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MED 107- MEDICAL PHYSICS II > 102. MRI: FLOW PHENOMENA > Flashcards

102. MRI: FLOW PHENOMENA Flashcards

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1
Q
  1. 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
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2
Q
  1. 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
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3
Q
  1. 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
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4
Q
  1. 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
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5
Q
  1. 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
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6
Q
  1. What kind of pulses do Stationery Nuclei receive?
A
  • they receive excitation pulses
  • they receive re-phasing pulses
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7
Q
  1. 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

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8
Q
  1. What happens if the moving nucleus receives the Excitation Pulse only, and no re-phasing pulse?
A
  • the nucleus does not produce a signal
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9
Q
  1. What happens if the moving nucleus receives the Excitation Pulse and the re-phasing pulse?
A
  • the nucleus does produce a signal
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10
Q
  1. What does the Time-of-Flight Phenomenon depend on?
A
  • the Velocity of the Flow
  • the Echo Time (TE)
  • the Slice Thickness
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11
Q
  1. 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

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12
Q
  1. 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
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13
Q
  1. 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
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14
Q

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

A
  1. Repetition Time (TE)
  2. Slice Thickness
  3. Velocity of the flow
  4. Direction of the flow
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15
Q
  1. Define: Co-Current.
A
  • this is the flow of nuclei that travels in the same
    direction as the Slice Selection
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16
Q
  1. 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
  1. Define: Countercurrent flow.
A
  • this is the flow of nuclei that travels in the opposite
    direction to the Slice Selection
18
Q
  1. 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
  1. 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
  1. 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
  1. What do Saturation Pulses do?
A
  • they make signals from unwanted flows insignificant
  • these signals do not interfere with the image
22
Q
  1. When do we use 3D Time-of-Flight Angiography
    (TOF-MRA)?
A
  • in high-velocity flow regions
23
Q
  1. When do we use 2D Time-of-Flight Angiography
    (TOF-MRA)?
A
  • in low-velocity flow regions
24
Q
  1. 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
  1. Which kind of spin produces a higher signal?
A
  • flowing spins
  • especially when compared to stationery spin
26
Q
  1. What is the amplitude of the sensitising gradient controlled by?
A
  • it is controlled by a Velocity-Encoding Technique
    (VNEC)
27
Q
  1. 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
  1. 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
  1. How does the flow of spins within a vessel appear in the Contrast Enhanced (CE) MRA?
A
  • it appears bright
30
Q
  1. How does the non-flow of spins within a vessel appear in the Contrast Enhanced (CE) MRA?
A
  • it appears dark
  • it is suppressed

MED 107- MEDICAL PHYSICS II (108 decks)

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