Abstract:
A phase-contrast imaging system and method. An embodiment of the invention includes a plurality of X-ray emitters for transmitting X-rays through an object to a detector. Adjacent X-ray emitters may be activated at different times to prevent confounding of X-ray striking on the detector. Each X-ray emitter can be operated independently to provide different flux outputs for reducing overall patient dose.

Description:
FIELD 
       [0001]    The invention relates to an X-ray radiographic imaging system and method, and more particularly, to an X-ray radiographic imaging system and method capable of imaging soft tissue. 
       BACKGROUND 
       [0002]    X-ray radiography is an imaging technique whereby X-ray radiation is applied to a patient or an object to produce images of its internal structures (on film or digital media). Conventional x-ray radiography has limited utility in discriminating between soft tissues with similar attenuation coefficients, thus making it less suited for imaging of this type of tissue. Specifically, this effect occurs because of the subtle differences in energy-dependent mass attenuation coefficients for various soft tissue types. These differences decrease at higher X-ray energies, thereby making it difficult to measure these differences accurately. 
         [0003]    Mathematical modeling of the interaction of X-rays with matter utilizes a construct known as the complex index of refraction, which comprises a real component modeling refractive characteristics of X-ray radiation and an imaginary component modeling absorptive characteristics of X-ray radiation. Depending upon the material type, thickness, and the spectrum of the applied X-ray radiation, the refractive component of the complex index of refraction may provide more and better information for identifying subtle differences in tissues properties at X-ray energies than conventional absorption imaging methods. Conventional radiography is sensitive to large differences in absorptive properties of X-rays, such as those between bones and tissue. For example, an X-ray image of a head will clearly reveal the bones of the skull, since they absorb much radiation. The image will not, however, reveal much of the internal brain structure, which will be presented as a relatively featureless region on the X-ray image. Unlike conventional radiography, which is based on the absorption of X-rays, phase-sensitive imaging has the potential to distinguish various types of soft tissue such as muscles and tendons, all in high contrast. 
         [0004]    With higher soft tissue contrast found in phase-sensitive imaging, imaged features within the scanned volume may be more clearly distinguished, including any tissue abnormalities such as the presence of tumorous tissue. Thus, phase-sensitive imaging has the potential to reveal the size and position of, for example, a tumor at an early stage, enabling doctors to determine the right treatment, including one or more of drug therapy, needle biopsy, and the appropriate dosage of radiation therapy. 
         [0005]    Since the real component (refractive component) of the complex index of refraction of materials is close to unity, it is typically characterized by 1−δ, where δ is the difference from unity. For nearly all elements in the periodic table, delta (δ) is larger than the imaginary part beta (β), where the complex index of refraction n is defined as n=1−δ−iβ. Data for breast tissue is shown in  FIG. 1 . Lewis et al., “Medical phase contrast x-ray imaging: current status and future prospects,” Phys. Med. Biol, Vol. 49, pp. 3573-3583, 2004. Here, δ is 10 3  to 10 5  times larger than the imaginary part β for X-ray photon energies over the range 20-150 keV that are typically used for diagnostic medical imaging. Although it is tempting to postulate that a large ratio of delta to beta will result in significant contrast boost in an image, one has to realize that beta is a signal absorptive term. Hence, the tissue contrast provided in an image depends on the tissue type that is imaged, the thickness of the tissue, and the spectrum of the applied X-ray radiation. The large difference between δ and β is true for most materials, including, for example, breast tissue, across a range of energy spectra from 20 keV to 150 keV. Thus, phase-sensitive imaging methods may be more sensitive to soft tissues than attenuation-based imaging methods. Since phase-sensitive imaging methods detect complementary information relative to standard attenuation imaging methods, and may provide higher soft tissue contrast, phase-sensitive imaging may provide the opportunity to expose persons to lower X-ray dosage. 
         [0006]    It is possible to simultaneously measure both absorption and phase shifts of X-rays due to attenuation and refraction, respectively. Like visible light and all electromagnetic radiation, X-rays can be regarded as both particles and waves. Conventional absorption-based radiography records the extent to which X-rays penetrate anatomy or not. Phase-sensitive imaging measures the extent to which the X-ray wavefront is modified with respect to its original position via passing through an object, due to refractive properties of the object. This phase shift is very revealing because it varies depending on the nature of the tissue through which the radiation is refracted. However, conventional X-ray imaging methods are very insensitive to the phase-shift of X-rays; therefore, different detection methods are required. 
