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Turn Off the Transmitter
What Happens?

• RF energy is retransmitted
- This is the "NMR" signal
- At the resonance frequency
- Signal proportional to Proton Density

• Mz begins to recover
- Exponential recovery of Mz
- Time constant is called T1
- Longitudinal or Spin-Lattice Relaxation

• Spins (Mxy) begin to dephase
- Exponential decay of signal
- Time constant is called T2 or T2*
- Transverse or Spin-Spin Relaxation
Once the RF transmitter is turned off three things begin to happen simultaneously.
1. The absorbed RF energy is retransmitted (at the resonance frequency).
2. The excited spins begin to return to the original Mz orientation. (T1 recovery to thermal equilibrium).
3. Initially in phase, the excited protons begin to dephase (T2 and T2* relaxation).

Each of these concepts will be gone over in more detail. Read on.

Rotating Magnetic
Vector = EM Radiation

Once Mz (a magnetization vector) has been tipped away from the Z axis, the vector will continue to precess around the external Bø field at the resonance frequency wø. A rotating magnetic field produces electromagnetic radiation. Since wø is in the radio frequency portion of the electromagnetic spectrum the rotating vector is said to give off RF waves.

So, just like phosphorescent paint glows in the dark, the absorbed RF energy is now being retransmitted, thereby producing the NMR signal.


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