GRASS

Gradient Recalled Acquisition using Steady States

GRASS/FISP sequences are among the oldest and simplest of the steady-state coherent gradient echo sequences. Although they are not as frequently used in their pure forms any more (having been largely replaced by TrueFISP and other “balanced” sequences), understanding how they create an MR signal and those signal properties are still very instructive.

GRASS/FISP may be performed in either 2D or 3D mode, usually with a short TR. Most often this technique is used to generate T2*-weighted images, although other weightings are possible.

Depending on what signals are sampled and used for image formation, steady-state sequences can be classified as follows

1. Postexcitation refocused steady-state sequences, in which only the FID (S+) component is sampled (eg, FISP [Siemens], GRASS [GE Medical Systems]