Improved resolution in x-ray tomography by super-resolution.

In this paper, super-resolution imaging is described and evaluated for x-ray tomography and is compared with standard tomography and upscaling during reconstruction. Blurring is minimized due to the negligible point spread of photon counting detectors and an electromagnetically movable micro-focus x-ray spot. Scans are acquired in high and low magnification geometry, where the latter is used to minimize penumbral blurring from the x-ray source. Sharpness and level of detail can be significantly increased in reconstructed slices to the point where the source size becomes the limiting factor. The achieved resolution of the different methods is quantified and compared using biological samples via the edge spread function, modulation transfer function, and Fourier ring correlation.

The governors of school markets? Local education authorities, school choice and equity in Finland and Sweden.

As one of the key elements of the Nordic welfare model, education systems are based on the idea of providing equal educational opportunities, regardless of gender, social class and geographic origin. Since the 1990s, Nordic welfare states have undergone a gradual but wide-ranging transformation towards a more market-based mode of public service delivery. Along this trajectory, the advent of school choice policy and the growing variation in the between-school achievement results have diversified the previously homogenous Nordic education systems. The aim of our paper is to analyse how Finnish and Swedish local education authorities comprehend and respond to the intertwinement of the market logic of school choice and the ideology of equality. The data consist of two sets of in-depth thematic interviews with staff from the local providers of education, municipal education authorities. The analysis discloses the ways in which national legislation has authorized municipal authorities to govern the provision of education.

Se-SAD serial femtosecond crystallography datasets from selenobiotinyl-streptavidin.

We provide a detailed description of selenobiotinyl-streptavidin (Se-B SA) co-crystal datasets recorded using the Coherent X-ray Imaging (CXI) instrument at the Linac Coherent Light Source (LCLS) for selenium single-wavelength anomalous diffraction (Se-SAD) structure determination. Se-B SA was chosen as the model system for its high affinity between biotin and streptavidin where the sulfur atom in the biotin molecule (C10H16N2O3S) is substituted with selenium. The dataset was collected at three different transmissions (100, 50, and 10%) using a serial sample chamber setup which allows for two sample chambers, a front chamber and a back chamber, to operate simultaneously. Diffraction patterns from Se-B SA were recorded to a resolution of 1.9 Å. The dataset is publicly available through the Coherent X-ray Imaging Data Bank (CXIDB) and also on LCLS compute nodes as a resource for research and algorithm development.

Selenium single-wavelength anomalous diffraction de novo phasing using an X-ray-free electron laser.

Structural information about biological macromolecules near the atomic scale provides important insight into the functions of these molecules. To date, X-ray crystallography has been the predominant method used for macromolecular structure determination. However, challenges exist when solving structures with X-rays, including the phase problem and radiation damage. X-ray-free electron lasers (X-ray FELs) have enabled collection of diffraction information before the onset of radiation damage, yet the majority of structures solved at X-ray FELs have been phased using external information via molecular replacement. De novo phasing at X-ray FELs has proven challenging due in part to per-pulse variations in intensity and wavelength. Here we report the solution of a selenobiotinyl-streptavidin structure using phases obtained by the anomalous diffraction of selenium measured at a single wavelength (Se-SAD) at the Linac Coherent Light Source. Our results demonstrate Se-SAD, routinely employed at synchrotrons for novel structure determination, is now possible at X-ray FELs.

Characterization of scintillator-based detectors for few-ten-keV high-spatial-resolution x-ray imaging.

PURPOSE: High-spatial-resolution x-ray imaging in the few-ten-keV range is becoming increasingly important in several applications, such as small-animal imaging and phase-contrast imaging. The detector properties critically influence the quality of such imaging. Here the authors present a quantitative comparison of scintillator-based detectors for this energy range and at high spatial frequencies. METHODS: The authors determine the modulation transfer function, noise power spectrum (NPS), and detective quantum efficiency for Gadox, needle CsI, and structured CsI scintillators of different thicknesses and at different photon energies. An extended analysis of the NPS allows for direct measurements of the scintillator effective absorption efficiency and effective light yield as well as providing an alternative method to assess the underlying factors behind the detector properties. RESULTS: There is a substantial difference in performance between the scintillators depending on the imaging task but in general, the CsI based scintillators perform better than the Gadox scintillators. At low energies (16 keV), a thin needle CsI scintillator has the best performance at all frequencies. At higher energies (28-38 keV), the thicker needle CsI scintillators and the structured CsI scintillator all have very good performance. The needle CsI scintillators have higher absorption efficiencies but the structured CsI scintillator has higher resolution. CONCLUSIONS: The choice of scintillator is greatly dependent on the imaging task. The presented comparison and methodology will assist the imaging scientist in optimizing their high-resolution few-ten-keV imaging system for best performance.

Intentions and knowledge shaping local safety policy: A comparison of two Swedish cities.

