X‐ray spectra by means of Monte Carlo simulations for imaging applications

X‐ray tubes have a broad range of applications worldwide, including several techniques for atomic physics, like X‐ray fluorescence, as well as for medical imaging, like computed tomography. The performances of X‐ray imaging detectors have shown to be significantly sensitive to the incident beam spectrum. Therefore, an accurate knowledge of the X‐ray beam becomes necessary for the emission source characterization and the whole imaging process comprehension. Direct measurements and suitable Monte Carlo simulations may be used to establish the X‐ray spectra. Dedicated Monte Carlo simulation routines, based on the PENELOPE code, have been developed to determine the Bremsstrahlung X‐ray spectra generated by conventional X‐ray tubes. The simulated spectra have been validated by comparison with the corresponding experimental data showing an overall good agreement. The incorporation of a suitably designed virtual grid allowed to assess the angular distribution of Bremsstrahlung yield, showing a remarkable anisotropy. In addition, a dedicated program has been developed for virtual imaging, which enables to perform suitable X‐ray absorption contrast images. Also, the developed program includes a user‐friendly graphic interface to allow the upload of required input parameters, which include setup arrangement, beam characteristics, sample properties and image simulation parameters (spatial resolution, tracks per run, etc.). The software includes dedicated subroutines which handle the physical process from X‐ray generation up to detector signal acquisition. The aim of the developed program is to perform virtual imaging by means of absorption contrast and using conventional X‐ray sources, which may be a useful tool for the study the X‐ray imaging techniques in several research fields as well as for educational purposes. The performed comparisons with experimental data have shown good agreement. The obtained results for X‐ray imaging may constitute useful information for the comprehension and improvement of X‐ray image quality, like absorption contrast optimization, detail visualization, definition and detectability. Copyright © 2010 John Wiley & Sons, Ltd.     

Multimodal hard X‐ray imaging of a mammography phantom at a compact synchrotron light source

The Compact Light Source is a miniature synchrotron producing X‐rays at the interaction point of a counter‐propagating laser pulse and electron bunch through the process of inverse Compton scattering. The small transverse size of the luminous region yields a highly coherent beam with an angular divergence of a few milliradians. The intrinsic monochromaticity and coherence of the produced X‐rays can be exploited in high‐sensitivity differential phase‐contrast imaging with a grating‐based interferometer. Here, the first multimodal X‐ray imaging experiments at the Compact Light Source at a clinically compatible X‐ray energy of 21 keV are reported. Dose‐compatible measurements of a mammography phantom clearly demonstrate an increase in contrast attainable through differential phase and dark‐field imaging over conventional attenuation‐based projections.     

Design,development and first experiments on the X‐ray imaging beamline at Indus‐2 synchrotron source RRCAT,India

A full‐field hard X‐ray imaging beamline (BL‐4) was designed, developed, installed and commissioned recently at the Indus‐2 synchrotron radiation source at RRCAT, Indore, India. The bending‐magnet beamline is operated in monochromatic and white beam mode. A variety of imaging techniques are implemented such as high‐resolution radiography, propagation‐ and analyzer‐based phase contrast imaging, real‐time imaging, absorption and phase contrast tomography etc. First experiments on propagation‐based phase contrast imaging and micro‐tomography are reported.     

Single‐cell resolution in high‐resolution synchrotron X‐ray CT imaging with gold nanoparticles

Gold nanoparticles are excellent intracellular markers in X‐ray imaging. Having shown previously the suitability of gold nanoparticles to detect small groups of cells with the synchrotron‐based computed tomography (CT) technique both ex vivo and in vivo, it is now demonstrated that even single‐cell resolution can be obtained in the brain at least ex vivo. Working in a small animal model of malignant brain tumour, the image quality obtained with different imaging modalities was compared. To generate the brain tumour, 1 × 10⁵ C6 glioma cells were loaded with gold nanoparticles and implanted in the right cerebral hemisphere of an adult rat. Raw data were acquired with absorption X‐ray CT followed by a local tomography technique based on synchrotron X‐ray absorption yielding single‐cell resolution. The reconstructed synchrotron X‐ray images were compared with images obtained by small animal magnetic resonance imaging. The presence of gold nanoparticles in the tumour tissue was verified in histological sections.     

