A rapid steady-state 3D spin-echo imaging pulse sequence, based on the
principle of nutating the spins by an angle greater than 90 degrees,
has been designed and implemented on a clinical 1.5-T whole-body MR sc
anner. The pulse sequence, denoted fast large-angle spin-echo (FLASE),
has been optimized for high-resolution imaging of tissues with short
T-2 and T-2. Features of FLASE include a minimum-phase Shinnar-Le Rou
x excitation pulse and distribution of phase- and slice-encoding gradi
ents before and after the 180 degrees refocusing pulse to minimize the
critical time delay between inversion and restoration of the residual
longitudinal magnetization and for minimizing echo time, A Bloch equa
tion analysis, corroborated by experimental data, shows FLASE signal-t
o-noise to be superior to its closest analog, 3D rapid spin-echo excit
ation (RASEE) (Jara et al., Magn Reson Medicine 29,528 (1993)), and 3D
gradient-recalled acquisition in steady state (GRASS). It is demonstr
ated that with judicious RF phase-cycling and steady state operation,
FLASE can produce high-quality microimages free of intravoxel phase di
spersion from susceptibility-induced background gradients. The perform
ance of the method is exemplified with ultra high-resolution images of
trabecular bone in vitro and in vivo in the human calcaneus and wrist
at voxel sizes as low as 98 x 98 x 200 mu m(3). Finally, the contrast
behavior of refocused FLASE can be altered by disrupting the steady s
tate analogous to gradient echo imaging.