Marco Lawrenz

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Stejskal and Tanner's ingenious pulsed field gradient design from 1965 has made diffusion NMR and MRI the mainstay of most studies seeking to resolve microstructural information in porous systems in general and biological systems in particular. Methods extending beyond Stejskal and Tanner's design, such as double diffusion encoding (DDE) NMR and MRI, may(More)
Experiments with two diffusion-weighting periods applied successively in a single experiment, so-called double-wave-vector (DWV) diffusion-weighting experiments, are a promising tool for the investigation of material or tissue structure on a microscopic level, e.g. to determine cell or compartment sizes or to detect pore or cell anisotropy. However, the(More)
Double-wave-vector diffusion-weighting experiments can detect diffusion anisotropy on a microscopic level which, e.g., could distinguish lower fiber densities from reduced fiber coherence. The underlying signal difference between parallel and orthogonal wave vector orientations has been observed on vertical-bore MR systems (≥500 mT m(-1) ); however,(More)
PURPOSE To demonstrate that rotationally invariant measures of the diffusion anisotropy on a microscopic scale can be mapped in human brain white matter in vivo. METHODS 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(More)
Diffusion-tensor imaging is widely used to characterize diffusion in biological tissue, however, the derived anisotropy information, e.g., the fractional anisotropy, is ambiguous. For instance, low values of the diffusion anisotropy in brain white matter voxels may reflect a reduced axon density, i.e., a loss of fibers, or a lower fiber coherence within the(More)
Human neuroimaging of tissue microstructure, such as axonal density and integrity, is 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 fiber orientation distribution, a more macroscopic tissue property.(More)
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(More)
The fractional anisotropy (FA) that can be derived from diffusion tensor imaging (DTI), is ambiguous because it not only depends on the tissue microstructure but also on the axon or fiber orientation distribution within a voxel. Measures of the microscopic diffusion anisotropy, like the microscopic anisotropy index (MA) that can be determined with so-called(More)
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