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180° RF pulse

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:

• Once the race starts (the relaxation begins), the turtle and the rabbit are at the same place (the starting line): they are in phase.
• As the rabbit runs faster, there is a distance between him and the turtle: they dephase.
• Then both have to turn around and go back (180° RF pulse)
• Assuming they are both going at the same speed as before, they arrive at the same time at the starting (finish) line : they rephase.

The 180° RF pulse restores phase coherence:

• After the 90° RF pulse spins dephase (during a time defined as TE/2)
• After the 180° RF pulse, spins are back in phase at time TE after the 90° RF pulse
• Then they dephase again.

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.