Abstract:
Superimposed X-ray and video images can be obtained by acquiring the respective images from the optically equivalent points in space. One or more mirrors may be used to acquire the images. Alignment of one camera with respect to the X-ray source may be achieved using images of reference points in space and their respective projections. Once the X-ray source and the video camera are positioned at the equivalent point in space, the resultant images can be superimposed through warping.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to the following applications simultaneously filed by the same inventors and incorporated by reference herein: 
     a) Apparatus for Superimposition of X-ray and Video Images; 
     b) Method for Aligning and Superimposing X-ray and Video Images; and 
     c) Laser-Based Method for Aligning Apparatus for Superimposing X-ray and Video Images. 
     BACKGROUND OF THE INVENTION 
     In addition to X-ray images of an object, it is often useful to have a corresponding video image. If the two could be combined into a composite image, then one could immediately see how the features revealed by the X-ray relate to the surface features displayed in a video image. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a conceptual diagram of the system; 
     FIGS. 2 and 3 are flow charts of procedures for aligning the video camera; and 
     FIG. 4 is a diagram of a laser alignment system. 
    
    
     DESCRIPTION OF THE INVENTION 
     One method of correlating a video image with an X-ray image of the same object is by acquiring the respective images from the same point in space. A video or optical camera can be placed at a point in space equivalent to that of the X-ray source by deflecting a portion of the optical image with an X-ray transparent mirror. The camera is oriented by an alignment procedure to insure that it is located at a point optically equivalent to the location of the X-ray source. Superimposition can then be achieved by warping one image onto the other. 
     In FIG. 1, a patient  10  is lying on a platform under which there is an X-ray detector plane  20 . An X-ray source  30  located above the patient emits X-ray energy from a point in space defined as a projection center  32 . The energy passes through the patient to the X-ray detector plane  20  to create an X-ray image. An optical camera  40 , such as a video camera, is also positioned to obtain an image that may be combined with the X-ray image to create a composite visual and X-ray image. 
     To obtain a video image that can be combined with the X-ray image, the optical center  42  of the video camera  40  is positioned at effectively the same point in space as the projection center  32  of the X-ray source  30 . Additionally, the optical axis of the camera  40  will also be aligned with an imaginary line running between the X-ray source projection center  32  and the center of the X-ray detector plane  20 . Since the X-ray source  30  and the video camera  40  cannot physically occupy the same space, mirrors are employed to provide the video camera  40  a vantage point or point of projection effectively the same as that of the X-ray source  20 . 
     A mirror M 1 , transparent to X-rays but reflective at visual wavelengths, is placed in the path of the X-ray source  30  at some angle, to deflect an optical image to a point away from the X-ray path. A second mirror M 2  can be positioned in the path of the reflected image, again at an angle, to deflect the optical image towards the video camera  40 . In FIG. 1, both mirrors M 1  and M 2  are at  45 E with respect to the path of the X-ray source  30 , although other angles could be employed. Thus, the visual image reflects off the surface of the mirror M 1  and is again reflected by mirror M 2 . 
     The location of the mirrors can be selected such that the length of the segment r 1  between mirrors M 1  and M 2  plus the length of the segment between the mirror M 2  and the optical center  42  of the video camera  40  is equal to the distance from the mirror M 1  to the center of X-ray projection  32  of the X-ray source  30 . 
     Alternatively, the second mirror M 2  could be dispensed with if the video camera  40  was positioned to one side of the X-ray path. Also, instead of using mirrors, a prism structure or another X-ray transparent light-bending mechanism could be employed to obtain the desired optical path length and angle of deflection. 
     Even with careful alignment of the mirrors M 1  and M 2 , it may be difficult to co-locate the X-ray source projection center  32  and the camera&#39;s optical center  42  at the equivalent point in space with any degree of precision. Thus, some means of accurately positioning the camera  40  with respect to the X-ray source  30  is desirable. 
     Two methods for correlating the two images use the procedure of warping one two-dimensional image on a first plane onto a second plane. The X-ray detector plane  20  is provided with a reference device such as a pattern of markers  12  arranged in a square or some other suitable configuration. In lieu of a marker, the borders of the X-ray image may be utilized. The markers  12 , fabricated from a material such as steel, appear as a series of dark point images in the X-ray and video images. Based on the aspect of the pattern of the markers  12  in the image that will be warped, the transformation that must be performed to warp the image to the second plane can be readily determined. 
     Warping of the X-ray image from the X-ray detector plane  20  to the video image of the markers  12  is accomplished by applying a planar transformation H to the X-ray image of the markers  12  such that it conforms to the aspect and dimensions of the pattern of the markers  12  as it appears in the video image. For each pixel in the X-ray image on the X-ray detector plane  20 , matrix H calculated for the particular location of the X-ray detector plane  20  is multiplied by the position of that pixel to produce the position of the corresponding pixel in the video image. 
     The warping operation can be represented by the following equation: 
     
