Synchrotron x ray induced axonal transections in the brain of rats assessed by high-field diffusion tensor imaging tractography

Since approximately two thirds of epileptic patients are non-eligible for surgery, local axonal fiber transections might be of particular interest for them. Micrometer to millimeter wide synchrotron-generated X-ray beamlets produced by spatial fractionation of the main beam could generate such fiber disruptions non-invasively. The aim of this work was to optimize irradiation parameters for the induction of fiber transections in the rat brain white matter by exposure to such beamlets. For this purpose, we irradiated cortex and external capsule of normal rats in the antero-posterior direction with a 4 mm×4 mm array of 25 to 1000 µm wide beamlets and entrance doses of 150 Gy to 500 Gy. Axonal fiber responses were assessed with diffusion tensor imaging and fiber tractography; myelin fibers were examined histopathologically. Our study suggests that high radiation doses (500 Gy) are required to interrupt axons and myelin sheaths. However, a radiation dose of 500 Gy delivered by wide minibeams (1000 µm) induced macroscopic brain damage, depicted by a massive loss of matter in fiber tractography maps. With the same radiation dose, the damage induced by thinner microbeams (50 to 100 µm) was limited to their paths. No macroscopic necrosis was observed in the irradiated target while overt transections of myelin were detected histopathologically. Diffusivity values were found to be significantly reduced. A radiation dose ≤ 500 Gy associated with a beamlet size of < 50 µm did not cause visible transections, neither on diffusion maps nor on sections stained for myelin. We conclude that a peak dose of 500 Gy combined with a microbeam width of 100 µm optimally induced axonal transections in the white matter of the brain.

In-situ X-ray micro-diffraction studies of heterogeneous interfaces between cementitious materials and geological formations

In the present study the challenge of analyzing complex micro X-ray diffraction (microXRD) patterns from cement–clay interfaces has been addressed. In order to extract the maximum information concerning both the spatial distribution and the crystal structure type associated with each of the many diffracting grains in heterogeneous, polycrystalline samples, an approach has been developed in which microXRD was applied to thin sections which were rotated in the X-ray beam. The data analysis, performed on microXRD patterns collected from a filled vein of a cement–clay interface from the natural analogue in Maqarin (Jordan), and a sample from a two-year-old altered interface between cement and argillaceous rock, demonstrate the potential of this method.

Hohlraum Target for Overcoming Refractive Losses in Plasma X-ray Lasers

Refractive losses in laser-produced plasmas used as gain media are caused by electron density gradients, and limit the energy transport range. The pump pulse is thus deflected from the high-gain region and the short wavelength laser signal also steers away, causing loss of collimation. A Hohlraum used as a target makes the plasma homogeneous and can mitigate refractive losses by means of wave-guiding. A computational study combining a hydrodynamics code and an atomic physics code is presented, which includes a ray-tracing modeling based on the eikonal theory of the trajectory equation. This study presents gain calculations based on population inversion produced by free-electron collisions exciting bound electrons into metastable levels in the 3d94d1(J = 0) → 3d94p1(J = 1) transition of Ni-like Sn. Further, the Hohlraum suggests a dramatic enhancement of the conversion efficiency of collisionally excited x-ray lasing for Ni-like Sn.

High-density large-scale field emitter arrays for x-ray free electron laser cathodes

High brightness electron sources are of great importance for the operation of the hard X-ray free electron lasers. Field emission cathodes based on the double-gate metallic field emitter arrays (FEAs) can potentially offer higher brightness than the currently used ones. We report on the successful application of electron beam lithography for fabrication of the large-scale single-gate as well as double-gate FEAs. We demonstrate operational high-density single-gate FEAs with sub-micron pitch and total number of tips up to 106 as well as large-scale double-gate FEAs with large collimation gate apertures. The details of design, fabrication procedure and successful measurements of the emission current from the single- and double-gate cathodes are presented.

