1. Section on Quantitative Imaging and Tissue Sciences, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892, United States
2. Materials Science and Engineering Division, National Institute of Standard and Technology, Gaithersburg, Maryland 20899, United States
3. Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland 20742, United States