Alzheimer's disease is the most common form of dementia among older people. 
Misfolding and aggregation of proteins (amyloid-β and tau) in the brain is the 
primary cause of neurodegeneration in the disease. Non-invasive detection of 
amyloid-β deposition can be realized using positron emission tomography probes, 
but a proportion of Aβ-positive subjects do not present with cognitive 
dysfunction, suggesting limitations in assessment using this method. 
Non-invasive detection of tau deposits in the brain can be used to diagnose, 
monitor, and predict Alzheimer's disease progression. Tau positron emission 
tomography radiolabelled probes such as T807, THK-5117, and PBB3 can image the 
pathology of the disease in vivo. The 18F-labeled tau imaging agents 
18F-THK-5351, 18F-T807 (18F-AV-1451), and 18F-RO6958948 are presently under 
evaluation in clinical studies and clinical trials worldwide. This imaging 
methodology could be applied to enable preclinical diagnoses and 
disease-modifying drugs for Alzheimer's disease. In this review, we provide an 
overview of the pathology and potential imaging of tau in Alzheimer's disease, 
development of a THK series among tau tracers, and the chemical, radiochemical, 
biological, and clinical features of tau probes.