Feasibility of 13N-ammonia production by 13C(p,n)13N reaction on a 7.5 MeV cyclotron

663 Objectives: 13N-ammonia is commonly produced using 16O(p, α)13N reaction but one of the limiting factor of this reaction is the relatively small reaction cross-section at proton energies of <10 MeV. An alternative N-13 ammonia production method using 13C(p,n)13N reaction is more suitable for a preclinical or small PET imagining facility equipped with a <10 MeV cyclotron which has a higher cross-section can be used. We report a novel practical and single-step method for the production of 13N-ammonia using 10% 13CH3OH/H2O solution target on a 7.5 MeV ABT BG-75 cyclotron. Methods: A solution (80 µl capacity) target constructed with tantalum consisting of a havar window supplied by ABT Molecular Inc for the production of 18F-fluoride was used without any modifications. Optimization of the reaction parameters was made thus; target solutions of various percentages of methanol (13C, 99% enrichment) solution were made with Milli-Q type 1 Ultrapure water and used to obtain maximum 13N-ammonia dose after bombardment. Target currents ranging from 0.5-2.5 µA were used as well as various bombardment times (2.0-10 min) were also investigated. Half-life determination and USP-monographed ion exchange analytical HPLC were used to identify the radionuclide. After the bombardment, an injectable solution was prepared by making it isotonic and filtering it through a 0.2 µm syringe filter. Doses were also assayed for the presence of methanol by gas chromatography (GC). Small animal PET/CT imaging was conducted in CD-1 mice (20-30 g) at 5 min after injection of 13N-ammonia (~8 MBq). Results: The product yield varied between 14.8-180.2 MBq, with decay correction, depending on the target current and duration of bombardment. Not much difference in the yield was observed with increased percentage of methanol (13C) in the target solution. A maximum yield of 180.2 MBq was obtained after a 10 min bombardment at a target current of 2.4 µA. We maintained target current at