MR DIFFUSION TENSOR SPECTROSCOPY AND IMAGING

This paper describes a new NMR imaging modality-MR diffusion tensor imaging. It consists of estimating an effective diffusion tensor, D-eff, within a voxel, and then displaying useful quantities derived from it. We show how the phenomenon of anisotropic diffusion of water (or metabolites) in anisotropic tissues, measured noninvasively by these NMR methods, is exploited to determine fiber tract orientation and mean particle displacements. Once D-eff is estimated from a series of NMR pulsed-gradient, spin-echo experiments, a tissue's three orthotropic axes can be determined. They coincide with the eigenvectors of D-eff while the effective diffusivities along these orthotropic directions are the eigenvalues of D-eff. Diffusion ellipsoids, constructed in each voxel from D-eff depict both these orthotropic axes and the mean diffusion distances in these directions. Moreover, the three scalar invariants of D-eff, which are independent of the tissue's orientation in the laboratory frame of reference, reveal useful information about molecular mobility reflective of local microstructure and anatomy. Inherently, tensors (like D-eff) describing transport processes in anisotropic media contain new information within a macroscopic voxel that scalars (such as the apparent diffusivity, proton density, T-1, and T-2) do not.