Abstract:
A marker system and method are provided that can determine a shape and an axial location of a rotationally symmetrical hole in an instrument having a plurality of instrument markers.

Description:
RELATED APPLICATION DATA  
       [0001]    This application claims priority of U.S. Provisional Application No. 60/882,772 filed on Dec. 29, 2006, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]    The present invention relates generally to medical navigation and, more particularly, to a marker system for determining a diameter and axial location of a bore or hole in an object. 
       BACKGROUND OF THE INVENTION  
       [0003]    Marker means are currently detected by means of a detection means (e.g., a camera or ultrasound detector). The marker means typically comprise three markers arranged in a fixed and predetermined location relative to each other and can be mechanically connected. The markers can be passive or active markers, wherein passive markers reflect signals (e.g., waves and/or radiation) emitted in their direction, and active markers are themselves the source of the signals (e.g., waves and/or radiation). The signals emitted from the (active or passive) markers, which can be wave signals or radiation signals, are detected by the detection means. In order to establish a position of the marker means relative to the detection means, the marker means is preferably moved to provide the detection means with different views of the marker means. On the basis of this, the location of the marker means relative to the detection means then can be determined in a known way, in particular in a spatial reference system. Reference is made in this respect to DE 196 39 615 A1 and the corresponding U.S. Pat. No. 6,351,659, each of which are hereby incorporated by reference in their entirety. 
       SUMMARY OF THE INVENTION  
       [0004]    A marker system is provided that enables a diameter and axial location of a rotationally symmetrical hole or bore in an object (referred to hereinafter as an instrument) to be determined. The term “rotationally symmetrical” as used herein relates to the axis of the hole or bore, i.e., the surface area which surrounds and delineates the hole has a shape which is rotationally symmetrical with respect to the axis in the longitudinal direction (insertion direction into the hole). Examples of rotationally symmetrical holes include cylindrical holes (bores or transit bores), but also can include, for example, conical holes or holes that are configured to be partially spherical (e.g., hemispheres). The term “hole” here includes both transit holes, which penetrate through the instrument, and a recess in the instrument. The marker system can include a body to which markers are attached (referred to as “body markers” below). The body markers can be situated in a predetermined arrangement relative to each other and can exhibit a prior-known size and shape (e.g., a spherical shape). The body thus can comprise a marker means. Preferably, the instrument also comprises a marker means, as will be discussed in more detail below. In general, the following applies to marker means. 
         [0005]    The location of the marker means is preferably determined by the position of the marker means in a predetermined reference system. The reference system can be a reference system in which the detection means lies. The location of the marker means can be determined by the positions of the markers, in particular center points of the markers, in the reference system. The positions, for example, can be described using Cartesian coordinates or spherical coordinates. The location of one part (e.g., the detection means or marker means) relative to another part (e.g., the marker means) can be described by spatial angles and/or distances and/or coordinates (in a reference system) and/or vectors and is preferably calculated from the positions describing the location, for example by means of a program running on a computer. 
         [0006]    The term “relative location” used here or the expression “location of a part A relative to a part B” thus comprises the concept of the relative positions between the two parts, in particular between the marker means and/or their markers or between a marker means (or its markers) and the detection means. In particular, centers of gravity or center points of the parts can be selected as a punctiform reference point for establishing a position. If the position of one part is known in a reference system, then it is possible, based on the relative location of two parts, to calculate the position of one of the two parts from the position of the other of the two parts. 
         [0007]    If the marker means comprises only two markers, a start position is preferably known, and the marker system then allows the location of the marker means to be tracked when the marker means is spatially moved. 
         [0008]    The marker means in accordance with the present invention preferably comprises at least two markers, and more preferably three markers. The dimensions of the markers and the location of the markers relative to each other are known and may be available as prior-known data of a data processing means. The shape of the markers is preferably also known. 
         [0009]    The marker system preferably also comprises a detection means that detects signals from the at least two markers. As stated above, these signals may be emitted from the markers (either actively emitted by the markers or reflected by the markers). In the latter case, a signal transmitting source, for example an infrared light source, is preferably provided that emits signals (e.g., ultrasound pulses or infrared light) toward the passive markers, wherein the passive markers reflect the signals. A data processing means, such as a computer, allows the location of the marker means relative to the detection means to be calculated, in particular the location of the marker means in a reference system in which the detection means lies, e.g., in a reference system which lies in an operating theater. 
