1. Department of Chemistry, Duke University, Durham, NC 27708, USA.
2. Department of Physics, Duke University, Durham, NC 27708, USA.
3. Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, USA.
4. Departments of Radiology and Biomedical Engineering, Vanderbilt University, Institute of Imaging Science, Nashville, TN 37232, USA.
5. Departments of Radiology and Biomedical Engineering, Duke University, Durham, NC 27708, USA.