Abstract:
A deformable marker device for adapting to a human or animal body includes a plurality of marker elements, and a connecting device that connects at least some marker elements of the plurality of marker elements to each other. The connecting device enables the at least some marker elements to be moved relative to each other so as to adapt a shape of the marker device to a course of a curved surface.

Description:
RELATED APPLICATION DATA 
       [0001]    This application claims priority of U.S. Provisional Application No. 60/891,794 filed on Feb. 27, 2007, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to medical markers and, more particularly, to a deformable marker device that can be adapted to a surface of a human or animal body and/or placed onto said body. 
       BACKGROUND OF THE INVENTION 
       [0003]    In medical navigation, markers are typically attached to an object to be tracked. These medical markers may be passive markers (e.g., light reflecting markers), active markers (e.g., light generating markers), or magnetic markers (e.g., coils). By tracking a location of the marker, the location of the object attached thereto also can be tracked. 
         [0004]    Conventional marker devices are not deformable. Instead, the individual marker elements of the marker device have a fixed position relative to each other, such as is for example known from so-called reference stars. 
       SUMMARY OF THE INVENTION 
       [0005]    A deformable marker device in accordance with the present invention preferably comprises a plurality of marker elements connected to each other, wherein the marker elements can be moved relative to each other. The markers can be configured such that they are impermeable to waves and/or radiation used in medical analysis (e.g., x-ray radiation), reflect said waves and/or radiation (e.g., infrared light or ultrasound) or emit waves and/or radiation themselves (e.g., light or infrared light). Images obtained using such imaging techniques (x-ray imaging, infrared light, ultrasound, etc.) are referred to herein as a medical image. The interaction between the marker elements and the waves and/or radiation can be verified by detection devices (e.g., x-ray radiation detectors or light-sensitive sensors). 
         [0006]    The deformability of the marker device is preferably achieved using a connecting means that connects the marker elements. The connecting means, for example, can be flexible (e.g., a material) and/or jointed (e.g., a mechanical joint connection between the marker elements). The distance between at least some of the marker elements can be variable. This is the case, for example, if the connecting means is a flexible cloth. The connecting means can, but need not, be elastic. The term “can be moved” means that the marker spheres can be moved relative to each other by a person, without a tool, by applying a minor or normal force, without destroying or damaging the connecting means. 
         [0007]    The marker device is preferably used to simplify the determination of a three-dimensional model of an anatomical structure from two two-dimensional images. In particular, the marker device can simplify the determination of correspondence points, wherein correspondence points can be used to determine the three-dimensional position of object points of a structure from two two-dimensional images of the structure from two different directions (in accordance with the principles of epipolar geometry). In particular, the marker device in accordance with the invention can enable the determination of a so-called fundamental or essential matrix (or also localization matrix), which describes properties of the geometry forming the basis of the at least two images. Further information regarding epipolar geometry and determining correspondence points can be found in co-pending U.S. application Ser. No. 12/029,716 filed on Feb. 12, 2008 and titled “Determining a Three-dimensional Model of a Rim of an Anatomical Structure”, the contents of which is hereby incorporated by reference in its entirety. 
         [0008]    Conventional localization techniques for determining a localization matrix use rigid objects for which the geometric relationship between the markers is exactly known. A deformable, in particular flexible marker device can be used in accordance with the invention as a localizer, wherein in accordance with one embodiment, the marker elements are visible in x-ray images. For a localization method, in particular for determining the localization matrix in accordance with the principles of epipolar geometry, the relative position between the marker elements should remain the same in the images. Knowledge of the exact geometric relationship between all of the marker elements is not compulsory. 
         [0009]    A localization algorithm can be used to determine the localization matrix, for example, by extracting relative camera movement from pairs of correspondence points, as is known from the field of “stereovision”. Such algorithms are known and can be used to extract three-dimensional information from video scenes (i.e., a sequence of images recorded by a moving camera), satellite images or images achieved by a specific stereo configuration (e.g., two cameras aligned in parallel that simultaneously capture images). Examples of such algorithms include the eight-point algorithm (Longuit-Higgins) or the five-point algorithm (Stewenius/Engels/Nister), which is preferably used. If the algorithms are used for video recordings or other “actual” images captured by a conventional lens system, image features such as edges and grey-color values are typically used to automatically find the matching correspondences (e.g., an edge of a traffic sign in a first image will correspond to the same edge in a second image). A different approach is preferred for x-ray images, since edge information is usually difficult to determine or unreliable because the images have a translucent or transparent property and/or because organic objects are rounded. The latter is in contrast to typical video recordings, which, for example, show buildings or cars that have identifiable edges. 
         [0010]    In accordance with an aspect of the invention, marker elements are inserted into the image so as to artificially produce “prominent” image portions that can be used as correspondence points. In particular, this enables the correspondence points to be automatically detected. 
