Double-wave-vector diffusion-weighting experiments with multiple concatenations at long mixing times
ABSTRACT MR sequences where two diffusion-weighting periods are applied successively in a single acquisition seem to be a promising tool for the investigation of tissue structure on a microscopic level such as the characterization of the compartment size or eccentricity measures of pores. However, the application of such double-wave-vector (DWV) experiments on whole-body MR systems is hampered by the long gradient pulses required that have been shown to reduce the signal modulation. In this work, it is demonstrated that involving multiple concatenations of the two diffusion-weighting periods can ameliorate this problem in experiments with long mixing times between the two wave vectors. The recently presented tensor equation is extended to multiple concatenations. As confirmed by Monte-Carlo simulations, this model shows a good approximation of the signals observed for typical whole-body gradient pulse durations and the derived anisotropy measures are obtained with good accuracy. Most importantly, the signal modulation is increased with multiple concatenations because shorter gradient pulses can be used to achieve the desired diffusion-weighting. Thus, the multiple concatenation approach may help to improve the applicability and reliability of DWV measurements with long mixing times on standard whole-body MR systems.
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ABSTRACT: Human neuroimaging of tissue microstructure, such as axonal density and integrity, are key in clinical and neuroscience research. Most studies rely on diffusion tensor imaging (DTI) and the measures derived from it, most prominently fractional anisotropy (FA). However, FA also depends on fibre orientation distribution, a more macroscopic tissue property. Recently introduced measures of so-called microscopic diffusion anisotropy, diffusion anisotropy on a cellular or microscopic level, overcome this limitation because they are independent of the orientation distributions of axons and fibres. In this study, we evaluate the feasibility of two measures of microscopic diffusion anisotropy IMA and MA indices, for human neuroscience and clinical research. Both indices reflect the eccentricity of the cells but while IMA also depends on the cell size, MA is independent of the cell size and, like FA, scaled between 0 and 1. In whole-brain measurements of a group of 19 healthy volunteers, we measured average values and variability, evaluated their reproducibility, both within and between sessions, and compared MA to FA values in selected regions-of-interest (ROIs). The within- and between-session comparison did not show substantial differences but the reproducibility was much better for the MA than IMA (coefficient of variation between sessions 10.5% vs. 28.9%). The reproducibility was less for MA than FA overall, but comparable in the defined ROIs and the average group sizes required for between-group comparisons was similar (about 60 participants for a relative difference of 5%). Group-averaged values of MA index were generally larger and showed less variation across white-matter brain ROIs than FA (mean±standard deviation of seven ROIs 0.83±0.10 vs. 0.58±0.13). Even in some gray-matter ROIs, MA values comparable to those of white matter ROIs were observed. Furthermore, the within-group variation of the values in white matter ROIs was lower for the MA compared to the FA (mean standard deviation over volunteers 0.038 vs. 0.049) which could be due to significant variability in the distribution of fibre orientation contributing to FA. These results indicate that MA (i) should be preferred to IMA, (ii) has a reproducibility and group-size requirements comparable to those of FA; (iii) is less sensitive to the fibre orientation distribution than FA; and (iv) could be more sensitive to differences or changes of the tissue microstructure than FA. R1.1. Copyright © 2015 Elsevier Inc. All rights reserved.NeuroImage 01/2015; 109. DOI:10.1016/j.neuroimage.2015.01.025 · 6.13 Impact Factor
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ABSTRACT: To demonstrate that rotationally invariant measures of the diffusion anisotropy on a microscopic scale can be mapped in human brain white matter in vivo. Echo-planar imaging experiments (resolution 3.0 × 3.0 × 3.0 mm(3) ) involving two diffusion-weighting periods (δ = 22 ms, Δ = 25 ms) in the same acquisition, so-called double-wave-vector or double-pulsed-field-gradient diffusion-weighting experiments, were performed on a 3 T whole-body magnetic resonance system with a long mixing time ( τm=45 ms) between the two diffusion weightings. The disturbing influences of background gradient fields, eddy currents, and the finite mixing time can be minimized using 84 direction combinations based on nine directions and their antipodes. In healthy volunteers, measures of the microscopic diffusion anisotropy ( IMA and MA indexes) could be mapped in white matter across the human brain. The measures were independent (i) of the absolute orientation of the head and of the diffusion directions and (ii) of the predominant fiber orientation. Compared to the fractional anisotropy derived from the conventional diffusion tensor, the double-wave-vector indexes exhibit a narrower distribution, which could reflect their independence of the fiber orientation distribution. Mapping measures of the microscopic diffusion anisotropy in human brain white matter is feasible in vivo and could help to characterize tissue microstructure in the healthy and pathological brain. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.Magnetic Resonance in Medicine 02/2015; 73(2). DOI:10.1002/mrm.25140 · 3.40 Impact Factor
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ABSTRACT: Since their introduction by Stejskal and Tanner, pulsed-field-gradient diffusion-weighted NMR experiments have been applied to characterize condensed matter ranging from liquids and rocks to biological tissue in vivo. In spite of their outstanding success, for example, in biomedical research and clinical diagnosis, the technique faces some inherent limitations, for instance the inability to detect diffusion anisotropy present on a microscopic scale in a macroscopically isotropic sample. Thus, the interest in an extended version of the diffusion-weighted NMR experiment where two (or more) diffusion-weighting periods are applied successively in a single acquisition has emerged in the past few years. Such experiments, sometimes referred to as two- or multiple-wave-vector experiments, have been shown to be a promising tool to investigate diffusion in confined geometries. They are able to assess information difficult or impossible to achieve with standard diffusion-weighting experiments and, thus, may help to provide deeper insight into the sample's microstructure. In this work, current developments in the theoretical modelling of multiple-wave-vector experiments and recent experimental results demonstrating the potential of the technique are summarized.