Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B6/00—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B6/00—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
  • A61B6/02—Devices for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
  • A61B6/03—Computerised tomographs
  • A61B6/032—Transmission computed tomography [CT]
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B6/00—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
  • A61B6/40—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment with arrangements for generating radiation specially adapted for radiation diagnosis
  • A61B6/4021—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment with arrangements for generating radiation specially adapted for radiation diagnosis involving movement of the focal spot
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B6/00—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
  • A61B6/42—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment with arrangements for detecting radiation specially adapted for radiation diagnosis
  • A61B6/4291—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment with arrangements for detecting radiation specially adapted for radiation diagnosis the detector being combined with a grid or grating
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B6/00—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
  • A61B6/48—Diagnostic techniques
  • A61B6/484—Diagnostic techniques involving phase contrast X-ray imaging
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B6/00—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
  • A61B6/40—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment with arrangements for generating radiation specially adapted for radiation diagnosis
  • A61B6/4064—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment with arrangements for generating radiation specially adapted for radiation diagnosis specially adapted for producing a particular type of beam
  • A61B6/4092—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment with arrangements for generating radiation specially adapted for radiation diagnosis specially adapted for producing a particular type of beam for producing synchrotron radiation

Definitions

• the present invention relates to X-ray image acquisition in general. More particularly, the present invention relates to image acquisition employing phase-contrast.
• the present invention relates to an apparatus for phase-contrast imaging comprising a displaceable X-ray detector element, an X-ray system, a method for acquiring phase-contrast image information and the use of an apparatus for phase-contrast imaging in one of an X-ray system, a CT system and a tomosynthesis system.
• an object to be examined e.g. a patient
• an X-ray generating device or X-ray source e.g. an X-ray tube
• an X-ray detector e.g. an X-ray detector
• a fan-beam or cone-beam is generated by the X-ray source, possibly employing collimation elements, in the direction of the X-ray detector.
• the object to be examined situated in the path of the X-radiation is spatially attenuating the X-ray beam, depending on its inner structure. The spatially attenuated X-radiation is subsequently arriving at the X-ray detector, with the intensity distribution of the X-radiation being determined and subsequently converted to electrical signals for further processing and display of an X-ray image.
• Both the X-ray generating device and the X-ray detector may be mounted on a gantry for rotation about the object to be examined.
• a three-dimensional reconstruction of the objects inner morphology may be obtained.
• a certain object may have only a minor attenuation of X-radiation or differences in attenuation even within different tissues in the inside of the object, thus resulting in a rather uniformly attenuated X-ray image having low contrast and so impeding
• phase-contrast imaging may be employed for visualization of phase information of X-radiation, in particular coherent X-rays, passing an object to be examined.
• phase-contrast imaging may not only determine absorption properties of an object to be examined along a projection line, but also the phase-shift of transmitted X-rays. A detected phase-shift may thus provide additional information that may be employed for contrast enhancement, determining a material composition, possibly resulting in a reduction in X- radiation dosage.
• the use of a cone-beam geometry may constitute a limitation of the usable size of an X-ray detector element, in particular when the phase and/or the absorption gratings are aligned with their trenches parallel to the optical axis.
• the point where the phase-sensitivity drops significantly with respect to the central region of the imaging system is about +-3cm off the optical axis.
• a field of view of under 6 cm, at least in one direction of a two-dimensional X-ray image may be too small to be feasibly reasonable.
• an apparatus for phase-contrast imaging with an increased field of view comprising an apparatus for phase-contrast imaging, a method for acquiring phase-contrast image information and the use of an apparatus for phase-contrast imaging in one of an X-ray system, a CT system and a tomosynthesis system according to the independent claims are provided.
• an apparatus for phase-contrast imaging comprising an X-ray source, an X-ray detector element having a detector size, a first grating element and a second grating element.
• An object is arrangeable between the X-ray source and the X-ray detector and the first grating element and the second grating element are also arrangeable between the X-ray source and the X-ray detector.
• the first grating element, the second grating element and the X-ray detector are operatively coupled such that a phase-contrast image of the object is obtainable.
