Abstract:
A method of forming a composite image from first and second digital images formed by recording first and second contiguous segments of a larger radiographic image in first and second overlapping storage phosphor members, exposed to a source of X-rays wherein the image content in the overlapped region is the same in both images and the end edge of the first member is present both on the first image and as a shadow edge in the second image, the method comprising: correcting for geometric distortion in the first and second digital images; determining any rotational displacement and any vertical displacement between the first and second images by matching the first member end edge in the first image to its shadow in the second image; correcting for image orientation based on any said rotational displacement; determining any horizontal displacement between the first and second images by correlating the image content in the overlapped region of the first and second images; and stitching said first and second images together along the first member end edge based on any said horizontal and vertical displacements.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuations under 35 USC §120 of the earlier filing date of U.S. patent application Ser. No.: 09/742,509, filed Dec. 20, 2000, entitled Elongated Computed Radiography Cassette. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to digital radiography, and in particular to the imaging of a long human body part, such as the spine or legs, using a storage phosphor-based computed radiography system. 
     BACKGROUND OF THE INVENTION 
     When a long segment of the human body is imaged using the conventional screen-film technique, special cassettes and films of extended length are used, such as 30×90 cm and 35×105 cm. As medical institutions are migrating from analog screen-film systems to digital modalities, such as computed radiography (CR), these types of exams impose a significant challenge. This is because the size of digital detector is limited. For example, the largest CR storage phosphor cassette from several major CR vendors is limited to 35×43 cm, which can only image a partial of the body part at a time. To address this problem, a method has been proposed that staggers several storage phosphor cassettes together in a specially made cassette holder (U.S. Pat. No. 5,986,279, European Patent App. EP0866342A1, EP0919856A1, and EP0919858A1). All the cassettes are exposed in a single x-ray exposure. Then image processing is applied to stitch all the partial images together. The advantage is that the method is compatible with the current CR readers. However, a pattern of reference markers needs to be imaged simultaneously with the patient in order to achieve precise geometric registration of the partial images. The shadow of the reference markers may obscure diagnostically important information in the stitched image. Also because of the overlapping of the cassettes, the metallic cassette frames introduce wide shadow artifacts in the resultant image that are sometimes objectionable. Moreover, the cassette holder is quite heavy and is typically mounted in a fixed position, which limits the users from moving it up and down for exact patient positioning. The cassette holder is bulky and does not conform to ISO/ANSI standards, which means that it can not be placed in the bucky grid holder that is designed for the current screen-film systems. U.S. patent application Ser. No.: 09/742,509 filed Dec. 20, 2000, discloses a method that is based on an extended length cassette with two 35×43 cm phosphor screens built inside. The two phosphor screens are slightly overlapped in the center of the cassette (FIGS.  1 - 3 ). The overall cassette size is about 35×85 cm, which nearly doubles the current largest cassette size and allows a fairly long segment of the human body to be imaged at a single exposure. The information recorded in either phosphor screen bears part of the desired final image. 
     During the readout process, one end of the cassette is placed in the CR reader and the first phosphor screen is scanned and stored, the cassette is then removed from the reader and inverted to allow the second phosphor screen to be read in the same manner as the first. The two images can then be processed into a composite full image if so desired. The length of the cassette can be designed to be shorter or longer in order to follow the ISO/ANSI standard, such as 36″ and 51″ inch long. The maximum cassette length is approximately twice the maximum allowable scan length of the CR reader. 
     Special digital image processing is required to construct a composite full image from the front and back images that are obtained from the two individual phosphor screens. The two phosphor screens are packed and partially overlapped inside the single cassette and are therefore not coplanar. This causes the image of the body part to be magnified differently for different locations in the cassette, and a demagnification operation is required as part of the process of registering the front and back images. In addition, the two phosphor screens will not be perfectly aligned inside the cassette, and there are translation and rotational displacements introduced by the CR reader during the image readout process. As a result, the placement of the pixels from the front and back images will not be perfectly aligned, and the images will require rotation and translation compensation. The aforementioned image registration processing can be accomplished by de-warping the front and back images to a set of reference markers (with known position) that are imaged in conjunction with the body part. However, it is desirable that the images be acquired without the use of any reference markers to preclude the possibility of obscuration of the important diagnostic regions of the image. It is therefore desirable to develop an image processing algorithm that can automatically (1) conduct image demagnification, (2) correct the translation and rotational displacements between the front and back images, and (3) make use of the information in the front and back images to form a composite full image that has high geometric fidelity without relying on any reference markers. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a solution to the problems discussed above. 
