Patent Number: 060318937
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a portion of a stray radiation grid that is composed of lead lamellae 1 and the carrier medium 2, here in the form of a paper layer. Two incident X-rays 3 are also shown in the schematic drawing. The computational description of the inventive stray radiation grid is recited below with reference to FIG. 1. The grid is constructed of parallel lead lamellae 1 having the characteristic quantities: d=thickness of the lead lamellae PA1 D=thickness of the carrier medium along the center line of the grid PA1 h=height of the lead lamellae PA1 f=focussing spacing. The shaft ratio r is calculated as follows along the center lie, i.e. in the middle of the grid: EQU r=h/d. Due to the oblique incidence of the X-rays (angle a relative to the center ray) of the image-producing rays and given the same height h of the lead lamellae 1, the same shaft ratio is achieved toward the grid edge when the spacing of the lead lamellae 1 is correspondingly enlarged. The following then applies according to FIG. 1: EQU h=r.multidot.D; EQU h=h/cos .alpha.; EQU D=D/cos .alpha.; EQU D=h.multidot.tan .alpha.. The lamella spacing D' dependent on the incident angle of the radiation then is derived as follows: EQU D'=D"+D"' EQU =D/cos .alpha.+H.multidot.tan .alpha. EQU =D/cos .alpha.+r.multidot.D.multidot.tan .alpha. EQU =D.multidot.(1/cos .alpha.+r.multidot.tan .alpha.). With F=(1/cos .alpha.+r.multidot.tan .alpha.), then D'=D.multidot.F. F is a factor dependent on the incident angle that increases with increasing distance from the middle toward the edge and exhibits its maximum value at the edge. Accordingly, d'.about.D' and, thus, EQU d'=d.multidot.(1/cos .alpha.+r.multidot.tan .alpha.)=d.multidot.F applies for a constant lead content per length unit. In the practical embodiment, however, the lead content of the grid will be advantageously sub-proportionally increased relative to the lamella spacing because the imaging radiation and the stray radiation decrease toward the edge (given extensive grids) as a result of the distance square law. The focussing of stray radiation grids with adapted line or element density additionally allows the grid to be adapted to the decreasing dose rate in the ray cone toward the outside. In the form of a table for different grid-focus spacings given a constant grid width k, i.e. the side length perpendicular to the lead lamellae 1, and a constant shaft ratio, FIG. 2 shows the different F-values arising therefrom, with the maximum F-value occurring at the grid edge being outlined. In addition, the table indicates the respective a-values at the extreme edge of the grid with which the calculation was implemented as an example; f and k are respectively indicated in millimeters, .alpha. in degrees, r and F have no dimension. As can be seen, the F-value decreases with increasing focus distance, due to the smaller and smaller incident angle. The same decrease also occurs with decreasing shaft ratio, i.e. the F-value also decreases here as the shaft ratio becomes smaller and smaller. FIG. 3 shows an exemplary embodiment of the invention wherein, in addition to an enlargement of the lamella spacing itself, the thickness of the lamellae also increases from the middle toward the edge. A section through a stray radiation grid 4 is shown in FIG. 3. Respective regions of interest in the middle of the grid (FIG. 3A) and at the edge of the grid (FIG. 3B) are shown enlarged. As can be seen, the spacing D of the lamellae in the region of the middle of the grid is clearly smaller than the spacing D' of the lamellae 6 at the edge of the grid, as is computationally derived from the above equation. In order to have a constant lead content per length unit, the lamella thickness is likewise increases toward the edge, proceeding from the middle, which, as the above equations indicate, can also be computationally determined. This means that the value d is smaller than the value d'. The following spacing and thickness values in the middle of the grid or at the edge of the grid, respectively, derive for the stray radiation grid having the parameters indicated dot-dashed in FIG. 2: ______________________________________ Grid middle: Grid edge: ______________________________________ D = 40 .mu.m D' = d .multidot. 2.69 = 107.6 .mu.m d = 8 .mu.m d' = d .multidot. 2.64 = 21.52 .mu.m ______________________________________ The invention is not limited to the illustrated exemplary embodiments of stray radiation grids composed of lead lamellae 1 and paper as carrier medium. The invention can likewise be applied to the recently developed silicon stray radiation grids, whereby the respective spacing increase and thickening of absorption elements can be unproblemmatically achieved using photolithographic means, so that corresponding stray radiation grids optimized in view of the focussing can also be easily produced for different focus distances. According to an alternative embodiment of the invention, the grid shown in FIG. 3 can be composed of a number of segments arranged next to one another with the lamella spacing and lamella thickness being constant with a segment, but changing from segment to segment. For each segment, the spacing therein is selected as the average of the edge values calculated at its edge, the same being true of the lamella thickness. The invention is not limited to the illustrated embodiments but can also be applied given different types of stray radiation grids having a different structure or composed of different materials. Although various minor modifications might be suggested by those skilled in the art, it should be understood that my wish to embody within the scope of the patent warranted hereon all such modifications as reasonably and properly come with the scope of my contribution to the art.