MRI Contrast And Vascular Imaging

Question 1

What are the methods of Contrast - enhanced MRA (CE-MRA) ?

Answer 1

• single-phase
• Time resolved

Question 2

What are the methods of Non-contrast MRA (NC-MRA) ?

Answer 2

• Time of flight (tof)
• Phase contrast (PC)

Question 3

What is CE - MRA and why do we use it?

Answer 3

Uses T1 shortening Gadolinium
Trade-off between temporal and spatial resolution

Why we need to use it?
- short TR
- Fat suppression
- Image subtraction

Question 4

What is single phase (CE-MRA technique) ?

Answer 4

• obtains images at a single points in time post contrast
• High spatial resolution images
• Needs the peak arrival time of the contrast, bolus to image at peak concentration

Question 5

What is Time resolved (4D) CE-MRA technique?

Answer 5

-repeatedly imaging, FOV, as contrast passes through the vessels
- High temporal resolution (fast imaging)

Question 6

What does Time resolved MRA involve?

Answer 6

+ Dynamic view of contrast, media similar to x-ray, DSA
+ Using fast, T1 weighted, grading echo sequences
- Fast 2D (<2sec /frame)
- Fast 3-D (5-7 sec) to do cE-MRA during a single breath hold
+ Uses good mask, subtraction and new K - space schemes
+ No need to estimate the bonus arrival time

Question 7

What are some of the Time resolved MRA techniques?

Answer 7

> Ultra fast spoiled GE
Time resolved imaging with kinetics (TRICKS)
Time resolved angiography with interleaved stochastic trajectories (TWIST)
4D time resolved angiography using keyhole ( 4D-TRAK)

Central K-space data is updated at a much faster rate
the main advantage is both high temporal and spatial resolution

Question 8

What are the Advantages of CE-MRA?

Answer 8

• very high SNR
• High spatial resolution (for small vessels - grading of arterial stenosis)
• no flow related artefacts
• well established ( so clinically proven)

Question 9

What are the Disadvantages of CE-MRA ?

Answer 9

• Use of gadolinium
• Bolus imaging ( scan time at the peak of the bolus)

Question 10

What is NC - MRA?

Answer 10

Relays on the effect of blood moving (flow) through (within) the slice
- affects the MR signal intensity
- can be observed even of standard imaging

Question 11

What are the methods of NC-MRA?

Answer 11

• Time of light (TOF)
• Phase contrast (PC)

Less common
Arterial spin labelling (ASL)
Fresh blood imaging (FBI)
Quiescent inflow single shot (QISS)

Question 12

Why are some of the NC-MRA hardware improvements?

Answer 12

Higher mRI field (3T) : higher SNR
Faster gradients : reduces artefacts (motion and dephasing from long TE)
Multi receiver RF coils : higher SNR

Question 13

What are the NC-MRA Reconstruction improvements?

Answer 13

Parallel imaging - SENSE- GRAPPA - C-SENCE
Radial under -sampling , spiral trajectories

Question 14

What is TOF - spin washout?

Answer 14

Outflow:
~High- velocity signal loss ‘dark’ or ‘black’ blood appearances
~ SE pulse sequences

Question 15

What is TOF flow related enhancement?

Answer 15

Inflow:
- ‘Bright’ blood appearance
- GE pulse sequences

Question 16

Where is TOF spin washout most prominent?
What sequence does TOF SW use?

Answer 16

Most prominent on long, TE spin – echo sequences
Uses 2D or 3-D FSE black blood sequences
Excited spins wash out of the slice before the 180° pulse is applied .

Question 17

Why is TOF - flow-related enhancement used?

Answer 17

Because primarily within GE pulse sequences (no spin, washout effect)
- Single slice
- Short TR to saturate the signal
Partially saturate, spins causes tissues to appear dark
Unsaturated blood entering the slice will be bright

Question 18

What happens as TOF flow-related enhancement increases?

Answer 18

Flow velocity increases
Slice thickness decreases
TR decreases

Question 19

Why are the differences between TOF MRA 2D and 3D acquisition?

Answer 19

2D TOF sensitive to slow flow – venography
3-D TOF Sensitive to fast flow - Arteriography

Question 20

What does short TR and TE during TOF MRA do?

Answer 20

Saturate static tissue
Minimises phase related signal loss

Question 21

What are the factors involved in 2D TOF MRA ?

Answer 21

+ plan multiple things overlapping slices
+ Perpendicular to vessels
+Slices acquired in sequential order
+Short TR
+ Very short TE to reduce flow effects due to dephasing

Question 22

What happens during 2D TOF MRA with spatial saturation?

Answer 22

Pre-saturation pulses Applied to volumes selected (Sat-bands)

Above slices = arteriography
Below slices = Venogram

Question 23

What does 2D TOF MRA generate?

Answer 23

Generates a single projected image from a volume data set

Based on the maximum pixel intensity detected using a ray-tracing algorithm

Can be performed in any direction

Question 24

What are the factors about 3D TOF MRA?

Answer 24

-Orientated perpendicular to the direction of flow

-Signal progressively, saturated as blood moves through the volume

-Limit thickness of volume to maximise inflow effect

-Usually for fast flow

Question 25

what are the 2 methods to reduce saturation effects in 3-D MRA?

Answer 25

Tilted optimised non-saturation excitation (TONE)
- RF, with variable flip angle, low angle at the entrance and high near the end

Multiple overlapping, thin slice acquisition (MOTSA)
- Instead of one thick slab, multiple smaller overlapped are acquired
- Can cover a larger area without losing signal

Question 26

What is PC-MRA and what is it used for?

Answer 26

PC-MRA change in the phase of moving spins (phase effects) to form an image

• Phase is proportional to velocity of moving spins (i.e. Blood)
• Signal is based on the phase shift (gained or lost) of moving spins
• Visualise and quantify, blood flow and velocity in the X ,Y and directions
• 2D and 3-D imaging
• Useful for slow flow, such as CSF flow studies , MR venography and extremities,

Question 27

What is Phase shift?

Answer 27

Spins phase increases/retards with time when moving into higher/lower magnetic field gradient.