Diffusion MRI
Diffusion MRI measures the diffusion of water molecules in biological tissues. In an isotropic medium (inside a glass of water for example) water molecules naturally move according to Brownian motion. In biological tissues however, the diffusion is very often anisotropic. For example a molecule inside the axon of a neuron has a low probability to cross a myelin membrane. Therefore the molecule will move principally along the axis of the neural fiber. Conversely if we know that molecules locally diffuse principally in one direction we can make the assumption that this corresponds to a set of fibers.
The recent development of Diffusion Tensor Imaging (DTI) enables diffusion to be measured in multiple directions (currently up to 99) and the Fractional Anisotropy in each direction to be calculated for each voxel. This enables researchers to make axonal maps to examine the structural connectivity of different regions in the brain (tractography) or to examine areas of neural degeneration and demyelinaton in diseases like Multiple Sclerosis.
Another application of diffusion MRI is diffusion-weighted imaging (DWI). Following an ischemic stroke, brain cells die. It is speculated that resultant areas of restricted diffusion are detectable. This finding appears within 5-10 minutes of the onset of stroke symptoms (as compared with computed tomography, which often does not detect changes of acute infarct for up to 4-6 hours) and remains for up to two weeks. As such, DWI sequences are extraordinarily sensitive for acute stroke.
Finally, it has been proposed that diffusion MRI may be able to detect minute changes in extracellular water diffusion and therefore could be used as a tool for fMRI. The nerve cell body enlarges when it conducts an action potential, hence restricting extracellular water molecules from diffusing naturally. Although this process works in theory, evidence is only moderately convincing. If it could be made to work, diffusion fMRI would not experience the temporal lag seen in BOLD fMRI.
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