Anisotropic diffusion in human white matter: Demonstration with MR techniques in vivo

Quantitative measurements of perfusion and molecular diffusion were made in human white matter in two orientations of the motion-sensitization gradient to document anisotropy of these parameters. Measurements were localized to a 10 × 10-mm tissue column oriented in an anterior-to-posterior direction in the left cerebral hemisphere just above the body of the left ventricle. This region was selected because of the relatively high directionality of white matter fibers. In this study of five healthy volunteers, strong diffusion anisotropy was observed in all cases. Twofold or greater anisotropy was commonly observed, with the higher diffusion value associated with motion sensitivity along the fiber directions. By combining data from both gradient orientations in all cases, diffusion values of solid tissue ranged from 0.38 × 10^-3 mm²/ sec to 1.12 × 10^-3 mm²/sec, and measured perfusion fractions were in the range of 2%-5% (excluding areas highly contaminated by cerebrospinal fluid). Little or no perfusion-fraction anisotropy was observed; however, perfusion measurements were limited by noise. Data were collected without cardiac gating by using a technique that offers good immunity to bulk tissue motion artifacts.