Learning-based Single-step Quantitative Susceptibility Mapping Reconstruction Without Brain Extraction

Quantitative susceptibility mapping (QSM) estimates the underlying tissue magnetic susceptibility from MRI gradient-echo phase signal and typically requires several processing steps. These steps involve phase unwrapping, brain volume extraction, background phase removal and solving an ill-posed inverse problem. The resulting susceptibility map is known to suffer from inaccuracy near the edges of the brain tissues, in part due to imperfect brain extraction, edge erosion of the brain tissue and the lack of phase measurement outside the brain. This inaccuracy has thus hindered the application of QSM for measuring the susceptibility of tissues near the brain edges, e.g., quantifying cortical layers and generating superficial venography. To address these challenges, we propose a learning-based QSM reconstruction method that directly estimates the magnetic susceptibility from total phase images without the need for brain extraction and background phase removal, referred to as autoQSM. The neural network has a modified U-net structure and is trained using QSM maps computed by a two-step QSM method. 209 healthy subjects with ages ranging from 11 to 82 years were employed for patch-wise network training. The network was validated on data dissimilar to the training data, e.g. in vivo mouse brain data and brains with lesions, which suggests that the network has generalized and learned the underlying mathematical relationship between magnetic field perturbation and magnetic susceptibility. AutoQSM was able to recover magnetic susceptibility of anatomical structures near the edges of the brain including the veins covering the cortical surface, spinal cord and nerve tracts near the mouse brain boundaries. The advantages of high-quality maps, no need for brain volume extraction and high reconstruction speed demonstrate its potential for future applications.Comment: 26 page

A Planar InGaAs/InP Geiger Mode Avalanche Photodiode with Cascade Edge Breakdown Suppression

A Geiger mode planar InGaAs/InP avalanche photodiode （APD） with a cascade peripheral junction structure to suppress edge breakdowns is designed by finite-element analysis. The photodiode breakdown voltage is reduced to 54.3V by controlling the central junction depth, while the electric field distribution along the device central axis is controlled by adjusting doping level and thickness of the lnP field control layer. Using a cascade junction structure at the periphery of the active area, premature edge breakdowns are effectively suppressed. The simulations show that the quadra-cascade structure is a good trade-off between suppression performance and fabrication complexity, with a reduced peak electric field of 5.2 × 10~5 kV/cm and a maximum hole ionization integral of 1. 201. Work presented in this paper provides an effective way to design high performance photon counting InGaAs/InP avalanche photodiodes

Effects of Chemical Short-Range Order and Lattice Distortion on Crack-Tip Behavior of Medium-Entropy Alloy by Atomistic Simulations

It is well demonstrated that the complex chemical fluctuations on high/medium-entropy alloys (H/MEAs) play critical roles in their deformation process, but there are few reports related to the effect of such complex chemical fluctuations on the crack behavior. In this paper, the effects of chemical short-range order (CSRO) and lattice distortion (LD) on the crack-tip behavior of CrCoNi MEAs under mode I loading at room temperature are investigated by carrying out molecular dynamics (MD) simulation, hybrid MD/Monte-Carlo (MC) simulation and the J-integral method. The results reveal that CSRO can improve the J-integral value without significant changes in the localized deformation zone size. On the contrary, LD can lower the J-integral value with an increase in the localized deformation zone size. The energetic analysis shows that CSRO improves the activation energy barrier of Shockley partial dislocation from the crack-tip while LD reduces the activation energy barrier. Our work is a step forward in understanding the effects of CSRO and LD on the crack-tip behavior and deformation mechanisms of CrCoNi MEAs

