or their pharmaceutically acceptable salts, which may be suitable for imaging tau aggregates, b-sheet aggregates, beta-amyloid aggregates or alpha-synuclein aggregates, and hence are useful in binding and imaging tau aggregates in Alzheimer' s patients. More specifically, this invention relates to a method of using the compounds of this invention as tracers in positron emission tomography (PET) imaging to study tau deposits in brain in vivo to allow diagnosis of Alzheimer' s disease and other neurodegenerative diseases characterized by tau pathology. The invention further relates to a method of measuring clinical efficacy of therapeutic agents for Alzheimer' s disease and other neurodegenerative diseases characterized by tau pathology.