MXPA06005562A - Position tracking using quasi-dc magnetic fields - Google Patents

Abstract

A method for tracking a position of a sensor includes generating a periodic magnetic field in a vicinity of the sensor, the field having a positive polarity phase and a negative polarity phase with respective constant positive and negative amplitudes. First and second field measurement signals are produced responsively to the magnetic field at the sensor during the positive and negative polarity phases, respectively. The position of the sensor is determined responsively to the first and second field measurement signals.

Description

TRACE POSITION USING MAGNETIC FIELDS QUAD- DIRECT CURRENT FIELD OF THE INVENTION The present invention relates generally to magnetic position tracking systems, and particularly to methods and systems for reducing measurement errors in magnetic position tracking systems.

BACKGROUND OF THE INVENTION Various methods and systems for tracking the coordinates of objects involved in medical procedures are known in the art. Some of these systems use magnetic field measurements. For example, the patents of E.U.A. 5,391, 199 and 5,443,489, the descriptions of which are incorporated herein by reference, describe systems in which the coordinates of an intracorporeal probe are determined using one more field transducer. Said systems are used to generate location information regarding a medical probe or catheter. A detector, such as a coil, is placed on the probe and generates signals in response to externally applied magnetic fields. The magnetic fields are generated by magnetic field transducers, such as radiator coils, fixed to an external reference frame in known mutually separate locations. Additional methods and systems that refer to magnetic position tracking are also described, for example, in PCT Patent Publication WO 96/05768, U.S. Patents. 6,690,963, 6,239,724, 6,618,612 and 6,332,089 and patent application publications 2002/0065455 A1, 2003/0120150 A1 and 2004/0068178 A1, all of which descriptions are incorporated herein by reference. These publications describe methods and systems that track the position of intracorporeal objects such as cardiac catheters, orthopedic implants and medical tools used in different medical procedures. Some position tracking systems, including some of the systems described in the aforementioned references, use magnetic fields of alternating current (AC). Other position tracking systems use direct current (DC) fields. For example, the patent of E.U.A. 4,945,305, the disclosure of which is incorporated herein by reference, discloses a system for measuring the position of receiving antennas with respect to transmitting antennas, using pulsed CD magnetic signals. The transmitting antennas are driven one at a time by a pulsed direct current signal. A receiving antenna measures the transmitted magnetic fields and the magnetic field of the Earth. A computer converts the received signals into location and orientation outputs. The patent of E.U.A. 5,453,686, the disclosure of which is incorporated herein by reference, discloses a system that generates a plurality of magnetic fields by applying time-division multiplexed pulsed CD signals to a plurality of field generating elements. The fields are detected by remote sensors to detect the rate of change of each of the electromagnetic fields generated. The outputs of the remote sensors are integrated in order to establish the constant state components of the generated electromagnetic fields. The constant state components are resolved in the position and orientation of the remote object.

