ABSTRACT
The diffusion magnetic resonance imaging (dMRI) signal in available clinical scanners is insensitive to signal attenuation caused by intra-axonal diffusion in the perpendicular direction. The observation of anisotropic diffusion in the white matter led to the idea of estimating the directions of white matter fiber bundles and visualizing the fiber trajectories. Maple syrup urine disease is another example where dMRI has yielded insightful clinical images. Diffusion in nervous tissue deviates from simple Gaussian distribution because of the presence of barriers such as cell membranes and myelin. In this setting, the diffusion propagator is influenced not only by the intrinsic diffusivity but also by the microstructural characteristics of the barriers. The microstructural underpinnings behind the diffusion abnormalities differ among diseases and are not completely elucidated yet. The organization of biological tissues, and therefore the diffusion process within them, is anisotropic, i.e., dependent on direction.
