Abstract:
An antiscatter grid for radiological imaging, the grid formed as a stack of two or more sheets having a flexible substrate, wherein each sheet has spaced-apart opaque cavities, each opaque cavity containing a radio-opaque material. The opaque cavities define a plurality of channels that extend through the sheets to allow ionizing radiation to pass therethrough. Magnets disposed along the sheets couple each sheet to one or more neighboring sheets of the stack.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to the field of diagnostic imaging and more particularly to apparatus and methods for providing a flexible grid to reduce scatter effects in a radiographic image. 
       BACKGROUND OF THE INVENTION 
       [0002]    Scattered radiation presents a particular challenge for radiographic imaging. In some cases, scatter can significantly reduce subject contrast, making it difficult to discern various anatomical features in the radiographic image. 
         [0003]    Linear grids have been devised in order to help correct this problem. Linear grids are antiscatter devices that are used to improve contrast and to improve the signal to noise ratio in radiographic images. A conventional antiscatter grid typically consists of a series of lead foil strips separated by spacers that are transmissive to x-rays. The spacing of the strips determines the grid frequency, and the height-to-interspacing distance between lead strips determines the grid aspect ratio. Grids can be oriented horizontally or vertically relative to the imaging medium. 
         [0004]    There are two general types of antiscatter grids: moving (Bucky-Potter configuration) and stationary. For moving type grids, the shadows of the lead strips are blurred out by motion, which can be either reciprocating or unidirectional (single stroke). For stationary grids, the shadows of the lead strips are imposed onto the radiographic image and can be reduced using programmed image processing methods. 
         [0005]    In general, antiscatter grids, equivalently termed “image contrast grids”, are required for most types of “thick” tissue medical imaging procedures; i.e., procedures in which the screen is not located close (within about the thickness of the screen) to body tissue during medical imaging procedures. 
         [0006]    Image contrast grids have been formed in a number of ways. Grids can be formed by laminating together foils of x-ray transparent material, such as aluminum and x-ray absorbing material, such as lead, to form an extended sandwich structure. The simplified schematic of FIG. 1  illustrates a known sandwich structure image contrast antiscatter grid  24  including aluminum foils  26  and lead foils  28  forming an alternating, parallel arrangement. 
         [0007]    Other methods of forming image contrast grids have been described, for example, in U.S. Pat. Nos. 5,581,592 and 5,557,650, which are incorporated herein by reference in their entirety. 
         [0008]    The various methods proposed for forming antiscatter grids have proved to be cumbersome or unsatisfactory for a number of reasons, including:
       (i) high cost, due in large part to complex and expensive fabrication;   (ii) significant weight, making the grid difficult to position;   (iii) grid visibility in the acquired image; [Known image contrast antiscatter grids, such as the image contrast grid  24  in  FIG. 1 , have a relatively coarse structure that produces grid lines in radiographic images. To reduce this problem, for example, the grids can be moved slightly back and forth in a direction approximately perpendicular to the normal (that is, slightly back and forth perpendicular to the direction of the x-rays) to blur the image of the grid lines formed on the receiver. This movement of the grids is known as the “Bucky system.” However, the Bucky system requires the imaging system to include additional components and, thus, increases the cost and complexity of the system.]   (iv) incomplete scatter compensation; [Known image contrast grids, such as the image contrast grid  24  of  FIGS. 1 and 2 , only remove the Compton-scattered, non-normal (off-z-axis) photons in one dimension (i.e., along either the x-axis or the y-axis). In order to provide two-dimensional photon removal using these grids, two grids, such as two of the image contrast grids  24 , must be stacked with their respective foils oriented orthogonally with respect to each other. Although the combined use of two grids may improve Compton-scattered photon removal in a second direction, the cost of the imaging system is significantly increased by the added cost of the second grid. Thus, the value of improving the performance of the imaging system by using two image contrast grids may not justify the associated added cost and space requirements to achieve the improved performance.]   (v) fragile structure, readily damaged by mishandling;   (vi) rigidity, making the grid unusable in some applications; [Flexibility of the grid can have value in particular imaging applications, but is not currently available.] and   (vii) not readily adaptable to different imaging conditions.       
 
         [0016]    In addition, grid use increases the required patient exposure, because of the needed compensation for absorption of primary radiation by the interspace material that forms the grid. 