BRIEF DESCRIPTION OF THE INVENTION In AC magnetic position tracking systems, the magnetic field is produced by driving field generators with alternating current, typically sinusoidal drive signals (hence, the name "AC field"). Position tracking systems that use CA fields (referred to herein as "CA systems" for simplicity purposes) are susceptible to measurement errors caused by metal items or field responders located in the vicinity of the tracked object. It is well known in the art that a magnetic field of AC (or any magnetic field having field resistance variable in time) induces eddy currents in said articles. The parasitic currents subsequently generate parasitic magnetic fields that distort the measurement of the position tracking system. Position tracking systems that use CD fields (ie, fields that have constant field resistances during a measurement period of interest) are less sensitive to eddy current distortion. On the other hand, position measurements based on CD fields are often less stable, because the measurements are subject to baseline measurement, as will be explained later. In addition, CD systems inevitably incorporate the Earth's magnetic field in their measurements, which constitutes an additional error factor in position measurement. The pulsed CD fields allow the effect of the Earth's magnetic field to be subtracted from the measurement, but still require a separate calibration procedure to adjust the baseline drift. Modes of the present invention provide improved methods and systems for tracking the position and orientation of an object using a "quasi-CD" magnetic field. The methods and systems described provide the parasitic current immunity characteristic of CD systems, while providing the ability to compensate for polarization drift and for the Earth's magnetic field. In some embodiments, a quasi-CD field is generated by a periodic driving signal that has the shape of a square wave. The driving signal (and the corresponding magnetic field) alternates between two phases that have positive and negative polarities. During each phase, the magnetic field can be considered as a CD field, eliminating the effects of eddy currents. The position and orientation tracking system combines measurements taken during the two phases to cancel polarization drift and measurement errors due to the Earth's magnetic field. Therefore, in accordance with one embodiment of the present invention, a method is provided for tracking a position of a sensor that includes: generating a periodic magnetic field in a neighborhood of the sensor, the field having a positive polarity phase and a phase of negative polarity with respective constant positive and negative amplitudes; producing first and second field measurement signals that respond to the magnetic field in the sensor during the positive and negative polarity phases, respectively; and determining the position of the sensor that responds to the first and second field measurement signals. In one embodiment, the sensor is implanted in the body of a patient. Additionally or alternatively, the sensor is coupled to a medical instrument that is used to treat a patient. In another embodiment, each of the phases of positive and negative polarity is constant for a duration of at least 10 milliseconds. In another modality more, the positive amplitude is equal to the negative amplitude. In yet another embodiment, determining the position of the sensor includes performing an arithmetic operation on the first and second field measurement signals. In another embodiment, performing the arithmetic operation includes adding the first and second field measurement signals to produce a position signal. In one embodiment, the first and second field measurement signals include transient ranges and the production of the first and second field measurement signals does not measure the signals outside the transient ranges. In another embodiment, the generation of the periodic magnetic field includes multiplexing two or more periodic magnetic fields generated in two or more different respective places. Further, in accordance with one embodiment of the present invention, a method for tracking a position transducer is provided, which includes: operating the position transducer to generate a periodic magnetic field having a positive polarity phase and a negative polarity phase with respective positive and negative amplitudes constant; detecting the magnetic field at a known location to produce, in response to the detected magnetic field, first and second field measurement signals during the positive and negative polarity phases, respectively; and determining the position of the position transducer that responds to the first and second field measurement signals. Also, in accordance with one embodiment of the present invention, an apparatus is provided for tracking a position of an object that includes: at least one locating pad, which is arranged to generate a periodic magnetic field in a vicinity of the sensor, the field having a positive polarity phase and a negative polarity phase with respective constant positive and negative amplitudes; a position sensor, which is coupled to the object and is arranged to produce first and second field measurement signals in response to the magnetic field during the phases of positive and negative polarity, respectively; and a processor, which is arranged to determine the position of the sensor in response to the first and second field measurement signals. In addition, in accordance with one embodiment of the present invention, an apparatus for tracking a position of an object is provided, which includes: a field generator, which is coupled to the object and is arranged to generate a periodic magnetic field that has a positive polarity phase and a negative polarity phase with respective constant positive and negative amplitudes; a location pad, which is arranged to produce first and second field measurement signals in response to the magnetic field during the phases of positive and negative polarity, respectively; and a processor, which is arranged to determine the position of the sensor in response to the first and second field measurement signals.