GRASS is a steady-state gradient echo technique that uses the entire combined signal from both the FID and the echo pathways to create an MR image. The gradient pulses in GRASS are balanced to ensure the maximum signal from the T2 echo pathways is collected. For long TR, the contribution to the signal from the T2 echo pathways is minimal, and image contrast for GRASS approaches that of the SPGR sequence discussed below.

GRASS is most useful for two short-TR applications: (1) to provide high SNR, T2*-weighted images, and (2) to selectively enhance fluids. Because all coherence pathways are allowed to contribute to the signal, under favorable conditions of a very homogeneous magnetic field, the steady-state signal can be quite large in comparison with the SPGR signal. T2* weighting is obtained because the images are acquired as gradient echo rather than spin echo images, so longer TEs can be used to increase the T2* weighting of the image. The second application is based on the driven equilibrium principle whereby alternating 90° pulses (i.e. flip angles of 90°, (-90°), 90°, (-90°)....) are applied successively at intervals of TR. In the presence of a homogeneous magnetic field, each 90° pulse tips all the regrown longitudinal magnetization into the transverse plane, of which whatever is left after T2 decay during TR is then transferred back to the longitudinal direction at the end of each TR. For TR << T2 and T1, it can be shown that the signal is then dependent on the ratio of T2/T1. Since T2 is comparable to T1 for fluids and much shorter for other tissues, this leads to selective fluid enhancement.

	GRASS is most often used in orthopedic imaging to provide T2* contrast for selected applications (Fig. 1.25). On most GRASS sequencing there is poor contrast between fat and muscle, but good contrast between tendons, ligaments and surrounding soft tissue. Bone trabecular patterns are accentuated on this sequence due to T2* effects.
(click on image for enlarged view) Figure 1.25 GRASS image of a knee acquired using T2* weighting. Note the decreased signal from bone relative to cartilage. This is due to T2* effects from susceptibility gradients around the trabeculae.