1. Nuclear and Radiological Engineering and Medical Physics Programs; The George W. Woodruff School of Mechanical Engineering; Georgia Institute of Technology; Atlanta GA 30332 USA
2. Department of Radiology; Stanford University; Palo Alto CA 94305 USA
3. Department of Medical Imaging; University of Arizona College of Medicine; Tucson AZ 85724 USA
4. Banner University Medical Center; Tucson AZ 85724 USA
5. Department of Modern Physics; School of Physical Sciences; University of Science and Technology of China; Hefei Anhui 230026 China