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood more fully from the following detailed description of the modalities thereof, taken together with the drawings in which: Figure 1 is a schematic pictorial illustration of a magnetic position tracking system, in accordance with one embodiment of the present invention; Figure 2 is a block diagram schematically illustrating a sensor unit, in accordance with an embodiment of the present invention; Figure 3A is a signal diagram schematically illustrating a magnetic field resistance, in accordance with one embodiment of the present invention; Figure 3B is a signal diagram schematically illustrating a detected magnetic field, in accordance with one embodiment of the present invention; and Figure 4 is a flow chart schematically illustrating a method for position tracking in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE MODALITIES Description of the system Figure 1 is a schematic pictorial illustration of a magnetic position and orientation tracking system 20, in accordance with one embodiment of the present invention. A surgeon 22 performs a medical procedure on a patient 26 using a medical tool 28. The tracking system that guides the surgeon comprises locating pads 32, which function as field generators. Locating pads typically comprise field generating coils, which generate quasi-CD magnetic fields in a predetermined work volume comprising the surgical site. The fields are generated in response to driving signals generated by a console 36. Magnetic fields are detected by miniature sensor units 34 inserted into the patient's body, as will be described in detail below. In the example shown in Figure 1, the sensor units are implanted in the patient's leg. Each sensor unit comprises position sensors that are designed to detect the magnetic field in its vicinity. The magnetic fields generated by location pads 32 cause the sensor units 34 to generate and transmit position signals that are indicative of the position and orientation of the sensor unit. The position signals are received by a wireless control unit, which is coupled to a computer 37, both located in the console 36. The computer 37, which serves as the central processor of the system 20, processes the received signals in order to calculate the relative location and orientation coordinates of the sensor units 34. the results are typically presented to the surgeon on a screen 38. (In the context of the present patent application and in the claims, the terms "position" and "coordinates" position "refer to both location and orientation of the sensor unit, typically positions are represented in terms of six-dimensional coordinates). The tracking system guides the surgeon in performing the procedure, in this example a knee joint operation, by measuring and presenting the positions and orientation of the sensor units 34. In some applications, a unit similar to the units sensor 34 is also adjusted in the tool 28. In said application, the tracking system can measure and present the position of the tool with respect to the intracorporeal sensor units. The system shown in Figure 1 is related to an orthopedic application. Further details regarding the position tracking system for orthopedic applications can be found in the provisional patent application of E.U.A. No. 60 / 550,924, filed March 5, 2004, now filed as a patent application of E.U.A. No. 11 / 062,258, which is assigned to the assignee of the present patent application and whose description is incorporated herein by reference. Nevertheless, the illustrative system was chosen purely for the purposes of conceptual clarity. Other system configurations and other applications will be apparent to those skilled in the art and are considered to be within the scope of the present invention. For example, any number of sensor units 34 and locator pads 32 may be used. The sensor units may be fitted on other types of implants and medical tools, as well as on invasive medical instruments such as catheters and endoscopes. The location pads can alternatively be attached to the patient's body. The location pads 32 and sensor units 34 may be designed to either transmit or receive magnetic fields. In other words, if the sensor unit 34 is configured to receive magnetic fields, then the location pads 32 are configured to generate fields. Alternatively, the locating pads may be configured to detect fields generated by field generators adjusted in the implants and / or the tool. In the following description, it is assumed that location pads 32 generate the magnetic fields, which are received by sensor units 34 in the implants and in the tool 28. In configurations in which the transmitter and receiver papers are reversed, the Principles of the present invention can be used to measure the positions of sensor units 34 by driving field transducers in the sensor units to generate quasi-CD fields, and detecting the fields in the pads of location.

