Abstract:
A system and method for tracking an object through a three dimensional space is provided that uses the generation and detection of various magnetic fields to provide three-dimensional location data. The integrity of the generated magnetic fields are monitored against a baseline in order to detect compromise by the unintentional introduction of a foreign metallic or magnetic object in the procedural space.

Description:
RELATED APPLICATIONS 
     The present application is related to and claims priority from provisional patent application Ser. No. 61/042,191, entitled “Magnetic Interference Detection System and Method” filed Apr. 3, 2008, the entirety of which is incorporated by reference herein; and is also related to and also claims priority from provisional patent application Ser. No. 61/042,578, entitled “Magnetic Interference Detection System and Method” filed Apr. 4, 2008, the entirety of which is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     Identifying and treating lung tissue abnormalities presents challenges that are somewhat unique to the lungs. If a tissue lesion or tumor is to be identified and excised surgically, the chest wall must be opened to provide access to the lungs. Opening the chest wall is a common procedure but one that presents risks of infection and lengthy recovery time, nonetheless. 
     A desirable alternative to surgery, in terms of reducing patient trauma, is to identify and excise the tumor endoscopically. Endoscopic surgery in the lungs, however, means that the complicated bronchial maze must be navigated. In order to assist in navigating the lungs, systems, such as that described in U.S. Pat. No. 7,233,820 to Gilboa, have been developed that include a sensor at the end of an endoscope. 
     The sensor is able to detect a plurality of magnetic fields generated by a location board, a flat mat on which the patient lies during the procedure. The magnetic fields collectively create an operable space known as a sensing volume. Each of the fields is oriented differently, such that three-dimensional coordinates of a sensor in the sensing volume can be determined and displayed. In order to overcome static interference such as the operating bed conducting parts, a mapping phase is performed when the system is installed. This mapping phase measures the actual shape of the magnetic field and generates correction or compensation parameters that enable the system to calculate the location of the sensor accurately taking into account the distorted magnetic field due to the static conducting parts. 
     Understandably, in order to provide useful, accurate data from within a body cavity, the sensor or sensors at the tip of the endoscope are very sensitive. Hence, if the magnetic field in the sensing volume is altered at all from the original field that was mapped during installation, such as by introduction of additional conducting objects into the sensing volume or changes to the previously mapped bed configuration the sensor will give data that is correspondingly altered. The result will be a sensor location indication that does not reflect the true location of the sensor. To the physician performing the procedure, it may not be evident that the magnetic field is being interfered with or that the sensor location indication is inaccurate. 
     It is evident that there is a need for a system and method of alerting a physician or other user of an endoscopic navigation system, such as that described above, that the magnetic field or data received has been compromised. 
     SUMMARY OF THE INVENTION 
     The system and method of the present invention detects magnetic interference within a location field of an endoscopic navigation system. An alert is generated warning the user of the system that the magnetic field has been distorted and that the location data being generated by the sensor may be unreliable. 
     The location board of an endoscopic navigation of the type addressed by the present invention has three coils. Each of the coils is shaped differently to provide location information along a separate axis. In order to prevent interference among the coils, each field is transmitted on a different frequency. Though each coil is supplied with electricity in order to generate a magnetic field, each coil necessarily has the capability of creating an electrical signal if subjected to a dynamic electrical field through the property of inductance. 
     The present invention uses a variety of techniques to use the location system in a way similar to a metal detector to monitor changes in the “magnetic environment”. If a metal, or other ferrous object, is introduced into the sensing volume and somehow influences the magnetic field, the location system detects that the magnetic field has been altered or compromised and creates a warning condition that alerts the user that the information being provided may be unreliable. 
