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1. Diffusion interactions between crossing fibers of the brain

   https://doi.org/10.1002/mrm.28702  

   Buldyrev, Sergey V. ; Meng, Xiangyi ; Reese, Timothy G. ; Mortazavi, Farzad ; Rosene, Douglas L. ; Stanley, H. Eugene ; Wedeen, Van J. ( February 2021 , Magnetic Resonance in Medicine)

   Recent observations of several preferred orientations of diffusion in deep white matter may indicate either (a) that axons in different directions are independently bundled in thick sheets and function noninteractively, or more interestingly, (b) that the axons are closely interwoven and would exhibit branching and sharp turns. This study aims to investigate whether the dependence of dMRI Q‐ball signal on the interpulse timecan decode the smaller‐than‐voxel‐size brain structure, in particular, to distinguish scenarios (a) and (b).

   High‐resolution Q‐ball images of a healthy brain taken with s/mm²for 3 different values ofwere analyzed. The exchange of water molecules between crossing fibers was characterized by the fourth Fourier coefficientof the signal profile in the plane of crossing. To interpret the empirical results, a model consisting of differently oriented parallel sheets of cylinders was developed. Diffusion of water molecules inside and outside cylinders was simulated by the Monte Carlo method.

   Simulations predict that, agreeing with the empirical results, must increase withfor largeb‐values, but may peak at a typicalthat depends on the thickness of the cylinder sheets for intermediateb‐values. Thus, the thickness of axon layers in voxels with 2 predominant orientations can be detected from empiricaltaken at smallerb‐values.

   Based on the simulation results, recommendations are made on how to design a dMRI experiment with optimalb‐value and range ofin order to measure the thickness of axon sheets in the white matter, hence to distinguish (a) and (b).

    

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2. Robust autocalibrated structured low‐rank EPI ghost correction

   https://doi.org/10.1002/mrm.28638  

   Lobos, Rodrigo A. ; Hoge, W. Scott ; Javed, Ahsan ; Liao, Congyu ; Setsompop, Kawin ; Nayak, Krishna S. ; Haldar, Justin P. ( December 2020 , Magnetic Resonance in Medicine)

   We propose and evaluate a new structured low‐rank method for echo‐planar imaging (EPI) ghost correction called Robust Autocalibrated LORAKS (RAC‐LORAKS). The method can be used to suppress EPI ghosts arising from the differences between different readout gradient polarities and/or the differences between different shots. It does not require conventional EPI navigator signals, and is robust to imperfect autocalibration data.

   Autocalibrated LORAKS is a previous structured low‐rank method for EPI ghost correction that uses GRAPPA‐type autocalibration data to enable high‐quality ghost correction. This method works well when the autocalibration data are pristine, but performance degrades substantially when the autocalibration information is imperfect. RAC‐LORAKS generalizes Autocalibrated LORAKS in two ways. First, it does not completely trust the information from autocalibration data, and instead considers the autocalibration and EPI data simultaneously when estimating low‐rank matrix structure. Second, it uses complementary information from the autocalibration data to improve EPI reconstruction in a multi‐contrast joint reconstruction framework. RAC‐LORAKS is evaluated using simulations and in vivo data, including comparisons to state‐of‐the‐art methods.

   RAC‐LORAKS is demonstrated to have good ghost elimination performance compared to state‐of‐the‐art methods in several complicated EPI acquisition scenarios (including gradient‐echo brain imaging, diffusion‐encoded brain imaging, and cardiac imaging).

   RAC‐LORAKS provides effective suppression of EPI ghosts and is robust to imperfect autocalibration data.

    

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3. Diffusion MRI tractography of the locus coeruleus‐transentorhinal cortex connections using GO‐ESP

   https://doi.org/10.1002/mrm.29088  

   Solders, Seraphina K. ; Galinsky, Vitaly L. ; Clark, Alexandra L. ; Sorg, Scott F. ; Weigand, Alexandra J. ; Bondi, Mark W. ; Frank, Lawrence R. ( November 2021 , Magnetic Resonance in Medicine)

   The locus coeruleus (LC) is implicated as an early site of protein pathogenesis in Alzheimer's disease (AD). Tau pathology is hypothesized to propagate in a prion‐like manner along the LC‐transentorhinal cortex (TEC) white matter (WM) pathway, leading to atrophy of the entorhinal cortex and adjacent cortical regions in a progressive and stereotypical manner. However, WM damage along the LC‐TEC pathway may be an earlier observable change that can improve detection of preclinical AD.

