Patent Number: 051484650
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows an X-ray examination apparatus including an X-ray source 1 for transmitting a beam of X-rays 3. The beam of X-rays 3 impinges upon an input screen 5 of an X-ray image intensifier tube 7. The input screen 5 has a phosphor layer located behind a glass or aluminium envelope and in which the X-ray beam 3 effects luminescence. With the aid of a photocathode the light quanta emitted by the phosphor are captured and electrons are released which are accelerated to, for example, 20 keV and are displayed on an output screen 9. Via a twin optical system 11 the luminous image from the output screen is displayed on an input screen of a television pick-up device 13. A partially transmitting mirror 17 which projects the luminous image originating from the output screen onto a photographic film in a film camera 19 is between the lenses of the twin optical system 11. The television pick-up device 13 generates a video signal which is proportional to the light intensity detected at its input screen. The video signal is applied to a television monitor 15 and to a control device 25. Arranged between the X-ray source 1 and the X-ray image intensifier tube 7 is an adjustable filter 21 which via a fluid pump 23 is adjustable by the control device 25 in dependence on the video signal. Then, fluid is supplied to or withdrawn from the filter by the pump 23 until on uniform irradiation of the input screen 5 the video signal for edges of an X-ray image is equal to the video signal for center portions of the X-ray image. FIG. 2 shows schematically how an object 27 extending in an x-direction located transversely of the optical axis 29 is displayed by a lens 31. An apertured diaphragm 30 is between the object 27 and the lens 31. When the object 27 is a uniform line source, then the flux passing through the aperture of the diaphragm 30 varies proportionally to (cos .theta.).sup.4, wherin .theta. is the angle of the radius between the portion of the object emitting the flux and the center of the aperture to the optical axis 29. For Gaussian systems, in which a distance in the x-direction is small compared to the dimensions of the system along the optical axis 29, the following approximation can be made for an object located at a distance L from the diaphragm 30: ##EQU1## An image 27' of the object 27 has, because of the bounding diaphragm 30 an intensity variation which quadratically decreases relative to the optical axis 29. For a length L of 5 cms. and a distance x to the optical axis of 1 cm this approximation is accurate to within 0.5%. FIG. 3 shows how a beam of X-rays impinging from the X-ray focus 33 onto the input screen 5 of the X-ray image intensifier tube has on the input screen a higher intensity than an X-ray beam 35 incident on the input screen 5 at an angle .alpha. relative to the optical axis. The intensity on the input screen varies as (cos .alpha.).sup.3, which for small distances of x with respect to Z can be approximated by: ##EQU2## Herein Z is the spacing between the X-ray focus 33 and the input screen 5. For a value of 1 meter for Z this approximation is accurate to within 0.02%. Both vignetting due to bounding apertures in the optical system of the X-ray imaging system and the decrease in intensity due to the geometry of the input screen of the X-ray image intensifier tube can be compensated for by the filter 21 between the X-ray source 1 and the X-ray image intensifier tube 7. For water, an attenuation length amounts to approximately 3 cm. for X-rays generated in an X-ray tube at approximately 80 kV. For a maximum filter thickness of 0.5 cm (in the center of the filter) the attenuation is linear within a margin of 0.5%. A layer of water having a thickness which decreases quadratically, provided between the source 1 and the X-ray image intensifier tube 7 can compensate for vignetting. Since vignetting varies versus the distance Z between the X-ray source 1 and the X-ray image intensifier tube 7, it is advantageous for the filter thickness to be variable. FIG. 4 shows a filter 21, two flexible X-ray radiation transparent walls 37 and 39 being clamped between two anular clamping members 40 and 41. The walls 37 and 39 may be rubber or thermoplastic, for example. FIG. 5 shows an X-ray radiation transparent filter 21, a flexible wall 43 being clamped between a rigid wall 45 of, for example, Perspex, an X-ray transparent thermoplastic material, and an anular clamping member 47. In this situation the filter has, for example, a diameter D of 10 cm. and a maximum height H of 2.5 cm. The filter is connected to the fluid pump 23, not shown, via a supply line 49. For an adequate form-retaining capacity of the flexible wall 43, not disturbed by force of gravity effects, the overpressure in the filter preferably exceeds 0.5 atmosphere. The walls curve in a lens-like arrangement in which the central region has the greatest spacing between the filter walls and the spacing decreases to the wall edges in a curve-like manner to a minimum similar to an optical lens.