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P2 PHYSICS > Origin of Image contrast > Flashcards

Origin of Image contrast Flashcards

1
Q

Where is histological slice obtained from?

A

Post-mortem

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2
Q

Where is there good contrast between?

A

Grey matter periphery and white mater centrally

Spaces that would’ve been filled by CSF

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3
Q

What does MRI allow you to look at?

A

Structure and function

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4
Q

What does longitudinal plane involve?

A

Z axis

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5
Q

What is Z axis typically used to denote?

A

Orientation of B0

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6
Q

What is the Transverse plane?

A

x-y plane

Orthogonal to the B0 direction

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7
Q

What plane can you get a measurable MR signal?

A

Transverse plane

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8
Q

How is the transverse magnetisation generated?

A

Manipulating the equilibrium condition

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9
Q

What is the Bloch Equation?

A

A phenomenological equation that was created and developed to describe behaviour that was observed for MRI/MR signal

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10
Q

Magnetisation in a magnetic field

A

There will be some change of magnetisation with time [pattern of behaviour over time]
Related to magnetisation vector and effective field that is applied (e.g. B0 and B1)

cross product

forces that are applied orthogonal to these directions

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11
Q

Longitudinal component

A

Difference to M0 (equilibrium magnetisation) and Mz component

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12
Q

What is T1?

A

Time constant that is describing the temporal behaviour of the magnetisation that is aligned along Z

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13
Q

What is T2 dependent on?

A

How large the transverse component is

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14
Q

What is the decay of transverse magnetisation described by?

A

T2

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15
Q

What is the approach to equilibrium in a static field described by?

A

T1

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16
Q

What is excitation?

A

The generation of Transverse component

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17
Q

What is the Bloch Equation broken down into?

A
  1. Precession

2. Relaxation

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18
Q

What does the rotating frame do?

A

Takes away all of the rotational component and allows us to focus solely on the relaxation parts
[How the magnetisation is recovering or decaying over time]

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19
Q

What are the features of Longitudinal Magnetisation Time Course?

A
  1. Exponential saturation curve
  2. The growth depends on amount present (protons)
  3. The time taken is characterised by a time constant T1
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20
Q

After T1

A

we will have 99% of the magnetisation recovered T=5 T1

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21
Q

What does different tissues have?

A

Different T1 values

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22
Q

What is the amount of recovery that occurs dependent on?

A

Tissue type we are looking at

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23
Q

What happens when different times that allow for different times of recovery?

A

There will be different signal intensity in the image

Generate an image that has different appearance depending on T1 time of the tissue

24
Q

What is a sequence parameter than can be used for different tissues?

A

Repitition time

25
Q

What maximises the differences between different tissue types?

A

TR

26
Q

What is TR?

A

Time between 2 RF excitation pulses

27
Q

Long TR

A

A lot of recovery happening

28
Q

Short TR

A

Little recovery happening

29
Q

Where does most recovery happen?

A

White Matter
It has the shortest TR
Appears bright

30
Q

Where does least recovery happen?

A

CSF
It has a very long TR
appears dark
Little magnetisation

31
Q

Where is there excellent contrast between?

A

Grey and White matter

32
Q

What will TI generate?

A

Transverse magnetisation component

33
Q

What happens in Inversion recovery?

A

Get an image where there is no signal from one tissue

34
Q

Choosing TI that goes to the crossing point for white matter

A

Know the cortical ribbon

35
Q

What is the important timing parameter for Inversion recovery?

A

Inversion time
Dictates what the amplitude of longitudinal magnetisation is and therefore what size transverse magnetisation that can be created

36
Q

When will you get no signal?

A

Choosing a TI that corresponds to 0 crossing of a particular tissue such as white matter
No transverse magnetisation is created

37
Q

What does FLAIR do?

A

Suppress the signal from CSF to make other pathologies visible

38
Q

What are the features of Transverse Magnetisation Time Course?

A
  1. Exponential decay curve
  2. The decay depends on the amount present
  3. The time taken is characterised by a time constant T2
39
Q

What is the relative length of T1 and T2?

A

T1 is longer

Transverse magnetisation must decay faster than the longitudinal magnetisation

40
Q

How can we exploit the differences in T2 times?

A

Sequence parameter used to capitalise on this difference: TE (echo time)

41
Q

What is Echo time?

A

Time after we have generated the transverse magnetisation – we can choose one that gives us large difference between the different tissue type
- Further along we go in time, the lower all of our signals are becoming

42
Q

Why is T2 a bit shorter?

A

Various dephasing effects that lead to a rapid decay of the signal

43
Q

What happens during a 90 degree pulse?

A
  • No longitudinal magnetisation
  • Maximum transverse magnetisation
  • Longitudinal magnetisation is going to recover when transverse magnetisation is going to decay
44
Q

What happens after a particular echo time?

A

We can sample the signal

ADC

45
Q

How can you manipulate the signal further?

A

weight some TR before we apply another excitation pulse

46
Q

What do you choose for T2 weighted imaging?

A

Fixed TE

47
Q

What will fixed TE determine?

A

The amplitude of the signal

48
Q

In T2 weighted Imaging what does CSF have?

A

Long T2

Bright

49
Q

In T2 weighted Imaging what does White matter have?

A

Short T2

Dark

50
Q

What has excellent contrast in T2 weighted Imaging?

A

Tissue and fluid e.g. oedema

51
Q

What happens when there is no magnetic field inhomogeneity?

A

There will be decays of the signal according to T2

52
Q

What does T2’ do?

A

accelerates decay such that 1/T2* is equal to 2 contributions: spin-spin relaxation [natural loss of coherence due to spins interacting with each other] and additional component that makes the decay happen more quickly

53
Q

In our blood what is the relative proportion between deoxygenated and oxygenated haemoglobin depend on ?

A

Neuronal activity

54
Q

Longer T2*

A

Higher signal intensity

55
Q

What leads to BOLD contrast?

A

Brain activation increases oxygen demand and overall results in a higher concentration of oxygenated haemoglobin in the activated area

56
Q

What can different tissues have?

A

Different number of protons per unit volume that results in different net magnetisation

P2 PHYSICS (16 decks)

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