Patent Publication Number: US-11391679-B2

Title: Holographic x-ray detection

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority of U.S. Application No. 62/971,677, filed Feb. 7, 2020, which is incorporated herein by reference for all purposes. 
    
    
     BACKGROUND 
     This disclosure relates generally to X-ray imaging. More specifically, the disclosure relates to differential phase contrast (DPC) X-ray imaging. Traditional X-ray imaging is only capable of measuring the absorptivity of an object. Such imaging detects bones but leaves much to be desired when differentiating soft tissues or telling the difference between powders and liquids. DPC X-ray imaging adds the ability to measure the differential phase and small angle scattering properties of materials in addition to absorption. The addition of phase and scatter images provides information about the refractive index and sub-resolution features. DPC scatter imaging has several useful capabilities such as distinguishing between powders and liquids, between healthy lung tissue and damaged, and between homogeneous and heterogeneous materials. Collection of this information is possible because X-rays are absorbed, refracted, and scattered when they pass through an object. Measuring the extent to which an object absorbs, refracts, or scatters light is made possible by spatially patterning the X-ray beam by adding a G1 phase grating. The phase grating causes portions of the X-ray beam to interfere and results in a modulation in X-ray intensity or X-ray fringes. The modulation caused by the G1 phase grating is the result of the interference between the diffractive orders from the G1 grating. The interference of the diffractive orders encodes both the phase shift and absorption of the X-ray beam by the object under inspection. 
     SUMMARY 
     In accordance with the invention, an apparatus for X-ray imaging is provided. An X-ray source provides an X-ray along an X-ray beam path. A holographic medium is along the X-ray beam path. An X-ray phase grating is between the X-ray source and the holographic medium along the X-ray beam path. A readout beam source provides a readout beam along a readout beam path. A readout detector is along the readout beam path, wherein the holographic medium is along the readout beam path. 
     In another manifestation, a method, for detecting a spatially varying X-ray intensity pattern is provided. X-rays are directed into a photorefractive medium. A spatially varying index of refraction modulation induced by the X-ray intensity pattern in the photorefractive medium is read out. 
     The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
         FIG. 1  is a schematic view of a holographic X-ray differential phase contrast detection system of an embodiment. 
         FIG. 2  is a high-level block diagram showing a computer system, which may be used to provide a controller. 
         FIG. 3  is a high level flow chart of an embodiment. 
         FIGS. 4A-E  schematically illustrate how a hologram is formed in a crystal material by X-rays. 
         FIG. 5  is a schematic view of a phase contrast optical microscope that is used in an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS 
     X-ray Differential Phase Contrast (DPC) imaging is the next generation of X-ray imaging in medical applications and non-destructive testing. Traditional X-ray imaging is only capable of measuring the absorptivity of an object. This process makes bones easy to detect but leaves much to be desired when differentiating soft tissues or telling the difference between powders and liquids. X-ray DPC adds in the ability to measure, in addition to absorption, the differential phase and small angle scattering properties of materials. The addition of phase and scatter images provides information about the refractive index and sub-resolution features. DPC scatter imaging has several useful capabilities such as distinguishing between powders and liquids, between healthy lung tissue and damaged, and between homogeneous and heterogeneous materials. The differential phase measurement from DPC provides an additional metric, in conjunction with absorption, for material identification and highlights changes across the image in thickness, density, or material type. Collection of this information is possible because X-rays are absorbed, refracted, and scattered when they pass through an object, just as any other type of electromagnetic wave. Measuring the extent to which an object absorbs, refracts, or scatters light is made possible by spatially patterning the X-ray beam by adding a G1 phase grating. The phase grating causes portions of the X-ray beam to interfere and results in a modulation in X-ray intensity or X-ray fringes. The modulation caused by the G1 phase grating is the result of the interference between the diffractive orders from the G1 grating. The interference of the diffractive orders encodes both the phase shift and absorption of the X-ray beam by the object under inspection. Thus the X-ray fringes are a hologram of the object being imaged. 