X-ray image calibration and its application to clinical orthopedics.

X-ray imaging is one of the most commonly used medical imaging modality. Albeit X-ray radiographs provide important clinical information for diagnosis, planning and post-operative follow-up, the challenging interpretation due to its 2D projection characteristics and the unknown magnification factor constrain the full benefit of X-ray imaging. In order to overcome these drawbacks, we proposed here an easy-to-use X-ray calibration object and developed an optimization method to robustly find correspondences between the 3D fiducials of the calibration object and their 2D projections. In this work we present all the details of this outlined concept. Moreover, we demonstrate the potential of using such a method to precisely extract information from calibrated X-ray radiographs for two different orthopedic applications: post-operative acetabular cup implant orientation measurement and 3D vertebral body displacement measurement during preoperative traction tests. In the first application, we have achieved a clinically acceptable accuracy of below 1° for both anteversion and inclination angles, where in the second application an average displacement of 8.06±3.71 mm was measured. The results of both applications indicate the importance of using X-ray calibration in the clinical routine.

Combining Monte Carlo methods with coherent wave optics for the simulation of phase-sensitive X-ray imaging

Phase-sensitive X-ray imaging shows a high sensitivity towards electron density variations, making it well suited for imaging of soft tissue matter. However, there are still open questions about the details of the image formation process. Here, a framework for numerical simulations of phase-sensitive X-ray imaging is presented, which takes both particle- and wave-like properties of X-rays into consideration. A split approach is presented where we combine a Monte Carlo method (MC) based sample part with a wave optics simulation based propagation part, leading to a framework that takes both particle- and wave-like properties into account. The framework can be adapted to different phase-sensitive imaging methods and has been validated through comparisons with experiments for grating interferometry and propagation-based imaging. The validation of the framework shows that the combination of wave optics and MC has been successfully implemented and yields good agreement between measurements and simulations. This demonstrates that the physical processes relevant for developing a deeper understanding of scattering in the context of phase-sensitive imaging are modelled in a sufficiently accurate manner. The framework can be used for the simulation of phase-sensitive X-ray imaging, for instance for the simulation of grating interferometry or propagation-based imaging.

Liquid–Vapor Interface of Formic Acid Solutions in Salt Water: A Comparison of Macroscopic Surface Tension and Microscopic in Situ X-ray Photoelectron Spectroscopy Measurements

The liquid–vapor interface is difficult to access experimentally but is of interest from a theoretical and applied point of view and has particular importance in atmospheric aerosol chemistry. Here we examine the liquid–vapor interface for mixtures of water, sodium chloride, and formic acid, an abundant chemical in the atmosphere. We compare the results of surface tension and X-ray photoelectron spectroscopy (XPS) measurements over a wide range of formic acid concentrations. Surface tension measurements provide a macroscopic characterization of solutions ranging from 0 to 3 M sodium chloride and from 0 to over 0.5 mole fraction formic acid. Sodium chloride was found to be a weak salting out agent for formic acid with surface excess depending only slightly on salt concentration. In situ XPS provides a complementary molecular level description about the liquid–vapor interface. XPS measurements over an experimental probe depth of 51 Å gave the C 1s to O 1s ratio for both total oxygen and oxygen from water. XPS also provides detailed electronic structure information that is inaccessible by surface tension. Density functional theory calculations were performed to understand the observed shift in C 1s binding energies to lower values with increasing formic acid concentration. Part of the experimental −0.2 eV shift can be assigned to the solution composition changing from predominantly monomers of formic acid to a combination of monomers and dimers; however, the lack of an appropriate reference to calibrate the absolute BE scale at high formic acid mole fraction complicates the interpretation. Our data are consistent with surface tension measurements yielding a significantly more surface sensitive measurement than XPS due to the relatively weak propensity of formic acid for the interface. A simple model allowed us to replicate the XPS results under the assumption that the surface excess was contained in the top four angstroms of solution.