For example, we did not include the following criteria: whether there were radiometabolites that would complicate PET quantification; whether a tracer is a substrate for blood–brain barrier efflux transporters that would affect net brain penetrance; whether a tracer is sensitive to 3R versus 4R tau, which would determine whether that tracer may be useful in diagnosing non-AD tauopathies; and estimates of whole-body or organ-specific radiation exposure. Here, MAPT is linked to Alzheimer disease.