Diffusion Tensor Imaging (DTI)

Information processing in the brain is performed by networks of neurons with the neurons connected to each other through axons. Groups of axons in the brain lead the formation of large scale white matter tracts which are important to investigate to understand human health and disease. Damage of the white matter tracts may affect neuronal communication and information processing and hence impaired cognition and disease. Magnetic resonance imaging (MRI) can be used to measure the white matter tracts and therefore obtain a map of the structural connectivity of the brain. Not only can the connectivity be measured, but also measures of the integrity of the white matter tracts can be obtained. A method that can be used for these measurements is diffusion tensor imaging (DTI).

721px-DTI_Brain_Tractographic_Image_Set

Anonymised clinical image provided by Aaron G. Filler, MD, PhD

Axons are long “wires” that are surrounded by a myelin sheath to protect the axon and allow communication between neurons. The myelin sheath is somewhat impermeable and hence water molecules move across the membrane at a very slow rate. In contrast, water molecules will diffuse more easily along the length of the axons (i.e. along the white matter tract) and hence water diffusion will be greater along the white matter tracts compared to across the myelin sheath. As an analogy, think of a drop of ink being placed in a glass of water. The ink will spread equally in all directions and hence the water diffusion is called isotropic. But if the ink drop is placed in a glass filled with water and, in addition, plastic straws are added into the glass, the ink drop will only diffuse along the length of the straws and not across the straws because the straw material is impermeable. Thus the water diffusion is anisotropic, referring to the fact that water diffusion is larger along one direction relative to another. Similarly in white matter tracts water diffusion is stronger along the length of the white matter tracts compared to across the white matter tracts.

The direction of water diffusion allows us to reconstruct the white matter tracts in the brain. Studies have shown that this technique allows one to reconstruct the major white matter tracts but caution is required, because the technique assumes during the image reconstruction, that there is only a single white matter tract at each measurement point. This is a limitation given that it is known that the brain has many regions with crossing fibres and complex connecting structure. More advanced techniques such as constrained spherical deconvolution and high angular resolution diffusion imaging techniques are available to map complex crossing fibres. But even with this limitation DTI provides useful information about the structural connectivity and integrity of the white matter tracts to be able to study normal brain function and disease.

Fractional anisotropy (FA) is one of the most common measures used in DTI to quantify the structural integrity of white matter tracts. FA is a function of the ratio of water diffusion along the 3 cardinal directions, with high values reflecting larger water diffusion along one direction relative to the other 2 directions. In the case of disease, the myelin sheath around the axon may be damaged and allow for greater water diffusion perpendicular compared to before damage to the myelin sheath. This leads to lower FA value compared to healthy tissue and therefore allows us to investigate changes due to disease.