Diffusion-weighted MRI (DWI) uses the movement of water molecules to create contrast in magnetic resonance images. Because water motion is influenced by microscopic tissue structure, diffusion measurements can reveal changes that may not yet be visible on conventional MRI.
DWI combines specially designed MRI sequences with processing software to estimate how readily water molecules move in each image voxel. A voxel is a small three-dimensional element of an image. The signal in a diffusion-weighted image is affected by molecular motion: greater diffusion generally causes greater signal attenuation when diffusion weighting is applied.
All flashcards
What does the b-value summarize in diffusion MRI?
The strength of diffusion weighting produced by the acquisition; a larger b-value increases sensitivity to diffusion-related signal loss.
What is the apparent diffusion coefficient (ADC)?
An effective estimate of diffusion in tissue, influenced by factors such as barriers, restricted spaces, and motion.
How many independent components does a symmetric three-dimensional diffusion tensor have?
Six, because symmetry makes each pair of off-diagonal components equal.
What does fractional anisotropy (FA) summarize?
How unequal diffusion is across directions; it does not identify the cause of that directional inequality.
What does tractography represent?
A model-based estimate of probable white-matter pathways, not a direct image or proof of anatomical connections.
Quiz questions
What effect does increasing the b-value generally have, with other acquisition factors held constant?Answer: It increases sensitivity to diffusion-related signal attenuation.
Why does brightness on a conventional diffusion-weighted image alone not prove restricted diffusion?Answer: The image retains relaxation-related contrast, including T2 effects.
Why are at least six non-collinear diffusion directions typically used to estimate a three-dimensional diffusion tensor?Answer: They provide measurements for the tensor’s six independent components.
Which tensor quantity is axial diffusivity (AD)?Answer: The largest eigenvalue, representing diffusion along the principal axis.
How should a streamline generated by tractography be interpreted?Answer: As an algorithmic estimate of a possible white-matter pathway.