     According to a feature of the present invention there is provided a method of forming a composite image from first and second digital images formed by recording first and second contiguous segments of a larger radiographic image in first and second overlapping storage phosphor members, exposed to a source of X-rays wherein the image content in the overlapped region is the same in both images and the end edge of said first member is present both on said first image and as a shadow edge in said second image, said method comprising: correcting for geometric distortion in said first and second digital images; determining any rotational displacement and any vertical displacement between said first and second images by matching said first member end edge in said first image to its shadow in said second image; correcting for image orientation based on any said rotational displacement; determining any horizontal displacement between said first and second images by correlating said image content in said overlapped region of said first and second images; and stitching said first and second images together along said first member end edge based on any said horizontal and vertical displacements. 
     ADVANTAGEOUS EFFECT OF THE INVENTION 
     The invention has the following advantages. 
     1. Enables the generation of a full composite image from two partial images that is free from artifacts by completely eliminating the use of references. 
     2. Preserves a high degree of geometric accuracy in the stitched image. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are diagrammatic views showing an extended length CR cassette with two storage phosphor screens built inside. 
     FIG. 2 is a perspective view showing one storage phosphor screen pulled from one end of the cassette as it would be during processing in CR reader. The other end of the cassette is capable of opening in a like manner. 
     FIG. 3 is a diagrammatic view showing two phosphor screens which partially overlap in the center of the cassette. The deflectors guide the screens as they approach the center of the cassette to force the overlap. 
     FIGS. 4A,  4 B and  4 C are diagrammatic views respectively showing how the extended length cassette is used to acquire images, how an object of rectangular shape placed in the patient location is deformed by magnification due to distance variation from the x-ray source to the storage phosphor screen, and how the acquired front and back image look. The CR reader over-scans both phosphor screens in the vertical direction of the cassette in order to make the screen ending edges fully visible in both images. 
     FIG. 5 is a flow diagram showing the image processing steps for automatic formation of a full composite image from first and second images. 
     FIG. 6 shows the major image processing steps that are used to automatically find the locations and orientations of the screen ending edges in both the front and the back images, and for finding the location and orientation of the shadow of the front screen ending edge in the back image. 
     FIG. 7 shows the major image processing steps that are used for finding the horizontal displacement between the front and back images by image-correlation. 
     FIG. 8 is a graphical illustration of the correlation function. 
     FIG. 9 shows the composite, stitched full image. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In general, the present invention relates to the radiographic imaging of an elongate object such as the full spine (for diagnosing scoliosis, for example) or leg of a human subject. 
     Two contiguous CR plates contained in an elongated cassette are exposed to a radiographic image of an elongate object to produce a latent image stored in the CR plates. The CR plates are removably mounted in the cassette and are sequentially fed to a CR reader where the latent radiographic images are converted to two electronic images which are combined to form an elongated image. The elongate image can be displayed on an electronic display or printed out on hard copy media. 
     Referring now to FIGS. 1A and 1B, there is shown an embodiment of the present invention. As shown, storage phosphor cassette  10  includes an elongate rectangular shell  12  having first and second open ends  14  and  16 . A first storage phosphor plate assembly  18  is detachably mounted in shell  12  from the first open end  14 . A second storage phosphor plate assembly  20  is detachably mounted on shell  12  from the second open end  16 . Each assembly  18 ,  20  includes a respective storage phosphor plate  22 ,  24  and a support and latching assembly  26 ,  28 . Plates  22 ,  24  are butt joined or overlapped in the central region  29  of shell  12 . Shell  12  includes upper and lower members  30 ,  32  and side extrusions  34 ,  36  which together form a rectangular shell. 
     FIG. 2 shows first storage phosphor assembly  18  partially detached from cassette  10  at a reading device (not shown). 
     FIG. 3 shows a cross-section of cassette  10  showing upper and lower members  30 ,  32  having respective opposed inner surfaces  40 ,  42  including deflectors  44 ,  46  extending therefrom for guiding the inner ends of assemblies  18 ,  20  to overlap. This results in an overlapping of storage phosphor plates  22  and  24  to form a composite storage phosphor plate for elongate radiographic images, such as the human spine and leg. A more detailed description of the latching and unlatching system and CR reader is given in U.S. patent application Ser. No.: 09/742,509 filed Dec. 20, 2000, the contents of which are incorporated herein by reference. 
     The first and second images read from first and second storage phosphor plates  22  and  24  are formed into a composite image according to the method of the present invention as follows. 
