D11 Structural neuroimaging (CT, MRI, DTI)

Topic

Brain tissue generates different types of contrast in structural magnetic resonance imaging based on its physical properties: the time it takes for each type of brain tissue to recover its longitudinal magnetization and the time it takes for its transverse magnetization to decay differ between gray matter, white matter, and cerebrospinal fluid; these differences in timing allow these three types of brain tissue to be distinguished from one another in the image.

Diffusion tensor imaging extends this same principle of tissue contrast, but instead of measuring relaxation times, it measures the anisotropic diffusion of water within white matter tissue—that is, it measures how water diffuses more easily in certain directions than in others within the white matter. This anisotropic diffusion is characterized by a diffusivity tensor, whose principal directions of diffusion allow for the calculation of the fractional anisotropy of the white matter tissue—a direct measure of the microstructural integrity of the axonal tracts running through that white matter. The greater the integrity of the axonal tracts, the higher the fractional anisotropy measured in that white matter region, since water diffuses more directionally along intact axons than along damaged ones. Furthermore, fractional anisotropy serves as a quantitative measure of the amount of myelin coating those axons—the same quantity of myelin used to describe the axon's myelin sheath in another model in this catalog.

In addition to fractional anisotropy, anisotropic water diffusion in white matter is also summarized by mean diffusivity, which quantifies the overall magnitude of water diffusion without distinguishing the direction in which it occurs. Based on the principal direction of diffusion at each point in the white matter tissue, tractography reconstructs the complete trajectory of axonal fiber bundles by integrating that principal direction throughout the white matter tissue. Beyond white matter, structural MRI also enables the volumetric segmentation of specific brain tissue structures, such as the hippocampus in Alzheimer's disease or the substantia nigra in Parkinson's disease. This volumetric segmentation allows for the objective quantification of atrophy or structural damage affecting each of these brain tissue structures. Ultimately, it is this very brain tissue—first differentiated by relaxation times and subsequently characterized in greater detail via white matter anisotropic diffusion—that enables the objective quantification, through volumetric segmentation, of the structural damage associated with the pathologies described in other models within this catalog.

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