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Fig. 1 | EJNMMI Physics

Fig. 1

From: Developing a novel positronium biomarker for cardiac myxoma imaging

Fig. 1

A pictorial illustration of the basic processes leading to the formation and decay of positronium in the intramolecular voids of a hemoglobin molecule. We have used a sodium 22Na isotope as an emitter of positrons (e +), considering the role of sodium fluoride in cardiovascular imaging [34, 37]. a 22Na radionuclide decays emitting a neutrino (brown arrow) and a positron (dark green arrow) (e +), and turns into an excited 22Ne* nucleus. It de-excites almost instantly (on an average in 3 ps) by the emission of the prompt photon (yellow arrow). b The positron thermalizes at a distance of about 1 mm [38], and annihilates into photons with one of the electrons (e) in the surrounding molecules. Positron–electron annihilation in the tissue undergoes direct annihilation into two photons (solid light green arrows) in roughly 60% of the cases. However, it proceeds via the formation of positronium atom in 40% of the cases. The latter may be trapped in the tissue in the intramolecular voids [39]. Positronium atom can be created in two forms: (i) short-lived (125 ps) para-Positronium (p-Ps indicated in purple), which decays into two photons (dotted pink arrows) or (ii) long-lived (142 ns) ortho-Positronium (o-Ps indicated in mint), which decays into three photons (dashed red arrows). In the tissue, o-Ps predominantly annihilates either through an interaction with an electron (e) from the surrounding molecule via pick-off process (dashed blue arrows) or through the conversion to p-Ps via an interaction with oxygen molecules, which subsequently decays into two photons (dashed black arrows) [16, 40]. These processes decrease the o-Ps lifetime, which becomes strongly dependent on the size of intramolecular voids and the concentration of bioactive molecules

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