         [0007]    Phase-contrast imaging (PCI) is a process for producing images on film or digital media using X-ray radiation, thereby visualizing the refractive properties of the imaged object or tissue. Several phase-sensitive imaging methods have been developed and are known to those skilled in the art, such as the propagation-based method, the interference method, the diffraction enhanced imaging method, and the X-ray differential phase-contrast imaging method. Such PCI processes are thus useful in medical diagnostic imaging techniques, such as, for example, mammography. 
         [0008]      FIGS. 2 and 3  schematically illustrate a known PCI system  10 . The PCI system  10  includes a potentially incoherent X-ray source  15 , having a width w, which transmits X-ray radiation  17  through an object  20 . The X-ray radiation  17  propagates through the object  20  and onto an imaging detector  25 . For film-based PCI systems, the imaging detector  25  is in communication with a processor which processes the film. For digital PCI systems, the imaging detector  25  is in communication with a processor which processes the data obtained by the imaging detector  25  to formulate images of regions of interest of the object  20 . 
         [0009]    The X-rays from the X-ray source are partially transmitted through an absorbing grating  30 , which may be formed by an alternating pattern of low- and high-attenuation materials (denoted as lines or rulings) such as silicon and gold, respectively. The thickness of each high-attenuation, individual line, or ruling, is sufficient to absorb the incident X-rays. The use of absorbing grating  30  provides a mechanism to generate pseudo-coherent wavefronts of electromagnetic radiation, thereby allowing the use of a standard X-ray source instead of a synchrotron. 
         [0010]    Absorbing grating  30  creates an array of individually coherent but mutually incoherent secondary X-ray sources. If the width w of the primary X-ray source  15  is sufficiently small, such that the X-ray source  15  is coherent itself, the absorbing grating  30  may be removed from the system  10 . Impingement of the X-rays  17  on the object  20  creates a slight refraction α of each of the coherent subsets of X-rays  17 . The refraction amount is proportional to the local differential phase gradient of the object  20 . A small angular deviation of the transmitted X-rays resulting from the refraction a results in a change of the locally transmitted intensity through the combination of gratings  35  and  40 . Grating  35  is a non-absorbing phase grating, formed of individual lines or rulings comprising silicon or nickel or other material having low attenuating properties while creating large phase shifts. Alternatively, grating  35  is an absorption grating. Although shown as being positioned between the object and grating  40 , grating  35  may be positioned between grating  30  and object  20 . Grating  40  is an absorbing grating, formed of individual grates comprising an alternating pattern of low- and high-attenuation materials, such as silicon and gold. This grating allows improved sampling of the phase-contrast signal utilizing detectors with relatively coarse resolution by repeated stepping of the grate and measurement of the detector signal. For detectors capable of completely resolving the phase-contrast signal, the grating  40  can be removed from the system  10  and the stepping procedure is then not required in the measurement procedure. 
         [0011]    The distance from the grating  30  to the grating  35  is l, and the distance from the grating  35  to the grading  40  is d. The measurement from the midpoint of one high-attenuation line or ruling of absorbing grating  30  to the midpoint of an adjacent high-attenuation line or ruling is the grating pitch p 0 . The grating pitch of non-absorbing grate  35  is p 1 ; the grating pitch of absorbing grate  40  is p 2 . 
         [0012]    To obtain high quality images of the object  20  at the detector  25 , it is necessary for each of the coherent subsets of X-rays  17  to contribute constructively to the image-formation process at the detector  25 . For that to occur, a geometry of the system  10  should satisfy the equation: 
         [0000]      p 0 =p 2   ×l/d.    
         [0013]    One disadvantage of the PCI system  10  is its size. Since practical gratings and detectors are planar in shape, the preferred X-ray beam is a plane-wave. The plane-wave is approximated by locating a conventional X-ray tube  15  at a relatively large distance from the object; a typical source-to-detector distance may be between about  150  and about 200 centimeters (cm). Such a distance is considerably longer than the source-to-object distance of approximately 65 cm, which is typical of the distance found in traditional mammography systems. Another disadvantage of the PCI system  10 , for practical use in diagnostic medical imaging, is the limited field-of-view (FOV). The FOV of the PCI system  10  is about five to six centimeters wide and about two centimeters in height. 
         [0014]    It is desired to implement an improved phase-contrast imaging system and method. Such an improved PCI system would desirably reduce the overall size of the system as well as increase the imaging field of view of known PCI systems. 
       SUMMARY 
       [0015]    An embodiment of the invention provides a phase-contrast imaging system having a plurality of X-ray emitters. 
