EFFECT OF THE RF PULSE ON THE HYDROGEN PROTONS
Inside the magnet, the protons are aligned in the direction of the main magnetic field (Bo) and precess around it. If at a given time it would be possible to stop them, each would show a transverse magnetization component (TM) pointing in a different direction. It is then said that the protons are out of phase. The sum of all TM is zero.
When an RF pulse is applied (90° pulse), the magnetization vector (μ) passesfrom a longitudinal to a transverse plane. So the longitudinal magnetization (LM) decreases to a minimum value (zero) and the transverse magnetization (TM) becomes maximum because all the protons are in phase (i.e. the magnetization vectors of all the protons point in the same direction). This phenomenon is called excitation.
Once the RF pulse stops, the excited hydrogen protons release the energy and the magnetization vector (μ) returns to its initial position. The LM recovers and the TM decreases. This phase is called relaxation.
If, instead of a 90° pulse, a longer RF pulse is applied (180° pulse), the magnetization vector (μ) becomes negative (-LM) and the protons are again out of phase.
The magnetic influence of the neighboring protons (spin-spin interactions) causes a loss in synchronization with each other, leading to a rapid out of phase stage during the relaxation time. In order to recover the TM and increase the signal, a 180° RF pulse is applied. This flips the magnetization vector and all the protons recover the phase at the same time, increasing the TM.
The need for a 180° RF pulse can also be explained by the following example:
Imagine that two runners, one fast and the other slow, start a race and you want them both to reach the finish line at the same time. We have to assume that their speed is constant. After a given time, the runners have to turn back and run towards the finish line. The faster is farther away and the slower is nearer the finish line so they cross the line at the same time.