2'-Deoxy-2'-[.sup.18F]-labeled and 3'-deoxy-3'-[.sup.18F]-labeled purine nucleoside analogs such as [.sup.18F]-FAA and [.sup.18F]-FXA have been found to have desirable properties for use as imaging agents. The analogs are particularly useful for in vivo imaging of biological material including organ tissues (e.g., heart, liver, brain and kidneys) and tumors. Methods for the preparation of the [.sup.18F]-FAA and [.sup.18F]-FXA are also provided.