     An overview illustration of the steps involved in the present invention is shown in FIGS. 4-9. The generation of a full composite image from the front and back images is comprised of the following steps: (1) demagnification of each image pixel based on the distance between the x-ray source and the physical location of the pixel in the individual phosphor screen, (2) determination of the rotational displacement and the vertical displacement between the front and back images by matching the front screen ending edge in the front image to it&#39;s shadow in the back image, (3) image orientation correction based on the rotational displacement, (4) determination of the horizontal displacement between the front and back images by correlating the image information in the overlapping screen regions, and (5) stitching the front and the back images together along the front screen ending edge based on the horizontal and vertical displacements. 
     As shown in FIG. 4A, during an x-ray exam, the patient  403  is positioned in the path of the x-ray beam  402  from the x-ray tube  401 . The extended length cassette  405  is placed behind the patient in order to record the image of the patient. The extended length cassette  405  can be used with an anti-scatter grid  404 , which is positioned behind the patient  403  but directly in front of the cassette. The grid can be either a stationary type or moving bucky. After the x-ray generator is fired and the cassette is exposed, the image of the patient is recorded by the front screen  406  and the back screen  407  that are enclosed inside the cassette. Each screen captures only a portion of the image of the patient, as indicated by element  420  and  421  (FIG.  4 C). Because the front screen  406  is not totally opaque to the incident x-rays, the back screen  407  can still record the image of the patient  403  in the screen overlap region  427 . However, the signal-to-noise ratio of the image captured on the back screen in the overlap region  427  will be relatively low because of the x-ray attenuation caused by the front screen. The image content recorded by the two screens in the overlap region is the same. This redundant information is then used to register the front and back images to produce a full patient image. The front screen ending edge  408  can impose a distinctive edge shadow on the back screen. By comparing the location and orientation of the front screen ending edge with its shadow in the back image, the relative orientation and vertical displacement between the two images is determined. The exposure process described in this paragraph corresponds to element  500  in FIG.  5 . 
     After the x-ray exposure, the extended length CR cassette is sent to the CR reader for image readout. The front phosphor screen  410  is scanned using a laser beam in a line-by-line format as described by element  412 . The depicted signal from the phosphor screen, which is linearly proportional to the magnitude of the recorded patient image signal, is extracted and converted into digital format. The CR reader may stop the reading process when the laser scan line nearly reaches the screen ending edge  414 . This does not guarantee that the complete information of the ending edge will be recorded in the acquired image, which is required by this invention for image registration. To address this issue, the CR reader must over-scan the phosphor screen, i.e., scan slightly beyond the end of the screen. In FIG. 4C, element  420  represents the image acquired from the front screen, and shows that the screen ending edge,  422 , is captured completely inside the image. The front image therefore is partitioned into two regions by the screen ending edge: the normal image area and the over-scanned image area. After the front screen is scanned and the image is stored in the CR memory, the cassette is removed from the reader and inverted to allow the back screen  407  ( 411 ) to be read in the same manner as the front screen. However, because of the inversion of the cassette, the laser beam conducts the scan in a format as indicated by element  413 . Therefore, to restore the correct orientation of the back image, the acquired image must be flipped once horizontally and once vertically after being stored in the CR memory. Element  421  shows the acquired back image after the flip operations. Since both the back and front screens are of the same size, the back screen will also be over-scanned beyond its ending edge  415  (FIG.  4 B). Consequently, the screen ending edge  423  will be captured completely inside the acquired back image  421 . Due to screen overlap, the front screen ending edge  414  is also recorded by the back screen, which is indicated by element  425 . The back image is therefore partitioned into three regions by the shadow of the front screen ending edge and further by the back screen ending edge. The end-to-end readout and storage process described in this paragraph corresponds to element  502 ,  504 ,  503 ,  505 , and  506  as shown in FIG.  5 . 
     Because the front and back storage phosphor screens are not exactly co-planar inside the extended cassette, there is a location dependent, although slight, geometric distortion (magnification) that is introduced, as indicated by elements  416  and  417 . For a nominal SID (x-ray source distance) of 180 cm, the mismatch between the front and back images in the overlap region can be as large as 0.5 mm in the image horizontal axis. This can significantly impact the stitching precision and introduce discontinuity adjacent to the seam line in the stitched image. It is therefore necessary to perform distortion correction, especially as the distortion conspicuity increases as the SID decreases. The distortion correction process is accomplished using a mathematical model that is based on the geometric placement of the phosphor screens inside the cassette. The design of the extended length cassette forces the top of the front screen and the bottom of the back screen to be coplanar such that there is no geometric distortion near the two ends of the cassette. As the distance from either cassette end to the center decreases, the magnification increases. This phenomenon is more dominant in the horizontal axis than that in the vertical axis. To correct for this distortion, each pixel in the front image is dewarped using the following equations: 
     
