Abstract:
The invention relates to X-ray imaging technology as well as image post-processing and analyzing. An X-ray imaging system element ( 3 ), an X-ray imaging system ( 2 ), the use of a system element in an X-ray imaging system and the method ( 40 ) of determining a deflection of a compression element is disclosed. The X-ray imaging system element ( 3 ) comprises two compression elements ( 8   a,    8   b ), which are movable relative to one another. An object ( 10 ) is introducible and compressible between the compression elements. At least one of the compression elements is adapted to alter its geometrical shape and/or alignment relative to the other during compression. At least one partly X-ray opaque marker element ( 24 ) is provided on one of the compression elements, which marker element is adapted to allow detection of an alteration of the geometrical shape of the respective compression element ( 8   a, b ).

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to X-ray imaging technology as well as image post-processing/analysis. 
         [0002]    In particular, the present invention relates to image-based determination of a geometrical alteration of the shape of a compression element. 
         [0003]    Particularly, the present invention relates to an image-based determination of compression paddle deflection for accurate breast density assessment in mammography applications. 
       BACKGROUND OF THE INVENTION 
       [0004]    Mammography is the modality of choice for screening for early breast cancer. In mammography, preferably low energy X-rays are employed for examining an object, e.g. human breast tissue, as a diagnostic and screening tool. The goal of mammography is the early detection of breast cancer, typically through detection of characteristic masses and/or micro-calcifications. 
         [0005]    In this regard, determining breast density is an important indicator for a cancer risk. With the large amount of images generated in mammography screening programs, it is beneficial to have a reliable automatic breast density assessment (BDA) in order to support the user, e.g. a radiologist. 
         [0006]      FIG. 1  shows an exemplary embodiment of a mammography system. Imaging system  2  comprises X-ray source  4  as well as an X-ray detector  6 . X-ray detector  6  also doubles as a second compression element  8   b,  while a first compression element  8   a , e.g. a compression paddle, is arranged between X-ray source  4  and X-ray detector  6 . The first and second compression element  8   a,b  are movable relative to one another, in particular, the first compression element  8   a  is movable relative to the second compression element  8   b /X-ray detector  6 . 
         [0007]    X-ray source  4  is exemplarily rotatable about an axis for obtaining horizontal/parallel as well as oblique image information. In standard mammography, the surface of the detector  6  is regularly always orthogonal to the line connecting X-ray detector  6  and X-ray source  4 . For oblique mammographic views, the entire system  4 , 6 , 8  is rotatable, as indicated by the circular arrow in  FIG. 1 . Angulations of X-ray source  4  with respect to a fixed position of the X-ray detector  6  may be used for tomosynthesis applications. 
         [0008]      FIG. 2  shows an exemplary mammography screening. An object  10 , e.g. human breast tissue, is arranged between the first and second compression elements  8   a,b , in particular compressed between the first and second compression elements  8   a,b . 
         [0009]    The first compression element  8   a  is attached to a movable arm element  18  allowing a relative movement of the first compression element  8   a  relative to the second compression element  8   b /X-ray detector  6 . By moving the first compression element  8   a  with force F acting in an object distal region  22 , object  10  is compressed between the first and second compression element  8   a,b . Object  10  is generating a counterforce F′ in an object proximal region  20  of the first compression element  8   a.  Since forces F, F′ do not coincide, resulting forces are acting on the first compression element  8   a.  In particular, a tilt about tilt axis  14  is conceivable as well as, in case the first compression element  8   a  is not made of an infinite rigid material, a deflection  16  by bending the first compression element  8   a . Accordingly, a known distance x between the first and the second compression element  8   a,b  at the object distal side  22  results in a distance x+Δx at the object proximal side  20  due to counterforce F′. 
         [0010]    For determining breast density, precise information about the force F applied to the first compression element  8   a  as well as the distance x between the first and the second compression element  8   a,b  is required. Thus, a precise measurement of breast thickness, i.e. distance x, further taking into account additional tilt and deflection of a compression element resulting in Ax is of importance when automatically determining volumetric or mammographic breast density, from image information of digital mammograms. This procedure is also referred to as breast density assessment (BDA). Due to tilt and deflection of a compression element, the actual distance x+Δx may show deviations of Δx=5 mm from the true thickness up to even Δx=15 mm. However, even small errors of 1 to 2 mm may be considered to have a large impact on the breast density assessment, resulting in a significant misjudgement of the density. 
       SUMMARY OF THE INVENTION 
       [0011]    Imprecision in compression thickness measurement may be considered significant for automatic breast density assessment. Such imprecision may result from the height or distance between the first and the second compression element being measured at the far end of the compression elements, i.e. at an object distal region or side, where compression force F is applied, while the height support or object to be examined typically only fills part of the base between the first and the second compression element near the chest wall or at an object proximal region. Additional impreciseness may be introduced by play in the connection of the gantry and paddle or arm and compression element, which may be as large as 15 mm, depending on the system and paddle. 