     The X-ray fringes are distorted due to the refraction of X-rays by the object being imaged. The extent of the distortion can be measured and results in a differential phase contrast image. The object also simultaneously absorbs some of the X-rays which results in an absorption image. Finally, the object causes some of the X-rays to scatter at small angles with scattering maximized at interfaces. In the example of a uniform sphere, this results in a scatter signal at the edges of the sphere. If the material was fibrous, sponge-like, or made of powder then those internal interfaces would result in a scatter signal from the bulk of the object as well. 
     The state of the art in X-ray DPC measurement is somewhat more complicated due to the lack of ultra high resolution X-ray detectors. When X-rays passing through a reasonably small object on the order of 1 cm are only refracted by a small angle, on the order of 10 −6  radians. This results in the X-ray fringes being distorted on the order of 10 −1  μm. In order to measure such a tiny distortion accurately, the X-ray fringes themselves must be within a couple of orders of magnitude in size to accurately measure the induced distortion, on the order of 1 μm to 10 μm. However, commercially available X-ray detectors are not capable of resolving features smaller than roughly 30 μm at 40 keV without significant (&gt;90%) loss in quantum efficiency. This problem can be partially resolved by adding a G2 absorption grating into the beam line directly in front of the X-ray detector. The G2 absorption grating, in conjunction with the X-ray fringes, forms a Moire pattern on the X-ray detector at a low resolution. Where the X-ray fringes are in phase with the G2 grating, the fringes can pass through the slits and a high intensity is detected by the X-ray detector. Where the X-ray fringes are out of phase with the G2 grating, the X-ray grating absorbs a large fraction of the incident X-rays and the X-ray detector detects low intensity. The use of a G2 grating also, unfortunately, comes with several drawbacks. The G2 grating has a 50% duty cycle, so it results in a 50% signal loss on average. In a medical context, this means the patient is being exposed to twice the radiation dose as is actually being measured by the X-ray detector. The G2 grating must also be precisely aligned and any misalignment or manufacturing irregularity results in image artifacts. Typically, the G2 grating must also be moved back and forth to separate measurements in phase from measurements in absorption as both are presented to the detector as shifts in intensity. These drawbacks of using a G2 grating hinder the adoption of X-ray DPC overall as an imaging technique. 
     X-ray DPC is of critical importance during and after the COVID-19 pandemic due to its ability to more clearly differentiate between healthy and damaged lung tissue. There is substantial evidence that even those who survive COVID-19, but require ventilation, are left with significant lung damage. X-ray DPC will be a critical tool in the early detection of the next respiratory disease, as SARS-CoV-2 is only the latest new virus in a century that has already seen SARS, MERS, and H1N1. However, despite a general acknowledgment of the benefits provided by X-ray DPC imaging, it has not been commonly adopted as a commercial instrument. The roadblock for DPC imaging lies not with the technique itself, but rather the lack of high resolution, high energy, and high quantum efficiency X-ray detectors. 
     The primary limitation in X-ray DPC imaging is not X-ray DPC imaging as a technique, but rather in the lack of sufficiently high resolution X-ray detectors which are also capable of operating at high energies and with high quantum efficiency. There are many methods of X-ray detection but spatially resolved digital X-ray detection is generally divided into direct and indirect X-ray detection. Direct X-ray detection uses a semiconductor material with an applied bias field. When an X-ray interacts with the material a charge cloud is produced which separates in the bias field and drifts to the anode and cathode of the detector. Direct X-ray detectors cannot come close to the desired resolution both due to defects in the semiconductor material and that the material must be roughly two orders of magnitude thicker than the desired resolution in order to maintain high quantum efficiency. The charge clouds diffuse and scatter too much over this thickness to achieve the desired resolution. High resolution X-ray detectors which are commercially available typically use indirect X-ray detection. Indirect X-ray detection uses a scintillator that converts a portion of incident X-ray energy into visible light which can be collected and imaged onto a digitizer such as a photodiode array, CMOS detector, or CCD camera. The high efficiency design must use a thick scintillator to have sufficient stopping power to absorb the majority of incident X-rays. However, this results in most of the scintillator being out of focus for the high NA (numerical aperture) optics, resulting in a blurring on the digitizer. The detector must use high NA optics because only a portion of the incident X-ray energy is converted into visible light. Any light which isn&#39;t collected results in a decrease in signal to noise ratio (SNR) of the detector. The resolution can be improved by using a thinner scintillator. However, then, the majority of X-rays pass through the scintillator without interacting and are never detected. This is a strict tradeoff. Choosing a different scintillation material results in a slightly different trade-off but does not provide the desired improvement. There are efforts to develop better scintillation materials. Micro-columnar structures attempt to bypass the efficiency to resolution trade-off but have met with only limited success. Extended depth of field techniques are not applicable in this case due to the statistical and incoherent properties of the X-ray beam and its interaction with the scintillator. In order for an X-ray detector to be useful for X-ray DPC imaging, it would need to achieve &gt;200 lp/mm and &gt;80% quantum efficiency at 40 keV. Indirect X-ray detectors using LuAG:Ce operating at 200 lp/mm would have only 2% quantum efficiency at 40 keV. 
