any other conductive material. It is also possible to use materials other than a conductive sheet such as a mesh or strips of material. A further possibility is a plastic of polymer film with a conductive coating. Shield device 120 should preferably be placed between the transmitter coil array and the disturbance. In an operating room, if the patient were to lay on the transmitter coil array, then shield device 120 could be placed under the array to block effects of the operating table.
• shield device 120 An additional enhancement could be made by bending the sides of shield device 120 up at an angle to negate the effects of the length of the operating table.
• Other arrangements for shield device 120 are possible, such as placing it to block the effects of microscopes or C-arms that may be present in the field.
• the device alters the fields produced by the transmitter coils.
• the effect of the device can either be computed from electromagnetic theory, as for example, from “Static and Dynamic Electricity” third edition, Taylor & Francis (1989) by William R. Smythe.
• the effect could also be measured in a calibration process. Both of these techniques correspond to steps 188 and 192 of FIGS. 18 and 19 respectively.
• a fixed geometry can be characterized by a fixed alteration of the transmitted field. If the device is to be moved, a dynamic correction of the effect should be performed.
• step 144 in FIGS. 13 and 14 was indicated as occurring after all of the coil sets were activated and measured. Step 144, however, can be placed after step 140 in both FIGS. 13 and 14 and is consistent with the present invention. Also, there may be a more efficient manner of processing measurements and calculations in parallel, rather than the linear schematic presented.
• the magnetic field was considered to have a sinusoidal waveform at an angular frequency ⁇ , but other examples of waveforms are possible consistent with an embodiment of the present invention, including sawtooth waves, or square waves.
• waveforms including sawtooth waves, or square waves.
• any numerical technique for solving an overspecified equation is consistent with the present invention.

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