         [0012]    Since even small errors may have a large impact on the estimated breast density, e.g. 1 mm may lead to about 10% misjudgement, not including other effects, e.g. varying compression height caused by paddle deflection or tilt, which cannot be estimated from a single height measurement, it may be beneficial to not only determine distance x between the first and the second compression element but also the resulting shape of a compression element in a compressed state. In other words, in a compressed state, i.e. with an object introduced between the first and second compression element, the geometry of a compression element may be altered, thereby resulting in additional variables, e.g. Δx, which have to be considered for a correct breast density assessment. To increase accuracy, marker elements are provided in one of the first and the second compression element, in particular in or on the first compression element or compression paddle, which may subsequently be detected in acquired X-ray image information and may be evaluated in an image processing step to derive a resulting geometrical shape of the respective compression element to allow determining a precise compression height x+Δx. 
         [0013]    Aspects, features and advantages of the present invention may further be derived from the detailed description of preferred embodiments described hereinafter which are explained with reference to the following drawings. 
         [0014]    Like elements may be referred to with like reference numerals. 
         [0015]    The figures are not drawn to scale, however may depict qualitative proportions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  shows an exemplary embodiment of a mammography system; 
           [0017]      FIG. 2  shows an exemplary mammography screening; 
           [0018]      FIGS. 3 to 5  show exemplary embodiments for detecting tilt and/or deflection of a compression element; 
           [0019]      FIGS. 6   a - e  show exemplary arrangements of marker elements; and 
           [0020]      FIG. 7  shows an exemplary embodiment of a method for determining a deflection of a compression element. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0021]    To allow a precise breast density assessment, marker elements, e.g. lead marker elements, which may be spherically, circularly or coin-shaped, may be incorporated into one of the compression elements, in particular the compression element, which is expected to be geometrically altered in the compressed state. 
         [0022]    The marker elements, in particular X-ray opaque elements or at least partly X-ray opaque elements, may subsequently be detected in the acquired X-ray image information and may be subjected to calculations to determine an occurring misalignment and/or geometrical alteration of the shape of the respective compression element. 
         [0023]    E.g., in case only a tilt of the compression element is assumed, three marker elements may be considered to be sufficient for determining the orientation of e.g. a planar compression element, i.e. the orientation of the plane of the compression element with respect to the other compression element. 
         [0024]    The X-ray source may be considered to be a point source allowing a precise projection of the respective marker elements onto the X-ray detector and thus into the X-ray image information. In this case, the three marker elements should not be arranged on a single line or marker line but should be arranged so as to unambiguously determine a plane. Since the distance and alignment of the marker elements on the compression element relative to one another is assumed to be known, due to the point source nature of the X-ray source, the precise angulation of the compression element may be determined by known mathematical methods. 
         [0025]    In case not only a tilt but also a deflection, bending or warping of the compression element has to be determined, employing only three marker elements spanning a plane may not be sufficient since the distance between the marker elements is not defined any more due to the deflection of the compression element. With small marker elements, e.g. metal spheres or coins, placed on either side of a compression element, it again may be possible to exploit the theorem or intersecting lines for deriving the compression height from the distance of the projected marker positions. 
         [0026]    Typically, an image-based detection of the marker elements within the X-ray image information may achieve sub-pixel accuracy, exploiting the detector&#39;s point spread function (PSF) as well as focal blur. However, even an accuracy of two pixels would be sufficient for deriving a height with an error smaller than 0.5 mm. 
         [0027]    Accordingly, an accuracy of one pixel would lead to below 0.25 mm precision while an accuracy of 4 pixels would be sufficient for a precision smaller than 1 mm. While it may be conceivable to employ the full area of a compression element, it may be beneficial to arrange the marker elements only in that part of the compression element, which would not interfere with a projection of tissue within the X-ray image information. Accordingly, a compression element may comprise an object proximal region as well as an object distal region. In the context of the present invention, the object distal region may be considered that region that would not allow an overlapping of a projected marker and tissue within the X-ray image information. 
         [0028]    Placing marker elements along an axis between chest wall and gantry or between the object proximal side and the object distal side, it is conceivable to derive position-specific heights thereby measuring paddle tilt or even deflection. Therefore, marker elements may be arranged in subgroups of at least three marker elements, which are arranged so as to constitute a single line or marker line. Since the distance between the marker elements within a single marker line is assumed to be known, a predefined model, e.g. a linear model or thin-plate-spline model or an interpolation through the measured heights as supporting points may be employed to determine the geometrical shape of the compression element and thus the deflection of the compression paddle. 
         [0029]    The true height x+Δx on a marker line can be computed via Equation 1: 
         [0000]    
       
