NPA_poster_CNS2011.lyx

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\begin_layout Title
Taking the 'T' out of DTI: model-free diffusion imaging anisotropy
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Diffusion-weighted MR imaging (dMRI) is a non-invasive technique which can
 reveal important information about the directional organisation of the
 white matter fibres of the brain.
 Local voxelwise measures based on dMRI - such as fractional anisotropy
 (FA), apparent diffusivity coefficient (ADC), or mean diffusivity (MD)
 - have been extensively adopted in clinical and applied research practice
 and have been shown to relate to a range of neurodegenerative pathologies.
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Measures such as FA, ADC and MD are based on the parametric simple diffusion
 tensor (SDT) model.
 SDT works well when there is a single dominant fibre direction.
 However SDT is also known not to give valid information if the local fibre
 organisation is more complex.
 This underlines the need to establish valid and reliable measures which
 can indicate the degree of local organisation of white matter in the brain
 in voxels where there are multiple fibre crossings as well as where there
 is a single dominant fibre direction.
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\begin_layout Standard
We show how model-free alternatives can be established which yield non-parametri
c anisotropy and directionality measures (NPA).
 These are constructed from the spin orientation distribution function (sODF).
 We apply exact analytical results which show the form of the sODF when
 the single tensor model is correct, and further indicate how the tensor's
 parameters may be estimated from this model-free approach.
 We generalise these to define the NPA measure in cases where the SDT is
 invalid.
 We compare the performance of these parametric and non-parametric measures
 for simulated and in vivo data and show that NPA is applicable over a wider
 range of white matter than DTI based measures.
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