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Acquisition and reconstruction methods for high-resolution, high-fidelity 3D multi-slab diffusion MRI
Get PDFDiffusion magnetic resonance imaging (dMRI) is a powerful tool for non-invasive mapping of brain microstructure and connectivity by measuring the random walk of water molecules. Conventional two-dimensional (2D) single-shot echo planar imaging (EPI) is widely used for dMRI due to its rapid acquisition and robustness against motion, but suffers from limitations such as long repetition times (TR) that reduce signal-to-noise ratio (SNR) efficiency and difficulty in achieving thin slice thickness, particularly for high-resolution imaging. To address these issues, three-dimensional (3D) multi-slab acquisition methods have been introduced, offering better SNR efficiency through shorter TR and the potential to achieve high, isotropic resolution dMRI for in-vivo human brain. However, key challenges remain for 3D multi-slab dMRI, including susceptibility to B0 inhomogeneity induced geometric distortions that compromise image anatomical fidelity, inefficiencies in current navigator-based methods for correcting motion-induced phase variations, and the need for careful considerations of blurring and noise to achieve true high spatial resolution.
This thesis develops novel acquisition and reconstruction methods to overcome these hurdles. First, sampling strategies and a joint reconstruction for blip-up/down data are designed to correct geometric distortions and slab boundary aliasing for 3D multi-slab imaging without increasing the scan time. Second, a self-navigated 3D multi-slab dMRI framework is developed, eliminating the need for navigator acquisitions through optimised self-navigation sampling and structured low-rank reconstruction for motion phase correction, which effectively shortens scan time and improves SNR efficiency. Lastly, we leverage in-plane segmented 3D multi-slab EPI and a denoiser-regularised reconstruction to achieve 0.5-0.6 mm isotropic resolutions for in-vivo dMRI with superior SNR, minimal blurring and distortions. These advances are expected to improve the image fidelity and scan efficiency of high-resolution 3D multi-slab dMRI, with the hope to ultimately benefit neuroscience research of the human brain
Nuclear dimension of extensions of commutative C ∗ -algebras by Kirchberg algebras
Get PDFWe compute the nuclear dimension of extensions of C∗-algebras involving commutative unital quotients and stable Kirchberg ideals. We identify the finite directed graphs whose C∗-algebras are covered by this theorem
Breaking Bonds at Tin(II): Reductive or Oxidative Addition?
Get PDFHere, the addition of H─H, Be─Be, B─B, and B─H bonds to SnII is studied by experimental and theoretical means. We report the first examples of B─B and Be─Be bond additions to a main group metal centre, along with the first example of borazine B─H bond addition to any element of the Periodic Table. Quantum chemical calculations suggest that the classification of these reactions as prototypical “oxidative” addition processes may be misleading and that a more nuanced description is warranted
Status Threat, Political Competition, and the Expansion of Voter Identification Laws in the United States
Get PDFObjective: Voter identification requirements in the U.S. are on a dramatic uptick and are becoming less forgiving. When do states phase in or adopt stricter voter ID laws? I propose an elite‐driven explanation. Building on the “racial threat” hypothesis, I argue that these provisions are a legal device exploited by legislators representing white majorities to safeguard the status quo in view of the “threat” posed to the establishment by the growing size of racial minorities, and that this effect is moderated by political competition. I introduce the concept of “majority gap,” the surplus of population belonging to the majority in relation to the size of all minority groups combined. Methods: I test my theory in the context of state legislatures, using two‐way fixed‐effects regression. Results: I show that, on average, Republicans present voter identification‐related proposals twice as often as Democrats and that governors rarely interfere with these proposals. I find that when Republicans control both chambers of a state assembly, the shrinking magnitude of the majority gap is associated with a significant expansion in ID requirements. Conclusion: This article contributes to the research on the disenfranchisement of racial minorities by shedding some light on the expansion of voter identification laws as a mechanism for White‐controlled legislatures to potentially “gatekeep” the polls
