Inter-study variability of AIF shape and Θ ( t ) = AUC ( t )/ c
( t ) in the available 15 data sets. The thick red lines represent the group-averaged curves. (A) AIF normalized to the mean value (time-average) of the respective curve. As can be seen, the shape of all AIF curves is very similar beyond t≈1 min. (B) Θ(t), the ratio of the AUC up to time t divided by c
(t), exhibits only small inter-study variability. Averaging the last two points of these curves (acquisition time from 50 to 60 min p.i.) yields the data presented in more detail below.