GRE

The GRE family of pulse sequences uses gradient reversal to create an echo. They can be acquired in either 2D, 3D, or Cine mode.

The GRE pulse sequence reverses gradient polarity to rephase protons and form echoes. This is unlike conventional SE imaging, which involves the use of a 180° RF pulse to refocus the echo and generally entails relatively long TRs for longitudinal relaxation of the spins. GRE sequences use a variable excitation pulse (less than 90° flip angle) to create magnetization in the transverse plane, followed by a gradient reversal pulse, which re-phases the magnetization to produce gradient echo signals.

GRE sequences can produce T1, T2, and PD-weighted images in shorter scan times than SE and FSE. Unfortunately, they are more sensitive to magnetic field inhomogeneities and paramagnetics than SE and FSE because of gradient rephasing. Gradient rephasing does not eliminate the effects of T2* dephasing. Air/tissue interfaces and bone/tissue interfaces, where the tissues are magnetized to different degrees, experience magnetic susceptibility artifacts.