1. Department of Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland 21218, United States
2. Biomolecular Measurement Division, Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States
3. Department of Oncology, Johns Hopkins University School of Medicine, Baltimore, Maryland 21287, United States
4. The Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, Maryland 21287, United States