     One technique used by the present invention is to monitor the self-inductance and mutual-inductance changes in the coils of the location board. Self-inductance of a coil will change when the ferromagnetic material in the coil&#39;s core or magnetic field changes. Changes in mutual-inductance between the three location board magnetic field generating coils will also occur because the location board&#39;s three coils are superimposed. The magnetic field generated by any of the individual coils will necessarily result in an induced voltage in the other two coils. If a ferromagnetic object or any object that influences magnetic fields enters the sensing volume and changes the magnetic field, the change in the magnetic field will result in a change in the induced voltage. Hence, when the location board is being set up for use on a patient, an initial step of recording a baseline self and/or mutual inductance will provide a standard off of which an alarm condition may be defined. Thus, if the inductance varies from the baseline above a threshold value, a warning will be displayed that the system is potentially inaccurate. The signals used for measuring the inductance will be the same ones used for generating the magnetic field, one or more of the three frequencies (2.5, 3 &amp; 3.5 Khz). 
     Another technique used by the present invention is to inject a signal into one or more of the sensor coils of the normally-passive locatable guide or patient sensor. This technique will be used to detect small changes in the magnetic field at the sensor vicinity, changes that the previous technique will not be able to detect. The self-inductance or mutual-inductance will be monitored the same way as in the location board, with two differences. The first difference is that the baseline inductances will be measured immediately after the sensor is manufactured and kept in an EEPROM that is part of the locatable guide and patient sensor. The second difference is that frequency of the signal used for the inductance measurement will be different than the one used by the system localization function. 
     The three coils of the location board are each creating magnetic fields on independent frequencies (e.g. 2.5 kHz, 3.0 kHz, and 3.5 kHz). Utilizing three different frequencies allows the sensors to distinguish between the three different fields and process the data accordingly. Injecting a signal into the sensor coils on a fourth frequency (e.g. 4.0 kHz) would create a small magnetic field that would cause an induced voltage in the other coils having a corresponding frequency. The system could monitor this induced voltage for changes above a threshold level, indicated some sort of magnetic interference. The electronic system reading the sensor signals is designed to prevent the 4 Khz signals from interrupting the localization signals (2, 3 &amp; 3.5 Khz) by means of analog filtering, digital filtering and, if needed, time domain differentiation (the 4 khz signals are generated at a low duty cycle at specific intervals when the localization signals are not measured). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an embodiment of a localization system of the present invention; 
         FIG. 2  is a diagram of an embodiment of a location board circuit of the present invention; 
         FIG. 3  is a diagram of an embodiment of a location board circuit of the present invention; 
         FIG. 4  is a diagram of an embodiment of a locatable guide of the present invention; 
         FIG. 5 a    is a cutaway perspective view of an embodiment of a locatable guide of the present invention; 
         FIG. 5 b    is a circuit diagram of the locatable guide of  FIG. 5   a;    
         FIG. 6  is an expanded view of an embodiment of loop antennas of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures and first to  FIG. 1 , there is shown a location system  10  of the present invention. The location system  10  generally includes a locatable guide  20 , a location board  40 , a plurality of patient sensors  70 , and a processing system  80 . 
     The locatable guide  20 , as shown in  FIGS. 4 and 5 , is a probe having a receiver that generally includes a plurality of (preferably three) field component sensors  22 ,  24  and  26 . Each of the field sensor components is arranged for sensing a different component of an electromagnetic field generated by the location board  40 . 
     In one embodiment, shown in  FIGS. 3 and 4 , each field component sensor  22 ,  24  and  26  includes two sensor elements,  22   a ,  22   b ,  24   a ,  24   b ,  26   a , and  26   b , respectively. Typically, the sensor elements are coils of wire, and the sensed components are independent magnetic field components. The coils may be formed by wrapping wire around a core. The core may then be removed to form an air core at the center of the coil or may be left in place, forming a solid core coil. Preferably, the solid core coils are made of a material such as ferrite or another material having similar magnetic properties. 
     Preferably, the sensor elements  22 ,  24  and  26  are arranged in the locatable guide  20  such that the sensor elements  22   a  and  22   b  are on opposite sides of, and equidistant from, a common reference point  28 . Similarly, sensor elements  24   a  and  24   b  are on opposite sides of, and equidistant from, point  28 , and sensor elements  26   a  and  26   b  also are on opposite sides of, and equidistant from, point  28 . In the illustrated example, the sensors  22 ,  24  and  26  are disposed collinearly along a longitudinal axis  30  of the locatable guide  20 , but other configurations are possible. 
