Table 1.

Definitions and possible biological interpretations of the neuroimaging metrics covered in this review.

T1-weighted (Gray matter) neuroimaging: morphometry of brain structure is measured.
Measure	Definition	Possible Biological Interpretation
Cortical Thickness	The thickness of the cerebral cortex, measured from the pial surface to the white/gray matter boundary in the cortex.	Number of neurons in ontogenetic columns (139).
Surface Area	Total area of a particular brain region	Measure of proliferation of radial unit progenitors (139).
Local Gyrification Index	Amount of cortex buried within the sulcal folds as compared with the amount of cortex on the outer visible cortex.	Believed to reflect early brain development, as cortical folding is determined in the first months of life.
Diffusion-weighted neuroimaging: measures how fast water diffuses across a space; greater diffusion indicates that there is more restriction in the opposing directions.
Measure	Definition	Possible Biological Interpretation
Fractional anisotropy	a general metric of overall white matter integrity	Greater fractional anisotropy is typically associated with higher white matter integrity.
Axial diffusivity	diffusion of water parallel to white matter fibers	Decreased axial diffusivity is associated with axonal damage (140).
Radial diffusivity	diffusion of water perpendicular to white matter tracts	Increased radial diffusivity is associated with demyelination (84,141,142).
Mean diffusivity	measure of the magnitude of diffusion	Greater mean diffusivity is typically associated with greater white matter pathology.
Resting-state functional magnetic resonance imaging: measures spontaneous fluctuations in neural activity. A functional MRI is acquired in the absence of a task (i.e., participant lies in the scanner while awake but does not engage in a task).
Measure	Definition	Possible Biological Interpretation
Network-Based Connectivity	Multiple individual brain regions that are more likely to correlate with each other are in the same network.	Varies based on network connectivity measure being assessed.
Seed-Based Connectivity	One region of the brain is chosen as the “seed” region (e.g., thalamus). Correlations between the time series extracted from this region and other regions of the brain are assessed. “Other regions” may consist of specific regions of interest or the entire rest of the brain.	Higher levels of correlation between two brain areas suggests that there is greater coupling between the two regions.