1. A method for simulating four-dimensional (4D) computed tomography (CT), comprising the steps of: generating a first set of volumes from a single thoracic CT scan; deforming the first set of volumes according to a biomechanical motion model to obtain a set of deformed volumes; constructing a second set of volumes using the set of deformed volumes; and deforming the second set of volumes according to a mass-spring model using the CT scan to produce the 4DCT, wherein the steps are performed in a processor.