This invention relates to a multiple frame imaging system including radiation source, a detector having an input area, a monitor configured to display detected images, means for determining at least one Region of Interest (ROI) of an object on the displayed image, and a collimator having means for projecting the at least one region of interest (ROI) on at least one selected fraction of the input area exposed by the x-ray source. The collimator including at least three essentially non-overlapping plates mounted in a plane generally parallel to the detector input surface plane, each plate includes a first edge in contact with an edge of a first neighboring plate and a second edge adjacent to the first edge in contact with an edge of a second neighboring plate； and means for moving each one of the plates.