Figure 2 is a block diagram schematically showing details of the sensor unit 34, in accordance with an embodiment of the present invention. The sensor unit 34 comprises position sensors 40, which are designed to detect and measure the magnetic field of its vicinity. The sensor unit typically comprises three position sensors 40 mounted in mutually orthogonal orientations. Each sensor 40 measures a component of the magnetic field, in accordance with the orientation of the sensor. The magnetic field detected during each phase of the quasi-CD field is substantially a CD field. Therefore, the position sensors 40 are designed to detect DC magnetic fields. In another embodiment, the sensors 40 comprise magnetoresistive transducers that change their electrical conductivity proportionally to the detected magnetic field. Alternatively, sensors 40 may comprise Hall-effect transducers that produce a voltage proportional to the detected magnetic field. Alternatively, in addition, any other sensor that is suitable for measuring DC magnetic fields can be used to implement position sensors 40. The position sensors 40 detect the components of the magnetic field and produce voltages that are processed by control circuit 42. circuit 42 produces position signals responsive to the voltages and transmits the signals to the wireless control unit in the console 36 using a transmission coil 44. A power unit 46 provides electrical power to operate a control circuit 42. In some embodiments, the power unit 46 comprises a battery. In other embodiments, the power unit 46 comprises a power coil, which receives radio frequency (RF) energy transmitted to the sensor unit from the external system. In these embodiments, the power unit rectifies the received RF signal and uses the resulting DC voltage to power the circuit 42. In some embodiments, the sensor unit 34 is connected by wires to the console 36. For example, in the sensor unit 34 can be adjusted at the distal end of a catheter or similar invasive instrument. The catheter comprises wires that connect the distal end to the external system. In such embodiments, the transmission coil 44 can be omitted and the position signals sent to the external system using the wired connection. Additionally or alternatively, power unit 46 can be similarly omitted, and power can be supplied to the control circuit through the wired connection. Figure 3A is a signal diagram schematically illustrating a magnetic field resistance of a quasi-CD magnetic field, in accordance with one embodiment of the present invention. A curve 50 shows the field strength of the magnetic field generated by one of the locating pads 32 that respond to a quasi-CD pulse signal. The generated field (also referred to as the "primary field") has the shape of a symmetric square wave. In this modality, the field comprises phases of positive and negative polarity, both having equal absolute magnitudes (denoted as A in the figure). Each phase of polarity has a duration denoted by T. The frequency of the impulse signal and the field is therefore defined as f = 1 / 2T. Although in the illustrative embodiment of Figure 3A the negative and positive polarities of the primary field are shown to have equal magnitudes and equal time durations, in other embodiments, the negative and positive polarities may be unequal. Similarly, the time durations of the positive and negative polarity phases do not need to be equal. Figure 3B is a signal diagram schematically illustrating a quasi-CD magnetic field detected, in accordance with one embodiment of the present invention. A curve 52 shows a typical signal magnitude of a signal produced by one of the position sensors 40 in one of the sensor units 34., which respond to the field shown by curve 50. As shown by curve 52, the signal is not symmetric. In the example illustrated by curve 52, the absolute magnitude of the negative phase (denoted by An in the figure) is greater than the absolute magnitude of the positive phase (denoted by Ap). The quasi-CD field asymmetry detected is mainly caused by two factors, namely the baseline drift and the Earth's magnetic field. The baseline drift is a term that refers to slow temporal variations in the field resistance measurement. Such variations may be caused, for example, by temperature variations and derivative of component value in the electronic circuit used to amplify, filter and sample the signals measured in the sensor unit and the external system. The baseline drift can be represented by an equivalent magnetic field vector that is added by vector to the primary field vector in the vicinity of the position sensor. Since each position sensor 40 detects a component of the mixed field, the polarization drift vector will reduce the value of a polarity phase of the detected field and increase the value of the opposite polarity by the same amount. The result of this effect is an asymmetry, or a phase shift, in the magnitudes of the positive and negative polarity phases, as shown in curve 52. The measurement of the quasi-CD primary field combined with the Earth's magnetic field causes an effect of similar asymmetry. A polarity phase of the quasi-CD field detected is increased by the contribution of the Earth's magnetic field, while the phase of opposite polarity is reduced by the same amount. In both cases, the error can be determined by subtracting the values of the positive and negative polarity phases of the detected field. A corrected field estimate can be produced by calculating the average between the positive and negative polarity phases detected. After the notation of Figures 3A and 3B, the error is given by e = (Ap-An) / 2. The corrected field estimate is given by A = Ap-e or A = An + e, or directly by A = (Ap + An) / 2. (All calculations assume that Ap and An are positive numbers, or that they represent the absolute values of the field resistances detected). The position tracking method described in Figure 4 below uses these measurements, taken during the two polarity phases of the quasi-CD field, to compensate for polarity drift and errors due to the Earth's magnetic field. In some embodiments, the opposite polarity measurements of the quasi-CD field can also be used to simplify the calibration of the sensor unit. In some cases, the calibration can be completely eliminated. In addition to the asymmetry effect, the detected field shown in the curve 52 comprises transient fluctuations 54 around the transitions between the positive and negative polarities. Transient fluctuations deviate from the well-defined square wave form of the primary field shown in curve 50. Transient fluctuations 54 are caused, for example, by eddy currents or other sources of parasitic fields that are excited by variations in the primary field , rather than the field itself (these parasitic effects are one of the main error contributors in location systems based on CA fields). When the magnetic field is detected in the quasi-CD system described using the sensors 40, transient fluctuations are avoided when the measurement is made after the transient fluctuations decrease and the resistance of the field has stabilized. Under these measurement conditions, the detected field can be safely considered as a CD field. The frequency of the quasi-CD field is also chosen with respect to transient responses such as transient fluctuations 54. As explained above, it is desirable to consider the magnetic field in each polarity phase as a CD field. To do so, each of the positive and negative polarity phases of the quasi-CD field remains constant for a sufficiently long interval, T, to allow parasitic effects such as eddy currents to decrease before detecting the field. T-values of 10 milliseconds or more (corresponding to square wave frequencies of 50 Hz or less) are typically considered sufficient for quasi-CD operation, although other ranges may also be used. Another factor that affects the choice of quasi-CD field frequency is the desired measurement recovery rate (ie, the number of position measurements per unit time). The recovery rate is typically determined based on the expected dynamics of the sensor unit and the desired measurement accuracy and resolution.

