1Biomedical Engineering, Tsinghua University, Beijing, China, 2Gordon Center for Medical Imaging, Radiology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, United States, 3Nuclear Medicine, West China Hospital, Sichuan University, Sichuan, China, 4Engineering Physics, Tsinghua University, Beijing, China, 5Key Laboratory of Particle and Radiation Imaging, Ministry of Education, Medical Physics and Engineering Institute, Tsinghua University, Beijing, China