Image optimisation Flashcards
When is an MRI image optimal?
- High signal
- Low noise
- High tissue contrast
- Good spatial resolution
- Good temporal resolution
- Short imaging time
- No (or few) artefacts
What are the basic image parameters?
- Sequence type:
- 2D
- 3D
- TR
- TE
- Flip angle, TI
What is the flip angle?
The amount of rotation that the net magnetisation experiences during application of RF pulse
What is the resultant flip angle approximately proportional to?
Frequency of the B1 field
What are other image parameters?
- Number of averages, FOV, matrix size, bandwidth
2. Number of shots, slice thickness, echo train length
What is a measure of true signal to noise?
signal to noise ratio
What does a lower signal-to-noise ratio generally result in?
Grainy appearance to images
What is the equation of SNR?
s/n
signal level/ noise level
What is a measure used to determine image quality?
CNR
Contrast to noise ratio
What are the 2 equations for CNR?
- SA-SB/standard deviation
2. Stissue1-Stissue2/n
What are MR images?
Complex valued (real and imaginary channels)
What is phase useful in?
Susceptibility mapping
What is Quantiative susceptibility mapping (QSM)?
Absolute concentration of iron, calcium and other subtanced may be measured in tissue based on changes in local susceptibility
What do we refer to for noise?
Thermal noise
i.e. stochastic thermal fluctuation of voltage induced in the receiver coils
What does the thermal fluctuations of voltage include?
Contributions from:
- Imaged object
- Coil
- Electronics
What is noise in each image channel?
- zero-mean
2. Guassian-distributed
What do magnitude images have?
More complex noise distributions such as Rician distributions
What are Rician distributions?
Models the path the scattered signals take to receiver
What is the SNR calculations?
- 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
What is noise-only acquisition calculation
- 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)
What is SNR calculation: method 5?
- S: use mean signal over the acquisition in each voxel
* N: use standard deviation of signal over acquisition in each voxel
Why should we watch out for spatially-variant noise?
The noise level can vary across the image
- Watch out for image ghosts in ‘‘air’’ areas
Why is the acquisition of multiple images be required as part of a quantitative study?
These images can then be exploited to estimate SNR
What is time-consuming?
Acquistion of noise-only images
What does Contrast,C, measure?
Differences between the intensity of signal from two different tissues
- Differences is usually normalised
How can the contrast be changed?
Varying TR, TE, FA
How can the noise level,n, be estimated?
Studying areas with no tissue (i.e. air)
What does CNR inform?
How relevant the contrast is with respect to random variations due to noise
-Aim: CNR > 1
How is the SNR/CNR altered?
- Voxel size
- Number of averages (NEX)
- Number of frequencies encode steps
- Number of phase encode steps
- Bandwidth
How can the total scan time be altered?
Changing parameters such as number of slices, number of shots, echo train length, acceleration factors
How can the resolution be altered?
The volume of the voxel
V= ∆x ∆y ∆z
What is bandwidth?
The inverse of dwell time
What is the dwell time?
Sampling time along the frequency encoding direction
i.e. time separating two points when filling k-space
What are examples of acceleration of acquistion?
- Partial Fourier Imaging
- Parallel Imaging
- Simultaneous multi-slice imaging
What is Partial Fourier Imaging?
- Only a fraction of k-space is sampled
2. It relied on symmetry properties of Fourier Transform
What is parallel imaging?
- The K space is sampled below the Nyquist limit
What is simultaneous multi-slice imaging?
- Two or more slices are excited together using specialised RF pulses
- The sensitivity of multiple coils is exploited to unfold slices
What implies trading off?
Designing an MRI sequence for practical use
What are the steps for trading off?
- Select contrast required for application
- Set up FOV and resolution
- Adjust other sequence parameters taking time into account
What is an artefact?
Feature that appears in MRI image but that is not present in image’s tissues
What is imaging artefact essential to?
Acquisition technique and how it interacts with underlying physiological processes
What may artefacts be misinterpreted for?
pathology
What are artefacts?
often systematic and in a way reproducible
What differs from thermal noise?
Imaging artefact
How to minimise motion artefact?
Use of fast imaging sequences
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
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
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
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
What are tissues characterised by?
Different magnetic susceptibility
What does magnetic susceptibility describe?
How a tissue interacts with the applied field
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
What is highly prone to susceptibility artefact?
Echo planar Imaging
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
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
What are Eddy Current Artefact?
Current loops induced in the metallic part of the scanner by fast switching gradients
What do Eddy Current Artefact cause?
Additional, undesired magnetic field that lead to distortion or imperfect rephasing
Where is Eddy Current Artefact an issue in?
Diffusion MRI
Mitigated with post-processing techniques
How to mitigate chemical shift artefact?
- Increase the bandwidth
2. Use dedicated fat suppression techniqye
What are the Wrap Around Artefact?
When the object producing MR signal is larger than the FOV or when the FOV is misplaced
What are the Gibbs Ringing or Truncation Artefacts?
- Commonly seen artefact
- Bright and dark bands at high contrast boundaries
- Caused by finite k-space sampling
How is the Gibs Ringing or Truncation Artefact reduced?
Increasing the matrix size or with post-processing techniques
- De-ringing filter
- K-space extrapolation
What are the other artefacts that exist?
- Gradient non-linearity
2. Phase-cancellation
What is gradient non-linearity?
When the gradients are non-linear
Additional image distortions are seen
What are phase-cancellation?
When the fat and water signal are 180 degree out of phase,
they cancel each out leading to signal loss
