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MED 107- MEDICAL PHYSICS II > 91. MRI: MAGNETIC ALIGNMENT > Flashcards

91. MRI: MAGNETIC ALIGNMENT Flashcards

1
Q
  1. What is the alignment of the MR active nuclei in their natural environment?
A
  • they point in random directions
  • this is because there is no external magnetic field
    being applied to them

THESE MR ACTIVE NUCLEI:
- do not produce an overall magnetic field in this
situation

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2
Q
  1. What happens to the MR active nuclei when they are placed in an External Magnetic Field?
A
  • their magnetic moments will line up with the magnetic
    field flux lines
  • this is known as ALIGNMENT

THESE MAGNETIC FIELD FLUX LINES:
- are produced by the external magnetic field

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3
Q
  1. Which two theories do we have when it comes to describing Alignment?
A
  1. Classical
  2. Quantum
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4
Q
  1. What is Parallel Alignment?
A
  • this is the alignment of the magnetic moments in the
    same direction as the main magnetic field
  • this is the case when we have low energy magnetic
    moments
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5
Q
  1. What is Anti-Parallel Alignment?
A
  • this is the alignment of the magnetic moments in the
    opposite direction to the Main Magnetic field
  • this happens when the magnetic moments have a high
    energy
  • this means that there is a need for them to change the
    alignment
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6
Q
  1. When we have MR Active Nuclei at room temperature, how do they tend to allign?
A
  • their magnetic moments will align parallel to the
    magnetic field more so than they align anti-parallel
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7
Q
  1. Define: The Net Magnetisation Vector (NMV)?
A
  • this is the net magnetism of the patient

IT REFLECTS:
- the balance between the Parallel and the anti-parallel
Magnetic Moments

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8
Q
  1. What is the alignments of the Net Magnetisation Vector at Room Temperature?
A
  • it is aligned parallel to the Main Magnetic Field
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9
Q
  1. What are the two energy states that the Hydrogen Nuclei possesses?
A
  • low energy nuclei
  • high energy nuclei
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10
Q
  1. Define: low energy nuclei.
A
  • these are the nuclei that align their magnetic moments
    PARALLEL to the External Magnetic Field
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11
Q
  1. Define: high energy nuclei.
A
  • these are the nuclei that align their magnetic moments
    anti-parallel to the External Magnetic Field
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12
Q
  1. What does every MR active nucleus spin on?
A
  • every MR active nucleus spins on its own axis
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13
Q
  1. What happens to the spin of the MR active nucleus when there is an External Magnetic Field acting on it?
A
  • the nuclei will produce a Secondary Spin
  • they can also produce a Spin Wobble

THIS SPIN WOBBLE:
- is known as Precession

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14
Q
  1. What does the Precession cause in an MR active nucleus?
A
  • it causes the magnetic moments of the MR active
    nucleus to travel in a circular path

THIS PATH:
- travels around the strength of the Magnetic Field (B₀)

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15
Q
  1. Define: Larmor.
A
  • this is also known as the Precessional Frequency

THIS IS THE SPEED:
- at which the magnetic moments wobble about the
external magnetic field

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16
Q
  1. What is the Larmor Equation?
A

ω₀ = this is the Precessional Frequency
= it is measured in Hertz

B₀ = this is the strength of the external magnetic
field
= it is expressed in Tesla (T)

y = this is the Gyromagnetic Ratio
= it is specific for different MR active nuclei
= it is in units of MHz / Tesla

17
Q
  1. What do we use the Larmor equation for?
A
  • it is used to calculate the frequency of the Precession
  • it also calculates the speed of the Precession
  • for a specific nucleus
  • in a specific magnetic field
18
Q
  1. What does the sign of the Gyromagnetic
    Ratio (+y / -y) determine?
A
  • it determines the sense of the precession
  • it determines the direction that the precession travels
    in

THIS CAN BE EITHER:
- clockwise
- anti-clockwise

19
Q
  1. What does a negative Gyromagnetic Ratio indicate?
A
  • the Magnetic Moment travels in a different direction
    from the Precession Path
  • it travels Counter-Clockwise
20
Q
  1. What does a positive Gyromagnetic Ratio indicate?
A
  • the Magnetic Moment travels in the same direction as
    the Precession Path
  • it travels Clockwise
21
Q
  1. What can be said about the Gyromagnetic Ratios of the most common nuclei (H1 and C13)?
A
  • they have positive Gyromagnetic Ratios
22
Q
  1. What are the common Precessional Frequencies of Hydrogen?
A
  • 21.285 MHz and 0.5 Teslas
  • 42.57 MHz at 1 Tesla
  • 63.86 MHz at 1.5 Teslas

NB:
- these are all found in clinical MRIs

23
Q
  1. What can be said about the energy and frequency at which Hydrogen precesses?
A
  • it precesses at Low Frequencies
  • it precesses at low energies
24
Q
  1. What can be said about the magnetic moments of the Hydrogen Nuclei at Room Temperature?
A
  • they are out of phase with each other
25
Q
  1. Define: Phase.
A
  • this is the position of the magnetic moments on their
    circular precessional path
26
Q
  1. What are the characteristics of Magnetic Moments that are Out of Phase?
A
  • the magnetic moments are in random directions
  • there are no Radio frequency paths or external
    magnetic field applies to the nucleus
27
Q
  1. What are the characteristics of Magnetic Moments that are In Phase?
A
  • Radio Frequency paths (external magnetic fields) are
    applied at the Larmor Frequency of the Hydrogen
  • they are applied to the nucleus
  • the MRI can detect this signal
  • there is now a Circular Precession Path

MED 107- MEDICAL PHYSICS II (108 decks)

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