High‐energy X‐ray diffraction using the Pixium 4700 flat‐panel detector

The Pixium 4700 detector represents a significant step forward in detector technology for high‐energy X‐ray diffraction. The detector design is based on digital flat‐panel technology, combining an amorphous Si panel with a CsI scintillator. The detector has a useful pixel array of 1910 × 2480 pixels with a pixel size of 154 µm × 154 µm, and thus it covers an effective area of 294 mm × 379 mm. Designed for medical imaging, the detector has good efficiency at high X‐ray energies. Furthermore, it is capable of acquiring sequences of images at 7.5 frames per second in full image mode, and up to 60 frames per second in binned region of interest modes. Here, the basic properties of this detector applied to high‐energy X‐ray diffraction are presented. Quantitative comparisons with a widespread high‐energy detector, the MAR345 image plate scanner, are shown. Other properties of the Pixium 4700 detector, including a narrow point‐spread function and distortion‐free image, allows for the acquisition of high‐quality diffraction data at high X‐ray energies. In addition, high frame rates and shutterless operation open new experimental possibilities. Also provided are the necessary data for the correction of images collected using the Pixium 4700 for diffraction purposes.     

Integrating 3D images using laboratory‐based micro X‐ray computed tomography and confocal X‐ray fluorescence techniques

The application of non‐destructive imaging to characterizing samples has become more important as the costs of samples increase. Imaging a sample via X‐ray techniques is preferable when altering or even touching the sample affects its properties, or when the sample is fielded after characterization. Two laboratory‐based X‐ray techniques used at Los Alamos include micro X‐ray computed tomography (MXCT) and confocal micro X‐ray fluorescence (confocal MXRF). Both methods create a 3D rendering of the sample non‐destructively. MXCT produces a high‐resolution (sub‐µm voxel) rendering of the sample based upon X‐ray absorption; the resulting model is a function of density and does not contain any elemental information. Confocal MXRF produces an elementally specific 3D rendering of the sample, but at a lower (30 × 30 × 65 µm) resolution. By combining data from these two techniques, scientists provided a more comprehensive method of analysis. We will describe a MATLAB routine written to render each of these data sets individually and/or within the same coordinate system. This approach is shown in the analysis of two samples: an integrated circuit surface mounted resistor and a machined piece of polystyrene foam. The samples chosen provide an opportunity to compare and contrast the two X‐ray techniques, identify their weaknesses and show how they are used in a complementary fashion. Copyright © 2010 John Wiley & Sons, Ltd.     

In vivo physiological saline‐infused hepatic vessel imaging using a two‐crystal‐interferometer‐based phase‐contrast X‐ray technique

Using a two‐crystal‐interferometer‐based phase‐contrast X‐ray imaging system, the portal vein, capillary vessel area and hepatic vein of live rats were revealed sequentially by injecting physiological saline via the portal vein. Vessels greater than 0.06 mm in diameter were clearly shown with low levels of X‐rays (552 µGy). This suggests that in vivo vessel imaging of small animals can be performed as conventional angiography without the side effects of the presently used iodine contrast agents.     

High‐resolution thermal imaging with a combination of nano‐focus X‐ray diffraction and ultra‐fast chip calorimetry

A microelectromechanical‐systems‐based calorimeter designed for use on a synchrotron nano‐focused X‐ray beamline is described. This instrument allows quantitative DC and AC calorimetric measurements over a broad range of heating/cooling rates (≤100000 K s^?1) and temperature modulation frequencies (≤1 kHz). The calorimeter was used for high‐resolution thermal imaging of nanogram‐sized samples subjected to X‐ray‐induced heating. For a 46 ng indium particle, the measured temperature rise reaches ～0.2 K, and is directly correlated to the X‐ray absorption. Thermal imaging can be useful for studies of heterogeneous materials exhibiting physical and/or chemical transformations. Moreover, the technique can be extended to three‐dimensional thermal nanotomography.     

