Spin-echo micro-MRI of trabecular bone using improved 3D fast large-angle spin-echo (FLASE)

Fast large-angle spin echo (FLASE) is a common pulse sequence designed for quantitative imaging of trabecular bone (TB) microarchitecture. However, imperfections in the nonselective phase-reversal pulse render it prone to stimulated echo artifacts. The problem is further exacerbated at isotropic resolution. Here, a substantially improved RF-spoiled FLASE sequence (sp-FLASE) is described and its performance is illustrated with data at 1.5T and 3T. Additional enhancements include navigator echoes for translational motion sensing applied in a slice parallel to the imaging slab. Whereas recent work suggests the use of fully-balanced FLASE (b-FLASE) to be advantageous from a signal-to-noise ratio (SNR) point of view, evidence is provided here that the greater robustness of sp-FLASE may outweigh the benefits of the minor SNR gain of b-FLASE for the target application of TB imaging in the distal extremities, sites of exclusively fatty marrow. Results are supported by a theoretical Bloch equation analysis and the pulse sequence dependence of the effective T(2) of triglyceride protons. Last, sp-FLASE images are shown to provide detailed and reproducible visual depiction of trabecular networks in three dimensions at both anisotropic (137 x 137 x 410 microm(3)) and isotropic (160 x 160 x 160 microm(3)) resolutions in the human distal tibia in vivo.