         [0011]    When analogously using the marker device (e.g., the marker device is attached or adapted to a human or animal body), at least some (preferably most) of the marker elements are preferably spaced apart from each other, while the marker elements also can be moved relative to each other. The connecting means can be configured such that the marker elements assume predetermined positions when the marker device is spread out. The marker device preferably is designed flat. 
         [0012]    Preferably, at least two of the marker elements are held at a fixed distance by the connecting means. To this end, the connecting means can be stiffened between these two marker elements or can comprise a rigid connecting member having a marker element attached to each of its ends. The known, fixed distance is preferably used to calibrate, in particular gauge, the geometry of the imaged object. The localization matrix can be gauged in this way. The distance between the remaining marker elements can be variable. 
         [0013]    The markers preferably have differing shapes and/or sizes. There can be at least two groups of markers, wherein the shape and/or size within the group is the same and the markers belonging to different groups differ in shape and/or size. Preferably, the at least two markers that are fixedly spaced apart from each other differ in shape and/or size from the remaining markers, or belong to a group of markers that differ in shape and/or size from the majority of the marker elements. 
         [0014]    The deformable marker device is preferably wound or attached at least partially around a part of a human or animal body. This means that when said part of the body is recorded, a first portion of the marker elements are then situated in front of the part of the human or animal body from the viewing direction of the imaging apparatus, and a second portion of the marker elements are situated behind the part of the human or animal body. The marker elements are preferably characteristically different in their shape, size and/or arrangement, such that it is possible to tell from the image which marker elements are in front of the part of the body and which are behind the part of the body. For instance, a different arrangement is given if a location of a foreground marker element relative to neighboring foreground marker elements differs from a location of a background marker element relative to neighboring background marker elements. The surrounded part of the body is also referred to as the “inner region”, since it lies within the region surrounded by the marker device. A characteristic arrangement, for example, would be an arrangement in lines, wherein the upper and lower line are in the foreground and the middle line is in the background. Alternatively or additionally, the marker elements in the foreground, for example, may be arranged in a zigzag shape, while the marker elements in the background may be arranged linearly. Another alternative would be for the marker elements arranged in the foreground to be cube-shaped, while the marker elements arranged in the background can be spherical, resulting in square or round areas in the image that allow the markers to be identified as foreground markers or background markers. Lastly, the marker elements arranged in the background, for example, can have a significantly different size relative to the marker elements arranged in the foreground. 
         [0015]    The connecting means also can be designed such that movement of the marker elements in a first direction is easier than movement of the marker elements in a direction perpendicular to the first direction. The marker device, for example, can be designed such that it is easy to deform the marker device into the shape of a cylindrical cloak, while a relative movement of the marker elements in the direction of the cylindrical axis requires a greater force to be applied. This increases the stability of the marker device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The forgoing and other features of the invention are hereinafter discussed with reference to the drawing. 
           [0017]      FIG. 1  shows an exemplary arrangement of marker elements in a marker device in accordance with the invention. 
           [0018]      FIG. 2  shows an exemplary x-ray recording of a pelvis, around which a marker device in accordance with the invention has been wound. 
           [0019]      FIG. 3  shows another exemplary x-ray recording under the same conditions as  FIG. 2  but from a different direction. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]      FIG. 1  shows a schematic arrangement of exemplary marker elements, which are shown as black circular areas  10  and  20  attached along strips a, b and c. The marker elements are preferably designed as marker spheres that are divided into two groups of different diameters. In the given example, the diameters measure 5 millimeters and 9 millimeters, and the marker spheres along a strip are each spaced apart by 2.5 centimeters. The sizes (diameters) of the marker elements can be arbitrarily selected. They can be larger than 1 millimeter and smaller than 3 centimeters. The distance between the marker elements within a strip can be larger than 2 millimeters and smaller than 10 centimeters. In the given example, the distance between the center points of the marker elements in the upper strip a and lower strip c measures 16.7 centimeters. This is purely by way of example. The distance can be greater than 3 centimeters and less than 30 centimeters. The outer dimensions of the arrangement shown in  FIG. 1  measure approximately 20×30 centimeters and are also purely by way of example. 
         [0021]    In accordance with one embodiment of the invention, marker elements are arranged flat and connected via a flat cloth  30 . The marker elements can be attached to predetermined positions on the flat cloth. Alternatively or in addition to cloth  30 , the marker elements may be connected to one another via a mechanical joint  31  (e.g., a hinge joint or other joint that enables movement in a first direction, but not in a direction perpendicular to the first direction, etc.). A hinge joint, for example, can include connecting elements (e.g., a first part and a second part) that are coupled together by a common shaft or the like. Such joints are well know and will not be further described herein. 
         [0022]    The cloth comprising the marker elements can be wound around a part of the body of a (human or animal) patient in the manner of a kidney belt or in the manner of a cuff. The flat cloth  30  shown in  FIG. 1 , for example, can be shaped into a cylindrical cloak, wherein the strips a and c should be on the front half of the cylindrical cloak, while the strip b should be on the rear half of the cylindrical cloak. The cloth shown in  FIG. 1  thus can be double-layered, wherein the strips a and c form part of the front layer and the strip b forms part of the rear layer. In other words, the strips a and c represent a view of the cuff-shaped marker device from the front, and the strip b represents a view from the rear. 
