Patent Publication Number: US-2010113917-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 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 microcoils 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 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 variable resistor and a magneto resistor coupled to the variable resistor. The variable resistor comprises an electronic device having a gate terminal, a source terminal and a drain terminal and a sensor coil coupled to the electronic device between the gate terminal and the source terminal. The sensor coil is coupled such 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. The voltage drop across the sensor coil when applied between the gate terminal and the source terminal of the electronic device induces a voltage drop between the source terminal and the drain terminal of the electronic device. The voltage drop between the source terminal and the drain terminal of the electronic device indicates the voltage drop across the two terminals of the variable resistor The magneto resistor is coupled to the variable resistor in series, such that a voltage drop is induced in the magneto resistor responsive to the electromagnetic fields applied to the body. The transponder further comprises a control unit coupled to the variable resistor and the magneto resistor. The control unit is configured to generate an output signal indicative of the voltage drop induced at the variable resistor 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 to a signal processing unit positioned outside the body for use in determining the coordinates. 
     In 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 variable resistor, a magneto resistor coupled to the variable resistor and a control unit coupled to the variable resistor and the magneto resistor. The variable resistor comprises an electronic device having a gate terminal, a source terminal and a drain terminal and a sensor coil coupled to the electronic device between the gate terminal and the source terminal. The sensor coil is configured to sense a voltage drop in response to exposure to the electromagnetic fields. The voltage drop across the sensor coil when applied between the gate terminal and the source terminal of the electronic device induces a voltage drop between the source terminal and the drain terminal of the electronic device. The voltage drop between the source terminal and the drain terminal of the electronic device indicates the voltage drop across the two terminals of the variable resistor The magneto resistor is coupled to the variable resistor 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 control unit coupled to the variable resistor and the magneto resistor is configured to generate and transmit an output signal indicative of the voltage drop induced at the variable resistor and the voltage drop induced at the magneto resistor. Further, the signal processing unit coupled to the transponder 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 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 variable resistor, a magneto resistor coupled to the variable resistor and a control unit coupled to the variable resistor and the magneto resistor. The variable resistor comprises a field effect transistor having a gate terminal, a source terminal and a drain terminal and a sensor coil coupled to the field effect transistor between the gate terminal and the source terminal. The sensor coil is configured to sense a voltage drop in response to exposure to the electromagnetic fields. The voltage drop across the sensor coil when applied between the gate terminal and the source terminal of the field effect transistor induces a voltage drop between the source terminal and the drain terminal of the field effect transistor. The voltage drop between the source terminal and the drain terminal of the field effect transistor indicates the voltage drop across the two terminals of the variable resistor The magneto resistor is coupled to the variable resistor 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 control unit coupled to the variable resistor and the magneto resistor is configured to generate and transmit an output signal indicative of the voltage drop induced at the variable resistor and the voltage drop induced at the magneto resistor. Further, the signal processing unit-coupled to the transponder 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 to the object, coupling to the object a transponder comprising a variable resistor and a magneto resistor coupled to the variable resistor, the variable resistor comprising an electronic device and a sensor coil coupled to the electronic device, 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 variable resistor and the magneto resistor, generating an output signal at the transponder indicative of the voltage drop across the variable resistor 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 schematic diagram of the transponder shown at  FIG. 1 ; 
         FIG. 3  shows a block diagram of an intra-operative tracking system, in another embodiment; 
         FIG. 4  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. 5  shows a flow diagram depicting a method of tracking an object, using the tracking system of  FIG. 3 . 
     
    
    
     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 variable resistor  110  and at least one magneto resistor  115  coupled to the variable resistor  110 . The variable resistor  110  comprises an electronic device  120  and a sensor coil  125  coupled to the electronic device  120 . The magneto resistor  115  is coupled to the variable resistor  110  in series such that the axis of the magneto resistor  115  is angled substantially perpendicular to the axis of the sensor coil  125 . One or more electromagnetic fields are applied to the body in a vicinity of the transponder  105 . The application of electromagnetic fields induce a voltage drop in each of the sensor coil  125  and the magneto resistor  115 . 
     A schematic diagram of the transponder  105  is shown at  FIG. 2 . As shown in  FIG. 2 , the electronic device  120  comprises a gate terminal  205 , a source terminal  210  and a drain terminal  215 . In one embodiment, the electronic device  120  may comprise a, filed effect transistor (FET). The field effect transistor  120  generally implies a depletion-mode field-effect transistor (FET) that includes one of a junction FET and a Metal Oxide Semi Conductor FET (MOSFET). The field-effect transistor  120  controls the current between the source terminal  210  and drain terminal  215  by the voltage applied between the gate terminal  205  and the source terminal  210 . In the field effect transistor  120 , a junction between the gate terminal  205  and the source terminal  210  is generally reverse biased for control of the current between the source terminal  210  and the drain terminal  215 . Generally, the field effect transistor  120  is in ON status. The application of a reverse biasing voltage causes the depletion region of that junction to expand, thereby pinching off the channel between source terminal  210  and the drain terminal  215  through which the controlled current travels. An example of the FET  120 , is the 2N5457 manufactured by Fairchild Semicondutor. 