         
           
             
               
                 
                   
                     x 
                     + 
                     
                       Δ 
                        
                       
                           
                       
                        
                       x 
                     
                   
                   = 
                   
                     h 
                     ( 
                     
                       1 
                       - 
                       
                         1 
                         
                           
                             ( 
                             
                               1 
                               
                                 1 
                                 - 
                                 
                                   x 
                                   h 
                                 
                               
                             
                             ) 
                           
                           + 
                           
                             Δ 
                              
                             
                                 
                             
                              
                             
                               
                                 D 
                                  
                                 
                                   [ 
                                   px 
                                   ] 
                                 
                               
                               · 
                               
                                 p 
                                 d 
                               
                             
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
       
         
           
             with: 
             h: the distance between X-ray source and detector; 
             ΔD[px]: the measured distance between the actual distance of the marker elements on the detector and the distance for Δx=0 mm; 
             p: the pixel size; 
             d: the distance between marker elements on the compression element; 
             x: the measured height at the object distal side. 
           
         
       
     
         [0036]    In case of a deflection of the compression element, true height can be computed likewise for several combinations of marker elements resulting in a plurality of supporting points with known true heights, that can be exploited in a thin-plate-spline or polynomial interpolation model. 
         [0037]    Using a plurality of marker lines may even allow to determine a two-dimensional  bending or warping of the compression element around the object to be determined, i.e. height x+Δx is a function of the two-dimensional position on the detector  6 . Such a determination may in particular take into account that the deflection or warping of the compression element is required to be a continuous modification of the geometrical shape of the compression element. Exemplary examples for a deviation are provided in the following table, determined by the following equations. 
         [0038]    For an exemplary mammography system with a distance between X-ray source and X-ray detector of h=650 mm, a detector area of 239Δ305 mm, d=250 mm, a true object thickness (x+Δx)=50 mm or measured height x=(50 mm−Δx) and a detector pixel size of p=0.085 mm, the deviation may be determined as follows. The exemplary marker element distance of d=250 mm exemplifies a case with substantially best accuracy when considering the exemplary dimensions of the detector area. 
         [0039]    For these values, the observed marker position difference for a paddle height difference of Δx amounts to 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           Δ 
                            
