Abstract:
In the present invention we propose a new device for x-ray optics which is an analogy to the zone plates but working for higher x-ray energies. This is achieved by using both refraction and diffraction of the x-rays and building the new device(s) in a three dimensional structure, contrary to the zone plates which are basically a two dimensional device. The three dimensional structure is built from a multitude of prisms, utilizing both refraction and diffraction of incoming x-rays to shape the overall x-ray flux. The result will be the first ever device achieving true two dimensional focusing in the x-ray energy range usually employed in medical imaging and may be used in a wide area of applications in this field and in other fields of x-ray imaging. The device will further be fairly straight forward to produce in large volumes.

Description:
TECHNICAL BACKGROUND 
       [0001]    In all imaging systems utilizing visible light, optics is an important tool to increase the performance for the imaging task. The optics can for example enable higher spatial resolution through magnification and also higher fluxes by collecting the light rays. 
         [0002]    In X-ray imaging this is not true, in e.g. medical x-ray imaging there is no x-ray optics in regular clinical use. The explanation is that for energies exceeding around 15 keV the difference in refraction index in any material compared to vacuum is very small, several orders of magnitude smaller than for visible light. This means that any optics is very hard to construct. At lower X-ray energies so called zone plates are successfully used in many applications while at higher energies they become increasingly inefficient and difficult to manufacture. In spite of the challenges some X-ray optics has been tested to work also at higher energies. Examples are grazing incidence optics as described in U.S. Pat. No. 6,949,748 where the x-rays are hitting a curved surface at a very small angle. Other examples are refractive optics as outlined in U.S. Pat. Nos. 6,668,040 and 6,091,798 and also the so called phase array lens as described in B. Cederström, C. Ribbing and M. Lundqvist, “Generalized prism-array lenses for hard X-rays”, J. Sync. Rad, vol 12 (3), pp. 340-344, 2005. 
         [0003]    A summary of state of the art x-ray optics can be found in “Soft X-Rays and Extreme Ultraviolet Radiation—Principles and Applications”, David Attwood ISBN-13: 9780521029971, Cambridge University Press 2007. The optics for higher energies are generally one dimensional which sometimes fits the application, such as imaging using scanning line detectors, but in most cases optics that works in two dimensions is desirable. This can be achieved by crossing two one dimensional lenses, putting one after the other. This however results in a bulky device with compromised performance since the absorption is increased and the two dimensional performance becomes sub-optimum by using one dimensional devices and this may be the reason why these arrangements are not in wide practical use, or in fact, are hardly used at all for any application. 
       SUMMARY 
       [0004]    The present invention overcomes these and other drawbacks of the prior art arrangements. 
         [0005]    In the present invention we propose an analogy to the zone plates but working for higher x-ray energies, normally exceeding 10 keV. This is achieved by using both refraction and diffraction and building the new device(s) in a three dimensional structure, contrary to the zone plates which are basically a two dimensional device. The three dimensional structure is built from a multitude of prisms, utilizing both refraction and diffraction of incoming x-rays to shape the overall x-ray flux. The result will be the first ever device achieving true two dimensional focusing in the x-ray energy range usually employed in medical imaging and may be used in a wide area of applications in this field and in other fields of x-ray imaging. The device will further be fairly straight forward to produce in large volumes. 
         [0006]    In another aspect of the invention, there is provided a method of manufacturing such x-ray optics devices. 
         [0007]    The invention also relates to an x-ray imaging system based on the novel x-ray optics device. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]      FIGS. 1A-C  are schematic diagrams illustrating examples of a new x-ray focusing device together with a cross-section of the device including the multitude of prisms and how they may be arranged relative to each other. 
           [0009]      FIGS. 2A-D  are schematic diagrams illustrating preferred embodiments of the design and manufacturing of a device assembled from a multitude of discs or plates and possible designs for the discs or plates are also outlined including the possibility to manufacture many devices in parallel. 
           [0010]      FIGS. 3A-F  are schematic diagrams illustrating the design and manufacturing of an exemplary embodiment of the device where a thin foil with a prism structure can be rolled to achieve the desired three-dimensional structure. 
           [0011]      FIG. 4  is a schematic block diagram of an x-ray imaging system according to an exemplary embodiment of the invention. 
           [0012]      FIG. 5  is a schematic flow diagram of an exemplary manufacturing method of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    In the following, the present invention will be described with reference to exemplary and non-limiting embodiments of a new x-ray optics device based on a three dimensional prism structure or arrangement utilizing both refraction and diffraction for shaping the incoming x-ray flux. 
         [0014]    In particular, the invention offers a solution to the challenges in state-of-the-art x-ray optics by offering means for efficient two dimensional focusing of x-rays with energy above around 10 keV with a device that is easy to align, handle and produce. 