Utilizing broadband X‐rays in a Bragg coherent X‐ray diffraction imaging experiment

A method is presented to simplify Bragg coherent X‐ray diffraction imaging studies of complex heterogeneous crystalline materials with a two‐stage screening/imaging process that utilizes polychromatic and monochromatic coherent X‐rays and is compatible with in situ sample environments. Coherent white‐beam diffraction is used to identify an individual crystal particle or grain that displays desired properties within a larger population. A three‐dimensional reciprocal‐space map suitable for diffraction imaging is then measured for the Bragg peak of interest using a monochromatic beam energy scan that requires no sample motion, thus simplifying in situ chamber design. This approach was demonstrated with Au nanoparticles and will enable, for example, individual grains in a polycrystalline material of specific orientation to be selected, then imaged in three dimensions while under load.     

Three‐dimensional imaging of chemical phase transformations at the nanoscale with full‐field transmission X‐ray microscopy

The ability to probe morphology and phase distribution in complex systems at multiple length scales unravels the interplay of nano‐ and micrometer‐scale factors at the origin of macroscopic behavior. While different electron‐ and X‐ray‐based imaging techniques can be combined with spectroscopy at high resolutions, owing to experimental time limitations the resulting fields of view are too small to be representative of a composite sample. Here a new X‐ray imaging set‐up is proposed, combining full‐field transmission X‐ray microscopy (TXM) with X‐ray absorption near‐edge structure (XANES) spectroscopy to follow two‐dimensional and three‐dimensional morphological and chemical changes in large volumes at high resolution (tens of nanometers). TXM XANES imaging offers chemical speciation at the nanoscale in thick samples (>20 µm) with minimal preparation requirements. Further, its high throughput allows the analysis of large areas (up to millimeters) in minutes to a few hours. Proof of concept is provided using battery electrodes, although its versatility will lead to impact in a number of diverse research fields.     

Coherent diffractive imaging: towards achieving atomic resolution

The next generation of X‐ray sources will feature highly brilliant X‐ray beams that will enable the imaging of local nanoscale structures with unprecedented resolution. A general formalism to predict the achievable spatial resolution in coherent diffractive imaging, based solely on diffracted intensities, is provided. The coherent dose necessary to reach atomic resolution depends significantly on the atomic scale structure, where disordered or amorphous materials require roughly three orders of magnitude lower dose compared with the expected scaling of uniform density materials. Additionally, dose reduction for crystalline materials are predicted at certain resolutions based only on their unit‐cell dimensions and structure factors.     

Phase-contrast X-ray imaging of the gas diffusion layer of fuel cells

The understanding of and in situ observation of the transport and distribution of water in carbon‐paper gas diffusion layers (GDLs) using non‐destructive imaging techniques is critical for achieving high performance in polymer electrolyte fuel cells (PEFCs). To investigate the behavior of water in GDLs of PEFCs, phase‐contrast X‐ray imaging via X‐ray interferometric imaging (XII) and diffraction‐enhanced imaging (DEI) were performed using 35 keV X‐rays. The XII technique is useful for the radiographic imaging of GDLs and in situ observations of water evolution processes in operating PEFCs. DEI provides a way for tomographic imaging of GDLs in PEFCs. Because high‐energy X‐rays are applicable to the imaging of both carbon papers and heavy materials, which make up PEFCs, phase‐contrast X‐ray imaging techniques have proven to be valuable for investigation of GDLs.     