• the phase- contrast image comprises phase-contrast image information, having a field of view larger than the detector size.
• the X-ray detector element is displaceable, wherein by the displacement of the X-ray detector a phase-contrast image of the field of view is obtainable.
• an X- ray system comprising an apparatus for phase-contrast imaging according to the present invention.
• a method for acquiring phase-contrast image information comprising the steps of acquiring first phase-contrast image information in a first phase stepping state, displacing, tilting and/or rotating an X-ray detector element relative to at least one of an object to be examined and an X-ray source and displacing a first grating element and a second grating element relative to one another.
• Second phase-contrast image information is acquired comprising a second phase stepping state.
• an apparatus for phase-contrast imaging according to the present invention is used in at least one of an X-ray system, a CT system and a tomosynthesis system.
• phase information of an X-ray beam an interferometer may be employed.
• coherent X-radiation passes through an object to be examined subsequently arriving at an X-ray detector. Since phase information may not be measured directly, the implications of a constructive or destructive interaction of two or more wave fronts, possibly resulting in an intensity modulation detectable by an X-ray detector, may be employed.
• An according interference may be obtained by providing a phase-shifting grating or a beam splitter grating between the object to be examined and the X-ray detector.
• X-radiation passing the beam splitter grating thus results in an interference pattern behind the beam splitter grating, containing information about a phase-shift within the X-ray beam in the relative positions of its minima and maxima, i.e. the respective local intensity of the X-ray beam.
• the resulting intensity pattern comprises minima and maxima having a distance typically in the order of several micrometers.
• an X-ray detector may only comprise a resolution in the order of ⁇ 50 to 150 ⁇ and may thus not be able to resolve an accordingly fine structure of the generated interference pattern.
• a phase analyzer grating or absorber grating may be employed, comprising a periodic pattern of transmitting and absorbing strip elements or trench regions and blocking regions, having a periodicity similar to that of the interference pattern.
• an interference pattern may be generated at the location of the analyzer grating, even in the absence of the latter.
• the analyzer grating may thus only be required due to x-ray detector elements employed, which do not provide a spatial resolution high enough to detect the interference pattern or fringes of the beam slitter grating directly. Because of this, the analyzer may be employed. In one phase- stepping position, it lets the fringe maxima pass through to the detector, after transverse displacement, the maxima may be absorbed in the gold trenches.
• a Moire pattern may be generated behind the analyzer grating on the surface of the X-ray detector.
• An according Moire pattern may have a substantially larger periodicity, which may thus be detectable by an X-ray detector having a resolution in the order of 50 to 150 ⁇ .
• the analyzer grating may be required to be shifted laterally, i.e.
• phase stepping An X-ray beam passing through the grating in a single phase stepping instance thus comprises an individual phase stepping state.
• the phase-shift may then be extracted from the intensity modulation observed in the X-ray detector element behind both grids during the phase stepping measured for each position, e.g. for each phase stepping state, of the analyzer grating. Due to an incident angle of the X-rays onto the gratings, the visibility may be seen as decreasing for larger off axes positions with regard to a lateral extension to the trenches of the gratings. To assure sufficient visibility and thus detectability of the x-ray phase by the X-ray detector, a field of view may be limited to the size of about 6 cm, e.g. in case of system lengths, the distance between X-ray source and X-ray detector element, of about lm and energies of about 20-30kVp.
• phase stepping i.e. e.g. 4 or 8 individual image acquisitions having a different phase stepping state, may be required an according movement, displacement, tilt or rotation of an X- ray detector combined with an according phase stepping may be a prolonged process.
• planar detectors may be limited in phase-contrast imaging like e.g. differential phase-contrast mammography.
• a solution to overcome the limitation of the field of view may be employing multi-tile detectors, e.g. detectors comprising multiple detector elements, possibly angled towards one another with respect to the detector plane, and/or scanning the tiles or detector over the field of view, thus acquiring multiple subsequent images constituting a larger field of view.
• An improvement in image quality may be obtained by distributing the total radiation dose over several angular views for improving depth information about the objects inner structure.
• An according technique may be referred to as tomosynthesis.
• An according system may require the X-ray source and the X-ray detector being arranged on a gantry for rotation about the object to be examined.