Vector-based loss functions for turbulent flow field inpainting
No full textWhen developing scientific machine learning (ML) approaches, it is often beneficial to embed knowledge of the physical system in question into the training process. One way to achieve this is by leveraging the specific characteristics of the data at hand. In the case of turbulent flows, fluid velocities can be measured and recorded as multi-component vectors at discrete points in space, using techniques such as particle image velocimetry (PIV) or computational fluid mechanics (CFD). However, the vectorised nature of the data is ignored by standard ML approaches, as widely-used loss functions such as the mean-square error treat each component of a velocity vector in isolation. Therefore, the aim of this work is to better preserve the physical characteristics of the data by introducing loss functions that utilise vector similarity metrics. To this end, vector-based loss functions are developed here and implemented alongside a U-Net model for a turbulent flow field inpainting problem, amounting to the prediction of velocity vectors inside large gaps in PIV images. The intention is for the inpainting task to pose a significant challenge for the ML models in order to shed light on their capabilities. The test case uses PIV data from the highly turbulent flow in the well-known Transparent Combustion Chamber III (TCC-III) engine. Loss functions based on the cosine similarity and vector magnitude differences are proposed; the results show that the vector-based loss functions lead to significantly improved predictions of multi-scale flow patterns, while a hybrid (vector and mean-square error) loss function enables a good compromise to be found between preserving multi-scale behaviour and pixel-wise accuracy
Hierarchical mechanical patterns in morphogenesis: from mollusc shells to plants, fungi and animals
Get PDFHierarchical patterns, made up of subunits of different sizes intercalated with each other, are present in diverse organisms spanning the plant, fungi, and animal kingdoms. Despite these structures appearing in different kingdoms, at different scales, at different levels of biological organization, and involving different developmental mechanisms, their sequential development follows a generic principle of recursive subdivision of space, associated with domain growth and an irreversibility condition. To investigate the morphogenesis of such hierarchical patterns, we develop a theoretical framework, based on morphoelasticity, the biomechanics of growth. In our model, the hierarchical pattern is modelled as a sum of Gaussians, each representing a subunit. The size and spacing of these Gaussians is then determined by minimizing the mechanical energy of the growing system. Our framework is simple enough to be analytically tractable, enabling us to identify the mechanisms necessary for hierarchical pattern formation and providing new insight into the developmental process involved. Our work is specifically motivated by and applied to the context of mollusc shells, in which the hierarchical ridge pattern is in some species dilated to form beautifully exuberant shell edges, which may take the form of a row of needle-like or fractal-like spines, nevertheless maintaining the hierarchical form
Education and learning: preliminary findings from the 2023–24 Young Lives Survey (round 7): India (in the states of Andhra Pradesh and Telangana)
Get PDFThis factsheet sets out headline findings on education and learning from Round 7 of the Young Lives survey in India, carried out in 2023–24 when the Younger Cohort was 22 years old and the Older Cohort was 29. It outlines changes in education and learning over time by comparing the Younger Cohort at age 22 with the Older Cohort at the same age in 2016, and exploring the Younger Cohort's progression from age 15 to 22. The headlines on education and learning in India are:
Over the last seven years, secondary school completion has improved.
University enrolment has increased among the 22-year-olds in 2023 compared to 2016.
Early life inequalities predict education outcomes, particularly higher education enrolment and school completion by age 22.
The gender gap in secondary completion has closed by age 22. However, women are less likely to enrol in university.
Women who married before the legal age, or had children at early ages achieve the lowest education outcomes.
Internet access has grown substantially, but a digital divide by gender, wealth and caste, though reduced, still persists.
More schooling years and higher completion rates have not translated into an improvement in reading comprehension test performance between ages 15 and 22.