A novel epitaxially grown LSO‐based thin‐film scintillator for micro‐imaging using hard synchrotron radiation

The efficiency of high‐resolution pixel detectors for hard X‐rays is nowadays one of the major criteria which drives the feasibility of imaging experiments and in general the performance of an experimental station for synchrotron‐based microtomography and radiography. Here the luminescent screen used for the indirect detection is focused on in order to increase the detective quantum efficiency: a novel scintillator based on doped Lu₂SiO₅ (LSO), epitaxially grown as thin film via the liquid phase epitaxy technique. It is shown that, by using adapted growth and doping parameters as well as a dedicated substrate, the scintillation behaviour of a LSO‐based thin crystal together with the high stopping power of the material allows for high‐performance indirect X‐ray detection. In detail, the conversion efficiency, the radioluminescence spectra, the optical absorption spectra under UV/visible‐light and the afterglow are investigated. A set‐up to study the effect of the thin‐film scintillator's temperature on its conversion efficiency is described as well. It delivers knowledge which is important when working with higher photon flux densities and the corresponding high heat load on the material. Additionally, X‐ray imaging systems based on different diffraction‐limited visible‐light optics and CCD cameras using among others LSO‐based thin film are compared. Finally, the performance of the LSO thin film is illustrated by imaging a honey bee leg, demonstrating the value of efficient high‐resolution computed tomography for life sciences.     

Enhanced renal image contrast by ethanol fixation in phase‐contrast X‐ray computed tomography

Phase‐contrast X‐ray imaging using a crystal X‐ray interferometer can depict the fine structures of biological objects without the use of a contrast agent. To obtain higher image contrast, fixation techniques have been examined with 100% ethanol and the commonly used 10% formalin, since ethanol causes increased density differences against background due to its physical properties and greater dehydration of soft tissue. Histological comparison was also performed. A phase‐contrast X‐ray system was used, fitted with a two‐crystal X‐ray interferometer at 35 keV X‐ray energy. Fine structures, including cortex, tubules in the medulla, and the vessels of ethanol‐fixed kidney could be visualized more clearly than that of formalin‐fixed tissues. In the optical microscopic images, shrinkage of soft tissue and decreased luminal space were observed in ethanol‐fixed kidney; and this change was significantly shown in the cortex and outer stripe of the outer medulla. The ethanol fixation technique enhances image contrast by approximately 2.7–3.2 times in the cortex and the outer stripe of the outer medulla; the effect of shrinkage and the physical effect of ethanol cause an increment of approximately 78% and 22%, respectively. Thus, the ethanol‐fixation technique enables the image contrast to be enhanced in phase‐contrast X‐ray imaging.     

Nondestructive rare earth element imaging of fish teeth from deep‐sea sediments

X‐ray fluorescence computed tomography is an emerging imaging modality that allows for the nondestructive reconstruction of the internal distribution of elements within a sample. The common use of X‐ray excitation energy (up to approximately 20 keV) has necessitated the use of l ‐shell fluorescence for heavy elements. In this study, based on high energy X‐ray at BL13W1 of the Shanghai Synchrotron Radiation Facility, we employed high‐energy excitation for tomographic imaging of the heavy metals (rare earth elements) in fish teeth from deep‐sea sediments on the micrometer scale using K‐shell X‐ray fluorescence. The virtual cross‐sectional distribution of La, Ce, Pm, Pr, Nd, and Sm were obtained, thereby providing a feasible approach for analyzing the enrichment mechanism of rare earth elements. Copyright © 2015 John Wiley & Sons, Ltd.     

Synchrotron‐radiation phase‐contrast imaging of human stomach and gastric cancer: in vitro studies

The electron density resolution of synchrotron‐radiation phase‐contrast imaging (SR‐PCI) is 1000 times higher than that of conventional X‐ray absorption imaging in light elements, through which high‐resolution X‐ray imaging of biological soft tissue can be achieved. For biological soft tissue, SR‐PCI can give better imaging contrast than conventional X‐ray absorption imaging. In this study, human resected stomach and gastric cancer were investigated using in‐line holography and diffraction enhanced imaging at beamline 4W1A of the Beijing Synchrotron Radiation Facility. It was possible to depict gastric pits, measuring 50–70 µm, gastric grooves and tiny blood vessels in the submucosa layer by SR‐PCI. The fine structure of a cancerous ulcer was displayed clearly on imaging the mucosa. The delamination of the gastric wall and infiltration of cancer in the submucosa layer were also demonstrated on cross‐sectional imaging. In conclusion, SR‐PCI can demonstrate the subtle structures of stomach and gastric cancer that cannot be detected by conventional X‐ray absorption imaging, which prompt the X‐ray diagnosis of gastric disease to the level of the gastric pit, and has the potential to provide new methods for the imageology of gastric cancer.     