• phase-contrast imaging a single projection may comprise superimposed structures and thus may also benefit from a tomosynthesis mode of operation. Accordingly, employing a phase-contrast system capable of tomosynthesis may overcome the diminished readability by superposition of anatomical structures.
• Extending the field of view by moving the X-ray detector thus scanning the X- ray detector through the field of view may require performing phase stepping for each position of the X-ray detector within the field of view.
• a phase stepping of 4 or 8 image acquisition steps each having a different phase stepping state may be required.
• the X-ray detector may be displaced to acquire a sub-region of the field of view substantially adjacent to the previous arrangement within the field of view subsequently employing phase stepping with 4 or 8 image acquisition steps for acquiring phase-contrast image information of the second sub-region of the field of view.
• the X-ray detector may not be required to be displaced in the magnitude of the extension or width of the X-ray detector itself, but may rather be displaced only by a fraction, like l/4 or l/8 of the extension of the X-ray detector or its active area for X-ray acquisition, with a concurrent phase stepping to acquire X-ray image information not only of a slightly different sub-region of the field of view, possibly overlapping with the previous sub-region by 3/4 or 7/8 but also having a different phase stepping state required for the subsequent generation of X-ray image information employing phase-contrast.
• An according displacement may be implemented by an additional translational element on either one of the X-ray detector, beam splitter grating or the analyzer grating or even possibly on a further source grating.
• a simple displacement of the X-ray detector, the beam splitter grating and the analyzer grating without altering the relationship of the individual elements relative to one another, may result in the incapability of acquiring image information for phase-contrast imaging.
• the trenches of the gratings may preferably be
• the beam-splitter grating and the analyzer grating may be manufactured from silicon wavers.
• an further electro-plating process may be required, in order to fill the trenches with a highly absorbing material, e.g. gold.
• the manufacturing process may e.g. start with the application of a passivation layer followed by an etching procedure.
• the regions covered by the passivation layer may not be affected by the etching process, thus resulting in a trench pattern typically required.
• the etch direction may depend strongly on the position on the waver, such that the trenches may be focused to a predefined position designed to coincide later with the x-ray source position.
• An according arrangement may be seen as being in particular responsible for reducing visibility of structures when departing from the optical axis in the range of about 6 cm.
• a distance of about 1 m between X-ray source and X-ray detector may limit the detector size to about 6 cm, e.g. in case of about 20-30keV.
• a tiled detector may be employed, possibly comprising individual detector elements being angled towards one another with regard to the X-ray source or the focal spot of an X-ray generating device.
• An according arrangement of detector elements may be seen as orienting the surface normal of the individual detector elements towards, thus in the direction of the X-ray source and its focal spot respectively, at least when considered in a two-dimensional cross-section perpendicular to the trenches of the gratings.
• each detector element can be seen as having its own, individual optical axis being directed towards and aligned with the X-ray source.
• the limitation of the detector size being about 6 cm may be seen as applying to each individual detector element individually.
• employing an X-ray detector being composed of at least two or a plurality of detector element introduces gaps in the detection area between the tiles or detector elements. Within the gaps, image acquisition may not be performed. In certain areas of X-ray image acquisition, e.g. mammography applications, where structures of a few ten to 100 ⁇ are to be detected, loss of image information due to undetectability within gaps between X-ray detector elements may not be acceptable.
• phase stepping may be combined with a movement, e.g. displacement, tilting or rotation of the X-ray detector in combination with the phase stepping.
• each geometrical array may thus only coincide once with a gap of a tiled detector array, thus providing a sufficient number of measurements for each geometrical ray for phase retrieval and for subsequent generation of a phase-contrast image.
• the detector as a whole may be moved with the focal spot as rotation axis, in particular tilted towards the focal spot or X-ray source such that the surface normals of the individual detector elements may be focused towards the focal spot of the X-ray source.
• each geometrical ray coincides only once during the entire acquisition with gaps between the tiles thus phase retrieval may be possible for the entire detector area with the gaps between the detector elements not being visible after image acquisition.
• the acquisition of medical images of diagnostic quality usually requires a gapless coverage of a certain field-of-view surrounding a given object of interest.
