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RAD512 > Fast MRI 1.2.24 > Flashcards

Fast MRI 1.2.24 Flashcards

1
Q

What does Partial k-space sampling cause?

A

Reduced spatial resolution
Low contrast
More artefacts

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2
Q

Why is fast MRI needed?

A

Shorter breath holds
Fewer artefacts
Reduced patient movement
Physiological imaging ( diffusion, perfusion, fMRI and dynamic MRA)
Real time imaging (cardiac imaging)
Scan more patients ( shorter wait times)

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3
Q

What does SMS stand for?

A

Simultaneous multi slice

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4
Q

What 3 things can reduce scan times?

A

Pulse sequences allowing shorter TR
K-space strategies - reduce data lines
Methods which reduce SNR

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5
Q

What is SNR?

A

Signal to noise ratio

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6
Q

What happens When the TR is reduced?

A

Tissues with long T1s don’t fully recover
The images become more T1 weighted
Compromised image quality

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7
Q

What does a Larger flip angle yield?

A

Larger MR signals

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8
Q

What does a Small pulse angle cause within tissues?

A

Tissues re-magnetise more rapidly

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9
Q

What is SENSE?
How does it acquire images?

A

Sensitivity Encoding
(Image space, unravel the aliased images )
Parallel imaging method

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10
Q

What is GRAPPA?
How does it acquire images?

A

Generalised Auto-calibrating
Partial parallel acquisition
(K-space - generates the missing lines of k-space)

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11
Q

What is shimming?

A

The process of making the main magnetic field more homogeneous

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12
Q

What does FLASH stand for?

A

Fast low angle shot

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13
Q

What are the 3 K space strategies?

A

Multiple Phase encoding
Partial k-space
Spiral imaging

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14
Q

What methods generate multiple echo’s?

A

Multiple spin echo’s (FSE)
Multiple gradient echos (EPI)

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15
Q

What does EPI stand for?

A

Echo- planar imaging

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16
Q

What does FSE stand for?

A

Fast spin echo

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17
Q

What are the Benefits of FSE?

A

Short imaging time
Allows breath hold imaging
Susceptibility decreases

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18
Q

What are some of the Consequences of FSE?

A

Lower SNR
Hight SAR ( heating)
Long ETL’s alter the contrast

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19
Q

What does ETL stand for?

A

Echo train length / turbo factor

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20
Q

What does SAR stand for?

A

specific absorption rate

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21
Q

What is EPI?

A
  • Single shot sequence (single RF excitation)
  • Scan time 30-100 ms/slice
  • Low RF deposition
  • Signal losses from reduced TR eliminated
  • Highly sensitive to susceptibility
  • Highly demanding on hardware
    - Very fast gradient switching
    - B0 Homogeneity
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22
Q

What is the Danger of fast gradient switching ?

A

Peripheral nerve stimulation

23
Q

What is Half -Fourier ?

A

When we Measure half of the matrix and estimate the other half

24
Q

Why do we use spiral imaging ?

A

More efficient k-space for fast MRI

Better immunity to flow artefacts ( no gradient at the centre of k-space)

Allows more room for magnetisation preparation such as diffusion weighting

25
Q

What is Spin echo’s sensitivity ?

A

T1 or T2
Based of TR, TE

26
Q

What are the advantages of Spin echo?

A

High SNR
High spatial resolution

27
Q

What are the disadvantages of Spin echo ?

A

Interval for 90-180° pulses
Delay to return Mz

28
Q

What is Gradient echo’s sensitivity ?

A

T2, T2* , chemical shift in FE direction
T1 in PE direction

29
Q

What are the advantages of Gradient echo ?

A

Sensitive for flow/blood. (High intensity signal)

30
Q

What are the disadvantages of Gradient echo?

A

Sensitive to in- homogeneities

31
Q

What are FSE (RARE) sensitivities?

A

Fast for T2 or PD weighted acquisition

32
Q

What are the advantages of FSE (RARE) ?

A

SNR comparable to SE
Rapid acquisition

33
Q

What are the disadvantage of FSE (RARE) ?

A

T2 effects along PE direction.
Insensitive to susceptibility.
High RF power deposition.

34
Q

What are FLASH (GE) sensitivities?

A

AS GE
Increased T1 weighting as flip angle decreases

35
Q

What are the advantages of FLASH (GE) ?

A

High spatial resolution
Low RF power deposition
Rapid acquisition

36
Q

What are the disadvantages of FLASH (GE)?

A

Very low SNR

37
Q

What are EPI-SE sensitivities?

A

Off centre 180° gives T2 weighted
Diffusion weighted

38
Q

What are the advantages of EPI -SE?

A

Rapid acquisition

39
Q

What are the disadvantages of EPI-SE?

A

Lower SNR
Poor spatial resolution
special gradient coils

40
Q

What are the advantages of EPI -GE?

A

Rapid acquisition

41
Q

What are the disadvantage of EPI-GE?

A

As EPI –S E
Chemical shift artefact
Sensitive to in-homogeneity

42
Q

What one Spiral scan sensitivities?

A

T2* weighting
Flow compensated

43
Q

What is the advantage Spiral scanning?

A

Rapid acquisition

44
Q

What is the Disadvantage Spiral scanning?

A

Sensitive to in-homogeneity

45
Q

What is Modified SENSE ?

A

Additional lines are Acquired at the centre of the K-space for each coil during the scan.
No separate calibration scan.

46
Q

What does stand for PI ?

A

Parallel imaging

47
Q

What are coil arrangements used for in PI?

A

1) signal reception
2) signal encoding

48
Q

What do phased array coils require for PI?

A

Multiple receiver channels
= amplifiers, AD converters, computer memory

Significant post processing to combine the images

49
Q

What are the Advantages of Phased array coils ?

A
  • good SNR
  • Large FOV
50
Q

What are the Disadvantages of Phased array coils ?

A
  • coil coupling - “magic separation” geometry
  • Multiple receiver channels ($)
51
Q

What do better gradients give?

A

Reduced TR

52
Q

What is the Slew rate?

A

It is the speed at which the gradients reach the maximum amplitude and is given by = G max / Rise time

53
Q

How to half the acquisition time?

A

The gradients slew rate is quadrupled (x4)

54
Q

What are the applications of Phased array (PA) coils?

A

Cardiovascular Imaging
- breath-hold
- realtime

Brain Imaging
- fMRI
- Single-shot EPI diffusion

RAD512 (26 decks)

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