Patent Number: 
Section: description

The X-ray examination apparatus 1 shown in FIG. 1 includes an X-ray source 2 which emits X-rays 3 which diverge in the direction of an object 4 to be irradiated which is, for example a patient or a material to be examined, for example a workpiece. The object 4 is arranged in a receiving space 5. An X-ray scatter grid 6 is arranged behind the object 4 to intercept the X-ray beam 3. In the direction of the beam axis 7 the X-ray scatter grid 6 is succeeded by a detector 8, for example a film, which serves to form a two-dimensional image of the object 4 to be examined. As is shown in FIG. 2, the X-ray scatter grid 6 comprises alternating regions of materials 9,10 having different X-ray absorption. Usually, the regions 9 are manufactured from a material having a very low X-ray absorption so that all X-rays spatially aligned with regions 9 are transmitted by the X-ray scatter grid 9. Such X-rays correspond to rays 3a, which did not undergo an X-ray scattering in the path between the X-ray source 2 and an element 9. On the other hand, the X-rays of the type 3b, which did undergo an X-ray scattering are no longer aligned with the cells 9 and are intercepted by the elements 10 and absorbed within the elements 10. The material for the elements 10 is chosen so that it has a high X-ray absorption. Thus, the X-ray absorption grid functions as a filter to intercept the scattered X-rays, which do not contribute to the attenuation information of the object. The strips 9 are made, for example of a polymeric material. Feasible materials in this respect are all thermoplastic polymers, like polymethyl methacrylate (PMMA) or polycarbonate which may be supplemented which may be supplemented with flow modifiers, for example plasticising agent DOP (dioctyl phthalate) A typical enrichment with plasticiser amounts to approximately 20%. It is also possible to use ABS (poly-(acrylo-nitrile-butadiene-styrene) with an addition of Kraton Liquid (hydroxy oligoethylene-butylene). Filling material, for example, aluminum oxide or carbon black (or other compounds with light nuclei) can be added to the material of the strip 9 so as to enhance the flow behavior in the multiplication cell at the cost of a slightly increased X-ray absorption. In general metals are suitable filling materials for manufacturing of strips 10, preferably metals with heavier nuclei, like W. It is also possible to utilize salts for manufacturing of the X-ray absorbing strips 10. Metal powders of nickel and/or tungsten, can also be used as the absorbing materials and hence as admixtures for the material strips 10. In that case the size of the powder particles is less than 10 xcexcm. The absorptivity of tungsten is approximately twice that of lead. Therefore, the thickness d of the material strip 10 may be kept small so as to avoid an excessive overall loss of intensity of the X-rays 3, thus also avoiding the occurrence of wide bands without information on the film 8. Another advantage of small strips 10 is that the dose delivered to the patient stays low. In an embodiment of FIG. 3, the material flows 13, 12 are co-extruded, the actual extrusion being succeeded by a device 11 for multiplying material strips 13, 12 that are situated one over the other. The flows 13,12 after subsequent multiplication will result in the material strips 9,10 of the layered structure. The stability of the interface between the various material strips 9, 10 is dependent on the flow behavior of the materials used. Therefore, special attention should be paid to such behavior. The choice of the share of the filling or binding agent, therefore, is dependent on the flow requirements. This choice can be made based on the ratio of the wall slip and the internal shear deformation of the two materials used for the co-extrusion. This ratio can be expressed by a so-called xcex2-value: xcex2=VS/xcfx84w*xcex7/R, where  Vs is a value of the slip velocity at the wall xcfx84w is a value of the shear stress at the wall xcex7 is a value of the true viscosity of the material R is the dimension of the channel in the multiplication element. In FIG. 3, the stock used to realize an X-ray scatter grid is formed by two material strips 12 and 13 of comparable viscosity that are melted and co-extruded in comparable circumstances. Such input stock strips 12, 13 can be fed to the multiplication device 11 in the form of the stacked layers or adjacently arranged layers. In FIG. 3 a cutting edge 14 of the multiplication device 11 separates the strips 12, 13 each time perpendicularly to their longitudinal direction; subsequently, a two-layer assembly of input stock strips 12 and 13 is transported upwards on a ramp 15 and is allowed to expand laterally so that the original width of the assembly 12, 13, that is, the width before cutting, is restored. The other part of the cut assembly 12, 13 travels downwards on a ramp 18 and, upon lateral expansion, takes in a position in the opposite direction underneath the previously described expanded two-layer assembly of input stock layers 12, 13. Subsequent to a first multiplication operation the two-layer assembly has thus become a four-layer assembly. By arranging a set of multiplication elements behind each other a higher degree layer multiplication can be achieved. This is also shown in the FIGS. 4 to 6, that is, rotated through 90xc2x0. FIG. 4 illustrates the cutting by the edge 14 as well as the subsequent upwards travel of one part