91. MRI: MAGNETIC ALIGNMENT

Question 1

1. What is the alignment of the MR active nuclei in their natural environment?

Answer 1

• 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

Question 2

2. What happens to the MR active nuclei when they are placed in an External Magnetic Field?

Answer 2

• 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

Question 3

3. Which two theories do we have when it comes to describing Alignment?

Answer 3

1. Classical
2. Quantum

Question 4

4. What is Parallel Alignment?

Answer 4

• 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

Question 5

5. What is Anti-Parallel Alignment?

Answer 5

• 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

Question 6

6. When we have MR Active Nuclei at room temperature, how do they tend to allign?

Answer 6

• their magnetic moments will align parallel to the
  magnetic field more so than they align anti-parallel

Question 7

7. Define: The Net Magnetisation Vector (NMV)?

Answer 7

• this is the net magnetism of the patient

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

Question 8

8. What is the alignments of the Net Magnetisation Vector at Room Temperature?

Answer 8

• it is aligned parallel to the Main Magnetic Field

Question 9

9. What are the two energy states that the Hydrogen Nuclei possesses?

Answer 9

• low energy nuclei
• high energy nuclei

Question 10

10. Define: low energy nuclei.

Answer 10

• these are the nuclei that align their magnetic moments
  PARALLEL to the External Magnetic Field

Question 11

11. Define: high energy nuclei.

Answer 11

• these are the nuclei that align their magnetic moments
  anti-parallel to the External Magnetic Field

Question 12

12. What does every MR active nucleus spin on?

Answer 12

• every MR active nucleus spins on its own axis

Question 13

13. What happens to the spin of the MR active nucleus when there is an External Magnetic Field acting on it?

Answer 13

• the nuclei will produce a Secondary Spin
• they can also produce a Spin Wobble

THIS SPIN WOBBLE:
- is known as Precession

Question 14

14. What does the Precession cause in an MR active nucleus?

Answer 14

• 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)

Question 15

15. Define: Larmor.

Answer 15

• this is also known as the Precessional Frequency

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

Question 16

16. What is the Larmor Equation?

Answer 16

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

Question 17

17. What do we use the Larmor equation for?

Answer 17

• 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

Question 18

18. What does the sign of the Gyromagnetic
    Ratio (+y / -y) determine?

Answer 18

• it determines the sense of the precession
• it determines the direction that the precession travels
  in

THIS CAN BE EITHER:
- clockwise
- anti-clockwise

Question 19

19. What does a negative Gyromagnetic Ratio indicate?

Answer 19

• the Magnetic Moment travels in a different direction
  from the Precession Path
• it travels Counter-Clockwise

Question 20

20. What does a positive Gyromagnetic Ratio indicate?

Answer 20

• the Magnetic Moment travels in the same direction as
  the Precession Path
• it travels Clockwise

Question 21

21. What can be said about the Gyromagnetic Ratios of the most common nuclei (H1 and C13)?

Answer 21

• they have positive Gyromagnetic Ratios

Question 22

22. What are the common Precessional Frequencies of Hydrogen?

Answer 22

• 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

Question 23

23. What can be said about the energy and frequency at which Hydrogen precesses?

Answer 23

• it precesses at Low Frequencies
• it precesses at low energies

Question 24

24. What can be said about the magnetic moments of the Hydrogen Nuclei at Room Temperature?

Answer 24

• they are out of phase with each other

Question 25

25. Define: Phase.

Answer 25

- this is the position of the magnetic moments on their circular precessional path

Question 26

26. What are the characteristics of Magnetic Moments that are Out of Phase?

Answer 26

- the magnetic moments are in random directions - there are no Radio frequency paths or external magnetic field applies to the nucleus

Question 27

27. What are the characteristics of Magnetic Moments that are In Phase?

Answer 27

- 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