Abstract:
In order to increase the ruggedness and the scattered radiation attenuation quality, a grid ( 3 ) with comb elements ( 12 ) which absorb electromagnetic radiation and are intended to form a grid is constructed, in such a manner that comb lamellae ( 11 ) extend transversely of an associated comb base surface which supports the comb lamellae ( 11 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a grid which includes comb elements which absorb electromagnetic radiation and are intended to form a grid. 
     SUMMARY OF THE INVENTION 
     Grids of this kind are used in the X-ray technique as anti-scatter grids for absorbing scattered radiation arising in the tissue of the patient before the characteristic X-ray signal, produced by the varying attenuation properties of the tissue examined, is incident on the X-ray detector. 
     U.S. Pat. No. 5,099,134 discloses a collimator (anti-scatter grid) and a method of manufacturing such a collimator. The collimator is formed by a frame which absorbs X-rays and in which first and second partition plates are arranged. Each of the partition plates is provided with slits which extend in the longitudinal direction of the partition plate and enable the first partition plates to be inserted into the second partition plates at the appropriate angle. The inner sides of the rectangular frame are provided with slits which serve to receive the respective ends of the partition plates. 
     The complexity of the partition plates imposes given limits on the manufacture of such anti-scatter grids. It has been found that the manufacture of anti-scatter grids of large dimensions, for example as used for large-area detectors, is difficult, because bow of the large partition plates impedes simple and correct meshing of the slits of the partition plates. 
     Large-area anti-scatter grids are used, for example for multi-line CT (Computed Tomography) apparatus. The length of the detector is then very large. The X-rays emitted by an X-ray source in computed tomography traverse the patient and are attenuated in conformity with the varying thickness and chemical composition of the tissue or bone to be examined. The X-ray signal at the same time is subject to scattered radiation. In order to reduce such scattered radiation which falsifies the primary X-ray image to be formed, the X-rays traverse an anti-scatter grid which is focused onto the focal spot of the radiation source. It is thus achieved that only the X-ray quanta that are characteristic of the attenuation of the irradiated object are detected during the detection of the X-ray quanta. 
     The construction of CT examination apparatus is such that the radiation source is mounted opposite the detector on a gantry which rotates about the patient, the patient being slowly displaced by means of a flat bed. Vibrations of the gantry, being transferred to the anti-scatter grid and the X-ray detector, have a negative effect on the quality of the image to be formed. Such negative effects cannot be imitated, so that such image falsifying effects can be reduced to a limited extent only during later image processing. 
     In order to achieve a fast X-ray procedure, the width of the X-ray beam is increased. A larger surface of the object to be examined is thus scanned in a single scan, and hence also a larger volume. However, this has the drawback that the scattered radiation component increases. In order to reduce such an increasing scattered radiation component, the height of the anti-scatter grid is increased. Known anti-scatter grids, however, are not sturdy enough for this purpose. 
     Therefore, it is an object of the invention to provide an anti-scatter grid which aims to reduce scattered radiation and can be simply manufactured with a sturdiness which also suffices for large-area anti-scatter grids. 
     This object is achieved in that comb lamellae extend transversely of an associated comb base surface which supports the comb lamellae. 
     An anti-scatter grid is arranged over the X-ray detector in such a manner that the primary X-rays are incident, via the grid, on a respective detector element arranged therebelow. 
     The anti-scatter grid consists of a plurality of comb elements which absorb X-rays, are provided with comb structures and are fixed in a frame. The comb elements preferably have a rectangular basic shape and comprise comb lamellae which extend transversely of the surface of a base plate and to the comb base surface formed thereby. These comb lamellae form the comb structure. The comb lamellae are focused onto the focal spot of the radiation source, with the result that the distance between the comb lamellae at the top of the comb element is less than that at the lower side. A plurality of such comb elements is arranged in such a manner that the comb lamellae which extend transversely of the comb base surface border or abut against the nearest comb element by way of the associated comb base surface. This results in a two-dimensional grid structure. The distance between the comb lamellae and the depth of the comb lamellae defines the resolution of the anti-scatter grid. The grid openings of this two-dimensional grid are oriented in the direction of the incident X-rays. 