         [0017]    A greatly enlarged cross sectional portion of a simple, conventional grid is schematically shown in  FIG. 2 . In the grid, slats of x-ray opaque lead foil  28  alternate with filler strips of x-ray transmissive aluminum foil  26  or fiber. The height of the grid is h, and the interspace width is w. The ratio r=h/w is known as the grid ratio. In practice, for this ratio, h/w=16/1 may be considered a maximum. To achieve this ratio without reducing a transmission magnitude of the grid requires a large number of slats (i.e., a small value of w), since the available h is limited by the current use and design of x-ray equipment to values of about two millimeters. Slats, generally formed of lead, add significantly to grid weight. 
         [0018]    Another type of grid, shown in U.S. Pat. No. 2,605,427 issued Jul. 29, 1952, to Delhumeau is a two-dimensional focusing grid, so called because the slats are aligned with the rays coming from the x-ray source. Two-dimensional grids can be nearly twice as heavy as one-dimensional grids due to the additional amounts of x-ray absorbent material that are needed. 
         [0019]    U.S. Pat. No. 6,408,054 to Rahn et al. describes a micromachined contrast grid having numerous tiny holes formed by etching and photolithography, for example. The holes can be of selected depths and angles. The holes are then filled with small amounts of lead or other x-ray absorbing material to form a grid pattern. Various coating processes are described for filling the cylindrical holes formed in the grid substrate. 
         [0020]    Among solutions proposed for grid fabrication is forming multiple sheets and aligning the sheets to each other to define the path of incident radiation through the grid. U.S. Pat. No. 4,951,305 to Moore et al. describes one approach to this problem using aligned sheets. Accurate alignment of multiple sheets to each other, however, proves to be difficult, even where extremely tight manufacturing tolerances are maintained. Moreover, subsequent handling of the grid can easily cause inadvertent misalignment of the successive sheets used in such an arrangement. 
         [0021]    Rigid grids do not adapt to nonplanar detector surfaces and allow only a very small focus range. Because grids have traditionally been formed using lead strips separated by aluminum spacing material, grids have been formed as rigid, planar devices that do not flex. However, there may be some applications for which some amount of grid flexure is of value. Existing approaches used for grid fabrication do not allow flexibility of the grid. 
         [0022]    It can thus be appreciated that there are advantages to grid design that helps to remedy one or more of the identified factors that make conventional grids cumbersome or unsatisfactory for use. 
       SUMMARY OF THE INVENTION 
       [0023]    An object of the present disclosure is to advance the art of radiography and to address the need for fabrication of a flexible grid. Features of the present disclosure include improved capabilities for alignment of sheets in a multiple-sheet grid. 
         [0024]    These objects are given only by way of illustrative example, and such objects may be exemplary of one or more embodiments of the invention. Other desirable objectives and advantages inherently achieved by the disclosed invention may occur or become apparent to those skilled in the art. The invention is defined by the appended claims. 
         [0025]    According to one aspect of the disclosure, there is provided an antiscatter grid for radiological imaging, the grid formed as a stack of two or more sheets of a flexible substrate, wherein each sheet has:
       (i) a plurality of spaced-apart opaque cavities, each opaque cavity containing a radio-opaque material and wherein the arrangement of opaque cavities within the stack of sheets defines a plurality of channels for ionizing radiation that extend through the sheets; and   (ii) a plurality of magnets that are disposed along the sheet and that couple the sheet to one or more neighboring sheets within the stack.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of the embodiments of the invention, as illustrated in the accompanying drawings. The elements of the drawings are not necessarily to scale relative to each other. 
           [0029]      FIG. 1  is a schematic view in perspective showing parts of an exemplary conventional antiscatter grid. 
           [0030]      FIG. 2  is a cross-sectional side view of the exemplary conventional grid of  FIG. 1 , showing dimensional definitions. 
           [0031]      FIG. 3  is a perspective view showing an antiscatter grid according to an exemplary embodiment of the present disclosure. 
           [0032]      FIG. 4  is a side view that shows an exemplary staggered arrangement of sheets that form an antiscatter grid. 
           [0033]      FIG. 5  is a side view that shows exemplary angled holes formed in a single sheet. 
