FAST 3D LARGE-ANGLE SPIN-ECHO IMAGING (3D FLASE)

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.