Patent Number: 054835720
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to an x-ray apparatus comprising a carrier supporting an x-ray source for producing an x-ray beam, an x-ray detector facing the x-ray source and an adjustable absorption filter which is arranged between the x-ray source and the x-ray detector, the carrier being moveable to direct the x-ray beam path. 2. Description of the Related Art An x-ray apparatus of said kind is known from the European patent application EP 0 496 438 which corresponds to U.S. Pat. No. 5,287,396. An x-ray apparatus as described in the cited European patent application is provided with absorption filters having the form of moveable wedge filters. To prevent areas in an x-ray image from being overexposed, wedge filters are positioned so that very low x-ray absorption areas adjacent to strong x-ray absorption areas are blocked out to some extent. Wedge filters are positioned in the known x-ray examination apparatus on the basis of contour recognition during the formation of the x-ray image. Consequently, complicated image processing means are required. SUMMARY OF THE INVENTION It is an object of the invention to provide an x-ray examination apparatus for producing an x-ray image in which overexposed areas are reduced. It is also an object of the invention to reduce a dose of x-radiation required for producing the x-ray image. To achieve this, an x-ray examination apparatus according to the invention is characterized in that the x-ray examination apparatus is provided with filter-control means for accepting a posture of the carrier and furnishing a position of the absorption filter so as to control the adjustment of the absorption filter. When a shadow-image is made of a part of a patient's body to be examined by means of x-irradiation, there are areas of the x-ray image in which overexposure is to be avoided in that these areas are blocked out by means of an absorption filter. The location of such areas with respect to a region of interest in the x-ray image is determined by the orientation of the x-ray beam path with respect to the patient. Since anatomical structures show a comparatively limited variation among various patients, the location of areas subjected to risk of overexposure is predominantly determined from the position of the carrier on which the x-ray source and the x-ray detector are mounted. Thus, absorption filters are adequately positioned by providing filter-control means that control the positioning of the moveable absorption filter on the basis of the posture of the carrier. In this manner, with respect to the patient and with respect to a patient support table, protracted irradiation of the patient is avoided when a person operating the x-ray examination apparatus adjusts the absorption filter. Thus, subjection of both patient on operating personnel to harmful x-radiation is reduced. A preferred embodiment of an x-ray examination apparatus in according to the invention is characterized in that the filter-control means comprises a memory device for storing pairs of position data, a pair consisting of a position data of the absorption filter and a posture of the carrier. The filter-control means is further adequately operated by providing a memory device in which positions of the carrier and corresponding position adjustment data of the absorption filter are stored. These data constitute one or several adjustment curves which represent the positioning of the absorption filter on the basis of the posture of the carrier. A further preferred embodiment of an x-ray examination apparatus according to the invention is characterized in that the filter-control means is arranged to furnish a translation of the absorption filter with respect to the x-ray beam path from said posture of the carrier. In particular, overexposure is avoided in an area in the x-ray image in that the absorption filter is moved into the x-ray beam to some extent by translating the absorption filter with respect to the x-ray beam path. In this manner part of the x-ray beam is attenuated to some extent. When the filter-control means is provided with a memory device, then the memory device will preferably contain an adjustment curve representing translation distances of the absorption filter corresponding to postures of the carrier. A further preferred embodiment of an x-ray examination apparatus according to the invention is characterized in that the absorption filter is rotatable and comprises an x-ray absorbing part and an x-ray transmitting part and that the filter-control means is arranged to furnish an orientation of the absorption filter with respect to an axis of rotation of the absorption filter. A sophisticated absorption filter comprises an x-ray absorbing part and an x-ray transmitting part which has the shape of a section of a substantially circular disk and such an absorption filter is slidable into or out of the x-ray beam path and is in addition rotatable about an axis of rotation making right angles to the plane of the disk section. By rotating of the absorption filter being within in or close to the x-ray beam path, portions of the x-ray beam having a comparatively more complicated form can be blocked out. In an x-ray examination apparatus in according to the invention, the absorption filter is controlled by the filter control means so as to adequately position the absorption filter and block out a relevant portion of the x-ray beam. When the filter-control means is provided with a memory device, this memory device will preferably contain an adjustment curve representing orientation angles of the absorption filter corresponding to postures of the carrier. A further preferred embodiment of an x-ray examination apparatus according to the invention is characterized in that the memory device is arranged to store a plurality of sets of said pairs of position data, each set corresponding to a class of objects to be examined. Although the anatomical structure of various patients may be comparatively similar, there is some variation between various groups of patients, such as e.g. corpulent or slender patients or adults or infants. When patients from either group are examined, an even more adequate adjustment of the absorption filter is achieved when various adjustment curves are provided in said memory device, each adjustment curve pertaining to one of said groups of patients. These and other aspects of the invention will become apparent from and will be elucidated with reference to the embodiments described hereinafter and with reference to the accompanying drawings.