Position detection method Figure 4 is a flow chart schematically illustrating a method for tracking the position, in accordance with a mode of the present invention. The description of the method below considers a single location pad 32 and a single sensor unit 34 for simplicity purposes. The generalized case of a system comprising several locating pads and several sensor units is described below. The method starts with the position tracking system that generates a quasi-CD magnetic field, in a field generation step 60. The console 36 generates a quasi-CD pulse signal which is used to drive the location pad 32, to generate a quasi-CD magnetic field in the entire work volume. The quasi-CD field generated by the location pad is detected by position sensors 40 of the sensor unit 34, in a field detection step 62. The control circuit 42 detects the voltages or currents corresponding to the polarity phases. positive and negative of the detected fields (the detected voltages or currents correspond to field resistances Ap and An in curve 52 of figure 3B above). The control circuit produces field measurement signals, corresponding to the measured values of Ap and An and produces a corrected field estimate, in an external calculation step 64. In one embodiment, the control unit then produces indicative position signals of the corrected field estimate and sends the position signals to the computer 37, as described above. In one embodiment, the control circuit comprises a filter that calculates the corrected field elimation using the relation A = (Ap + An) / 2 given above. In an alternative embodiment, the field measurement signals indicative of the values of Ap and An are sent by the control circuit to the computer 37, and the computation of the corrected field estimate and the position signals is performed by the computer. Alternatively, any other suitable method can be used to calculate the corrected field estimate using the measured values of Ap and An. Such methods may comprise either software or hardware implementations. The corrected field estimate "is then used by the computer 37 to calculate the position coordinates of the sensor unit 34. In many practical cases, the system 20 comprises several location pads 32. In said embodiments, each location pad 32 it generates its quasi-CD field separately, while the other pads of location do not generate any magnetic field, any allocation of time division multiplexing (TDM) between different pads can be used to satisfy this condition. , it is desirable that the positive and negative polarity phases generated by a given location pad can be temporarily adjacent to each other Adjacency ensures that the primary field is similar in both phases, and that the polarization drift remains approximately constant. In one embodiment, steps 60-64 are repeated for each location pad ion 32, according to a predetermined sequence (TDM). The computer 37 receives multiple position signals from the sensor unit in response to the detected field of each location pad. The computer uses the position signals to calculate the position coordinates of the sensor unit using position calculation methods known in the art. The methods described above can be used without change in the systems comprising multiple sensor units 34, since each sensor unit performs its measurements independently of other sensor units. Although the methods and systems described here are mainly directed to the use of quasi-CD magnetic fields in medical position tracking systems, the principles of the present invention can also be used in non-medical position tracking systems, as well as in other applications. Therefore, it will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been shown and described in particular above. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described above, as well as variations and modifications thereof that would occur to those skilled in the art upon reading the above description and which are not described. in the prior art.

