92. MRI: RESONANCE

Question 1

1. Define: Resonance.

Answer 1

• this is an energy transition
• it occurs when an object is subjected ti a frequency
  that is the same as its own frequency

Question 2

2. How is Resonance induced in MR?

Answer 2

• a Radio Frequency (RF) Pulse is applied

THIS RADIO FREQUENCY IS APPLIED:
- at the same frequency as the precessing Hydrogen
Nuclei
- at 90° to the External Field (B₀)

Question 3

3. What does the applies Radio Frequency pulse result in?

Answer 3

• it causes the Hydrogen Nuclei to resonate
• the other mR nuclei will not resonate

Question 4

4. What two phenomena happen at Resonance?

Answer 4

1. Energy Absorption
   2 Phase Coherence

Question 5

5. Define: Phase Coherence.

Answer 5

• this is when the magnetic moments of the nuclei will
  move into phase with each other

AS THE MAGNETIC MOMENTS ARE IN PHASE:
- the net effect is the one of Precession
- the Net Magnetisation Vector precesses into the
Transverse Plane of the Larmor Frequency

Question 6

6. What happens when the Net Magnetisation Vector is in the Transverse Plane?

Answer 6

• it is able to detect the MR signal
• it cannot do this in the Longitudinal plane

Question 7

7. What apparatus can be found in the Transverse Plane?

Answer 7

• the receiver coil

Question 8

8. What happens as the Net Magnetisation Vector rotates around the Transverse Plane?
   (due to Resonance)

Answer 8

• the Net Magnetisation Vector will pass across the
  receiver coil
• this induces a Voltage in the coil

THIS VOLTAGE:
- is the MR signal

Question 9

9. What happens after the MR signal has been created?

Answer 9

• the Radio Frequency pulse will be removed

THE NET MAGNETISATION VECTOR:
- will try to align with the external magnetic field
- it will try to go back into the Longitudinal Direction
- the voltage will be reduced
- this is because the signal in the Receiver Coil begins to
decrease

Question 10

10. Why does the signal in the Receiver Coil begin to decrease?

Answer 10

• the In-Phase component of the Net Magnetisation
  Vector begins to decrease
• the Net Magnetisation Vector is still in the Transverse
  plane at this point

Question 11

11. What happens when the in-phase component of the Net Magnetisation Vector begins to decrease?

Answer 11

• the amplitude of the voltage will decrease
• this is called the Free Induction Decay (FID)

Question 12

12. Why is this process called the Free Induction Decay?

Answer 12

1. IT IS FREE
   - because of the absence of the Radio Frequency Pulse
2. IT IS A TYPE OF INDUCTION DECAY
   - because of the decay of the Induced signal in the
   Receiver Coil