[1] MIT, Whitehead Inst Biomed Res, BioImaging Ctr, Cambridge, MA 02142 USA
[2] Weizmann Inst Sci, Dept Biol Struct, IL-76100 Rehovot, Israel
[3] MIT, Dept Elect Engn & Comp Sci, Cambridge, MA 02139 USA
[4] Univ Calif San Francisco, Sch Med, San Francisco, CA 94143 USA