text: A gradient echo sequence does not use a 180 degrees RF pulse to make the spins of particles coherent. Instead, it uses magnetic gradients to manipulate the spins, allowing the spins to dephase and rephase when required. After an excitation pulse, the spins are dephased, no signal is produced because the spins are not coherent. When the spins are rephased, they become coherent, and thus signal (or "echo") is generated to form images. Unlike spin echo, gradient echo does not need to wait for transverse magnetisation to decay completely before initiating another sequence, thus it requires very short repetition times (TR), and therefore to acquire images in a short time. After echo is formed, some transverse magnetisations remains. Manipulating gradients during this time will produce images with different contrast. There are three main methods of manipulating contrast at this stage, namely steady-state free-precession (SSFP) that does not spoil the remaining transverse magnetisation, but attempts to recover them (thus producing T2-weighted images); the sequence with spoiler gradient that averages the transverse magnetisations (thus producing mixed T1 and T2-weighted images), and RF spoiler that vary the phases of RF pulse to eliminates the transverse magnetisation, thus producing pure T1-weighted images.
  For comparison purposes, the repetition time of a gradient echo sequence is of the order of 3 milliseconds, versus about 30 ms of a spin echo sequence.