     The sides of the individual comb elements are secured in the frame by way of grooves. The number of comb elements to be linked is determined by the dimensions of the X-ray detector used. In the case of CT apparatus, the length of the X-ray detector usually mounts to a number of times its width. It has been found that the comb elements advantageously are so rugged and stable that a large number of comb elements can be arranged in a frame, thus forming a large-area anti-scatter grid which covers a large-area X-ray detector. 
     In the case of X-ray exposures the X-rays which are characteristic of the region examined are converted in an X-ray detector, for example, into light which itself is either read out by a light-sensitive sensor or is used to expose a film accordingly. 
     In the case of digital X-ray detectors the image information is read out by sensors. In these discrete exposures it is important that the X-ray quanta of a relevant examination zone to be imaged on a pixel are converted only in the associated detector element and are detected only by the corresponding sensor which is situated therebelow the X-ray quanta that are characteristic of the examination zone corresponding to the resolution of the detector reach the associated detector element directly via the relevant grid opening of the anti-scatter grid. The X-ray quanta that are characteristic of an examination zone corresponding to the resolution of the detector are conducted directly to the associated detector element via the corresponding grid opening in the anti-scatter grid. The radiation scattered in the transverse direction is absorbed by the grid structure of the anti-scatter grid. 
     The anti-scatter grid in a further embodiment is made of comb elements having a double comb structure and plane lamellae. The comb elements have comb lamellae which extend in the direction transversely of the base plate on both sides of the base plate. The comb lamellae of these double comb elements extend transversely of the two comb base surfaces to both sides of the base plate. For the anti-scatter grid a double comb element and a plane lamella are linked each time in an alternating fashion. This again results in a grid. The double comb elements and the lamellae are retained by the frame. 
     The comb lamellae of the comb elements are oriented so as to be focussed onto the focal spot of the radiation source. The X-rays are incident on the anti-scatter grid at a given angle. Because the direct X-rays should pass the anti-scatter grid without impediment, the orientation of the grid must be adapted to the radiation angle. To this end, the distances between the comb lamellae at the top of the comb elements are smaller than the distances between the comb lamellae at the bottom of the comb elements. 
     Moreover, in the case of curved X-ray detectors it is also necessary to adapt the anti-scatter grid to the curvature of the X-ray detector. To this end, the depth of the comb lamellae increases towards the lower side of the comb element, so that a curvature corresponding to the curvature of the X-ray detector is obtained upon assembly of a plurality of comb elements. 
     The frame in which the comb elements are secured is adapted to the shape of the X-ray detector. Grooves are provided on the inner sides of the frame. The thickness of the grooves corresponds to the wall thickness of the comb elements, so that they are retained by the shape of the grooves. Additionally, the comb elements can be glued in said grooves. 
     The object according to the invention is also achieved by means of a detector with a grid for the absorbtion of X-rays. 
     Furthermore, the object of the invention is also achieved by means of an X-ray apparatus which includes a grid for the absorption of X-rays which is arranged in front of the detector. 
     The object according to the invention is also achieved by means of a method of manufacturing a grid which includes comb elements which absorb electromagnetic radiation, the comb elements with comb lamellae extending perpendicularly to an associated comb base surface supporting the comb lamellae being arranged in such a manner that they form a two-dimensional grid. 
     Embodiments of the invention will be described in detail hereinafter with reference to the drawing. Therein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a computed tomography apparatus with a grid arranged over the detector, 
     FIG. 2 is a plan view of a one-sided comb element, 
     FIG. 3 is a side elevation of a one-sided comb element, 
     FIG. 4 is a front view of a one-sided comb element, 
     FIG. 5 is a perspective view of a one-sided comb element, 
     FIG. 6 is a side elevation of a plurality of one-sided comb elements arranged over the detector, 
     FIG. 7 shows an anti-scatter grid consisting of one-sided comb elements, 
     FIG. 8 shows a part of a frame with grooves, 
     FIG. 9 is a plan view of a two-sided comb element, 
     FIG. 10 is a plan view of lamellae, 
     FIG. 11 shows an anti-scatter grid consisting of two-sided comb elements and lamellae, and 
     FIG. 12 is a perspective view of a two-sided comb element. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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  1 I 1  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 1×1 to 2×5 mm 2 . 
     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 D o  between the comb lamellae  11  at the upper edge of the base plate  10  is smaller than the distance D u  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.