1. A method of deformable image registration for thoracic 4-D computed tomography (CT) images, comprising: receiving, by a processing device, a set of thoracic 4-D CT images; and iteratively solving, by the processing device, an energy function applied to subsequent images of the set of thoracic 4-D CT images to transform the subsequent images into respective optical flow fields between the subsequent images, the energy function enforcing the following constraints on the subsequent images: intensity constancy; mass conservation; gradient constancy; and spatio-temporal smoothness.