In order to explain the observed in vivo click performance, we examined the data received from in silico calculations, pretargeted autoradiography, in vivo brain uptake and metabolism studies. In vitro contrast from pretargeted autoradiography experiments inversely correlated with the relative imaging contrast (r 0.69, Figure ). We speculate that this inverse correlation was detected as tested 18 F-Tzs are still within the optimal lipophilicity interval with respect to BBB uptake and non-displaceable binding. In this interval, increased lipophilicity would only result in improved BBB permeability and not majorly increase the nondisplaceable binding component. Only after a certain lipophilicity threshold the relative imaging contrast would negatively be affected, and this threshold was not exceeded within this study. Consequently, relative in vivo imaging contrast correlated with clogP (r 0.88, Figure ).