       
         m i N=Hm i   
       
     
     where: 
     m i N are the pixels in the video image; 
     H is the planar transformation matrix mapping pixels in the X-ray image to the video image; and 
     m i  are the pixels in the X-ray image. 
     The matrix H is calculated by using techniques well known in the art. Such methods are described in U.S. Pat. No. 5,821,943 and U.S. Pat. No. 5,845,639, incorporated herein by reference, and in Wolberg, “Digital Image Warping,” IEEE Computer Society Press, Los Alamitos, Calif. 1990. 
     In the both of the methods utilizing warping, at least two additional markers  80  are positioned off the detector plane  20  between the X-ray source  30  and the X-ray detector plane  20  (see FIG.  1 ). One way of accomplishing this is to place the “off-plane” markers  80  on a piece of plexiglass above the X-ray detector plane  20 . 
     In the first of these methods, illustrated in the flow chart of FIG. 2, an X-ray image is taken of the off-plane markers  80  as well as the markers  12  on the X-ray detector plane  20 . Next, a video image is taken of the off-plane markers  80  and the markers  12  on the X-ray detector plane  20 . Using the video image of the markers  12  on the X-ray detector plane  20 , a value for H is computed. Then, the X-ray image is warped onto the video image, and the locations of the projected and visually-detected off-plane markers  80  are compared. If these locations coincide, the optical center  42  of the video camera  40  is then at a point in space equivalent to that of the projection center  32  of the X-ray source  30 . However, if these locations do not coincide, then the orientation of the video camera  40  is adjusted to bring its optical center  42  towards the projection center  32  of the X-ray source  30 . The process is repeated until the images coincide and the orientation of the video camera  40  is then fixed. 
     In the second of these methods, shown in the flow chart of FIG. 3, an X-ray image is taken of the off-plane markers  80 . Then, a second set of markers  90 , which will be referred to as “projection markers,” are placed on the X-ray detector plane  20  at the points at which the off-plane markers  80  are projected by the energy from the X-ray source  30 . On an X-ray image, therefore, the off-plane markers  80  and the corresponding on-plane projection markers  90  will appear as one on the X-ray image. 
     Now, a video image is taken of the X-ray detector plane  20 . Since the off-plane markers  80  are suspended above the detector plane  20 , they will also appear in the video image. The video image is examined to determine whether the video images of the on-plane projection markers  90  coincide with the corresponding off-plane markers  80 . If they do, then the optical center  42  and projection center  32  effectively share the same point in space. If, however, the images do not coincide, then the orientation of video camera  40  is adjusted to bring the images of the off-plane markers  80  and on-plane projection markers  90  together, and another video image is acquired and evaluated, repeating until the images coincide, at which point the orientation of the camera is fixed. Finally, using the on-plane markers  12 , a value of H is computed and the X-ray image is warped onto the video image to achieve superimposition. 
     A third method for positioning the video camera  40  uses a laser. A source  70  of laser light is placed at the center of the X-ray detector plane  20  and aimed at the projection center  32  of the X-ray source  30 , as shown in FIG.  4 . 
     The mirrors M 1  and M 2  reflect the laser light causing it to travel to the video camera  40  and reflect off the surface of lens  44  of the video camera  40 . The position of the video camera  40  is adjusted until the laser light returning to the source  70  is coincident (or nearly coincident) with the light issuing from the source  70 . This may be confirmed visually by observing where the reflected beam lands on the X-ray detector plane  20 . To align the images and achieve superimposition, the X-ray image can then be warped onto the video image. 
     Variations of the foregoing may be employed to suit the application. For example, one may use two X-ray source and video camera combinations to achieve a stereo representation of the object of interest. In lieu of the off-plane markers  80 , on may substitute any object or objects that presents at least two points of reference visible by X-ray and optically. Also, instead of warping the X-ray image onto the video image, one could warp the video image onto the X-ray image, to achieve superimposition. In the configurations discussed above, the optical image is acquired by a video camera. In reality, any optical camera—still, digital, CCD, or other—may be employed with the apparatus and method described above. Additionally, X-ray images should be understood to include single, X-ray exposures as well as real-time, X-ray fluroscopic exposures. Finally, it should be understood that the methods described here may be executed in real time.