           [0034]      FIG. 6  is a side view that shows one exemplary method for cutting angled holes in a sheet. 
           [0035]      FIG. 7A  is a side view that shows an exemplary starting substrate to be fabricated into a sheet of an antiscatter grid. 
           [0036]      FIG. 7B  is a side view that shows cavities formed in the exemplary starting substrate of  FIG. 7A . 
           [0037]      FIG. 7C  shows filling the cavities in the exemplary substrate with radio-opaque material. 
           [0038]      FIG. 7D  shows covering the filled cavities to form an exemplary finished sheet. 
           [0039]      FIGS. 8A and 8B  show an alternate exemplary sequence for filling holes and for sealing an individual sheet. 
           [0040]      FIG. 9A  shows a side view of two exemplary grid sheets assembled to use magnet-assisted alignment. 
           [0041]      FIG. 9B  shows magnet-assisted alignment between portions of two exemplary sheets. 
           [0042]      FIG. 10A  is an exploded view that shows how magnetized material in the cavities may be used to magnetically align sheets that are placed in order. 
           [0043]      FIG. 10B  is a perspective view that shows an exemplary stacking of sheets to form a grid, along with a number of columns of magnetized material. 
           [0044]      FIG. 10C  is an enlarged perspective view that shows exemplary channels that extend between sheets. 
           [0045]      FIG. 11  shows a perspective view of a portion of an exemplary flexible grid formed according to an embodiment of the present disclosure. 
           [0046]      FIG. 12  is a schematic view showing components of an exemplary x-ray apparatus that uses the grid of the present disclosure. 
           [0047]      FIG. 13  is a schematic view showing exemplary components of an x-ray apparatus that has a grid mounted on a transport apparatus with rollers. 
           [0048]      FIG. 14  is a schematic diagram showing an exemplary separation mechanism used to temporarily separate sheets in order to relieve grid stiffness for transport about a radius. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0049]    The following is a detailed description of the preferred embodiments, reference being made to the drawings in which the same reference numerals identify the same elements of structure in each of the several figures. 
         [0050]    Where they are used herein, the terms “first”, “second”, and so on, do not necessarily denote any ordinal, sequential, or priority relation, but are simply used to more clearly distinguish one element or set of elements from another, unless specified otherwise. 
         [0051]    In the context of the present disclosure, the terms “viewer”, “operator”, “viewing practitioner”, “observer”, and “user” are considered to be equivalent and refer to the viewing practitioner or other person who views and manipulates an x-ray image on a display monitor or other viewing apparatus. 
         [0052]    As used herein, the term “energizable” relates to a device or set of components that perform an indicated function upon receiving power and, optionally, upon receiving an enabling signal. 
         [0053]    The term “actuable” has its conventional meaning, relating to a device or component that is capable of effecting an action in response to a stimulus, such as in response to an electrical signal, for example. 
         [0054]    The term “subject” refers to the patient who is being imaged and, in optical terms, can be considered equivalent to the “object” of the corresponding imaging system. 
         [0055]    The term “set”, as used herein, refers to a non-empty set, as the concept of a collection of elements or members of a set is widely understood in elementary mathematics. The term “subset”, unless otherwise explicitly stated, is used herein to refer to a non-empty proper subset, that is, to a subset of the larger set, having one or more members. For a set S, a subset may comprise the complete set S. A “proper subset” of set S, however, is strictly contained in set S and excludes at least one member of set S. 
         [0056]    In the context of the present disclosure, the term “oblique” means at an angle that is not an integer multiple of 90 degrees. Two lines, linear structures, or planes, for example, are considered to be oblique with respect to each other if they diverge from or converge toward each other at an angle that is at least about 10 degrees or more away from parallel, or at least about 10 degrees or more away from orthogonal. 
         [0057]    In the context of the present disclosure, the term “coupled” is intended to indicate a mechanical association, connection, relation, or linking, between two or more components, such that the disposition of one component affects the spatial disposition of a component to which it is coupled. For mechanical coupling, two components need not be in direct contact, but can be linked through one or more intermediary components. 
         [0058]    Radio-opaque materials are those that absorb and thus attenuate the x-ray beam significantly enough for detection and radiographic imaging and are considered to be non-transparent to x-rays at given energy levels. A well known radio-opaque material typically used for grids is lead. Radio-lucent or transmissive materials do not significantly absorb or attenuate the x-ray radiation. 
