Neuroimaging Techniques

While sleep stages are defined by neurophysiological criteria (i.e., electroencephalographic recording), sleep has been explored with functional brain imaging techniques to characterize the distribution of brain activity across and within the various stages of sleep. Positron emission tomography (PET) shows the neural distribution of compounds labeled with positron-emitting isotopes. For instance, PET using oxygen-15 labeled water (H215O) gives an assessment of regional cerebral blood flow (rCBF), while PET with 18F-fluorodeoxyglucose (18F-FDG) measures brain glucose metabolism (CMRglu). PET has been used to compare brain perfusion or glucose metabolism patterns between different stages of sleep and wakefulness. More recently, functional magnetic resonance imaging (fMRI) has also been used to study brain activity during sleep. This technique measures the variations in brain perfusion related to neural activity, by assessing the blood-oxygen-level-dependent (BOLD) signal. The latter relies on the relative decrease in deoxyhemoglobin concentration that follows the local increase in cerebral blood flow in an activated brain area. Although improvements have been made in the spatial resolution of PET, fMRI still benefits from a superior temporal resolution, which has allowed to assess with fMRI brain activations related to specific neural oscillations within sleep stages, and thus to characterize the neural correlates of sleep micro-architecture.