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P2 PHYSICS > Image optimisation > Flashcards

Image optimisation Flashcards

1
Q

When is an MRI image optimal?

A
  1. High signal
  2. Low noise
  3. High tissue contrast
  4. Good spatial resolution
  5. Good temporal resolution
  6. Short imaging time
  7. No (or few) artefacts
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2
Q

What are the basic image parameters?

A
  1. Sequence type:
    - 2D
    - 3D
    - TR
    - TE
    - Flip angle, TI
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3
Q

What is the flip angle?

A

The amount of rotation that the net magnetisation experiences during application of RF pulse

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4
Q

What is the resultant flip angle approximately proportional to?

A

Frequency of the B1 field

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5
Q

What are other image parameters?

A
  1. Number of averages, FOV, matrix size, bandwidth

2. Number of shots, slice thickness, echo train length

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6
Q

What is a measure of true signal to noise?

A

signal to noise ratio

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7
Q

What does a lower signal-to-noise ratio generally result in?

A

Grainy appearance to images

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8
Q

What is the equation of SNR?

A

s/n

signal level/ noise level

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9
Q

What is a measure used to determine image quality?

A

CNR

Contrast to noise ratio

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10
Q

What are the 2 equations for CNR?

A
  1. SA-SB/standard deviation

2. Stissue1-Stissue2/n

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11
Q

What are MR images?

A

Complex valued (real and imaginary channels)

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12
Q

What is phase useful in?

A

Susceptibility mapping

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13
Q

What is Quantiative susceptibility mapping (QSM)?

A

Absolute concentration of iron, calcium and other subtanced may be measured in tissue based on changes in local susceptibility

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14
Q

What do we refer to for noise?

A

Thermal noise

i.e. stochastic thermal fluctuation of voltage induced in the receiver coils

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15
Q

What does the thermal fluctuations of voltage include?

A

Contributions from:

  1. Imaged object
  2. Coil
  3. Electronics
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16
Q

What is noise in each image channel?

A
  1. zero-mean

2. Guassian-distributed

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17
Q

What do magnitude images have?

A

More complex noise distributions such as Rician distributions

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18
Q

What are Rician distributions?

A

Models the path the scattered signals take to receiver

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19
Q

What is the SNR calculations?

A
  • S: use mean signal in a homogenous region (e.g. within white matter)
  • N: use mean signal in an area with air
  • N: use standard deviation of signal in an area with air
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20
Q

What is noise-only acquisition calculation

A
  • S: use mean signal in a homogenous region (e.g. within white matter)
  • N: use mean signal from noise-only acquisition (e.g. within same white matter region)
  • S: use mean signal in a homogenous region-of-interest (e.g. within white matter)
  • N: use standard deviation of signal from noise-only acquisition (e.g. within same white matter region)
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21
Q

What is SNR calculation: method 5?

A
  • S: use mean signal over the acquisition in each voxel

* N: use standard deviation of signal over acquisition in each voxel

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22
Q

Why should we watch out for spatially-variant noise?

A

The noise level can vary across the image

- Watch out for image ghosts in ‘‘air’’ areas

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23
Q

Why is the acquisition of multiple images be required as part of a quantitative study?

A

These images can then be exploited to estimate SNR

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24
Q

What is time-consuming?

A

Acquistion of noise-only images

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25
Q

What does Contrast,C, measure?

A

Differences between the intensity of signal from two different tissues
- Differences is usually normalised

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26
Q

How can the contrast be changed?

A

Varying TR, TE, FA

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27
Q

How can the noise level,n, be estimated?

A

Studying areas with no tissue (i.e. air)

28
Q

What does CNR inform?

A

How relevant the contrast is with respect to random variations due to noise

-Aim: CNR > 1

29
Q

How is the SNR/CNR altered?

A
  1. Voxel size
  2. Number of averages (NEX)
  3. Number of frequencies encode steps
  4. Number of phase encode steps
  5. Bandwidth
30
Q

How can the total scan time be altered?

A

Changing parameters such as number of slices, number of shots, echo train length, acceleration factors

31
Q

How can the resolution be altered?

A

The volume of the voxel

V= ∆x ∆y ∆z

32
Q

What is bandwidth?

A

The inverse of dwell time

33
Q

What is the dwell time?

A

Sampling time along the frequency encoding direction

i.e. time separating two points when filling k-space

34
Q

What are examples of acceleration of acquistion?

