Patent Publication Number: US-2010113918-A1

Title: System and method for tracking object

Description:
FIELD OF INVENTION 
     The invention generally relates to intrabody tracking systems and more particularly to methods and devices for tracking the position and orientation of an object in the body. 
     BACKGROUND OF THE INVENTION 
     Many surgical, diagnostic, therapeutic and prophylactic medical procedures require the placement of objects such as sensors, treatment units, tubes, catheters, implants and other objects within the body. 
     In many instances, insertion of the object is for a limited time, such as during a surgery or catheterization. In other cases, objects such as feeding tubes or orthopedic implants are inserted for long-term use. A need exists for providing real-time information, for accurately determining the location and orientation of objects within a patient&#39;s body, while minimizing the use of X-ray imaging. 
     It is known in the art to use sensor coils as magnetic field transmitters and as magnetic field receivers. Further, the use of magnetic field sensors in determining the location and orientation of an object inside the patient&#39;s body is well known. Typically, the magnetic field sensor is located at the tip of a guidewire or a catheter and a plurality of leads connect the magnetic field sensor to an outside processing circuitry. The size of the magnetic field sensor located at the tip of the guidewire or the catheter is desired to be small and the number of leads connecting the magnetic field sensor to the outside processing circuitry is desired to be less. 
     Generally, a tracking system adapted for determining the location and orientation of an object, employs at least one magnetic field sensor, the at least one magnetic field sensor comprising a plurality of coils. A first coil provides five degrees of freedom (five location and orientation coordinates) and a second coil provides the sixth degree of freedom at the price of twice as many leads and twice as much space. 
     One of the prior art methods provides a magnetic field sensor using three co-located flux-gate magnetometers. A major disadvantage associated with this method is, the magnetic field sensor becomes bulky and employs a large number of leads thereby consuming more space and resource. 
     A number of other methods suggested in the prior art use three co-located coils and/or two non-coaxial coils (which may be co-located or positioned in Hazeltine configuration). This again is associated with a common disadvantage of using more space and resource. 
     Thus, there also exists a need for reducing the size of the magnetic field sensor used in tracking, as well as the number of leads used in the tracking system. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The above-mentioned shortcomings, disadvantages and problems are addressed herein which will be understood by reading and understanding the following specification. 
     In one embodiment, a position transponder for operation inside the body of a subject is provided. The transponder comprises a sensor coil and a magneto resistor coupled in series to the sensor coil. 
     In another embodiment, a position transponder for operation inside the body of a subject is provided. The transponder comprises a sensor coil, coupled so that a voltage drop is induced in the sensor coil responsive to one or more electromagnetic fields applied to the body in a vicinity of the transponder, a magneto resistor coupled to the sensor coil in series, such that a voltage drop is induced in the magneto resistor responsive to the electromagnetic fields applied to the body and a control unit coupled to the sensor coil and the magneto resistor so as to generate an output signal indicative of the voltage drop induced at the sensor coil and the voltage drop induced at the magneto resistor, such that the output signal is indicative of coordinates of the transponder inside the body. The control unit is further configured to transmit the output signal, so that the output signal is received by a signal processing unit positioned outside the body for use in determining the coordinates. 
     In yet another embodiment, a tracking system for tracking an object is provided. The tracking system comprises a radio frequency driver, adapted to transmit a radiofrequency driving current to the object, a plurality of transmitters adapted to generate electromagnetic fields at different respective frequencies in a vicinity of the object, a transponder coupled to the object and a signal processing unit coupled to the transponder. The transponder comprises a sensor coil, the sensor coil configured to sense a voltage drop in response to exposure to the electromagnetic fields and a magneto resistor coupled to the sensor coil in series, such that the magneto resistor is adapted to sense the electromagnetic field at a direction substantially perpendicular to the axis of the sensor coil and thereby experience a voltage drop. The transponder further comprises a control unit coupled to the sensor coil and the magneto resistor. The control unit is configured to generate and transmit an output signal, the output signal indicative of the voltage drop induced at the sensor coil and the voltage drop induced at the magneto resistor. Further, the signal processing unit is adapted to receive the output signal transmitted by the control unit and responsive thereto to determine the coordinates of the object. 
