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

Authors

  • J.F. Magland,

    Corresponding author
    1. Laboratory for Structural NMR Imaging, Department of Radiology, University of Pennsylvania Medical Center, Philadelphia, Pennsylvania, USA
    • 1 Founders, University of Pennsylvania Medical Center, 3400 Spruce Street Philadelphia, PA 19104
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  • M.J. Wald,

    1. Laboratory for Structural NMR Imaging, Department of Radiology, University of Pennsylvania Medical Center, Philadelphia, Pennsylvania, USA
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  • F.W. Wehrli

    1. Laboratory for Structural NMR Imaging, Department of Radiology, University of Pennsylvania Medical Center, Philadelphia, Pennsylvania, USA
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  • This effect of readout bandwidth on SNR efficiency is sometimes overlooked in the literature. For example, in Ref.7, the authors state that for b-SSFP, the signal magnitude is largely independent of TR (when TR << T2), and therefore the shortest possible TR should be used. However, it is to be noted that when the acquisition time per repetition is constrained by TR, true SNR efficiency does not increase with decreased TR since the bandwidth must be increased proportional to the decrease in TR.

Abstract

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 T2 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 × 137 × 410 μm3) and isotropic (160 × 160 × 160 μm3) resolutions in the human distal tibia in vivo. Magn Reson Med, 2009. © 2009 Wiley-Liss, Inc.

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