Abstract
1948
Objectives We propose to distinguish the biodistributions of two simultaneously injected PET isotopes where one is a pure positron emitter that generates two-photon coincidences and the other emits a positron and a γ-ray in cascade, yielding a triple coincidence. However triple coincidence (TC) detection methods suffer from very low sensitivity. This work presents a method to enhance the sensitivity of TC for multi-isotope PET (MIP) by adding an extra detector dedicated of the third prompt γ in coincidence with the 511s photons.
Methods We performed simulations and measurements of two positron emitting isotopes simultaneously. A possible candidate for MIP is Zr-89, which emits a 909 keV γ-ray. For the simulations we used a phantom comprising 1cm diameter spheres, half of them filled with F-18 and the other half with Zr-89. The phantom was acquired in a simulated microPETR4 system 5cm thick detector slab of BGO. For the measurements we used a Na22 positron source that produces a 1275 keV γ-ray in cascade. We arranged two LYSO crystals coupled to two silicon photomultipliers to detect the 511 keV photons and one large 8cm diameter, 2cm thick detector slab of LYSO coupled to a PMT for the detection of the 1275 keV γ-ray.
Results Our simulations indicate the two- vs. three-photon coincidence method enables the Zr89 spheres to be distinguished from the F18 spheres with a 3.8 increase in sensitivity for TC resulting from the extra detector added to the microPETR4 system. The measured ratio of the triple coincidence detection counts divided by the two-photon coincidence detection counts is 0.0370±0.003, which compares well with the calculated ratio of 0.042 estimated from intrinsic and geometric efficiency considerations.
Conclusions Simulations and measurements we have demonstrated that two PET radionuclides can be spatially and spectrally distinguished in the same study