     In yet another embodiment, a method for tracking an object is provided. The method comprises positioning a radio frequency (RF) driver to transmit an RF driving current at a first frequency, to the object, coupling to the object a transponder comprising a sensor coil and a magneto resistor, driving a plurality of transmitters to generate electromagnetic fields at respective frequencies in a vicinity of the object that induce a voltage drop across the sensor coil and the magneto resistor, generating an output signal at the transponder indicative of the voltage drop across the sensor coil and the voltage drop across the magneto resistor, transmitting the output signal from the transponder and receiving and processing the output signal at a signal processing unit to determine coordinates of the object. 
     Systems and methods of varying scope are described herein. In addition to the aspects and advantages described in this summary, further aspects and advantages will become apparent by reference to the drawings and with reference to the detailed description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a block diagram of a transponder employed in a tracking system, in one embodiment; 
         FIG. 2  shows a block diagram of an intra-operative tracking system using the transponder shown at  FIG. 1 , in another embodiment; 
         FIG. 3  shows a schematic diagram of the intra-operative tracking system of  FIG. 2  used in conjunction with an imaging system, in yet another embodiment; and 
         FIG. 4  shows a flow diagram depicting the method of tracking an object using the tracking system of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments, which, may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the embodiments. The following detailed description is, therefore, not to be taken in a limiting sense. 
     In one embodiment, shown in  FIG. 1 , a position transponder  105  for operation inside the body of a subject is provided. The transponder  105  comprises at least one sensor coil  110  and at least one magneto resistor  115  coupled in series to the sensor coil  110 . One or more electromagnetic fields are applied to the body in a vicinity of the transponder  105 . The application of electromagnetic fields induces a voltage drop in each of the sensor coil  110  and the magneto resistor  115 . The transponder  105  further comprises a control unit  120 , coupled to the sensor coil  110  and the magneto resistor  115  so as to generate an output signal indicative of the voltage drop induced at the sensor coil  110  and the voltage drop induced at the magneto resistor  115 . The output signal is indicative of coordinates of the transponder  105  inside the body. The control unit  120  is further configured to transmit the output signal to a signal processing unit positioned outside the body, such that the output signal is received by the signal processing unit for use in determining the coordinates of the transponder  105 . 
     In practice, the transponder  105  is tracked against a plurality of transmitters. The plurality of transmitters emit at different respective frequencies including a second frequency. Further, a radiofrequency driver is configured to drive the transponder  105  with a sine wave at a first frequency. This is further explained in conjunction with  FIG. 2 . 
     Accordingly, in one embodiment, as shown in  FIG. 2 , a tracking system  200  for tracking, an object (not shown) is provided. The tracking system  200  comprises a radio frequency driver  210 , adapted to transmit a radiofrequency driving current, to the object (not shown) via one or more connecting leads connecting the transponder  105  to an outside circuitry comprising the radio frequency driver  210 , a plurality of transmitters  215  adapted to generate electromagnetic fields at different respective frequencies in a vicinity of the object (not shown), a transponder  220  coupled to the object (not shown) and a signal processing unit  230  coupled to the transponder  220 . 
     The plurality of transmitters  215  generate electromagnetic fields composed of a plurality of differently oriented field components each having a different known frequency in the range of 2-10 kHz. Each of these field components are sensed by each of the sensor coil  222  and the magneto resistor  224  which each produce a signal comprising one or more frequency components having different amplitudes and phases depending on the relative distance and orientation of the particular sensor coil  222  or the magneto resistor  224  from the particular transmitter which transmits a particular frequency. The contributions of each of the transmitters  215  are used to solve a set of field equations, which are dependent upon the field form. Solving these equation sets produces the location and orientation of the transponder  220 . 
     The transponder  220  is typically about 2-5 mm in length and about 2-3 mm in outer diameter, enabling it to fit conveniently inside the object (not shown). The sensor coil  222  is optimized to receive and transmit high-frequency signals, in the range of 1 MHz. However, the sensor coil  222  is designed for operation in the range of 1-3 kHz, the frequencies at which the transmitters  215  generate the electromagnetic fields. Alternatively, other frequency ranges may be used, as dictated by application requirements. 
