Diffusion Tensor Imaging DTI is a sequence of an MRI scan that examines the microstructure of the white matter (axons) of the brain. DTI works by measuring the distribution of water through portions of the brain. DTI is based upon the known physics of the flow of water.

On a purely smooth surface, water will flow equally in all directions in a manner called an isotropic distribution. If, however, there are barriers to flow (such as found in the white matter of the brain), water will move unequally in all directions, called anisotropic distribution.

Water distribution in healthy, intact white matter tends to be anisotropic. But as white matter is damaged, the outer membranes are broken down, causing the water to diffuse in a more isotropic distribution. DTI divides the brain into thousands of voxels. Voxels are like pixels of a digital camera, except they are three dimensional.

DTI measures the distribution of water through each voxel in the brain and provides a score between 0 and 1. In the medical literature, that score is referred to as “FA” (fractional anisotropy). A lower score means that the distribution of water is more isotropic (equal in all directions), with a score of 0 representing pure isotropic distribution.

A higher score means the distribution is more anisotropic, with a score of 1 being close to a straight line. It is well known that axonal injury will result in decreased FA scores. As per the guidelines published by the American Society of Functional Neuroradiology, DTI has been approved for clinical use.