1. Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA
2. Department of Physics, Washington University in St. Louis, St. Louis, Missouri 63130, USA
3. Department of Biomedical Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, USA
4. Division of Biology and Biomedical Sciences, Washington University in St. Louis, St. Louis, Missouri 63130, USA