Patent ID: 7170975

Claim:
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; and during 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.