of the assembly 12, 13 on the ramp 15 and the parallel downwards travel of the separated part of the assembly 12, 13 on the ramp 18. FIG. 5 shows the position in which the lateral expansion of the input stock strips 12, 13 commences; at the exit of a multiplication element they have become stacked on one another as four layers of the same initial width (or height in the rotated representation) so that the two-layer assembly has been converted into a four-layer assembly. FIG. 7 illustrates how a set of devices 11 (not shown) convert the input stock strips 12, 13, by repeated multiplication in the described manner or a similar manner, overall into a multi-layer assembly with superposed layers 12, 13 which constitute the material strips 9, 10 in the X-ray scatter grid 6 after a subsequent cutting operation (not shown). Referring to FIG. 8, the input stock strips 12, 13 (not shown) are maintained in a molten or in a melt-like condition during the multiplication. The material strips 9, 10 obtained at the end of the multiplication process pass through an extrusion device 16, from which the layered structure is finally extruded. The extrusion device 16 schematically shows two operations, where the flow is transformed in two dimensions simultaneously. It is also possible that these two operations are performed one after the other, so that the deformation of the material in the extrusion die in order to convert the multilayer into a plate with a correct width and height is split into two steps each of which being a uniaxial deformation. The device 16, shown in the FIG. 8 performs a pressing operation in the direction transversely of the longitudinal direction of the material strips 9, 10, thus forming a wide flat member which height h is in the range of up to a few millimeters. An example for the thickness dimension h parallel to the beam axis 7 is in the range of from 0.5 mm to 2 mm. FIG. 9 schematically illustrates the steps 17,18 of a further processing of the layer structure in case the structure must show a certain degree of convergence towards its central ray. The flat member initially formed (FIG. 9a) contains the material strips 9, 10 with different absorption coefficients in the direction transversely of its width. The pressing device 20 schematically shown in FIG. 9b is constructed in such a manner that at the same time it imparts to the flat member body being formed in a shape that deviates from a plane by viscous deformation. This step can be integrated in the continuous extrusion step by adding a transition to a curved shape in the extrusion die. After that the curved structure is flattened again, for example by means of pressing in the elastic state (as a separate operation). After deformation into the flat shape, the assembly is cooled so that the flat shape is frozen in. The material strips 9, 10 thus assume the inclined position shown, as is shown in the FIG. 9c. The subsequent cooling and elastic deformation of the overall surface convert the assembly of the strips 9, 10 into a plane assembly again, the transmission direction of the strips 9 and the direction of the strips 10 being directed essentially towards a point 2 that corresponds to the X-ray source in the operational condition. Scattered radiation that does not follow the direction of propagation of the rays 3 (FIG. 1), therefore, cannot traverse the element 6 acting as a grid, because they cannot pass through the grid 6 in the direction parallel to the longitudinal direction of the strips 9 but are incident at an angle on the absorbing strips 10. As a result, the scattered radiation is absorbed as fully as possible. Only rays that travel through the grid 6 in the direction parallel to the longitudinal direction of the non-absorbing material strips 9 are transmitted without being absorbed and hence become available to the detector (not shown) for imaging. It must be noted that it is also possible to proceed in a different way. In this case, a thicker initial flat member 6 is extruded during the step illustrated in the FIG. 9a. Then instead of step 9b a machining of the upper surface of the member is applied so that to form a spherically shaped concave surface. During the step 9c the concave surface is flattened to form a flat scatter grid with inclined neighboring cells. Application of the machining step has an advantage that by the removal of a surface layer to shape a concave surface, the surface layer exhibiting minor irregularities in the thickness of the stripes 9,10 is removed resulting in a better quality of the scatter grid. The combining of strips 9, 10 at a later stage can thus be dispensed. An assembly that acts as an X-ray scatter grid 6 can be formed by means of a manufacturing process involving co-extrusion and strip multiplication. The alignment of the strips in a direction corresponding to the divergence of the rays, which alignment can be realized by means of an extrusion die, ensures at the same time that the grid 6 is effective over a large width of the diverging radiation beam 3. It must be noted, that alternatively to what is shown in the FIGS. 7 to 9, the thickness d of the absorbing material strips 10 and the thickness D of non-absorbing material strips 9 may be different (FIG. 2). The thickness d of an absorbing material strip 10 is typically in the range of from 15 xcexcm to 50 xcexcm, whereas the thickness of a non-absorbing material strip 9 typically lies in the range of from 150 xcexcm to 350 xcexcm.