The effects of positron binding and annihilation mechanisms in biomolecules on PET resolution

The angular correlation distribution of electron-positron annihilation photons is presented for polar biological molecules, and the associated noncollinearity effect on resolution in positron emission tomography (PET) is discussed. The image resolution in PET is known to be limited by a number of factors such as radioactive decay statistics, attenuation, scatter, or random coincidence of annihilation photons. This paper deals with the resolution limits on the side of the measured system. Our calculated distribution of annihilation momentum belongs to the intrinsic resolution factors together with the positron range in biological tissue; the other factors are due to the interactions along the photon path from the tissue to the detector. Here we demonstrate that the angular distribution of annihilation trajectories is not necessarily Gaussian, especially when the positron temporarily binds in a molecular dipole moment field before annihilation. The theoretical framework treats the electron and positron on the same footing by using the multireference single- and double-excitation configuration interaction algorithms applicable also to other bound-state systems. Our results form a subset of parameters relevant to the statistical image reconstruction algorithms, which may eventually increase the sensitivity of PET imaging in clinical setting