Grain boundary phase transformation in a CrCoNi complex concentrated alloy

The phase-transformation-like behavior of grain boundaries, where their chemistry, structure and properties change discontinuously, is emerging as a fundamental interest in the context of grain boundary engineering. The cases studied so far pertain primarily to elemental metals and dilute alloy systems. The next step of complexity would be a system still of a single phase structure but involving multiple and concentrated elements. The recently emerging high/medium entropy alloys (H/MEAs), alternatively known as complex concentration alloys (CCAs), fit the bill in this regard. Here we use CoCrNi as a model CCA to highlight that intricate interatomic interactions influence the grain boundary element distribution and consequently phase transitions. By combining classical molecular dynamics simulations and first-principles density functional theory calculations, we reveal that grain boundary phase transition temperature is sensitive to the grain boundary atomic configuration. The CCA grain boundary with random atomic distribution is more apt to transform due to a larger thermodynamic driving force and faster diffusion kinetics, compared to a hypothetical reference that bears the same bulk properties but no distinguishable constituent components. In contrast, when the three elements redistribute with local Ni-clustering and Co & ndash;Cr ordering in the grain boundary structure, diffusion is rendered more sluggish, delaying grain boundary phase transformation. Our work is a step forward in understanding critical phenomena in grain boundaries and CCAs, and enriches the knowledge base for materials design via grain boundary engineering. (c) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Crystal Plasticity Model Analysis of the Effect of Short-Range Order on Strength-Plasticity of Medium Entropy Alloys

Numerous studies have demonstrated the widespread presence of chemical short-range order (SRO) in medium and high entropy alloys (M/HEAs). However, the mechanism of their influence on macroscopic mechanical behavior remains to be understood. In this paper, we propose a novel dislocation-based model of crystal plasticity, by considering both the dislocation blocking and coplanar slip induced by SRO. The effect of SRO on the plastic deformation of CoCrNi MEAs was investigated. We found that the yield strength increases monotonically with increasing SRO-induced slip resistance, but the elongation first appeared to increase and then decreased. Further analysis suggested that the plastic elongation is a result of the competition between grain rotation-induced deformation coordination and stress concentration, which depends on the slip resistance of the SRO

A new Al5Cu6(Li,Mg)2 cubic phase in an Al-Cu-Li-Mg-X alloy

Among cubic metals, the cubic phase has excellent heat-resistance potential owing to its cube-on-cube orientation relationship with the matrix. In this study, a new cubic phase, Al5Cu6(Li,Mg)2, was identified in an Al-CuLi-Mg-X alloy after ageing at 165 degrees C for 15 h. It has a Pm3 structure similar to that of both Al5Cu6Li2 and Al5Cu6Mg2. The density functional theory (DFT) calculations noted that the precipitation of Al5Cu6Li2 is more energetically favorable than that of Al5Cu6Mg2 in the Al-Cu-Li-Mg-X alloy. Moreover, the formation enthalpy may be further reduced when the Mg atoms replace some Li sites in Al5Cu6Li2, in favor of the formation of Al5Cu6(Li,Mg)2

Effect of non-isothermal creep aging on the microstructure, mechanical properties and stress corrosion cracking resistance of 7075 alloy

The effect of non-isothermal creep aging (NICA) on the microstructure, mechanical properties, and stress corrosion cracking (SCC) resistance of 7075 alloy was investigated. The results showed that the tensile strength of the alloy increased to 565 MPa when the alloy was heated to 210 °C (CH210) and reached 580 MPa when it was subsequently cooled to 120 °C (CC120). Simultaneously, the SCC susceptibility of rtf increased from 50.8 % to 98.4 %. As compared with traditional creep aging process [1], a large strength increment with excellent stress corrosion resistance have been obtained by NICA. The microstructure revealed that a lot of dislocations have been introduced by creep during the heating stage which could improve the precipitates volume fraction and accelerate the diffusion of solutes; while during the cooling stage, η′ was greatly refined, and GPI and GPⅡ were re-precipitated from the matrix due to the decreased solid solubility and increased critical radius R*; both of them are responsible for the continuous strength increase during NICA. Moreover, the width of the precipitate free zone (PFZ) was narrowed from 46.1 nm (CH210) to 28.6 nm (CC120). The microchemical analysis reveals that solutes were more homogenously distributed in grain boundary precipitates (GB-ppts), matrix precipitates, the PFZ, and the matrix with the help of creep. The narrower PFZ and homogeneous solute distribution are responsible for improving the SCC susceptibility in the CC120 alloy