INTRODUCTION: This article explores how intentions and knowledge shape two Swedish cities' local safety policy (LSP). METHOD: The applied framework is derived from the integration of governance and implementation research and the theory of knowledge and its use. RESULTS: The study shows that LSPs are shaped by a mix of intentions and different kinds of knowledge, and intentions and knowledge interplay and intertwine in many ways. Key-persons construct LSPs when they work out solutions to urgent safety problems and take departure in the local context, its pre-conditions, and their experience-based and professional knowledge. The state governs LSP softly through management by objectives in the background, but more often key-actor intentions and commitments, local safety problems, and events initiated and influenced LSPs. CONCLUSIONS: The article contributes to a better understanding of conditions for LSPs in multi-level governance. PRACTICAL APPLICATIONS: The article can be used to improve governance, identify implementation problems and knowledge needs that will improve LSPs and the overall safety situation in the community. The study has implications for how LSPs should be evaluated; many different evaluation criteria can be appropriate, such as relevance, legitimacy, achievement of key actors' objectives, sustainability of policy solutions, and creation of a local safety culture.

Speckle-based x-ray phase-contrast imaging with a laboratory source and the scanning technique.

The speckle-based scanning method for x-ray phase-contrast imaging is implemented with a liquid-metal-jet source. Using the two-dimensional scanning technique, the phase shift introduced by the object is retrieved in both transverse orientations, and the limitations on spatial resolution inherent to the speckle-tracking technique are avoided. This method opens up possibilities of new high-resolution multimodal applications for lab-based phase-contrast x-ray imaging.

Laboratory x-ray fluorescence tomography for high-resolution nanoparticle bio-imaging.

We demonstrate that nanoparticle x-ray fluorescence computed tomography in mouse-sized objects can be performed with very high spatial resolution at acceptable dose and exposure times with a compact laboratory system. The method relies on the combination of the 24 keV line-emission from a high-brightness liquid-metal-jet x-ray source, pencil-beam-forming x-ray optics, photon-counting energy-dispersive detection, and carefully matched (Mo) nanoparticles. Phantom experiments and simulations show that the arrangement significantly reduces Compton background and allows 100 µm detail imaging at dose and exposure times compatible with small-animal experiments. The method provides a possible path to in vivo molecular x-ray imaging at sub-100 µm resolution in mice.

First application of liquid-metal-jet sources for small-animal imaging: high-resolution CT and phase-contrast tumor demarcation.

PURPOSE: Small-animal studies require images with high spatial resolution and high contrast due to the small scale of the structures. X-ray imaging systems for small animals are often limited by the microfocus source. Here, the authors investigate the applicability of liquid-metal-jet x-ray sources for such high-resolution small-animal imaging, both in tomography based on absorption and in soft-tissue tumor imaging based on in-line phase contrast. METHODS: The experimental arrangement consists of a liquid-metal-jet x-ray source, the small-animal object on a rotating stage, and an imaging detector. The source-to-object and object-to-detector distances are adjusted for the preferred contrast mechanism. Two different liquid-metal-jet sources are used, one circulating a Ga∕In∕Sn alloy and the other an In∕Ga alloy for higher penetration through thick tissue. Both sources are operated at 40-50 W electron-beam power with ∼7 µm x-ray spots, providing high spatial resolution in absorption imaging and high spatial coherence for the phase-contrast imaging. RESULTS: High-resolution absorption imaging is demonstrated on mice with CT, showing 50 µm bone details in the reconstructed slices. High-resolution phase-contrast soft-tissue imaging shows clear demarcation of mm-sized tumors at much lower dose than is required in absorption. CONCLUSIONS: This is the first application of liquid-metal-jet x-ray sources for whole-body small-animal x-ray imaging. In absorption, the method allows high-resolution tomographic skeletal imaging with potential for significantly shorter exposure times due to the power scalability of liquid-metal-jet sources. In phase contrast, the authors use a simple in-line arrangement to show distinct tumor demarcation of few-mm-sized tumors. This is, to their knowledge, the first small-animal tumor visualization with a laboratory phase-contrast system.

Phase retrieval in X-ray phase-contrast imaging suitable for tomography.

In-line phase-contrast X-ray imaging provides images where both absorption and refraction contribute. For quantitative analysis of these images, the phase needs to be retrieved numerically. There are many phase-retrieval methods available. Those suitable for phase-contrast tomography, i.e., non-iterative phase-retrieval methods that use only one image at each projection angle, all follow the same pattern though derived in different ways. We outline this pattern and use it to compare the methods to each other, considering only phase-retrieval performance and not the additional effects of tomographic reconstruction. We also outline derivations, approximations and assumptions, and show which methods are similar or identical and how they relate to each other. A simple scheme for choosing reconstruction method is presented, and numerical phase-retrieval performed for all methods.