Ralf Lützkendorf1, Robin M. Heidemann2, Thorsten Feiweier2, Michael Luchtmann3, Sebastian Baecke1, Joern Kaufmann4, Joerg Stadler5, Eike Budinger5, and Johannes Bernarding1
1Biometry and Medical Informatics, University of Magdeburg, Magdeburg, Germany, 2Siemens Healthcare GmbH, Erlangen, Germany, 3Department of Neurosurgery, University of Magdeburg, Magdeburg, Germany, 4Department of Neurology, University of Magdeburg, Magdeburg, Germany, 5Leibniz Institute for Neurobiology, Magdeburg, Germany
Synopsis
Diffusion
anisotropy in cortical gray matter (GM) and adjacent white matter (WM) provides
microanatomic information about the course of the neuronal structures within GM
and when connecting to other brain regions. However, interwoven neuronal fiber
orientations and complex folded structures render the analysis difficult.
Ultra-high-field diffusion MR imaging (dMRI) overcomes these limitations as the
improved SNR allowed acquiring 1.4 mm isotropic voxel with increased
diffusion-weighting. Applying constrained spherical deconvolution (1) enabled
resolving radial and tangential anisotropic diffusion in cortical gray matter
confirming recent reports of reduced radial anisotropy in primary somatosensory
cortex as compared to other cortical areas (2).Purpose:
We analyzed high-resolved whole-brain diffusion MRI
maps acquired at 7T using constrained spherical deconvolution (CSD) for
resolving radial diffusion anisotropy in cortical gray matter and adjacent
white matter.
Methods:
Data were measured on a research 7 Tesla whole-body MR scanner (Siemens
Healthcare GmbH, Germany), equipped with a 70 mT/m gradient coil (slew rate of
200 T/m/s). A 32-channel phased-array head coil (Nova Medical, USA) was used
for head imaging. The protocol consisted of a high-resolution anatomic scan
(MPRAGE, 0.8 mm isotropic resolution, covering the whole head including the
cerebellum), diffusion-weighted MR images (dMRI) using a prototype
single-shot-EPI sequence employing a modified Stejskal Tanner diffusion
encoding gradient scheme (3,4). Additionally, a Gradient Echo sequence was acquired
serving for B0 field mapping (5). Wwe optimized the diffusion gradients by
employing a web application for multiple-shell protocol design provided by
Caruyer (http://www.emmanuelcaruyer.com/q-space-sampling.php), consisting of 128
diffusion gradients per shell and different gradients in each shell. The dMRI
protocol compromised 137 volumes with 1.4 mm isotropic resolution. We acquired 128
diffusion-weighted data sets (b=3000 s/mm2) with different combinations of
gradient directions (6), and nine non-diffusion-weighted data sets (b=0 s/mm2,
b0 images) interspersed with every 17th diffusion-weighted data set. EPI
acquisition was accelerated using GRAPPA factor 3, 36 reference lines, 6/8
partial Fourier mode . Other imaging parameters were;bandwidth 1526 Hz/Pixel,
echo spacing of 0.76 ms, TE = 73 ms, base resolution 156*156, 98 slices, field
of view 220 mm), dMRI measurements coverd the whole brain including the cerebellum. Duration
of the measurement was 50 minutes.
Results and discussion:
Comparing anatomic maps and fODF maps confirmed that diffusion in GM could be clearly differentiated from diffusion in adjacent WM due to the spatially different diffusion characteristics (fig. 1): in GM fiber orientations vary strongly according to the complex three-dimensional folded structure while in WM the fibers appear more homogeneously directed along larger distances. Compared to standard direction-encoded color maps (DEC) where only the direction of the largest Eigenvector is depicted the fODF maps show more clearly crossing and bending fibers which allows following better complex fiber courses. In all volunteers, primary somatosensory cortex (fig.1; S1, fig. 2) exhibited strongly reduced radial anisotropy compared to opposite parts of primary motor cortex (fig. 1; M1, fig. 2) as well as to other cortical parts. Inspecting the data in all orthogonal views (fig. 2 a,b,c) revealed that this tissue characteristics is not an artifact due to digitizing the complex three-dimensional cortical surface but is an inherent feature of the central part of the primary somatosensory cortex probably reflecting the well-known different micro-anatomic tissue architecture of M1 as compared to S1. We found that the resolution of 1.4 mm isotropic was the optimum when acquiring whole head dMRI data with high diffusion-weighting of b=3000 s/mm2 in a single measurement (i.e., without averaging). Few studies were published with higher spatial resolution (1,7,8,9,10) but our study adds new evidence by analyzing isotropic whole head data of a larger cohort of 12 volunteers. The results suggest strongly that 1.4 mm isotropic resolution is sufficient to analyze GM diffusion characteristics with a sufficient degree of detail. The acquisition time of about 50 min renders the protocol acceptable for patients thus opening the application of the technique for clinical examinations. (Part of the results presented in the abstract was recently submitted for publication.)
Acknowledgements
No acknowledgement found.References
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