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Diffusion tensor imaging, or DTI, is a type of magnetic resonance imaging (MRI) that measures the diffusion of water molecules in biological tissues. In a biological environment, water molecules are not static but are in constant random motion. This random motion is called diffusion. In tissues, the movement of water molecules is influenced by the underlying structure. For example, in white matter tracts of the brain, which are composed of tightly packed nerve fibers, water diffusion is restricted in directions perpendicular to the fiber orientation and more free along the direction of the fibers. DTI exploits this directional diffusion to infer the organization and integrity of these white matter pathways.

The core principle of DTI involves applying a series of magnetic field gradients during the MRI scan. These gradients sensitize the image to the motion of water molecules. By acquiring images with diffusion gradients applied in multiple directions, DTI can estimate a tensor for each voxel (a 3D pixel) in the image. This tensor mathematically describes the magnitude and directionality of water diffusion within that voxel. From this tensor, various metrics can be derived, such as fractional anisotropy (FA), which quantifies the degree of directional diffusion, and mean diffusivity (MD), which measures the overall magnitude of diffusion. High FA values typically indicate well-organized and intact white matter, while low FA can suggest damage or degeneration. DTI is widely used in neuroscience research and clinical settings for studying conditions affecting white matter, including stroke, multiple sclerosis, traumatic brain injury, and neurodegenerative diseases. It allows for non-invasive visualization and characterization of the brain's structural connectivity.