Abstract
PurposeIn living organisms, the positron-electron annihilation (occurring during the PET imaging) proceeds in about 30% via creation of a metastable ortho-positronium atom. In the tissue, due to the pick-off and conversion processes, over 98% of ortho-positronia annihilate into two 511 keV photons. In this article, we assess the feasibility for reconstruction of the mean ortho-positronium lifetime image based on annihilations into two photons. The main objectives of this work include the (i) estimation of the sensitivity of the total-body PET scanners for the ortho-positronium mean lifetime imaging using 2γ annihilations and (ii) estimation of the spatial and time resolution of the ortho-positronium image as a function of the coincidence resolving time (CRT) of the scanner.MethodsSimulations are conducted assuming that radiopharmaceutical is labeled with 44Sc isotope emitting one positron and one prompt gamma. The image is reconstructed on the basis of triple coincidence events. The ortho-positronium lifetime spectrum is determined for each voxel of the image. Calculations were performed for cases of total-body detectors build of (i) LYSO scintillators as used in the EXPLORER PET and (ii) plastic scintillators as anticipated for the cost-effective total-body J-PET scanner. To assess the spatial and time resolution, the four cases were considered assuming that CRT is equal to 500 ps, 140 ps, 50 ps, and 10 ps.ResultsThe estimated total-body PET sensitivity for the registration and selection of image forming triple coincidences (2γ+γprompt) is larger by a factor of 13.5 (for LYSO PET) and by factor of 5.2 (for plastic PET) with respect to the sensitivity for the standard 2γ imaging by LYSO PET scanners with AFOV = 20 cm. The spatial resolution of the ortho-positronium image is comparable with the resolution achievable when using TOF-FBP algorithms already for CRT = 50 ps. For the 20-min scan, the resolution better than 20 ps is expected for the mean ortho-positronium lifetime image determination.ConclusionsOrtho-positronium mean lifetime imaging based on the annihilations into two photons and prompt gamma is shown to be feasible with the advent of the high sensitivity total-body PET systems and time resolution of the order of tens of picoseconds.
Highlights
In the positron emission tomography (PET), the density distribution of points of positronelectron annihilations, reflecting an image of metabolism rate of the radiopharmaceutical administered to the patient, is used for the diagnosis
Ortho-positronium mean lifetime imaging based on the annihilations into two photons and prompt gamma is shown to be feasible with the advent of the high sensitivity total-body PET systems and time resolution of the order of tens of picoseconds
In this article, based on the Monte Carlo simulations, we argue that with the time resolution in the order of tens of picoseconds and the advent of the high sensitivity total-body PET systems [37,38,39], the 2γ + γprompt mean lifetime positronium imaging based on time measurements may become possible in the future
Summary
In the positron emission tomography (PET), the density distribution of points of positronelectron annihilations, reflecting an image of metabolism rate of the radiopharmaceutical administered to the patient, is used for the diagnosis. The in vitro studies indicated that positronium mean lifetime and its production probability as well as the average time of direct annihilation differ for healthy and cancerous uterine tissues operated from the patients [4, 9]. Another in vitro measurements, performed with blood taken from patients before and after the chemotherapy or radiotherapy, demonstrated dependence of positronium properties in blood on the time after anti-neoplastic therapy [5].
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