       
         
           x′=x, 
         
       
     
     
       
           y ′=( y−y   c )× g   f   ×x/x   max   +y   c ,  (1) 
       
     
     where x and y are image pixel coordinates in the vertical and horizontal axes, respectively, and x′ and y′ are the new image pixel coordinates, respectively, g f &gt;=1 is a constant specific to the front image and specific to the distance from the x-ray tube to the cassette, x max  is the pixel coordinate maximum in the vertical axis, y c  is the center coordinate in the horizontal axis of the image. The origin of the image pixel coordinate is defined at the image upper-left comer, with the downward-pointing vertical axis being the positive x-axis and the right-pointing horizontal axis being the y-axis. Eq. 1 essentially conducts variable correction for each image row but ignores the very small distortion in the vertical direction. The correction is conducted symmetric to the middle column of the image, which is valid because during the x-ray exposure the central x-ray beam is normally centered with the cassette. Similarly, the back image can be corrected using the formula given by: 
     
       
         
           x′=x, 
         
       
     
     
       
           y ′=( y−y   c )× g   b ×( x   max   −x )/ x   max   +y   c ,  (2) 
       
     
     where g b &lt;=1 is a constant specific to the back image and specific to the distance from the x-ray tube to the cassette. This image demagnification process is indicated by elements  507  and  508  in FIG.  5 . This processing step can be ignored when the SID becomes large (&gt;&gt;180 cm), as the distortion introduced by the magnification factor is negligible. 
     In order to calculate the parameters that are used for stitching the front and back images, the screen ending edges in both the front and the back images must be located. This operation is shown by elements  509  and  510  in FIG.  5 . The pixel values in the image region that is beyond the screen ending edge reflect the baseline noise level of the CR reader. This is because there is no signal contribution from the phosphor screen. Consequently, the pixel values in these regions are relatively low in comparison to those in the normally exposed image regions, therefore there is an abrupt pixel value decrement/discontinuity across the screen ending edge in the image. This pixel value discontinuity is used to detect the location and orientation of the screen ending edges, which can be accomplished in many ways. In the preferred embodiment of the present invention, the detection is carried out by (1) computing all the significant edge transition pixels in the proximity of the screen ending edge location, and (2) performing line delineation of the screen ending edge pixels. 
     Using the front image as an example, FIG. 6 describes the preferred embodiment of the detection process. First, a narrow band  602  is extracted from the end of the front image  600 . Depending on how the phosphor screen is being scanned in the CR reader, the orientation of the screen ending edge  601  can have a variation of several degrees in the acquired image from one scan to the next scan. Therefore, the size of the narrow band must be large enough such that the entire screen ending edge is reliably extracted. For an image that has a width of 2,048 pixels, the size of the narrow band should be approximately 200×2,048 pixels. 
     Second, the one-dimensional derivative of the image which is computed in the vertical direction using an operator [−1, 0, 1]. A one-dimensional derivative operator is preferred because the pixel value discontinuity only occurs across the edge direction, which is always nearly horizontal, and because of the computational efficiency advantages. A predefined threshold is used to select only those candidate edge transition pixels which are of greater magnitude and of falling slope. Element  603  shows the results from this step. 
     Third, a linear function is fitted to the candidate edge pixels and the best fitting parameters are obtained when the least square error is reached. Element  604  shows the fitted linear function overlaid on top of the edge transition pixels. The fitting parameters describe the ending edge location and orientation: 
     
       
