Table 2.

Neuroimaging Modalities

Imaging Modality	Brain Feature Measured	Description
High-resolution structural MRI	Macrostructural anatomy	Allows for the accurate quantification of macrostructural anatomical features of the brain, including regional volume, surface area, cortical thickness, or shape. Brain regions of interest can be delineated manually or using automated parcellation software.
DTI	White matter microstructure	Used to examine properties of brain tissue microstructure through the quantification of local water diffusion. DTI provides four main measures of white matter microstructure that are derived from the three eigenvalues of the diffusion tensor, including FA (describes the degree of diffusion anisotropy, scaled from 0 to 1), MD (a direction-independent measure of average diffusivity), AD (quantifies diffusion along the main axis of diffusion, ie, the primary eigenvalue), and RD (quantifies diffusion perpendicular to the main axis of diffusion, ie, the mean of the secondary and tertiary eigenvalues). Differences in DTI measures can be examined using white matter tract-based or voxel-based approaches.
MRS	Neurochemicals	Measures the concentration of diverse chemicals and metabolites in the brain based on known proton resonance frequencies, including NAA (a marker of neuronal density and viability), Cho (a component of membrane phospholipids), mI (a marker of astrocytes), Cr (a metabolite involved in brain energetics), and Glu (an excitatory neurotransmitter).
SWI	Microhemorrhages	Enables the identification of hemosiderin (iron storage complex) foci that develop following both large and small hemorrhages, given the different magnetic susceptibilities of these foci as compared to the surrounding tissue. Using SWI, a hypointensity burden can be calculated, which provides a normalized estimate of the total volume of hypointensities belonging to nonblood vessel hypointense voxel clusters. The hypointensity burden is believed to reflect the burden of microhemorrhages in the brain.
fMRI	Brain region or network activation	Detects changes in local levels of paramagnetic deoxyhemoglobin via the BOLD contrast, thus providing insight into changes in localized blood flow, oxygen concentration, and neuronal activity. FMRI can provide information about brain activity when individuals are in a state of rest (allowing for the evaluation of stable “resting-state” brain networks) or performing a task.

Abbreviations: AD, axial diffusivity; BOLD, blood oxygen level-dependent; Cho, choline; Cr, creatine; DTI, diffusion tensor imaging; FA, fractional anisotropy; fMRI, functional MRI; Glu, glutamate; MD, mean diffusivity; mI, myo-Inositol; MRS, magnetic resonance spectroscopy; NAA, N-acetyl aspartate; RD, radial diffusivity; SWI, susceptibility weighted imaging.

Note: A description of the neuroimaging methods that have been employed to examine differences in brain structure, function, and neurochemistry following sports-related concussion.