Assess Fibrin in Brains With AD/ADRD

Inclusion Criteria: * Age between 55 and 90 years * Ability to provide informed consent * Specific to healthy volunteers: no history of ADRD * Specific to ADRD subjects: clinical diagnosis of AD - either amnestic or atypical, clinical severity ranging from MCI to moderate dementia (CDR 0.5-2.0), MMSE score greater than or equal to 15 and/or MOCA greater than or equal to 12 * Specific to BAnD subjects: referred through the Brain Aging and Dementia (BAnD) research registry Exclusion Criteria: * MR contraindications such as: electrical implants such as cardiac pacemakers or perfusion pumps; ferromagnetic implants such as aneurysm clips, surgical clips, prostheses, artificial hearts, valves with steel parts, metal fragments, shrapnel, tattoos near the eye, or steel implants, ferromagnetic objects such as jewelry or metal clips in clothing * Preexisting medical conditions including a likelihood of developing seizures or claustrophobic reactions, and any greater than normal potential for cardiac arrest * Subjects whose estimated glomerular filtration rate (eGFR) \< 60 mL/min will be excluded from receiving the gadolinium-based contrast agent * Research-related radiation exposure exceeding current Massachusetts General Hospital (MGH) Radiology Department guidelines (i.e. 50 millisievert in the prior 12 months) * 3\. In line with published MGH IRB guidelines for pregnancy must be ruled out by urine ß-HCG if answers to screening questions suggest that pregnancy is possible and if female participants are premenopausal and of child-bearing age. Subjects will not be able to enroll if they are breastfeeding.