1. A method for building a 4D reconstruction of a region of interest exhibiting multiple phases of periodic motion, the method comprising: (i) building, with a processor, a plurality of 3D reconstructions using a plurality of subsets of 2-D projections which are identified from a set of 2-D projections, wherein each of the plurality of subsets of the 2-D projections corresponds to a different motion phase of the region of interest and each of the 3D reconstructions corresponds to a different one of the plurality of subsets; and (ii) deriving, with the processor, one or more 3D model segments from each of said plurality of 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 plurality of 3D reconstructions, wherein, the plurality of derived 3D model segments forms a 4D reconstruction of the region of interest.