A dual-purpose transmission source for use in a nuclear medicine imaging system is described. A Cs-137 point source is used to transmit radiation having a 662 keV photopeak to a corresponding detector during a transmission scan of an object. The Cs-137 source can be used in transmission scans for correcting either coincidence or single-photon emission data for non-uniform attenuation. When performing a transmission scan to correct single-photon emission data, a collimator may remain mounted to the detector, since a substantial portion of the transmitted radiation completely penetrates the radiation-absorbing material of the collimator to reach the detector.