Ultra-high energy spectral prompt PET.

Objective: Utilizing prompt gammas in preclinical pinhole-collimated PET avoids image degradation due to positron range blurring and photon down scatter, enables multi-isotope PET and can improve counting statistics for low-abundance positron emitters. This was earlier reported for 124I, 89Zr and simultaneous 124I -18F PET using the VECTor scanner (MILabs, The Netherlands), demonstrating sub-mm resolution despite long positron ranges. The aim of the present study is to investigate if such sub-mm PET imaging is also feasible for a large variety of other isotopes including those with extremely high energy prompt gammas (>1 MeV) or with complex emission spectra of prompt gammas.

Approach: We use Monte Carlo simulations to assess achievable image resolutions and uniformity across a broad range of spectrum types and emitted prompt gamma energies (603 keV - 2.2 MeV), using 52Mn, 94Tc, 89Zr, 44Sc, 86Y, 72As, 124I, 38K, and 66Ga.

Main Results: Our results indicate that sub-millimeter resolution imaging may be feasible for almost all isotopes investigated, with the currently used cluster pinhole collimators. At prompt gamma energies of 603 keV of 124I, an image resolution of ~0.65 mm was achieved, while for emissions at 703, 744, 834, and 909 keV of 94Tc, 52Mn, 72As, and 89Zr, respectively, ~0.7 mm resolution was obtained. Finally, at ultra-high energies of 1.2 (44Sc) and 1.4 MeV (52Mn) resolutions of ~0.75 mm and ~0.8 mm could still be achieved although ring artifacts were observed at the highest energies (1.4 MeV). For 38K (2.2 MeV), an image resolution of 1.2 mm was achieved utilizing its 2.2 MeV prompt emission.

Significance: This work shows that current cluster pinhole collimators are suitable for sub-mm resolution prompt PET up till at least 1.4 MeV. This may open up new avenues to developing new tracer applications and therapies utilizing these PET isotopes.