Analysis, Tractography and Applications in Diffusion Imaging Flashcards
What can be exploited by diffusion MRI?
Entire white matter tracts which can be several millimetres across, often have a coherent orientation in any particular part of brain
What is magnetic resonance signal sensitive to?
Random thermal motion of water molecules within neural tissue
Since structures such as cell walls and myelin impede such motion, observing the characteristics of this self-diffusion in a particular part of brain provides insight into the underlying tissue microstructure
What confers a strong orientiental dependence of anisotropy on the mobility of free water ?
Very linear structure of white matter tracts
When are water molecules freer to move along cyclinders than across them?
Very loosely, a millimetre or so of tract may be thought of as resembling a bundle of cyclinders with a single orientation
What does MR brain diffusion utilise?
The principle of microscopic water movement in the brain tissue
Under normal conditions, water in the brain tissue moves rather freely. When a disruption to the blood supply of the brain occurs, the tissue will swell
The swelling will decrease the freedom with which the water can move about
The restriction in movement is visible on the MT diffusion scan images
What is diffusivity in the brain?
- Mean diffusivity which can be collected from ADC analysis
- Get bright area In the centre of the brain at the surface of cerebrospinal fluid where the diffusivity is high
- Dark areas in the remaining areas
What are we interested from tractography?
Anisotropy
What do we want to get the orientation of?
White matter structures
Why do we rely on the anisotropy of diffusion?
We can’t see the white matter structures directly
What is found in CSF?
Relatively equal freedom to move in all directions
What is fractional anisotropy?
A scalar value between zero and one that describes the degree of anisotropy of a diffusion process
A value of zero means that diffusion is isotropic I.e. it is unrestricted (or equally restricted) in all directions
A value of one means that diffusion occurs only along one axis and is fully restricted along all other directions
What is FA thought to measure in diffusion imaging?
- Fiber density
- Axonal diameter
- Myelination in white matter
What occurs in structured spaces?
The probability of water diffusion is constrained in some directions but unconstrained in others
What happens in brain tissue?
Water molecules diffuse more freely along the axons but are relatively constrained from escaping it or moving across the walls of axons
How can the diffusion tensor be calculated?
By calculating the diffusivity along multiple directions and it becomes possible to estimate the orientation of axon bundles
What does white matter have?
Relatively high FA
What do CSF and grey matter have?
Low FA
What are the basis of anisotropy in tissue?
- Neural white matter, arranged in bundles of fibres, is particularly linear
- Diffusion perpendicular to bundles is more impeded
- Myelin and neural filaments contribute