         [0059]    Non-magnetic materials are materials that are negligibly affected by magnetic fields and that exhibit no perceptible magnetic attraction and are thus not perceptibly pulled toward a magnet. In general, non-magnetic materials have a low relative magnetic permeability, typically not exceeding 1.0 at room temperature. Some exemplary non-magnetic materials include copper, aluminum, standard stainless steel, and most metals and alloys; sapphire; various ceramics; wood and paper composite materials; glass; water; plastics and other polymers; fiberglass; and various composite materials such as phenolic materials. Magnetic materials have higher relative permeability and are considered to be “magnetically responsive”, exhibiting magnetic attraction that can be readily perceived without requiring instrumentation; this includes ferromagnetic materials and various compounds of rare earth materials, for example. 
         [0060]    There are two general classes of ferromagnetic materials. Permanent magnets are made from “hard” ferromagnetic materials such as alnico and ferrite that are subjected to special processing in a powerful magnetic field during manufacture, to align their internal microcrystalline structure to exhibit a magnetic flux field. Magnetically “soft” materials like annealed iron, on the other hand, can be magnetized for a period of time, but do not tend to stay magnetized. To demagnetize a saturated magnet, a magnetic field of a given threshold must be applied, and this threshold depends on coercivity of the respective material. “Hard” materials that behave as permanent magnets have high coercivity, whereas “soft” materials have low coercivity. By way of example, electrical steel, used as a flux carrier in many electrical devices, exhibits coercivity values in the range of about 0.5 oersteds; samarium cobalt, used for rare earth permanent magnets, has coercivity in the range of about 40,000 oersteds. 
         [0061]    In the context of the present disclosure, a flexible sheet is a sheet that can be bent from a substantially flat planar form over a bend radius of 100 millimeters or less and can be restored to substantially flat planar form without damage. 
         [0062]    Embodiments of the present invention address a number of problems that relate to grid fabrication and can be used to form a grid that is flexible. The methods and apparatus of the present invention can be used to configure a wide range of grid patterns to allow for various radiographic imaging applications. 
         [0063]    The perspective view of  FIG. 3  shows schematic representation of a grid  30  formed according to an embodiment of the present disclosure. Features are not shown to scale, but are exaggerated in size to represent their physical arrangement. Grid  30  is formed as a stack  48  of sheets  32  that are aligned in a suitable arrangement for a particular imaging application. There are nine sheets  32  according to an embodiment of the present disclosure shown in  FIG. 3 . The number of sheets  32  that are used can be varied, allowing grid  30  to be formed from a stack  48  of as few as two sheets  32 , or more. Each sheet  32  is formed from a flexible material that has numerous radio-opaque cavities  34  that are filled with radio-opaque material  42 . Successive sheets  32  in the stack  48  are registered to each other so that their respective cavities  34  align to provide a suitable pattern of clear channels bounded and defined by the opaque cavities  34 . Thus, each neighboring sheet  32  can be the same or can be separately configured. 
         [0064]    As shown in  FIG. 4 , alignment of cavities  34  defines how clear channels  46  are arranged in the stack  48 . Groups of aligned cavities  34  in successive sheets  32  can each be formed along an axis, with the axes of the groups of cavities converging toward a focal point that lies some distance outside the sheet. Sheets  32  stacked to form a grid can have opaque cavities  34  directed toward the focal point so that, when the full set of sheets for a grid are stacked, clear channels  46  are formed extending from the top of the stack to the bottom, with the channels  46  inclined toward the focal point. Cavities  34  themselves can each be formed along an axis A that is normal (angle N) to the sheet surface, with focus provided by staggering respective axes A of each of the cavities from one sheet to the next, so that cavities on each sheet are offset slightly from those on the neighboring sheet, effectively forming a substantially linear channel  46  for radiation when the sheets are stacked, as shown in  FIG. 4 . 
