  PET is an established medical diagnostic imaging method. Continuous
improvements are aimed at refining image reconstruction, reducing the amount of
radioactive tracer and combining with targeted therapy. TOF-PET provides the
localization of the tracer through improved time resolution, nuclear physics
may contribute to this goal via selection of radioactive nuclei emitting
additional $\gamma$-rays. This additional radiation, when properly detected,
localizes the decay of the tracer at the line of response determined by two
detected 511 keV quanta. Selected candidates are presented. Some are
particularly interesting, as they are strong candidates for theranostic
applications.
