Fiber orientation-dependent white matter contrast in gradient echo MRI

Abstract

Recent studies have shown that there is a direct link between the orientation of the nerve fibers in white matter (WM) and the contrast observed in magnitude and phase images acquired using gradient echo MRI. Understanding the origin of this link is of great interest because it could offer access to a new diagnostic tool for investigating tissue microstructure. Since it has been suggested that myelin is the dominant source of this contrast, creating an accurate model for characterizing the effect of the myelin sheath on the evolution of the NMR signal is an essential step toward fully understanding WM contrast. In this study, we show by comparison of the results of simulations and experiments carried out on human subjects at 7T, that the magnitude and phase of signals acquired from WM in vivo can be accurately characterized by (i) modeling the myelin sheath as a hollow cylinder composed of material having an anisotropic magnetic susceptibility that is described by a tensor with a radially oriented principal axis, and (ii) adopting a two-pool model in which the water in the sheath has a reduced T 2 relaxation time and spin density relative to its surroundings, and also undergoes exchange. The accuracy and intrinsic simplicity of the hollow cylinder model provides a versatile framework for future exploitation of the effect of WM microstructure on gradient echo contrast in clinical MRI. G radient echo (GE) MRI is widely used in imaging the human brain, because both the phase and magnitude of the complex NMR signal measured with GE sequences can be used to create high-resolution images that show strong contrast between different types of brain tissue (1). Recent studies have shown that there is a direct link between the orientation of the nerve fibers in white matter (WM) with respect to the magnetic field and the contrast observed in magnitude and phase images (2-6). Although the origin of this link is currently not fully understood, orientation-dependent contrast is of great interest because it could offer researchers access to a new diagnostic tool for investigating tissue microstructure using MRI.It has recently been suggested that the myelin sheaths that surround axons are the dominant source of WM contrast in GE MRI (7,8). Creating an accurate model for characterizing the effect of the myelin sheath on the evolution of the magnitude and phase of the NMR signal is consequently an essential step toward fully understanding WM contrast and its relationship to fiber orientation. Such a model must incorporate two main features: (i) a representation of the microscopic spatial variation of resonant frequency, due to the myelin compartment-isotropic and anisotropic magnetic susceptibility effects (2, 9, 10) and chemical exchange of protons between water and macromolecules (11, 12), have been proposed as mechanisms through which myelin could perturb the resonant frequency in WM; (ii) a signal-weighting scheme to account for the reduced T 2 relaxation time constant of the myelin water relative to that of water found outs...

Combining phase images from array coils using a short echo time reference scan (COMPOSER)

“…Neglecting small nonlinear effects caused by multiple water compartments and white matter anisotropy 29, incompletely compensated flow, as well as wraps and noise, the total measured phase θ l in the l th coil at TE , is ${\theta }_l=2\pi \gamma \Delta B_0{T}_E+{\theta }_{0,l},$where Δ B 0 is the local deviation from the static magnetic field, γ is the gyromagnetic ratio, and ${\theta }_{0,l}$ is the phase offset. When the assumption of linearity holds, a multi‐echo acquisition allows ${\theta }_{0,l}$ to be calculated from two or more measurements of ${\theta }_l$, although these need to be unwrapped 26.…”

Effects of white matter microstructure on phase and susceptibility maps

“…The derivation of the Fourier expression in Eq. [4], which can be used to calculate the field due to a zero-trace, cylindrically symmetric susceptibility tensor is described in our previous work (19,39). For this approach to be valid, the sample must have homogenous magnetic properties: That is, the values of x I and x A , as well as the direction of the principal axis of the susceptibility tensor, must be uniform over the entire sample.…”

“…where x I and x A are scalar values describing the isotropic and anisotropic components of a cylindrically symmetric susceptibility tensor (19), x , representing the magnetic properties of the sample relative to the surrounding medium such that where x jj and x ? are the magnetic susceptibility of the sample parallel and perpendicular to the principal axis of the cylindrically symmetric susceptibility tensor.…”

“…Recently, however, several research groups have described additional mechanisms that could give rise to phase contrast in GE images: (i) exchange processes (15); (ii) nuclear magnetic resonance (NMR)-invisible microstructure (16); (iii) anisotropic magnetic susceptibility (17,18). Despite an increasing effort to quantify the contributions of each of these contrast mechanisms to phase measurements (19)(20)(21)(22)(23), the effect that these contributions have on susceptibility maps, calculated using QSM methods that assume isotropic magnetic susceptibility is the only cause of phase variation, has not been characterized. Only by understanding the impact of the additional phase contrast mechanisms on susceptibility mapping can the true value of QSM and other related methods be fully appreciated.…”

“…Recent studies have suggested that the source of the anisotropic susceptibility of WM is the myelin sheath (19,(30)(31)(32). This is composed of multiple lipid bilayers in which the lipid chains are radially oriented with respect to the approximately cylindrical sheath.…”