Yoshitaka Bito¹, Yuko Kawai², Koji Hirata¹, Toshihiko Ebisu³, Toru Shirai¹, Satoshi Hirata¹, Yoshihisa Soutome¹, Hisaaki Ochi¹, Masahiro Umeda², Toshihiro Higuchi⁴, and Chuzo Tanaka^4
1Central Research Laboratory, Hitachi, Ltd., Kokubunji-shi, Tokyo, Japan, ²Medical Informatics, Meiji University of Integrative Medicine, Kyoto, Japan, ³Neurosurgery, Nantan General Hospital, Kyoto, Japan, ⁴Neurosurgery, Meiji University of Integrative Medicine, Kyoto, Japan

Diffusion tensor spectroscopic imaging (DTSI), using diffusion-weighted echo-planar spectroscopic imaging with a pair of bipolar diffusion gradients (DW-EPSI with BPGs), was developed. Diffusion tensor (DT) images of N-acetylaspartate (NAA) in rat brains were measured by using this DTSI technique. The measured DT images of NAA and water were compared by calculating tensor correlation coefficient and difference of fractional anisotropy. The DT of NAA and DT of water show high similarity in most brain regions but show differences in the detail, i.e., the base, corpus callosum, and cortex. These results suggest that this DTSI technique is effective in investigating microstructures of the nervous system.

Jeff Kershaw^1,2, Christoph Leuze³, Takayuki Obata¹, Iwao Kanno¹, and Ichio Aoki^1
1Molecular Imaging Centre, National Institute of Radiological Sciences, Chiba, Japan, ²School of Bioscience and Biotechnology, Tokyo Institute of Technology, Yokohama, Japan, ³Department of Neurophysics, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany

A sequence with oscillating motion-probing gradients was applied to investigate the restricted or hindered motion of water in in vivo rat brain tissue by observing changes to the apparent diffusion tensor, fractional anisotropy (FA) and mean diffusivity (MD) as the effective diffusion-time is decreased. Imaging was performed in a sagittal slice through the rat cerebellum and corpus callosum, which contain most of the white matter in the rat brain. Paired-t tests found significant differences in the FA of the corpus callosum and cerebellar white matter after the effective diffusion-time was decreased from 7.5 ms to 3.75 ms. Significant differences were also found for the MD in both grey and white matter. It is anticipated that normal and pathological in vivo tissue structure can be probed with this technique.

Matthew D Budde^1,2, Lindsay Janes², Eric Gold², L. Christine Turtzo², and Joseph A Frank^1,2
1Radiology and Imaging Sciences, National Institutes of Health, Bethesda, MD, United States, ²Center for Neuroscience and Regenerative Medicine at the Uniformed Services University, Bethesda, MD, United States

Most studies applying diffusion tensor imaging (DTI) following traumatic brain injury (TBI) have demonstrated a decrease of white matter fractional anisotropy (FA), but increased FA has occasionally been observed. We performed DTI on rats following controlled cortical impact and developed a method to assess the microscopic anisotropy of immunostained histological sections using Fourier analysis. In the injured white matter, FA was decreased and associated with axonal and myelin degeneration. In contrast, FA increased in the peri-lesion gray matter and was associated with coherent astrogliosis. The results demonstrate that astrocytes are a potential source of anisotropy in injured brain tissue.

Brian Hansen¹, Jeremy J. Flint^2,3, Choong Heon-Lee^3,4, Michael Fey⁵, Franck Vincent⁵, Michael A. King⁶, Peter Vestergaard-Poulsen¹, and Stephen J. Blackband^7,8
1Center for Functionally Integrative Neuroscience (CFIN), Aarhus University, Aarhus, Denmark, ²Department of Neuroscience, University of Florida, ³McKnight Brain Institute, University of Florida, Gainesville, Florida, United States, ⁴Department of Electrical Engineering, University of Florida, Gainesville, Florida, United States, ⁵Bruker Biospin, ⁶Department of Pharmacology and Therapeutics, University of Florida, ⁷Department of Neuroscience, Center for Structural Biology & National High Magnetic Field Laboratory, University of Florida, ⁸McKnight Brain Institute, University of Florida

Diffusion tensor imaging (DTI) and tractography (DTT) are regularly used for investigating tissue structure and for delineating white matter tracts. Recently, we proposed a method for comparing DTT results to the histology of the actual tissue on which DTI experiments were performed. Here we present new results obtained using these methods on samples of the ventral horn in human spinal cord. Specifically, we extract the primary eigenvector from DTI measurements at microscopic resolution and compare this microstructural information to myelin-stained histology of the tissue samples employed. Our results confirm that the primary eigenvector reflects microstructure clearly. These results are relevant to techniques in which tissue structure is investigated using the primary eigenvector.

Jennifer Andrea McNab¹, Jonathan R Polimeni¹, and Lawrence L Wald^1,2
1A.A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, United States, ²Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, United States

We present a surface-based analysis that characterizes the principal diffusion eigenvectors from 1 mm isotropic diffusion measurements relative to the cortical surface normal. We investigate DTI changes as a function of cortical depth, surface curvature and partial volume effects. We find that much of the cortex is predominately radial, but replicate the finding that S1 cortex is strongly tangential. Cortical regions S2 and A1 share S1’s tangential orientation. The ability to parcellate folded cortex by diffusion metrics provides new possibilities for studying brain organization as it relates to function in the healthy and diseased brain.

