Week 3 part 2 Flashcards
What is diffusion?
The simple random movement of water molecules. During the time water molecules cover a larger distance.
What is diffusion coefficient?
Property directly related to the mobility of water molecules and its kinetic energy and therefore its temperature.
What equation is needed to understand diffusion?
Einstein equation: [r squared] = 6Dt;
- r2 the mean squared displacement of the particle in three dimensions
- 6 numerical constant which depends on the dimensionality
- D - diffusion coefficient
t- time
What is apparent diffusion coefficient (ADC)?
It represents an index of mobility of the water molecules inside biological tissues.
What is diffusion MRI?
It is a technique that allows us the measurement of molecular diffusivity of water inside biological tissues.
What do we need to measure diffusion?
Two types of images: are T2-weighted and diffusion-weighted images. Both are very similar, only the diffusion-weighted image is a T2 image with the added diffusion contrast.
Isotropy diffusion
Shows same diffusion property along every direction.
Anisotropy diffusion
Shows different diffusion properties along different directions.
Where we can see anisotropy?
In the axon and white matter, because axonal membranes, myelin sheets create well organized structure that restricts diffusion across fibre.
What is diffusion tensor?
Diffusion tensor is a mathematical formula that gives us a 3D or 3D model of water molecular diffusion inside each voxel of the brain.
What is diffusion ellipsoid?
A graphical representation of the diffusion tensor.
How the size and shape of the diffusion is defined by the three eigenvalues?
Lambda 1, lambda 2, lambda 3, can be three different shapes:
- axial anistrophy (cigar shape)
- planar anistropy (no more dominant orientation, circle)
- Isotropy
What type of quantitative information we can extract from DTI?
- Mean diffusivity (MD)
2. Fractional anisotropy (FA)
What does the mean diffusivity (MD) identify?
Average mobility or diffusivity of water molecules without worrying about direction. MD is a rotational invariant index: it doesn’t change with the orientation of the subject in the scanner. MD can detect small changes in the brain.
What does the fractional anisotropy (FA) identify?
FA tells about the organisation of the brain microstructure.
Tells about white matter diffusion properties (eg the level of the myelinisation). FA is rotational invariant like MD.
How MD and FA are related?
Changes you see in FA might not be visible in MD and vice versa.
What is RGB map?
It is like FA map in which colors are added to describe the orientation of white matter.
What is the main assumption under streamline tractogrophy?
The main eigenvector is tangential to the underlying trajectory of the white matter.
What are potential issues with tractography?
- generate surplus streamlines
- show invalid or missing connections
- noisy artefacts generate spurious artefacts
- cannot resolve multiple fibre orientations or fibre connections
What is spherical deconvolution?
pherical deconvolution (SD) is a particularly appealing HARDI method, as it is able to provide estimates of the full fibre orientation distribution function (fODF) in each brain voxel, regardless of the number of underlying fibre orientations