         [0010]    If the body has then been provided with a marker means and the instrument to be examined has also been provided with a marker means, then it is possible to determine the relative location of the body relative to the instrument. The location of the axis of the hole relative to the marker means attached to the instrument, and the shape of the hole, are however still unknown. 
         [0011]    In order to determine the location of the axis, body markers can be provided on the body as well as rotationally symmetrical extensions. These extensions, for example, can have a cylindrical configuration or a conical configuration (including a truncated cone). 
         [0012]    An operator who wishes to measure a characteristic (e.g., a diameter) of a hole in an (arbitrary) instrument uses an instrument that is provided with a marker means. The markers of the marker means are referred to here as “instrument markers”, in order to distinguish them from the body markers. The operator searches out the extension that has a diameter that is substantially the same as the diameter of the hole and rotates the instrument around this hole while the signals emitted by the instrument markers are detected by the detection means. As used herein, substantially the same diameter refers to diameters that are nearly identical (e.g., an exact fit that permits movement of the hole relative to the extension, with little to no interference). Due to the embodiment of the marker system in accordance with the invention, this approach allows both the shape of the hole (e.g., the diameter of a cylindrical hole) and the location of the axis of the hole to be determined. 
         [0013]    The detection means in accordance with the invention is embodied to detect signals emitted both by the body markers and by the instrument markers. This allows the data processing means to determine the location of the body markers relative to the instrument markers. In order to make this and/or other determinations, the detection means preferably converts the detected signals into data signals and outputs them to the data processing means. 
         [0014]    The data processing means is preferably designed to perform the functions (described in more detail below) of receiving, storing, calculating and determining. The data processing means receives the data signals output by the detection means. In other words, the data processing means receives data signals that represent the location of the body markers relative to the detection means, and data signals that represent the location of the instrument markers relative to the detection means. As already stated above, the instrument is preferably rotated by an operator around an extension that has been fit into the hole in the instrument. A number of locations of the instrument markers relative to the detection means thus arise during this rotational movement. Preferably, at least three locations of the instrument markers relative to the detection means are provided in accordance with the invention. The data processing means can be designed to receive and further process at least three different locations of the instrument markers relative to the detection means. When further processing the different relative locations of the instrument markers (relative to the detection means), it is then preferably assumed that these relative locations lie on a circular trajectory. 
         [0015]    An approximating method (fitting method such as for example a least-square fit) is preferably performed to calculate a circular trajectory from the three relative locations, on which the three relative locations at least approximately lie. The data processing means thus has the function of calculating a circular trajectory and the assumption that the at least three different relative locations lie on a circular trajectory. In order to mathematically describe a circular trajectory, the location of the center point and the radius of the circular trajectory, and in particular trigonometric functions (e.g., sine, cosine), are preferably used. Calculating the circular trajectory thus also calculates the location of the center point of the circular trajectory and determines the center point relative to the detection means. Since the location of the center point of the circular trajectory matches the location of the axis of the hole in the instrument, the location of the axis of the hole relative to the detection means is thus also determined. 
         [0016]    However, the aforementioned method step of fitting an extension into an instrument hole and then rotating the instrument around said extension not only allows the relative location of the axis of the hole to be determined, but also allows the shape of the hole to be determined. The shape of the hole is preferably described by one or more characteristics. In the case of a cylindrical hole and thus a cylindrical extension, this at least one characteristic, for example, can be the diameter or radius of the hole, and additionally the height of the cylindrical extension. It is, however, also possible to provide rotationally symmetrical extensions of different shapes on the body, e.g., cylindrically shaped, conically shaped or spherically shaped extensions. In this case, the at least one characteristic preferably also includes a characteristic that indicates the configuration, e.g., cylindrical, conical or spherical. If, for example, there are only conically configured extensions comprising a conical tip (i.e., not truncated cones), then the at least one characteristic preferably includes the aperture angle of the cone. In the case of a truncated cone, the height of the truncated cone and/or the radii or diameters of the faces enclosing the truncated cone, for example, may be other characteristics. 
