title: NMR diffusion-encoding with axial symmetry and variable anisotropy: Distinguishing between prolate and oblate microscopic diffusion tensors with unknown orientation distribution.
abstract: We introduce a nuclear magnetic resonance method for quantifying the shape of axially symmetric microscopic diffusion tensors in terms of a new diffusion anisotropy metric, DΔ, which has unique values for oblate, spherical, and prolate tensor shapes. The pulse sequence includes a series of equal-amplitude magnetic field gradient pulse pairs, the directions of which are tailored to give an axially symmetric diffusion-encoding tensor b with variable anisotropy bΔ. Averaging of data acquired for a range of orientations of the symmetry axis of the tensor b renders the method insensitive to the orientation distribution function of the microscopic diffusion tensors. Proof-of-principle experiments are performed on water in polydomain lyotropic liquid crystals with geometries that give rise to microscopic diffusion tensors with oblate, spherical, and prolate shapes. The method could be useful for characterizing the geometry of fluid-filled compartments in porous solids, soft matter, and biological tissues.
authors.affiliation: Division of Physical Chemistry, Department of Chemistry, Lund University, Lund, Sweden.
authors.forename: Stefanie
authors.initials: S
authors.lastname: Eriksson
authors.affiliation: CR Development AB, Lund, Sweden.
authors.forename: Samo
authors.initials: S
authors.lastname: Lasič
authors.affiliation: Lund University Bioimaging Center, Lund University, Lund, Sweden.
authors.forename: Markus
authors.initials: M
authors.lastname: Nilsson
authors.affiliation: Department of Radiology, BWH, Harvard Medical School, Boston, Massachusetts MA 02215, USA.
authors.forename: Carl-Fredrik
authors.initials: CF
authors.lastname: Westin
authors.affiliation: Division of Physical Chemistry, Department of Chemistry, Lund University, Lund, Sweden.
authors.forename: Daniel
authors.initials: D
authors.lastname: Topgaard
mesh_terms: D016880:Anisotropy; D004058:Diffusion; D050866:Liquid Crystals / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008954:Models, Biological