     The sensor coil  222  in the transponder  220  has an inner diameter, of about 0.5 mm and has approximately 800 turns of about 16 micrometer diameter to provide an overall diameter in the range of 1-1.2 mm. Skilled artisans shall however appreciate that these dimensions may vary over a considerable range and are only representative of a range of dimensions. The effective capture area of the sensor coil  222  is about 400 mm.sup.2. The effective capture area is desired be made as large as feasible, consistent with the overall size requirements. Though the shape of the sensor coil  222  used in one embodiment is cylindrical, other shapes can also be used depending on the geometry of the object (not shown). An example of the sensor coil  222  is the T30AA01 passive telecoil manufactured by the Sonion division of Pulse Engineering. 
     The electromagnetic fields produced by the transmitters  215  induce a voltage drop in the sensor coil  222 . The voltage drop at the sensor coil  222  comprises a component at the second frequency, the frequency of the electromagnetic fields produced by the transmitters  215 . The voltage components are proportional to the strengths of the components of the respective magnetic fields produced by the transmitters  215  in a direction parallel to the axis of the sensor coil  222 . Thus, the amplitudes of the voltages indicate the position and orientation of the sensor coil  222  relative to the fixed transmitters  215 . 
     The magneto resistor  224  is coupled to the sensor coil  222  in series using one of a single twisted-pair and a coaxial cable, such that the magneto resistor  224  is adapted to sense the electromagnetic field at a direction substantially perpendicular to the axis of the sensor coil  222 . This configuration is aimed at minimizing the field coupling between the sensor coil  222  and the magneto resistor  224 . 
     An example of the magneto resistor  224  is an extraordinary magneto resistance (EMR) device. Extraordinary magneto resistance (EMR) devices have been fabricated and characterized at various magnetic fields, operating temperatures, and current excitations. The extraordinary magneto resistance devices are comprised of nonmagnetic high mobility semiconductors and low resistance metallic contacts and shunts. The resistance of the extraordinary magneto resistance device is modulated by magnetic fields due to the Lorentz force steering an electron current between a high resistance semiconductor and a low resistance metallic shunt. 
     The magneto resistor  224  comprises a first portion, where the resistance does not significantly change with the electromagnetic field. Therefore, the voltage drop at the magneto resistor  224  comprises a component at the first frequency, the frequency of the driving currents flowing through the transmitters  215 . 
     On the other hand, the magneto resistor  224  comprises a second portion, where the electrical resistance of the magneto resistor  224  varies responsive to the changing electromagnetic field. Following Ohm&#39;s law, V=IR, the magneto resistor  224  develops a voltage drop that varies with the product of the applied electromagnetic field and the current through the magneto resistor  224 . As the driving current is at the first frequency, with a zero direct current component, and the electromagnetic field is at the second frequency, the voltage drop at the magneto resistor  224  comprises components at the sum of the first frequency and the second frequency and at the difference between the first frequency and the second frequency 
     As the voltage drops induced at the sensor coil  222  and the magneto resistor  224  due to the electromagnetic field are at different frequencies, the two voltage drops can be distinguished when measuring their sum through two connecting leads. 
     The control unit  226  coupled to the sensor coil  222  and the magneto resistor  224  comprises suitable circuitry for reading the signals from the sensor coil  222  and the magneto resistor  224 . For example, in one embodiment, the control unit  226  comprises at least one of a balanced bridge and hybrid-circuit electronics to read the signals, in the presence of the signal from the radio frequency driver  210 . Skilled artisans shall however appreciate other suitable circuits and methods for signal processing. 
     Responsive to reading the signals from the sensor coil  222  and the magneto resistor  224 , the control unit  226  generates an output signal indicative of an amplitude of the voltage drop induced at the sensor coil  222 , an amplitude of the voltage drop induced at the magneto resistor  224  and a phase of the voltage drop relative to a phase of the electromagnetic fields. The signal processing unit  230  is adapted to determine the coordinates and an orientation of the object (not shown), responsive to the amplitude and the phase of the voltage drop indicated by the output signal. 
     Skilled artisans shall however appreciate that both analog and digital embodiments of signal processing are possible. The signal processing unit  230  represents an assemblage of units to perform intended functions. For example, such units may receive information or signals, process information, function as a controller, display information, and/or generate information or signals. Typically the signal processing unit  230  may comprise one or more microprocessors. 