         [0015]      FIG. 1A  illustrates an example of a device including a multitude of prisms which are traversed by incoming x-rays. The prisms ( 1 A) are preferably arranged in one or more layers along an axis of symmetry, the so called optical axis ( 1 B), and for x-rays entering substantially parallel to the optical axis there will be a focusing effect. The device will also work for x-rays entering the lens which are not entirely parallel to the optical axis, in this case with a slight reduction in the efficiency. As shown in  FIG. 1B , the orientation of the “lens” is preferably such that the flat back of the prisms ( 1 C) is oriented to be substantially parallel to the optical axis, the obtuse corner ( 1 D) is pointing in substantially right angle to the optical axis while the sharp angles ( 1 E) is pointing substantially along the optical axis  1 A. The number of prisms in cross-section (i.e. orthogonal to the optical axis) is changing when moving along the optical axis and a corresponding void is also changing in diameter; the reason is that x-rays further away from the optical axis requires more deflection than x-rays close to the optical axis. The important thing is that the prisms are arranged in such a way to achieve the desired focusing effects which is in turn decided by the amount of material and the number of surfaces traversed by any single x-ray. The three-dimensional prism structure is thus arranged such that x-rays further away from the optical axis will traverse more prisms than x-rays close to the optical axis. The optimum design of the device will depend on the x-ray energy and has to be decided through experiments and/or calculations in each case. 
         [0016]    Typically, mechanical support structures are included to hold the individual prisms. It is beneficial to make the prisms and/or the support structures out of plastic or any other material which is mainly transparent to x-rays. 
         [0017]    It should be understood that the number of prisms is normally relatively large, compared to the schematic diagrams of  FIGS. 1A-B . An example of a more realistic configuration is shown in  FIG. 1C , which illustrates part of an exemplary three-dimensional prism arrangement of the invention. 
         [0018]    As an example, for an optimum effect at around 27 keV the length of each prism ( 1 F) should be around 140 micrometers while the height ( 1 G) should be around 7 micrometers. In a particular exemplary realization, the number of prisms orthogonally to the optical axis may be around 60 and the number of prisms along the optical axis may be around 230, yielding an outer diameter of the device of around 0.5 millimeters and a length of about 33 millimeters, including support structures. One may think that increasing the diameter of the device would yield an increase in the so called aperture and a corresponding increase in collecting incoming x-rays but this is not the case since the absorption will increase towards the edges and approaches one hundred percent. Increasing the diameter beyond what is indicated in the example above for 27 keV will for example not be very useful. 
         [0019]    In general x-ray absorption in the device decreases its efficiency and to minimize this effect a light element of low atomic number should be used, as for example a polymer made of Hydrogen, Oxygen and Carbon. 
         [0020]    The prisms should be fabricated to as high surface finish and form tolerance as possible to work well. 
         [0021]    Since the ideal structure may be hard to manufacture one or more of a number of practical approaches may be taken:
       1) Divide the device in discs or slices along the optical axis.   2) Make these (ideally circular) discs not circular but hexagonal or other shapes. It should thus be understood that the discs are not necessarily circular, but may have other forms.   3) Sub-dividing the discs into sectors.   4) Divide the device in layers orthogonally to the optical axis.   5) Divide the individual prisms in two or more parts to be assembled later.   6) Introduce a radius for the edges of the prisms—they will not be infinitely sharp.   7) Introduce space between the individual prisms and rearrange them while keeping the projected amount of material and the number of prism surfaces traversed as seen by the incoming x-rays.   8) Add material to mechanically support the individual prisms.       
 
         [0030]    In a preferred exemplary embodiment of the device, as mentioned above, it can be built from slices such as discs or plates arranged or assembled side by side along the optical axis according to  FIG. 2A . 
         [0031]    A corresponding cross-section view is illustrated in  FIG. 2B . Each disc preferably has a rotationally symmetric or near-symmetric (e.g. hexagonal) form, and accordingly the overall prism arrangement also has a rotationally symmetric or near-symmetric (e.g. hexagonal) form. The discs arranged along the optical axis are preferably grouped, and the number of prisms (seen in a direction orthogonal to the optical axis) in a first group of discs generally differs from the number of prisms in a second group of discs. In this way, the number of prisms in cross section (i.e. orthogonal to the optical axis) will be different at different positions along the optical axis. In addition, the distance of a given layer of prisms in relation to the optical axis may differ between different discs within a group of discs, as can be seen from  FIG. 2C . 
         [0032]    It should though be understood that the groups, having the same number of prisms in a direction orthogonal to the optical axis, may be re-arranged in any arbitrary order along the optical axis. 
         [0033]    In fact, the discs may optionally be arranged in any arbitrary order, without any concept of groups. 
         [0034]    Each disc may have one or more layers of at least one prism. With many layers, each layer typically has one or more prisms. It is even possible to build discs that contain only a fraction of a prism. Preferably, however, an entire prism or several layers of one or more prisms is/are contained in a disc. Generally, each disc includes at least one layer of at least part of a prism. 
