A Direct Comparative Study of Tau Tracer in Patients With Alzheimer's Disease

Inclusion Criteria: * Age 55-80, male or female. * The legal authorizer or caregiver of the subject (if applicable) needs to sign the informed consent before any assessment. * Based on the NINCDS / ADRDA and DSM-IV standards, it is likely to have AD dementia, with mild severe amnesia. * Screening CDR score = 0.5, and MMSE score is between 24-26 (AD patients in prodromal stage). * Or, * CDR score = 0.5 or 1, and MMSE score is between 20-23 (mild AD patients) -Or, * CDR score = 2, and MMSE score is between 15-21 (for moderate AD patients). * It has received PET imaging examination of amyloid protein or \[18F\] florbetapir imaging examination within 12 months before screening, and it is confirmed that there is amyloid binding based on qualitative analysis (visual read) and quantitative analysis. The results of amyloid PET imaging will be shared with participants, and the scanning results may be used by participants for future research. * There is no obvious evidence of other neuropathology in brain MRI supporting AD diagnosis. * Before the screening visit date, patients taking symptomatic treatment drugs must maintain a stable maintenance dose for at least 30 days. * Female subjects must have medical records or doctor's records for surgical infertility (through hysterectomy, bilateral ovariectomy or tubal ligation) or at least 1 year after menopause, otherwise, pregnancy test is required in screening period and every scanning visit and should be negative. Male subjects and their fertile partners must promise to use two methods of contraception during the study period, and one of them is barrier contraception for male subjects. * Male subjects are not allowed to donate sperm during the study and within 90 days after the completion of the study. * Willing and able to cooperate with the research process. * A qualified subject's Research Companion needs regular and sufficient contact between the research companion and the subject (weekly time ≥ 10 hours), can provide accurate information about the subject's cognition and function, and agrees to accompany the subject and provide relevant information during the visit. The study companion must have sufficient cognitive ability to accurately report the behavior, cognition and function of the subjects. The same study companion should be able to accompany the subjects to participate in the whole study process. Exclusion Criteria: * Any existing or 3-year history of alcohol or drug abuse (self statement) * Laboratory examination or ECG indicates significant clinical abnormalities and / or important unstable clinical diseases. * In the past year, the amount of radiation received by participating or clinical clinics in combination with this research institute has exceeded 50mSV (the annual limit allowed by FDA for research volunteers). * Serious gastrointestinal, cardiovascular, liver and kidney, blood, tumor, endocrine, potential nervous system, immune deficiency (including HIV positive), pneumonia and other diseases. Stable, treated chronic disease conditions such as high blood pressure, hyperlipidemia, diabetes, non metastatic skin cancer or prostate cancer, which researchers believe will not cause cognitive impairment or restrict participation in the study, are acceptable. * In addition to AD, the history or current status of neurological diseases that may affect cognition, including but not limited to Parkinson's disease, cortical basal ganglia degeneration, dementia of Lewy body, CJD, Huntington's disease, normal intracranial pressure hydrocephalus, tic disease, hypoxia, or other serious CNS trauma with significant clinical significance. * Other diseases or causes may cause the subjects to fail to complete the whole study. * MRI exclusion criteria include: evidence that may affect cognition, such as significant evidence of cerebrovascular disease (more than two lacunar infarcts, any infarct larger than 1cm3, or deep white matter abnormality, equivalent to Fazekas score level 3, at least one fusion high signal lesion on FLAIR sequence, i.e. ≥20mm in any dimension), infectious diseases, space occupying lesions, normal pressure hydrocephalus, central nervous system injury or any other structural abnormality that may affect cognition. * Internal implants such as cardiac pacemaker or defibrillator, insulin pump, cochlear implant, metal eye foreign body, implanted nerve stimulator, central nervous system aneurysm clip and other medical implants that can not receive MRI, or MRI examination has a history of claustrophobia. * Daily use of anticholinergic antidepressants, typical antipsychotics or barbiturates. Infrequent use of typical antipsychotics for vomiting or barbiturates for migraine treatment is permitted, provided that no dose is taken during the 5 half lives prior to screening or any neurocognitive assessment. * Daily use of benzodiazepines, opioids or opioids. However, intermittent short-term treatment is permitted, except for use in the 5 half-lives prior to screening or any neurocognitive assessment. * Use hypnotics, stimulants, atypical antipsychotics, central anticholinergic antihistamines, or anticholinergic antispasmodics with central effect, unless (a) they are administered daily, so that they will not start or stop treatment or dose change within the first five half lives of screening or at any time during the study period, or (b) they are administered intermittently and in a short period, while in screening or neurocognition Not used in the first 5 half lives of the assessment. * In the 12 months before screening, use any therapeutic molecule or treatment method targeting Aβ, or use the treatment targeting tau in the 24 months before screening