Abstract:
A motion correction method includes two steps. The first step includes a global motion correction using the bilinear warping technique and a rough delineation of the lung fields. One of the native images (low energy image, high energy image) is deformed to match the other image. In a second step, local motion corrections are applied to the globally motion corrected image by computing a proximity value in small overlapping tiles. Only tiles with a sufficient high proximity value are taken into account. The maximum shift applied in this second step is limited to a few pixels to avoid strong deformations of the native images.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a 371 National Stage Application of PCT/EP2015/079757, filed Dec. 15, 2015. This application claims the benefit of European Application No. 14198241.3, filed Dec. 16, 2014, which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
       [0002]    The invention relates to dual energy radiography and more particularly to a motion correction method to reduce misregistration artifacts in tissue type specific derivative images. 
       2. Description of the Related Art 
       [0003]    The diagnostic reading of chest examinations is impeded by anatomical noise. Thoracic abnormalities are hard to recognize due to obstruction by overlying bone structures. In standard chest radiography the contrast of the bone structures is partly reduced by taking exposures with high-energy photons (ca. 120 kVp). 
         [0004]    The technique of dual energy subtraction allows separating bone and soft tissue by using a dual exposure: one with a high energy spectrum and one with a low energy spectrum. The outcome is a standard posterior-anterior chest image and 2 tissue-type specific images. This technique aids in identifying and characterizing nodules, masses, lung diseases and bone abnormalities. 
         [0005]    Although the two exposures are made within less than a second (typically 200 ms) motion artifacts from respiration, heart motion and physiological motion requires a motion correction method to reduce the motion artifacts. 
         [0006]    The overlapping structures in the projection radiographic images can show different displacements. For example the motion due to respiration is complex and structures in the lung fields (e.g. the bronchi) can move into other directions compared to the overlapping rib structures. Even the shadows of the anterior ribs and posterior ribs can show different displacements. 
         [0007]    To optimize the quality of the soft tissue derivative image, it is important to suppress the bone structures. Motion artifacts due to misalignment of the bone structures are disturbing and must be avoided. However the displacement of soft tissue in the images must be taken into account to avoid strong deformations of the soft tissue areas. 
         [0008]    A proposal for a soft tissue type driven image registration method is described in U.S. Pat. No. 7,257,245 B2. The method relies on mask images indicating tissue type specific structures in the high energy (HE) and the low energy (LE) image to steer the image alignment. This approach strongly depends on the ability to identify corresponding tissue type specific structures in the dual exposure native images. 
       SUMMARY OF THE INVENTION 
       [0009]    Preferred embodiments of the present invention provide a method for correction of motion in a dual energy image recording process that overcomes the above-described disadvantages. 
         [0010]    The above-mentioned aspects are realised by a method having the method steps set out below. 
         [0011]    Specific features for preferred embodiments of the invention are also set out below. 
         [0012]    The present invention is generally implemented as a computer program product adapted to carry out the method of any of the claims when run on a computer and is stored on a computer readable medium. 
         [0013]    Further advantages and embodiments of the present invention will become apparent from the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows a quadrangle fitted to the lung fields. 
           [0015]      FIG. 2  shows the local proximity measure for multiple translation offsets. 
           [0016]      FIG. 3  shows the actual (final) corner control points. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    In dual energy radiography 2 images are generated by an x-ray imaging system: a low energy native image (LE) and a high energy native image (HE). 
         [0018]    A log-subtraction technique allows decomposing the low and high energy native images into material specific images, e.g. a bone and soft tissue image in chest radiography, using a simplified physical model that does not take into account spatially dependent effects as the heel effect, beam hardening and scatter: 
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         [0019]    With μ S,L  and μ B,L  the attenuation coefficients of soft tissue and bone for the low-energy spectrum, μ S,H  and μ B,H  the attenuation coefficients of soft tissue and bone for the high-energy spectrum and z S  and z B  the thicknesses of soft tissue and bone in the patient for pixel position (x, y). 
         [0020]    By choosing the appropriate weight parameters w, a soft tissue image can be reconstructed (w=w B =μ B,H /μ B,L ) and a bone image can be reconstructed (w=w S =μ S,H /μ S,L ). 
         [0021]    In case of a dual-exposure system, the low and high energy native images must be spatially registered to reduce misalignment artifacts due to cardiac, respiratory, bowel and patient motion. 
         [0022]    Preprocessing operations can be applied to the low and high energy image to reduce the noise, remove the anti-scatter grid line-artifacts, etc. 
         [0023]    According to a preferred embodiment of the present invention, motion correction is achieved by applying a global motion correction step to one of the low energy or high energy native images followed by a local motion correction step applied to the globally motion corrected (intermediate) image. 
         [0024]    The global motion correction is based on bilinear warping of one of the low energy or high energy native images controlled by a number of control points. 
         [0025]    In a specific embodiment the radiographic image is a chest image. 
         [0026]    In this embodiment these control points are the 4 control points, more specifically 4 corners of a quadrangle that roughly delineates the lung fields in the chest image (see  FIG. 1 , points A, B, C, D). 
         [0027]    These corner control points are found by analyzing horizontal and vertical average profiles of one of the native images. 
         [0028]    The corner control points are chosen near the edge of the lung fields. 
         [0029]    At the periphery of the lung fields, the motion of the shadows of bone structures and soft tissue are more or less aligned. Moreover the discrepancy between the displacements of the anterior and posterior ribs is limited in this region. So near the edge of the lung fields local motion can accurately be computed taking into account all the pixels. 
         [0030]    At these corner control points the local proximity value between the dual exposure native images (High energy (HE) image and low energy (LE) image) is computed for multiple translation offsets using local image patches centred at the corner control points (as illustrated in  FIG. 2 ). 
