A method and apparatus for magnetic resonance (MR) imaging of a moving object, and in particular MR imaging by retrospectively selecting and sorting the MR data acquired with the known prior positions of an orthopedic joint. A mechanical apparatus limits the motion of the joint while an optical or electronic apparatus determines the instantaneous articulation position of the joint, for example its rotational angle. Either partial or complete MR data are acquired asynchronously with respect to various positions of joint movement, and the MR data are then retrospectively selected and sorted to form images of the joint at the various articulated positions. This retrospective selecting and sorting allows sharp images to be acquired using MR data acquired over several flexions and extensions of the joint, regardless of the periodicity or velocity of its motion.