     As shown in  FIG. 2 , the sensor coil  125  is coupled to the electronic device  120  between the gate terminal  205  and the source terminal  210 . Therefore, the voltage drop induced at the sensor coil  125  is applied between the gate terminal  205  and the source terminal  210  of the electronic device  120 . The application of voltage between the gate terminal  205  and the source terminal  210  of the electronic device  120  controls the resistance between the source terminal  210  and the drain terminal  215  of the electronic device  120 . The resistance influences the current flow between the source terminal  210  and the drain terminal  215  of the electronic device  120  thereby directly controlling the voltage drop across the source terminal  210  and the drain terminal  215  of the electronic device  120 . The voltage drop between the source terminal  210  and the drain terminal  215  of the electronic device  120  indicates the voltage drop across the two terminals of the variable resistor  110 . 
     As shown in  FIG. 1 , the transponder  105  further comprises a control unit  130 , coupled to the variable resistor  110  and the magneto resistor  115 , so as to generate an output signal indicative of the voltage drop induced at the variable resistor  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  130  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. Further, a radiofrequency driver is configured to drive the transponder  105  with a sine wave at a selected frequency. This is further explained in conjunction with  FIG. 3 . 
     Accordingly, in one embodiment, as shown in  FIG. 3 , a tracking system  300  for tracking an object (not shown) is provided. The tracking system  300  comprises a radio frequency driver  310 , adapted to transmit a radiofrequency driving current to the object (not shown), a plurality of transmitters  315  adapted to generate electromagnetic fields at different respective frequencies in a vicinity of the object (not shown), a transponder  320  coupled to the object (not shown) and a signal processing unit  325  coupled to the transponder  320 . 
     The transponder  320  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 plurality of transmitters  315  emit the electromagnetic field, in the range of 2-10 kHz. The sensor coil  330  is optimized to receive and transmit high-frequency signals, in the range of 1 MHz. However, the sensor coil  330  is designed for operation in the range of 1-3 kHz, the frequencies at which the transmitters  315  generate the electromagnetic fields. Alternatively, other frequency ranges may be used, as dictated by application requirements. 
     Further, the sensor coil  330  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  330  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  330  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  330 , is the T30AA01 passive telecoil manufactured by the Sonion division of Pulse Engineering. 
     With the movement of the object (not shown), the transponder  320  coupled to the object (not shown) is exposed to varying electromagnetic fields. Changing magnetic fields induce a voltage drop in the sensor coil  330 . The voltage components are proportional to the strengths of the components of the respective magnetic fields produced by the transmitters  315  in a direction parallel to the axis of the sensor coil  330 . The voltage drop developed at the sensor coil  330  is applied between the gate terminal  205  and the source terminal  210  of the FET  340 . The current between the source terminal  210  and the drain terminal  215  of the FET  340  is controlled by the voltage applied between the gate terminal  205  and source terminal  210 , thereby changing the resistance between the source terminal  210  and the drain terminal  215  of the FET  340 . Thus, the variable resistor  345  comprising the sensor coil  330 -and-FET  340  combination is a variable (change-of-magneto) resistor  345 , where the two resistor leads are the drain terminal  215  and the source terminal  210  of the FET  340 . Thus, the FET  340  along with the sensor coil  330  forms a voltage-to-resistance converter. Skilled artisans shall however appreciate that other suitable integrated circuits can be employed in place of FET  340 . 
     The magneto resistor  335  is coupled to the variable resistor  345  in series using one of a single twisted-pair and a coaxial cable. The magneto resistor  335  is sensitive to the electromagnetic field such that the magneto resistor  335  is adapted to sense the electromagnetic field at a direction substantially perpendicular to the axis of the sensor coil  330 . This configuration is aimed at minimizing the field coupling between the sensor coil  330  and the magneto resistor  335 . 
     An example of the magneto resistor  335  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. 
     In order to record a significant change in the resistance of the magneto resistor  335 , it is desired to drive the variable resistor  345  circuit with a current substantially below the limiting current of the FET  340 , so that the FET  340  functions as a voltage-controlled resistor. However, this makes the gain of the FET  340  low. 