         [0016]    One aspect of the invention provides a phase-contrast imaging system that includes a detector and a non-absorbing grating positioned between the object to be imaged and the detector or between the plurality of X-ray emitters and the object to be imaged. 
         [0017]    An embodiment of the invention provides a method for phase-contrast imaging an object that includes transmitting X-rays from a plurality of X-ray emitters through an object to a detector. 
         [0018]    One aspect of the invention provides a method for phase-contrast imaging that includes optionally transmitting the X-rays through the first absorbing grating into the object, propagating the X-rays through the object, and transmitting the X-rays through a central non-absorbing or absorbing grating, and optionally through a second absorbing grating to the detector. 
         [0019]    These and other features, aspects and advantages of the present invention may be further understood and/or illustrated when the following detailed description is considered along with the attached drawings. 
         [0020]    DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a graph plotting a parameter of the real part and the imaginary part of the complex index of refraction against energy. Lewis et al., “Medical phase contrast x-ray imaging: current status and future prospects,” Phys. Med. Biol, Vol. 49, pp. 3573-3583, 2004. 
         [0022]      FIG. 2  is a schematic view of a known phase-contrast imaging system. Pfeiffer et al., “Phase retrieval and differential phase contrast imaging with low-brilliance X-ray sources,” Nature Physics, Vol. 2, pp. 258-261, 2006. 
         [0023]      FIG. 3  is a schematic top view of the phase-contrast imaging system of  FIG. 2 . Pfeiffer et al., “Phase retrieval and differential phase contrast imaging with low-brilliance X-ray sources,” Nature Physics, Vol. 2, pp. 258-261, 2006. 
         [0024]      FIG. 4  is a schematic top view of a phase-contrast imaging system in accordance with an embodiment of the invention. 
         [0025]      FIG. 5  is a graph plotting image score against mean glandular dose. 
         [0026]      FIG. 6  is a graph plotting absorption and exposure of radiation energy against operating energy. 
         [0027]      FIG. 7  illustrates a process for phase-contrast imaging an object in accordance with an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION 
       [0028]    The present specification provides certain definitions and methods to better define the embodiments and aspects of the invention and to guide those of ordinary skill in the art in the practice of its fabrication. Provision, or lack of the provision, of a definition for a particular term or phrase is not meant to imply any particular importance, or lack thereof; rather, and unless otherwise noted, terms are to be understood according to conventional usage by those of ordinary skill in the relevant art. 
         [0029]    Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. The terms “first”, “second”, and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, the terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item, and the terms “front”, “back”, “bottom”, and/or “top”, unless otherwise noted, are merely used for convenience of description, and are not limited to any one position or spatial orientation. If ranges are disclosed, the endpoints of all ranges directed to the same component or property are inclusive and independently combinable (e.g., ranges of “up to about 25 wt. %, or, more specifically, about 5 wt. % to about 20 wt. %,” is inclusive of the endpoints and all intermediate values of the ranges of “about 5 wt. % to about 25 wt. %,” etc.). 
         [0030]    The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., includes the degree of error associated with measurement of the particular quantity). Reference throughout the specification to “one embodiment”, “another embodiment”, “an embodiment”, and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and may or may not be present in other embodiments. In addition, it is to be understood that the described inventive features may be combined in any suitable manner in the various embodiments. 
         [0031]    A phase-contrast imaging (PCI) system  100  illustrated in  FIG. 4  includes an X-ray source which transmits X-rays, potentially through an absorbing grating  30  to an object  20 . After propagating through the object  20 , the X-rays extend through non-absorbing or absorbing central grating  35  and potentially through absorbing grating  40  to X-ray detector  25 . Alternatively, the non-absorbing grating  35  may be located between the X-ray source and the object  20 . 
         [0032]    The X-ray source of PCI system  100  includes an array  110  of X-ray focal spots or emitters. The X-ray spots can be of any type of X-ray emitting device, including but not limited to X-rays generated from electron beams provided by tungsten filaments, cold-cathode emission devices, field emitters, and carbon nanotubes, comprising both reflection or transmission sources. In one embodiment of a PCI system  100  for use in mammography operations, the individual emitters in the array  110  may have a width of about 0.3 millimeters. In one embodiment, the pitch between emitters is approximately 1.3 centimeters. For a linear array of 12 emitters having an individual emitter size of 0.3 centimeters and a distance of 1.3 centimeters between spots provides a field-of-view of approximately 20 cm at the array  110 . 
