An assembly including a base and first and second spaced-apart shafts secured to the base and extending generally perpendicular to an elongated aperture of the base. A carrier includes a first support member slidingly receiving the first shaft, a second support member receiving the second shaft, and an elongated opening extending between the support members and generally perpendicular to the shafts. The elongated opening of the carrier is for aligning with the aperture of the base and an elongated slit of a collimator supported on the carrier for passage of an x-ray beam therethrough. A backlash-resistant nut assembly is threadingly received on a threaded portion of the second shaft and secured to the second support member.