The three-photon yield from e⁺annihilation in various fluids

Abstract

Positronium in the triplet state decays by the emission of three photons and it has been proposed that their simultaneous detection can be used for medical imaging. The three-photon yield has been observed to be enhanced in low O(2) levels in some fluids but has never been measured in biologically relevant liquids. In this study, the delayed three-photon decay yield, at both high and low O(2) levels, has been extracted by fitting the time dependence of the two-photon yield to a set of coupled differential equations. The differential equations, in a simple yet seemingly satisfactory fashion, account for the e(+) capture to form positronium, its decay and the interconversion of the two spin configurations. Our results indicate that the delayed three-photon fraction is 0.25% in water (or blood-like) samples and exhibits no (or exceedingly small) dependence on the dissolved oxygen content. If one assumes that the direct component contributes a fraction expected by annihilation on free electrons (1/372), then the total three-photon fraction is 0.52% in the samples of biological relevance.

Recovery and normalization of triple coincidences in PET

“…15 Finally, there are other processes which can also yield tri-ple coincidences, like random triple events caused by three different decays, or positron annihilations generating three γ-rays via the formation of orthopositronium. 16 These events do not provide information about the LORs in which a decay occur, but as they are typically one or two orders of magnitude less frequent 8,17 than the aforementioned triple coincidences, we did not consider them in this work.…”

Variation of 3-to-2 ratio from 18F in haematological components measured using the GAMMASPHERE

“…Unlike CC, the SE sample did not contain haemoglobin, whether free or bound. The haemoglobin in venous blood, free O 2 , free CO 2 , oxyhaemoglobin, carboxyhaemoglobin, other chemical species as well as the oxygenation/deoxygenation handling could have contributed to previously reported insensitivity [2].…”

“…The prospect of detecting oxygenation levels in body tissues using three-quanta annihilations has been proposed [1] and refuted [2]. The proposal was based on the theoretical expectation, backed by experimental evidence, that a positronium is created when a positron binds with an electron, giving rise to the possibility of three-quanta [3,4] in addition to two-quanta annihilations, which have so far served as the basis for positron emission tomography (PET).…”

SPLIT: Statistical Positronium Lifetime Image Reconstruction via Time-Thresholding

“…There are two kinds of positroniums: parapositronium (p-Ps) and ortho-positronium (o-Ps). The p-Ps has a relatively short lifetime of 0.125 ns [5] in vacuum before annihilating into two 511-keV photons; however, the lifetime of o-Ps varies greatly from 142 ns [6] in vacuum to several ns in tissues [7,8], and the o-Ps annihilates into three photons in vacuum but predominantly two photons in tissues [9]. This variability of o-Ps behavior is due to two effects-pick-off annihilation and spin-exchange interaction [10]-which react much faster than the three-photon annihilation and thus become dominant in materials.…”