Microscopic and compartment shape anisotropies in gray and white matter revealed by angular bipolar double-PFG MR

Diffusion MR has become one of the most important tools for studying neuronal tissues. Conventional single-pulsed-field-gradient methodologies are capable of faithfully depicting diffusion anisotropy in coherently ordered structures, providing important microstructural information; however, it is extremely difficult to characterize randomly oriented compartments using conventional single-pulsed-field-gradient MR. The angular double-pulsed-field-gradient methodology can potentially overcome the limitations of conventional single-pulsed-field-gradient MR, and offer microstructural information on microscopic anisotropy and compartment shape anisotropy even when anisotropic compartments are completely randomly oriented. Here, we used angular double-pulsed-field-gradient MR at different mixing times to study isolated gray matter and white matter, respectively, and the results are compared with phantoms in which compartments are randomly oriented and coherently organized, respectively. We find that angular bipolar double-pulsed-field-gradient MR offers novel microstructural information, especially in the gray matter, depicting the local microscopic and compartment shape anisotropies present. Furthermore, direct comparison between the angular dependencies arising from white and gray matter at different mixing times reveals signatures for these tissues that are based on compartment shape anisotropy. These findings demonstrate that microstructural information can indeed be obtained from gray matter, and therefore, angular double-pulsed-field-gradient MR is promising for future application in MRI of the central-nervous-system.