1. Apparatus for radiological imaging, the apparatus comprising: an imaging positron emitting tomography (PET) detector array capable of detecting and locating two-photon emission events resulting from positron annihilation to provide PET detector signals; one or more prompt gamma detectors responsive to a prompt gamma ray from isotopes that produce both a positron and a prompt gamma ray in cascade, wherein the prompt gamma detectors are not responsive to positron annihilation photons; and a processor coupled to the imaging PET detector array and to the prompt gamma detectors and configured to provide a classification of the PET detector signals as 2-photon events or as 3-photon events according to signals provided by the PET detector array and by the prompt gamma detectors; wherein an object being imaged by the apparatus for radiological imaging includes at least a first radio-isotope that provides two-photon emission events without associated prompt gamma rays and a second radio-isotope that provides two-photon emission events with associated prompt gamma rays; wherein the apparatus is configured to simultaneously and independently image the object with the first and second radio-isotopes according to the classification of the PET detector signals as 2-photon events or as 3-photon events.