Advanced MRI- 2

Question 1

How can the axonal structure of anisotropic diffusion be described in a neurone and why is this the case?

Answer 1

• Thimble structure

- Greater diffusion along the axon than across it.

Question 2

What ADC value is measured across the axon?

Answer 2

-Low

Question 3

What ADC value is measured along the axon?

Answer 3

-High

Question 4

Why is the direction of the diffusion sensitisation altered?

Answer 4

-So we can measure ADC in any direction

Question 5

Where is the greatest diffusivity observed when diffusion is measured from left-right?

Answer 5

-Corpus callosum

Question 6

Where is the greatest diffusivity observed when diffusion is measured from anterior-posterior?

Answer 6

-White matter pathways

Question 7

Where is the greatest diffusivity observed when diffusion is measured from superior-inferior?

Answer 7

-Internal capsules

Question 8

Which tissue reflects similar diffusivity in all directions?

Answer 8

-CSF

Question 9

What does DTI stand for?

Answer 9

-Diffusion tensor imaging

Question 10

What is the purpose of DTI?

Answer 10

-To determine whether we have ellipsoid or sphere representing the root mean squared displacement

Question 11

What is the DTI process?

Answer 11

• 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

Question 12

What is purpose of mean diffusivity?

Answer 12

-Quantifies magnitude of isotropic diffusion

Question 13

What are two characteristics of MD?

Answer 13

• Rotationally invariant

- Uniform across grey and white matter

Question 14

What is usual MD of white matter?

Answer 14

0.8 x 10-3 mm2s-1

Question 15

What can MD be used to measure?

Answer 15

-Measures ultrastructural damage

Question 16

What is purpose of FA?

Answer 16

-To measure how anisotropic diffusion is at each point in an image

Question 17

What type of diffusion do dark and light areas represent?

Answer 17

• Dark is isotropic

- Light is anisotropic

Question 18

Name on characteristic of FA

Answer 18

-Rotationally invariant

Question 19

Purpose of FA?

Answer 19

• Measures structural integrity and tissue damage

- Healthier matter has more anisotropy

Question 20

What causes FA to decrease?

Answer 20

-FA decreases with neuronal damage?

Question 21

What is FA a measure of?

Answer 21

-Measure of disease severity in :

• Cerebrovascular small vessel disease
• Multiple sclerosis
• Normal aging
• Lupus

Question 22

What is white matter damage on DTI imaging correlative with?

Answer 22

-White matter damage on diffusion tensor imaging correlates with age-related cognitive decline.

Question 23

How is MD and FA affected as the brain develops?

Answer 23

• MD decreases

- FA increases

Question 24

How is MD and FA affected as the brain ages?

Answer 24

• MD increases

- FA decreases

Question 25

What is observed in Small Vessel disease?

Answer 25

- Leukoaraiosis as a result of white matter degeneration | - This is a result of axonal degeneration, demyelination and gloss

Question 26

What changes to MD and FA are seen in small vessel disease?

Answer 26

- Increase in MD | - Decrease in FA

Question 27

What changes to MD and FA is observed in demyelination and remyelination phase of MS?

Answer 27

Remyelination: Increase MD, Decrease FA Demyelination: Decrease MD, Increase FA

Question 28

What does the primary orientation of white matter mean?

Answer 28

-Primary direction that white matter grows in

Question 29

What does the primary orientation of white matter indicate?

Answer 29

-Principle diffusion direction

Question 30

What is tractography?

Answer 30

- Process by which white matter pathways are tracked | - Connect voxels on the basis of their principle diffusion direction

Question 31

What is a potential Clinical Use Of Tractography?

Answer 31

-Presurgical planning for tumour resection

Question 32

What are the limitations of DTI?

Answer 32

- 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

Question 33

What is it meant by the statement that DTI produces secondary correlates of tissue microstructure?

Answer 33

- Measures diffusion characteristics, it is not a direct measure - Structural tissue changes are inferred from diffusion characteristics

Question 34

What is an advantage of DTI?

Answer 34

-Only in-vivo technique currently capable of providing insight to tissue structural integrity and white matter pathway anatomy and connectivity