                           
                               
                           
                            
                           
                             D 
                              
                             
                               [ 
                               mm 
                               ] 
                             
                           
                         
                         = 
                           
                          
                         
                           
                             
                               h 
                               
                                 h 
                                 - 
                                 
                                   ( 
                                   
                                     x 
                                     + 
                                     
                                       Δ 
                                        
                                       
                                           
                                       
                                        
                                       x 
                                     
                                   
                                   ) 
                                 
                               
                             
                             · 
                             d 
                           
                           - 
                           
                             
                               h 
                               
                                 h 
                                 - 
                                 x 
                               
                             
                             · 
                             d 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               
                                 
                                   650 
                                    
                                   
                                       
                                   
                                    
                                   mm 
                                 
                                 
                                   
                                     650 
                                      
                                     
                                         
                                     
                                      
                                     mm 
                                   
                                   - 
                                   
                                     50 
                                      
                                     
                                         
                                     
                                      
                                     mm 
                                   
                                 
                               
                               · 
                               250 
                             
                              
                             
                                 
                             
                              
                             mm 
                           
                           - 
                         
                       
                     
                   
                   
                     
                       
                           
                          
                         
                           
                             
                               
                                 650 
                                  
                                 
                                     
                                 
                                  
                                 mm 
                               
                               
                                 
                                   650 
                                    
                                   
                                       
                                   
                                    
                                   mm 
                                 
                                 - 
                                 
                                   50 
                                    
                                   
                                       
                                   
                                    
                                   mm 
                                 
                                 + 
                                 
                                   Δ 
                                    
                                   
                                       
                                   
                                    
                                   x 
                                 
                               
                             
                             · 
                             250 
                           
                            
                           
                               
                           
                            
                           mm 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   2 
                 
               
             
           
         
       
     
         [0000]    with ΔD in [mm] or converted to 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       D 
                        
                       
                         [ 
                         px 
                         ] 
                       
                     
                   
                   = 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           D 
                            
                           
                             [ 
                             mm 
                             ] 
                           
                         
                       
                       p 
                     
                     = 
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           D 
                            
                           
                             [ 
                             mm 
                             ] 
                           
                         
                       
                       
                         0.085 
                          
                         
                             
                         
                          
                         mm 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   2 
                 
               
             
           
         
       
     
         [0000]    with ΔD in detector pixels [px]. 
         [0040]    Table 1 contains the observed marker distances in the image for a given height difference Δx and thus derives the necessary accuracy to achieve a certain precision of height measurement, e.g. 1.3 pixels marker localization accuracy for 0.25 mm precision in height measurement. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 deviation  
                 magnification  
                 difference  
                 difference  
               
               
                   
                 Δx[mm] 
                 D[mm] 
                 ΔD[mm] 
                 ΔD[px] 
               
               
                   
               
             
             
               
                   
                 0.00 
                 270.8 
                 — 
                 — 
               
               
                   
                 1.00 
                 270.4 
                 0.5 
                 5.3 
               
               
                   
                 0.50 
                 270.6 
                 0.2 
                 2.7 
               
               
                   
                 0.25 
                 270.7 
                 0.1 
                 1.3 
               
               
                   
                 0.20 
                 270.7 
                 0.1 
                 1.1 
               
               
                   
               
             
          
         
       
     
