Inclusion Criteria:
* Age 55-90 years (inclusive).
* Willing and able to participate in baseline assessment and longitudinal follow-up; voluntary provision of biospecimens and personal information, and commitment to complete all follow-up visits.
* Able to undergo MRI scanning (no contraindications to MRI).
* Group-specific cognitive criteria:
CN: No subjective memory complaints beyond age expectation (confirmed by study partner); MMSE score 26-30 (inclusive; exceptions permitted for participants with \<8 years of education with principal investigator approval; CDR = 0, memory box = 0; Normal cognitive and daily functioning, no significant impairment.
MCI: Subject, partner, or physician reports subjective memory concerns; MMSE criteria same as CN group; CDR = 0.5 (memory box ≥0.5); General cognition and function relatively preserved; does not meet criteria for AD.
AD: Subject, partner, or physician reports subjective memory concerns; MMSE score \<26 (inclusive; exceptions as above); CDR = 0.5 or 1.0; Meets National Institute of Neurological and Communicative Disorders and Stroke/Alzheimer's Disease and Related Disorders Association (NINCDS/ADRDA) probable AD diagnostic criteria or 2024 National Institute on Aging-Alzheimer's Association (NIA-AA) criteria (e.g., positive Pittsburgh compound B (PIB) and tau).
Exclusion Criteria:
* Self-reported or MRI-detected major neurological diseases other than AD: including but not limited to stroke (cerebral hemorrhage, infarction), congenital intellectual disability, intracranial tumors, epilepsy, Parkinson's disease, Huntington's disease, normal pressure hydrocephalus, progressive supranuclear palsy, multiple sclerosis, severe head trauma with persistent deficits, etc.
* Severe psychiatric disorders (e.g., schizophrenia requiring medication control) or other known brain structural abnormalities.
* Significant organ failure (heart, liver, kidney, etc.), malignant tumors, or short life expectancy making completion of follow-up unlikely.
* Contraindications to MRI (e.g., claustrophobia, incompatible pacemaker, aneurysm clip, artificial heart valve, cochlear implant, or other metallic implants).
* Other clinical history or examination findings judged by investigators as potentially unsafe for MRI or follow-up.
* Use of investigational drugs within one month prior to enrollment or during the study.
