[ 18 F]12 and [ 18 F]1 showed the best stability, 50% of intact tracer could be detected. Interestingly, [ 18 F]18 displayed an intermediate metabolism rate. Approx. 30% intact tracer could be detected (Figure ). All detected radiometabolites were much more hydrophilic than the parent (SI Figure ) indicating that these metabolites do not contribute to the brain uptake as they are unlikely to pass the BBB. In general, rapid metabolism appeared to strongly influence the brain uptake of the studied Tzs -mainly due the fact that less intact F-Tz is available to diffuse into the brain (Figure ). Percentage of intact Tz correlated with brain uptake (SI Figure ). This explains the higher uptake of [ 18 F]1 and [ 18 F]18 as well as the lower uptake of [ 18 F]14 and [ 18 F]10 (Figure ).