Gradient Echo Versus Spin Echo

Abstract

Spin echo and gradient echo pulse sequences follow similar contrast behaviour but there are some key differences. The spin echo pulse sequence uses an excitation pulse that is normally 90°, and a 180° refocusing pulse that reverses the effect of field inhomogeneities. These two attributes make the spin echo technique ideally suited when the primary goal is to achieve good image quality, with a reduced sensitivity to artefacts caused by magnetic field distortions. The gradient echo pulse sequence uses an excitation pulse with a flip angle that is normally less than 90°. This causes only a proportion of the z-magnetisation is transferred into the xy plane, allowing faster recovery of the z -magnetisation towards the equilibrium shorter repetition times. This is known as low flip angle imaging and is the reason why gradient echo pulse sequences are suited to fast imaging. The absence of the 180° refocusing pulse in the gradient echo sequence leads to a greater influence of magnetic susceptibility effects, different appearances of the chemical shift artefact and flow blood in comparison with spin echo.