1. Department of Gastroenterology Xinqiao Hospital Army Medical University Chongqing 400037 P. R. China
2. Department of Mechanical Engineering University of Manitoba Winnipeg MB R3T 2N2 Canada
3. School of Biomedical Engineering and Medical Imaging Army Medical University Chongqing 400038 P. R. China
4. School of Medicine West Virginia University Morgantown WV 26506 USA