[1] Lab Neuro Imaging, Los Angeles, CA 90095 USA
[2] Univ Calif Los Angeles, Dept Neurol, Multiple Sclerosis Program, Los Angeles, CA USA
[3] Duke Univ, Ctr Vivo Microscopy, Durham, NC 27706 USA
[4] Univ Winnipeg, Dept Phys, Winnipeg, MB R3B 2E9, Canada
[5] CALTECH, Beckman Inst, Pasadena, CA 91125 USA