         [0065]    Cavities  34  can be tiny and may not be visible to the unaided eye, so that each sheet  32  can have as many as a few million cavities  34 , each extending within sheet  32  in the depth direction (h in  FIG. 2 ) from one major surface (upper or lower) to the other major surface (lower or upper). The exploded side view of  FIG. 4  shows a portion of one possible stack  48  arrangement, in which cavities  34  are formed along axes A normal to a surface of each sheet  32 , which sheets  32  are successively staggered in the horizontal direction with respect to each neighbor sheet  32 , for the orientation shown. This type of arrangement may provide oblique clear channels  46  formed by the stack  48  for focusing of incident radiation to form a focused grid  30 , for example. Other arrangements can be envisioned for special purpose imaging, taking into account factors such as source-to-image distance (SID), grid resolution, amount of scatter compensation that is needed, and the like. 
       Sheet Substrate 
       [0066]    In order to provide a flexible grid  30 , sheet  32  may be formed from a material that is substantially transparent to radiation and that is also flexible. Substrates that can be used for forming sheet  32  include, but would not be limited to, various polymer materials including polyester and Polyethylene Terephthalate (PET), for example. The substrate sheet material can be dimensionally stable in the plane of the sheet but flexible out of the plane. 
       Forming Sheet Cavities 
       [0067]    Cavities  34  can be formed in any of a number of ways, depending on size and spacing requirements and properties of the substrate that is used. According to an embodiment of the present disclosure, cavities  34  and other features (e.g., holes) are formed by sheet treatment with a laser, such as a CO 2  laser, to form holes or cavities through a sheet initially. Lasers from Universal Laser Systems, Trotec Laser Systems, or other manufacturers can be used.  FIG. 5  shows a cross section of finished sheet  32  formed with angled cavities  34 . 
         [0068]    The side view of  FIG. 6  shows one method for forming holes  33  through one or more sheets  32 . Using this technique, one or more sheets  32  are bent about a radius having a preselected magnitude; then, a laser is used to form holes  34  through the one or more sheets  32 . When a sheet  32  that has been treated in this way is straightened, the resulting holes  33  may be disposed at variably controlled angles. 
         [0069]    Holes  33  or cavities  34  in a plurality of stacked sheets may also be drilled or otherwise formed simultaneously. Where this is done, the holes  33  may extend fully through several sheets. Other methods can be used for forming sheet cavities  34 , including molding, perforation, drilling, and etching, for example. Each cavity or hole may have a diameter of about 2/1000 of an inch or less. Cavities  34  can be uniformly sized and spaced. Alternately, cavities  34  may have variable size and/or spacing along the sheet. 
       Filling Sheet Cavities 
       [0070]    The fabrication sequence shown in  FIGS. 7A through 7D  shows how sheet  32  can be formed, according to an embodiment of the present disclosure. A flexible substrate  40  is shown in  FIG. 7A . A cavity-forming operation is then used to form an array of tiny cavities  34 , as shown in  FIG. 7B . As noted previously, each cavity  34  can have a central axis A that may be normal to the substrate  40  surface or may be oblique with respect to the substrate  40  surface. Then, as shown in  FIG. 7C , a filling operation is performed, filling each cavity  34  with a radio-opaque material  42 .  FIG. 7D  shows addition of an x-ray transparent sealant layer  44 , which may be a thin sheet of the same substrate material, for example, or some other suitable sealing material to form a finished sheet  32 . The additional sealant layer  44  can also be applied to the opposite side of flexible substrate  40 . Where holes are drilled completely through the starting substrate sheet, sealing material or treatment is applied to both surfaces of the sheet, thereby transforming the filled hole into a filled cavity. 
         [0071]    The radio-opaque material  42  that is used for filling holes or cavities may be any of a number of materials and can be provided in nanoparticulate or powdered form, or provided in solution, such as an emulsion or colloidal solution, for example. Finely ground metals such as tungsten (W) or lead (Pb) can be used. Tungsten particulate is an advantageous alternative to lead. One commercially available tungsten powder (Buffalo Tungsten, type SR) has apparent powder density 8.5-10 g/cm 3 . By way of reference, solid lead has a density of 11.3 g/cm 3 . 
