The present invention is directed to pyrrolopyridine compounds of formula (I) 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.