Abstract
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.
Highlights
Molecular self-diffusion measured with nuclear magnetic resonance (NMR) [1, 2] can be used to non-invasively probe the microstructure of porous materials [3,4,5] and tissues [6]
We have recently shown that microscopic anisotropy can be efficiently detected with an acquisition protocol including singleshot isotropic diffusion weighting (DW) using magic-angle spinning of the q-vector (q-MAS) [34]
We introduce a new parameter, the microscopic fractional anisotropy, for quantification of the microscopic anisotropy, and suggest a method to estimate the value of μFA by analysis of a set of diffusion MRI data acquired with both isotropic and conventional DW
Summary
Molecular self-diffusion measured with nuclear magnetic resonance (NMR) [1, 2] can be used to non-invasively probe the microstructure of porous materials [3,4,5] and tissues [6]. The apparent self-diffusion coefficient, as measured in a pulsed gradient spin echo (PGSE) experiment, reflects the average diffusivity, which is a sum of contributions from different water compartments in a complex system. The anisotropy of the tissue morphology renders the water self-diffusion anisotropic, a feature that is the basis for noninvasive mapping of muscle and nerve fiber orientations by diffusion tensor imaging (DTI) [13, 14]. DTI is commonly used to study the white matter (WM) of the brain, where the nerve fibers have a dominant direction on macroscopic length scales. Due to the significance of accurate quantification of the level of anisotropy in the brain, techniques for detecting fiber orientation dispersion are being developed [16, 17]
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