David Stout, Ph.D. is the director of the Crump Small Animal Imaging Facility at the UCLA Crump Institute for Molecular Imaging, 700 Westwood Plaza, A222-A, Los Angeles, CA 90095. He can be reached at voice (310) 267-2646, fax: (310) 206-8975, dstout@med-net.ucla.edu. This paper is modified from the original published on-line at Mol Imaging Biol (2005), DOI: 10.1007/s11307-005-0015-2.