Microanisotropy imaging: quantification of microscopic diffusion anisotropy and orientational order parameter by diffusion MRI with magic-angle spinning of the q-vector

• Main Authors: Samo eLasič, Filip eSzczepankiewicz, Stefanie eEriksson, Markus eNilsson, Daniel eTopgaard
• Format: Article
• Language: English
• Published: Frontiers Media S.A. 2014-02-01
• Series: Frontiers in Physics
• Subjects: diffusion MRI; fractional anisotropy; Order parameter; microscopic diffusion anisotropy; single shot isotropic diffusion weighting; q-MAS
• Online Access: http://journal.frontiersin.org/Journal/10.3389/fphy.2014.00011/full

Diffusion tensor imaging (DTI) is the method of choice for non-invasive investigations of the structure of human brain white matter. The results are conventionally reported as maps of the fractional anisotropy (FA), which is a parameter related to microstructural features such as axon density, diameter, and myelination. The interpretation of FA in terms of microstructure becomes ambiguous when there is a distribution of axon orientations within the image voxel. In this paper, we propose a procedure for resolving this ambiguity by determining a new parameter, the microscopic fractional anisotropy (µFA), which corresponds to the FA without the confounding influence of orientation dispersion. In addition, we suggest a method for measuring the orientational order parameter (OP) for the anisotropic objects. The experimental protocol is capitalizing on a recently developed diffusion NMR pulse sequence based on magic-angle spinning of the q-vector. Proof-of-principle experiments are carried out on microimaging and clinical MRI equipment using lyotropic liquid crystals and plant tissues as model materials with high µFA and low FA on account of orientation dispersion. We expect the presented method to be especially fruitful in combination with DTI and high angular resolution acquisition protocols for neuroimaging studies of grey and white matter.