         [0072]    According to an alternate embodiment of the present disclosure, as shown in the partial fabrication sequence of  FIGS. 8A and 8B , a squeegee  62  is used to press a tungsten-based paste  64  as radio-opaque material  42  into holes  33 . The high density emulsion can then be at least partially dried. Excess paste  64  is then scraped from the top and bottom of the filled substrate and a protective film  66  applied to both top and bottom surfaces to form the finished sheet. Alternately, particulate or nanoparticulate material could be forced into the holes  33  as radio-opaque material  42 . According to an alternate embodiment of the present invention, radio-opaque material  42  is a particulate and is combined with a curing material that binds the particulate to the side walls of holes  33  (or cavities  34 ) so that no sealant layer  44  or film  66  is needed. 
         [0073]    Once holes  33  have been filled and sealed, assembly of the sheet  32  with other sheets  32  to form the grid can be performed. Sealing can also be performed using heat to encapsulate material within the cavity or hole, or by using an adhesive, a hardener, or other material or process to encapsulate the radio-opaque material or to harden the material into place. 
       Sheet Coupling and Alignment 
       [0074]    Grid  30  can be formed from a number of suitably prepared sheets  32 . To provide close registration between neighboring sheets  32  and all of the sheets  32  for a particular grid  30 , holes  33  for all of the sheets  32  used for a grid  30  can be cut through at the same time. Angled cavities may be created in a sheet  32  by forming a substrate in radius, then cutting holes  33  through the substrate, then re-flattening the substrate. Specialized, focused grid patterns may be formed, such as patterns that are suitably randomized to reduce or eliminate fringe patterns in the image that is obtained using the grid  30 . 
         [0075]    The area of the pattern of cavities on the sheet can be a standard grid size, such as 435 mm×435 mm [17.1″×17.1″], for example. The stack of sheets forming the grid can be about 42 cm×34 cm or about 42 cm×42 cm. 
         [0076]    Sheet alignment is a particular challenge with cavities  34  of the number and size needed. Sheets can be coarsely aligned by permanently fastening together, either at one end or in the middle of the sheet or at the perimeter. 
         [0077]    According to an embodiment of the present disclosure, sheets  32  are coarsely aligned in some way, then precision aligned and held together or coupled using magnetic attraction. Referring to the partial cross-section view of  FIG. 9A , a pattern of cavities  34  are filled with a magnetized material  38  that give the corresponding magnetic cavities, now numbered cavities  36  in  FIGS. 9A  and following, a particular magnetic polarity arrangement. Mutual attraction between magnetic cavities  36  that have magnetized material  38  causes these cavities  36  to align with each other and couple the sheets together as shown in  FIG. 9B . 
         [0078]    According to an embodiment of the present disclosure, magnetized material  38  is selectively deposited into a patterned distribution of magnetic cavities  36 , so that the pattern of cavities  36  is the same for adjacent sheets  32 . The pattern can be a matrix of cavities  36 , such as having a magnetized cavity  36  at every increment of a given number of millimeters from a given origin, for example. The material  38  is then magnetized (typically, permanently magnetized) so that magnetic poles north (N) and south (S) are oriented to correctly attract each sheet  32  to its adjacent neighboring sheet  32  for a magnetic coupling. The magnetization step can be performed using a fixture that is designed with magnetizing elements in accordance with the given pattern. This magnetization processing can be performed prior to filling cavities or holes with high density materials to form grid channels  46 . 
         [0079]    According to an alternate embodiment of the present invention, the patterned distribution of magnetic cavities  36  includes a matrix having both magnets and magnetic materials, such as ferrous materials that are magnetically responsive but are not themselves magnetized. 
         [0080]    The exploded view of  FIG. 10A  shows an alternate arrangement in which magnets  50   a,    50   b,    50   c  that are embedded within or coupled to the sheet can be used to magnetically couple and align neighboring sheets that may be placed in order, such as to provide radio-lucent channels  46  that are orthogonal to the sheet  32  surface or to provide oblique radio-lucent channels  46  using staggered-pattern sheets that allow the channeling of radiation in a focused manner The same principles described with reference to magnets  50   a,    50   b,  and  50   c  in  FIG. 10A  apply when using magnetized material  38  that is embedded within cavities  36 , as described previously. In the example of  FIG. 10A , three sheets are shown, labeled as sheets  32   a,    32   b,  and  32   c.  Neighboring sheets  32   a  and  32   b  have corresponding magnets  50   a  and  50   b,  respectively, that are attracted to each other for coupling and alignment. Similarly, adjacent or neighboring sheets  32   b  and  32   c  have corresponding magnetic cavities  50   b  and  50   c  that have magnetic attraction to support alignment. Where each sheet  32  in the stack that forms grid  30  is similarly constructed and the order of sheets  32  does not matter, the same pattern of magnet placement can be used on each of the neighboring sheets. In fabricating grid  30 , the individual sheets  32  that form the grid can be sequentially fitted onto the stack in a sequence, such as by first registering only the edge of each subsequent sheet  32  to the existing stack  48  of sheets and then unrolling the surface of the new sheet onto the existing stack to increasingly engage magnets in succession on each added sheet  32  and to reinforce magnetic attraction along the sheet as it is rolled into position. 
