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

Marco Pizzolato,Mariam Andersson,Erick Jorge Canales-Rodríguez,Jean-Philippe Thiran,Tim B Dyrby

doi:10.1016/j.mri.2021.11.012

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.

Highlights

One of the fundamental goals in magnetic resonance imaging (MRI) is the production of brain maps that provide, at each voxel, the quantification of the proportions and of the biophysical characteristics of the constituents of the brain’s tissue microstructure, such as neuronal cell bodies, axons, myelin, oligodendrocytes, astrocytes, etc.(Alexander, Dyrby, Nilsson and Zhang, 2019)
Simulations The experiments based on synthetic data are designed to illustrate the potential bias arising from estimating of the axonal transverse relaxation time 2a based on the spherical mean in the presence of an isotropically-restricted compartment, and to evaluate the robustness of the estimates based on the spherical variance
The histograms shown in fig. 2 reveal that the spherical mean ( 2m) estimates are centered around a value in between the axonal transverse relaxation time and that of the isotropically-restricted compartment

Summary

Introduction

One of the fundamental goals in magnetic resonance imaging (MRI) is the production of brain maps that provide, at each voxel, the quantification of the proportions and of the biophysical characteristics of the constituents of the brain’s tissue microstructure, such as neuronal cell bodies, axons, myelin, oligodendrocytes, astrocytes, etc.(Alexander, Dyrby, Nilsson and Zhang, 2019). This quantification holds the promise of identifying biomarkers that are sensitive to the presence and type of alterations and/or pathology. Since diffusion inside of axons is assumed to be more restricted than that outside of them (extra-axonal), a “sufficiently high” b-value leads to a diffusion signal for which the spherical mean over the acquired diffusion gradient directions mainly contains signal contributions from spin-bearing water particles inside of axons

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Discussion

Conclusion