1. School of Mathematics Science, Liaocheng University, Liaocheng 252000, China
2. School of Computer Science and Technology, Shandong Jianzhu University, Jinan 250101, China
3. College of Mathematics and Statistics, Sichuan University of Science and Engineering, Zigong 643000, China
4. Department of Radiology and BRIC, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA