Optimizing the Liquid Target Production of the PET-radionuclide Ti-45 from a Natural Scandium Target

Abstract

The search for novel PET-nuclides is an ongoing process as some applications require decay characteristics that are different from the conventional ones. The use of novel radionuclides will fill the gap between conventional PET-nuclides and broaden the spectrum of diseases that can be diagnosed by imaging modalities and treated by radiotherapy. Ti-45 (t1/2 = 3.08 h) is a promising radionuclide with suitable decay characteristics for PET-imaging. It has a high positron branching ratio of 85 % and a half-life that is suitable for imaging slower physiological processes. The cyclotron production of Ti-45 using a solid scandium target is already an established method. In this thesis, the liquid target production of Ti-45 was studied using a PETtrace 860 cyclotron equipped with a PETtrace 800 68Ga Liquid target. Scandium nitrate in dilute nitric acid was used as target and irradiated with 14.3 MeV protons to follow the Sc-45(p,n)Ti-45 nuclear rection. Different irradiations times (1 – 3 h) and currents (20 – 30 μA), and concentrations of nitric acid (0.1 – 0.2 M) and scandium nitrate (1.0 – 2.0 M) were tested and analyzed using an experimental design with the aim of optimizing the production. Based on the model constructed from the full factorial design, the most optimal irradiation parameters were found to be a high scandium nitrate and nitric acid concentration combined with low irradiation currents and low to medium irradiation time. Using these irradiation parameters, the production of Ti-45 was increased with 24.4 to 81.5 %, but there were several indications that the model was inadequate and required modifications. Possible explanations for why these irradiation parameters are optimal may be unavailability of target caused by high irradiation currents, and the decay and production rate reaching an equilibrium somewhere between 1 to 2 h of irradiation. Activities of Ti-45 ranging from 104 to 1240 MBq with a radionuclidic purity of 85.5 to 99.6 % were obtained during the experiments. In conclusion, the process was optimized, and it is feasible to produce Ti-45 in a cyclotron by using a liquid target.