Patent Number: 052573052
Section: summary

The invention relates to a slit radiography device, comprising an X-ray source which is capable, when in operation, of scanning a body under examination, via a slit of a slit diaphragm, with a fan-shaped X-ray beam in a direction transverse to the longitudinal direction of the slit, an absorption device comprising a number of movable absorption elements placed next to each other being provided, which absorption elements can be moved into the fan-shaped X-ray beam to a greater or lesser extent under the influence of suitable control signals in order to influence, when in operation, the X-ray radiation incident on the body per sector of the X-ray beam. A device of the type described above is, for example, known from U.S. Pat. No. 4,715,056. U.S. Pat. No. 4,715,056 shows and describes diverse types of absorption elements which can be moved up and down in order to influence a fan-shaped X-ray beam, transmitted or to be transmitted through the slit of a slit diaphragm, per sector thereof. The absorption elements may be composed of small plates, situated next to each other, of material which attenuates or even completely absorbs X-ray radiation, which small plates are placed on the free ends of cantilever-mounted tongue-shaped devices. The tongue-shaped devices may advantageously be piezoelectric tongues, it being possible to control the position of the free ends, and consequently of the absorption elements, directly by electrical signals. Tongue-shaped devices controlled in a different manner may also be used, however, as can, for example, diverse types of means which are able to cause the absorption elements to perform a rectilinear to-and-fro sliding movement. In all cases it is important that the absorption elements can be moved independently of each other in a direction transverse to the longitudinal direction of the slit of the slit diaphragm. Furthermore, adjacent absorption elements should adjoin each other in a manner such that the X-ray radiation cannot pass freely between two elements. The absorption elements with rectangular cross section shown in U.S. Pat. No. 4,715,056 are only able to satisfy this last requirement if adjacent elements are situated so as to fit tightly against each other. However, this adversely affects the free movement capability. The trapezoidal elements and the elements provided with tongue and groove also shown in U.S. Pat. No. 4,715,056 can indeed be fitted with a small gap without X-ray radiation being able to pass freely between two adjacent elements. However, the radiation is then not absorbed to the same extent in the region of the edge parts of the elements which overlap each other as it is by the central part of the elements. This may produce strip-like artefacts in a radiograph to be made, which is undesirable. The applicant has carried out an investigation into a slit radiography apparatus provided with an absorption device which is constructed with tongue-shaped elements which are provided at the free end with small plates of material, which absorbs X-ray radiation, placed transversely to the longitudinal direction of the tongues. In this case, the tongues were alternately relatively short and relatively long and the small plates of absorbing material were chosen so wide that adjacent small plates, which are consequently situated at different distances from the X-ray source, overlap each other to some extent. A drawback of this arrangement is, however, that the overlapping parts of the small plates give rise to strip-like artefacts in the radiographs to be made. The varying distances of the small plates from the X-ray source also result in a varying influencing of the X-ray beam with positions of the small plates which are otherwise identical. The object of the invention is to eliminate the drawbacks outlined and to provide, in general, a slit radiography apparatus having an advantageous absorption device. For this purpose, an apparatus of the type described is characterized, according to the invention, in that at least the edge sections facing each other of adjacent absorption elements are of matching construction and, viewed from the X-ray source, overlap each other, the edge section of an absorption element which, seen from the X-ray source, overlaps an edge section of an adjacent absorption element always being a small distance nearer the X-ray source than the matchingly constructed edge section of the other absorption element and the total material thickness at the site of the overlapping edge sections being equal to the material thickness between the edge sections. A procedure for producing absorption elements for an absorption device of a slit radiography apparatus is characterized according to the invention in that the shape of the absorption elements is constructed by defining, between two equidistant lines which have a mutual spacing which is equal to the desired absorption thickness of the material to be used for the elements, at least two cut lines extending, at least not over the entire length, transversely to the equidistant lines; by defining, on either side of each cut line, two edge regions, which are each bounded by the cut line, a boundary line extending transversely to the equidistant lines and a part of at least one of the equidistant lines; by moving the edge regions thus determined on either side of each cut line over a small distance in opposite directions along the boundary lines, with the result that a gap is produced at the position of each cut line; and by using the shape situated between two gaps thus obtained as a template for the cross section of the absorption elements.