     An embodiment using Holographic X-ray detection is capable of both high resolution at 208 lp/mm and high quantum efficiency X-ray detection at 80% even for high energy X-rays in the 40 keV range. Holographic X-ray detection used in an embodiment operates in three stages; an X-ray hologram is written into a photorefractive crystal, the recorded hologram is read using visible light, and finally, the hologram is erased using uniform UV illumination. The separation of holographic X-ray detection into these three stages avoids the limitations of indirect X-ray detection since the number of visible photons detected by the digitizer is not limited by the energy of incident X-rays. 
     To facilitate understanding,  FIG. 1  is a schematic view of a holographic X-ray differential phase contrast (DPC) detection system  100 . The holographic X-ray differential phase contrast detection system  100  comprises an X-ray source  104  for providing an X-ray beam  108 . A source amplitude grating  110 , an X-ray phase grating  112 , and a holographic medium  116  are placed along the path of the X-ray beam  108 . An object support  118  supports an object  120  to be imaged between the X-ray phase grating  112  and the holographic medium  116  along the X-ray beam  108  path. 
     A readout beam source  128  provides a readout beam  132 . Readout forming optics  136 , such as lenses or mirrors are along the path of the readout beam  132 . A beam combiner  140 , the holographic medium  116 , an optical mirror  144 , an imaging system  148 , and a readout detector  152  are along the readout beam  132  path. 
     An erasure beam source  160  provides an erasure beam  164  along an erasure beam path. The beam combiner  140  directs the erasure beam  164  to the holographic medium  116 . 
     A controller  172  is configured to be electrically connected to the X-ray source  104 , the readout detector  152 , the erasure beam source  160 , and the readout beam source  128 . In other embodiments, more or fewer components may be configured to be electrically connected to the controller  172 . For example, the readout forming optics  136  and the imaging system  148  may also be configured to be electrically connected to and controlled by the controller  172 . The controller  172  may be a single computer system or separate computer systems. The separate computer systems may be stand-alone or may be networked together with each other or with other computer systems. The controller  172  may be used to control various components, such as the X-ray source  104 , the readout detector  152 , the erasure beam source  160 , the readout forming optics  136 , the imaging system  148 , and the readout beam source  128 . The controller  172  may receive and process data from various components such as the X-ray source  104 , the readout detector  152 , the erasure beam source  160 , the readout forming optics  136 , the imaging system  148 , and the readout beam source  128 . 
       FIG. 2  is a high-level block diagram showing a computer system  200 , which may be used to provide the controller  172 . The computer system may have many physical forms ranging from an integrated circuit, a printed circuit board, and a small handheld device up to a computer. The computer system  200  includes one or more processors  202 , and further can include an electronic display device  204 , a main memory  206  (e.g., random access memory (RAM)), data storage device  208  (e.g., hard disk drive), removable storage device  210  (e.g., optical disk drive), user interface devices  212  (e.g., keyboards, touch screens, keypads, mice or other pointing devices, etc.), and a communication interface  214  (e.g., wireless network interface). The communication interface  214  allows software and data to be transferred between the computer system  200  and external devices via a link. The system may also include a communications infrastructure  216  (e.g., a communications bus, cross-over bar, or network) to which the aforementioned devices/modules are connected. 
