Our initial assumption that in vivo click performances correlate with the brain uptake/washout -as determined by the Peak/AUC ratio of the tracer -did not reveal any clear trend (Figure ). However, tracer washout (AUC values) strongly correlated with absolute imaging contrast (r 0.99, Figure ). Interestingly, absolute imaging contrast for all F-Tzs -except for [ 18 F]12 -also correlated to the metabolic stability (Figure ). Slow metabolism appeared to be beneficial for higher absolute imaging contrast. We believe that rapid metabolism results in less tracer available to enter the brain from the blood. This explains the low absolute imaging contrast of [ 18 F]14 and [ 18 F]10, for example. Both tracers possess a rapid metabolism (Figure and 5A,B,F). In contrast, [ 18 F]18 displays an intermediate fast metabolism profile (Figure ) and consequently, a higher absolute imaging contrast is observed. Absolute imaging contrast of [ 18 F]12 is not aligned with this observation. Its slow metabolism is not resulting in a high absolute imaging contrast. We believe that [ 18 F]12 is possible to polar for sufficient BBB permeability (Figure ). Our results indicate that fast tracer washout is not necessarily required for high absolute imaging contrast in vivo. However, slow metabolism resulting in high brain uptake results in good imaging contrast as the tracers can continuously enter the brain (Figures and). Increasing lipophilicity -at least to clogP values of 1.55 -results in higher absolute imaging contrast. The imaging contrast increases also correlated well with the BBB / CNS MPO score (r 0.91-0.93, Figure ).