Abstract:
An achromatic phase-contrast imaging apparatus for examining an object of interest is provided which comprises two different phase gratings which have different pitches. Thus, the imaging apparatus yields phase-contrast information for two different energies. Thus, phase-information over a wider energy band can be used.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to achromatic phase-contrast imaging. In particular, the invention relates to a phase-contrast imaging apparatus for examining an object of interest, to a phase grating module for a phase contrast imaging apparatus and to a method of phase-contrast imaging for examining an object of interest. 
       BACKGROUND OF THE INVENTION 
       [0002]    A severe limitation of grating based phase-contrast imaging systems, such as X-ray phase-contrast imaging systems for hard X-rays (15 to 30 keV), is that the system usually has to be designed for a certain mean energy E M . Consequently, such systems will use phase-information in a more or less narrow band with a bandwidth of approximately 10% around the design-energy. For mammography systems typical spectra are much wider in bandwidth and the necessary choice of design-energy may significantly limit the fraction of x-ray power in the spectrum contributing to the phase-contrast imaging process. 
       SUMMARY OF THE INVENTION 
       [0003]    It may be desirable to provide for an imaging system and method which uses a larger amount of the power available. 
         [0004]    The invention relates to a phase-contrast imaging apparatus for examining an object of interest, to a phase grating module for a phase contrast imaging apparatus and to a method of phase-contrast imaging for examining an object of interest according to the features of the independent claims. Further features of exemplary embodiments of the invention are stated in the dependent claims. 
         [0005]    It should be noted that the features which are in the following described for example with respect to the imaging apparatus or the module may also be implemented as method steps in the method according to exemplary embodiments of the invention and vice versa. 
         [0006]    According to an exemplary embodiment of the invention, a phase-contrast imaging apparatus for examining an object of interest is provided, the apparatus comprising a source, a detector, a first phase grating (G 1 ) and a second phase grating (G 1 ′). The source is designed for generating a beam of radiation, in particular a beam of polychromatic X-rays. The detector is adapted for detecting the radiation after it has passed the object of interest and the first and the second phase gratings. The first phase grating is positioned between the source and the detector and has a first pitch and the second phase grating is also positioned between the source and the detector and has a second pitch, wherein the first pitch differs from the second pitch. 
         [0007]    In other words, an achromatic phase-contrast imaging apparatus may be provided which uses at least two different phase gratings. This allows for the use of phase-information over a wider energy band for example in X-ray based differential phase-contrast imaging (DPCI). 
         [0008]    Furthermore, the two phase gratings may have a different height, i.e. different thicknesses and/or trenches of different depths. 
         [0009]    According to another exemplary embodiment of the present invention, the second phase grating is positioned adjacent to the first phase grating. For example, the two phase gratings are forming an integral part of a phase grating module. 
         [0010]    According to another exemplary embodiment of the present invention, the second phase grating is positioned a predetermined distance from the first phase grating. This may also help to use phase-contrast information over a larger bandwidth than the usual 10%. 
         [0011]    It should be noted that further phase gratings may be provided which all differ in their respective pitch and/or heights. 
         [0012]    According to another exemplary embodiment of the present invention, a third grating is provided, wherein the third grating is an absorption grating which forms part of the detector or is positioned right before the detector. The third grating has a third pitch and/or height which is different from the first pitch and the second pitch of the first and second phase gratings. 
         [0013]    According to another exemplary embodiment of the present invention, the imaging apparatus further comprises a fourth grating positioned between the source and the object of interest, wherein the fourth grating is an absorption grating and has a fourth pitch which is different from the first pitch and the second pitch. 
         [0014]    This grating is used to generate coherent x-rays. The x-rays from a tube (even each monochromatic component cannot be used for interference because of the lack of coherence. This grating “generates” coherent” rays by slicing the source into smaller pieces (line sources). 
         [0015]    According to another exemplary embodiment of the present invention, the first pitch corresponds to a first energy of the radiation emitted by the source. The second pitch corresponds to a second energy of the radiation. Furthermore, the first phase grating has a Talbot distance for the first energy and the second phase grating has the same Talbot distance for the second energy. 
         [0016]    Thus, the intensities for the two different design energies (first energy and second energy, E 1 , E 2 ) are superimposed at the location of the detector for the same value of the Talbot distance d T . 
