The goal of this observational study is to compare cerebral perfusion patterns with pseudo-continuous arterial spin labeling (pCASL) and brain metabolism patterns with fluorodeoxyglucose-position emission tomography (FDG-PET) in patients with mild cognitive impairment (MCI) and subjective cognitive decline (SCD). The main questions it aims to answer are: * Do pCASL sequences identify hypoperfusion patterns that correlate well with FDG-PET hypometabolic patterns? * Are there differences in this correlation in terms of cerebrospinal fluid (CSF) profiles? * Can hypoperfusion patterns in pCASL predict conversion to dementia? Participants will undergo brain 3 Tesla magnetic resonance imaging (MRI), FDG-PET, lumbal puncture and blood collection to analyze amyloid beta and tau, yearly detailed neuropsychological tests for three years.