K Mercurio et al 2006 Phys. Med. Biol. 51 N323 doi:10.1088/0031-9155/51/17/N05
K Mercurio1, P Zerkel2, R Laforest2,4, L G Sobotka1,3 and R J Charity3
Show affiliationsPositronium 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 O2 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 O2 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.
87.57.uk Positron emission tomography (PET)
87.53.-j Effects of ionizing radiation on biological systems
Issue 17 (7 September 2006)
Received 16 March 2006, in final form 6 July 2006
Published 15 August 2006
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