1. A method for building a 4D reconstruction of a region of interest exhibiting multiple phases of periodic motion, the method comprising: (i) building one or more 3D reconstructions using a set of 2-D projections; and (ii) deriving one or more 3D model segments from each of said one or more 3D reconstructions, wherein a plurality of 3D model segments are formed thereby, and wherein each of the one or more 3D model segments is derived from a single one of the one or more 3D reconstructions, wherein, the plurality of derived 3D model segments forms a 4D reconstruction of the region of interest.