Focusing synchrotron radiation using a polycapillary half‐focusing X‐ray lens for imaging

An imaging system based on a polycapillary half‐focusing X‐ray lens (PHFXRL) and synchrotron radiation source has been designed. The focal spot size and the gain in power density of the PHFXRL were 22 µm (FWHM) and 4648, respectively, at 14.0 keV. The spatial resolution of this new imaging system was better than 5 µm when an X‐ray charge coupled device with a pixel size of 10.9 × 10.9 µm was used. A fossil of an ancient biological specimen was imaged using this system.     

High‐efficiency coherence‐preserving harmonic rejection with crystal optics

This work reports a harmonic‐rejection scheme based on the combination of Si(111) monochromator and Si(220) harmonic‐rejection crystal optics. This approach is of importance to a wide range of X‐ray applications in all three major branches of modern X‐ray science (scattering, spectroscopy, imaging) based at major facilities, and especially relevant to the capabilities offered by the new diffraction‐limited storage rings. It was demonstrated both theoretically and experimentally that, when used with a synchrotron undulator source over a broad range of X‐ray energies of interest, the harmonic‐rejection crystals transmit the incident harmonic X‐rays on the order of 10^?6. Considering the flux ratio of fundamental and harmonic X‐rays in the incident beam, this scheme achieves a total flux ratio of harmonic radiation to fundamental radiation on the order of 10^?10. The spatial coherence of the undulator beam is preserved in the transmitted fundamental radiation while the harmonic radiation is suppressed, making this scheme suitable not only for current third‐generation synchrotron sources but also for the new diffraction‐limited storage rings where coherence preservation is an even higher priority. Compared with conventional harmonic‐rejection mirrors, where coherence is poorly preserved and harmonic rejection is less effective, this scheme has the added advantage of lower cost and footprint. This approach has been successfully utilized at the ultra‐small‐angle X‐ray scattering instrument at the Advanced Photon Source for scattering, imaging and coherent X‐ray photon correlation spectroscopy experiments. With minor modification, the harmonic rejection can be improved by a further five orders of magnitude, enabling even more performance capabilities.     

Towards tender X‐rays with Zernike phase‐contrast imaging of biological samples at 50 nm resolution

X‐ray microscopy is a commonly used method especially in material science application, where the large penetration depth of X‐rays is necessary for three‐dimensional structural studies of thick specimens with high‐Z elements. In this paper it is shown that full‐field X‐ray microscopy at 6.2 keV can be utilized for imaging of biological specimens with high resolution. A full‐field Zernike phase‐contrast microscope based on diffractive optics is used to study lipid droplet formation in hepatoma cells. It is shown that the contrast of the images is comparable with that of electron microscopy, and even better contrast at tender X‐ray energies between 2.5 keV and 4 keV is expected.     

X‐ray grating interferometer for biomedical imaging applications at Shanghai Synchrotron Radiation Facility

An X‐ray grating interferometer was installed at the BL13W beamline of Shanghai Synchrotron Radiation Facility (SSRF) for biomedical imaging applications. Compared with imaging results from conventional absorption‐based micro‐computed tomography, this set‐up has shown much better soft tissue imaging capability. In particular, using the set‐up, the carotid artery and the carotid vein in a formalin‐fixed mouse can be visualized in situ without contrast agents, paving the way for future applications in cancer angiography studies. The overall results have demonstrated the broad prospects of the existing set‐up for biomedical imaging applications at SSRF.