     Information transferred via communications interface  214  may be in the form of signals such as electronic, electromagnetic, optical, or other signals capable of being received by communications interface  214 , via a communication link that carries signals and may be implemented using wire, cable, fiber optics, a phone line, a cellular phone link, a radio frequency link, and/or other communication channels. With such a communications interface, it is contemplated that the one or more processors  202  might receive information from a network, or might output information to the network in the course of performing the above-described method steps. Furthermore, method embodiments of the present invention may execute solely upon the processors or may execute over a network such as the Internet in conjunction with remote processors that share a portion of the processing. 
     The term “non-transient computer readable media” is used generally to refer to media such as main memory, secondary memory, removable storage, and storage devices, such as hard disks, flash memory, disk drive memory, CD-ROM, and other forms of persistent memory and shall not be construed to cover transitory subject matter, such as carrier waves or signals. Examples of computer code include machine code, such as produced by a compiler, and files containing higher level code that are executed by a computer using an interpreter. Computer readable media may also be computer code transmitted by a computer data signal embodied in a carrier wave and representing a sequence of instructions that are executable by a processor. 
       FIG. 3  is a high level flow chart of an embodiment. An X-ray DPC holographic image is created in the holographic medium  116  (step  304 ). The X-ray source  104  provides the X-ray beam  108  through the source amplitude grating  110  and the X-ray phase grating  112 , which passes through the object  120  and then to the holographic medium  116 . The X-ray beam  108 , X-ray phase grating  112 , and the object create a holographic DPC X-ray image in the holographic medium  116 . When this step is completed, the irradiation of the holographic medium  116  by the X-ray beam  108  is stopped. 
     After the X-ray DPC holographic image is created, the X-ray DPC holographic image is read out (step  308 ). A readout beam  132  from the readout beam source  128  passes through the holographic medium  116  to the readout detector  152 . Readout forming optics  136  and an imaging system  148  are used to focus and direct the readout beam  132  to allow the reading out of the X-ray DPC holographic image. In this embodiment, the optical mirror  144  is used to bend the readout beam  132  to the readout detector  152 , while allowing the X-ray beam  108  to pass to the holographic medium  116 . The readout detector  152  records the readout beam  132  and sends recorded readout data to the controller  172 , where the controller  172  creates a DPC X-ray image of the object. After the X-ray DPC holographic image readout is completed, the irradiation of the holographic medium  116  by the readout beam  132  may be stopped. 
     After the X-ray DPC holographic image readout is completed, the X-ray DPC holographic image may be erased (step  312 ). The erasure beam source  160  irradiates the holographic medium  116  with the erasure beam  164 . The beam combiner  140  in this embodiment allows both the erasure beam  164  and the readout beam  132  to be directed to the holographic medium  116 . By erasing the X-ray DPC holographic image in the holographic medium  116 , the holographic medium  116  is available to provide another image for the same object or provide an image of another object (step  316 ). In some embodiments, an image of the same object may be made at a different position or angle. 
     In some embodiments, one or more of the steps may be controlled by the controller  172 . The controller  172  may comprise computer readable code for performing one or more or all of the steps. The controller may further comprise computer readable code for generating and displaying an X-ray DPC image generated from the data from the readout detector  152 . The controller  172  may further contain computer readable code for moving at least one of the object  120 , X-ray beam  108 , and holographic medium  116  with respect to each other. 
     In an embodiment, the X-ray phase grating  112  is an absorption grating to avoid interaction effects between the X-ray phase grating  112  performance in the holographic X-ray differential phase contrast detection system  100  and holographic X-ray detector performance. In an embodiment, the readout beam source  128  is a 635 nm laser. The hologram is read out using a 635 nm readout beam  132 , which is aligned and focused to satisfy the Bragg interference condition within the hologram in the holographic medium  116 . The hologram then diffracts a portion of the readout beam  132  which is detected by the readout detector  152 , which in this embodiment is a CMOS camera. The readout beam  132  reads out a spatially varying index of refraction modulation induced by the X-ray. In this embodiment, the erasure beam source  160  is a 340 nm UV light source. The erasure beam  164  is a UV beam. The hologram in the holographic medium  116  can be erased by illuminating the holographic medium, which in this embodiment is a photorefractive medium, such as a photorefractive crystal, with uniform intensity UV light. In this embodiment, the imaging system makes the combination of the imaging system  148  and readout detector  152  act as a variable phase contrast optical microscope. Variable phase contrast optical microscopes are known in the art and are described in more detail in Goodman, Joseph W. Introduction to Fourier optics. Roberts and Company Publishers, 2005. Pages 312-314, which is incorporated by references for all purposes. 