         [0017]    According to another exemplary embodiment of the present invention, the absorption grating G 2  has a third pitch equal to the harmonic mean of the pitches of the two gratings of 2×(first pitch*second pitch)/(first pitch+second pitch). 
         [0018]    A superposition of two intensity modulations will therefore occur at the detector. This will result in a beating phenomenon with a large and a small frequency component. The phase retrieval process will have to take the effect of the beatings into account. In the conventional system (Pfeiffer et al.) the modulation has a trigonometric form. A superposition of two such profiles with different frequency occurs in the more general case scope described here, which are nevertheless detectable if the phase stepping is performed over one entire period of the beating. 
         [0019]    According to another exemplary embodiment of the present invention, the first energy is two times the second energy. 
         [0020]    According to another exemplary embodiment of the present invention, the imaging apparatus is adapted as a mammography imaging apparatus for examination of the female breast. 
         [0021]    According to another exemplary embodiment of the present invention, the source is an X-ray source, wherein the apparatus is adapted as an X-ray based differential contrast imaging apparatus. 
         [0022]    According to another exemplary embodiment of the present invention, the imaging apparatus is adapted as an optical imaging apparatus, where the beam of energy used for probing the object is an optical radiation beam with a wavelength within the range of for example 400 to 1400 nm. 
         [0023]    According to another exemplary embodiment of the present invention, a phase grating module for a phase contrast imaging apparatus for examining an object of interest is provided, the module comprising: a first phase grating for being positioned between a source of the imaging apparatus and a detector of the imaging apparatus, wherein the first phase grating has a first pitch and a second phase grating for being positioned between the source and the detector, wherein the second phase grating has a second pitch, wherein the first pitch is different from the second pitch. 
         [0024]    The phase grating module may be designed such that the distance between the first and second phase gratings can be adjusted electronically by a control unit. Furthermore, both phase gratings may be replaced by different phase gratings or further phase gratings may be added to the module. According to an exemplary embodiment of the present invention, the module comprises three different phase gratings which can all be moved by the control unit in order to adjust their distance to each other. 
         [0025]    According to another exemplary embodiment of the present invention, a method of phase-contrast imaging for examining an object of interest is provided, the method comprising the steps of generating a beam of radiation by a source, using a first phase grating having a first pitch and being positioned between the source and a detector for creating a first Talbot image corresponding to a first energy of the radiation at the Talbot distance from the first grating, using a second phase grating having a second pitch and being positioned between the source and the detector for creating a second Talbot image corresponding to a second energy of the radiation at the same Talbot distance from the first grating, and detecting, by a detector, the beam after it has passed the object of interest and the first and second gratings, wherein the first pitch is different from the second pitch. 
         [0026]    According to another exemplary embodiment of the present invention, the method further comprises the step of using a third phase grating having a third pitch and being positioned between the source and the detector for creating a third Talbot image corresponding to a third energy of the radiation at the same Talbot distance from the first grating, wherein the third pitch is different from the first pitch and the second pitch. 
         [0027]    An absorption grating G 0  may be used for generating the coherence, and another absorption grating G 2  may be used for detecting the fringes altogether. Conventional detectors may not offer high enough spatial resolution to directly detect the fringes. 
         [0028]    According to another exemplary embodiment of the present invention, the method further comprises the step of using an absorption grating having a pitch which is equal to equal to the harmonic mean of the pitches of the two gratings 2×(first pitch*second pitch)/(first pitch+second pitch), wherein the absorption grating is part of the detector. 
         [0029]    It may be seen as the gist according to an exemplary embodiment of the invention, that a further phase grating G 1 ′ is placed just before or behind the first phase grating G 1  with a pitch different from the pitch of G 1 . The idea is to superimpose the intensities for two different design energies E 1  and E 2  at the location of the detector for the same value of the Talbot distance. 
         [0030]    The invention is based on the observation that the phase shift of a given slab of material is inversely proportional to the energy of the radiation (for X-rays). Therefore, a phase grating designed to generate a regular phase shift pattern of height π at a design energy E 2  will result in a phase shift of 2π for half the design energy. Thus, there may be no measurable effect on the monochromatic wave component with energy E 1 =E 2 /2. In order to design a system which also yields phase-contrast for E 1  another grating G 1 ′ is placed right behind (or in front of) G 1 . 