Claims (9)

NOVELTY OF THE INVENTION CLAIMS

1. - A method for tracking a position of a sensor, comprising: generating a periodic magnetic field in a neighborhood of the sensor, the field having a positive polarity phase and a negative polarity phase with respective negative and positive constant amplitudes; producing first and second field measurement signals that respond to the magnetic field in the sensor during the positive and negative polarity phases, respectively; and determining the position of the sensor that responds to the first and second field measurement signals. 2 - The method according to claim 1, further characterized in that the sensor is implanted in the body of a patient. 3. The method according to claim 1, further characterized in that the sensor is coupled to a medical instrument that is used to treat a patient. 4. The method according to claim 1, further characterized in that each of the phases of positive and negative polarity is constant for a duration of at least 10 milliseconds. 5. The method according to claim 1, further characterized in that the positive amplitude is equal to the negative amplitude. 6. The method according to claim 1, further characterized in that the determination of the position of the sensor comprises performing an arithmetic operation on the first and second field measurement signals. 7. The method according to claim 6, further characterized in that the performance of the arithmetic operation comprises adding the first and second field measurement signals to produce a position signal. 8. The method according to claim 1, further characterized in that the first and second field measurement signals comprise transient intervals, and wherein the production of the first and second field measurement signals comprises measuring the signals outside the transient intervals. 9. The method according to claim 1, further characterized in that the generation of the periodic magnetic field comprises multiplexing two or more periodic magnetic fields generated in two or more different respective places. 10. A method for tracking a position of a position transducer, comprising: operating the position transducer to generate a periodic magnetic field having a positive polarity phase and a negative polarity phase with respective constant positive and negative amplitudes; detecting the magnetic field at a known location to produce, in response to the detected magnetic field, first and second field measurement signals during the positive and negative polarity phases, respectively; and determining the position of the position transducer in response to the first and second field measurement signals. 11. An apparatus for tracking a position of an object, comprising: at least one locating pad, which is arranged to generate a periodic magnetic field in a neighborhood of the sensor, the field having a positive polarity phase and a phase of negative polarity with respective constant positive and negative amplitudes; a position sensor, which is coupled to the object and is arranged to produce first and second field measurement signals responsive to the magnetic field during the phases of positive and negative polarity, respectively; and a processor, which is arranged to determine the position of the sensor in response to the first and second field measurement signals. 1

2. The apparatus according to claim 11, further characterized in that the sensor is implanted in the body of a patient. 1

3. The apparatus according to claim 11, further characterized in that the sensor is coupled to a medical instrument that is used to treat a patient. 1

4. The apparatus according to claim 11, further characterized in that each of the phases of positive and negative polarity is constant for a duration of at least 10 milliseconds. 1

5. The apparatus according to claim 11, further characterized in that the positive amplitude is equal to the negative amplitude. 1

6. The apparatus according to claim 11, further characterized in that the processor is adapted to determine the position of the sensor when prming an arithmetic operation on the first and second field measurement signals. 1

7. The apparatus according to claim 16, further characterized in that the arithmetic operation comprises adding the first and second field measurement signals to produce a position signal. 1

8. The apparatus according to claim 11, further characterized in that the first and second field measurement signals comprise transient intervals, and wherein the processor is adapted to measure the field measurement signals outside the transient intervals. 1

9. The apparatus according to claim 11, further characterized in that at least one location pad comprises a plurality of locating pads in two or more different respective places, which are multiplexed to generate two or more periodic magnetic fields. 20. An apparatus for tracking a position of an object, comprising: a field generator, which is coupled to the object and is arranged to generate a periodic magnetic field having a phase of positive polarity and a phase of negative polarity with amplitudes positive and negative respective constants; a location pad, which is arranged to produce first and second field measurement signals responsive to the magnetic field during the phases of positive and negative polarity, respectively; and a processor, which is arranged to determine the position of the sensor that responds to the first and second field measurement signals.