1. A method for determining a reference radiograph, comprising the steps of: providing a Four-Dimensional Cone Beam Computed Tomography (4D-CBCT) apparatus, comprising a source and a detector configured to be selectively positioned at different angular positions around a patient, the 4D-CBCT apparatus generating a plurality of CBCT images corresponding to different times; providing a radiographic apparatus comprising a radiographic source and a radiographic detector, the radiographic apparatus being configured to generate a sequence of radiographs corresponding to different times; positioning a patient for generating CBCT images of the patient with the 4D-CBCT apparatus and radiographs of the patient with the radiographic apparatus; performing a 4D-CBCT scanning sequence with the 4D-CBCT apparatus to generate a plurality of CBCT images of a portion of the patient's body, the CBCT images corresponding to different temporal phases associated with the patient; performing a radiographic sequence with the radiographic apparatus so as to generate a plurality of first radiographs of patient, the first radiographs corresponding to different temporal phases associated with the patient; providing a synchronization device configured to correlate a CBCT image corresponding to a given temporal phase with a radiograph corresponding to an equivalent temporal phase; generating a correlated radiograph by correlating, with the synchronization device, one of the first radiographs with a designated one of the CBCT images, the designated CBCT image corresponding to a designated temporal phase; and identifying the correlated radiograph as the reference radiograph.