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Time‐dependent diffusion in undulating thin fibers: Impact on axon diameter estimation

Diffusion MRI may enable non‐invasive mapping of axonal microstructure. Most approaches infer axon diameters from effects of time‐dependent diffusion on the diffusion‐weighted MR signal by modeling axons as straight cylinders. Axons do not, however, propagate in straight trajectories, and so far the...

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Autores principales: Brabec, Jan, Lasič, Samo, Nilsson, Markus
Formato: Online Artículo Texto
Lenguaje:English
Publicado: John Wiley and Sons Inc. 2019
Materias:
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7027526/
https://www.ncbi.nlm.nih.gov/pubmed/31868995
http://dx.doi.org/10.1002/nbm.4187
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author Brabec, Jan
Lasič, Samo
Nilsson, Markus
author_facet Brabec, Jan
Lasič, Samo
Nilsson, Markus
author_sort Brabec, Jan
collection PubMed
description Diffusion MRI may enable non‐invasive mapping of axonal microstructure. Most approaches infer axon diameters from effects of time‐dependent diffusion on the diffusion‐weighted MR signal by modeling axons as straight cylinders. Axons do not, however, propagate in straight trajectories, and so far the impact of the axonal trajectory on diameter estimation has been insufficiently investigated. Here, we employ a toy model of axons, which we refer to as the undulating thin fiber model, to analyze the impact of undulating trajectories on the time dependence of diffusion. We study time‐dependent diffusion in the frequency domain and characterize the diffusion spectrum by its height, width, and low‐frequency behavior (power law exponent). Results show that microscopic orientation dispersion of the thin fibers is the main parameter that determines the characteristics of the diffusion spectra. At lower frequencies (longer diffusion times), straight cylinders and undulating thin fibers can have virtually identical spectra. If the straight‐cylinder assumption is used to interpret data from undulating thin axons, the diameter is overestimated by an amount proportional to the undulation amplitude and microscopic orientation dispersion of the fibers. At higher frequencies (shorter diffusion times), spectra from cylinders and undulating thin fibers differ. The low‐frequency behavior of the spectra from the undulating thin fibers may also differ from that of cylinders, because the power law exponent of undulating fibers can reach values below 2 for experimentally relevant frequency ranges. In conclusion, we argue that the non‐straight nature of axonal trajectories should not be overlooked when analyzing and interpreting diffusion MRI data.
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spelling pubmed-70275262020-02-24 Time‐dependent diffusion in undulating thin fibers: Impact on axon diameter estimation Brabec, Jan Lasič, Samo Nilsson, Markus NMR Biomed Research Articles Diffusion MRI may enable non‐invasive mapping of axonal microstructure. Most approaches infer axon diameters from effects of time‐dependent diffusion on the diffusion‐weighted MR signal by modeling axons as straight cylinders. Axons do not, however, propagate in straight trajectories, and so far the impact of the axonal trajectory on diameter estimation has been insufficiently investigated. Here, we employ a toy model of axons, which we refer to as the undulating thin fiber model, to analyze the impact of undulating trajectories on the time dependence of diffusion. We study time‐dependent diffusion in the frequency domain and characterize the diffusion spectrum by its height, width, and low‐frequency behavior (power law exponent). Results show that microscopic orientation dispersion of the thin fibers is the main parameter that determines the characteristics of the diffusion spectra. At lower frequencies (longer diffusion times), straight cylinders and undulating thin fibers can have virtually identical spectra. If the straight‐cylinder assumption is used to interpret data from undulating thin axons, the diameter is overestimated by an amount proportional to the undulation amplitude and microscopic orientation dispersion of the fibers. At higher frequencies (shorter diffusion times), spectra from cylinders and undulating thin fibers differ. The low‐frequency behavior of the spectra from the undulating thin fibers may also differ from that of cylinders, because the power law exponent of undulating fibers can reach values below 2 for experimentally relevant frequency ranges. In conclusion, we argue that the non‐straight nature of axonal trajectories should not be overlooked when analyzing and interpreting diffusion MRI data. John Wiley and Sons Inc. 2019-12-23 2020-03 /pmc/articles/PMC7027526/ /pubmed/31868995 http://dx.doi.org/10.1002/nbm.4187 Text en © 2019 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd This is an open access article under the terms of the http://creativecommons.org/licenses/by/4.0/ License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
spellingShingle Research Articles
Brabec, Jan
Lasič, Samo
Nilsson, Markus
Time‐dependent diffusion in undulating thin fibers: Impact on axon diameter estimation
title Time‐dependent diffusion in undulating thin fibers: Impact on axon diameter estimation
title_full Time‐dependent diffusion in undulating thin fibers: Impact on axon diameter estimation
title_fullStr Time‐dependent diffusion in undulating thin fibers: Impact on axon diameter estimation
title_full_unstemmed Time‐dependent diffusion in undulating thin fibers: Impact on axon diameter estimation
title_short Time‐dependent diffusion in undulating thin fibers: Impact on axon diameter estimation
title_sort time‐dependent diffusion in undulating thin fibers: impact on axon diameter estimation
topic Research Articles
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7027526/
https://www.ncbi.nlm.nih.gov/pubmed/31868995
http://dx.doi.org/10.1002/nbm.4187
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