95. MRI: T1 RECOVERY & T2 DECAY

Question 1

1. What is the cause of T1 Recovery?

Answer 1

• 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

Question 2

2. What happens as the nuclei energy starts to dissipate?

Answer 2

• the energy of the magnetic moments of the nuclei
• starts to relax
• it returns to the original strength of the External
  Magnetic Field (B⁰)

Question 3

3. What happens when the magnetic of the nuclei returns to the original strength of the External Magnetic Field (B⁰)?

Answer 3

• 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

Question 4

4. What can be said about the T1 time of a tissue?

Answer 4

• it is specific to the tissue
• it is a contrast parameter

Question 5

5. Define: T1 Time?

Answer 5

• this is the time it takes
• for 63% of the longitudinal magnetisation to recover

Question 6

6. What is the time period in which 63% of the longitudinal magnetisation recovers?

Answer 6

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

Question 7

7. What does the Repetition Time determine?

Answer 7

• it determines how much T1 Recovery occurs in the
  tissue

Question 8

8. Define: T1 Relaxation.

Answer 8

• 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

Question 9

9. 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

Answer 9

• it determines the T1 Time of the tissue
• this is the tissue in which the nuclei are situated

Question 10

10. What can be said about the T1 time of fat?

Answer 10

• it is very short
• fat can absorb energy quickly

Question 11

11. What does a short T1 Relaxation Time mean in the case of Fat?

Answer 11

• 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)

Question 12

12. What can be said about the T1 Relaxation time of water?

Answer 12

• 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)

Question 13

13. What is the T1 Relaxation time of water?

Answer 13

• 2500 ms

Question 14

14. What is the T1 Relaxation time of fat?

Answer 14

• 200 ms

Question 15

15. What is the T1 Relaxation time of Cerebro-Spinal Fluid?

Answer 15

• 2000 ms

Question 16

16. What is the T1 Relaxation time of White Matter?

Answer 16

• 500 ms

Question 17

17. What does the Repetition Time (TR) control?

Answer 17

• 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

Question 18

18. What can be said about having a short Repetition Time (TR)?

Answer 18

• it does not permit the full longitudinal plane recovery
  in most tissues

Question 19

19. What happens when the Repetition Time is short and there is no full longitudinal plane recovery?

Answer 19

• there will be a contrast in the images
• this contrast is between fat and water
• it is due to their different T1 Recovery Times

Question 20

20. What can be said about having a long Repetition Time (TR)?

Answer 20

• 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

Question 21

21. What causes T2 Decay?

Answer 21

• 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

Question 22

22. What causes spin-spin transfer energy?

Answer 22

• the magnetic fields of the nuclei will interact with one
  another

Question 23

23. What does spin-spin transfer energy result in?

Answer 23

• 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

Question 24

24. What is the T2 Decay Time?

Answer 24

• 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%

Question 25

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

Answer 25

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

Question 26

26. What does the Echo Time (TE) determine?

Answer 26

- it determines how much T2 decay occurs in the tissue

Question 27

27. What causes T2 Relaxation?

Answer 27

- the spins of adjacent nuclei will interact with each other - they exchange energy

Question 28

28. What dictates the efficiency of T2 Relaxation? EFFICACY= how much energy is exchanged between the adjacent nuclei

Answer 28

- how closely packed the molecules are to each other

Question 29

29. What can be said about the T2 Relaxation time of fat?

Answer 29

- it is very short - the molecules in fat are very closely packed together - the spin-spin effect is efficient

Question 30

30. What can be said about the T2 Relaxation Time of Water?

Answer 30

- it is very long - the water molecules are spaced very far apart - the spin-spin effect is not efficient

Question 31

31. What is the T2 Decay time in the following: 31.1. Water 31.2. Fat 31.3. Cerebro-Spinal Fluid 31.4. White Matter

Answer 31

(31.1) 2500 ms (31.2.) 100 ms (31.3) 300 ms (31.4) 100 ms

Question 32

32. What does the Echo Time control in Terms of T2 decay?

Answer 32

- 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

Question 33

33. What happens when we have a short Echo Time (TE)?

Answer 33

- 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

Question 34

34. What happens when we have a long Echo Time (TE)?

Answer 34

- 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

Question 35

35. 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)?

Answer 35

(35.1) The Repetition Time is short - EG: 500 ms (35.2) The Echo Time is short - EG: 14 ms

Question 36

36. 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)?

Answer 36

(36.1) The Repetition Time is long - EG: 4000 ms (36.2) The Echo Time is long - EG: 90 ms