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Axonal T2 estimation using the spherical variance of the strongly diffusion-weighted MRI signal

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author Pizzolato Marco, Andersson Mariam, Canales-Rodríguez Erick Jorge, Thiran Jean-Philippe, Dyrby Tim B.,
Project Exploring brain communication pathways by combining diffusion based quantitative structural connectivity and EEG source imaging : application to physiological and epileptic networks
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Original article (peer-reviewed)

Journal Magnetic Resonance Imaging
Publisher Elsevier
Volume (Issue) 86
Page(s) 118 - 134
Title of proceedings Magnetic Resonance Imaging
DOI 10.1016/j.mri.2021.11.012

Open Access

URL https://doi.org/10.1016/j.mri.2021.11.012
Type of Open Access Publisher (Gold Open Access)

Abstract

In magnetic resonance imaging, the application of a strong diffusion weighting suppresses the signal contributions from the less diffusion-restricted constituents of the brain's white matter, thus enabling the estimation of the transverse relaxation time T2 that arises from the more diffusion-restricted constituents such as the axons. However, the presence of cell nuclei and vacuoles can confound the estimation of the axonal T2, as diffusion within those structures is also restricted, causing the corresponding signal to survive the strong diffusion weighting. We devise an estimator of the axonal T2 based on the directional spherical variance of the strongly diffusion-weighted signal. The spherical variance T2 estimates are insensitive to the presence of isotropic contributions to the signal like those provided by cell nuclei and vacuoles. We show that with a strong diffusion weighting these estimates differ from those obtained using the directional spherical mean of the signal which contains both axonal and isotropically-restricted contributions. Our findings hint at the presence of an MRI-visible isotropically-restricted contribution to the signal in the white matter ex vivo fixed tissue (monkey) at 7T, and do not allow us to discard such a possibility also for in vivo human data collected with a clinical 3T system.
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