     For example,  FIG. 5  shows a locatable guide  10  having field sensor components  22 ,  24  and  26 ′. Field sensor components  22  and  24  each have two sensor elements  22   a  and  22   b , and  24   a  and  24   b , respectively. Sensor elements  22   a  and  22   b  are on opposite sides of, and equidistant from, point  28 . Sensor elements  24   a  and  24   b  are on opposite sides of, and equidistant from, point  28 . However, field sensor component  26 ′ consists of a single coil centered on point  28 . 
     Referring again to  FIG. 1 , the location system  10  also includes the location board  40 . The location board  40  is a transmitter of electromagnetic radiation. The location board  40  includes a stack of three substantially planar rectangular loop antennas  42 ,  44  and  46  connected to drive circuitry  48 .  FIG. 6  provides an expanded view of the loop antennas  42 ,  44  and  46  of the location board  40  in an expanded view to show the details of their configurations. 
     Antenna  42  is skewed in a y direction in that the loops on one side of the antenna  42  are closer together than the loops on the opposite side. Hence, antenna  42  creates a magnetic field that is stronger on the side where the loops are close together than it is on the opposite side. By measuring the strength of the current induced by the antenna  42  in the locatable guide  20 , it can be determined where the locatable guide  20  is located in a y direction over the antenna  42 . 
     Antenna  44  is similarly skewed but in an x direction. Hence, the antenna  44  also creates a magnetic field that is stronger on the side where the loops are closer together than it is on the opposite side. By measuring the strength of the current induced by the antenna  44  in the locatable guide  20 , it can be determined where the locatable guide  20  is located in an x direction over the antenna  44 . 
     Antenna  46  is not skewed. Rather, it creates a uniform field that naturally diminishes in strength in a vertical direction when the location board is horizontal. By measuring the strength of the field induced in the locatable guide  20 , it can be determined how far the locatable guide is located above the antenna  46 . 
     In order to distinguish one magnetic field from another, the fields of each antenna  42 ,  44  and  46  are generated using independent frequencies. For example, antenna  42  might be supplied with alternating current oscillating at 2.5 kHz, antenna  44  might be supplied with alternating current oscillating at 3.0 kHz, and antenna  46  might be supplied with alternating current oscillating at 3.5 kHz. Hence, each of the field sensors  22 ,  24 , and  26  of the locatable guide will have three different alternating current signals induced in its coils. A person having ordinary skill in the art will realize that the location board design of the present invention is desirable in that the entire field generating device may be placed under the patient and out of the way. However, the interference detection aspect of the present invention may be practiced with other field generating designs, such as those incorporating field generating elements located in various positions around the patient. 
     Referring to  FIG. 2 , driving circuitry  48  includes appropriate signal generators and amplifiers for driving current in each of the loop antennas  42 ,  44  and  46  at their corresponding frequencies. The electromagnetic waves generated by the location board  40  are received by the locatable guide  20  and converted into electrical signals that are then sent to the processing system  80 , shown diagrammatically in  FIGS. 1 and 3 . 
     The processing system  80  generally includes reception circuitry  82  that has appropriate amplifiers and A/D converters. The reception circuitry  82  and the driving circuitry  48 , which may be considered part of the control system  80 , are controlled by a controller/processor  84  that typically is an appropriately programmed micro-controller and logic circuits. The controller/processor  84  directs the generation of transmitted signals by driving circuitry  48 . 
