Evaluation of [18F]PI-2620 as a Potential Positron Emission Computed Tomography Radioligand for Imaging Tau Protein in the Brain

Inclusion Criteria: 1. General Subject Inclusion Criteria In order to be eligible for participation in this trial, the subject must: * Be ≥ 40 and \< 80 years of age at the Screening Visit. * Be able to read at a 6th grade level or equivalent, (as determined by the investigator, and must have a history of academic achievement and/or employment sufficient to exclude mental retardation.) * Be able to speak, read, hear, and understand the language of the trial staff, and the informed consent form, and possess the ability to respond verbally to questions, follow instructions, and complete questionnaires and detailed neuropsychological test. * Have results of clinical laboratory tests/physical examination, vital signs, and ECG within normal limits (at 90 days prior to \[18F\]PI-2620 positron emission computed tomography) or clinically acceptable to the investigator at screening. * If female, not be of childbearing potential as indicated by one of the following * Each subject (or legal representative) must sign the informed consent form in accordance with local requirements after the scope and nature of the investigation have been explained to them, and before screening assessments. 2. Cognitively Healthy Subjects 3. Subjects with Alzheimer's disease 4. Subjects with frontotemporal dementia 5. Subjects with Parkinson's disease Exclusion Criteria: The subject must be excluded from participating in the trial if the subject fulfil any single criteria described below: 1. General Exclusion Criteria * Based on the investigators' judgement, if the patient is not capable of communicating with the site personnel, if the patient is not proficient in the language in which the psychometric tests will be completed, or if the patient is not sufficient for compliance with the study procedures. * The patient has an abnormal physical examination or abnormal laboratory test results at the screening that are clinically significant to affect results of the research, as judged by the investigator. * If the patient has or is suspicious of having a hypersensitivity or allergy to \[18F\] PI-2620 or its derivatives. * The patient is pregnant, is attempting to become pregnant, or is nursing (breast-feeding) children. * The patient has a history of alcoholism or drug dependency/abuse within the last 2 years before screening. * The patient has contraindications to undergo positron emission computed tomography or MRI, which include but are not restricted to the examples below: claustrophobia, cardiac pacemaker, metal devices around the eye or spinal cord, cochlear implant, etc.) at the screening visit. * The patient has been treated with any investigational medicinal product (IMP) within 30 days prior to the screening visit. 2. Cognitively Healthy Subjects 3. Subjects with Alzheimer's disease 4. Subjects with frontotemporal dementia 5. Subjects with Parkinson's disease