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 demylinaton 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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