1. An imaging system, comprising: a closed annular ring with an aperture defining an examination region; a set of detector modules affixed to the closed annular ring, wherein the set of detector modules detect gamma rays, which have energy in a range of 40 to 140 keV and 511 keV, emitted by a radioisotope in an examination region, wherein 511 keV gamma rays are detected in singles mode in which individual 511 keV gamma rays, and not coincidence pairs of 511 keV gamma rays, are detected; an energy discriminator that bins detected gamma rays into a first energy bin corresponding to 511 keV energy gamma rays and a second energy bin corresponding to 40 to 140 keV energy gamma rays; and a reconstructor that reconstructs the 511 keV energy gamma rays thereby generating a first image of a distribution of a first radionuclide and that reconstructs the gamma rays in the one or more ranges between 40 and 140 keV thereby generating a second image of a distribution of a second radionuclide.