1. Sixth Medical Department, Hunan Chest Hospital, Changsha, Hunan 410013, P.R. China
2. Endoscopy Center, Hunan Chest Hospital, Changsha, Hunan 410013, P.R. China
3. Department of Ultrasound, The First Affiliated Hospital of University of South China, Hengyang, Hunan 421001, P.R. China