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MED 107- MEDICAL PHYSICS II > 95. MRI: T1 RECOVERY & T2 DECAY > Flashcards

95. MRI: T1 RECOVERY & T2 DECAY Flashcards

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1
Q
  1. What is the cause of T1 Recovery?
A
  • the exchange of energy
  • this energy is exchanged from the nuclei to their
    surrounding environment

THIS ENERGY CAN ALSO BE EXCHANGED:
- from the nuclei to their surrounding lattice
- this is called the Spin Lattice Energy Transfer

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2
Q
  1. What happens as the nuclei energy starts to dissipate?
A
  • the energy of the magnetic moments of the nuclei
  • starts to relax
  • it returns to the original strength of the External
    Magnetic Field (B⁰)
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3
Q
  1. What happens when the magnetic of the nuclei returns to the original strength of the External Magnetic Field (B⁰)?
A
  • their Net Magnetisation Vector returns to the
    longitudinal plane
  • this means that there will no longer be a signal emitted
    from the nuclei
  • this happens at different rates in different tissues
  • it is an exponential process
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4
Q
  1. What can be said about the T1 time of a tissue?
A
  • it is specific to the tissue
  • it is a contrast parameter
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5
Q
  1. Define: T1 Time?
A
  • this is the time it takes
  • for 63% of the longitudinal magnetisation to recover
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6
Q
  1. What is the time period in which 63% of the longitudinal magnetisation recovers?
A

IT IS THE TIME BETWEEN:
- one excitation pulse to the next excitation pulse
- this is known as the Repetition Time (TR)

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7
Q
  1. What does the Repetition Time determine?
A
  • it determines how much T1 Recovery occurs in the
    tissue
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8
Q
  1. Define: T1 Relaxation.
A
  • this is a process
  • it occurs when the nuclei exchange the energy that was
    given to them
  • they exchange it with their surrounding environment

THIS ENERGY WAS GIVEN TO THEM:
- by the Radio Frequency pulse

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9
Q
  1. What does the efficacy of T1 Relaxation determine?

EFFICACY= the ability to reach the desired result
= in this case it is the exchange of energy from
the nuclei to the environment

A
  • it determines the T1 Time of the tissue
  • this is the tissue in which the nuclei are situated
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10
Q
  1. What can be said about the T1 time of fat?
A
  • it is very short
  • fat can absorb energy quickly
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11
Q
  1. What does a short T1 Relaxation Time mean in the case of Fat?
A
  • the nuclei give off their energy to the surrounding fat
    tissue over a short period of time
  • the Net Magnetisation Vector of fat rapidly returns to
    the Longitudinal Plane
  • it returns to the direction and strength of the External
    Magnetic Field (B₀)
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12
Q
  1. What can be said about the T1 Relaxation time of water?
A
  • it is long
  • water is very inefficient at receiving energy from nuclei

THIS MEANS THAT:
- the nuclei give off their energy to the surrounding
water over a long period of time
- the Net Magnetisation Vector of water slowly returns
to the Longitudinal Plane
- it returns to the direction and strength of the External
Magnetic Field (B₀)

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13
Q
  1. What is the T1 Relaxation time of water?
A
  • 2500 ms
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14
Q
  1. What is the T1 Relaxation time of fat?
A
  • 200 ms
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15
Q
  1. What is the T1 Relaxation time of Cerebro-Spinal Fluid?
A
  • 2000 ms
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16
Q
  1. What is the T1 Relaxation time of White Matter?
A
  • 500 ms
17
Q
  1. What does the Repetition Time (TR) control?
A
  • it controls how much of the Net Magnetisation Vector
    has recovered to the Longitudinal plane
  • before the application of the next Radio Frequency
    Pulse