         [0023]      FIG. 2  is an x-ray recording of a pelvis. In front of the x-ray recording, a belt configured in accordance with the device of  FIG. 1  has been wound around the patient&#39;s pelvis and attached to the human body (e.g., by a waistband, buttons and/or by designing the cloth to be elastic) and/or is held on the human body by tension. The marker elements  10  and  20 , which are visible in the x-ray recordings, can again be seen as black circles in  FIG. 2  in their characteristic arrangement known from  FIG. 1 . They are arranged along the lines a, b and c, which correspond to the strips a, b and c in  FIG. 1 . The lines are clearly identifiable, since the distance between the marker elements within a line is preferably less than the distance between the lines. Further, the strips a, b and c are arranged such that marker elements do not lie one directly behind the other when the device is formed as a closed area (e.g., when formed as an area resembling a cylinder or elliptic cylinder, or other shape that conforms to an outer surface of the patient&#39;s body). 
         [0024]    The larger marker spheres  20  are conspicuous and clearly distinguished from the smaller marker spheres  10 . A rod  40  also can be seen, which is not shown in  FIG. 1 . The rod  40  preferably consists of a material that is at least partially permeable to x-rays, e.g., a plastic such as PVC. The rod  40  is preferably designed rigid and defines a fixed distance between two marker spheres situated on the same side of the part of the body. In the given example, this is the connection between the two large marker spheres  20   a  and  20   c  situated on the front side of the pelvis. Using a spacer  40  allows the localization matrix to be calibrated or gauged in terms of size. 
         [0025]    While  FIG. 2  shows a frontal recording of the pelvis,  FIG. 3  is an x-ray recording taken from the view of the observer from obliquely front-right. In other words, the right-hand hip joint has been rotated forwards while the recording apparatus remains stationary. 
         [0026]    The marker spheres corresponding to each other in  FIGS. 2 and 3  can be easily determined. The large marker spheres  20  can serve as starting points. The large marker sphere  20   a,  for example, has five small marker spheres  10   a  located to its right. Each of the identical marker spheres have been provided with reference signs in  FIGS. 2 and 3 . The left-hand marker sphere of the two large marker spheres  20   b  in  FIG. 2  can be seen in the middle row b in  FIG. 3 . The large marker spheres  20   a  and  20   c  are shown in both images. The translucently visible spacer  40  provides an additional identification aid. 
         [0027]    Due to the relative shift in position between the marker spheres, it is possible when comparing  FIGS. 2 and 3  to determine the different recording geometry in each case. As can be seen, the middle group of markers ( 10   b,    20   b ) is shifted to the right from  FIG. 2  to  FIG. 3  relative to the upper ( 10   a,    20   a ) and lower ( 10   c,    20   c ) group of markers. This is due to the fact that the marker spheres of the strip b are behind the imaged pelvis, while the marker spheres of the strips a and c are in front of the imaged pelvis. Changing the imaging direction appears to shift the position of the marker spheres in the images. In reality, however, the marker spheres are stationary relative to the anatomical structure while the two x-ray recordings are taken, since the marker device is fixedly strapped to the patient. 
         [0028]    The changed imaging direction can be determined from the relative shift from  FIG. 2  to  FIG. 3 . For example the imaging direction can be determined based on the shift of the spheres  20   b  relative to the spheres  20   a  and  20   c,  and on the known distance between the marker spheres  20   a  and  20   c.  The distances between the marker spheres within a group or “line” also can be adduced, particularly if the cloth is a flexible but inelastic cloth. 
         [0029]    In summary, it is possible to determine information on the change in the imaging conditions from image to image, in particular on the change in the imaging direction, from the images of the marker elements. The so-called essential matrix or localization matrix can be determined, which contains essential information on the imaging geometry that changes from image to image. If this matrix is determined, then it is possible to produce three-dimensional models of the imaged anatomical structure from the two images, based on the principle of epipolar geometry. 
         [0030]    As shown in  FIG. 2 , the middle row b of markers contains two large marker spheres  20   b.  This is only one example embodiment. In accordance with another embodiment, one larger marker sphere is also sufficient. Arranging the larger marker spheres  20   b  to the left and right of the center lying at the rod  40  simplifies handling. In particular, the belt does not have to be rotated about an axis running normal to and through the center in  FIG. 1 , depending on whether the left-hand or right-hand side of the anatomical structure is to be more precisely examined. 
         [0031]    If a recording protocol is defined for the x-ray recording in which the part to be treated is to be rotated forwards or backwards, then it is possible to automatically determine which side is the side to be treated from the shift in the rows of marker spheres relative to each other. In other words, due to the recording protocol, the side which is to be treated can be deduced from the polarity of the rotational angle between the two images, as determined from the images. This can be utilized within the framework of an evaluation software. 
         [0032]    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.