         [0017]    In accordance with the invention, the data processing means is designed to determine the at least one characteristic. To this end, the relative locations between the body markers and the axes of the rotationally symmetrical extensions are preferably known and stored in the data processing means. The aforesaid characteristics also can be stored in association with the aforesaid relative locations of the axes relative to the body markers. In particular, at least one characteristic is stored in association with each individual axis. 
         [0018]    It is thus possible to calculate, from the data signals, which of the axes match the calculated center point. It is in particular possible to calculate the relative location of the center point and, therefore, of the axis relative to the detection means. From the detected location of the body markers and the stored data concerning the location of the body markers relative to the axes, it is then possible to determine which of the axes matches the center point. 
         [0019]    Detecting the instrument markers thus allows the location of the center point of the circular trajectory relative to the detection means to be determined. Detecting the body markers allows the location of the axes relative to the detection means to be determined. Thus, it is possible to determine which of the respective determined locations of the axes relative to the detection means matches the determined location of the center point relative to the detection means. Further, it is possible to determine which axis matches the center point, so as to identify the axis. 
         [0020]    If the axis has then been identified, it is then possible to determine, from the stored association between the axis and the characteristics, the characteristic that is associated with the identified axis. The aforesaid method step thus enables both the location of the axis relative to the detection means and therefore also relative to the instrument markers to be determined. In addition, it is simultaneously possible to determine the characteristic of the hole in the instrument. The instrument is thus registered and can be used in any way. The determined location of the axis of the hole relative to the instrument markers, and the at least one characteristic which characterizes the shape of the hole can be stored for a later application of the instrument. 
         [0021]    As already stated above, the arrangement of the body markers relative to each other may be known and preferably stored. This arrangement differs from the arrangement (and as applicable the shape) of the instrument markers, which also may be stored. The markers are preferably all spherical. Due to the characteristic arrangement of the body markers and the characteristic arrangement of the instrument markers, it is possible to distinguish the body markers from the instrument markers. When processing the data signals, the data processing means can thus recognize which data signals are to be associated with the body markers and which are to be associated with the instrument markers. 
         [0022]    The distance between two extensions is preferably larger than the diameter of one of the extensions, in particular larger than the maximum diameter of the extensions. A preferred range is around 100 to 300% of the diameter of the extension. 
         [0023]    The surface from which the extensions protrude is preferably convexly curved or planar. This enables the instrument to be placed directly onto the surface and simultaneously to not obstruct the instrument from being rotated around the extension when the extension has been inserted into the hole in the instrument. A planar embodiment of the surface is preferred, since it is possible to laterally guide the instrument during the rotational movement around the axis of the extension. The intersection points between the axis of the extensions and the aforesaid surface preferably represent points that lie on a trajectory. The trajectory, for example, can be a straight line or a curved line, in particular a closed line (for example a circle or ellipse). This arrangement means that rotating the instrument around one of the axes of the extensions is not obstructed by other extensions. 
         [0024]    Alternatively or additionally, the extensions also can be arranged one above the other, such that the axes of the extensions transition into each other, wherein the diameter of the extensions decreases as the distance from the aforesaid surface increases. The diameter preferably decreases in steps (see  FIG. 4 ). 
         [0025]    The body on which the extensions are provided also can be designed in a number of parts. One part of the body, which is referred to here as the “extension body” and on which the extensions are provided, can in particular be separate from another part of the body which is referred to here as the “marker unit”. A marker means can be provided on the marker unit. In this way, differently configured marker means can be connected to the extension body, so as to be able to use a marker means, which is suitable for the respective application purpose, as the body marker. On the other hand, a number of different extension bodies can of course be connected to a particular marker means. It is then for example possible to select, depending on the shaping of the hole, an extension body comprising cylindrical extensions or one comprising conical extensions, or to select extension bodies having different diameters or characteristics. The connection between the two parts is preferably releasable and, when the parts are connected, spatially fixed, i.e., a relative movement between the marker unit and the extension body is prevented. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0026]    The forgoing and other features of the invention are hereinafter discussed with reference to the drawings. 
           [0027]      FIG. 1  illustrates an exemplary extension body with markers coupled thereto. 
           [0028]      FIG. 2   a  is a front view of the extension body of  FIG. 1 . 
           [0029]      FIG. 2   b  is a lateral view of the extension body of  FIG. 1 . 
