102. MRI: FLOW PHENOMENA

Question 1

1. What type of Flow can be seen in this image?

Answer 1

• this is Laminar Flow
• the fluid occurs in parallel layers
• there is no disruption between the fluid particles

Question 2

2. What type of Flow can be seen in this image?

Answer 2

• 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

Question 3

3. What type of Flow can be seen in this image?

Answer 3

• 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

Question 4

4. What type of Flow can be seen in this image?

Answer 4

• this is Turbulent Flow
• the fluid particles are exposed to irregular fluctuations
• they also mix randomly with each other

Question 5

5. What is needed to produce an MR signal?

Answer 5

• a nucleus
• it must receive an excitation pulse
• it must also receive a re-phasing pulse

Question 6

6. What kind of pulses do Stationery Nuclei receive?

Answer 6

• they receive excitation pulses
• they receive re-phasing pulses

Question 7

7. What kind of pulses do Flowing nuclei receive?

Answer 7

• 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

Question 8

8. What happens if the moving nucleus receives the Excitation Pulse only, and no re-phasing pulse?

Answer 8

• the nucleus does not produce a signal

Question 9

9. What happens if the moving nucleus receives the Excitation Pulse and the re-phasing pulse?

Answer 9

• the nucleus does produce a signal

Question 10

10. What does the Time-of-Flight Phenomenon depend on?

Answer 10

• the Velocity of the Flow
• the Echo Time (TE)
• the Slice Thickness

Question 11

11. Define: the Entry-Slice Phenomenon.

Answer 11

• 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

Question 12

12. What happens to Stationery Nuclei within a slice?

Answer 12

• they become saturated
• this happens after the repeated Radio Frequency
  pulses
• this means that they give a very low signal

Question 13

13. What happens to the Nuclei that flow perpendicular to the slice?

Answer 13

• 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

Question 14

14 . What does the magnitude of the Entry Slice Phenomenon depend on?

Answer 14

1. Repetition Time (TE)
2. Slice Thickness
3. Velocity of the flow
4. Direction of the flow

Question 15

15. Define: Co-Current.

Answer 15

• this is the flow of nuclei that travels in the same
  direction as the Slice Selection

Question 16

16. What happens to the Nuclei that have a Co-current direction?

Answer 16

• 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

Question 17

17. Define: Countercurrent flow.

Answer 17

• this is the flow of nuclei that travels in the opposite
  direction to the Slice Selection

Question 18

18. What happens to the Nuclei that have a Countercurrent direction?

Answer 18

• 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

Question 19

19. Define: Time-of-Flight (TOF) angiography.

Answer 19

• 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

Question 20

20. What is the results of Time-of-Flight (TOF) angiography?

Answer 20

• it will produce images where there are different
  contrasts
• this is done without any contrast agents being injected
  into the patient

Question 21

21. What do Saturation Pulses do?

Answer 21

• they make signals from unwanted flows insignificant
• these signals do not interfere with the image

Question 22

22. When do we use 3D Time-of-Flight Angiography
    (TOF-MRA)?

Answer 22

• in high-velocity flow regions

Question 23

23. When do we use 2D Time-of-Flight Angiography
    (TOF-MRA)?

Answer 23

• in low-velocity flow regions

Question 24

24. What are Phase Contrast MRAs (PC-MRA)?

Answer 24

• they are scanning mechanisms
• they make use of gradients
• they use them sensitise the sequence to flow

Question 25

25. Which kind of spin produces a higher signal?

Answer 25

- flowing spins - especially when compared to stationery spin

Question 26

26. What is the amplitude of the sensitising gradient controlled by?

Answer 26

- it is controlled by a Velocity-Encoding Technique (VNEC)

Question 27

27. What are the characteristics of the images produced by 2D Phase Contract (PC) MRA?

Answer 27

- 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

Question 28

28. What are Contrast-Enhanced (CE) MRAs?

Answer 28

- 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

Question 29

29. How does the flow of spins within a vessel appear in the Contrast Enhanced (CE) MRA?

Answer 29

- it appears bright

Question 30

30. How does the non-flow of spins within a vessel appear in the Contrast Enhanced (CE) MRA?

Answer 30

- it appears dark - it is suppressed