Abstract:
A surgical navigation system is disclosed herein. The surgical navigation system includes a tracking system, a field generator operatively connected to the tracking system, a field sensor operatively connected to the tracking system, and an electrostatic shield circumscribing the field sensor. The electrostatic shield is adapted minimize capacitive coupling between the field generator and the field sensor.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to Provisional Application No. 60/938,608 filed on May 17, 2007, and is hereby incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The subject matter disclosed herein relates to a surgical navigation system with an electrostatic shield adapted to prevent or reduce capacitive coupling. 
         [0003]    Surgical navigation systems incorporating electromagnetic (EM) tracking technology can be implemented to determine the position and orientation of a medical instrument and to convey this information to a user. The position and orientation information may, for example, be conveyed by virtually superimposing a graphic representation of the distal end of the medical instrument onto a patient image. Accordingly, the user receives visual feedback to help navigate or guide the medical instrument to a target site. 
         [0004]    EM tracking systems generally include a field generator and a field sensor configured to operate in combination in order to obtain the position and orientation information. The field generator and the field sensor each comprise an electrical conductor, and they are separated by atmospheric gasses that can act as an electrical insulator. It is generally well known that two electrical conductors separated by an electrical insulator define a capacitor, and that the field generator and field sensor of the EM tracking system can function as a capacitor. 
         [0005]    “Capacitive coupling” is a term of art referring to the transfer of electricity between the conductors and through the insulator of a capacitor. One problem with EM tracking systems is that the amount of capacitive coupling is difficult to predict, and the algorithms implemented to calculate position and orientation are therefore commonly predicated on a zero capacitive coupling assumption. This assumption is potentially problematic in that capacitive coupling is generally unaccounted for and can lead to imprecision in the tracking system position and orientation estimates. 
       SUMMARY OF THE INVENTION 
       [0006]    The above-mentioned shortcomings, disadvantages and problems are addressed herein which will be understood by reading and understanding the following specification. 
         [0007]    In an embodiment, a surgical navigation system includes a tracking system, a field generator operatively connected to the tracking system, a field sensor operatively connected to the tracking system, and an electrostatic shield circumscribing the field sensor. The electrostatic shield is adapted minimize capacitive coupling between the field generator and the field sensor. 
         [0008]    In another embodiment, a surgical navigation system includes a computer, an imaging device connected to the computer, a display connected to the computer, and a tracking system connected to the computer. The tracking system is adapted to estimate a position and/or orientation of a medical instrument. The surgical navigation system also includes a field generator connected to the tracking system, a field sensor connected to the tracking system, and an electrostatic shield completely surrounding the field sensor. The electrostatic shield is adapted minimize capacitive coupling between the field generator and the field sensor. 
         [0009]    In yet another embodiment, a surgical navigation system includes a computer, an imaging device connected to the computer, a display connected to the computer, and a tracking system connected to the computer. The tracking system is adapted to estimate a position and/or orientation of a medical instrument. The surgical navigation system also includes a field generator connected to the tracking system, a field sensor connected to the tracking system, and a housing assembly completely surrounding the field sensor. The housing assembly includes a structural portion adapted to protect the field sensor, and an electrostatic shield applied as a coating to the structural portion. The electrostatic shield is adapted minimize capacitive coupling between the field generator and the field sensor. 
         [0010]    Various other features, objects, and advantages of the invention will be made apparent to those skilled in the art from the accompanying drawings and detailed description thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic diagram of a navigation system in accordance with an embodiment; and 
           [0012]      FIG. 2  is a detailed isometric illustration of a field sensor in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    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 that 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 as limiting the scope of the invention. 
         [0014]    Referring to  FIG. 1 , a navigation system  10  is shown in accordance with one embodiment. The navigation system  10  includes an electromagnetic (EM) tracking system  12  operatively connected to a plurality of tracking elements  14 ,  16 ; an imaging device  18 ; a computer  20  and a display  22 . 
         [0015]    The tracking element  14  is adapted for attachment to a medical instrument such as, for example, the instrument  24 . The tracking element  16  can be rigidly attached to an internal anatomy (e.g., the heart  26 ) or to the external body of the patient  28  in a known manner. A tracking element secured to directly to a patient may be referred to as a “dynamic reference” because it is adapted to move along with the patient. The present invention will hereinafter be described in accordance with an embodiment wherein the tracking element  16  comprises a field generator  30 , and the tracking element  14  comprises a field sensor  32 . It should, however, be appreciated that according to alternate embodiments the tracking element  16  may include a field sensor and the tracking element  14  may include a field generator. 