     The transponder  220 , as described above, can be employed to provide all six position and orientation coordinates (X, Y, Z yaw, pitch and roll) of the object (not shown). The single sensor coil  222  shown in  FIG. 2 , in conjunction with one or more transmitters  215 , enables the signal processing unit  230  to generate three dimensions of position and two dimensions of orientation information. The third dimension of orientation (typically rotation of the object (not shown) about its longitudinal axis) can be inferred from the magneto resistor  224 . Although the signal from the magneto resistor  224  is smaller than the signal from the sensor coil  222 , the signal from the magneto resistor  224  is large enough to provide the roll information. 
     The description above primarily concerns with acquiring information by a set of a sensor coil  222  and a magneto resistor  224 , used to determine the position and orientation of a remote object (not shown) such as a medical device or instrument. It is also within the scope of the invention that the transponder  220  may comprise more than one set of sensor coils or magneto resistors that will provide sufficient parameters to determine the configuration of the remote object (not shown), relative to a reference frame. 
     Accordingly, in one embodiment, one or more magneto resistors can be combined with one or more sensor coils to obtain six position and orientation coordinates for the object (not shown). For example, a plurality of magneto resistors can be used along with one or more sensor coils or a plurality of sensor coils can be used along with one or more magneto resistors to form a transponder  220 . Further, each magneto resistor  224  can be connected to a single sensor coil  222  using a single pair of leads 
     In an alternative embodiment, the transponder  220  can be tracked against a plurality of receivers. Accordingly, the tracking system  200  can comprise a plurality of receivers and the sensor coil  222  can be selected to be a five degree of freedom (“5DOF”) sensor. Further, similar to the tracking system  200  described above, the magneto resistor  224  can be employed to provide the roll information 
     In yet another alternative embodiment, the transponder  220  can be tracked against an array comprising at least one transmitter and at least one receiver. Further, each receiver can comprise a magnetic field sensor such as but not limited to a magneto resistor  224 . 
     The tracking system  200  described in various embodiments can be used as a part of a surgical navigation product. For this application, the transponder  220  is adapted to be inserted, together with the object (not shown), into the body of the subject, while one or more transmitters  215  and the RF driver  210  are placed outside the body. 
     In an exemplary embodiment, shown at  FIG. 3 , an object  305  includes an elongate probe, for insertion into the body of a subject  310  positioned on a patient positioning system  312 . A transponder  315  is fixed to the probe so as to enable an externally located signal processing unit  318  to determine the coordinates of a distal end of the probe. Alternatively, the object  305  includes an implant, and the transponder  315  is fixed in the implant so as to enable the signal processing unit  318  to determine the coordinates of the implant within the body. Further, the transponder  315  may be fixed to other types of invasive tools, such as endoscopes, catheters and feeding tubes, as well as to other implantable devices, such as orthopedic implants. 
     An externally-located radio frequency driver  320  sends a radio frequency (RF) signal, having a frequency in the kilohertz range, to drive the transponder  315 . Additionally, a plurality of electromagnetic transmitters  325  positioned in fixed locations outside the body produce electromagnetic fields at different, respective frequencies, typically in the kilohertz range. These fields induce voltage in the sensor coil  222  and the magneto resistor  224  of the transponder  315 , which depend on the spatial position and orientation of the sensor coil  222  and the magneto resistor  224  relative to the transmitters  325 . The control unit  226  converts the voltages into high-frequency signals, which are transmitted by the control unit  226 , in the form of output signal, to the externally-located signal processing unit  318 . The signal processing unit  318  processes the output signal to determine the position and orientation coordinates of the transponder  315 , for display and recording. 
     Typically, prior to performing a medical procedure, the image of the subject  310  is captured using an imaging device  330  (such as an X-ray imaging device) and is displayed on a computer monitor. The transponder  315  is visible in the X-ray image, and the position of the transponder  315  in the image is registered with the respective location coordinates, as determined by the signal processing unit  318 . During the medical procedure, the movement of the transponder  315  is tracked by the tracking system  335  and is used to update the position of the transponder  315  in the image on the computer monitor, using image processing techniques known in the art. The updated image can be used to achieve desired navigation of the object  305  during the medical procedure, without the need for repeated X-ray exposures during the medical procedure. 
