Feasibility study of dual isotope PET

Positron emission tomography (PET) is a highly effective functional imaging modality. Unfortunately, PET cannot be easily used for dual-isotope imaging (which would allow for simultaneous investigation of two different biological processes), because positron-electron annihilation products from different tracers are indistinguishable in terms of energy. Methods have been proposed for dual-isotope PET based on different half-lives of the participating isotopes, however, those approaches are based on many assumptions concerning kinetic behavior of the tracers and may not always lead to optimal results. In this manuscript we propose another approach for dualisotope PET and investigate its effectiveness using GATE simulations. Our method requires that one of the two radioactive isotopes is a pure positron emitter while the second isotope emits an additional high-energy photon in a cascade (simultaneously) with positron emission. Measurement of this auxiliary photon in coincidence with the annihilation event allows us to identify the corresponding 511 keV photon pair as originating from the same isotope. Two list-mode datasets are created: a primary dataset that contains all detected 511 keV photon pairs from both isotopes; and a second, auxiliary (much smaller) dataset that contains only those PET events for which coincident auxiliary gamma has also been detected. An image reconstructed from the auxiliary dataset reflects the distribution of the second radiotracer and serves as a prior for the reconstruction of the primary dataset. Our preliminary simulation study with partially overlapping ¹⁸F and ²²Na radiotracer distributions suggest that this method allows for separation of two activities with relative errors less than 0.07.