         [0041]      FIG. 3  shows an exemplary embodiment of a tilt of a compression element relative to a further compression element. 
         [0042]    Exemplarily, a first compression element  8   a  is tilted relative to a second compression element  8   b,  e.g. an X-ray detector  6 . The first compression element  8   a  comprises an object proximal region or side  20  and an object distal region or side  22 . An arm element or a mounting element for the first compression element  8   a,  not depicted in  FIG. 3 , may be assumed at the left side of  FIG. 3 . On the right side of  FIG. 3 , an object  10  is arranged between the first and second compression elements  8   a,b , e.g. human breast tissue. 
         [0043]    The first compression element  8   a  is forced downwards by force F, depicted at the left side of  FIG. 3 , while a resistance force F′ due to the object  10  being compressed between the first and second compression element  8   a,b  occurs at the right side. The thickness or distance between the first and second compression elements  8   a,b , is regularly measured at the left side, constituting a measured thickness x  28 . Due to force F′ acting on the right side of  FIG. 3 , the first compression element  8   a  is tilted upwards with regard to the second compression element  8   b,  resulting in a tilt Δx 15 . 
         [0044]    Accordingly, the actual occurring compression thickness  30  equates to the measured thickness x  28  and the tilt Δx i . Accordingly, compression thickness 30=x+Δx 1 . 
         [0045]    In  FIG. 3 , it is assumed that the first compression element  8   a  is only tilted with regard to the second compression element  8   b  without a further geometrical alteration, e.g. a bending or warping of the first compression element. The first compression element  8   a  comprises exemplarily two marker elements  24 , which are arranged in the plane of cross-section of  FIG. 3 . The distance between marker elements  24  is assumed to be known, thereby allowing, by the projection of the marker elements  24  on the X-ray detector element  6 , to reconstruct a tilt angle, thus allowing to determine Δx 1 . 
         [0046]    By determining the tilt Δx 1  from the projection of the marker elements  24  on detector element  6  by knowing the distance D between the marker elements on compression element  8   a  and the projected distance D of the marker elements  24  on the X-ray detector  6 , the compression thickness  30  may be determined by the measured thickness  28  and Δx 1 . 
         [0047]      FIG. 4  shows an embodiment where not only a tilt but (also) a deflection or warping of the first compression element occurs. Such a deflection occurs in case the compression element is not made of a stiff, but a to some extent flexible material. 
         [0048]    In the plane of the cross-sectional view of  FIG. 4 , exemplarily three marker elements  24  are arranged. The difference between the measured thickness x  28  and the compression thickness x+Δx 2 30 is Δx 2 , resulting from deflection  16 . 
         [0049]    Due to the deflection  16  of compression element  8   a,  the geometrical shape of the compression element is altered as well, an alteration of the distance of the marker elements relative to one another occurs. Since now the distance between the marker elements is not precisely known any more, using only two marker elements would result in an inaccurate determination of Δx 2  and consequently the compression thickness  30 . However, by employing at least three marker elements arranged in a single line, a so-called marker line  26 , and employing the knowledge that the deformation of the compression element has to be a continuous deformation, said deformation or deflection  16  may be determined and thus Δx 2  may be determined. In this regard, known methods of curvature approximation using known structure support points, i.e. marker elements, the shape of the deflected compression element may be determined with sufficient accuracy to allow determining Δx 2 . 
         [0050]    The cross-sectional view of  FIG. 4  is further depicted in  FIG. 5  in the context of an imaging system  2 . X-ray source  4 , e.g. embodies as a point source, is generating X-radiation  12  having a cone shape. Exemplarily three marker elements  24  are arranged in a marker line  26  in the plane of cross-section depicted in  FIG. 5 . 
         [0051]    Force F is applied to the object distal side  22 , while object  10  is compressed between compression elements  8   a,b  on the object proximal side  20 , thereby generating force F′ acting to deflect  16  compression element  8   a  by Δx. The distance between two adjacent marker elements  24  is indicated as d 1  and d 2 , while the distance between the outer marker elements  24  is designated as d 3 . X-ray detector  6  obtains projections  24 ′ of marker elements  24  having the respective distances D 1  to D 3 . 