         [0033]    In one embodiment, the array  110  has a first set of X-ray focal spots  112  interleaved with a second set of X-ray focal spots  114 . Each set  112  includes  112   a  to  112   n  number of X-ray focal spots. The X-ray focal spots  112   a  through  112   n  emit, respectively, X-rays  117   a  through  117   n.  Each set  114  includes  114   a  to  114   n  number of X-ray focal spots. The X-ray focal spots  114   a  through  114   n  emit, respectively, X-rays  119   a  through  119   n.  It should be understood that the X-ray flux of each individual focal spot can be varied individually, i.e. different spots of the same set of focal spots can be operated to provide different X-ray intensities on portions of object  20 . This allows adaptation of the emitted radiation to the patient, thereby achieving optimal image quality at the lowest possible patient dose. Moreover, individual X-ray focal spots  112   a  to  112   n,  or  114   a  to  114   n,  may be operated simultaneously, or they may be operated sequentially. Both sets of X-ray focal spots are identified in  FIG. 4 . This is only one possible embodiment; two or more subsets of X-ray focal spots may be identified. 
         [0034]    Although the array  110  is shown to be in one dimension, it should be understood that the array may be arranged in two dimensions. In one embodiment, the array  110  comprises a linear array of approximately 10 emitters. In another embodiment, the array  110  comprises a two dimensional array of approximately 16 emitters. It should be understood that the number of emitters can be two or more. It should be further understood that a two dimensional array may have three or more emitters, and may be formed in a 2×2, 3×3, 4×4, etc. square array or an interleaved 1×2 triangular array or an interleaved 2×3, 2×4, etc. rectangular array. In addition, the array  110  may be formed in a non-planar fashion, for example, curved in one direction. 
         [0035]    Furthermore, each emitter may include a microfocus or an array of individual sub-sources. Each of the sub-sources is individually coherent but mutually incoherent to the other sub-sources. The array of sub-sources may be generated by placing an array of slits, i.e. an additional amplitude grating close to the source or creating an array of sub-microfoci (for examples with carbon nanotubes). 
         [0036]    For a PCI system  100  to be used in mammography, the high-attenuation lines or grates of grating  30  are made of a certain material and thickness to block approximately all of the X-rays incident on the lines accounting for a total blockage of 50 percent or more of X-rays incident on the grating  30 . 
         [0037]    As shown in  FIG. 4 , there is an overlap of emitted X-rays  117 ,  119  between adjacent X-ray focal spots  112 ,  114 . For example, X-rays  117   a  overlap with X-rays  119   a  and X-rays  117   n  overlap with X-rays  119   n.  The sequenced operation of X-ray focal spots  112  and  114  will be described below. 
         [0038]    The utilization of numerous X-ray focal spots overcomes the deficiency in conventional PCI systems of a limited FOV. In forming a phase-contrast image, each X-ray focal spot can be considered independently. Further, data can be acquired at the detector  25  when several X-ray focal spots are emitting. 
         [0039]    Additionally, the PCI system  100  overcomes the deficiency of conventional PCI systems, such as PCI system  10 , in that the distance from the emitters  110  to the detector  25  can be reduced from the 100 to 200 centimeters found in PCI system  10  to the distance found in conventional mammography systems. Lateral dimensions, such as focal spot width w and the grating pitches p i  are also scaled in proportion. 
         [0040]    Abutment or overlapping of adjacent X-ray emissions is necessary to ensure complete coverage of the object being imaged. To alleviate any potential confusion regarding the data signals at the detector from multiple X-ray emissions, however, one embodiment has adjacent X-ray focal spots operating at separate times. For example, X-ray focal spots  112  emit at a first time and X-ray focal spots  114  emit at a second time different than the first time. Specifically, X-ray focal spots  112 , including  112   a,  are operated and emit X-rays  117 , including X-rays  117   a.  X-rays  117   a  impinge, after transmission through the grating  30 , object  20 , and gratings  35  and  40 , on section  25   a  of the detector  25 . Data is acquired from section  25   a  of the detector by a processor (not shown). After the readout to the processor, X-ray focal spots  114 , including X-ray focal spot  114   a  are operated. X-rays  119   b  impinge on section  25   b  of the detector  25 . As illustrated, section  25   b  overlaps with section  25   a  of the detector  25 . It should be understood that the emitters  110  and gratings  30 ,  35 ,  40  and detector  25  can be positioned relative to one another such that adjacent sections of the detector  25 , like sections  25   a  and  25   b,  abut one another instead of overlap one another. 
