**2.4.2 PET imaging of tau neurofibrillary tangle burden**

Non-invasive functional molecular imaging techniques such as PET imaging have the potential to become the future diagnostic standard for Alzheimer's disease and related tauopathies as they would allow for earlier and more definitive diagnosis of such diseases, and provide an effective method for monitoring possible treatments. One such approach being aggressively explored is the development of tau specific radiopharmaceuticals that would allow for the non-invasive quantification of tau NFTs in the brain. Developing appropriate biomarkers for detecting tau has proven challenging as they must cross the blood brain barrier (BBB), bind selectively to tau, demonstrate safe biodistribution, and exhibit low non-specific binding. Nevertheless, several tau-targeting radiopharmaceuticals, radiolabeled with fluorine-18 or carbon-11, are in various stages of clinical and pre-clinical development. Experimental radiopharmaceuticals including BF-158, FDDNP and T808 are possible candidates for PET imaging of tau, and are outlined below for proof-of-concept purposes.
