1. Imaging Physic Laboratory, Biophysics and Biochemistry Center, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892, USA
2. Applied Research Laboratory, Penn State University, State College, Pennsylvania 16804, USA
3. X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA
4. National Institute of Biomedical Imaging and Bioengineering, National Institutes of Health, Bethesda, Maryland 20892, USA