1. Department of Physics, Harvard University, Cambridge, Massachusetts 02138
2. Harvard−MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology E19-502D Cambridge, Massachusetts 02139
3. Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
4. Division of Medicine, Brigham and Women’s Hospital, Boston, Massachusetts 02115
5. Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
6. Harvard−Smithsonian Center for Astrophysics, 60 Garden Street, MS 59, Cambridge, Massachusetts 02138