Fast MRI 1.2.24

Question 1

What does Partial k-space sampling cause?

Answer 1

Reduced spatial resolution
Low contrast
More artefacts

Question 2

Why is fast MRI needed?

Answer 2

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)

Question 3

What does SMS stand for?

Answer 3

Simultaneous multi slice

Question 4

What 3 things can reduce scan times?

Answer 4

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

Question 5

What is SNR?

Answer 5

Signal to noise ratio

Question 6

What happens When the TR is reduced?

Answer 6

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

Question 7

What does a Larger flip angle yield?

Answer 7

Larger MR signals

Question 8

What does a Small pulse angle cause within tissues?

Answer 8

Tissues re-magnetise more rapidly

Question 9

What is SENSE?
How does it acquire images?

Answer 9

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

Question 10

What is GRAPPA?
How does it acquire images?

Answer 10

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

Question 11

What is shimming?

Answer 11

The process of making the main magnetic field more homogeneous

Question 12

What does FLASH stand for?

Answer 12

Fast low angle shot

Question 13

What are the 3 K space strategies?

Answer 13

Multiple Phase encoding
Partial k-space
Spiral imaging

Question 14

What methods generate multiple echo’s?

Answer 14

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

Question 15

What does EPI stand for?

Answer 15

Echo- planar imaging

Question 16

What does FSE stand for?

Answer 16

Fast spin echo

Question 17

What are the Benefits of FSE?

Answer 17

Short imaging time
Allows breath hold imaging
Susceptibility decreases

Question 18

What are some of the Consequences of FSE?

Answer 18

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

Question 19

What does ETL stand for?

Answer 19

Echo train length / turbo factor

Question 20

What does SAR stand for?

Answer 20

specific absorption rate

Question 21

What is EPI?

Answer 21

• 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

Question 22

What is the Danger of fast gradient switching ?

Answer 22

Peripheral nerve stimulation

Question 23

What is Half -Fourier ?

Answer 23

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

Question 24

Why do we use spiral imaging ?

Answer 24

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

Question 25

What is Spin echo’s sensitivity ?

Answer 25

T1 or T2 Based of TR, TE

Question 26

What are the advantages of Spin echo?

Answer 26

High SNR High spatial resolution

Question 27

What are the disadvantages of Spin echo ?

Answer 27

Interval for 90-180° pulses Delay to return Mz

Question 28

What is Gradient echo’s sensitivity ?

Answer 28

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

Question 29

What are the advantages of Gradient echo ?

Answer 29

Sensitive for flow/blood. (High intensity signal)

Question 30

What are the disadvantages of Gradient echo?

Answer 30

Sensitive to in- homogeneities

Question 31

What are FSE (RARE) sensitivities?

Answer 31

Fast for T2 or PD weighted acquisition

Question 32

What are the advantages of FSE (RARE) ?

Answer 32

SNR comparable to SE Rapid acquisition

Question 33

What are the disadvantage of FSE (RARE) ?

Answer 33

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

Question 34

What are FLASH (GE) sensitivities?

Answer 34

AS GE Increased T1 weighting as flip angle decreases

Question 35

What are the advantages of FLASH (GE) ?

Answer 35

High spatial resolution Low RF power deposition Rapid acquisition

Question 36

What are the disadvantages of FLASH (GE)?

Answer 36

Very low SNR

Question 37

What are EPI-SE sensitivities?

Answer 37

Off centre 180° gives T2 weighted Diffusion weighted

Question 38

What are the advantages of EPI -SE?

Answer 38

Rapid acquisition

Question 39

What are the disadvantages of EPI-SE?

Answer 39

Lower SNR Poor spatial resolution special gradient coils

Question 40

What are the advantages of EPI -GE?

Answer 40

Rapid acquisition

Question 41

What are the disadvantage of EPI-GE?

Answer 41

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

Question 42

What one Spiral scan sensitivities?

Answer 42

T2* weighting Flow compensated

Question 43

What is the advantage Spiral scanning?

Answer 43

Rapid acquisition

Question 44

What is the Disadvantage Spiral scanning?

Answer 44

Sensitive to in-homogeneity

Question 45

What is Modified SENSE ?

Answer 45

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

Question 46

What does stand for PI ?

Answer 46

Parallel imaging

Question 47

What are coil arrangements used for in PI?

Answer 47

1) signal reception 2) signal encoding

Question 48

What do phased array coils require for PI?

Answer 48

Multiple receiver channels = amplifiers, AD converters, computer memory Significant post processing to combine the images

Question 49

What are the Advantages of Phased array coils ?

Answer 49

- good SNR - Large FOV

Question 50

What are the Disadvantages of Phased array coils ?

Answer 50

- coil coupling - “magic separation” geometry - Multiple receiver channels ($)

Question 51

What do better gradients give?

Answer 51

Reduced TR

Question 52

What is the Slew rate?

Answer 52

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

Question 53

How to half the acquisition time?

Answer 53

The gradients slew rate is quadrupled (x4)

Question 54

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

Answer 54

Cardiovascular Imaging - breath-hold - realtime Brain Imaging - fMRI - Single-shot EPI diffusion