Quantification of radicals in aqueous solution by positronium lifetime: an experiment using a clinical PET scanner

Positrons entered into living organisms can form positronium (Ps), a bound state with electrons. Most of the triplet Ps (ortho-Ps) in insulating materials annihilate with electrons in surrounding molecules, and then the ortho-Ps lifetime varies depending on the surrounding electron density. The ortho-Ps lifetime may add new biological information to positron emission tomography (PET) scan information. In order to discuss the feasibility of quantifying (free) radicals in vivo by the Ps lifetime, we used a clinical PET system to make ortho-Ps lifetime measurements in aqueous solutions containing radicals. The results suggested that differences in radical concentrations in aqueous solutions of the order of a few mM could be quantified by the Ps lifetime if the counting statistic of the detection time difference spectra was more than 10 8 events. This concentration was much higher than the radical concentration generated in the physiological functions of living organisms. Therefore, we concluded that quantification of radicals generated in vivo by using the Ps lifetime is very difficult employing the current technology.