         [0035]    Each disc or plate ( 2 A) can be fabricated through standard techniques such as mechanical tooling, ablation for example with a laser, hot embossing, UV embossing or molding using a master or other methods. It has been recognized that a master for molding may be fabricated through etching in e.g. Silicon or through laser ablation. 
         [0036]    In the magnified cross-section view of  FIG. 2C , a preferred example of a design for mechanical support ( 2 A,  2 B) of the prisms is illustrated. The advantage with this design is that all prisms in a layer is in one peace and not in two or more peaces, which will need alignment later. The different discs or plates can in the assembly process be aligned relative to each other either in an assembly machine or through built-in structures, so called passive alignment, or they may be aligned manually. A great advantage with this manufacturing process is that many individual “lenses” or x-ray optics devices can be fabricated in parallel as indicated in  FIG. 2D . As illustrated in  FIG. 2D , a number of independent discs are produced on a common substrate. It is possible to produce two or more x-ray optics devices in parallel by stacking a number of such substrates in proper alignment and mechanically attaching them and finally extracting individual three-dimensional prism structures.  FIG. 2D  also illustrates the principle of constructing the prisms in several (e.g. two) pieces that will subsequently be assembled in order to provide a full prism or one or more layers of full prisms. 
         [0037]    Another embodiment of the invention is based on preparing a thin foil with a layer of prisms as illustrated in  FIG. 3A . The advantage with this method it that it is easy to manufacture a film or similar thin substrate with the desired structure since the height of the prisms above the film is relatively small. The prisms on the foil may for example be manufactured through hot embossing or UV embossing. For example, the prisms may be manufactured by embossing from a laser-abladed, etched or machined master, and then arranged on the foil. Alternatively, the prisms may be formed directly into the foil by any of the above-mentioned methods (e.g. laser ablation, etching, machining). Preferably, the foil is of the same type as now used for holography. There exist commercial foils for embossing that are used for hologram markings on e.g. credit cards. Before rolling the foil it is preferably cut in a general diagonally curved form (see  FIG. 3F ), preferably into a stair-like structure (see  FIGS. 3B and 3F ), in order to obtain the desired three-dimensional structure (when rolled). The foil is subsequently rolled, for example into a cylindrical or similar rotationally symmetric or near-symmetric structure according to  FIG. 3C , in order to assume the desired shape of the device (see  FIG. 3D ). After the rolling is completed the foil is fixed with for example glue. The rolling can be performed manually under a microscope or in dedicated machines. As can be seen from the cross-section view of  FIG. 3E , the cross-section number of prisms (i.e. the number of prisms stacked orthogonal to the optical axis) will differ at different positions along the optical axis. Preferably, with the manufacturing procedure of  FIGS. 3A-F , the number of prisms in cross section of the device will change successively along the optical axis. 
         [0038]      FIG. 4  is a schematic block diagram of an x-ray imaging system using an x-ray optics device of the present invention. The x-ray imaging system basically comprises an x-ray source ( 4 A), x-ray optics ( 4 B) and a detector ( 4 C) connectable to image processing circuitry ( 4 D). The x-ray optics, and more particularly the optical axis of the three-dimensional prism structure, is preferably aligned with the general direction of incoming x-rays from the x-ray source. In particular the x-ray optics comprises a three dimensional structure of a multitude of prisms for both refraction and diffraction of incoming x-rays in order to focus radiation from the x-ray source. The detector is configured for registering radiation from the x-ray source that has been focused by said x-ray optics and has passed an object ( 4 E) to be imaged. The detector is preferably connectable to image processing circuitry to obtain a useful image. The imaging system may for example be used for medical imaging, e.g. to obtain diagnostic images. 
         [0039]    In a preferred exemplary embodiment of the invention, the prisms are arranged in at least one layer along an optical axis for incoming x-rays to achieve the desired focusing effect. Advantageously, the three-dimensional prism structure is arranged such that x-rays further away from the optical axis will traverse more prisms than x-rays close to the optical axis. Specific embodiments of the prism structure that can be used have been discussed above. 
         [0040]      FIG. 5  is a schematic flow diagram of a method for manufacturing an x-ray optics device. In step S 1 , a multitude of prisms is provided. In step S 2  the prisms are arranged in at least one layer along an optical axis for incoming x-rays to provide a three-dimensional prism structure for both refraction and diffraction of x-rays to shape the x-ray flux. The overall manufacturing procedure covers different methods including that described above in connection with  FIGS. 2A-D  as well as that described in connection with  FIGS. 3A-F . For example, a number of discs, each having at least one layer of prisms, may: be assembled side by side in alignment along the optical axis to form the three-dimensional prism structure. Alternatively, it is possible to prepare a foil containing the prisms, and then rolling the foil into the three-dimensional prism structure. 
         [0041]    The embodiments described above are merely given as examples, and it should be understood that the present invention is not limited thereto. Further modifications, changes and improvements which retain the basic underlying principles disclosed and claimed herein are within the scope of the invention.