Fast MRI 1.2.24 Flashcards
What does Partial k-space sampling cause?
Reduced spatial resolution
Low contrast
More artefacts
Why is fast MRI needed?
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)
What does SMS stand for?
Simultaneous multi slice
What 3 things can reduce scan times?
Pulse sequences allowing shorter TR
K-space strategies - reduce data lines
Methods which reduce SNR
What is SNR?
Signal to noise ratio
What happens When the TR is reduced?
Tissues with long T1s don’t fully recover
The images become more T1 weighted
Compromised image quality
What does a Larger flip angle yield?
Larger MR signals
What does a Small pulse angle cause within tissues?
Tissues re-magnetise more rapidly
What is SENSE?
How does it acquire images?
Sensitivity Encoding
(Image space, unravel the aliased images )
Parallel imaging method
What is GRAPPA?
How does it acquire images?
Generalised Auto-calibrating
Partial parallel acquisition
(K-space - generates the missing lines of k-space)
What is shimming?
The process of making the main magnetic field more homogeneous
What does FLASH stand for?
Fast low angle shot
What are the 3 K space strategies?
Multiple Phase encoding
Partial k-space
Spiral imaging
What methods generate multiple echo’s?
Multiple spin echo’s (FSE)
Multiple gradient echos (EPI)
What does EPI stand for?
Echo- planar imaging
What does FSE stand for?
Fast spin echo
What are the Benefits of FSE?
Short imaging time
Allows breath hold imaging
Susceptibility decreases
What are some of the Consequences of FSE?
Lower SNR
Hight SAR ( heating)
Long ETL’s alter the contrast
What does ETL stand for?
Echo train length / turbo factor
What does SAR stand for?
specific absorption rate
What is EPI?
- 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
What is the Danger of fast gradient switching ?
Peripheral nerve stimulation
What is Half -Fourier ?
When we Measure half of the matrix and estimate the other half
Why do we use spiral imaging ?
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
What is Spin echo’s sensitivity ?
T1 or T2
Based of TR, TE
What are the advantages of Spin echo?
High SNR
High spatial resolution
What are the disadvantages of Spin echo ?
Interval for 90-180° pulses
Delay to return Mz
What is Gradient echo’s sensitivity ?
T2, T2* , chemical shift in FE direction
T1 in PE direction
What are the advantages of Gradient echo ?
Sensitive for flow/blood. (High intensity signal)
What are the disadvantages of Gradient echo?
Sensitive to in- homogeneities
What are FSE (RARE) sensitivities?
Fast for T2 or PD weighted acquisition
What are the advantages of FSE (RARE) ?
SNR comparable to SE
Rapid acquisition
What are the disadvantage of FSE (RARE) ?
T2 effects along PE direction.
Insensitive to susceptibility.
High RF power deposition.
What are FLASH (GE) sensitivities?
AS GE
Increased T1 weighting as flip angle decreases
What are the advantages of FLASH (GE) ?
High spatial resolution
Low RF power deposition
Rapid acquisition
What are the disadvantages of FLASH (GE)?
Very low SNR
What are EPI-SE sensitivities?
Off centre 180° gives T2 weighted
Diffusion weighted
What are the advantages of EPI -SE?
Rapid acquisition
What are the disadvantages of EPI-SE?
Lower SNR
Poor spatial resolution
special gradient coils
What are the advantages of EPI -GE?
Rapid acquisition
What are the disadvantage of EPI-GE?
As EPI –S E
Chemical shift artefact
Sensitive to in-homogeneity
What one Spiral scan sensitivities?
T2* weighting
Flow compensated
What is the advantage Spiral scanning?
Rapid acquisition
What is the Disadvantage Spiral scanning?
Sensitive to in-homogeneity
What is Modified SENSE ?
Additional lines are Acquired at the centre of the K-space for each coil during the scan.
No separate calibration scan.
What does stand for PI ?
Parallel imaging
What are coil arrangements used for in PI?
1) signal reception
2) signal encoding
What do phased array coils require for PI?
Multiple receiver channels
= amplifiers, AD converters, computer memory
Significant post processing to combine the images
What are the Advantages of Phased array coils ?
- good SNR
- Large FOV
What are the Disadvantages of Phased array coils ?
- coil coupling - “magic separation” geometry
- Multiple receiver channels ($)
What do better gradients give?
Reduced TR
What is the Slew rate?
It is the speed at which the gradients reach the maximum amplitude and is given by = G max / Rise time
How to half the acquisition time?
The gradients slew rate is quadrupled (x4)
What are the applications of Phased array (PA) coils?
Cardiovascular Imaging
- breath-hold
- realtime
Brain Imaging
- fMRI
- Single-shot EPI diffusion