     The resistance of the variable resistor  345  and the magneto resistor  335  combination varies with the magnetic field applied to the magneto resistor  335  in addition to the change of the magnetic field applied to the sensor coil  330 . For known time dependence of the magnetic field, the voltage drop across the variable resistor  345  and the voltage drop across the magneto resistor  335  can be distinguished mathematically. For example, when the electromagnetic field is a sinusoidal wave of selected frequency the resistance of the magneto resistor  335  changes sinusoidally and the resistance of the variable resistor  345  changes consinusoidally. Following ohm&#39;s law V=IR, the voltage drop across the variable resistor  345  and the voltage drop across the magneto resistor  335  are directly proportional to the resistance of the variable resistor  345  and the resistance of the magneto resistor  335  respectively. Thus, the variable resistor  345  and the magneto resistor  335  can be configured to act as two sensors with distinguishable signals connected in series across a single pair of leads. 
     For a sinusoidal electromagnetic field, the variation in the resistance of the magneto resistor  335  is in phase with the electromagnetic field. However, the variation in the resistance of the variable resistor  345  is out of phase with the electromagnetic field by approximately ninety degrees. Thus the two signals can be distinguished by the difference in the phases of the respective voltage drops. 
     The control unit  350  coupled to the variable resistor  345  and the magneto resistor  335  comprises suitable circuitry for reading the signals from the variable resistor  345  and the magneto resistor  335 . For example, in one embodiment, the control unit  350  comprises at least one of a balanced bridge and hybrid-circuit electronics to read the signals, in the presence of the signals from the radio frequency driver  310 . Skilled artisans shall however appreciate other suitable circuits and methods for signal processing. 
     Responsive to reading the signals from the variable resistor  345  and the magneto resistor  335 , the control unit  350  generates an output signal indicative of an amplitude of the voltage drop induced at the variable resistor  345 , an amplitude of the voltage drop induced at the magneto resistor  335 , a phase of the voltage drop induced at the variable resistor  345  relative to the phase of the electromagnetic fields and a phase of the voltage drop induced at the magneto resistor  335  relative to a phase of the electromagnetic fields. The signal processing unit  325  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  325  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  325  may comprise one or more microprocessors. 
     Thus, the transponder  320 , 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 magneto resistor  335  shown in  FIG. 3 , in conjunction with one or more transmitters  315 , enables the signal processing unit  325  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 variable resistor  345 . 
     When operating at low frequencies, the sensor coil  330  is less sensitive than the magneto resistor  335 . Thus the magneto resistor  335  can be employed as a first receiver providing five degree of freedom (“5DOF”) location information and consequently the variable resistor  345  can be used as a second receiver employed to track roll when operating at higher frequencies. Accordingly, it is desirable to assign the highest frequencies to the transmitters  315  useful for providing roll determination. For example, the three highest frequencies can be assigned to three transmitters  315  providing relatively uniform fields in the X, Y, and Z directions. 
     The voltage drop at the sensor coil  330  is small and so is the voltage between the gate terminal  205  and the source terminal  210  of the FET  340 . Assuming the conductance (1/resistance) is linear, the change of resistance in the variable resistor  345  is small. Thus, the signal representing the voltage drop at the variable resistor  345  is small, however, sufficient for providing the roll information. Since the position, azimuth, and elevation are determined by the signal from the magneto resistor  335 , the noise in the signal from the variable resistor  345  is present only in determining the roll information. 
     Thus, the magneto resistor  335 , which is comparatively more sensitive than the variable resistor  345  can be used as a five degree of freedom (“5DOF”) electromagnetic tracker sensor. Subsequently, the variable resistor  345  can be employed to provide the sixth degree of freedom or to track roll. 
     In an alternative embodiment, the variable resistor  345  can be employed to provide five degree of freedom (“5DOF”) location information and subsequently the magneto resistor  335  can be employed to provide the roll information. 
     The description above primarily concerns acquiring information by a combination of a variable resistor  345  and a magneto resistor  335 , used in determining 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  320  may comprise more than one set of variable resistors or magneto resistors that 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 variable resistors 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 variable resistors or a plurality of variable resistors can be used along with one or more magneto resistors to form a transponder  320 . Further, each magneto resistor can be connected to a single variable resistor using a single pair of leads. 
     In an alternative embodiment, the transponder  320  can be tracked against a plurality of receivers. Accordingly, the tracking system  300  can comprise a plurality of receivers and the magneto resistor  335  can be selected to be a five degree of freedom (“5DOF”) transmitter. Further, similar to the tracking system  300  described above, the variable resistor  345  can be employed to provide the roll information 
     In yet another alternative embodiment, the transponder  320  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 variable resistor  345 . 
     The tracking system  300  described in various embodiments can be used as a part of a surgical navigation product. For this application, the transponder  320  is adapted to be inserted, together with the object (not shown), into the body of the subject, while one or more transmitters  315  and the RF driver  310  are placed outside the body. 