         [0041]    After a full cycle of operation of all the X-ray focal spots, the grating  40  may be moved relative to the detector  25  and another full cycle of operation of the X-ray focal spots is performed. The movement of the grating  40  is a small distance, based upon the equation p 2 /n, where n equals the desired oversampling of the phase-contrast signal. It is important for the detector  25  to be able to detect the phase modulation, and one option for that is utilizing the absorbing grating with stepping. As mentioned previously, if a detector with suitable resolution to sample the phase-contrast signal is available, grating  40  can be eliminated and only one data collection is needed for each set of X-ray focal spots. 
         [0042]    Referring now to  FIG. 5 , conventional mammography operates at a low X-ray photon energy value, typically 10-40 keV, where the absorption contrast between different soft tissues is larger. At this lower energy level, the absorption contrast is higher. Since PCI systems, such as PCI system  100 , do not operate on an X-ray absorption basis but instead operate on an X-ray phase-contrast basis, PCI systems can operate at higher energy levels, such as 60 keV. At such a level, the absorbed dose is lower leading to less exposure to harmful ionizing radiation for a patient. Further, as indicated in  FIG. 5 , experience with diffraction-enhanced imaging, which is a particular type of phase-contrast imaging indicates that radiologists are able to detect features in images from phase-contrast imaging at a much lower X-ray dose, in comparison with conventional absorption x-ray images. The top plot in  FIG. 6  show the energy-dependent absorption of various tissue types: adipose tissue, breast tissue, muscle and blood. The bottom plot in  FIG. 6  shows the incremental dose per flux density as a function of photon energy. For the bottom plot, one can see the incremental dose per flux density is minimized at an approximate X-ray energy of 60 keV. 
         [0043]    Referring now to  FIG. 7 , a method is described for imaging an object, such as a patient, with a PCI imaging system, such as PCI system  100 . At Step  200 , the object is positioned at a location between a plurality of X-ray emitters and a central non-absorbing or absorbing grating. When present in the system, the first absorbing grating is positioned relative to a plurality of X-ray emitters and the high-attenuation lines or grates are manufactured so as to block more than 50 percent of the emitted X-rays. Ideally, all X-rays that impinge upon the high-attenuation (absorbing) part of the grating are attenuated, and all X-rays impinging on the low-attenuation lines or grates are transmitted. 
         [0044]    At Step  205 , the plurality of X-ray emitters transmit X-rays into the object, potentially through a first absorbing grating, which absorbs some of the X-rays allowing the remainder to be transmitted into the object. Step  205  may be performed numerous times. For example, the plurality of X-ray emitters may be divided up into a first set of emitters interleaved with a second set of emitters. The first set of emitters may fire at a first time and the second set of emitters may fire at a second time different than the first time. 
         [0045]    At Step  210 , the X-rays propagate through the object and continue through the non-absorbing or absorbing grating, and potentially through a second absorbing grating, to a detector. The plurality of X-ray emitters are positioned relative to one another and relative to the gratings and the detector such that impingement of X-rays from adjacent emitters at least abuts one another at the detector. Specifically, the X-rays from one emitter will strike the detector at a first detector portion and the X-rays from an adjacent emitter will strike the detector at a second detector portion. The first and second detector portions will at least abut one another but may overlap one another. Since confusion at the detector over the origin of signals is to be avoided, adjacent emitters likely should fire at different time periods if the respective X-ray impingement areas will overlap at the detector. Like Step  205 , Step  210  may be performed numerous times. 
         [0046]    At Step  215 , the second absorbing grating, when present in the system, may be moved relative to the detector and Steps  205  and  210  may be performed again. Alternatively, the central grating or the source grating, when present in the system, may instead be moved and then Steps  205  and  210  may be performed again. It should be appreciated that there are, in principle, several alternatives to this step which accomplish the same goal. Step  215  as described is not meant to be limiting, but comprises one mechanism for sampling the phase-contrast signal, as is known to those skilled in the art. The multiple imaging steps are performed to collect data formed at the detector that is used to construct the image that is presented to the radiologist. The above steps may further be repeated for different positions of the system with respect to the patient in order to perform tomosynthesis or tomography. For example, the process can be repeated at multiple angles of the source  15 , multiple angles of the gratings  30 ,  35 ,  40 , and multiple angles of the detector  25  relative to object  20  to reconstruct volumetric phase-contrast computed tomography images. As with Steps  205  and  210 , Step  215  may be performed numerous times. Finally, at Step  220  the signals from the detector are forwarded to a processor to formulate the phase-contrast images of the object. 
         [0047]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. For example, while embodiments have been described in terms that may initially connote singularity, it should be appreciated that multiple components may be utilized. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.