Patent Number: 
Section: claims

1. A method for operating a computed tomography apparatus having an x-ray radiator, which emits an x-ray beam from a focus, a radiation detector at which said x-ray beam is incident on an active detection field of said radiation detector and a diaphragm disposed proximate said radiation detector in said x-ray beam, said diaphragm having oppositely disposed absorber elements linearly movable in said diaphragm toward and away from each other, each of said absorber elements being curved in said x-ray beam relative to said focus, and a movable support adapted to receive an examination subject thereon, said method comprising the steps of:acquiring computed tomography data from the examination subject in a spiral scan of said subject by rotating said x-ray source and said radiation detector around said subject and around a system axis while irradiating said examination subject with said x-ray beam, and while moving said support with the examination subject thereon through said x-ray beam in a direction substantially parallel to said system axis; andduring at least one of a beginning of said spiral scan or an end of said spiral scan, reducing exposure of said examination subject to said x-ray beam by dynamically varying a spacing between said absorber elements by asymmetrically displacing said absorber elements independently of each other in said diaphragm with respect to a line connecting said focus and a center of said active detection field. 2. A method as claimed in claim 1 comprising, before said beginning of said spiral scan, placing one of said absorber elements in a completely closed position in said diaphragm relative to said line, and placing the other of said absorber elements in a completely open position in said diaphragm relative to said line. 3. A method as claimed in claim 2 comprising after said beginning of said scan, opening said absorber element in said closed position in synchronization with said movement of said patient support during said spiral scan. 4. A method as claimed in claim 2 comprising, before an end of said spiral scan, closing said absorber element in said open position in synchronization with said movement of said support. 5. A method as claimed in claim 1 comprising displacing a first of said absorber elements in said diaphragm with a first drive connected to said first of said absorber elements, and displacing a second of said absorber elements in said diaphragm with a second drive, operated independently of said first drive, connected to said second of said absorber elements. 6. A method as claimed in claim 5 comprising mounting the respective absorber elements in said diaphragm for linear movement thereof by said first and second drives. 7. A method as claimed in claim 6 comprising moving said first of said absorber elements in said diaphragm along a first linear guide in said diaphragm and moving said second of said absorber elements along a second linear guide in said diaphragm. 8. A method as claimed in claim 6 comprising employing a first linear motor as said first drive and employing a second linear motor as said second drive. 9. A method as claimed in claim 1 comprising curving each of said absorber elements in a plane perpendicular to said system axis. 10. A method as claimed in claim 1 comprising curving each of said absorber elements along an arc of a circle having a center coinciding with said focus of said x-ray radiator. 11. A method as claimed in claim 1 comprising offsetting said absorber elements with respect to each other along said line by curving said absorber elements with respectively different curvature radii. 12. A method as claimed in claim 11 comprising differing said curvature radii from each other in amount in a range between 0.5% and 10% of an offset spacing between said absorber elements along said line.