title: Microstructural parameter estimation in vivo using diffusion MRI and structured prior information.
abstract: OBJECTIVE
  Diffusion MRI has recently been used with detailed models to probe tissue microstructure. Much of this work has been performed ex vivo with powerful scanner hardware, to gain sensitivity to parameters such as axon radius. By contrast, performing microstructure imaging on clinical scanners is extremely challenging.
  
  
  METHODS
  We use an optimized dual spin-echo diffusion protocol, and a Bayesian fitting approach, to obtain reproducible contrast (histogram overlap of up to 92%) in estimated maps of axon radius index in healthy adults at a modest, widely-available gradient strength (35 mT m(-1)). A key innovation is the use of influential priors.
  
  
  RESULTS
  We demonstrate that our priors can improve precision in axon radius estimates--a 7-fold reduction in voxelwise coefficient of variation in vivo--without significant bias. Our results may reflect true axon radius differences between white matter regions, but this interpretation should be treated with caution due to the complexity of the tissue relative to our model.
  
  
  CONCLUSIONS
  Some sensitivity to relatively large axons (3-15 μm) may be available at clinical field and gradient strengths. Future applications at higher gradient strength will benefit from the favorable eddy current properties of the dual spin-echo sequence, and greater precision available with suitable priors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance.
authors.affiliation: UCL Institute of Child Health, University College London, London, UK.
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Clayden
authors.affiliation: Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, University College London, London, UK.|Laboratory for Social and Neural Systems Research, University of Zurich, Zurich, Switzerland.
authors.forename: Zoltan
authors.initials: Z
authors.lastname: Nagy
authors.affiliation: Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, University College London, London, UK.
authors.forename: Nikolaus
authors.initials: N
authors.lastname: Weiskopf
authors.affiliation: Centre for Medical Image Computing, University College London, London, UK.
authors.forename: Daniel C
authors.initials: DC
authors.lastname: Alexander
authors.affiliation: UCL Institute of Child Health, University College London, London, UK.
authors.forename: Chris A
authors.initials: CA
authors.lastname: Clark
mesh_terms: D000328:Adult; D001499:Bayes Theorem; D003337:Corpus Callosum / Q000000981:diagnostic imaging; D038524:Diffusion Magnetic Resonance Imaging / Q000379:methods*; D005260:Female; D006801:Humans; D008297:Male; D009010:Monte Carlo Method; D012815:Signal Processing, Computer-Assisted; D055815:Young Adult
keywords: Bayesian; MCMC; dual spin-echo; microstructure; prior; twice-refocused spin-echo