1. Biomedical Engineering Department, University of Michigan, Ann Arbor, Michigan 48109
2. Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, State College, Pennsylvania 16802
3. Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan 48109
4. Biomedical Engineering Department, University of Michigan, Ann Arbor, Michigan 48109 and Mechanical Engineering Department, University of Michigan, Ann Arbor, Michigan 48109