102. MRI: FLOW PHENOMENA Flashcards
1
Q
- What type of Flow can be seen in this image?
A
- this is Laminar Flow
- the fluid occurs in parallel layers
- there is no disruption between the fluid particles
2
Q
- What type of Flow can be seen in this image?
A
- this is Spiral Flow
- the fluid particles have a tangent-like component of
velocity - this exists about an axis
- it combined with an axial component of velocity
3
Q
- What type of Flow can be seen in this image?
A
- this is Vortex Flow
- the motion of a fluid in a curved path
- this happens when the vessel containing the fluid is
rotated about a vertical axis
4
Q
- What type of Flow can be seen in this image?
A
- this is Turbulent Flow
- the fluid particles are exposed to irregular fluctuations
- they also mix randomly with each other
5
Q
- What is needed to produce an MR signal?
A
- a nucleus
- it must receive an excitation pulse
- it must also receive a re-phasing pulse
6
Q
- What kind of pulses do Stationery Nuclei receive?
A
- they receive excitation pulses
- they receive re-phasing pulses
7
Q
- What kind of pulses do Flowing nuclei receive?
A
- they receive excitation pulses
- they may exit the slice before they experience re-
phasing pulses
NB:
- this is known as the time-of-flight phenomenon
- the nucleus is in motion
- it may receive a re-phasing pulse
- it may not receive a re-phasing pulse
8
Q
- What happens if the moving nucleus receives the Excitation Pulse only, and no re-phasing pulse?
A
- the nucleus does not produce a signal
9
Q
- What happens if the moving nucleus receives the Excitation Pulse and the re-phasing pulse?
A
- the nucleus does produce a signal
10
Q
- What does the Time-of-Flight Phenomenon depend on?
A
- the Velocity of the Flow
- the Echo Time (TE)
- the Slice Thickness
11
Q
- Define: the Entry-Slice Phenomenon.
A
- it is related to the Excitation History of the Nuclei
- it relates to the different signals that are produced by
saturated and unsaturated nuclei
THIS OCCURS WHEN:
- unsaturated spins first enter into a slice or slices
12
Q
- What happens to Stationery Nuclei within a slice?
A
- they become saturated
- this happens after the repeated Radio Frequency
pulses - this means that they give a very low signal
13
Q
- What happens to the Nuclei that flow perpendicular to the slice?
A
- they enter the slice fresh
- they did not experience the Repeated Radio Frequency
pulses and excitations - they are not saturated
- they produce a different, stronger signal than the
Stationery Nuclei
14
Q
14 . What does the magnitude of the Entry Slice Phenomenon depend on?
A
- Repetition Time (TE)
- Slice Thickness
- Velocity of the flow
- Direction of the flow
15
Q
- Define: Co-Current.
A
- this is the flow of nuclei that travels in the same
direction as the Slice Selection
16
Q
- What happens to the Nuclei that have a Co-current direction?
A
- they are flowing nuclei
- they receive repeated Radio Frequency excitations as
they move from one slice to the next - they become saturated very quickly
- the Entry-Slice Phenomenon decreases
- the nuclei produce lower signals
17
Q
- Define: Countercurrent flow.
A
- this is the flow of nuclei that travels in the opposite
direction to the Slice Selection
18
Q
- What happens to the Nuclei that have a Countercurrent direction?
A
- they are flowing nuclei
- they stay fresh as they enter the slice
- they are less likely to receive previous radio frequency
excitation pulses - the Entry-Slice Phenomenon increases
- the nuclei produce stronger signals
- these signals can be seen deep within the slice stack
19
Q
- Define: Time-of-Flight (TOF) angiography.
A
- it is not the same as the Time-of-Flight Phenomenon
IT IS A TECHNIQUE:
- it produces images
- where there are unsaturated spins that come into the
slice
- these spins produce a higher signal intensity
- than the stationary spins found in the slice
20
Q
- What is the results of Time-of-Flight (TOF) angiography?
A
- it will produce images where there are different
contrasts - this is done without any contrast agents being injected
into the patient
21
Q
- What do Saturation Pulses do?
A
- they make signals from unwanted flows insignificant
- these signals do not interfere with the image
22
Q
- When do we use 3D Time-of-Flight Angiography
(TOF-MRA)?
A
- in high-velocity flow regions
23
Q
- When do we use 2D Time-of-Flight Angiography
(TOF-MRA)?
A
- in low-velocity flow regions
24
Q
- What are Phase Contrast MRAs (PC-MRA)?
A
- they are scanning mechanisms
- they make use of gradients
- they use them sensitise the sequence to flow
25
Q
- Which kind of spin produces a higher signal?
A
- flowing spins
- especially when compared to stationery spin
26
Q
- What is the amplitude of the sensitising gradient controlled by?
A
- it is controlled by a Velocity-Encoding Technique
(VNEC)
27
Q
- What are the characteristics of the images produced by 2D Phase Contract (PC) MRA?
A
- they have a better Signal to Noise Ratio
- they have a better resolution
- they have a longer scan time
- it does not need any contrast agents
NB:
- this is all in comparison to the 2D PC-MRA
28
Q
- What are Contrast-Enhanced (CE) MRAs?
A
- it is a scanning mechanism
- it labels the flow spins within a vessel
- it make use of Gadolinium
- this is a contrast agent
GADOLINIUM:
- shortens the period of time taken to produce a T1
weighted image
29
Q
- How does the flow of spins within a vessel appear in the Contrast Enhanced (CE) MRA?
A
- it appears bright
30
Q
- How does the non-flow of spins within a vessel appear in the Contrast Enhanced (CE) MRA?
A
- it appears dark
- it is suppressed