         [0081]      FIG. 10B  shows an arrangement of grid  30  formed from a stacking of sheets  32 , not drawn to scale. Magnetized materials  38  are provided in cavities  36  that align in columns  54 . As is particularly shown in the enlarged view of  FIG. 10C , columns  54  that provide magnetic coupling and alignment may be arranged along axes A 1 . Axes A 1  may be at angles that are offset from the angle A 2  of cavities  34  that collectively form channels  46 . For example, angle A 2  may be normal while angle A 1  is oblique, or vice versa, for portions of a focused grid in which cavities  36  are angled at normal, or other than a normal, to the sheet  32  surface. Columns  54  are shown as cylindrical and are shown distributed at spaced intervals along the stacked sheets  32 . 
         [0082]      FIG. 10B  shows some details of grid  30  according to an embodiment of the present disclosure. In the exemplary embodiment shown, the section of grid  30  shown has approximate dimensions of about 10.0×5.0 mm There are eight layered sheets  32  shown, each sheet having approximately 0.20 mm thickness; there can be some incremental spacing between sheets due to cavity features, giving a total grid stack height of about 1.79 mm Protective film thickness of sealant layer  44  ( FIG. 7D ) is about 0.012 mm Cavity  34  diameter is approximately 0.05 mm Cavity  34  centerline is about 0.25 mm, nominal, so that 40 cavities/cm are provided in this example. Magnet column  54  diameter is approximately 0.10 mm. 
         [0083]    One or more electromagnets can alternately be used to supplement the positional registration and coupling provided by permanent magnet materials of magnets  50   a,    50   b,    50   c  ( FIG. 10A ) between two or more sheets  32 . 
         [0084]    Magnet-assisted coupling and alignment between neighboring sheets  32  is advantageous because it allows some amount of flexure to the stack, so that rigidity is not a requirement for maintaining sheet  32  alignment. Instead, grid  30  can be flexibly bent or curved in order to adapt more closely to the path of incident radiation through a material object or a human or animal subject and to a receiver. As the stack  48  is bent, magnetic attraction still holds the sheets  32  together and supports restoring the grid  30  to a flat, planar state. The magnetic coupling between sheets  32  allows some measure of flexibility and movement between sheets, so that a roller of a given radius can be used to move the grid into and out from the image path. The flexible grid  30  can thus be transported about the radius of the roller, yet remain in the bucky  52 , for example, as shown subsequently. Tensioned wires can be used to help separate sheets when bending or otherwise in flexure, such as about a roller. 
         [0085]    One exemplary application in which flexure can be particularly useful is for x-ray imaging of large pipes, such as utility or chemical processing piping. The capability to wrap around the curved surface of the pipe or other structure has a number of advantages for non-destructive testing, for example. 
         [0086]    Magnets can be formed by filling a subset of the cavities  36  with a liquid magnetizable material, then applying a blade or squeegee to the surface of the sheet, and then magnetizing the material after it sets. Sheets  32  can be polarized so that all cavities  36  are N-polarized on one side of the sheet and S-polarized on the opposite side. Alternately, magnetization itself can be patterned, so that, from the same side of the sheet  32 , a portion of the filled cavities  36  are N-polarized and the remaining portion of the filled cavities  36  are S-polarized. 
         [0087]    Other methods can alternately be used for coupling sheets  32  to each other in the stack. These include use of adhesives, fasteners, frames and other holders, and other devices. Magnetic coupling using micro-magnet structures such as cavities  36  is particularly advantaged, due to its capability for effecting precision alignment between sheets, at multiple points along the sheet surface. 
