Orientation dependence of magnetization transfer parameters in human white matter

Abstract

Quantification of magnetization-transfer (MT) experiments is typically based on a model comprising a liquid pool "a" of free water and a semisolid pool "b" of motionally restricted macromolecules or membrane compounds. By a comprehensive fitting approach, high quality MT parameter maps of the human brain are obtained. In particular, a distinct correlation between the diffusion-tensor orientation with respect to the B0-magnetic field and the apparent transverse relaxation time, T2(b), of the semisolid pool (i.e., the width of its absorption line) is observed. This orientation dependence is quantitatively explained by a refined dipolar lineshape for pool b that explicitly considers the specific geometrical arrangement of lipid bilayers wrapped around a cylindrical axon. The model inherently reduces the myelin membrane to its lipid constituents, which is motivated by previous studies on efficient interaction sites (e.g., cholesterol or galactocerebrosides) in the myelin membrane and on the origin of ultrashort T2 signals in cerebral white matter. The agreement between MT orientation effects and corresponding forward simulations using empirical diffusion imaging results as input as well as results from fits employing the novel lineshape support previous suggestions that the fiber orientation distribution in a voxel can be modeled as a scaled Bingham distribution.

High-resolution three-dimensional macromolecular proton fraction mapping for quantitative neuroanatomical imaging of the rodent brain in ultra-high magnetic fields

“…T 2 B measurements obtained in this study are highly consistent with the literature data based on the analysis with the Super-Lorentzian lineshape (Morrison and Henkelman, 1995; Sled and Pike, 2001; Yarnykh, 2002; Sled et al, 2004; Stanisz et al, 2005; Underhill et al, 2009; Levesque et al, 2010; Pohmann et al, 2011; Underhill et al, 2011; Samsonov et al, 2012; Yarnykh, 2012; Thiessen et al, 2013; Mossahebi et al, 2014; Pampel et al, 2015; Varma et al, 2015b) and do not suggest any field dependence of this parameter. Generally, T 2 B values demonstrate minimal distinctions between WM and GM and a narrow range of variations (typically about 8–12 μs) in the brain.…”

“…Generally, T 2 B values demonstrate minimal distinctions between WM and GM and a narrow range of variations (typically about 8–12 μs) in the brain. Residual tissue contrast on T 2 B maps has been recently attributed to the directional dependence of this parameter in WM (Yarnykh, 2012; Pampel et al, 2015). The visual appearance of T 2 B maps of the rat brain at 11.7T (Fig.…”

Optimization of inhomogeneous magnetization transfer (ihMT) MRI contrast for preclinical studies using dipolar relaxation time (T1D) filtering

“…Previous MT experiments have also reported a dependence of quantitative MT‐derived parameters (e.g. T 2B – T 2 of bound protons) on WM anisotropy . Hence, the results obtained here warrant further investigations in order to interpret the WM anisotropy effect on ihMT and to evaluate to what extent it is a potential confounder for ihMT quantification.…”

Quantitative analysis of the z-spectrum using a numerically simulated look-up table: Application to the healthy human brain at 7T

“…It is interesting to note that the NOE and MT values appear to be uncoupled in the corpus callosum, with the MT/NOE ratio varying between the truncus and the splenium and genu. Previous investigators have suggested that the truncus and genus have more myelinated axons than the splenium (31), so this may indicate a difference in sensitivity of the NOE and MT signals to myelination; alternatively, this result may relate to the previously reported dependence of MT on fiber orientation (20). In practice, a small positive APT pool was measured in all GM and WM ROIs.…”