A 180° RF pulse can rephase spins and reverse static field inhomogeneities. After a 90° RF pulse, spins dephase and transverse magnetization decreases. If we apply a 180° RF pulse, spins rephase and transverse magnetization reappears.
How can a 180° RF pulse rephase spins? Consider the following race:
The 180° RF pulse restores phase coherence:
At time TE (Echo Time), the signal is not as high as the initial transverse magnetization intensity. As the 180° RF pulse reverses dephasing due to static field inhomogeneities but not spin-spin relaxation, the signal loss is due to pure T2 effect.
The signal enveloppe joining maximums of echos after 180° RF pulses is corresponding to the pure T2 decay curve.