Patent Number: 
Section: claims

1. A diaphragm device for gating a moving x-ray beam in an x-ray apparatus having a radiation detector on which the x-ray beam is incident, said diaphragm device comprising:at least two diaphragms, each disposed to interact with said moving x-ray beam during a scan of a subject;a control unit that operates a first of said at least two diaphragms to gate said x-ray beam to define an area extent of said x-ray beam that is substantially unchanging during said scan, and that operates a second of said at least two diaphragms to dynamically mask at least a portion of said area extent relative to said radiation detector dependent on movement of said x-ray beam during the scan. 2. A diaphragm device as claimed in claim 1 wherein said control unit operates said first of said at least two diaphragms to gate said x-ray beam with a high adjustment precision and operates said second of said at least two diaphragms to dynamically mask said area extent of said x-ray beam with a high adjustment speed. 3. A diaphragm device as claimed in claim 1 wherein said x-ray beam emanates from a focus, and wherein said second of said at least two diaphragms is disposed closer to said focus than said first of said at least two diaphragms. 4. A diaphragm device as claimed in claim 1 comprising an adjustment mechanism operated by said control unit that adjusts said first of said at least two diaphragms and said second of said at least two diaphragms in parallel with each other. 5. A diaphragm device as claimed in claim 1 wherein at least one of said first of said at least two diaphragms and said second of said at least two diaphragms comprises adjustable diaphragm elements. 6. A diaphragm device as claimed in claim 1 wherein at least one of said first of said at least two diaphragms and said second of said at least two diaphragms is a slit diaphragm. 7. An x-ray examination apparatus for scanning a subject, comprising:an x-ray source that emits an x-ray beam emanating from a focus;a radiation detector disposed in a path of said x-ray beam, said x-ray source and said radiation detector being configured to receive a subject therebetween and being movable to conduct a scan of the subject with said x-ray beam, thereby also producing movement of said x-ray beam during said scan;a diaphragm device disposed in said path of said x-ray beam preceding the subject, said diaphragm device comprising at least two diaphragms each disposed to interact with the moving x-ray beam during said scan; anda control unit that operates a first of said at least two diaphragms to define an area extent of said x-ray beam, that is substantially unchanging during said scan, and that operates a second of said at least two diaphragms to dynamically mask a portion of said area extent of said x-ray beam, relative to said radiation detector, dependent on the movement of the x-ray beam during the scan. 8. An x-ray examination apparatus as claimed in claim 7 wherein said x-ray source and said radiation detector are movable to conduct a spiral scan of the subject, as said scan of the subject. 9. An x-ray examination apparatus as claimed in claim 7 wherein said diaphragm device comprises a first adjustment mechanism that is operated by said control unit to adjust said first of said at least two diaphragms with a high adjustment precision, and a second adjustment mechanism that is operated by said control unit to adjust said second of said at least two diaphragms with a high adjustment speed. 10. An x-ray examination apparatus as claimed in claim 7 comprising an image reconstruction computer that receives attenuation data from said radiation detector and that calculates an image of the subject from said attenuation data using a reconstruction algorithm, and wherein said control unit operates said first of said at least two diaphragms to define said area extent of said x-ray beam so that said x-ray beam irradiates only a region of the subject that contributes to reconstruction of said image in said reconstruction algorithm. 11. An x-ray examination apparatus as claimed in claim 7 wherein said scan comprises a leading movement, and wherein said control unit operates said first of said at least two diaphragms to gate said x-ray beam during said leading movement of said scan. 12. An x-ray examination apparatus as claimed in claim 7 wherein said scan comprises a trailing movement, and wherein said control unit operates said first of said at least two diaphragms to gate said x-ray beam during said trailing movement of said scan. 13. An x-ray examination apparatus as claimed in claim 7 comprising a system axis, and wherein said control unit operates said second of said at least two diaphragms to dynamically mask said x-ray beam dependent on a position in said scan in a direction of said system axis. 14. An x-ray examination apparatus as claimed in claim 7 wherein said second of said at least two diaphragms is disposed closer to said focus than said first of said at least two diaphragms. 15. An x-ray examination apparatus as claimed in claim 7 wherein said diaphragm device comprises an adjustment mechanism that is operated by said control unit to adjust said first of said at least diaphragms and said second of said at least two diaphragms in parallel with each other. 16. An x-ray examination apparatus as claimed in claim 7 wherein at least one of said first of said at least two diaphragms and said second of said at least two diaphragms comprises adjustable diaphragm elements. 17. An x-ray examination apparatus as claimed in claim 7 wherein at least one of said first of said at least two diaphragms and said second of said at least two diaphragms is a slit diaphragm. 18. A method for scanning a subject with an x-ray beam, comprising the steps of:emitting an x-ray beam from a focus of an x-ray source;moving said x-ray source and a radiation detector disposed in a path of said x-ray beam, with a subject therebetween, to conduct a scan of the subject with said x-ray beam, thereby also moving said x-ray beam;disposing a diaphragm device in a path of said x-ray beam preceding said subject, said diaphragm device comprising at least two diaphragms; andautomatically adjusting a first of said at least two diaphragms to define an area extent of said x-ray beam that is substantially unchanging during said scan, and automatically operating a second of said two diaphragms to dynamically mask a portion of said area extent of said x-ray beam, relative to said radiation detector, dependent on the movement of the x-ray beam during the scan. 19. A method as claimed in claim 18 comprising moving said x-ray source and said radiation detector to conduct a spiral scan of the subject, as said scan. 20. A method as claimed in claim 18 comprising, in said diaphragm device, adjusting said first of said at least two diaphragms with a high adjustment precision, and adjusting said second of said at least two diaphragms with a high adjustment speed. 21. A method as claimed in claim 18 comprising computationally reconstructing an image of the subject from attenuation data from the radiation detector using a reconstruction algorithm, and comprising, with said first of said at least two diaphragms, defining said area extent of said x-ray beam so that said x-ray beam irradiates only a region of the subject that contributes to reconstruction of said image in said reconstruction algorithm. 22. A method as claimed in claim 18 wherein said scan comprises a leading movement, and comprising, with said first of said at least two diaphragms, gating said x-ray beam during said leading movement of said scan. 23. A method as claimed in claim 18 wherein said scan comprises a trailing movement, and comprising, with said first of said at least two diaphragms, gating said x-ray beam during said trailing movement of said scan. 24. A method as claimed in claim 18 comprising conducting said scan around a system axis, and comprising, with said second of said at least two diaphragms, dynamically masking said x-ray beam dependent on a position in said scan in a direction of said system axis. 25. A method as claimed in claim 18 comprising placing said second of said at least two diaphragms closer to said focus than said first of said at least two diaphragms. 26. A method as claimed in claim 18 comprising adjusting said first of said at least diaphragms and said second of said at least two diaphragms in parallel with each other.