         [0031]    If the second phase grating G 1 ′ is removed, an additional image can be acquired which then can be subtracted from the original image generated by using both phase gratings in order to resolve E 2 . It should be noted that distinguishing between the contributions from the two energies may also be possible during the phase stepping with both gratings G 1  and G 1 ′ present. 
         [0032]    These and other aspects of the present invention will become apparent from and elucidated with reference to the embodiments described hereinafter. 
         [0033]    Exemplary embodiments of the present invention will be described in the following, with reference to the following drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  shows two X-ray spectra after 8 cm breast tissue. 
           [0035]      FIG. 2  shows a grating and detector setup according to an exemplary embodiment of the present invention. 
           [0036]      FIG. 3  shows an imaging system according to an exemplary embodiment of the present invention. 
           [0037]      FIG. 4  shows a flow-chart of a method according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0038]    The illustration in the drawings is schematically and not to scale. In different drawings, similar or identical elements are provided with the same reference numerals. 
         [0039]      FIG. 1  shows two X-ray spectra after the X-rays have passed 8 cm of breast tissue. The X-ray source is Mo (Molybdenum) at 35 keV with 13 μm Mo filtration. 
         [0040]    The horizontal axis  112  shows the energy of the X-rays ranging from 10 keV to 35 keV. The vertical axis  113  shows the number of counts in arbitrary units. 
         [0041]    The upper curve  114  shows the number of counts in case the breast tissue contains 33% water and 67% lipid and the lower curve  115  shows the number of counts when the breast tissue contains 50% water and 50% lipid. 
         [0042]    Two characteristic peaks  116 ,  117  can be seen at approximately 17.3 keV and 17.4 keV and 19.7 keV and 20.0 keV, respectively. 
         [0043]    As can be seen from the spectra depicted in  FIG. 1 , the apparatus according to the invention may advantageously use both peaks  116 ,  117  for imaging, thereby using a large amount of the power available for the phase-contrast imaging process. 
         [0044]      FIG. 2  shows a measurement setup comprising four gratings and a radiation detector  109  according to an exemplary embodiment of the present invention. 
         [0045]    The X-rays or other rays of electromagnetic energy  110 , for example emitted by an X-ray source (not depicted in  FIG. 2 ), first pass the initial grating  107  (G 0 ). Then, the rays pass the object of interest  103 , for example a breast or another kind of tissue to be examined. Then, the rays pass the first and second phase gratings  104 ,  105  (G 1 , G 1 ′), which are arranged before the detector  109  and a fourth grating  106 , which is an absorption grating (G 2 ). The distances between the two phase gratings G 1 , G 1 ′ and the detector  109  corresponds to the respective Talbot distance of each phase grating corresponding to different design energies. 
         [0046]    The imaging method according to the invention is based on the method by Pfeiffer et al., “Phase retrieval and differential phase-contrast imaging with low-brilliance X-ray source”, Nature Physics, 2006, using three different gratings G 0 , G 1  and G 2  in order to detect phase shifts introduced by an object by observing interference fringes with the use of the absorption gratings G 2  on a detector. 
         [0047]    The additional piece of hardware consists of a phase grating G 1 ′ which is placed right behind (or in front of) the standard phase grating G 1  with a pitch p 2  different from the pitch p 1  of G 1 . The setup is such that the Talbot images for two different design energies E 1  and E 2  at the location of the detector are superimposed at the same location, i.e. for (more or less) the same value of the Talbot distance d T . 
         [0048]    The setup is based on the observation that the phase shift of a given slab of material is inversely proportional to the X-ray energy. Therefore, a phase grating designed to generate a regular phase shift pattern of height π at a design energy E 2  will result in a phase shift of 2π for half the design energy. Thus, there will be no measurable effect on the monochromatic wave component with energy E 1 =E 2 /2. However, since another grating G 1 ′ is put right behind (or in front of) G 1 , the imaging system also yields phase-contrast information for the first energy E 1 . 
         [0049]    Beating phenomena will be observed in the intensities measured by the detector but the Talbot distance d T  will be the same for both design energies E 1  and E 2 . 
         [0050]    In the following it is assumed that the imaging system has a Talbot distance d T  both for E 1 =E 2 /2 and E 2  by using two phase gratings G 1  and G 1 ′ with pitch p 1  and p 2 , respectively. The phase shift π generated by G 1  for the energy E 2  result in a shift of 2π at the energy E 1 . The Talbot effect for the energy E 1  would be observable at a distance d T =p 2   2 /8λ 1 . The height of G 1 ′ is designed to give a phase shift of π at the energy E 1 . The superposition of two different phase grids at a single location will yield a beating effect in the phase shift with a high frequency corresponding to a pitch of p eff =2p 1 p 2 /(p 1 +p 2 ). Talbot images of this periodic structure will be observed at the same distance d T  if 
         [0000]    
       