     One embodiment of the present invention involves detecting the presence of a disturbance in the magnetic fields created by the location board  40  by monitoring the self or mutual inductance of the loop antennas  42 ,  44  and  46 . As stated above, each loop antenna  42 ,  44  and  46  operates on a different frequency in order to allow distinction by the control system  80 . Because each loop antenna  42 ,  44  and  46  is creating an independent magnetic field, AC voltages at the respective frequencies of the loop antennas are also being induced into each loop antenna from the other two and also a self induced voltage created by the loops own current (self-inductance). In other words, if loop antenna  42  is operating a 2.5 kHz, for example, it is creating a magnetic field that oscillates at 2.5 kHz. This oscillating magnetic field then will induce an alternating electrical voltage in loop antennas  44  and  46  and also onto itself that has a frequency of 2.5 kHz. The magnetic fields created by loop antennas  44  and  46  similarly create alternating electrical voltages in the other loop antennas. 
     These induced voltages are proportional to characteristics called self inductance (for the self induced voltage) and mutual inductance (for the voltage induced in one loop due to current flowing in another loop). They are dependant on the geometric configuration and materials of the location board and on the conducting materials in the environment close to the location board. If a conductive object enters the magnetic fields close enough to the location board loops the self and mutual inductances will change, the induced signals in each loop antenna will correspondingly change. Hence, by monitoring the baseline induced voltage in one, two, or all three loop antennas  42 ,  44  and  46 , a change attributable to the presence of a foreign object may be used as an indicator that the data being generated by the system  10  should be considered compromised. 
     The circuit enabling measurement of the induced voltages includes voltage pickup circuits  52 , connected between the location board loops  42 ,  44  and  46  and the system current drivers  48 . They pick up the voltages induced and feed them into an amplifier/converter  54  that sends the signals for processing in the signal processing  84  part of the system. The processing system  80  includes circuitry to control the voltage pickup circuitry as to which of the loop&#39;s signals will be fed into the amplifier  54 . 
     Since the system is installed on a bronchoscopy table which has metal parts and has an effect on the magnetic field and the location board&#39;s self and mutual inductance. Upon system installation a threshold level equivalent to the self and/or mutual inductance is set. Changes of the self and/or mutual inductance which are above the threshold level by a certain level, depending on the noise characteristics will result in an alert. Preferably, this alert will be in the form of an audible tone, a video signal, or both. The alert will signal the physician that the bronchoscopy table configuration has changed from the original installation configuration, or a metal, or other interfering object, has entered the magnetic field or is in close enough proximity thereto to compromise the integrity of the data being generated by the system. The physician then knows to determine the source of the interference and remove it, or proceed with the procedure without giving undue deference to the compromised data. 
     A second embodiment of the present invention involves detecting the presence of a disturbance in the magnetic fields in the vicinity of the sensor (LG or patient sensor) by monitoring the self and/or mutual inductance change of sensors components  22 ,  24  and  26 . Similar to the first embodiment, currents are driven into one of the coils and the voltage induced is measured on all the coils. 
     The sensor section of the localization system does not include mechanisms for driving current into the sensor coils; a circuitry is added that drives this current into the coils—injected signal. The circuitry included an alternating current generator (in frequency F 4  different from F 1 , F 2  &amp; F 3 )  88  and controlled current feeder circuits  86 . The signal processing section  84  controls the current feeders  86  and determine into which of the coils the current is injected. The sensor&#39;s amplifiers and converter circuits  82  that are used for measuring the localization signals (induced from the location board  40 ) are also used to amplify the F 4  induced voltage. 
     In order to prevent interference with the magnetic fields being generated by the location board  40 , the injected signal will preferably have a frequency that is distinct from the operating frequencies of the loop antennas  42 ,  44  and  46 . For example, if the loop antennas  42 ,  44  and  46  are operating at 2.5, 3.0, and 3.5 kHz (F 1 , F 2  &amp; F 3 ), the injected signal could be at 4.0 kHz (F 4 ). 
     Preferably, since the location amplifier converters  82  may get interfered by the strong level of the F 4  signal compared to the localization signals (F 1 , F 2  &amp; F 3 ) induced from the location board  40 , and create a false location measurement, the injected signal will be sent to the locatable guide sensor  20  or patient sensor  70  at very low duty cycles, every few seconds for a few milliseconds each time, during that time the calculation of the location is ignored by the system. A baseline inductance response in each field component sensor  22 ,  24  and  26  will be recorded in a magnetically clean environment during the production of the sensor (locatable guide or patient sensor). Hence, by monitoring the inductance response to the injected signal and comparing it to the baseline induced signal in one, two, or all three field component sensors  22 ,  24  and  26 , a change attributable to the presence of a foreign object may be used as an indicator that the data being generated by the system  10  should be considered compromised and an alert is generated. 
