**2. Neuroimaging techniques**

Functional neuroimaging, broadly defined as techniques that provide measures of brain activity, has further increased our ability to study the neural basis of cognition and behavior. Functional neuroimaging dates back to the use of electrophysiological methods such as electroencephalography (EEG). However, the lack of spatial resolution in these methods (i.e., often limited to hemispheric or anterior-posterior differences) did not allow testing hypotheses regarding the precise anatomical location of a brain region subserving a given cognitive process (D'Esposito & Wills, 2000, Galin & Ornstein, 1972).

The modern era of functional brain imaging, bringing markedly improved spatial resolution, was introduced first in the mid-1970s using the xenon cerebral blood flow technique and later in the mid-1980s using positron emission tomography (Mazziotta & Phelps, 1984, Petersen et al., 1988). In more recent years, functional magnetic resonance imaging (fMRI) has rapidly emerged as an extremely powerful technique with many advantages over positron emission tomography (PET) for studying cognition.

In dementia evaluation, the most commonly used tracers are 99mTc-HMPAO for SPECT brain perfusion and 18F-FDG for PET glucose's metabolism. For practical purposes, the clinical analysis of brain perfusion and glucose metabolism are equivalent, so that hypoperfusion areas in brain SPECT roughly correspond to glucolytic hypometabolism in PET.

The different syndromes and diseases presenting with dementia have different patterns of brain perfusion abnormalities, so we can distinguish them with good specificity.