         [0031]    There are several ways to compute the local proximity (also called similarity) between images. 
         [0032]    In a preferred embodiment the local proximity value is the (normalized) cross-correlation coefficient of a patch in one of the low or high energy native images (e.g. the low energy native image) and a template of the other native image (e.g. the high energy native image) for multiple translation offsets. 
         [0033]    An implementation of the normalized cross-correlation coefficient calculation is described in “Fast Normalized Cross-Correlation”, J. P. Lewis, Vision Interface, 1995. 
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         [0034]    Where
   t is the template     t  is the mean of the template   ƒ is the image patch     ƒ   u,v  is the mean of the image patch under the region of the template   
 
         [0039]    In a specific embodiment, the images are multiplicatively demodulated. In a typical implementation, the multiplicative demodulation of the dual exposure input images is performed by dividing the original pixel values by its low pass filtered version. The multiplicative demodulated images are preferably clipped to limit the impact of high signal contrasts to the proximity value. 
         [0040]    In a preferred embodiment the size of the template is a square with a physical extent between 3 cm and 6 cm. The template is shifted over a maximum distance of 4 mm in every direction. 
         [0041]    In another embodiment the proximity value is computed directly on the grey value native images using the normalized cross-correlation coefficient. 
         [0042]    In still another embodiment the Euclidean distance or mutual information can be used as proximity values. 
         [0043]    In every control point the translation offset for which the proximity value reaches its maximum value and exceeds a predefined threshold is chosen as displacement vector. Thresholding avoids introducing unreliable displacement vectors due to noise contamination in the native images. The chosen threshold is related to the expected noise level in the native images. 
         [0044]    To improve the robustness, the maximum proximity value and corresponding displacement vector can be computed in additional control points. In a preferred embodiment the additional control points are the midpoints of the pairs (A,B), (B,C), (C,D), (A,D) and the centre of quadrangle ABCD (see  FIG. 1 , points  0  to  4 ). 
         [0045]    Control points  0 ,  1  and  3  are located near the spine. Taking into account these additional control points will avoid unwanted deformations in the centre of the image. 
         [0046]    In a preferred embodiment the final position and displacement of the 4 corners of the quadrangle ABCD are computed as weighted average using the position and displacement of each corner control point and 3 additional control points with the shortest distance to the said corner control point. 
         [0047]    This results in a new quadrangle with corner points A′, B′, C′ and D′ (see  FIG. 3 ). 
         [0048]    The position of these corner points A′, B′, C′ and D′ and the corresponding final displacements are computed using as following combinations:
   A′ based on A,  0 ,  1  and  4     B′ based on B,  0 ,  1  and  2     C′ based on C,  0 ,  2  and  3     D′ based on D,  0 ,  3  and  4     
 
         [0053]    In a preferred embodiment the weights assigned to the different control points are determined by the proximity values and a predefined weight distribution. 
         [0054]    Only control points with a proximity value above a predefined threshold are taken into account. The weight distribution over the different control points ensures that the weight of a corner control points is at least twice the weight of the additional control points. 
         [0055]    Given the positions of the corner points A′, B′, C′ and D′ and the corresponding displacements, the motion correction is applied using warping, e.g. bilinear warping. 
         [0056]    Bilinear warping transforms pixel coordinates (x,y) in the original image to new coordinates (x′, y′) according to the formulas: 
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         [0057]    The 8 unknown coefficient a x  and b x  can be computed out of the given x and y coordinates and displacements of the 4 corner points A′, B′, C′ and D′ using well known matrix algebra operations. 
         [0058]    The pixel values of the warped image are computed using interpolation such as cubic interpolation. 
         [0059]    In a second step the (bilinear) warped image is locally corrected for further improving the content matching with the other, non-modified native image. 
         [0060]    To compute the local displacements, proximity values are computed using overlapping tiles in the globally corrected image and a non globally correct (high or low energy) native image. The tiles are computed for every N-th pixel in horizontal and vertical direction. A typical value for N is 16. 
         [0061]    In a preferred embodiment the local value of proximity is the normalized cross-correlation coefficient of a tile in the warped image, e.g. bilinear warped low energy image and a template of the other native image (e.g. the HE image) for multiple translation offsets or vice-versa. (Images may be multiplicatively demodulated similar as described with regard to the global correction steps). 
         [0062]    Typical tile size is 63×63 pixels. Translation offsets are preferably limited to shifts in both directions of maximum 2 pixels. 
         [0063]    The translation offset for which the proximity value reaches its maximum value and exceeds a predefined threshold is chosen as local displacement vector. 
         [0064]    The result is a sparse sampled local motion correction displacement map. 
         [0065]    The displacement vector for each intermediate pixel is computed using bilinear interpolation of the values in the sparse displacement map. 
         [0066]    In another embodiment cubic interpolation is used to compute the displacement vectors for each intermediate pixel. 
         [0067]    The local motion correction displacement map is applied to the globally corrected image (e.g. the bilinear warped LE image) using bilinear interpolation. 
         [0068]    In another embodiment cubic interpolation is used to apply the displacement map to the (bilinear) warped (LE) image. 
         [0069]    The motion corrected dual exposure images are used to compute tissue type specific images with suppressed motion artifacts. 
         [0070]    Such a method to compute the tissue type specific images is the calibration free dual energy radiography method as described in patent application WO/2013/037659. 
         [0071]    Post-processing operations can be applied to the tissue type specific images to reduce noise, to apply scatter corrections and to convert the pixel values to values suitable for reproduction or displaying, e.g. using known multiscale image processing methods as there are multiscale gradation processing (EP1341125) and multiscale contrast enhancement processing (EP1347413).