     Preferably, this alert will be in the form of an audible tone, a video signal, or both. The alert will signal the physician that a metal, or other interfering object, has entered the magnetic field or is in close enough proximity to the sensor and thereto to compromise the integrity of the data being generated by the system. The physician then knows to determine the source of the interference and remove it, or proceed with the procedure without giving undue deference to the compromised data. 
     A third embodiment of the present invention involves detecting the presence of a disturbance in the magnetic fields created by the location board  40  by monitoring the inductance generated in the field component sensors  22 ,  24  and  26 , similar to the second embodiment, except that the inductance being monitored occurs in the loop antennas  42 ,  44  and  46  of the location board  40 . As stated above, each loop antenna  42 ,  44  and  46  operates on a different frequency in order to allow distinction by the control system  80 . Because each loop antenna  42 ,  44  and  46  is creating an independent magnetic field, AC currents at the respective frequencies of the loop antennas are also being induced into each of the field component sensors  22 ,  24 , and  26 . For example, if antenna  42  operates at 2.5 kHz, antenna  44  operates at 3.0 kHz, and antenna  46  operates at 3.5 kHz, then each field component sensor  42 ,  44  and  46  has corresponding alternating current signals in all three frequencies being induced and sent to the control system  80 . The field component sensors are passive in that electricity is not being supplied to them, rather, all of the current traveling through them is induced. 
     However, according to the second embodiment of the present invention, a magnetic field can be generated by one or more of the field component sensors  22 ,  24  and  26 , by sending electricity through their coils. If a short, electric signal is sent to one or more of the sensors, the result will be the generation of a magnetic field having a corresponding frequency. This magnetic field will, in turn, induce an electrical signal in the loop antennas  42 ,  44  and  46  of the location board  40 . The induced electrical signal in loop antennas  42 ,  44  and  46  will have the same frequency as the electrical signal sent to the locatable guide  20 . 
     In order to prevent interference with the magnetic fields being generated by the location board  40 , the injected signal will preferably have a frequency that is distinct from the operating frequencies of the loop antennas  42 ,  44  and  46 . For example, if the loop antennas  42 ,  44  and  46  are operating at 2.5, 3.0, and 3.5 kHz, the injected signal could be at 4.0 kHz. 
     Preferably, the injected signal will be sent to the locatable guide  20  every few seconds for a few milliseconds each time. A baseline inductance response in each loop antenna  42 ,  44  and  46  will be recorded in a magnetically clean environment with the location board  40  operating. The magnetic fields of the location board  40  will impact the induced signals corresponding to the injected signal. Hence, if an object enters the magnetic fields and changes them, the induced signals in the loop antennas  42 ,  44  and  46  will correspondingly change. Hence, by monitoring the inductance response to the injected signal and comparing it to the baseline induced signal in one, two, or all three loop antennas  42 ,  44  and  46 , a change attributable to the presence of a foreign object may be used as an indicator that the data being generated by the system  10  should be considered compromised and an alert is generated. 
     Preferably, this alert will be in the form of an audible tone, a video signal, or both. The alert will signal the physician that a metal, or other interfering object, has entered the magnetic field or is in close enough proximity thereto to compromise the integrity of the data being generated by the system. The physician then knows to determine the source of the interference and remove it, or proceed with the procedure without giving undue deference to the compromised data. 
     Although the invention has been described in terms of particular embodiments and applications, one of ordinary skill in the art, in light of this teaching, can generate additional embodiments and modifications without departing from the spirit of or exceeding the scope of the claimed invention. Accordingly, it is to be understood that the drawings and descriptions herein are proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.