Schematic diagram of the motion model. Realtime 2D free breathing images of each pig are registered to a corresponding end expiration 3D prior volume. All images are cardiac gated and acquired along with synchronized respiratory physiology data. Image registration was used to extract motion parameters consisting of rotations and translations along the x,y,z imaging axes. Each parameter was then fitted as a linear function of the physiology data to produce a subject-specific respiratory motion model.