The Influence of Sequential Tau Protein and Amyloid Plaque Imaging Changes on Stroke Prognosis and Cognitive Outcome

Inclusion Criteria: 1. Inclusion criteria for acute stroke/TIA patients (Group A, n=200) * Males or females with age \>= 50 years old * Having acute cerebral stroke or transient ischemic attack in recent 1 month * Female subjects of childbearing potential must practice effective contraception during the study and be willing and able to continue contraception after the final study * Provision of signed informed consent 2. Inclusion criteria for chronic stroke/TIA patients (Group B, n=200) * Males or females with age \>= 50 years old * Having cerebral stroke or transient ischemic attack in the past 1.5 years * Having had tau PET imaging study within 1 year after the index stroke/TIA event * Female subjects of childbearing potential must practice effective contraception during the study and be willing and able to continue contraception after the final study * Provision of signed informed consent 3. Inclusion criteria for healthy elderly controls (Group C, n=30) * Males or females with age \>= 50 years old * Without history of cerebral stroke or transient ischemic attack * Ability to participate in cognitive and neuroimaging assessments * Female subjects of childbearing potential must practice effective contraception during the study and be willing and able to continue contraception after the final study * Provision of signed informed consent Exclusion Criteria: Exclusion criteria for all subjects * Presence of dementia diagnosis before the index stroke or at the initial screening * History of vascular MCI (VaMCI) * The Chinese version of the Informant Questionnaire on Cognitive Decline in the Elderly (IQCODE) score \>=104 at the initial screening 47. * Life expectancy less than 1 year. * Clinically significant abnormal laboratory values. * Clinically significant or unstable medical or psychiatric illness. * Epilepsy history. * Cognitive impairment resulting from trauma or brain damage. * Substance abuse or alcoholism in the past 3 months. * General MRI, and / or PET exclusion criteria. * Pregnant or becoming pregnant during the study (as documented by pregnancy testing at screening or at any date during the study according to the PI discretion) or current breast feeding. * History of allergy to 18F-labelled radionucleic agents, \[18F\]AV45 or \[18F\]THK5351. * Subjects having high risks for the study according to the PI discretion.