Alexandru Vlad Avram^1,2, Arnaud Guidon^1,2, Chunlei Liu², and Allen W Song^2
1Biomedical Engineering Department, Duke University, Durham, NC, United States, ²Brain Imaging and Analysis Center, Duke University Medical Center, Durham, NC, United States

We investigate the echo time (TE) dependence of white matter diffusion anisotropy using a novel stimulated echo based (STE) self-navigated interleaved spiral (SNAILS) DTI sequence capable of imaging with a wide range of TEs (22 – 82 ms) while maintaining adequate b-value (600 s/mm2). We quantify the dependence of FA, axial and radial diffusivities on TE for the first time with an in vivo experiment and present a clinical subtraction-based technique for achieving short T2 (myelin) specificity by acquiring DTI datasets at only two TEs.

Chu-Yu Lee¹, and Josef P Debbins^2
1Electrical Engineering, Arizona State University, Tempe, Arizona, United States, ²Neuroimaging Research, Barrow Neurological Institute, Phoenix, Arizona, United States

The distance of water movement during a DWI experiment is beyond the microstructural dimension (4-100μm), a fact that is demonstrated as a non-monoexponential decay when b-value is high. Considering the multiple physical compartments in tissues, there can be more than two diffusion components. More generally, the signal can be given by a summation of a statistical distribution of diffusion rates. The stretched exponential (α-DWI) [1] and diffusion kurtosis imaging (DKI) [2] models can be used to characterize the distribution of diffusion rates. Another approach is to model the signal decay with a statistical distribution: the truncated Gaussian distribution [3] and gamma distribution [4] models. Those phenomenological models fit the data well with only two parameters within a certain range of b-value. Their model parameters can quantify the diffusion heterogeneity, related to the width of the distribution of diffusion rates. However, their theoretical underpinnings are very different and how to infer the tissue structures from the measured diffusion heterogeneity is unclear. In this work, we created a simulation where cell sizes were statistically distributed, and the cellular volume fraction, mean cell sizes, and membrane permeability were varied to study how the measured diffusion heterogeneity correlated with the changes. We focused on three fitted parameters: α, Kapp, and σgamma (standard deviation of gamma distribution) of α-DWI, DKI, and gamma distribution models. The diffusion models were also applied to a clinical case of recurrent tumor to compare with the simulation results.

Ido Tavor¹, Shir Hofstetter¹, Shani Ben-Amitay¹, and Yaniv Assaf^1
1Neurobiology, Tel Aviv University, Tel Aviv, Israel

Characterizing brain plasticity with DTI is gaining interest in the last years. The cellular mechanism that leads to this observation is unclear. Alternative to DTI, models that disintegrate the diffusion MRI signal to different cellular sources were suggested. In the present work we have utilized CHARMED to study structural plasticity following a short term spatial memory task. It was found that MD decrease following 2 hours of spatial navigation computer game is characterized by a more significant increase in the fraction of restricted diffusion. This study shows the utility of the high b-value framework to study brain dynamics.

Thijs Dhollander^1,2, Wim Van Hecke^1,3, Frederik Maes^1,2, Stefan Sunaert^1,3, and Paul Suetens^1,2
1Medical Imaging Research Center (MIRC), K.U.Leuven, Leuven, Belgium, ²Department of Electrical Engineering (ESAT), K.U.Leuven, Leuven, Belgium, ³Department of Radiology, University Hospitals of the K.U.Leuven, Leuven, Belgium

The number of atlas construction methods for diffusion weighted imaging is at least equal to the number of possible models which can be reconstructed from the original data in q-space. Therefore, we aim at constructing an atlas before the reconstruction of any of these models: a hybrid diffusion imaging atlas containing signal functions for multiple shells of q-space. In this work, we show how this can be achieved by creating such an atlas (containing signal functions for b = 700, 1000, 2800) and reconstructing some possible models of diffusion (diffusion tensor and fiber orientation distribution function) from the result.

Robin Martin Heidemann¹, David A Porter², Alfred Anwander¹, Thorsten Feiweier², Fernando Calamante³, Jaques-Donald Tournier³, Gabriele Lohmann¹, Heiko Meyer², Thomas R Knösche¹, and Robert Turner^1
1Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany, ²Siemens Healthcare, Erlangen, Germany, ³Brain Research Institute, Melbourne, Australia

Diffusion-weighted imaging using single-shot EPI at 7T is prone to a high level of susceptibility and blurring artefact, even when parallel imaging with a large acceleration factor is used. Multi-shot imaging using readout-segmented EPI has been shown to provide a substantial reduction in these artefacts, but previous applications of the method were restricted to localized studies of small brain regions. In this study, an extension of the technique is used, which provides whole brain coverage with high isotropic resolution and sufficient angular resolution to estimate fibre orientation density functions using spherical deconvolution.