1. Harvard-MIT (HST) Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Harvard Medical School, Charlestown, Massachusetts 02129;
2. Division of Mathematics, University of Dundee, Dundee DD1 4HN, United Kingdom
3. Department of Pathology, University of New Mexico, Albuquerque, New Mexico 87131
4. Department of Chemical and Biomedical Engineering, University of New Mexico, Albuquerque, New Mexico 87131