• This coverage may automatically be established by employing large-area pixelated detection units, thus X-ray detectors, so that for all neighbouring geometrical rays, falling within the solid angle covered by the detector or at least a detector pixel, imaging information is available.
• This may cede to be the case for several detector elements aligned next to each other, in particular possibly comprising a gap or separation distance.
• a geometrical ray may be considered to be a fixed line in the reference frame attached to the object of interest.
• a geometrical ray may be a fixed line of sight that coincided, in one instance or image acquisition step, with an X-ray detector element pixel, pixel row or pixel column.
• image information e.g. for phase contrast imaging
• the same geometrical ray in particular with respect to the X-ray source and a certain inner structure of the object to be examined, may be required to be acquired.
• An according geometrical ray may in particular have a dimensional extension related to the size of an X-ray detector element pixel, taking further into account a distance between the X-ray source and its focal spot respectively.
• Phase-contrast imaging may be implemented beneficially when employing a coherent X-ray source.
• a coherent X-ray source may in particular only be provided by e.g. a synchrotron, a further grating, a source grating, may be employed between the X-ray source and the object in the beam path of the X-rays for generating a plurality of individual coherent X-ray sources.
• phase-contrast imaging may be performed by also employing two absorption gratings, instead of one phase and one absorption grating.
• a phase stepping in accordance with the present patent application may thus be required as well.
• X-radiation may be required to collimate X-radiation dynamically with regard to a moving X-ray detector element for assuring that only X-radiation detected is allowed to pass through the object.
• each of the grating tiles may be focused along the propagation directions of the x-ray cone-beam.
• further embodiments of the present invention are described referring in particular to the apparatus for phase-contrast imaging.
• the object may also be arranged between a first grating element and a second grating element, in particular between a beam splitter grating and an analyzer grating.
• the displacement of the X-ray detector element may comprise rotating about at least one of the X-ray source and a focal spot of the X-ray source.
• the angle of the X-ray source and/or a focal spot of the X-ray source with regard to the trench structure of the grating elements, in particular the sidewalls of the trench structure remains substantially identical during acquisition of phase contrast image information.
• the angle is substantially zero with regard to the sidewalls of the grating elements, with a cone beam or fan beam of X-radiation impinging directly parallel to the sidewalls of the grating elements, at least with regard to the center of the grating elements.
• the beam splitter grating and the analyzer grating are displaceable relative to one another for providing phase stepping and/or the beam splitter grating and the analyzer grating may be arranged parallel to one another and to the X-ray detector element.
• Employing phase stepping for acquiring individual phase-contrast image information, e.g. intensity modulations during phase stepping, may allow reconstructing a representation of an inner structure of an object to be examined.
• the displacement of the splitter grating and the analyzer grating is preferably such that a plurality of displacements is arranged within one period of a grating. It is in particular beneficial to employ a displacement having e.g. 1/4 or 1/8 of the period of a grating.
• the X- ray source may be displaceable relative to the beam splitter grating, the analyzer grating and/or the X-ray detector element. Furthermore, the X-ray source, the beam splitter grating, the analyzer grating and the X-ray detector element may be rotatable about the object to be examined.
• Such a displacement may be seen as positioning the X-ray source and the focal spot for generating X-radiation respectively in a different alignment with regard to the object to be examined and its inner morphology.
• a tomosynthesis image acquisition may be provided.
• At least one of the first grating element and the second grating element may comprise a trench structure, wherein the trench structure may comprise a first extension parallel to the trench structure and the trenches respectively, and wherein the displacement of the X-ray source may be parallel to the first extension.
• an angle between the X-ray source and the sidewalls of the grating elements may remain substantially unchanged during acquisition of phase contrast image information, at least with regard to a single phase contrast image.
• the X- ray detector element may be displaceable from a first position for acquiring a first sub-region of the field of view to a second position for acquiring a second sub-region of the field of view and the beam splitter grating and the analyzer grating may be displaceable relative to one another for providing a first phase stepping state at the first position and a second phase stepping state at the second position.