         [0016]    The field generator  30  generates a magnetic field  34  in an area that includes the site at which a given procedure is to be performed. The field sensor  32  is adapted to measure the magnetic field  34 , and to transmit the magnetic field measurements to the tracking system  12 . The tracking system  12  implements the magnetic field measurements to calculate the position and orientation of the tracking element  14 . After calculating the position and orientation of the tracking element  14 , the position and orientation of the instrument  24  attached thereto can also be calculated in a known manner. 
         [0017]    The tracking system  12  transmits the medical instrument position and orientation data to the computer  20 . The computer  20  registers the position and orientation data to a patient image  40  obtained from the imaging device  18 . The imaging device  18  may, for example, include a CT imaging device, a MR imaging device, a PET imaging device, an ultrasound imaging device, an X-ray imaging device, or any other known imaging device, as well as any combinations thereof. The medical instrument position and orientation data can be visualized on the display  22 . According to one embodiment, a graphic representation corresponding to the instrument  24  can be virtually superimposed on the patient image  40  in a manner adapted to convey the position and orientation of the instrument  24 . In the embodiment of  FIG. 1 , the graphic representation includes the cross hairs  42  which may, for example, represent the distal end portion of the instrument  24 . Alternate embodiments may include a more complete rendering showing the instrument  24  in detail. 
         [0018]    Referring to  FIG. 2 , a more detailed representation of the field sensor  32  is shown in accordance with an embodiment. For illustrative purposes, the field sensor  32  will hereinafter be described as comprising industry-standard-coil-architecture (ISCA) type coils, however it should be appreciated that alternate coil architectures may be envisioned. The illustrative ISCA coils of the field sensor  32  include the coils  50 ,  52  and  54  that are approximately collocated, approximately orthogonal, and approximately dipole coils. The coils  50 ,  52  and  54  may optionally be wound around a cube shaped bobbin  56  composed of an electrically insulative material such as plastic. 
         [0019]    The coils  50 ,  52  and  54  may be disposed within a housing assembly  58 . The housing assembly  58  comprises an electrically insulative structural portion  60  and an electrically conductive electrostatic shield  62 . The structural portion  60  is adapted to protect the coils  50 ,  52  and  54 . The electrostatic shield  62  is adapted to prevent capacitive coupling between the field generator  30  (shown in  FIG. 1 ) and the field sensor  32 , and to thereby improve the precision of the tracking system  12  (shown in  FIG. 1 ). The electrostatic shield  62  may be grounded so that it conducts electricity in a manner that does not excessively accumulate charge. The field sensor  32  is shown disposed within the electrostatic shield  62  such that the electrostatic shield  62  completely surrounds the field sensor  32 . It should, however, be appreciated that according to alternate embodiments, the electrostatic shield  62  may circumscribe only a discrete portion of the field sensor  32 . 
         [0020]    The electrostatic shield  62  may comprise any material that is conductive enough to prevent capacitive coupling and that is resistive enough to avoid the formation of eddy currents. Eddy currents can be formed by changes in the magnitude or direction of the magnetic field  34  (shown in  FIG. 1 ) and an intersecting conductor such as the electrostatic shield. Changes in the magnitude of the magnetic field occur in the normal operation of the tracking system, and induce the voltages measured in the receiver coils. As eddy currents are a potential source of tracking system imprecision, it is important to avoid their formation by selecting a sufficiently resistive electrostatic shield material. It has been observed that materials exhibiting a resistance of approximately several ohms as measured at a distance of approximately one centimeter apart are both conductive enough to prevent capacitive coupling and resistive enough to avoid the formation of eddy currents. 
         [0021]    According to one embodiment, the electrostatic shield  62  is applied as a coating to the internal surface of the structural portion  60 . In this manner, the electrostatic shield  62  can effectively eliminate capacitive coupling by preventing the transmission of electricity between the field generator  30  (shown in  FIG. 1 ) and the field sensor  32 , and the electrostatic shield  62  is also protected by the structural portion  60 . Alternatively, the electrostatic shield  62  may be applied as a coating to the external surface of the structural portion  60 . The electrostatic shield  62  may, for example, be applied as a coating to the structural portion  60  by spraying an electrically conductive paint onto the structural portion  60 , or by vacuum depositing an electrically conductive coating onto the structural portion  60 . A non-limiting list of potentially appropriate electrostatic shield coating materials includes nickel loaded paint, silver loaded paint, carbon loaded paint, and carbon loaded plastic. 
         [0022]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention 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.