1. Center for Interdisciplinary Magnetic Resonance, National High Magnetic Field Laboratory, 1800 E. Paul Dirac Drive, Tallahassee, Florida 32310, Department of Electrical and Computer Engineering, Florida A&M University and Florida State University, Tallahassee, Florida 32310, U.S. Army Communications-Electronics Command, Ft. Monmouth, New Jersey 07703, and Wuhan Institute of Physics and Mathematics, the Chinese Academy of Sciences, Wuhan 430071, P. R. China