           [0030]      FIG. 2   c  is a rear view of the extension body of  FIG. 1 . 
           [0031]      FIG. 3  illustrates an exemplary marker system in accordance with the invention. 
           [0032]      FIG. 4  illustrates an alternative arrangement of the extensions on an extension body. 
       
    
    
     DETAILED DESCRIPTION  
       [0033]    The exemplary extension body  1  shown in  FIG. 1  comprises a number of extensions  8 ,  9 ,  10  and  11 . The extension  8  has an axis A 8 , the extension  9  has an axis A 9 , the extension  10  has an axis A 10  and the extension  11  has an axis A 11 . As shown in  FIG. 1 , the extensions preferably protrude from a level plane  30  of the extension body  1 . In the exemplary extension body of  FIG. 1 , the extensions are designed cylindrically, i.e., they have a constant diameter along their axes. The diameters of the respective extensions preferably differ, and the extensions are preferably arranged in a row. Further, the distance between each extension is preferably at least as large as the diameter of the adjacent extension, and more preferably larger than the diameter of the extension with the largest diameter. The extensions preferably protrude from the plane  30  by the same distance. 
         [0034]    The marker unit  2  comprises three markers  3 ,  4 , and  5  having a preferably spherical configuration, wherein a diameter of each marker is the same. The location of the markers  3 ,  4  and  5  relative to each other is preferably known and in particular characteristic of the marker unit  2 . It is thus possible to recognize, from the characteristic arrangement of the markers  3 ,  4  and  5 , the marker unit  2 . 
         [0035]    In the embodiment shown in  FIG. 1 , the marker unit  2  is designed as a calibrating device, wherein for calibrating purposes, instruments can be placed on planes of the marker unit  2  or inserted into openings to perform a calibration. This secondary function of the marker unit  2 , however, is not further discussed herein. 
         [0036]      FIG. 2   a  shows a front view of the extension body  1 , wherein the extensions  8 ,  9 ,  10  and  11  are shown in a top view as circles of different diameters.  FIG. 2   b  shows a lateral view of the extension body  1 , wherein to the right of the plate  32  that includes the front plane  30 , the extensions  8 ,  9 ,  10  and  11  are arranged in a row from top to bottom. A coupling member  12  is situated on the rear side  34  of the plate  32  and protrudes from the planar rear side  34 . The coupling member  12  is shown in more detail in  FIG. 2   c .  FIG. 2   c  is a rear view of the extension body  1 , in which the coupling member  12  is shown in a top view. It consists of a circular portion  12   b  and an extension  12   a  which protrudes from the circular member  12   b  and blocks rotation. 
         [0037]    As shown in  FIG. 1 , the circular portion  12   b  is inserted into the circular opening  6  of the marker unit  2 , as indicated by the arrow B. In addition, the extension  12   a  is inserted into the complementarily designed recess  7  of the marker unit  2 . The extension  12   a , in cooperation with the recess  7 , thus prevents the extension body  1  from rotating relative to the marker unit  2 . The coupling member  12  is preferably configured in an exact fit to ensure a stable relative location between the extension body  1  and the marker unit  2 . Additionally or alternatively, a catch-like coupling can also be provided. 
         [0038]      FIG. 3  shows the assembly of a marker system in accordance with the invention. Identical reference signs duly designate identical parts, as in the preceding figures. In  FIG. 3 , the extension body  1  is coupled and positionally fixed to the marker unit  2  such that the extension body  1  cannot be moved relative to the marker unit  2 . An exemplary instrument  14  is also shown, which can be measured in conjunction with the marker system. The rod-shaped configuration of the instrument  14  is merely by way of example. Other embodiments of the instrument, such as for example a triangular configuration, are of course possible. The extension  11  penetrates through a cylindrical opening (transit bore) in the instrument  14  in an exact fit, such that the axis of the extension  11  matches the axis of the cylindrical opening in the instrument  14 . A marker means  13  is provided at an end of the instrument  14  opposite the end near which the cylindrical opening is situated. The instruments are preferably configured such that the marker means  13  is situated as far away as possible from the cylindrical opening to be measured to more exactly determine the center point of the circular trajectory  20  on which the marker means  13  is moved. The distance between the marker means  13  and the center point of the rotational movement is preferably a multiple of the diameter of the cylindrical opening, and more preferably more than five times or ten times the diameter. The present invention is applicable to instruments comprising a marker means that are suitably configured in this way. 
