Advanced MRI- 2 Flashcards
How can the axonal structure of anisotropic diffusion be described in a neurone and why is this the case?
- Thimble structure
- Greater diffusion along the axon than across it.
What ADC value is measured across the axon?
-Low
What ADC value is measured along the axon?
-High
Why is the direction of the diffusion sensitisation altered?
-So we can measure ADC in any direction
Where is the greatest diffusivity observed when diffusion is measured from left-right?
-Corpus callosum
Where is the greatest diffusivity observed when diffusion is measured from anterior-posterior?
-White matter pathways
Where is the greatest diffusivity observed when diffusion is measured from superior-inferior?
-Internal capsules
Which tissue reflects similar diffusivity in all directions?
-CSF
What does DTI stand for?
-Diffusion tensor imaging
What is the purpose of DTI?
-To determine whether we have ellipsoid or sphere representing the root mean squared displacement
What is the DTI process?
- Acquire diffusion sensitised images in at least 6 different directions
- Also acquire an image with no diffusion-sensitisation
- Compute ADC in each diffusion direction
- Build a 3D model of the diffusion at each image voxel
- Each image voxel allows us to determine the shape of the diffusivity taking place.
- This 3D model is called the Diffusion Tensor model
- This provides Diffusion Tensor Images (DTI)
- This allows us to quantify anisotropic and isotropic diffusion
What is purpose of mean diffusivity?
-Quantifies magnitude of isotropic diffusion
What are two characteristics of MD?
- Rotationally invariant
- Uniform across grey and white matter
What is usual MD of white matter?
0.8 x 10-3 mm2s-1
What can MD be used to measure?
-Measures ultrastructural damage
What is purpose of FA?
-To measure how anisotropic diffusion is at each point in an image
What type of diffusion do dark and light areas represent?
- Dark is isotropic
- Light is anisotropic
Name on characteristic of FA
-Rotationally invariant
Purpose of FA?
- Measures structural integrity and tissue damage
- Healthier matter has more anisotropy
What causes FA to decrease?
-FA decreases with neuronal damage?
What is FA a measure of?
-Measure of disease severity in :
- Cerebrovascular small vessel disease
- Multiple sclerosis
- Normal aging
- Lupus
What is white matter damage on DTI imaging correlative with?
-White matter damage on diffusion tensor imaging correlates with age-related cognitive decline.
How is MD and FA affected as the brain develops?
- MD decreases
- FA increases
How is MD and FA affected as the brain ages?
- MD increases
- FA decreases
What is observed in Small Vessel disease?
- Leukoaraiosis as a result of white matter degeneration
- This is a result of axonal degeneration, demyelination and gloss
What changes to MD and FA are seen in small vessel disease?
- Increase in MD
- Decrease in FA
What changes to MD and FA is observed in demyelination and remyelination phase of MS?
Remyelination: Increase MD, Decrease FA
Demyelination: Decrease MD, Increase FA
What does the primary orientation of white matter mean?
-Primary direction that white matter grows in
What does the primary orientation of white matter indicate?
-Principle diffusion direction
What is tractography?
- Process by which white matter pathways are tracked
- Connect voxels on the basis of their principle diffusion direction
What is a potential Clinical Use Of Tractography?
-Presurgical planning for tumour resection
What are the limitations of DTI?
- DTI provides a single principal diffusion direction but the model does not allow for multiple fibre crossings
- Principal direction may be inaccurately orientated
- Secondary correlates of tissue microstructure
- Macroscopic measurement (voxels ~2mm3) of microscopic effect
- Ambiguous nature of parameters. For instance FA is affected by myelination, axon density or axon diameter
- Difficult to separate disease process from diffusion characteristics
What is it meant by the statement that DTI produces secondary correlates of tissue microstructure?
- Measures diffusion characteristics, it is not a direct measure
- Structural tissue changes are inferred from diffusion characteristics
What is an advantage of DTI?
-Only in-vivo technique currently capable of providing insight to tissue structural integrity and white matter pathway anatomy and connectivity
