A very small concentration at which a radiotracer is administered, such that the presence of the radiotracer itself does not noticeably influence the pharmacology or pharmacokinetics of the process being imaged. For rever-sibly binding ligands, the usual convention is that the peak binding of the radioligand does not exceed 5% of the available receptor pool. Because the equilibrium bound to free ligand ratio is B/F = Bmax/(F + KD) and the free ligand associated with 5% occupancy is approximately 5% of KD, this restriction insures that binding potential will be measured accurately to within 5%. From the data-fitting perspective, the use of tracer dose also insures that the linearization of the mass action law applied in the compartment model is justified.
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