• the beam splitter grating and the analyzer grating are further repositioned relative to one another, possibly by a fraction of the period of the grating of one of the beam splitter grating and the analyzer grating for obtaining a difference in phase stepping, thus different phase stepping states when acquiring a first image information at the first position and a second image information at the second position for allowing
• the apparatus further comprises at least two X-ray detector elements, at least two beam splitter gratings and at least two analyzer gratings.
• the at least two X-ray detector elements may be adjacently arranged and may be separated by a separation distance, possibly constituting a gap between the detector element pixels of the at least two detector elements, thus constituting a region or an area that may not be capable of acquiring image information.
• Each of the at least two X-ray detector elements may comprise a surface normal vector in the direction of the X- ray source and in the separation distance no image information may be acquired.
• the field of view of the X-ray detector comprising at least two detector elements may be enlarged, e.g. enlarged over 6 cm.
• the at least two beam splitter gratings and the at least two analyzer gratings may be individual elements or may also be adjacently arranged and thus connected to one another.
• one of a plurality of beam splitter gratings and analyzer gratings constitute a combined element, possibly having individual orientations in parallel to the at least two X-ray detector elements with the other plurality of the beam splitter gratings and the analyzer gratings being individual elements separated from one another and from the respective other grating for allowing a phase stepping.
• the surface normal vector at a center of each of the X-ray detector elements may be considered to point towards the focal spot, at least in a 2dimensional plane parallel to the grating trenches.
• a detector element, a beam splitter grating and/or an analyzer grating may be provided having a cylindrical shape or a spherical shape with the X-ray source being arranged in the focus, thus equidistant to the surface of the X-ray detector element, the beam splitter grating and/or the analyzer grating.
• the size of the at least two detector elements may substantially comprise the field of view of the image to be acquired.
• an image of a field of view larger than a single detector element may be acquired.
• an image having a field of view larger than what would normally be allowable with the respect of the dimensions of the individual detector elements may be acquired.
• the X- ray source may be displaceable about the object to be examined while the orientation of the surface normal vector of the at least two X-ray detector elements in the direction of the X-ray source may be maintained during the displacement of the X-ray source.
• X-ray source when displacing the X-ray source may allow acquiring a tomosynthesis phase- contrast image having a field of view larger than a single detector element, e.g. larger than 6 cm, for example 12, 18, 20, 24 or 30 cm.
• the orientation of the surface normal vector in the direction of the X-ray source may at least be maintained in a two-dimensional plane perpendicular to the grating trenches, thus a plane parallel to the surface normal vector.
• the at least two X-ray detector elements may be displaceable from a first position and/or orientation for acquiring a first phase-contrast image to a second position and/or orientation for acquiring a second phase-contrast image and wherein each of the at least two first grating elements and each of the respective other of the at least two second grating elements may be displaced relative to one another for providing a first phase stepping state when acquiring the first phase-contrast image and a second phase stepping state when acquiring the second phase- contrast image.
• a different phase stepping state may be provided when acquiring a first phase- contrast image and a second phase-contrast image.
• Each of the first grating elements is associated with a respective second grating element for acquiring phase contrast image information.
• the at least two detector elements may be displaced, tilted and/or rotated between acquisition of two different phase-contrast images such that loss of image information that may not be acquired in the separation distance is minimized, in particular wherein the at least two X-ray detector elements may be displaced, tilted and/or rotated during acquisition of a phase contrast image such that each geometrical ray falls maximally into the unavoidable gaps between detector elements.
• the two detector elements may be moved such that each geometrical ray in a successive number of phase-contrast image acquisitions, employing individual phase stepping states, coincides only once of the e.g. 4 or 8 individual image acquisition steps with the gap between the at least two detector elements and the separate distance respectively.
• each geometrical ray in a successive number of phase-contrast image acquisitions, employing individual phase stepping states coincides only once of the e.g. 4 or 8 individual image acquisition steps with the gap between the at least two detector elements and the separate distance respectively.
• 9 different measurements may be required to be performed.
• n different measurements employing different phase stepping states for a respective geometrical ray for each geometrical ray at least n-1 different measurement values may be obtained for subsequently determining a phase contrast image.
• the apparatus may further comprise a source grating.
