What is the purpose of the 180 degree refocusing pulse in a spin echo sequence?

The purpose of the 180-degree refocusing pulse in a spin echo sequence is to minimize magnetic field inhomogeneities. In an MRI scan, protons in different magnetic environments can lose coherence and dephase over time due to variations in the local magnetic field. This dephasing can lead to loss of signal and contrast in the resulting images.

The 180-degree refocusing pulse is applied after the initial excitation pulse. It effectively reverses the dephasing of the spins that has occurred due to these magnetic field inhomogeneities. By refocusing the spins, the sequence generates echoes of coherent signals, thus enhancing the image quality. This process allows for improved spatial resolution and better overall contrast of the MRI images without the complications arising from T1 relaxation or harmonic distortions caused by inhomogeneities in the magnetic field.

Understanding this mechanism is crucial for optimizing MRI protocols and ensuring high-quality imaging for diagnostic purposes.