           x=k   f   ×y+a   f ,  (3) 
       
     
     where k f  and a f  are the fitting parameters with k f  the orientation and a f  the offset of the front screen ending edge in the front image. Similarly, this process is conducted for the back image  610 , except rising edge transition pixels are searched instead inside a narrow band  614  at the beginning of the processed back image. A new function is obtained by least-square-error fit: 
     
       
           x=k   b   ×y+a   b ,  (4) 
       
     
     where k b  and a b  are the fitting parameters with k b  the orientation and a b  the offset of the back screen ending edge in the back image. 
     Once the screen ending edge location is successfully found in the front image, it is compared with its shadow in the back image for image registration. To locate the shadow of the front screen ending edge in the back image (FIG. 5, element  511 ), a similar approach to element  509  is used. This is possible because the pixel values in the back image also undergo a strong signal intensity decrement in the screen overlap region  427  (FIG. 4C) due to the high attenuation of the incident x-rays by the front screen during the x-ray exposure. In order to locate the shadow of the front screen ending edge, the location of the narrow band needs to be defined in the back image. This can be calculated based on the size of the overlap regions (D in mm), which is determined by cassette design, the image pixel size (psize in mm), and the average location of the identified back screen ending. The distance from the center of the narrow band to the beginning of the back image is given by: 
     
       
           d=D /psize+( k   b   ×y   c   +a   b ).  (5) 
       
     
     The function that is obtained using the least-square-error fit to describe the shadow of the front screen ending edge in the back image can be depicted as: 
     
       
           x=k×y+a,   (6) 
       
     
     where k and a are the fitting parameters with k the orientation and a the offset. 
     Theoretically, parameters k f  and k should be equal because they both represent the orientation of the front screen ending edge. However, they may differ by as much as several degrees in practice for several reasons such as misalignment between the two phosphor screens in the cassette or screen positioning variations in the CR reader during the readout process. The deviation between k f  and k represents the orientation misalignment between the front and back images. To assure a seamless composite image after stitching, and to preserve high geometric fidelity, this misalignment must be corrected. 
     Misalignment correction is accomplished in one of three ways: (1) rotating the front image by θ=a tan(k)−a tan(k f ) while keeping the back image unchanged, (2) rotating the back image by θ=a tan(k f )−a tan(k) while keeping the front image unchanged, or (3) rotating the front and back image by −a tan(k f ) and the back image by −a tan(k), respectively. The first and the second methods have the advantage of reduced computation because only one of the two images must be rotated. However, the orientation of the resultant front screen ending edge, which is also the orientation of the seam line in the composite stitched image, may still contain some residual mis-registration in the horizontal direction which can cause the seam line in the stitched image to appear jagged. The third method overcomes this disadvantage. FIG. 5, element  513  shows the effect of rotating the back image. Element  512 , which shows the effect of rotating the front image, is optional depending on whether method 2 or method 3 was used Since the parameters that are used for aligning the front and back images, e.g., k a , k b , k, a a , a b , and a, are calculated before image rotation, the parameters must be transformed accordingly to reflect the new values in the rotated image(s). The parameters are modified by placing Eqs. 3, 4, and 6 into the transform given by: 
     
