Bayesian Image Reconstruction Algorithm Based on a Novel Nonlocal Quadratic MRF Prior Model ( 1.Laboratory of Image Science and Technology,Department of Computer Science and Engineering,Southeast University,Nanjing,Jiangsu 210096,China; 2.Center of Medical Engineering Support,Chinese PLA General Hospital,Beijing 100853,China; 3.School of Biomedical Engineering,Southern Medical University,Guangzhou,Guangdong 510515,China)