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MED 107- MEDICAL PHYSICS II > 94. MRI: RELAXATION > Flashcards

94. MRI: RELAXATION Flashcards

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1
Q
  1. Define: Relaxation.
A
  • it is a term that refers to Energy Loss
  • it is when we remove the Radio Frequency Pulse
  • the Net Magnetisation Vector will try to return to the
    Longitudinal Direction
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2
Q
  1. When do we remove the Radio Frequency Pulse?
A
  • when the Radio Frequency Excitation Pulse has been
    applied
  • and then Resonance and Flip angle are achieved
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3
Q
  1. What happens once we remove the Radio Frequency Pulse?
A
  • the signal in the Receiver Coil begins to decrease
  • the amplitude of the Voltage in the coil gradually
    decreases
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4
Q
  1. Why does the Net Magnetisation Vector in the Transverse Plane begin to decrease?
A
  1. Relaxation Processes
  2. Field Inhomogenities
  3. Susceptibility Effects
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5
Q
  1. Define: T2* Decay.
A
  • this is the cumulative dephasing of the spin-spin
    interactions
  • it is also the dephasing of the inhomogenities
  • the field becomes more homogeneous
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6
Q
  1. What happens to the magnetisation in each tissue?
A
  • it relaxes at different rates
  • this is one of the factors that contribute to image
    contrast
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7
Q
  1. What are the effects that take place when the Radio Frequency is withdrawn?
A
  1. THE NUCLEI EMIT ENERGY
    - this energy is absorbed from the Radio Frequency
    Pulse
    - this is known as Spin Lattice Energy Transfer
  2. THE NUCLEI SHIFT THEIR MAGNETIC MOMENTS
    • from the high energy state
    • to the low energy state
  3. THE NET MAGNETISATION VECTOR
    - recovers to its original position
    - it aligns with the direction of the magnetic field (B₀)
    - this is the 90° Longitudinal Plane

NB:
- this whole relaxation process is known as the
T1 Recovery

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8
Q
  1. Define: T2 Decay.
A
  1. THE NUCLEI
    - lose their precessional coherence
    - they dephase
  2. THE NET MAGNETISATION VECTOR
    - decays in the Transverse Plane
    - this is a Relaxation Process
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9
Q
  1. How do Nuclei lose their consistency?
A
  1. THEY HAVE INTERACTIONS
    - with the naturally-occurring magnetic fields
    - of the adjacent nuclei
    - spin-spin energy is transferred from one nuclei to the
    other
  2. THIS CAUSES INHOMOGENITIES
    - of the external magnetic field
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10
Q
  1. What does Relaxation result in?
A
  1. T1 Recovery
  2. T2 Decay
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11
Q
  1. Briefly state what T1 Recovery is.
A
  • this is the Recovery of the magnetisation
  • this happens in the Longitudinal Field

NB:
- there is no Radio Frequency signal in the Longitudinal
Plane
- the paths are removed

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12
Q
  1. Briefly state what T2 Decay is.
A
  • this is the Decay of Magnetisation in the Transverse
    Plane

NB:
- the Net magnetisation Vector is aligned to the External
Magnetic Field
- the Net Magnetisation Vector is at 90°
- energy has been lost due to T1 Recovery

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13
Q
  1. Can we avoid Inhomogenities of the External Magnetic Field?
A
  • no
  • they are inevitable
  • they directly affect the T2 Decay

NB:
- we can become more homogenous by using the Skin-
shining effect

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14
Q
  1. What do Inhomogenities in the External Magnetic Field alter?
A
  • they alter the magnitude of the Magnetic Field (B₀)

THIS IS BECAUSE:
- some small areas in the field will have a magnetic field
strength that is slightly more or slightly less
- than the main field strength

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15
Q
  1. How do we get areas in the Magnetic Field that are weaker than others?
A
  • they have a lower frequency compared to the applied
    Magnetic Field
  • they will be weaker
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16
Q
  1. What is the Precessional Frequency of a Spin proportional to?
A
  • it is proportional to the Magnetic Field (B₀)
17
Q
  1. What happens when a Spin passes through inhomogenities?
A
  • it will experience a magnetic Field strength that differs
    from the direction of the External Magnetic Field (B₀)

THIS CAUSES:
- the Precessional Frequencies to change

18
Q
  1. What happens when the Precessional Frequencies of the Spin change?
A
  • the Spins change their phase
  • the Net Magnetisation Vector is now dephased
19
Q
  1. What is the results of the Net Magnetisation Vector dephasing?
A
  • the Transverse Magnetisation Decays
  • this decay occurs exponentially
  • this means that it continues more and more rapidly
  • this is known as T2* Decay
20
Q
  1. What do Magnetic Field Inhomogenities result in?
A
  • they cause the Net Magnetisation Vector to dephase
  • this happens before the intrinsic Magnetic Fields
    influence the Dephasing

THIS MEANS THAT:
- T2* Decay happens before T2 Decay

21
Q
  1. Can you summarise T2 Decay in three simple points?
A
  1. This is the decay in the Transverse Plane
  2. It is caused by inhomogenities
  3. It removes the Radio Frequency signals
  4. It is the decay of the Net Magnetisation Vector in the
    Transverse Plane
22
Q
  1. What do we use in order to produce images where the T2 Contrast can be visualised?
A
  • we make use of Pulse Sequences

THESE PULSE SEQUENCES:
- allow us to control the way in which the system
- applies the Radio Frequency Pulses and gradients

MED 107- MEDICAL PHYSICS II (108 decks)

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