A
  1. Partial Fourier Imaging
  2. Parallel Imaging
  3. Simultaneous multi-slice imaging
35
Q

What is Partial Fourier Imaging?

A
  1. Only a fraction of k-space is sampled

2. It relied on symmetry properties of Fourier Transform

36
Q

What is parallel imaging?

A
  1. The K space is sampled below the Nyquist limit
37
Q

What is simultaneous multi-slice imaging?

A
  1. Two or more slices are excited together using specialised RF pulses
  2. The sensitivity of multiple coils is exploited to unfold slices
38
Q

What implies trading off?

A

Designing an MRI sequence for practical use

39
Q

What are the steps for trading off?

A
  1. Select contrast required for application
  2. Set up FOV and resolution
  3. Adjust other sequence parameters taking time into account
40
Q

What is an artefact?

A

Feature that appears in MRI image but that is not present in image’s tissues

41
Q

What is imaging artefact essential to?

A

Acquisition technique and how it interacts with underlying physiological processes

42
Q

What may artefacts be misinterpreted for?

A

pathology

43
Q

What are artefacts?

A

often systematic and in a way reproducible

44
Q

What differs from thermal noise?

A

Imaging artefact

45
Q

How to minimise motion artefact?

A

Use of fast imaging sequences

46
Q

What happens when you move during an acquistion?

A
  • Tissues moves from one place to another – has the ‘wrong’ frequency and phase
  • This can lead to signal losses (imperfect rephasing) or signal increases
47
Q

What is physiological noise?

A

• Related to motion due to normal physiological processes
- Pulsation of cerebrospinal fluid
- Respiration
• Lead to signal variability over time – have physiological origin
• Can be mitigated by synchronizing the acquisition with cardiac or respiratory cycle (gating)
- Variable TR (i.e. variable T1-weighting)
- The acquisition is usually less efficient and can last longer

48
Q

What is spike noise?

A
  • Artefact caused by loose cable in gradient system
  • Causes noise spike in k-space leading to oscillatory patterns in image space
  • Call engineer
49
Q

What is zipper artefact?

A
  • Artefact caused by leakage of RF radiation at a specific frequency
  • Can be internal – scanner fault
  • Or external – leakage through shielding
50
Q

What are tissues characterised by?

A

Different magnetic susceptibility

51
Q

What does magnetic susceptibility describe?

A

How a tissue interacts with the applied field

52
Q

Where do strong gradients arise?

A

At boundaries of tissues with different magnetic susceptibility

  • Rapid dephasing of spins - signal loss
  • This changes the effective gradient - distortion, signal pile up/drop out
53
Q

What is highly prone to susceptibility artefact?

A

Echo planar Imaging

54
Q

What are magnetic susceptibility artefacct?

A

Refer to a variety of MRI artifacts that share distortions or local signal change due to local magnetic field inhomogeneities from a variety of compounds

55
Q

How to mitigate susceptibility artefact?

A

• Reduced EPI echo train length
• Use multiple EPI shots
• Correct image after acquisition with a B0 field map
- Estimated studying the phase at different echo times
- Estimated from 2 magnitude images with reversed phase encode direction

56
Q

What are Eddy Current Artefact?

A

Current loops induced in the metallic part of the scanner by fast switching gradients

57
Q

What do Eddy Current Artefact cause?

A

Additional, undesired magnetic field that lead to distortion or imperfect rephasing

58
Q

Where is Eddy Current Artefact an issue in?

A

Diffusion MRI

Mitigated with post-processing techniques

59
Q

How to mitigate chemical shift artefact?

A
  1. Increase the bandwidth

2. Use dedicated fat suppression techniqye

60
Q

What are the Wrap Around Artefact?

A

When the object producing MR signal is larger than the FOV or when the FOV is misplaced

61
Q

What are the Gibbs Ringing or Truncation Artefacts?

A
  1. Commonly seen artefact
  2. Bright and dark bands at high contrast boundaries
  3. Caused by finite k-space sampling
62
Q

How is the Gibs Ringing or Truncation Artefact reduced?

A

Increasing the matrix size or with post-processing techniques

  • De-ringing filter
  • K-space extrapolation
63
Q

What are the other artefacts that exist?

A
  1. Gradient non-linearity

2. Phase-cancellation

64
Q

What is gradient non-linearity?

A

When the gradients are non-linear

Additional image distortions are seen

65
Q

What are phase-cancellation?

A

When the fat and water signal are 180 degree out of phase,

they cancel each out leading to signal loss

P2 PHYSICS (16 decks)

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