1. Department of Radiology, University of California, San Diego, La Jolla, California 92093, United States
2. Department of Pharmacology, University of California, San Diego, La Jolla, California 92093, United States
3. Department of Radiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
4. School of Chemical Sciences, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
5. Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, United States
6. Howard Hughes Medical Institute, University of California, San Diego, La Jolla, California 92093, United States