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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. Temporally resolved parametric assessment of Z‐magnetization recovery (TOPAZ): Dynamic myocardial T 1 mapping using a cine steady‐state look‐locker approach

   https://doi.org/10.1002/mrm.26887  

   Weingärtner, Sebastian ; Shenoy, Chetan ; Rieger, Benedikt ; Schad, Lothar R. ; Schulz‐Menger, Jeanette ; Akçakaya, Mehmet ( August 2017 , Magnetic Resonance in Medicine)

   To develop and evaluate a cardiac phase‐resolved myocardial T₁mapping sequence.

   The proposed method for temporally resolved parametric assessment of Z‐magnetization recovery (TOPAZ) is based on contiguous fast low‐angle shot imaging readout after magnetization inversion from the pulsed steady state. Thereby, segmented k‐space data are acquired over multiple heartbeats, before reaching steady state. This results in sampling of the inversion‐recovery curve for each heart phase at multiple points separated by an R‐R interval. Joint T₁andestimation is performed for reconstruction of cardiac phase‐resolved T₁andmaps. Sequence parameters are optimized using numerical simulations. Phantom and in vivo imaging are performed to compare the proposed sequence to a spin‐echo reference and saturation pulse prepared heart rate–independent inversion‐recovery (SAPPHIRE) T₁mapping sequence in terms of accuracy and precision.

   In phantom, TOPAZ T₁values with integratedcorrection are in good agreement with spin‐echo T₁values (normalized root mean square error = 4.2%) and consistent across the cardiac cycle (coefficient of variation = 1.4 ± 0.78%) and different heart rates (coefficient of variation = 1.2 ± 1.9%). In vivo imaging shows no significant difference in TOPAZ T₁times between the cardiac phases (analysis of variance:P = 0.14, coefficient of variation = 3.2 ± 0.8%), but underestimation compared with SAPPHIRE (T₁time ± precision: 1431 ± 56 ms versus 1569 ± 65 ms). In vivo precision is comparable to SAPPHIRE T₁mapping until middiastole (P > 0.07), but deteriorates in the later phases.

   The proposed sequence allows cardiac phase‐resolved T₁mapping with integratedassessment at a temporal resolution of 40 ms. Magn Reson Med 79:2087–2100, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

    

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3. Realistic Electron Diffusion Rates and Lifetimes Due to Scattering by Electron Holes

   https://doi.org/10.1029/2021JA029380  

   Shen, Yangyang ; Vasko, Ivan Y. ; Artemyev, Anton ; Malaspina, David M. ; Chu, Xiangning ; Angelopoulos, Vassilis ; Zhang, Xiao‐Jia ( August 2021 , Journal of Geophysical Research: Space Physics)

   Plasma sheet electron precipitation into the diffuse aurora is critical for magnetosphere‐ionosphere coupling. Recent studies have shown that electron phase space holes can pitch‐angle scatter electrons and may produce plasma sheet electron precipitation. These studies have assumed identical electron hole parameters to estimate electron scattering rates (Vasko et al., 2018,https://doi.org/10.1063/1.5039687). In this study, we have re‐evaluated the efficiency of this scattering by incorporating realistic electron hole properties from direct spacecraft observations into computing electron diffusion rates and lifetimes. The most important electron hole properties in this evaluation are their distributions in velocity and spatial scale and electric field root‐mean‐square intensity (). Using direct measurements of electron holes during a plasma injection event observed by the Van Allen Probe at, we find that when4 mV/m electron lifetimes can drop below 1 h and are mostly within strong diffusion limits at energies below10 keV. During an injection observed by the THEMIS spacecraft at, electron holes with even typical intensities (1 mV/m) can deplete low‐energy (a few keV) plasma sheet electrons within tens of minutes following injections and convection from the tail. Our results confirm that electron holes are a significant contributor to plasma sheet electron precipitation during injections.

    

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4. High‐resolution microstructural and compositional analyses of shock deformed apatite from the peak ring of the Chicxulub impact crater

   https://doi.org/10.1111/maps.13541  

   Cox, Morgan A. ; Erickson, Timmons M. ; Schmieder, Martin ; Christoffersen, Roy ; Ross, Daniel K. ; Cavosie, Aaron J. ; Bland, Phil A. ; Kring, David A. ; IODP–ICDP Expedition 364 Scientists ( August 2020 , Meteoritics & Planetary Science)

   The mineral apatite, Ca₅(PO₄)₃(F,Cl,OH), is a ubiquitous accessory mineral, with its volatile content and isotopic compositions used to interpret the evolution of H₂O on planetary bodies. During hypervelocity impact, extreme pressures shock target rocks resulting in deformation of minerals; however, relatively few microstructural studies of apatite have been undertaken. Given its widespread distribution in the solar system, it is important to understand how apatite responds to progressive shock metamorphism. Here, we present detailed microstructural analyses of shock deformation in ~560 apatite grains throughout ~550 m of shocked granitoid rock from the peak ring of the Chicxulub impact structure, Mexico. A combination of high‐resolution backscattered electron (BSE) imaging, electron backscatter diffraction mapping, transmission Kikuchi diffraction mapping, and transmission electron microscopy is used to characterize deformation within apatite grains. Systematic, crystallographically controlled deformation bands are present within apatite, consistent with tilt boundaries that contain the <c> (axis) and result from slip in <> (direction) on(plane) during shock deformation. Deformation bands contain complex subgrain domains, isolated dislocations, and low‐angle boundaries of ~1° to 2°. Planar fractures within apatite form conjugate sets that are oriented within either {, {, {, or. Complementary electron microprobe analyses (EPMA) of a subset of recrystallized and partially recrystallized apatite grains show that there is an apparent change in MgO content in shock‐recrystallized apatite compositions. This study shows that the response of apatite to shock deformation can be highly variable, and that application of a combined microstructural and chemical analysis workflow can reveal complex deformation histories in apatite grains, some of which result in changes to crystal structure and composition, which are important for understanding the genesis of apatite in both terrestrial and extraterrestrial environments.

    

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5. Measuring distributions by phase encoding

   https://doi.org/10.1002/mrm.26114  

   Jordanova, Kalina V. ; Nishimura, Dwight G. ; Kerr, Adam B. ( January 2016 , Magnetic Resonance in Medicine)

   We propose a method to acquiredistribution plots by encoding ininstead of image space. Using this method,data is acquired in a different way from traditional spatialmapping, and allows for quick measurement of high dynamic rangedata.

   To encode in, we acquire multiple projections of a slice, each along the same direction, but using a different phase sensitivity to. Using a convex optimization formulation, we reconstruct histograms of thedistribution estimates of the slice.

   We verify in vivodistribution measurements by comparing measured distributions to distributions calculated from reference spatialmaps using the Earth Mover's Distance. Phantom measurements using a surface coil show that for increased spatialvariations, measureddistributions using the proposed method more accurately estimate the distribution than a low‐resolution spatialmap, resulting in a 37% Earth Mover's Distance decrease while using fewer measurements.

   We propose and validate the performance of a method to acquiredistribution information directly without acquiring a spatialmap. The method may provide faster estimates of afield for applications that do not require spatiallocalization, such as the transmit gain calibration of the scanner, particularly for high dynamicranges. Magn Reson Med 77:229–236, 2017. © 2016 Wiley Periodicals, Inc.

    

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