北京市卫生系统高层次卫生技术人才培养计划项目%首都卫生发展科研专项项目(2009-3145)%北京市科技计划项目(Z121107001012159)%This study was supported by High-level Technical Training Project Funding of Beijing Health System,Capital Medical Development Foundation of China(2009-3145)%Beijing Science and Technology Plan Project(Z121107001012159)