Patent Number: 051484650
Section: summary

FIELD OF THE INVENTION The invention relates to an X-ray examination apparatus comprising an X-ray source and a detector for detecting X-rays to be transmitted by the X-ray source for forming an X-ray image, an X-ray absorbing filter being arranged between the X-ray source and the X-ray detector, which includes a liquid reservoir placeable in the X-ray beam and having a first and a second wall, the first wall being flexible. The invention also relates to a filter suitable for use in such an X-ray examination apparatus. BACKGROUND OF THE INVENTION Such an X-ray examination apparatus and filter are disclosed in DE 689 868. In this Patent Specification a filter is described for use in a medical X-ray examination apparatus, the filter being formed by a liquid space between two flexible walls. The filter is arranged between an X-ray source and an X-ray detector. An X-ray absorbing liquid can be inserted between the walls. The walls have such a curvature, that X-ray radiation received from an X-ray focus is spatially uniformly attenuated by the filter. The path length of X-rays passing through the filter at its edges, is equal to the path length of X-rays passing through the center portion of the filter. The filter has for its object to remove the low-energetic X-ray radiation, which contributes only little to the image being formed, from the beam. This reduces the dose of radiation to which a patient is exposed during a medical examination. In X-ray examination apparatuses in which an X-ray image is displayed as a televison picture on a monitor with the aid of an X-ray image intensifier tube and a television pick-up device coupled thereto, vignetting occurs due to the geometry of an input screen of the X-ray image intensifier tube and the presence, if any, of optical means between the television pick-up device and the X-ray image intensifier tube. As a result thereof, the edges of the X-ray image become darker than center portions of the X-ray image, even in the case of an uniform irradiation of the X-ray image intensifier tube. The invention has inter alia for its object to provide an X-ray absorbing filter, which reduces the spatial brightness variation in an X-ray image, produced by vignetting. SUMMARY OF THE INVENTION Therefore, according to the invention, an X-ray examination apparatus, comprises a liquid reservoir which has a wall spacing which decreases from the reservoir center to the edges in order to reduce vignetting when the detector is irradiated by a uniform X-ray beam. The liquid pressure causes the flexible wall to become curved, with a maximum deviation relative to the second wall in the center portion of the filter. The brightness variation of the X-ray image due to vignetting shows in a first approximation a quadratic decrease from the center of the X-ray image towards the edges. When a filter thickness is used which is small with respect to an attenuation length of the X-ray radiation on attenuation by the filter fluid, the attenuation increases approximately linearly with filter thickness and the spatial brightness variation due to vignetting is compensated for by the filter. A preferred embodiment of an X-ray examination apparatus in accordance with the invention, is one where, the second wall of the filter is a rigid wall. A rigid wall can, for example, be fitted to an exterior side of the X-ray source and permits the filter can be handled with greater ease on, for example, exchange of the filter. A material for the rigid wall can be chosen such, that a desired filtration of the X-ray spectrum is effected. A proper seal between the flexible wall and the rigid wall can be obtained by using an anular clamping member. An embodiment of an X-ray examination apparatus in accordance with the invention, in which the second wall of the filter is flexible, comprises one where, the second wall has a curvature opposite to the curvature of the first wall. Two flexible walls which have their edges sealed together in a fluid-tight manner, for example by clamping them between two anular clamping members, assume a paraboloid shape, the walls curving in opposite directions. For small filter thicknesses for which the exponential attenuation varies approximately linearly versus the filter thickness, the attenuation is twice as high when two flexible walls are used than when one flexible wall is utilized. Consequently, a higher attenuation can be obtained at a smaller stretch of the flexible walls, so that the paraboloid shape of the filter is maintained to an improved extent. A further preferred embodiment of an X-ray examination apparatus in accordance with the invention, comprises a filter connected to a fluid pump. Via a pump which is controlled, for example, in dependence on a video signal from the television pick-up device, a filter thickness setting can be effected by the supply and discharge of fluid to and from the reservoir. The filter is adjusted such, that on irradiation of the X-ray detector by a uniform X-ray beam, the video signal from the picture edges of the X-ray image is approximately equal to the video signal from the centre portions of the X-ray image. Such a setting is necessary since the vignetting changes when the spacing between the X-ray source and the X-ray detector is varied.