     In this embodiment, the holographic medium  116  is undoped y-cut 500 μm thick lithium niobate. Lithium niobate is a useful material for this application because it is both photorefractive and ferroelectric. The photorefractive property uses the electro-optic effect to form a change in the index of refraction of the crystal when illuminated at photon energies above the bandgap of lithium niobate. This allows X-rays, in the 40 keV range compared with the 3.8 eV bandgap of lithium niobate, to result in a spatially varying shift in the index of refraction. This results in a spatially varying index of refraction modulation induced by the X-ray in the holographic medium  116 . 
     The hologram itself is written by spatially patterned X-rays which interact with the crystal material in the manner depicted in  FIGS. 4A-E , creating an X-ray intensity pattern. First, detected X-rays  408  interact with the semiconductive, photorefractive crystal  404  by generating a high energy electron-hole pair that can down-scatter to form charge clouds as shown in  FIG. 4A-C .  FIG. 4A  schematically illustrates X-rays  408  interacting with the semiconductive, photorefractive crystal  404 . X-rays have greater energy than the bandgap B of the photorefractive medium, 3.8 eV in the case of lithium niobate.  FIG. 4B  shows how the X-rays interact with the semiconductive, photorefractive crystal, exciting electrons  420  across the photorefractive medium&#39;s bandgap  416 , leaving behind high energy holes  424 . An electric field E exists within the semiconductor, photorefractive crystal  404 . The electric field can be either intrinsic due to a ferroelectric material or externally applied. A bandgap  416  energy of 3.8 eV is the minimum energy B needed to excite an electron  420  into a conductive band. All X-rays, by the definition of X-ray as a photon with a wavelength of less than 10 nm, have an energy greater than 124 eV. Thus all incident X-rays, upon interaction with the photorefractive crystal, are capable of imparting sufficient energy to excite an electron across the bandgap  416  of the photorefractive crystal  404 . The excited electrons  420  create corresponding holes  424 . 
       FIG. 4C  is a schematic illustration of how X-ray interactions  432  form electron-hole pairs of electrons  420  and corresponding holes  424 . The charge clouds dissociate in the bias field due to drift.  FIG. 4D  schematically illustrates how high energy charge pairs scatter to form electron charge clouds  440  and hole charge clouds  444 .  FIG. 4E  shows how the electron charge clouds  440  and the hole charge clouds  444  further dissociate and form a space charge field  448  causing localized changes to the electric field and the index of refraction. The charges become trapped in local crystal defects to form a space charge field  448  with an associated spatially varying electric field  FIG. 4E . This spatially varying electric field results in a varying shift in the permittivity along the c-axis of the photorefractive crystal. The hologram forms in real time but is also recorded semi-statically. Experimentally, the resulting hologram showed no observable change in its diffraction efficiency even months after exposure when it was not erased. 
     In this embodiment, the hologram is written with X-rays that have a mean energy of 40 keV using a microfocus X-ray source with an 80 kV accelerating potential. The broad spectrum produced by an X-ray tube means the hologram that is written is effectively a white light hologram with a central wavelength of 0.03 nm. In this embodiment, the hologram is read out using visible light, in order to make use of commonly available optics and photodetectors. Thus a hologram written at 0.03 nm is read using a beam with a wavelength over 4 orders of magnitude longer, at 635 nm. Reading out this hologram is possible by satisfying the Bragg condition for the thick hologram which requires precise alignment of the read out beam with the X-ray fringes. For a set fringe pitch of the X-ray hologram, Λ, the Bragg condition of the thick hologram is satisfied when Λ=λ/2 sin θ, where λ is the wavelength of the readout beam and θ is the relative angle between the X-ray beam and optical readout beam. The hologram is read out using 635 nm light which, according to the previously mentioned equation, should be diffracted at an angle of 0.066 radians. The amount of light that is diffracted is described by the diffraction efficiency. 
