Figure 1. Vortex map used to identify unrealistic motion. Displacement field when mapping exhale and inhale datasets of a 4D CT scan (a) is represented in (b) with yellow arrows, their direction and intensity proportional with displacement and describing motion during the breathing cycle. At region #1, voxels engage in a circular motion, while in region #2 voxels inside the liver suddenly change direction. Neither displacement represents expected anatomical motion and both are identified as unnatural by the vortex map (c).