       
           x′=x  cos θ+ y  sin θ, 
       
     
     
       
           y′=−x  sin θ+ y  cos θ,  (7) 
       
     
     where (x′, y′) are the new coordinates in the rotated image, and θ is the rotation angle. For the simplicity of the description, the symbols k a , k b , k, a a , a b , and a will be used to represent the new transformed values. 
     The vertical displacement between the front and back image, x_offset, is defined as the vertical distance from each pixel in the back image to origin of the front image and is given by: 
     
       
           x _offset= a   f   −a   b .  (8) 
       
     
     Using the vertical displacement guarantees that the front and the back images are stitched along the ending edge of the front screen. This process is indicated by element  530  in FIG.  5 . 
     Once the back screen ending edge, as described by k b  and a b , and the shadow of the front screen ending edge, as described by k a  and a a , are successfully identified, the location of the screen overlap region  427  (FIG. 4C) in the back image can be defined. The screen overlap region in the back image is located between the back screen ending edge and the shadow of the front screen ending edge. The size of the region is calculated based on the equation given by: 
     
       
         overlap_size=( k×y   c   +a )−( k   b   ×y   c   +a   b ),  (9) 
       
     
     and the vertical displacement from the back image origin is: 
     
       
         overlap_offset b =( k   b   ×y   c   +a   b ).  (10) 
       
     
     Element  515  shows the aforementioned process. Using the computed value of overlap_size, the corresponding region in the front image is derived. This is the region of the same size but with a vertical displacement from the image origin defined by: 
     
       
         overlap_offset f   =x   max −( k   a   ×y   c   +a   a )−overlap_size.  (11) 
       
     
     This process is suggested by element  514 . After the screen overlap regions are extracted from each image, as shown by elements  516  and  517 , they are compared in the next step to find the horizontal displacement between the front and back images. 
     The image content recorded in the overlap regions are the same except for some horizontal displacement, y_offset, between the corresponding pixels. A one-dimensional correlation function is computed to find the displacement using the formula given by 
     
       
           c (Δ)=Σ ij   F ( x   i   , y   j )× B ( x   i   , y   j +Δ),  (12) 
       
