Fig. 1From: Developing a novel positronium biomarker for cardiac myxoma imagingA 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 moleculesBack to article page