New Imaging Technology: Diffusion Tensor Imaging (DTI)

Diffusion tensor imaging (DTI) is an emerging imaging expression method developed based on conventional magnetic resonance imaging technology. DTI is a special form of MRI that not only displays the signal intensity of each voxel but also calculates specific data on tensor directions in three-dimensional space. Each voxel’s individual arrow is then organized based on the differences in neural bundle directions, generating a set of directional arrows. Through image data processing, each set of directional arrows is processed to form a linear image of the neural bundles.

The white matter fiber bundles in the brain are arranged geometrically, and diffusion magnetic resonance imaging can clearly display the white matter fiber bundles, allowing for multi-angle studies of their arrangement. It is now possible to observe the anisotropy of water molecule movement within white matter fiber bundles. The density of white matter bundle arrangement is positively correlated with the anisotropy of that area. In brain tissue, the anisotropy of white matter is higher compared to that of the cerebral cortex, which has a relatively lower anisotropy. Due to the presence of cell membranes, molecules must complete transmembrane movement during the process of moving between the inside and outside of cells and tissues, and diffusion occurs in this process for water molecules.

Due to the obstruction of nerve cell membranes, the movement or diffusion of water molecules along white matter fibers is faster than their movement or diffusion perpendicular to them, resulting in anisotropy. The differences in diffusion between parallel and perpendicular movements of water molecules form the basis for the images produced by diffusion tensor imaging. The fractional anisotropy (FA) value is expressed in the range of 0 to 1, representing the ratio of the anisotropic components of water molecules to the entire diffusion tensor. A smaller value indicates less restricted diffusion, while a larger value indicates increasing regularity and directionality of the tissue, which enhances neural conduction function. Therefore, FA values can be used to infer the degree of cell structure arrangement within brain white matter fiber bundles and the integrity of tissue structure.

DTI describes the directionality of water molecule diffusion within tissues not only through fractional anisotropy (FA) but also through the apparent diffusion coefficient (ADC), mean diffusivity (MD), axial diffusivity (AD), and radial diffusivity (RD) to provide a more specific description of the direction and magnitude of water molecule diffusion within tissues. Among them, the ADC value measures the diffusion movement of water molecules within tissues, reflecting the degree of displacement of water molecules in the diffusion-sensitive gradient direction. ADC is influenced by the temperature of water inside and outside cells, cell membrane permeability, viscosity, and proportions.

MD reflects the total water content in tissues, expressing the total diffusion activity of water molecules and molecular replacement; in the parallel axis position, it is described by AD, which reflects the direction in which the diffusion and movement of water molecules encounter the least obstruction, corresponding to the maximum value displayed in the tensor. AD is more sensitive to the integrity and degeneration of axons. The RD is described in the perpendicular axis position, where the value depends on the average of two relatively lower tensor values. RD can express the integrity of myelin. DTI has unique advantages in identifying and estimating neural bundles at subcortical levels and is widely used in clinical diagnosis and research exploration, as well as in the study of many brain diseases, such as stroke, Alzheimer’s disease, and brain aging.