1. School of Mathematics, Shandong University, Jinan, Shandong, China
2. Biomedical Imaging Center, Rensselaer Polytechnic Institute, Troy, NY, USA
3. Department of Medical Engineering, Shandong Provincial Chest Hospital, Jinan, Shandong, China
4. School of Mechanical Engineering and Automation, Beijing University of Aeronautics and Astronautics, Beijing, China
5. Department of Mathematics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
6. Department of Mathematics, The University of Iowa, Iowa City, IA, USA