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

Abstract

Diffusion tensor imaging (DTI) is the method of choice for non-invasive investigations of the structure of human brain white matter (WM). 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 nuclear magnetic resonance (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 gray and white matter.

Axon Diameters and Myelin Content Modulate Microscopic Fractional Anisotropy at Short Diffusion Times in Fixed Rat Spinal Cord

“…Methods other than D(O)DE, targeting µFA such as tailoring b-tensor shapes are emerging, with many potential applications [77][78][79][80]. However, such methods may be confounded by time-dependent diffusion effects [27,[81][82][83], whereas D(O)DE at long mixing times naturally avoids these confounds [43].…”

Microstructures of Learning: Novel methods and approaches for assessing structural and functional changes underlying knowledge acquisition in the brain

“…The total scan time was 10:12 min. DTI was performed on the data acquired with anisotropic encoding to obtain maps of FA, while the full data was used to estimate mFA using the methods described by Lasič et al (2014). The analysis was also applied on data simulated by calculating the MR signal from a collection of diffusion tensors in order to demonstrate the theoretical response of FA and mFA when increasing RD with fixed AD, increasing orientation dispersion, and varying fiber crossing angle.…”

“…The echo time was 160 ms, repetition time 2000 ms, acquisition matrix 96 × 96, spatial resolution 3 mm × 3 mm × 3 mm, partial Fourier factor 0.8, and SENSE factor 2. Anisotropic encoding was performed in 15 directions for each b-value, using harmonically modulated gradients according to Lasič et al (2014). The isotropic encoding was repeated 15 times per b-value in order to acquire an equal number of images for both types of encoding tensors.…”

“…The microscopic anisotropy can be estimated by considering the difference in m 2 when estimated from data acquired with conventional and partial or complete isotropic diffusion encoding (Lasič et al, 2014;Westin et al, 2014). In the following analysis, we will assume that S(b) is independent of the orientation of the sample, i.e., that P(D) is independent of the direction of the diffusion encoding gradients.…”

“…Such methods compare the MR signal acquired with the two blocks either in a parallel or perpendicular orientation, resulting in anisotropic or partially isotropic diffusion encoding, respectively. Lasič et al (2014) proposed an alternative that involved complete instead of partial isotropic encoding, based on magic angle spinning of the q-vector (qMAS; Eriksson et al, 2013). Here, we demonstrate quantification of microscopic anisotropy (mFA) in a healthy volunteer using the qMAS technique, and we elucidate how the measures of anisotropy (mFA and FA) respond to various changes in micro-architecture.…”