         [0052]    By knowing the pre-given distances d 1  to d 3 , determining projected distances D 1  to D 3  in the image information and employing the assumption that the deformation of the compression element  8   a  is a continuous deflection  16  caused by a single resulting force F′, the shape and alignment of the compression element  8   a  along the marker line of the three marker elements depicted in  FIG. 5  may be determined. In case a completely symmetrical deformation or deflection  16  of compression element  8   a  is assumed with regard to the plane of cross-section of  FIG. 5 , using only a single marker line as depicted in  FIG. 5  may be considered to be sufficient to determine the overall shape of deflected compression element  8   a.    
         [0053]    However, in case a non-symmetrical deformation is assumed, a plurality of marker elements or marker lines may be employed to determine the overall three-dimensional shape or deformation of a dedicated compression element. In particular, for each marker element  24 , a specific Δx i  may be determined, e.g. Δx a,b , which subsequently allows, by determining the shape of deformed compression element  8   a,  Δx c  at the object proximal side  20  and thus compression height x+Δx c . 
         [0054]    The sections between two marker elements may either be approximated as a straight line or the bent shape of the compression element may be taken into account by an iterative algorithm. 
         [0055]      FIGS. 6   a - e  depict different marker element configurations for determining a three-dimensional alteration of the shape of compression element  8   a.  The marker elements  24  in  FIG. 6   a - 6   d  are arranged in an object distal region  22  so not to interfere with projected object information, e.g. by projecting a marker element into the tissue projection of object  10 . In  FIG. 6   e , marker elements  24  are substantially distributed evenly over compression element  8   a  though still being arranged so as to not interfere with a tissue projection of object  10 . 
         [0056]    In  FIG. 6   a , seven marker elements  24  are incorporated into compression element  8   a,  constituting altogether five marker lines  26 , each marker line  26  comprising three marker elements  24 . 
         [0057]    The accuracy of determining a paddle deflection in the direction of a dedicated marker line may be further increased by employing more than three marker elements  24  within an individual marker line  26 , e.g. four, five, six, seven, eight, nine, ten or more marker elements contained in a single marker line.  FIG. 6   b  employs altogether eight marker elements  24  which are arranged so as to constitute six individual, non-identical marker lines  26 . 
         [0058]      FIGS. 6   c  and  d  each comprise nine individual marker elements  24 , which however are arranged differently with regard to the inner three marker elements  24 , thereby resulting in nine individual marker lines  26  in  FIG. 6   c  and eight individual marker lines  26  in  FIG. 6   d . 
         [0059]    The arrangement of marker elements according to  FIG. 6   e  substantially corresponds to the arrangement of  FIG. 6   a , with the exception that the markers are spread over the entire area of compression element  8   a,  whereas in  FIG. 6   a  marker elements  24  are only spread over the object distal region  22 . It is to be noted that in both arrangements, according to  FIG. 6   a  and  e , marker elements  24  arranged so as to not interfere with a tissue projection of object  10 . 
         [0060]    Now taking reference to  FIG. 7 , a method of determining a deflection of a compression element is depicted. 
         [0061]    Method  40  comprises receiving  42  image information of an object being compressed between a first compression element and a second compression element while performing image acquisition, wherein the image information comprises image information of at least one marker element; and determining  44  a geometrical alteration of the at least one compression element by analyzing the marker element image information in the received image information. Further, from the geometrical alteration of at least one compression element, a thickness of the object compressed between the first compression element and the second compression element may be determined  46 , in particular for an object density assessment. 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
           2  Imaging system 
           3  Imaging system element 
           4  X-ray source 
           6  X-ray detector 
           8   a,b  First, second compression element 
           10  Object 
           12  X-radiation/X-ray beam 
           14  Tilt axis 
           15  Tilt 
           16  Deflection 
           18  Arm element 
           20  Object proximal region/side 
           22  Object distal region/side 
           24  Marker element 
           26  Marker line 
           28  Measured thickness 
           30  Compression thickness 
           40  Method of determining a deflection of a compression element 
           42  STEP: Receiving image information 
           44  STEP: Determining a geometrical alteration 
           46  STEP: Determining object thickness