• a possibly incoherent X-ray source or an at least partially incoherent X-ray source may be employed for phase-contrast imaging.
• steps b; and c may be repeated a defined number of times, in particular are repeated 8 times, constituting an acquisition cycle, wherein each geometrical ray may coincide maximally only once during the acquisition cycle with gaps between the tiles.
• Figs, la-c show an exemplary embodiment of an apparatus for phase-contrast imaging according to the present invention
• Fig. 2 shows an exemplary embodiment of an interference pattern
• Figs. 3a,b show exemplary phase-contrast images acquired according to the present invention
• Fig. 4 shows exemplary visibility of interference fringes versus off axis position of a detector element pixel according to the present invention
• Figs. 5a,b show exemplary embodiments of tomosynthesis according to the present invention
• Figs. 6a,b show a three-dimensional and a two-dimensional representation of an exemplary embodiment of an X-ray detector comprising a plurality of detector elements according to the present invention
• Figs. 7a-d show an exemplary displacement of the focal spot with regard to a tiled X-ray detector according to the present invention
• Fig. 8 shows an exemplary embodiment of a method for acquiring phase- contrast image information according to the present invention.
• FIG. 1 An exemplary embodiment of an apparatus for phase-contrast imaging according to the present invention is depicted.
• Fig. la shows a three-dimensional representation of an exemplary embodiment of an apparatus for phase-contrast imaging.
• a rather large X-ray source 2 is arranged adjacent to a source grating 4. Since X-ray source 2 may be considered to be incoherent due to its size with respect to the wavelength of the radiation emitted, the source grating Go 4 is employed for providing a plurality of single coherent X-ray sources as depicted by the two arrows in Fig. lb.
• X-radiation 5 is emanating from X-ray source 2 in the direction of the optical axis 7 possibly constituting a fan-beam or cone-beam of X-rays.
• the respective shape of the X-ray beam is not depicted in Fig. la.
• X-radiation 5 is arriving at object 6, penetrating object 6, subsequently arriving at a beam splitter grating Gi 8.
• the trenches or gaps of the beam splitter grating 8 alter the phase of passing electromagnetic radiation with respect to the solid areas of the beam splitter grating, the blocking region. Accordingly, a phase shift by ⁇ , in particular by ⁇ , is performed.
• An analyzer grating 10 G 2 is arranged between the beam splitter grating Gi 8 and the X-ray detector 12.
• the distance between the source grating and the beam splitter grating 8 is depicted as 1 whereas the distance between the beam splitter grating 8 and the analyzer grating 10 is depicted as distance d.
• the multiple waves originating from the beam splitter grating 8 Gi in the direction of the X-ray detector are arriving at the analyzer grating 10 G 2 , subsequently producing an intensity modulation pattern (see Fig. 2) on the surface of the X-ray detector 12.
• a plurality of intensity modulations induced by the phase stepping may be obtainable by the image detector 12, since the individual phase stepping states are different between individual phase steppings, i.e. alignment of Gi versus G 2 . Accordingly, by a plurality of Moire patterns, an X-ray image of the object to be examined may be generated.
• Distance 1 may be of the order of 50 - 150cm and distance d may be of the order of 2 - 20cm, depending on the Talbot order chosen in the design of the interferometer.
• Gratings G 0 and G 2 may in particular be filled with gold (Au).
• Gratings Gi and G 2 may be implemented by etching a silicon based material for providing the trenches of the gratings.
• the grating period p 0 of the source grating may be in the order of 200 ⁇ , even smaller, the grating period pi of Gi may exemplary be 4 ⁇ and the grating period p 2 of G 2 may exemplary be 2 ⁇ .
• FIG. 2 an exemplary embodiment of an interference pattern according to the present invention is depicted.
• Fig. 2 depicts an interference pattern created between beam splitter grating Gi 8 and analyzer grating G 2 10, demonstrating the self imaging effect of the grid in characteristic distances di, d 2 and d3 (Talbot effect).
• the relative position of the minima and maxima may in particular depend on the phase-shift of the wave front incident on beam splitter grating Gi. di may in particular be in the order of several cm. If a monochromatic plane wave is incident on the beam-splitter grating that induces a phase shift of ⁇ , in particular by ⁇ , the intensity is split into two main diffraction orders, cancelling the zeroth order.
