Quantitative mapping of the per-axon diffusion coefficients in brain white matter

  • Kaden E
  • Kruggel F
  • Alexander D
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PURPOSE: This article presents a simple method for estimating
the effective diffusion coefficients parallel and
perpendicular to the axons unconfounded by the intravoxel
fiber orientation distribution. We also call these parameters
the per-axon or microscopic diffusion coefficients. THEORY AND
METHODS: Diffusion MR imaging is used to probe the underlying
tissue material. The key observation is that for a fixed
b-value the spherical mean of the diffusion signal over the
gradient directions does not depend on the axon orientation
distribution. By exploiting this invariance property, we
propose a simple, fast, and robust estimator of the per-axon
diffusion coefficients, which we refer to as the spherical
mean technique. RESULTS: We demonstrate quantitative maps of
the axon-scale diffusion process, which has factored out the
effects due to fiber dispersion and crossing, in human brain
white matter. These microscopic diffusion coefficients are
estimated in vivo using a widely available off-the-shelf pulse
sequence featuring multiple b-shells and high-angular gradient
resolution. CONCLUSION: The estimation of the per-axon
diffusion coefficients is essential for the accurate recovery
of the fiber orientation distribution. In addition, the
spherical mean technique enables us to discriminate
microscopic tissue features from fiber dispersion, which
potentially improves the sensitivity and/or specificity to
various neurological conditions. Magnetic Resonance in
Medicine published by Wiley Periodicals, Inc.

Author-supplied keywords

  • diffusion MR imaging; fiber orientation distributi

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  • Enrico Kaden

  • Frithjof Kruggel

  • Daniel C Alexander

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