     For a thick hologram, the diffraction efficiency, η goes as η=sin 2 (πdΔn/λ cos θ). α is the absorption coefficient of the crystal, d is the thickness of the hologram, λ is the wavelength of the readout beam, and θ is the angle at which the Bragg condition is satisfied. The absorption of the diffracted beam is negligible for undoped lithium niobate at 635 nm. For small diffraction efficiencies, the diffraction efficiency can be approximated using the small angle approximation as η=(πdΔn/λ cos θ) 2 . The definition of the electro-optic effect relative to the change in the index of refraction is
 
Δ(1/π 2 ) ij   =Σr   ijk   E   k   (1)
 
where i, j, and k refer to x, y, and Z. E k  is the electric field along that axis and r ijk  refers to the electro-optic coefficient tensor. Combining the electro-optic induced change to the index of refraction with the diffraction efficiency of a thick hologram yields Δ(E) is linearly proportional to the incoming X-ray fringe intensity ϕ X-ray . Δ(E)=Aϕ X-ray  where A is a constant representing material properties such as the scattering efficiency of the X-ray, mean free path of charge carriers, and predominant carrier type.
 
     
       
         
           
             
               
                 
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     Thus, the linear change in the space-charge field results in a linear change to the diffraction efficiency. This linear dependence would break down at saturation or when the small angle approximation no longer holds true, which occurs at a diffraction efficiency of approximately 30%. Because the diffraction efficiency is relatively low, at around 10 −6 , the detector response is linear as a function of detected X-ray dose. 
     In this embodiment, the hologram stored in the photorefractive crystal can be erased by illuminating the crystal with uniform UV radiation. The UV light has high enough energy to generate freed charges within the crystal. Effectively erasure is the same as writing a new, completely uniform hologram using UV light. This removes non-uniformities in the space charge field and clears the way for recording new X-ray holograms. 
     In an embodiment, directly imaging the hologram requires a variable phase contrast optical microscope. In this embodiment, the microscope is integrated with the rest of the holographic X-ray detector module. 
     The pattern recorded in the holographic medium  116  can be read out using a phase contrast optical microscope. The phase contrast optical microscope transforms information which is encoded in the phase modulation of the readout beam  132  into an amplitude modulation which can be digitized using a CCD, CMOS, or photodiode array device. The shaped readout beam  132 , after passing through the holographic medium  116  is collected using a microscope objective assembly. The microscope objective is designed such that the readout beam  132  is focused at the stop plane of the microscope objective. Also at the stop plane is a phase and amplitude modulator. The phase and amplitude modulator consists of a localized area of optical attenuation and all other areas coated to impart a π/2 or 3π/2 phase shift relative to the attenuated area. In some instantiations, the phase modulation is accomplished using a liquid crystal phase modulator. In some instantiations, the optical attenuation is accomplished using a liquid crystal amplitude modulator. The attenuated area is placed such that the readout beam  132 , when not modified by the hologram, will be focused on that area. Modification by the hologram is meant to include all diffraction, refraction, and reflection which are caused by the hologram. All holographically modified portions of the beam pass through the phase modulated portion of the phase and amplitude modulator. The resulting light can additionally pass through a tube lens to assist with focusing before the light is detected by the readout detector  152 . The holographically modified and subsequently, phase shifted light and the holographically unmodified light which has been attenuated, interfere on the readout detector  152  to form an amplitude modulated image of the recorded X-ray hologram. Linear changes in the phase modulation by the X-ray hologram can thus result in linear changes in detected image amplitude at the readout beam detector. 
     To facilitate understanding,  FIG. 5  is a schematic view of a phase contrast optical microscope  500  that provides more detail to the readout forming optics  136  and imaging system  148  between readout beam source  128  and the readout detector  152 , used in an embodiment. In this embodiment, the readout forming optics  136  comprises a readout forming beam forming lens  508 , along the readout beam  132 . The readout beam  132  passes through the holographic medium  116  to the imaging system  148 . The readout beam  132  passes to a microscope objective assembly  512  of the imaging system  148 . The microscope objective assembly  512  focuses the readout beam  132  at a stop plane  516 . A phase and amplitude modulator  520  is placed at the stop plane  516 . The readout beam  132  passes from the stop plane  516  through a tube lens  528  to the readout detector  152 . 