• the interference effects lead to an effect of self-imaging of the wave-front incident on Gi at discrete distances downstream from Gi.
• the Talbot effect is referred to as the Talbot effect.
• the phase modulation of the incident wave-front induced by Gi is transformed into an intensity modulation with double frequency.
• the analyzer grating samples these modulations and allows to measure the phase-gradient induced by an object onto the x-ray wave-front via phase- stepping.
• exemplary phase-contrast images acquired according to the present invention is depicted.
• exemplary four images are acquired of an object comprising individual bubbles by phase stepping employing four phase steps and thus four individual phase stepping states a-d.
• Distances X1-X4 relate to a displacement of grids Gi versus G 2 for creating an intensity modulation.
• the full movement from X1-X4 is within one period of grating G 2 ( ⁇ 2 ⁇ ).
• the absorber grid or analyzer grid G 2 10 is shifted in a direction x parallel to the grating planes.
• the difference in the wave front phase at two positions "1" and "2" may be extracted from the phase-shift ⁇ ⁇ - ⁇ 2 of the measured intensity modulation, e.g. for four sampling positions X1-X4 in Fig. 3a.
• Fig. 4 exemplary visibility of interference fringes versus off axis position of a detector element pixel according to the present invention is depicted.
• the degradation of the fringe visibility as a function of the off axis position of detector pixels may be taken from Fig. 4.
• a fringe visibility of .5 or greater may be considered to provide reasonable phase-contrast for image generation and processing.
• Three functions are provided in Fig. 4, depending on the height H 2 of the grating structure of grating G 2 (see Fig. lc) providing deeper trenches in the grating, e.g. 35 ⁇ , results in a diminished off axis visibility over a shallower grating depth H 2 of e.g. 15 ⁇ .
• Fig. 4 Three functions are provided in Fig. 4, depending on the height H 2 of the grating structure of grating G 2 (see Fig. lc) providing deeper trenches in the grating, e.g. 35 ⁇ , results in a diminished off axis visibility over a shallower grating depth H 2 of e.g. 15 ⁇ .
• a two- sided collimation has to be below 6 cm, thus ⁇ should be ⁇ 3 cm, thereby limiting the usable size of planar detectors in phase-contrast imaging like e.g. differential phase-contrast mammography, to about 6 cm.
• FIGs. 5a,b exemplary embodiments of tomosynthesis according to the present invention are depicted.
• Figs. 5a,b depict two possible realizations of phase-contrast tomosynthesis.
• the X-ray source 2 or focal spot 14 is displaced in a linear movement 16 about object 6 employing a cone-beam of X-radiation 5 for acquiring different X-ray views through object 6.
• Movement 16 is substantially parallel to the trenches of the gratings employed for phase-contrast imaging, which are not depicted in Figs. 5a,b.
• the X-ray detector 12 Since the X-ray detector 12 has as an extension perpendicular to the trenches of the gratings of e.g. 6 cm, a scanning movement of the X-ray detector through the field of view FOV is required for obtaining an X-ray image of the object 6 which is sufficiently large. E.g. in mammography applications, a field of view of typically 20x30 or 30x40 cm may be required.
• the X-ray source 2 or the focal spot 14 may be considered to be moved independently of the X-ray detector 12, which is only performing the field of view scanning movement as depicted by the arrow in Fig. 5a.
• Both the X-ray source 2/focal spot 14 and the X-ray detector 12 may e.g. be mounted on a gantry for rotation about an axis 18, subsequently rotating both the X-ray source 2 and the X-ray detector 12 about object 6.
• An according movement may be compared to a regular movement in a computed tomography system.
• X-ray tube and X-ray detector are thus rotated simultaneously about object 6.
• a phase stepping is to be implemented for acquiring phase-contrast information.
• the X-ray detector 12 may be displaced substantially about its extension, thus e.g. 6 cm, subsequently providing phase stepping image information acquisition exemplary employing 4, 8 or 9 phase steps or may only be displaced by a fraction of the aforementioned 6 cm, e.g. 1/4, 1/8 or 1/9 of its extension of 6 cm, with an accompanying, simultaneous phase stepping for providing an individual phase stepping state.
