Uptake Kinetics and Biodistribution of (14)C-D:-Luciferin-a Radiolabeled Substrate for the Firefly Luciferase Catalyzed Bioluminescence Reaction: Impact on Bioluminescence Based Reporter Gene Imaging European Journal of Nuclear Medicine and Molecular Imaging Berger, F., Paulmurugan, R, Bhaumik, S, Gambhir, SS 2008