         [0039]    The markers  13   a ,  13   b  and  13   c  can be active markers that emit signals (e.g., light or ultrasound). Preferably, however, the markers are passive markers that reflect signals emitted in their direction. An infrared light source, for example, can be provided that continuously or intermittently emits infrared light, which is reflected by the marker spheres  13   a ,  13   b ,  13   c ,  3 ,  4  and  5 . The camera  100 , which is preferably designed as a camera comprising two spatially separate detection elements, receives the signal (e.g., infrared light) emitted by the marker spheres  3 ,  4 ,  5 ,  13   a ,  13   b  and  13   c . The marker spheres  3 ,  4  and  5  and the marker spheres  13   a ,  13   b  and  13   c , respectively, are in a characteristic positional relationship with respect to each other. This characteristic positional relationship is preferably known. For example, the center points of the marker spheres  3 ,  4  and  5  enclose a triangle, the side lengths and angles of which are preferably known. The center points of the marker spheres  13   a ,  13   b  and  13   c  correspondingly enclose a triangle, the side lengths and angles of which are known. If both the marker array comprising the spheres  3 ,  4  and  5  and the marker array comprising the spheres  13   a ,  13   b  and  13   c  have thus been characterized by the side lengths and angles of the respective triangle, then the data processing means  200  can ascertain, from the data signals transmitted from the camera  100 , which marker array the signals originated from. 
         [0040]    In order to determine the position of the axis of the cylindrical opening and the diameter of the cylindrical opening in the instrument  14 , the following procedure can be performed. The instrument  14  can be rotated around the axis of the extension  11  along the circular trajectory  20 , such that at least three positions are detected by the camera  100  and thus also by the data processing means  200 . 
         [0041]    From the data signals emitted from the marker means  13 , the data processing means  200  then calculates the center point of the circular trajectory along which the marker means  13  has moved. The center point of this circular trajectory matches the axis of the extension  11 . The relative location between the marker array comprising the spheres  3 ,  4  and  5  and the individual extensions, in particular the extension  11 , is also known. The location of the extension  11  is thus also known from the detected signals from the marker array comprising the spheres  3 ,  4  and  5  and from the stored relative location between the spheres  3 ,  4  and  5  and the respective extensions, in particular the extension  11 . The data processing means can thus verify whether the center point of the circular trajectory  20  matches the location of the axis of the extension  11 . If this is the case, then the location of the axis of the cylindrical opening is thus determined, since it matches the center point of the circular trajectory. 
         [0042]    The location, for example, can be described in a reference system in which the detection means lies. Due to the known location of the center point of the circular trajectory  20  and the known location of the extension  11 , it is also known that the instrument has rotated around the extension  11 . Since the particular geometric properties of the extension  11 , in particular the diameter of the extension  11 , are stored in the data processing means, the diameter of the cylindrical opening in the instrument  14  is thus also known. Thus, both the location of the cylindrical opening and its diameter can be determined by means of the method in accordance with the invention and the marker system in accordance with the invention. 
         [0043]      FIG. 4  shows another alternative embodiment for an extension body  1 ′. The extension body  1 ′ is preferably also connected to a marker means (not shown) which can for example be designed like the array  13  (reference star). 
         [0044]    In the embodiment shown in  FIG. 4 , a cylindrical extension protrudes from the front face  30 ′ of the extension body  1 ′, wherein the diameter of said extension decreases in steps as the distance from the surface  30 ′ increases. The height of the steps is preferably larger than the diameter of the largest extension  8 ′. A cylindrical opening in an instrument can also be placed onto the extension body  1 ′ shown in  FIG. 4 , wherein the instrument can be moved up against one of the steps or against the front plane  30 ′, depending on the size of the diameter of the opening. Here, too, the instrument is then preferably rotated around the respectively fitting extension  8 ′,  9 ′ or  10 ′. By detecting the signals of the marker array attached to the instrument, it is in turn possible to determine the center point of the rotational movement. It is then possible to determine from the determined location of the center point of the rotational movement which of the extensions  8 ′,  9 ′ or  10 ′ the instrument has been rotated around. 
         [0045]    Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.