Tipping the Net Magnetization Vector from Equilibrium

The nuclear precession (Larmor) frequency is the same frequency as that of photons that can make the spins of the ensemble undergo transitions between spin states. 

The precession of the net magnetization vector M at the Larmor frequency (500 MHz in the preceding example) gives a clue as to how RF can be used to tip the vector from its equilibrium position. 

Electromagnetic radiation consists of a stream of photons. Each photon is made up of an electric field component and a magnetic field component, and these two components are mutually perpendicular. The frequency of a photon determines how fast the electric field component and magnetic field component will pulse, or beat. Radio-frequency electromagnetic radiation at 500 MHz will thus have a magnetic field component that beats 500 million times a second, by definition. 

RF pulse. The abrupt turning on of a sinusoidal waveform with a specific phase for a specific duration, followed by the abrupt turning off of the sinusoidal waveform. 

If we do not have a well-timed application of the magnetic field component from our RF, then the net magnetization vector will not be effective in tipping the net magnetization vector. In particular, if the RF frequency is not just randomly mistimed but is consistently higher or lower than the Larmor frequency, the errors between when the push should and does occiu will accumulate before too long our pushes will actually serve to decrease the amplitude of the net magnetization vector M s departure from equilibrium. The accumulated error caused by poorly synchronized beats of RF with respect to the Larmor frequency of the spins is well known to NMR spectroscopists and is called pulse roll-off. 

---