Disclosed are imidazo-pyridine compounds represented by general formulae (1), (2), (3) and (4). Examples of the compounds include 6-tributylstannyl-2-(4'-fluorophenyl)imidazo[1,2-a]pyridine and 2-(4'iodophenyl)-6-methoxyimidazo[1,2-a]pyridine. The compounds are used in the diagnosis of Alzheimer's disease.