         
           
             
               
                 p 
                 2 
               
               
                 p 
                 1 
               
             
             = 
             
               
                 
                   
                     E 
                     2 
                   
                   
                     E 
                     1 
                   
                 
                 - 
                 1. 
               
             
           
         
       
     
         [0051]    For E 2 /E 1 =2 this gives p 2 /p 1 ≅1.8 and p eff /p 1 ≅1.3. The absorption grating may be fabricated with a pitch of p eff /2 to step the fringes generated by the wave fields at E 1  and E 2  individually. The phase retrieval procedure may also be modified in order to account for the beating phenomenon of two monochromatic components. In other words, since two modulated signals corresponding to two energies are superimposed, two phases have to be determined from the intensity modulation. 
         [0052]    Although the method may work best for ratios of design energies of 2, in principle it can also be applied to other ratios with somewhat decreasing fringe visibility. 
         [0053]    Finally, it may also be possible to superimpose the Talbot images of more than two coherent monochromatic components at one and the same distance from the first phase grating by using more than one additional phase grating. 
         [0054]      FIG. 3  shows in imaging system  300  according to an exemplary embodiment of the invention. The imaging system  300  depicted in  FIG. 3  may be adapted as an optical imaging system or an X-ray imaging system, for example a mammography imaging system. The object of interest  103  may be a breast of a patient which is disposed between two pressure plates  301 ,  302  for applying pressure to the patient&#39;s breast. 
         [0055]    The source  101  may be an X-ray source or, for instance, an optical energy source. 
         [0056]    The radiation emitted by the source  101  first passes the grating  107  and then the object of interest to be imaged  103 . After that, the radiation passes the two or more phase gratings  104 ,  105 . The two or more phase gratings  104 ,  105  may be integrated in a corresponding housing  310  and may thus form a module. The module is connected to the control unit  303  such that the gratings  105 ,  104  can be moved upwards and downwards along arrows  308 ,  309 . 
         [0057]    Furthermore, a detector  102  with an absorption grating  106  is provided for detecting the radiation. Both the source  101  and the detector  102  are connected to the control unit  303  via the lines  304 ,  305 , respectively. 
         [0058]    The detector  102  comprises a Talbot interferometer  311 . 
         [0059]    Furthermore, a data line  307  connects the control unit  303  to an input and output device  306 , which can be used for inputting control information for controlling the imaging system  300  and which can also be used for outputting visual information relating to the final image. 
         [0060]    An advantage of the imaging system depicted in  FIG. 3  is that a relatively high portion of the X-ray spectrum can be used for the phase-contrast imaging process by using one or more additional phase gratings. 
         [0061]      FIG. 4  shows a flow-chart of a method according to an exemplary embodiment of the invention. In step  401  a beam of radiation is generated by a source, for example an X-ray source. Then, in step  402  two phase gratings are used for creating two Talbot images corresponding to different energies of the radiation at a Talbot distance from the first phase grating. Then, in step  403 , the final beam is detected after it has passed the object of interest and the two phase gratings. 
         [0062]    It should be noted that the term “comprising” does not exclude other elements or steps and the “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments may be combined. 
         [0063]    It should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims.