Patent Number: 063058421
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to an X-ray examination apparatus which includes an X-ray source and a diaphragm unit which is connected to the X-ray source and is provided with shutters for limiting a radiation cone beam emanating from a focal spot of the X-ray source, and also includes a light source for generating a light cone beam which traverses the shutters via a reflector device. The invention also relates to a diaphragm device for an X-ray source. 2. Description of the Related Art An X-ray examination apparatus and a diaphragm unit of the kind set forth are known from U.S. Pat. No. 5,844,962. Therein, the light cone serves as an aid which facilitates the adjustment by the user of the irradiated field exposed to the X-rays during a subsequent X-ray exposure. The light cone then illuminates either the patient to be examined or, in the case of an X-ray examination apparatus provided with a grid drawer, the grid drawer when the X-ray source and the grid drawer with the film to be exposed are temporarily pulled to a position to the side of the table top on which the patient is accommodated. The illuminated surface should then correspond to the surface exposed to the cone beam of the X-ray source during a subsequent X-ray exposure. The light source is then situated at the same optical distance from the shutter plates as the focal spot of the radiation source. The light source should be as inexpensive as possible, have a long service life and a high brightness, and the light cone defined thereby should correspond as accurately as possible to the radiation cone formed during a subsequent X-ray exposure. The dimensions of light sources, i.e. the structure emitting the light, for example the filament of the light source, however, are large in comparison with those of the focal spot of the X-ray source. Consequently, the surface illuminated by the light source is larger than the surface subsequently exposed to the X-rays. In order to match the surface irradiated by the X-ray source and the surface irradiated by the light source, the known X-ray examination apparatus is provided with a motor drive for the shutters, which drive is controlled in such a manner that during the adjustment by means of the light source the diaphragm aperture is smaller than during a subsequent X-ray exposure. This solution necessitates the presence of an appropriately controlled motor drive. SUMMARY OF THE INVENTION It is an object of the present invention to provide an X-ray examination apparatus as well as a diaphragm unit which do not require a motor drive so as to achieve matched illuminated and irradiated fields. This object is achieved according to the invention in that the shutters are provided with correction shutters which are transparent to X-rays but impervious to light and limit the light cone. A correspondingly constructed diaphragm unit is a further object of the present invention. Because they are transparent to X-rays the correction shutters according to the invention do not affect the radiation cone but limit the light cone. When the correction shutters are suitably proportioned, the illuminated field and the irradiated field can thus be matched. Exact matching of this kind can be achieved only for a defined distance between the focal spot and the image converter (for example, a film). For other distances (Bucky exposure apparatus usually operates with a defined distance, for example 1.15 m) such matching is no longer exact, but is significantly better than in the absence of correction shutters. Simple X-ray examination apparatus may have one or more different diaphragm apertures, each one of which limits the radiation cone. In that case the correction shutters should constrict the diaphragm apertures on all sides. Contemporary X-ray examination apparatus, however, comprise two preferably adjustable pairs of diaphragm plates for limiting the radiation cone in two mutually perpendicular directions. An embodiment of the invention which is suitable for such X-ray examination apparatus is such that the shutters include two preferably adjustable pairs of shutter plates having pair-wise parallel shutter edges for limiting the radiation cone beam in two mutually perpendicular directions, and that a correction shutter is connected to each shutter plate in such a manner that its edge extends parallel to the shutter edge. The correction shutters can then be arranged in different manners. In another embodiment, the correction shutters connected to a pair of shutter plates are situated in the same plane. The correction shutters are then situated at the same side for each pair of diaphragm plates. However, in that case the correction shutters obstruct the complete closing of the pair of diaphragm plates. Such complete closing, however, usually is not necessary. In a further embodiment which enables complete closing of the pairs of diaphragm plates, the correction shutter are situated in different planes. In an additional embodiment, the focal spot of the X-ray source is inclined relative to the planes of the correction shutters, and the offset between the planes of the correction shutters is chosen to be such that the asymmetry of the radiation cone beam, imposed by the inclination, is at least partly compensated for by a corresponding asymmetry of the light cone beam. The non-symmetry occurring for one of the two pairs of diaphragm plates in an X-ray tube having a strip focus can thus be removed.