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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
What does Contrast,C, measure?
Differences between the intensity of signal from two different tissues - Differences is usually normalised
26
How can the contrast be changed?
Varying TR, TE, FA
27
How can the noise level,n, be estimated?
Studying areas with no tissue (i.e. air)
28
What does CNR inform?
How relevant the contrast is with respect to random variations due to noise -Aim: CNR > 1
29
How is the SNR/CNR altered?
1. Voxel size 2. Number of averages (NEX) 3. Number of frequencies encode steps 4. Number of phase encode steps 5. Bandwidth
30
How can the total scan time be altered?
Changing parameters such as number of slices, number of shots, echo train length, acceleration factors
31
How can the resolution be altered?
The volume of the voxel | V= ∆x ∆y ∆z
32
What is bandwidth?
The inverse of dwell time
33
What is the dwell time?
Sampling time along the frequency encoding direction | i.e. time separating two points when filling k-space
34
What are examples of acceleration of acquistion?
1. Partial Fourier Imaging 2. Parallel Imaging 3. Simultaneous multi-slice imaging
35
What is Partial Fourier Imaging?
1. Only a fraction of k-space is sampled | 2. It relied on symmetry properties of Fourier Transform
36
What is parallel imaging?
1. The K space is sampled below the Nyquist limit
37
What is simultaneous multi-slice imaging?
1. Two or more slices are excited together using specialised RF pulses 2. The sensitivity of multiple coils is exploited to unfold slices
38
What implies trading off?
Designing an MRI sequence for practical use
39
What are the steps for trading off?
1. Select contrast required for application 2. Set up FOV and resolution 3. Adjust other sequence parameters taking time into account
40
What is an artefact?
Feature that appears in MRI image but that is not present in image's tissues
41
What is imaging artefact essential to?
Acquisition technique and how it interacts with underlying physiological processes
42
What may artefacts be misinterpreted for?
pathology
43
What are artefacts?
often systematic and in a way reproducible
44
What differs from thermal noise?
Imaging artefact
45
How to minimise motion artefact?
Use of fast imaging sequences
46
What happens when you move during an acquistion?
- 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
What is physiological noise?
• 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
What is spike noise?
* Artefact caused by loose cable in gradient system * Causes noise spike in k-space leading to oscillatory patterns in image space * Call engineer
49
What is zipper artefact?
* Artefact caused by leakage of RF radiation at a specific frequency * Can be internal – scanner fault * Or external – leakage through shielding
50
What are tissues characterised by?
Different magnetic susceptibility
51
What does magnetic susceptibility describe?
How a tissue interacts with the applied field
52
Where do strong gradients arise?
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
What is highly prone to susceptibility artefact?
Echo planar Imaging
54
What are magnetic susceptibility artefacct?
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
How to mitigate susceptibility artefact?
• 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
What are Eddy Current Artefact?
Current loops induced in the metallic part of the scanner by fast switching gradients
57
What do Eddy Current Artefact cause?
Additional, undesired magnetic field that lead to distortion or imperfect rephasing
58
Where is Eddy Current Artefact an issue in?
Diffusion MRI | Mitigated with post-processing techniques
59
How to mitigate chemical shift artefact?
1. Increase the bandwidth | 2. Use dedicated fat suppression techniqye
60
What are the Wrap Around Artefact?
When the object producing MR signal is larger than the FOV or when the FOV is misplaced
61
What are the Gibbs Ringing or Truncation Artefacts?
1. Commonly seen artefact 2. Bright and dark bands at high contrast boundaries 3. Caused by finite k-space sampling
62
How is the Gibs Ringing or Truncation Artefact reduced?
Increasing the matrix size or with post-processing techniques - De-ringing filter - K-space extrapolation
63
What are the other artefacts that exist?
1. Gradient non-linearity | 2. Phase-cancellation
64
What is gradient non-linearity?
When the gradients are non-linear | Additional image distortions are seen
65
What are phase-cancellation?
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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