Positron range effects of 66Ga in small-animal PET imaging.

Abstract

PURPOSE\nGallium-66 is a non-conventional positron emitter that stands out not only for its high potential to label peptides, proteins and antibodies, but also because it can provide spatio-temporal information of relatively slow physiological processes in the body due to its conveniently long half-life of 9.5\u202fh. However, 66Ga emits the most energetic positrons for PET imaging. The lack of information of the positron range effect on spatial resolution for this positron emitter is an issue, particularly in preclinical imaging.\n\n\nMETHODS\nThe line spread function (LSF) in tissue-equivalent materials with densities between 0.2 and 1.93\u202fg/cm3 was obtained with 66Ga and 18F. A complementary study with the NEMA NU 4-2008 image quality phantom is also included.\n\n\nRESULTS\nHigh-energy positrons moving in lower density materials produce far-reaching activity distributions. The LSFs were characterized with Lorentzian-Gaussian fits, with spatial resolution (FWHM) in the 2.14-3.2\u202fmm range, and long tails extending a few tens of mm depending on the material type and density. A narrowing of the LSF was observed for lung-equivalent materials, indicating the lack of enough material for the positron annihilation to take place. The NEMA NU 4-2008 image quality phantom produced blurred images, notoriously observed in the hot and cold cylinders used for evaluation of recovery coefficients (RC) and spill-over ratios (SOR), producing very low RC and very large SOR.\n\n\nCONCLUSIONS\nQuantitative PET imaging with the non-conventional 66Ga is hampered due to the large range of its high-energy positrons affecting both spatial resolution and activity concentration quantification.