The majority of PET radiohalogenations reported in the literature are nucleophilic fluorinations utilizing [18F]fluoride. The reasons for this include the availability of high amounts of [18F]fluoride from low- and medium-energy proton-only cyclotrons utilizing [18O]water target material and the generally higher specific activities achievable using [18F]fluoride. A variety of chemical reaction types are amenable to ra-diolabeling using [18F]fluoride. These can be divided into two principal catagories: SN2-type (substitution nucleophilic bimolecular) reactions with substrates containing leaving groups such as halides or alkyl sul-fonate esters, and aromatic nucleophilic substitution reactions utilizing activated aromatic systems with leaving groups such as nitro or trimethylammonium.
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