• grating Gi may perform the field of view scanning movement slightly faster than the remaining elements of a sliding arm, e.g. by an additional translational element on the detector/Gi/G2 arrangement.
• e.g. grating Gi is displaced by the same distance or angle, depending on a linear or rotational movement, plus an additional ⁇ value for providing a further, new phase stepping state.
• Figs. 6a,b a three-dimensional and a two-dimensional representation of an exemplary embodiment of an X-ray detector comprising a plurality of detector elements according to the present invention is depicted.
• Fig. 6a a tiled X-ray detector comprising exemplary five detector elements 12a-e is depicted.
• X-ray source 2 is emanating a cone-beam of X-rays 5, which, in the case of Fig. 6a, may be considered to constitute substantially the desired field of view.
• Gaps 20 are arranged between the individual detector elements 12a-e, possibly being in the order of magnitude of 1 mm to 100 ⁇ .
• a typical resolution, thus X-ray detector element pixel size, may be seen as 50 to 250 ⁇ .
• Collimation elements may dynamically collimate fan- beam 5 to substantially correspond to the area or current position of the of X-ray detector 12.
• Fig. 6b a two-dimensional image, cross-section along lines A- A' is depicted only showing detector elements 12b-d. Gaps 20 are arranged between detector 12b and c as well as between detector element 12c and d. A surface normal vector 22a,b,c is arranged on each surface of the individual X-ray detector element 12b,c,d in the direction of focal spot 2, possibly crossing focal spot 2. The individual detector elements 12b,c,d are angled towards one another by angles a and ⁇ , which in particular may be identical. Gratings Gi, G 2 and possibly G 0 are not depicted in Figs. 6a,b. As may be seen in Figs. 6a,b, gaps 20 between detector elements 12a-e are arranged such that no image information may be acquired within the gaps.
• FIGs. 7a-d an exemplary displacement of the focal spot with regard to a tiled X-ray detector according to the present invention is depicted.
• focal spot 14/X-ray source 2 is moved linearly for a tomosynthesis acquisition in accordance with Fig. 5a.
• a further rotation in accordance with Fig. 5b may be feasible as well.
• the X-ray detector 12 comprising individual detector elements 12a,b,c is tilted so that surface normal vectors (22a,b,c,) of the individual X-ray detector elements 12a,b,c, are focused towards X-ray source 2, while X-ray source 2 is performing a translatory or linear movement.
• the detector is moved, displaced and/or tilted with respect to the focal spot 14, in particular with the focal spot 14 as rotation axis or tilting axis.
• each geometrical ray coincides only once during the entire acquisition with gaps between the tiles. Accordingly phase retrieval is possible for the entire detector area here comprising detectors 12a,b,c with the gaps subsequently not being visible after phase retrieval in the image so obtained.
• Grids Gi, G 2 and possibly G 0 are not depicted in Figs. 7a-d, however are required for an additional phase stepping between individual image acquisitions 7a,b,c,d as explained earlier.
• a rotatory tomosynthesis movement in accordance with Fig. 5b is feasible as well.
• FIG. 8 an exemplary embodiment of a method for acquiring phase-contrast image information according to the present invention is depicted.
• a method 30 for acquiring phase-contrast image information comprising the steps of acquiring 32 first phase-contrast information in a first phase stepping state, displacing, tilting and/or rotating 34 an X-ray detector element relative to at least one of an object and an X-ray source and displacing a beam splitter grating and an analyzer grating relative to one another and acquiring 36 second phase-contrast image information in a second phase stepping state.
• Steps 34a,b and 36 may be repeated x times, e.g. 8 times, for a total of e.g. 9 acquisition steps with different, individual phase stepping states, for arriving at a full acquisition cycle, in which each geometrical ray coincides maximally only once during the entire acquisition with gaps between the tiles.
• the displacement of the X-ray detector element and the displacement of the beam splitter grating versus the analyzer grating may be performed subsequently or concurrently.

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• 2010
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