         [0088]    The perspective view of  FIG. 11  shows a portion of flexible grid  30  that is formed from individual sheets  32  according to an embodiment of the present disclosure. Numerous sheets  32  can be used, depending on the desired thickness and other characteristics. It can be appreciated that the use of individual sheets  32  allows grid  30  to have any of a number of desirable arrangements, for focused or unfocused grids. 
         [0089]    The schematic view of  FIG. 12  shows a focused grid  30 , wherein channels  46  that extend through the grid  30 , from sheet to sheet as described previously, substantially converge toward a focal point, shown as x-ray source  60 . The position of a patient  14  and an x-ray detector  56  are also shown in  FIG. 12 . The radiation detector  56  that is used for obtaining an image can be a digital radiography (DR) receiver or can be a film or computed radiography (CR) receiver, for example. 
         [0090]    The schematic view of  FIG. 13  shows an optional bucky  52  that has a transport apparatus  70  with rollers  68  or other mechanics that can be energized to move grid  30  from a first position in front of detector  56  to a second position, behind detector  56 . Because grid  30  can be formed from separable polymer sheets that can be magnetically coupled and aligned, grid  30  is flexible and can be transported around the radius of a roller or other curved surface. Grid channels  46  ( FIG. 12 ) can be slightly out of alignment during transport, since registration of magnetic cavities  36  to each other can be restored automatically by the magnetic alignment feature. An actuator  72  rotates one or both rollers  68  in order to transport grid  30  between positions. Bucky  52  can also be energizable to use rotation in order to change grids. In such an embodiment, bucky  52  moves a second grid  30   b  (shown in dashed outline) having a different focal distance or arrangement of channels  46  in front of the detector  56  after the first grid  30  is moved out of position. Alternately, other types of transport apparatus can be used, providing U-shaped movement about a roller radius, for example. 
         [0091]    The top view schematic view of  FIG. 14  shows a grid  30  in transport apparatus  70  wherein a separation mechanism  80  is used to temporarily separate sheets  32   a,    32   b,    32   c,  etc. in order to relieve grid  30  stiffness and allow grid  30  to be transported about a radius around detector  56 . According to the embodiment shown in  FIG. 14 , one or more combs  76  of wires  78  extend through grid  30  orthogonal to the radius of curvature, shown as dots in the cross-section top view of  FIG. 14 . When grid  30  is moved past rollers  74  (to the right in the  FIG. 14  view), combs  76  are manipulated by an optional actuator (not shown) or by spring tension that spreads the comb wires  78  outwards. This relaxes grid  30  thickness by causing sheets  32   a,    32   b,    32   c,  etc. to separate slightly from each other as grid  30  is driven around the turn radius. Wires  78  lie outside the corresponding image area of grid  30  and can be formed of stiff wire, such as piano wire or other stiff structure. According to an alternate embodiment of the present disclosure, combs  76  are held in place while grid  30  moves through the turn radius. Because the sheets  32   a,    32   b,    32   c,  etc. are magnetically attracted to each other, reversing the turn direction allows the grid  30  to reconstruct itself, with its cavities properly aligned for imaging when the grid  30  is transported back into imaging position. 
         [0092]    The pattern of cavities  34  can be adapted to suit particular imaging requirements, with variable cavity angle and width and depth dimensions. Cavity shape and size can be different within the same sheet or in adjacent sheets  32  in the stack that forms grid  30 . The channels  46  that extend through the stacked sheets  32  can substantially converge toward a focal point, such as toward the location of the x-ray source, as shown in  FIG. 12 . Grids having different focal lengths can be formed by placing inter-sheet spacers of different thicknesses between each sheet  32  in a grid. An inter-sheet spacer can be an untreated sheet of the same substrate that is used for forming sheet  32 , for example. 
         [0093]    The flexibility of grid  30  is a function of factors such as the substrate  40  that is selected for sheet  32  material, density and size of cavities  34 , material  42  ( FIG. 7C ) used to fill cavities  34 , sheet thickness, number of sheets  32  stacked together ( FIG. 10B ), and how the sheets  32  are coupled to each other. 
         [0094]    While the invention has been described with reference to exemplary embodiments for flexible grid fabrication and use, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.