Biophysical mechanisms of myelin‐induced water frequency shifts

Yablonskiy, Dmitriy A.; Sukstanskiı̆, A. L.

doi:10.1002/mrm.25214

Cited by 30 publications

(58 citation statements)

References 18 publications

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“…This observation was explained not only by the isotropic magnetic susceptibility of myelin but also by another myelin susceptibility property, susceptibility anisotropy, discovered in separate studies described below . To further understand the characteristics of the frequency shifts and T 2 * anisotropy of white matter, research has been continued to develop more realistic models, such as large numbers of hollow cylinders, realistic fiber shapes, and multiple layers of myelin sheaths . Using the T 2 * anisotropy, a new fiber orientation mapping method has been proposed as a potential application of the observation .…”

Section: Sources Of T2* and T2* Anisotropy In White Mattermentioning

confidence: 99%

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Mechanisms of T₂* anisotropy and gradient echo myelin water imaging

et al. 2016

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In MRI, structurally aligned molecular or micro-organization (e.g. axonal fibers) can be a source of substantial signal variations that depend on the structural orientation and the applied magnetic field. This signal anisotropy gives us a unique opportunity to explore information that exists at a resolution several orders of magnitude smaller than that of typical MRI. In this review, one of the signal anisotropies, T * anisotropy in white matter, and a related imaging method, gradient echo myelin water imaging (GRE-MWI), are explored. The T * anisotropy has been attributed to isotropic and anisotropic magnetic susceptibility of myelin and compartmentalized microstructure of white matter fibers (i.e. axonal, myelin, and extracellular space). The susceptibility and microstructure create magnetic frequency shifts that change with the relative orientation of the fiber and the main magnetic field, generating the T * anisotropy. The resulting multi-component magnitude decay and nonlinear phase evolution have been utilized for GRE-MWI, assisting in resolving the signal fraction of the multiple compartments in white matter. The GRE-MWI method has been further improved by signal compensation techniques including physiological noise compensation schemes. The T * anisotropy and GRE-MWI provide microstructural information on a voxel (e.g. fiber orientation and tissue composition), and may serve as sensitive biomarkers for microstructural changes in the brain. Copyright © 2016 John Wiley & Sons, Ltd.

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Section: Sources Of T2* and T2* Anisotropy In White Mattermentioning

confidence: 99%

“…The interaction of the structure and neighboring water molecules may become substantially different when the relative orientation between the structure and the B 0 field is altered. This difference in interaction may generate the susceptibility anisotropy .…”

Section: Sources Of T2* and T2* Anisotropy In White Mattermentioning

confidence: 99%

Mechanisms of T₂* anisotropy and gradient echo myelin water imaging

et al. 2016

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“…With this model, the change of signal according to fiber orientation can be investigated. Recently, several studies have been conducted to suggest more realistic models . For example, in Ref 37, the microscopic effects caused by the heterogeneous architecture of the myelin sheath or varing water distributions within the myelin sheath are modeled .…”

Section: Discussionmentioning

confidence: 99%

“…For example, in Ref 37, the microscopic effects caused by the heterogeneous architecture of the myelin sheath or varing water distributions within the myelin sheath are modeled . In adittion, several researchers considered different fiber shapes, number of fibers, and multiple layers of myelin sheaths . Such modeling can represent more realistic conditions and can have the advantage of including additional effects in the model.…”

Section: Discussionmentioning

confidence: 99%

Echo time‐range effects on gradient‐echo based myelin water fraction mapping at 3T

Lee

Nam

Kim

2018

Magnetic Resonance in Med

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Purpose To investigate the effect of the range of TE on multi‐echo gradient echo–based myelin water fraction (MWF) mapping at 3 T. Methods Myelin water fraction estimation was performed using 3 widely used models: magnitude multipool, magnitude 3‐pool, and complex 3‐pool model. Simulations were conducted using a hollow cylinder model with susceptibility anisotropy. The TE range was varied, and its effect on MWF estimation was observed. The same analysis was also performed with in vivo data from 4 healthy volunteers. Results Different MWFs were obtained depending on the range of TE data acquired. In the simulations, the results of both magnitude models had a bias with respect to the true values: a maximum bias of up to 20% for the multipool model, and a maximum bias of up to 8% for the 3‐pool model. This bias was reduced using the complex 3‐pool model (maximum bias was under 2%). These effects were also found in the in vivo data. The MWF values using the complex model was most stable with respect to the TE range. Conclusion The results of the simulation and in vivo data suggest that MWF values fitted using magnitude models are more influenced by the TE range collected for fitting than using the complex model. The complex model is helpful in mitigating bias due to dependencies over the TE range selection.

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“…In a subsequent publication , Yablonskiy and Sukstanskii extended their model to account for diffusion of water between myelin and intermyelin layers. In this case, there is a significant difference between our model and theirs.…”

Section: Discussionmentioning

confidence: 99%

Heterogeneous anisotropic magnetic susceptibility of the myelin‐water layers causes local magnetic field perturbations in axons

2016

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One goal of magnetic resonance imaging (MRI) is to determine the myelin water fraction in neural tissue. One approach is to measure the reduction in T2* arising from microscopic perturbations in the magnetic field caused by heterogeneities in the magnetic susceptibility of myelin. In this paper, analytic expressions for the induced magnetic field distribution are derived within and around an axon assuming the myelin susceptibility is anisotropic. Previous models considered the susceptibility to be piecewise continuous, whereas this model considers a sinusoidally varying susceptibility. Many conclusions are common in both models. When the magnetic field is applied perpendicular to the axon, the magnetic field in the intraaxonal space is uniformly perturbed, the magnetic field in the myelin sheath oscillates between the lipid and water layers, and the magnetic field in the extracellular space just outside the myelin sheath is heterogeneous. These field heterogeneities cause the spins to dephase, shortening T2*. When the magnetic field is applied along the axon, the field is homogeneous within water-filled regions, including between lipid layers. Therefore the spins do not dephase and the magnetic susceptibility has no effect on T2*. Generally, the response of an axon is given as the superposition of these two contributions. The sinusoidal model uses a different set of approximations compared to the piecewise model, so their common predictions indicate that the models are not too sensitive to the details of the myelin-water distribution. Other predictions, such as the sensitivity to water diffusion between myelin and water layers, may highlight differences between the two approaches.

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Biophysical mechanisms of myelin‐induced water frequency shifts

Cited by 30 publications

References 18 publications

Mechanisms of T₂* anisotropy and gradient echo myelin water imaging

Mechanisms of T₂* anisotropy and gradient echo myelin water imaging

Echo time‐range effects on gradient‐echo based myelin water fraction mapping at 3T

Heterogeneous anisotropic magnetic susceptibility of the myelin‐water layers causes local magnetic field perturbations in axons

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Biophysical mechanisms of myelin‐induced water frequency shifts

Cited by 30 publications

References 18 publications

Mechanisms of T2* anisotropy and gradient echo myelin water imaging

Mechanisms of T2* anisotropy and gradient echo myelin water imaging

Echo time‐range effects on gradient‐echo based myelin water fraction mapping at 3T

Heterogeneous anisotropic magnetic susceptibility of the myelin‐water layers causes local magnetic field perturbations in axons

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Mechanisms of T₂* anisotropy and gradient echo myelin water imaging

Mechanisms of T₂* anisotropy and gradient echo myelin water imaging