Fast 3D large-angle spin-echo imaging (3D FLASE)

✍ Scribed by Jingfei Ma; Felix W. Wehrli; Hee Kwon Song

• Publisher: John Wiley and Sons
• Year: 1996
• Tongue: English
• Weight: 881 KB
• Volume: 35
• Category: Article
• ISSN: 0740-3194
• DOI: 10.1002/mrm.1910350619

✦ Synopsis

Abstract

A rapid steady‐state 3D spin‐echo imaging pulse sequence, based on the principle of nutating the spins by an angle greater than 90°, has been designed and implemented on a clinical 1.5‐T whole‐body MR scanner. 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 Roux excitation pulse and distribution of phase‐ and slice‐encoding gradients before and after the 180° refocusing pulse to minimize the critical time delay between inversion and restoration of the residual longitudinal magnetization and for minimizing echo time. A Bloch equation analysis, corroborated by experimental data, shows FLASE signal‐to‐noise to be superior to its closest analog, 3D rapid spin‐echo excitation (RASEE) (Jara et al., Magn Reson Medicine 29, 528 (1993)), and 3D gradient‐recalled acquisition in steady state (GRASS). It is demonstrated that with judicious RF phase‐cycling and steady state operation, FLASE can produce high‐quality microimages free of intravoxel phase dispersion from susceptibility‐induced background gradients. The performance 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 × 98 × 200 μm^3^. Finally, the contrast behavior of refocused FLASE can be altered by disrupting the steady state analogous to gradient echo imaging.