A multiple frame imaging system comprising: 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 comprising means for projecting the at least one region of interest (ROI) on at least one selected fraction of the input area exposed by said x-ray source, the collimator comprising at least three essentially non-overlapping plates mounted in a plane generally parallel to the detector input surface plane, wherein each plate comprises a first edge in contact with an edge of a first neighboring plate and a second edge adjacent to said first edge in contact with an edge of a second neighboring plate; and means for moving each one of the plates.
