Patent Number: 051484650
Section: claims

1. An X-ray examination apparatus comprising an X-ray source and a detector for detecting a beam of X-rays transmitted by the X-ray source for forming an X-ray image, an X-ray absorbing filter between the X-ray source and the X-ray detector, said filter including a liquid reservoir adapted for placement in the X-ray beam and having a first and second wall, at least the first wall being flexible, said first and second walls for receiving said liquid reservoir therebetween and having a wall spacing which decreases from the reservoir center to the reservoir edges, said wall spacing being dimensioned to reduce vignetting in response to irradiation of the detector by a uniform X-ray beam. 2. An X-ray examination apparatus as claimed in claim 1, wherein the second wall is rigid. 3. An X-ray examination apparatus as claimed in claim 2, the first wall of the filter is connected at said edges to the second wall via an anular clamping member. 4. An X-ray examination apparatus as claimed in claim 1 wherein both walls of the filter are flexible, the first wall has a curvature opposite to the curvature of the second wall. 5. An X-ray examination apparatus as claimed in claim 4, wherein the walls of the filter are clamped between two anular clamping members in a liquid-tight manner. 6. An X-ray examination apparatus as claimed in claim 1 including a pump connected to the filter. 7. An X-ray examination apparatus as claimed in claim 5 including a pump connected to the filter. 8. A filter for use with an X-ray examination apparatus comprising an X-ray source and an X-ray detector for detecting a beam of X-rays transmitted by the X-ray source for forming an image, said image exhibiting vignetting, said filter comprising: a first wall;  a second wall, at least one of said walls being flexible; and  means for securing the filter in said beam and the walls to each other at their edges to form a liquid receiving cavity therebetween, said walls being so spaced and so dimensioned such that said cavity has a spacing which decreases from the cavity central region towards said edges in a manner to reduce said vignetting.  a first wall;  a second wall, at least one of said walls being flexible; and  means for securing the filter in said beam and the walls to each other at their edges to form a liquid receiving cavity therebetween, said walls being spaced so that said cavity has a spacing which decreases from the cavity central region towards said edges in a manner to reduce said vignetting, both said walls being flexible. 9. The filter of claim 8 wherein the at least one wall curves from said central region towards said edges. 10. The filter of claim 8 wherein both walls are flexible. 11. The filter of claim 8 further including pump means coupled to said cavity for pumping liquid to and from said cavity for adjusting the spacing of said walls. 12. The filter of claim 10 wherein the walls are mirror images of each other curving from the central region to their edges. 13. The filter of claim 8 including filter adjust means coupled to said cavity for adjusting the spacing of said walls and thus the size of said cavity to thereby adjust the amount of vignetting that is reduced. 14. The filter of claim 13 wherein the filter adjust means includes means for adjusting the pressure of said liquid in said cavity to thereby flex said at least one wall. 15. The filter of claim 8 wherein the spacing between the walls at said central region decreases to the spacing of the walls at said edges according to a quadratic relationship. 16. An X-ray examination apparatus comprising an X-ray source and a detector for detecting a beam of X-rays transmitted by the X-ray source for forming an X-ray image, an X-ray absorbing filter between the X-ray source and the X-ray detector, said filter including a liquid reservoir adapted for placement in the X-ray beam and having a first and second wall, at least the first wall being flexible, said first and second walls for receiving said liquid reservoir therebetween and having a wall spacing which decreases from the reservoir center to the reservoir edges to reduce vignetting in response to irradiation of the detector by a uniform X-ray beam, both walls of the filter being flexible, the first wall having a curvature opposite to the curvature of the second wall. 17. A filter for use with an X-ray examination apparatus comprising an X-ray source and an X-ray detector for detecting a beam of X-rays transmitted by the X-ray source for forming an image, said image exhibiting vignetting, said filter comprising: