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 qualityMT 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., thewidth 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 themyelinmembrane to its lipid constituents, which ismotivated by previous studies on efficient interaction sites (e.g., cholesterol or galactocerebrosides) in themyelinmembrane and on the origin of ultrashort T2 signals in cerebral white matter. The agreement betweenMT 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 bemodeled
as a scaled Bingham distribution.
André Pampel, Dirk K. Müller, Alfred Anwander, Henrik Marschner, Harald E. Möller
NeuroImage (2015) 114: 136-146.