     In an exemplary embodiment, shown at  FIG. 4 , an object  405  includes an elongate probe, for insertion into the body of a subject  410  positioned on a patient positioning system  412 . A transponder  415  is fixed to the probe so as to enable an externally located signal processing unit  418  to determine the coordinates of a distal end of the probe. Alternatively, the object  405  includes an implant, and the transponder  415  is fixed in the implant so as to enable the signal processing unit  418  to determine the coordinates of the implant within the body. Further, the transponder  415  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  420  sends a radio frequency (RF) signal, having a frequency in the kilohertz range, to drive the transponder  415 . Additionally, a plurality of electromagnetic transmitters  425  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  330  and the magneto resistor  335  of the transponder  415 , which depend on the spatial position and orientation of the sensor coil  330  and the magneto resistor  335  relative to the transmitters  425 . The voltage drop induced at the sensor coil  330  due to varying electromagnetic field is applied between the gate terminal  205  and the source terminal  210  of the FET  340 . The FET  340  converts the sensor coil  330  into a variable resistor  345 . In other words, the FET  340  operates as a variable resistor  345  controlled by the sensor coil  330 . Since the voltage drop induced at the sensor coil  330  is dependent on the varying electromagnetic field, the resistance developed at the FET  340  is sensitive to the rate of change of the electromagnetic field. Further, the resistances developed across the variable resistor  345  and the magneto resistors  335  are directly proportional to the voltage drops induced at the variable resistor  345  and the magneto resistor  335  respectively. 
     The control unit  350  converts the voltages into high-frequency signals, which in the form of the output signal is transmitted by the control unit  350  to the externally-located signal processing unit  418 . The signal processing unit  418  processes the output signal to determine the position and orientation coordinates of the transponder  415  for display and recording. 
     Typically, prior to performing a medical procedure, the image of the subject  410  is captured using an imaging device  430  (such as an X-ray imaging device) and is displayed on a computer monitor. The transponder  415  is visible in the X-ray image, and the position of the transponder  415  in the image is registered with respective location coordinates, as determined by the signal processing unit  418 . During the medical procedure, the movement of the transponder  415  is tracked by the tracking system  435  and is used to update the position of the transponder  415  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  405  during the medical procedure, without the need for repeated X-ray exposures during the medical procedure. 
     In another embodiment shown at  FIG. 5 , a method  500  for tracking an object  405  is provided. The method  500  comprises positioning the radio frequency (RF) driver  420  to transmit an RF driving current to the object  405  step  505 , coupling to the object  405  the transponder  415  comprising the variable resistor  345  and the magneto resistor  335  coupled to the variable resistor  345  step  510 , driving the plurality of transmitters  425  to generate electromagnetic fields at respective frequencies in a vicinity of the object  405  that induce a voltage drop across the variable resistor  345  and the magneto resistor  335  step  515 , generating an output signal at the transponder  415  indicative of the voltage drop across the variable resistor  345  and the voltage drop across the magneto resistor  335  step  520 , transmitting the output signal from the transponder  415  to the signal processing unit  418  step  525  and receiving and processing the output signal at the signal processing unit  418  to determine coordinates of the object  405  step  530 . 
     In some embodiments, the method  500  includes inserting the transponder  415 , together with the object  405 , into a body of a subject  410 , wherein positioning the plurality of the transmitters  425  and the RF driver  420  includes placing the one or more transmitters  425  and the RF driver  420  outside the body. 
     In an exemplary embodiment, to operate the transponder  415 , a subject  410  is placed in a magnetic field generated, for example, by situating under the subject  410  a pad containing a plurality of transmitters  425  for generating a magnetic field. The plurality of transmitters  425  are configured to generate electromagnetic fields at different, respective frequencies. A reference electromagnetic field sensor (not shown) is fixed relative to the subject  410 , for example, taped to the back of the subject  410 , and the object  405  with the transponder  415  coupled thereto, is advanced into the body of the subject  410 . Signals received from the transponder  415  are conveyed to the signal processing unit  418 , which analyzes the signals and then displays the results on a monitor. By this method, the precise location of transponder  415 , 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  410 . In this way, the acquired position and orientation 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  330  and a field effect transistor with a magneto resistor  335  to obtain a transponder  320 . The magneto resistor  335  replaces a second sensor coil  330  typically employed in prior art systems, thereby eliminating the use of the second sensor coil  330 . A major advantage associated with the magneto resistor  335  is its ability to be fabricated as a miniature device. Thus, replacing the second sensor coil  330  with a magneto resistor  335  smaller than the second sensor coil  330  reduces the space needed. 
     Further, the magneto resistor  335  and the variable resistor  345  can share a single pair of leads. This allows for a simplified guide wire fabrication as the number of leads employed in connecting two components is reduced by half. Thus, the combination of the variable resistor  345  and the magneto resistor  335  enables the transponder  320  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.