   Diffusion‐weighted MRI (dMRI) allows reconstruction of WM pathways in vivo, offering promising potential to examine this pathway and enhance our understanding of neural mechanisms underlying the preclinical phase of AD. However, standard dMRI analysis tools have generally been unable to reliably reconstruct this pathway. We apply a novel method, geometric‐optics based entropy spectrum pathways (GO‐ESP) and produce a new measure of connectivity: the equilibrium probability (EP).

   We demonstrated reliable reconstruction of LC‐TEC pathways in 50 cognitively normal older adults and showed a negative association between LC‐TEC EP and cerebrospinal fluid tau. Using Human Connectome Project data, we demonstrated replicability of the method across acquisition schemes and scanners. Finally, we compared our findings with the only other existing LC‐TEC tractography template, and replicated their pathway as well as investigated the source of these discrepant findings.

   AD‐related tau pathology may be detectable within GO‐ESP‐identified LC‐TEC pathways. Furthermore, there may be multiple possible routes from LC to TEC, raising important questions for future research on the LC‐TEC connectome and its role in AD pathogenesis.

    

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4. Chronic exposure to inhaled vaporized cannabis high in Δ9-THC alters brain structure in adult female mice

   https://doi.org/10.3389/fnins.2023.1139309  

   Taylor, Autumn ; Nweke, Amanda ; Vincent, Veniesha ; Oke, Marvellous ; Kulkarni, Praveen ; Ferris, Craig F. ( March 2023 , Frontiers in Neuroscience)

   The medical and recreational use of cannabis has increased in the United States. Its chronic use can have detrimental effects on the neurobiology of the brain—effects that are age-dependent. This was an exploratory study looking at the effects of chronically inhaled vaporized cannabis on brain structure in adult female mice.

   Adult mice were exposed daily to vaporized cannabis (10.3% THC and 0.05% CBD) or placebo for 21 days. Following cessation of treatment mice were examined for changes in brain structure using voxel-based morphometry and diffusion weighted imaging MRI. Data from each imaging modality were registered to a 3D mouse MRI atlas with 139 brain areas.

   Mice showed volumetric changes in the forebrain particularly the prefrontal cortex, accumbens, ventral pallidum, and limbic cortex. Many of these same brain areas showed changes in water diffusivity suggesting alterations in gray matter microarchitecture.

   These data are consistent with much of the clinical findings on cannabis use disorder. The sensitivity of the dopaminergic system to the daily exposure of vaporized cannabis raises concerns for abuse liability in drug naïve adult females that initiate chronic cannabis use.

    

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5. Reduced White Matter Integrity and Deficits in Neuropsychological Functioning in Adults With Autism Spectrum Disorder

   https://doi.org/10.1002/aur.2271  

   Haigh, Sarah M. ; Keller, Timothy A. ; Minshew, Nancy J. ; Eack, Shaun M. ( February 2020 , Autism Research)

   Autism spectrum disorder (ASD) is currently viewed as a disorder of cortical systems connectivity, with a heavy emphasis being on the structural integrity of white matter tracts. However, the majority of the literature to date has focused on children with ASD. Understanding the integrity of white matter tracts in adults may help reveal the nature of ASD pathology in adulthood and the potential contributors to cognitive impairment. This study examined white matter water diffusion using diffusion tensor imaging in relation to neuropsychological measures of cognition in a sample of 45 adults with ASD compared to 20 age, gender, and full‐scale‐IQ‐matched healthy volunteers. Tract‐based spatial statistics were used to assess differences in diffusion along white matter tracts between groups using permutation testing. The following neuropsychological measures of cognition were assessed: processing speed, attention vigilance, working memory, verbal learning, visual learning, reasoning and problem solving, and social cognition. Results indicated that fractional anisotropy (FA) was significantly reduced in adults with ASD in the anterior thalamic radiation (P= 0.022) and the right cingulum (P= 0.008). All neuropsychological measures were worse in the ASD group, but none of the measures significantly correlated with reduced FA in either tract in the adults with ASD or in the healthy volunteers. Together, this indicates that the tracts that are the most impacted in autism may not be (at least directly) responsible for the behavioral deficits in ASD.Autism Res2020, 13: 702–714. © 2020 International Society for Autism Research, Wiley Periodicals, Inc.

   White matter tracts are the data cables in the brain that efficiently transfer information, and damage to these tracts could be the cause for the abnormal behaviors that are associated with autism. We found that two long‐range tracts (the anterior thalamic radiation and the cingulum) were both impaired in autism but were not directly related to the impairments in behavior. This suggests that the abnormal tracts and behavior are the effects of another underlying mechanism.

    

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