Concentration dependence of f
for a hypothetical functioning PET tracer (solid line, Eq.2). Blue and red areas represent the proportions of target-bound tracer and non-specific tracer in brain tissue, respectively. At low concentrations (Cu,brainISF < < Kd), ftb is at a plateau maximum value, which is high for functioning tracers and low for non-functioning tracers. At excessive concentrations (Cu,brainISF > > Kd), the specific binding is saturated and ftb negligible also for a good tracer.