Patent Number: 
Section: description

FIG. 1 shows a computed tomography apparatus which includes a gantry 1 on which a radiation source 2 is mounted. The X-ray detector 8 with the anti-scatter grid 3 arranged thereabove is mounted so as to face the radiation source 2. A patient 5 on a table 6 is introduced into the beam path 4. The gantry 1 rotates about the patient 5. An examination zone 7 is thus irradiated from all sides. The patient 5 is slid through the rotating gantry in the horizontal direction, so that a volume image is acquired by way of a plurality of cross-sectional images. The zone scanned during one rotation is substantially larger in the case of two-dimensional X-ray detectors 8 than in the case of single-line X-ray detectors. As a result, the patient 5 can be slid through the gantry 1 faster. The FIGS. 2 to 5 show a one-sided comb element 12 in several views. FIG. 2 is a plan view of a one-sided comb element 12. This one-sided comb element 12 is made of a material absorbing X-rays, for example brass, molybdenum, tungsten. The comb structure of the comb element 12 is formed by comb lamellae 1I1 which extend transversely of a base plate 10. The height of the comb element 12 is dependent on the specific application. A decisive criterion in this respect is the surface area irradiated by one scan. The ratio of useful radiation to scattered radiation becomes worse as the width of the surface irradiated by the X-rays per scan increases. The comb elements 12 typically have a height of from approximately 2 to 6 cm. The more scattered radiation is contained in the overall signal, the higher the anti-scatter grid must be. The width of the comb element 12, or also of the base plate 10, is governed by the width of the X-ray detector 8. An anti-scatter grid 3 as constructed from such comb elements 12 must completely cover the X-ray detector 8. In the case of large-area flat X-ray detectors, therefore, the comb elements 12 are wider than in the case of the narrower multi-line or two-dimensional X-ray detectors 8 used in computed tomography. The depth of the comb lamellae 11 and the distance D between the individual comb lamellae 11 define the pixel size of such an anti-scatter grid 3. In the case of two-dimensional X-ray detectors 8 for computed tomography apparatus the pixel size amounts to from approximately 1xc3x971 to 2xc3x975 mm2. A plurality of comb elements 12 are oriented relative to the incident X-rays in such a manner that the X-rays pass through the grid openings formed by the comb lamellae 11 and the base plate 10. X-rays are emitted by the X-ray source with a focal spot and emanate at a radiation angle from this spot. In order to achieve effective filtering or an as good as possible primary radiation transparency, the comb lamellae 11 are arranged on the base plate 11 so as to be oriented towards or focused on this focal spot. This is shown in FIG. 4. The distance Do between the comb lamellae 11 at the upper edge of the base plate 10 is smaller than the distance Du between the comb lamellae 11 at the lower edge of the base plate 10. Because the X-ray detectors 8 in computed tomography apparatus are adapted to a curvature, it is necessary to adapt the anti-scatter grid 3 accordingly. FIG. 3 shows that the depth of the comb lamellae 11 at the upper edge is less than that at the lower edge of the base plate 10. Piece-wise assembly of small anti-scatter grid segments is possible in the case of long X-ray detectors. FIG. 6 illustrates the linking of a plurality of one-sided comb elements 12. Due to the different depths of the comb lamellae 11 at the upper edge and the lower edge (FIG. 3), the anti-scatter grid 3 can be readily adapted to the curvature of the X-ray detector 8. The curvature of the anti-scatter grid 3 is also imposed by the arrangement of the grooves 14 in the frame 13. FIG. 7 illustrates the arrangement of a plurality of one-sided comb elements 12 in a frame 13 which produces an X-ray shadow. The inner side of the frame 13 is provided with grooves 14 which are shown in FIG. 8. The grooves 14 receive the sides of the base plates 10 of the plurality of one-sided comb elements 12. The comb elements 12 can be glued in or be secured in any other feasible manner. Mechanical fixation by pressing in the comb elements 12 is also feasible. An anti-scatter grid 3 is formed by linking a plurality of onesided comb elements 12. The comb lamellae 11 of one base plate 10 then adjoin the rear side of a neighboring base plate 10. The length of such an anti-scatter grid 3 can be increased at will by selection of the number of comb elements 12. A further embodiment of an anti-scatter grid 3 will be described in detail hereinafter. The FIGS. 9 to 12 show a two-sided comb element 15 and an anti-scatter grid 3 assembled from such elements and lamellae 19. FIG. 9 shows a two-sided comb element 15 with a double comb structure. It consists of a base plate 17 on both sides of which there are provided lamellae 16 and 18. The comb lamellae 16 and 18 are arranged on both sides of the base plate 17 so as to extend transversely of the comb base surface formed by the base plate 17. The above configurations for the focusing of the one-sided comb element 12 are to be used accordingly for this two-sided comb element 15. Moreover, in order to imitate the curvature of the X-ray detector 8, the comb lamellae 16 and 18 are deeper at the lower side of the base plate 17 than the comb lamellae 16 and 18 at the upper edge of the base plate 17. FIG. 11 shows the assembly of plane lamellae 19 (FIG. 10) and two-sided comb elements 15. Two-sided comb elements 15 and lamellae 19 are fitted in an alternating arrangement in a frame 13, thus forming an anti-scatter grid 3. The comb lamellae 16 and 18 adjoin the respective neighboring lamellae 19. The length of the anti-scatter grid 3 can again be increased by increasing the number of two-sided comb elements 15 and lamellae 19 used. Anti-scatter grids are used not only for computed tomography but also for radiology. In that case the anti-scatter grid 3 need not be curved, because the X-ray detector 8 is flat. Such anti-scatter grids typically have dimensions other than the grids described thus far. In these fields of application, however, fewer vibrations occur. The frames of these anti-scatter grids are larger and the comb elements 12 or 15 to be used are also larger. Because of the very high natural stability of the comb elements 15, such an embodiment of an anti-scatter grid is suitable for a very large range of applications. Several methods are available for the manufacture of such comb elements 15. Depending on the resolution or pixel size of the anti-scatter grid, the comb elements 12 or 15 can be formed, for example by means of milling, sintering or injection molding. In the case of the injection-molding method materials absorbing X-rays can be added to a basic material. An anti-scatter grid 3 can also be formed by linking two-sided comb elements 15 without arranging lamellae 19 therebetween. Instead of using a frame 13, the comb elements 12 or 15 can also be arranged while using spacers in such a manner that an anti-scatter grid is formed. Such an anti-scatter grid can be adapted to special applications by varying the distances between the comb lamellae of the comb elements. For example, it is feasible to realize a higher resolution for an inner or core area of an anti-scatter grid; this can be achieved by means of a grid with very fine meshes. The resolution could be lower at the edge area of the X-ray detector that is covered by the anti-scatter grid, so that at this area the grid openings in the anti-scatter grid may be larger.