Characterization of anisotropy in high angular resolution diffusion-weighted MRI.
The methods of group theory are applied to the problem of characterizing the diffusion measured in high angular resolution MR experiments. This leads to a natural representation of the local diffusion in terms of spherical harmonics. In this representation, it is shown that isotropic diffusion, anisotropic diffusion from a single fiber, and anisotropic diffusion from multiple fiber directions fall into distinct and separable channels. This decomposition can be determined for any voxel without any prior information by a spherical harmonic transform, and for special cases the magnitude and orientation of the local diffusion may be determined. Moreover, non-diffusion-related asymmetries produced by experimental artifacts fall into channels distinct from the fiber channels, thereby allowing their separation and a subsequent reduction in noise from the reconstructed fibers. In the case of a single fiber, the method reduces identically to the standard diffusion tensor method. The method is applied to normal volunteer brain data collected with a stimulated echo spiral high angular resolution diffusion-weighted (HARD) acquisition.
Department of Radiology, University of California-San Diego, San Diego, California, USA. lfrank@ucsd.edu
Lawrence R
LR
Frank
D016880:Anisotropy; D001921:Brain / Q000033:anatomy & histology* / Q000502:physiology; D004058:Diffusion; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008433:Mathematics; D008962:Models, Theoretical