THIS IS CHECKED IN BOTH:
- water
- fat

18
Q
  1. What can be said about having a short Repetition Time (TR)?
A
  • it does not permit the full longitudinal plane recovery
    in most tissues
19
Q
  1. What happens when the Repetition Time is short and there is no full longitudinal plane recovery?
A
  • there will be a contrast in the images
  • this contrast is between fat and water
  • it is due to their different T1 Recovery Times
20
Q
  1. What can be said about having a long Repetition Time (TR)?
A
  • it allows for the full recovery to the Longitudinal plane
    in most tissues
  • there is no difference in the magnitude of the
    longitudinal components
  • there is no contrast difference in the images
21
Q
  1. What causes T2 Decay?
A
  • the exchange of energy from one nucleus to another
    nucleus

THIS IS CALLED:
- the spin-spin transfer energy

THE T2 DECAY TIME:
- is specific to each tissue
- it is a contrast parameter

22
Q
  1. What causes spin-spin transfer energy?
A
  • the magnetic fields of the nuclei will interact with one
    another
23
Q
  1. What does spin-spin transfer energy result in?
A
  • it results in the loss of phase coherence
  • this means that it results in de-phasing

THIS RESULTS IN:
- the decay of the Net Magnetisation Vector in the
Transverse Plane
- this process is exponential
- it occurs at different rates

24
Q
  1. What is the T2 Decay Time?
A
  • it is the time is takes for 63% of the Transverse
    Magnetisation to be lost due to de-phasing

THIS MEANS THAT:
- the transverse magnetisation is reduced to 63% of its
original value
- it leaves behind only 37%

25
Q
  1. Over which period of time is 63% of the Transverse Magnetisation lost due to de-phasing?
A

THIS HAPPENS BETWEEN:
- the excitation pulse
- and the MR signal
- this is known as the Echo Time (TE)

26
Q
  1. What does the Echo Time (TE) determine?
A
  • it determines how much T2 decay occurs in the tissue
27
Q
  1. What causes T2 Relaxation?
A
  • the spins of adjacent nuclei will interact with each
    other
  • they exchange energy
28
Q
  1. What dictates the efficiency of T2 Relaxation?

EFFICACY= how much energy is exchanged between the
adjacent nuclei

A
  • how closely packed the molecules are to each other
29
Q
  1. What can be said about the T2 Relaxation time of fat?
A
  • it is very short
  • the molecules in fat are very closely packed together
  • the spin-spin effect is efficient
30
Q
  1. What can be said about the T2 Relaxation Time of Water?
A
  • it is very long
  • the water molecules are spaced very far apart
  • the spin-spin effect is not efficient
31
Q
  1. What is the T2 Decay time in the following:

31.1. Water
31.2. Fat
31.3. Cerebro-Spinal Fluid
31.4. White Matter

A

(31.1) 2500 ms

(31.2.) 100 ms

(31.3) 300 ms

(31.4) 100 ms

32
Q
  1. What does the Echo Time control in Terms of T2 decay?
A
  • it controls how much the transverse magnetisation has
    been allowed to decay
  • this happens when the signal is read

THIS HAPPENS IN BOTH:
- fat
- water

33
Q
  1. What happens when we have a short Echo Time (TE)?
A
  • full dephasing cannot happen
  • it cannot happen in water or in fat

THIS MEANS THAT:
- there will be very little contrast between the fat and the
water in the image
- this is due to the very small differences in the T2 Decay
times

34
Q
  1. What happens when we have a long Echo Time (TE)?
A
  • full dephasing takes place
  • this happens in both the water and the fat

THIS MEANS THAT:
- there will be a contrast difference between the fat and
the water in the image
- this is because there is a large difference in the T2
Decay times

35
Q
  1. Answer the following questions with regards to
    T1 Weighted Images:

35.1. What is the Repetition Time (TR)?
35.2. What is the Echo Time (TE)?

A

(35.1) The Repetition Time is short
- EG: 500 ms

(35.2) The Echo Time is short
- EG: 14 ms

36
Q
  1. Answer the following questions with regards to
    T2 Weighted Images:

36.1. What is the Repetition Time (TR)?
36.2. What is the Echo Time (TE)?

A

(36.1) The Repetition Time is long
- EG: 4000 ms

(36.2) The Echo Time is long
- EG: 90 ms

MED 107- MEDICAL PHYSICS II (108 decks)

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