Cone Beam CT Imaging with Limited Angle of Projection and Prior Knowledge for Volumetric Verification of Non-Coplanar Beam Radiation Therapy: A Proof of Concept Study Phys Med Biol Meng, B., Xing, L., Han, B., Koong, A., Chang, D., Cheng, J., Li, R. 2013; 58 (21): 7777-89