Which pulse is used for refocusing in a spin echo pulse sequence?

In a spin echo pulse sequence, the 180-degree RF pulse is critical for refocusing the spins that have dephased over time. When the initial 90-degree pulse is applied, it tips the net magnetization into the transverse plane, where the spins start to precess and gradually lose coherence due to magnetic field inhomogeneities or differences in local magnetic environments. This dephasing is what leads to the loss of signal in the transverse plane over time.

The 180-degree RF pulse is then applied at a specific time after the initial 90-degree pulse, which inverts the spins. By reversing the phase of the spins, this pulse effectively causes the dephased spins to re-align and refocus at a later time, resulting in the generation of the echo signal. The echo is what ultimately contributes to the MR image, making the 180-degree pulse essential for achieving a spin echo.

The other types of RF pulses are not used for this purpose in the context of a spin echo sequence. For instance, a 270-degree pulse does not serve a standard function in most spin echo sequences, and a 360-degree pulse, which effectively offers a full rotation of the magnetization back to its original state, is unnecessary for ref