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P2 PHYSICS > Image formation 2: Basic Pulse Sequence > Flashcards

Image formation 2: Basic Pulse Sequence Flashcards

1
Q

What is it difficult to reconstruct?

A

Artefact free images from projections

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2
Q

What is technically difficult to measure?

A

MR signal immediately after a 90 degree pulse

- can be overcome by collecting a spine echo (SE)

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3
Q

What is pulse sequence?

A

Succession of commands that the computer gives the scanner to obtain desired signal, hence image

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4
Q

What is spin echo?

A

Regeneration of spin phase information apparently lost during decay of FID
– Insensitive to magnetic field B0 inhomogeneity, but longer scan times

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5
Q

What is Gradient echo?

A
  1. Applying external dephasing gradient field across the specimen or tissue
    - calibrated change in local magnetic field and hence alter resonance frequencies slightly across the specimen
    - Accelerated dephasing of FID
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6
Q

What is the first step in the formation of a spin echo?

A

90˚ pulse is applied along +x’ which rotates the longitudinal magnetization MS into the transverse plane along y’ axis

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7
Q

What is the second step in the formation of a spin echo?

A

180˚ RF pulse is applied again along the postive x’ axis

-This reverses the direction of the magnetisation so now it is along the -y’ axis

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8
Q

What is the third step in the formation of a spin echo?

A

The spins continue to move in the same direction as they were travelling before the 180˚ pulse

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9
Q

What is the fourth step in the formation of a spin echo?

A

There is a time point (the ‘spin echo’) where the isochromats rephase and the magnetisation of the entire spin ensemble reaches a maximum again

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10
Q

What is the fifth step in the formation of a spin echo?

A

The centre of the spin echo (where the signal will reach a maximum) occurs at a time referred to as the “echo time” or TE of the sequence – The sampling or acquisition window will be centred around this time point • Finally the isochromats begin to dephase again

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11
Q

What happens after the spin . echo has formed?

A

The magnetisation will continue to precess and therefore dephase

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12
Q

What is the basis of spin echo Imaging?

A

Combining the effect of a field gradient (to localise the signal) with the concept of refocusing transverse magnetisation

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13
Q

What is Spin Echo?

A

Add another pulse to generate an ‘‘echo’’ that occurs later in time and happens to reverse the T2* effects to leave us with T2 only

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14
Q

Why do we rotate the protons with a 180-degree pulse?

A

They continue precessing in the same direction but will refocus along the -y axis

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15
Q

What is the spin dephasing effect due to?

A

Dispersion in the resonance frequency produced

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16
Q

What is the rephase lobe on the gradient needed?

A

Counteract the dephasing of spins during the application of pulse

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17
Q

What is the k-space sampling?

A
  1. Apply a 90 degree RF pulse (produces an FID)
  2. Apply large, negative phase encode gradient along y direction
  3. Apply a positive dephase gradient along the x axis
  4. At a time TE/2 after the 90 degree pulse w eapply a 180 degree pulse
  5. Apply a readout gradient and start measuring the echo signal
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18
Q

What will produce different degrees of contrast between tissues?

A

Different combinations of TR and TE

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19
Q

What will changing TR change the contrast between?

A

Tissues with different T1 relaxation times

20
Q

What will changing TE change the contrast between?

A

Tissues with different T2 values

21
Q

What does T1 weighting have?

A
  1. Short TE
  2. Short TR
    - PD weighting
22
Q

What does T2 have?

A
  1. Long TE
  2. long TR
    - PD weighting aswell
23
Q

What generally produces no signal?

A
  1. Long TE

2. Short TR

24
Q

What does fluids have?

A
  1. Long T1

2. Long T2

25
Q

What does water-vased tissues have?

A
  1. Mid-range T1

2. Short T2

26
Q

What does Fat-based tissue have?

A
  1. Short T1

2. Short T2

27
Q

What happens in PD-weighted images?

A

Tissues with highest PD are brighter

28
Q

What happens in T2-weighted images?

A

Tissues with longest T2 are brighter

-Takes longer for transverse magnetisation to decay

29
Q

What happens in T1-weighted imageS?

A

Tissues with longest T1 are darker

-Takes longer for longitudinal magnetisation to recover

30
Q

What are the spin echo tissue signal intensity?

A 
•	Short TE
-	Grey matter is white 
-	White matter is darker
-	CSF bright 
-	The contrast is from PD
•	Longer TE
-	GM still white
-	WM is still darker
-	Contrast greater than short TE 
-	CSF is very bright 
-	The contrast is from PD and T2

• A very long TE

  • WM and GM are black
  • Only CSF remains
31
Q

What is Dual echo sequence?

A

Include images with different weightings and/or echo times and are used to obtain both proton density and T2 weighted images without increasing the measurement time

32
Q

What do different compartment in tissues have?

A

Different relaxation characteristics

33
Q

What can be detected by fitting the transverse magnetisation decay curve (TMDC)?

A

Multi-exponential relaxation

34
Q

How is a gradient echo formed?

A

First dephasing and subsequently rephasing the FID so that all isochromat magnetisation vectors are back in phase with one another at the centre of gradient echo

35
Q

Where does the gradient echo occur at?

A

the point in time where the net phase is zero, i.e. the amount of dephasing is equal to the amount of rephasing – Signal reaches a maximum at the gradient echo

36
Q

What is gradient echo signal?

A

T2* dependent

There is no 180 degree pulse to refocus the dephasing of isochromat due to B0 field inhomogeneities

37
Q

What is the first step in the formation of a gradient echo?

A

90° pulse is applied and isochromats are tipped into the transverse plane

38
Q

What is the second step in the formation of a gradient echo?

A

Dephasing gradient is applied and the isochromat dephase as a result of this grafient

39
Q

What is the third step in the formation of a gradient echo?

A

Equal and opposite gradient is applied (a rephasing gradient)
-isochromat phases are reversed and a gradient echo is formed at the point in time where the amounts of rephasing and dephasing by gradients are exactly equal

40
Q

What does T2* describe?

A

Total transverse relaxation due to both spin-spin interaction and B0 field inhomogeneities

41
Q

What does magnet homogeneity directly relate to?

A

Image quality and various artefacts

42
Q

What are the types of problem associated with poor magnet homogeneity?

A
  • Shading
  • Spatial distortion
  • Blurring
  • Intensity loss
  • Curved slice profiles
  • Zebra banding artefacts
43
Q

What is inversion recovery pulse sequence?

A

spin echo pulse sequence preceded by a 180° RF pulse

44
Q

What does the Inversion recovery pulse sequence do?

A

inverts longitudinal magnetisation (Mz), namely, it flips Mz to its negative value, -Mz.Tissues regain Mz at different longitudinal (T1) relaxation rates determined by their T1 relaxation times

45
Q

What are the Inversion recovery contrast?

A
  1. Short T1
    - GM is white
    - WM is darker
    - contrast: T1 and PD
  2. Longer T1
    - WM is white
    - GM is darker
  3. Longer still T1
    - WM and GM are iso-intense
  4. Very long T1
    - GM is white
    - WM darker
    - contrast: mainly due to PD

P2 PHYSICS (16 decks)

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