MRI Contrast And Vascular Imaging Flashcards
What are the methods of Contrast - enhanced MRA (CE-MRA) ?
- single-phase
- Time resolved
What are the methods of Non-contrast MRA (NC-MRA) ?
- Time of flight (tof)
- Phase contrast (PC)
What is CE - MRA and why do we use it?
Uses T1 shortening Gadolinium
Trade-off between temporal and spatial resolution
Why we need to use it?
- short TR
- Fat suppression
- Image subtraction
What is single phase (CE-MRA technique) ?
- 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
What is Time resolved (4D) CE-MRA technique?
-repeatedly imaging, FOV, as contrast passes through the vessels
- High temporal resolution (fast imaging)
What does Time resolved MRA involve?
+ 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
What are some of the Time resolved MRA techniques?
> 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
What are the Advantages of CE-MRA?
- very high SNR
- High spatial resolution (for small vessels - grading of arterial stenosis)
- no flow related artefacts
- well established ( so clinically proven)
What are the Disadvantages of CE-MRA ?
- Use of gadolinium
- Bolus imaging ( scan time at the peak of the bolus)
What is NC - MRA?
Relays on the effect of blood moving (flow) through (within) the slice
- affects the MR signal intensity
- can be observed even of standard imaging
What are the methods of NC-MRA?
- Time of light (TOF)
- Phase contrast (PC)
Less common
Arterial spin labelling (ASL)
Fresh blood imaging (FBI)
Quiescent inflow single shot (QISS)
Why are some of the NC-MRA hardware improvements?
Higher mRI field (3T) : higher SNR
Faster gradients : reduces artefacts (motion and dephasing from long TE)
Multi receiver RF coils : higher SNR
What are the NC-MRA Reconstruction improvements?
Parallel imaging - SENSE- GRAPPA - C-SENCE
Radial under -sampling , spiral trajectories
What is TOF - spin washout?
Outflow:
~High- velocity signal loss ‘dark’ or ‘black’ blood appearances
~ SE pulse sequences
What is TOF flow related enhancement?
Inflow:
- ‘Bright’ blood appearance
- GE pulse sequences
Where is TOF spin washout most prominent?
What sequence does TOF SW use?
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 .
Why is TOF - flow-related enhancement used?
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
What happens as TOF flow-related enhancement increases?
Flow velocity increases
Slice thickness decreases
TR decreases
Why are the differences between TOF MRA 2D and 3D acquisition?
2D TOF sensitive to slow flow – venography
3-D TOF Sensitive to fast flow - Arteriography
What does short TR and TE during TOF MRA do?
Saturate static tissue
Minimises phase related signal loss
What are the factors involved in 2D TOF MRA ?
+ 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
What happens during 2D TOF MRA with spatial saturation?
Pre-saturation pulses Applied to volumes selected (Sat-bands)
Above slices = arteriography
Below slices = Venogram
What does 2D TOF MRA generate?
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
What are the factors about 3D TOF MRA?
-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
what are the 2 methods to reduce saturation effects in 3-D MRA?
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
What is PC-MRA and what is it used for?
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,
What is Phase shift?
Spins phase increases/retards with time when moving into higher/lower magnetic field gradient.
