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Quantification of microscopic anisotropy with diffusion MRI

Markus Nilsson1*, Filip Szczepankiewicz2 and Daniel Topgaard3
  • 1 Lund University, Lund University Bioimaging Center, Sweden
  • 2 Lund University, Dept. of Medical Radiation Physics, Sweden
  • 3 Lund University, Dept. of Chemistry, Sweden

The anisotropy of water diffusion in brain tissue can be quantified by means of the fractional anisotropy (FA) using diffusion tensor imaging (DTI). Studies of brain plasticity have reported elevated FA values in white matter as a response to learning. Such observations indicate that FA reflects properties of axons, in contexts where the diffusion is anisotropic. However, FA is also sensitive to the orientation dispersion of axons. Here we report on efforts to disentangle these effects by using a combination of isotropic and anisotropic diffusion encoding. The result is a parameter denoted ‘microscopic fractional anisotropy’ (μFA). In white matter, values of μFA are uniform and high, but in grey matter, the values are low. In regions of crossing fibres, μFA has been observed to be high and FA low. Using simulations, we demonstrate the response of FA and μFA to changes in micro-geometry, such as fibre crossing angle. We conclude that FA from DTI reflects both the diffusion anisotropy and orientation dispersion. However, microscopic anisotropy and orientation dispersion can be quantified separately, which allows for more specific analyses of microstructural change.

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Acknowledgements

This research was supported by the Swedish Foundation for Strategic Research (grant no. AM13-0090).

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Keywords: Diffusion Tensor Imaging, Anisotropy, Magic angle spinning, White Matter Integrity, microscopic anisotropy, diffusion MRI, plasticity

Conference: Microstructures of Learning: Novel methods and approaches for assessing structural and functional changes underlying knowledge acquisition in the brain, Lund, Sweden, 23 May - 23 May, 2014.

Presentation Type: Oral presentation

Topic: Neuroscience

Citation: Nilsson M, Szczepankiewicz F and Topgaard D (2015). Quantification of microscopic anisotropy with diffusion MRI. Front. Neurosci. Conference Abstract: Microstructures of Learning: Novel methods and approaches for assessing structural and functional changes underlying knowledge acquisition in the brain. doi: 10.3389/conf.fnins.2015.88.00004

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Received: 04 Mar 2015; Published Online: 05 Mar 2015.

* Correspondence: Dr. Markus Nilsson, Lund University, Lund University Bioimaging Center, Lund, Sweden, Markus.Nilsson@med.lu.se