Policies addressing the additional barriers faced by women from disadvantaged
This factsheet is one of a series sharing headline findings from Round 7. In addition to education and learning, there are factsheets on health, nutrition and well-being, and work and family lives in Ethiopia, India and Peru. You can access all the factsheets on our publications page
Industrial policy revisited
Get PDFIn the past decade, there has been a global resurgence in attention to industrial policy (IP), a resurgence that cuts across political ideologies and geographic regions. IPs are inherently political, intimately connected to the roles of the state in the economy and of states within an international economic system. This review demonstrates that while overt IPs have waxed and waned in their political acceptability in the aftermath of World War II, IPs have always remained part of the policy tool kit. In using IP, policymakers have had to navigate three common governance domains: building coalitions to support productive investments, building the state’s capacity to collaborate with and discipline the private sector, and creating political incentives for credible commitments to firms. Nonetheless, the political dynamics in each of these domains have shifted over time. Historically, IPs focused on export-based catch-up strategies, requiring the mobilization of coalitions around manufacturing investment and export discipline. Today’s IPs often target frontier technologies and aim to address perceived vulnerabilities in global supply chains and new geopolitical competition, demanding greater experimentation with more uncertain economic outcomes and higher risks of failure.We trace the evolution of the literature on IP through four phases: state-led developmental policies, the changing coalitions and institutions in a globally fragmented production system, neoliberalism, and the more recent renewed focus on transformative IP
Work and family lives: preliminary findings from the 2023–24 Young Lives Survey (round 7): Peru
Get PDFThis factsheet sets out headline findings on work and family lives from Round 7 of the Young Lives survey in Peru, carried out in 2023–24 when the Younger Cohort was 22 years old and the Older Cohort was 29. It outlines changes in education and learning over time by comparing the Younger Cohort at age 22 with the Older Cohort at the same age in 2016, and exploring the Younger Cohort's progression from age 15 to 22. The headlines on work and family lives in Peru are:
- More Younger Cohort participants are studying at age 22 compared to the Older Cohort at the same age in 2016.
- Most young people who are working are engaged in poor-quality jobs, without a written contract and working long hours.
- There is a significant gender difference in the distribution of work, with women less likely to be employed than men and spending more time on unpaid care work.
- Early marriage and early parenthood for women have decreased over time.
-Current employment, fertility and marital status of the participants are closely related to early life inequalities.
This factsheet is one of a series sharing headline findings from Round 7. In addition to work and family lives, there are factsheets on health, nutrition and well-being, and education and learning in Ethiopia, India and Peru. You can access all the factsheets on our publications page
External validity of randomized clinical trials in vascular surgery: systematic review of demographic factors of patients recruited to randomized clinical trials with comparison to the National Vascular Registry
Get PDFBackground: Evidence-based medicine relies on randomized clinical trials, which should represent the patients encountered in clinical practice. Characteristics of patients recruited to randomized clinical trials involving vascular index operations (carotid endarterectomy, abdominal aortic aneurysm repair, infrainguinal bypass and major lower limb amputations) were compared with those recorded in the National Vascular Registry across England and Wales. Methods: MEDLINE, Embase, Web of Science, CENTRAL, clinicaltrials.gov and World Health Organization International Trials Registry Platform (CRD42021247905) were searched for randomized clinical trials involving the index operations. Demographic (age, sex, ethnicity) and clinical (co-morbidities, medications, body mass index, smoking, alcohol, cognition) data were extracted, by operation. Characteristics of operated on patients were extracted from publicly available National Vascular Registry reports (2014–2020). All findings are reported according to PRISMA guidelines. Rayyan.AI, Excel and GraphPad Prism were used for screening and analysis. Results: A total of 307 randomized clinical trials (66 449 patients) were included and compared with National Vascular Registry data for 119 019 patients. Randomized clinical trial patients were younger across all operations; for carotid endarterectomy, bypass and major lower limb amputation randomized clinical trials, there were differences in female patient representation. Further comparisons were limited by the insufficient baseline data reporting across randomized clinical trials, though reporting improved over decades. National Vascular Registry reports lacked information on patient factors such as patient ethnicity or body mass index. Conclusions: There are significant differences in demographic and clinical factors between patients recruited to vascular surgery randomized clinical trials and the real-world National Vascular Registry vascular surgery patient population. Minimum reporting standards for baseline data should be defined to allow future randomized clinical trials to represent real-world patient populations and ensure the external validity of their results