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Clinical Neuro > Advanced MRI- 2 > Flashcards

Advanced MRI- 2 Flashcards

1
Q

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

A
  • Thimble structure

- Greater diffusion along the axon than across it.

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2
Q

What ADC value is measured across the axon?

A

-Low

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3
Q

What ADC value is measured along the axon?

A

-High

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4
Q

Why is the direction of the diffusion sensitisation altered?

A

-So we can measure ADC in any direction

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5
Q

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

A

-Corpus callosum

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6
Q

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

A

-White matter pathways

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7
Q

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

A

-Internal capsules

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8
Q

Which tissue reflects similar diffusivity in all directions?

A

-CSF

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9
Q

What does DTI stand for?

A

-Diffusion tensor imaging

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10
Q

What is the purpose of DTI?

A

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

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11
Q

What is the DTI process?

A
  • 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
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12
Q

What is purpose of mean diffusivity?

A

-Quantifies magnitude of isotropic diffusion

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13
Q

What are two characteristics of MD?

A
  • Rotationally invariant

- Uniform across grey and white matter

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14
Q

What is usual MD of white matter?

A

0.8 x 10-3 mm2s-1

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15
Q

What can MD be used to measure?

A

-Measures ultrastructural damage

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16
Q

What is purpose of FA?

A

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

17
Q

What type of diffusion do dark and light areas represent?

A
  • Dark is isotropic

- Light is anisotropic

18
Q

Name on characteristic of FA

A

-Rotationally invariant

19
Q

Purpose of FA?

A
  • Measures structural integrity and tissue damage

- Healthier matter has more anisotropy

20
Q

What causes FA to decrease?

A

-FA decreases with neuronal damage?

21
Q

What is FA a measure of?

A

-Measure of disease severity in :

  • Cerebrovascular small vessel disease
  • Multiple sclerosis
  • Normal aging
  • Lupus
22
Q

What is white matter damage on DTI imaging correlative with?

A

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

23
Q

How is MD and FA affected as the brain develops?

A
  • MD decreases

- FA increases

24
Q

How is MD and FA affected as the brain ages?

A
  • MD increases

- FA decreases

25
Q

What is observed in Small Vessel disease?

A
  • Leukoaraiosis as a result of white matter degeneration

- This is a result of axonal degeneration, demyelination and gloss

26
Q

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

A
  • Increase in MD

- Decrease in FA

27
Q

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

A

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

28
Q

What does the primary orientation of white matter mean?

A

-Primary direction that white matter grows in

29
Q

What does the primary orientation of white matter indicate?

A

-Principle diffusion direction

30
Q

What is tractography?

A
  • Process by which white matter pathways are tracked

- Connect voxels on the basis of their principle diffusion direction

31
Q

What is a potential Clinical Use Of Tractography?

A

-Presurgical planning for tumour resection

32
Q

What are the limitations of DTI?

A
  • 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
33
Q

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

A
  • Measures diffusion characteristics, it is not a direct measure
  • Structural tissue changes are inferred from diffusion characteristics
34
Q

What is an advantage of DTI?

A

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

Clinical Neuro (6 decks)

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