     The time required for capturing an X-ray image is often limited by the desired signal to noise of the image. A high power X-ray source, capable of providing a higher flux often results in a shorter integration time. 
     The overall speed of this X-ray detector could be very fast. It has been shown that the X-ray generated charge scattering time is on the order of less than 100 fs and hologram formation in lithium niobate can occur in less than 2 ns when written using a high power source. Thus, with a sufficiently high flux X-ray source, the X-ray detector could form a hologram for readout in as little as 2 ns. Readout itself is only limited by the speed of the readout detector  152  as undoped lithium niobate is transparent at 635 nm, so the readout beam has no obvious power limitation. The fastest readout detector  152  available currently is capable of 1 million FPS, which is still far slower than the hologram formation time. Hologram erasure follows the same path as hologram formation and thus should take an equivalent amount of time. Thus, the factor limiting holographic X-ray detection speed is the readout detector  152  speed. 
     An interesting feature of this X-ray detection technique is its relatively low resource requirement. Advances in resolution and efficiency are the product of the overall technique and actually relaxes the requirements on the optics, photo-detector, and X-ray interaction material relative to indirect X-ray detection. For instance, the readout detector  152 , whether that is a CMOS camera, CCD camera, or photodiode array, does not need to have a particularly high quantum efficiency as any decrease in quantum efficiency can be easily compensated for by increasing the power of the readout beam. Undoped lithium niobate is already produced at industrial scales with optical quality, polished wafers that could capture a large field of view. Thus this technique could easily scale to capturing gigapixel X-ray images. 
     Holographic X-ray detection enables differential phase contrast X-ray imaging by achieving both the high resolution and the high quantum efficiency necessary for clinical viability. The X-ray detector in an embodiment achieves a quantum efficiency of 80% at 40 keV while also being able to resolve 208 lp/mm features. For comparison, our calculations indicate that an indirect X-ray detector using LuAG:Ce, capable of resolving 208 lp/mm, would have a quantum efficiency of roughly 2% at 40 keV. Furthermore, the ability to directly image the X-ray fringes which are necessary for X-ray DPC imaging, means that a G2 absorption grating is no longer required which decreases alignment requirement, improves quantum efficiency, and decreases imaging artifacts while providing a G2 free system. The experimental findings previously shown demonstrate that this X-ray detection technique opens several exciting avenues of research using high energy, high resolution, and high efficiency, phase sensitive X-ray imaging. 
     In various embodiments, the holographic (DPC) X-ray detector may be used in systems that use DPC X-ray detection such as a computed tomography system, a mammography system, a chest X-ray system, a dual energy X-ray absorptiometry (DEXA) system, an industrial nondestructive testing system, a food industry analysis system, or a security scanning system. The invention may be used in various high resolution imaging X-ray systems. In various embodiments, the readout detector  152  may be at least one of a CMOS camera, a CCD camera, a photodiode array, or a similar detector. In various embodiments, the holographic medium  116  may be at least one a photoreactive crystal and a photopolymer or similar rewritable photorecordable medium. In various embodiments, the photorefractive crystal may be at least one of a lithium niobate crystal, a barium titanate crystal, a bismuth silicon oxide crystal, a germanium silicon oxide crystal, or a strontium barium niobate crystal. In various embodiments, the readout beam source  128  is an optical light source, where optical light includes infrared, visible, and ultraviolet light. In various embodiments, the erasure beam source may be at least one of a UV light source or an X-ray source, or any beam that has a higher energy than the band gap of the recording medium. In some embodiments, the readout beam source  128  and the readout detector  152  may be separated from the rest of the system or the erasure beam source  160  may be separate from the rest of the system. 
     While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, modifications, and various substitute equivalents, which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, modifications, and various substitute equivalents as fall within the true spirit and scope of the present invention.