1. Division of Medical and Biological Measurement; National Institute of Metrology; No.18 Beisanhuandonglu Beijing Chaoyang District 100029 China
2. Department of Biomedical Engineering; School of Medicine; Tsinghua University; Beijing Haidian District 100084 China
3. Division of Ionizing Radiation Metrology; National Institute of Metrology; No.18 Beisanhuandonglu Beijing Chaoyang District 100029 China