     
     where F(x i , y j ) and B(x i , y j ) is the pixel value at (x i , y j ) in the extracted overlap region from the front and back images, respectively, and Δ is the horizontal displacement parameter for correlation. The Δ value at which c(Δ) reaches a maximum is the optimal value for y_offset. 
     FIG. 7 describes the preferred implementation of this operation. First, the overlap region  702  and  703  are extracted from the front and back images respectively. Second, element  704  is obtained by extracting a portion of  702 , then is correlated with  703  to create the correlation function c(Δ),  706 . Similar results can be achieved by correlating a portion of  703  with  702 . Third, the maximum of function c(Δ) is searched and the corresponding value of Δ is identified as y_offset,  707 . Because the edge information in  702  and  703 , including skin line, tissue boundaries, bone edges, collimation boundaries, and hardware labels etc, contribute the most useful information to the correlation, the low frequency content is removed from  702  and  703  in order to improve the correlation robustness. Normally the correlation function is smooth, as indicated by element  810  (FIG.  8 ). However, if stationary grid lines are present in the image, small periodic peaks can appear in the function, as indicated by element  811  (FIG.  8 ). The stationary grid imposes a periodic line pattern artifact in the acquired images, the artifact is particularly dominant when the grid is orientated in the vertical direction, and can correlate with itself, causing small spikes to be introduced on top of the back ground correlation function. This artifact will negatively impact the accuracy of the determination of the location of the true function maximum. To address this issue, low-pass filtering of the correlation function is used before searching for the maximum. The process described in this paragraph is represented by element  531  (FIG.  5 ). 
     After both the front and the images have been demagnified, the relative orientation of the two images has been aligned, and x_offset and y_offset have been found, the back image is stitched to the front image. Each pixel of the front image is copied to the stitched image buffer except those pixels that are beyond the screen ending edge line. Each pixel in the back image is copied to the stitched image buffer with an displacement defined by x_offset and y_offset except those pixels before the shadow of the front screen ending edge. The resultant image is shown in FIG.  9 . The process conducted in this paragraph is represented by element  532  (FIG.  5 ). 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
       10  storage phosphor cassette 
       12  elongated rectangular shell 
       14  first open end 
       16  second open end 
       18  first phosphor plate assembly 
       20  second phosphor plate assembly 
       22 , 24  storage phosphor plate 
       26 , 28  latching assembly 
       29  central region 
       30 , 32  upper and lower members 
       34 , 36  side extrusions 
       40 , 42  inner surfaces 
       44 , 46  deflectors 
       401  x-ray tube 
       402  x-ray beam 
       403  patient 
       404  x-ray anti-scatter grid 
       405  extended length cassette 
       406  front screen (lateral view) 
       407  back screen (lateral view) 
       408  ending edge of front screen (lateral view) 
       410  front screen (front view) 
       411  back screen (front view) 
       412  indication of laser scan direction for front screen 
       413  indication of laser scan direction for back screen 
       414  front screen ending edge (front view) 
       415  back screen ending edge (front view) 
       416  deformation of a rectangular object in patient position in front screen 
       417  deformation of a rectangular object in patient position in back screen 
       420  acquired image from front screen—front image 
       421  acquired image from back screen—back image 
       422  front screen ending edge 
       423  back screen ending edge 
       425  shadow of front screen ending edge in the back image 
       427  screen overlap region in the back image 
       500  exposure patient 
       502  read front image in the CR reader 
       503  read back image in the CR reader 
       504  store front image in the CR reader 
       505  store back image in the CR reader 
       506  flip back image once in horizontal direction and once in vertical direction 
       507  optional image demagnification of the front image 
       508  optional image demagnification of the back image 
       509  detect back screen ending edge in back image 
       510  detect front screen ending edge in front image 
       511  detect shadow of front screen ending edge in back image 
       512  optional rotation of front image 
       513  image rotation of back image 
       514  define screen overlap region in front image 
       515  define screen overlap region in back image 
       516  extract screen overlap region from front image 
       517  extract screen overlap region from back image 
       530  calculate vertical displacement—x_offset 
       531  calculate horizontal displacement—y_offset by image correlation 
       532  image stitching 
       600  acquired front image 
       601  front screen ending edge 
       602  extracted narrow band at the end of front image for identifying screen ending edge 
       603  candidate edge transition pixels (falling slope) in  602   
       604  fitted line overlaid on top of candidate edge transition pixels 
       610  acquired back image 
       611  back screen ending edge 
       612  shadow of front screen ending edge in the back image 
       614  extracted narrow band at the beginning of back image for identifying screen ending edge 
       615  candidate edge transition pixels (rising slope) in  614   
       616  fitted line overlaid on top of candidate edge transition pixels 
       622  extracted narrow band for searching of shadow of front screen ending edge 
       623  candidate edge transition pixels (rising edge) in  622   
       624  fitted line overlaid on top of candidate edge transition pixels 
       702  extract screen overlap region from front image 
       703  extracted screen overlap region from back image 
       704  portion of  702   
       705  process for conducting image correlation 
       706  correlation function 
       707  location of maximum in the correlation function 
       810  low-pass filtered  811   
       811  spikes in the correlation function due to the use of grid.