     In another embodiment shown at  FIG. 4 , a method  400  for tracking an object  305  is provided. The method  400  comprises positioning a radio frequency (RF) driver  320  to transmit an RF driving current to the object  305  step  405 , coupling to the object  305  a transponder  315  comprising a sensor coil  222  and a magneto resistor  224  step  410 , driving a plurality of transmitters  325  to generate electromagnetic fields at respective frequencies in a vicinity of the object  305  that induce a voltage drop across the sensor coil  222  and the magneto resistor  224  step  415 , generating an output signal at the transponder  315  indicative of the voltage drop across the sensor coil  222  and the voltage drop across the magneto resistor  224  step  420 , transmitting the output signal from the transponder  315  step  425  and receiving and processing the output signal at the signal processing unit  318  to determine coordinates of the object  305  step  430 . 
     In some embodiments, the method  400  includes inserting the transponder  315 , together with the object  305 , into the body of the subject  310 . Further, positioning the plurality of the transmitters  325  and the RF driver  320  includes placing one or more transmitters  325  and the RF driver  320  outside the body. 
     In an exemplary embodiment, to operate the transponder  315 , the subject  310  is placed in a magnetic field generated, for example, by situating under the subject  310  a pad containing the plurality of transmitters  325  for generating the electromagnetic field. The plurality of transmitters  325  generate electromagnetic fields at different, respective frequencies. A reference electromagnetic field sensor (not shown) is fixed relative to the subject  310 , for example, taped to the back of the subject  310 , and the object  305  with the transponder  315  coupled thereto is advanced into the body of the subject  310 . Signals received from the transponder  315  are conveyed to the signal processing unit  318 , which analyzes the signals and then displays the results on a monitor. By this method, the precise location of transponder  315 , relative to the reference sensor (not shown), can be ascertained and visually displayed. Furthermore, the reference sensor (not shown) may be used to correct for breathing motion or other movement in the subject  310 . In this way, the acquired position and orientation of the object  305  may be referenced to an organ structure and not to an absolute outside the reference frame, which is less significant. 
     As described in various embodiments, the invention combines a sensor coil  222  with a magneto resistor  224  to obtain a transponder  220 . The magneto resistor  224  replaces a second sensor coil typically employed in prior art systems, thereby eliminating the use of the second sensor coil. A major advantage associated with the magneto resistor  224  is its ability to be fabricated as a miniature device. Thus, replacing the second sensor coil with a magneto resistor  224  smaller than the second sensor coil reduces the space needed. 
     Further, the magneto resistor  224  and the sensor coil  222  can share a single pair of leads. Thus, using the magneto resistor  224 , allows for a simplified guidewire fabrication as the number of leads employed in connecting two components is reduced by half. Thus, the use of the magneto resistor  224  in the transponder  220  enables the transponder  220  to obtain six degrees of freedom (“6DOF”) without causing much burden on resource or space. 
     In various embodiments, system and method for tracking an object are described. However, the embodiments are not limited and may be implemented in connection with different applications. The application of the invention can be extended to other areas. For example, in cardiac applications such as in catheter or flexible endoscope for tracking the path of travel of the catheter tip, to facilitate laser eye surgery by tracking the eye movements, in evaluating rehabilitation progress by measuring finger movement, to align prostheses during arthroplasty procedures and further to provide a stylus input for a Personal Digital Assistant (PDA). The invention provides a broad concept of tracking an object in obscure environment, which can be adapted to track the position of items other than medical devices in a variety of applications. That is, the tracking system may be used in other settings where the position of an object in an environment is unable to be accurately determined by visual inspection. For example, tracking technology may be used in forensic or security applications. Retail stores may use tracking technology to prevent theft of merchandise. Tracking systems are also often used in virtual reality systems or simulators. Accordingly, the invention is not limited to a medical device. The design can be carried further and implemented in various forms and specifications. 
     This written description uses examples to describe the subject matter herein, including the best mode, and also to enable any person skilled in the art to make and use the subject matter. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.