Traditional Posters: Diffusion & Perfusion - ismrm

1563. The Extracellular Diffusion Weighted Signal Predicts Axon Diameter Distribution
Parameters
Hubert Martinus Fonteijn 1 , Matt G. Hall 1 , Daniel C. Alexander 1
1 Computer Science, Centre for Medical Image Computing, London, United Kingdom
The estimation of axon diameter distribution parameters remains a big challenge for diffusion-weighted imaging. Generally, only
intracellular diffusion is considered to be influenced by axon diameter. Extracellular diffusion on the other hand is considered to be
approximately Gaussian in the long diffusion time limit and to be independent of axon diameter. In this abstract, we perform Monte
Carlo simulations of diffusion in the extracellular compartment for a wide range of diffusion times and we construct a non-parametric
model of extracellular diffusion using Gaussian Process Regression. We then show that axon diameter distribution parameters can be
estimated from this model.
1564. Polynomial Models of the Spatial Variation of Axon Radius in White Matter
Gemma Louise Morgan 1 , Rexford D. Newbould 2 , Brandon Whitcher 2 , Daniel C.
Alexander 1
1 Centre for Medical Image Computing, University College London, London, United Kingdom; 2 Clinical
Imaging Centre, GlaxoSmithKline, London, United Kingdom
Axon radius r is a potentially useful clinical biomarker that can be derived from diffusion weighted imaging. However its estimation
in a clinical setting is hampered by poor signal-to-noise ratio and limited sensitivity to small axon radii at low gradient strengths. In
this study we introduce a technique for estimating a mean radius index ρ that exploits the spatial coherence of axon radii across the
corpus callosum. Specifically, we fit a polynomial model of the spatial variation of ρ. This significantly reduces the total number of
parameters to estimate and provides sensitivity to axon radius, even at typical clinical gradient strengths.
1565. Can AxCaliber Be Extended to Estimate Axonal Radius and Orientation at the
Same Time?
Jaime E. Cisternas 1
1 Engineering and Applied Sciences, Universidad de los Andes, Santiago, RM, Chile
Diffusion tensor MRI provides biomarkers that have been shown to indicate microstructural features in the brain and other organs.
These biomarkers, even though contain information about development, ageing and disease progression, lack specificity and don't give
direct measures of axon density and radius. Several approaches, within the framework of diffusion weighted MR, have been proposed
to extract radii, assuming previous knowledge of the orientation of the axons. In this work we extend AxCaliber, to measure axon
diameter distribution along multiple orientations, and use numerical simulations to evaluate the capacity of the model to estimate
radius and orientation reliably under the presence of noise.
1566. The Effect of Beading and Permeable Axons on Water Diffusion Properties: A
Monte Carlo Simulation of Axonal Degeneration and Its Effect on DTI and Q-Space
Contrasts
Jonathan Andrew David Farrell 1,2 , Bennett A. Landman 3,4 , Jiangyang Zhang 1 , Seth A.
Smith 5,6 , Daniel S. Reich 1,7 , Peter A. Calabresi 8 , Peter C.M. van Zijl 1,2
1 Dept. of Radiology, Johns Hopkins University School of Medicine, Baltimore, MD, United States; 2 Kennedy
Krieger Institute, F.M. Kirby Research Center for Functional Brain Imaging, Baltimore, MD, United States;
3 Biomedical Engineering, Johns Hopkins University School of Medicine, Baltimore, MD, United States;
4 Electrical Engineering, Vanderbilt University, Nashville, TN, United States; 5 Dept. of Radiology, Vanderbilt
University, Nashville, TN, United States; 6 Institute of Imaging Science, Vanderbilt University, Nashville, TN,
United States; 7 Neuroimmunology Branch (NINDS), National Institutes of Health, Bethesda, MD, United
States; 8 Dept. of Neurology, Johns Hopkins University School of Medicine, Baltimore, MD, United States
Axonal injury can produce constrictions and enlargements (“beading”) of axon membranes and increase their permeability. Here we
investigate the effect of these morphological parameters on diffusion properties measured with diffusion tensor and q-space imaging.
Degenerating axons are modeled as the union of cylinders and spheres of varying radii. Using Monte Carlo simulations, with intraand
extra-cellular compartments, we show that beading and increased permeability can act in concert to produce increased
perpendicular diffusion. However, while parallel diffusion is decreased by beading, non-Gaussian behavior is mitigated by increased
permeability. This study may aid the development of contrasts specific for axonal injury.
1567. Diffusion MRI on Undulating Versus Straight Axons: Reduced Fractional
Anisotropy and Increased Apparent Axonal Diameter
Håkan Hagslätt 1,2 , Markus Nilsson 3 , Henrik Hansson 3 , Jimmy Lätt 1,3 , Danielle van
Westen 1,2
1 Center for Medical Imaging and Physiology, Lund University Hospital, Lund, Sweden; 2 Department of
Diagnostic Radiology, Lund University Hospital, Lund, Sweden; 3 Department of Medical Radiation Physics,
Lund University, Lund, Sweden
Axons in fibre tracts may be non-straight and have an undulating, approximately sinusoidal course. It is known that axonal
undulations are present in the peripheral nervous system and in some parts of the central nervous system that are subjected to strain
during locomotion, for instance, the optic nerve. These undulations might affect parameters estimated using diffusion MRI, such as the
fractional anisotropy. Furthermore, measurements attempting to estimate the axonal sizes might be biast towards an overestimated
axonal size when undulations are present.