Multi-energy X-ray Research Articles

Liquid-explosives scanners stand trial in airports

Air passengers may once more be allowed to pack beverages, lotions, and hair spray in their carry-on luggage, if imaging technologies to detect liquid explosives can prove their worth. Several competing systems, including multi-energy x-ray systems and a low-field magnetic resonance imaging (MRI) scanner, are undergoing field tests at some airports worldwide.

Synchrotron X-ray Tomography for 3D Chemical Distribution Measurement of a Flame Retardant and Synergist in a Fiberglass-Reinforced Polymer Blend

A fiberglass-reinforced polymer blend with a new-generation flame retardant is studied with multienergy synchrotron X-ray tomography to assess the blend homogeneity. Relative to other composite materials, this sample is difficult to image due to low X-ray contrast between the fiberglass reinforcement and the polymer blend. Also, the glass fibers are only slightly larger than the 3.26 microm voxels and, due to their high concentration, exist as partially aligned bundles in the polymer matrix. To investigate the chemical composition surrounding the glass fibers, new procedures were developed to find and mark the fiberglass and then assess the flame retardant distribution near the fiber bundles. On the basis of the multienergy imaging across Br and Sb K-edges, the absorbance values were converted to volume percent concentrations. Besides the basic question of the successful and stable blending of the flame retardant and synergist within the polymer matrix, we are also interested in precipitation reactions that might concentrate or diminish concentrations in the close vicinity of the fiberglass reinforcement. Thus, a procedure was developed to analyze radial concentrations about selected, well-isolated fiberglass bundles. Overall, the results show a nicely homogeneous system to the level of the tomography resolution, 3.26 microm, with some enhanced concentration near, approximately 20 microm, the fiber bundles.

Photon counting multienergy x‐ray imaging: Effect of the characteristic x rays on detector performance

The purpose of this work was to investigate the effect of characteristic x rays on the performance of photon counting detectors for multienergy x-ray imaging. X-ray and CT systems with photon counting detectors have compelling advantages compared to energy integrating detectors, and cadmium zinc telluride (CZT) detector is the detector of choice. However, current CZT detectors exhibit several limitations that hamper their practical applications. These limitations include hole trapping, high leakage current, and charge sharing between detector pixels. Charge sharing occurs due to the diffusion of charge when it drifts toward the pixel electrodes. It also occurs due to nonlocal reabsorption of characteristic and scattered x rays created in the detector volume. Hole trapping, leakage current, and charge diffusion may potentially have technical solutions. Characteristic x-ray escape and scatter, however, are fundamental in nature and cannot be easily addressed. The x-ray scatter in the CZT material is small at photon energies used in x-ray imaging. Therefore, the remaining major factor is characteristic x ray. Monte Carlo simulations were used for this study. An experimental photon counting multienergy x-ray imaging system was used to compare simulations to experimental results. An x-ray spectrum at 120 kVp tube voltage was used. The x-ray energy range was split into five subregions (energy bins) and Monte Carlo simulations were performed at average x-ray energies corresponding to these energy bins. The detector pixel size was changed within the 0.1-1 mm range, which covered all possible applications including radiography and CT imaging. The pixel shapes included square and strip pixels. For strip pixels, tilted angle irradiation of the CZT detector was also investigated. The characteristic x rays escaped the pixels in approximately 70% of all x-ray interactions for the smallest pixel size of 0.1 mm. The escape fraction decreased to 20% for the largest pixel size of 1 mm. All escape fractions, for all pixel sizes, at five energies, for square and strip pixels, and at three tilt angles were calculated and presented in tables. Simulated and measured spectra at 120 kVp were compared. Characteristic x-ray escape deteriorates energy and spatial resolution, particularly for small pixel sizes. Correction methods should be developed based on the results of the simulations and experimental study.

Cramér–Rao lower bound of basis image noise in multiple-energy x-ray imaging

We present an analytical method to compute the basis image noise in the context of multi-energy x-ray imaging based on the Cramér–Rao lower bound (CRLB). The proposed formalism extends the original idea of Alvarez and Macovski (1976 Phys. Med. Biol. 21 733) to estimate the noise in the photo-effect and Compton-effect basis images in the case of dual-energy imaging. It includes an arbitrary number of independent, spectrally distinct attenuation measurements and also goes beyond the two-dimensional decomposition of the attenuation, including, e.g., a contrast agent as a third basis material. To illustrate our method, we consider three simple applications. The first application is to study the influence of the exact values for the energy thresholds on the basis image noise for a binned photon-counting system. The second application relates to the same detector system as the first and is an investigation of the dependence of the basis image noise on the energy resolution of the detector system. Finally, the third application provides an example for the case of an energy-integrating detector: the aim is to optimize the front-scintillator layer thickness of a dual-crystal detector for dual-energy imaging. The CRLB is used to minimize the noise of a photo-effect/Compton-effect basis material decomposition.

Observation of electron transport dynamics in high intensity laser interactions using multi-energy monochromatic x-ray imaging

We describe recent measurements in which a novel imaging technique was used to investigate the transport of high energy electrons produced by the interaction of a femtosecond laser pulse with a three-layer target at an intensity of 5 × 1019 W cm−2. The imaging system was configured to work in a single-photon detection regime to identify the energy of the x-ray photons and to discriminate among Kα photons generated in each target layer. Electrons emerging from the rear side after propagation through all the target layers were also detected using a custom developed detector. The results on fast electron propagation are combined with the information obtained from electron diagnostics and are modelled using analytical and numerical codes to obtain a detailed description of electron propagation dynamics.

An Integrated Broad Area Coverage Fusion Neutron/X-Ray Interrogation Unit

Homeland security has an urgent need for an advanced detecting system to accurately and quickly search for nuclear and explosive materials in a wide variety of situations. An integrated broad area coverage neutron/x-ray interrogation unit is proposed here to meet such needs. This system will use a unique cylindrical Inertial Electrostatic Confinement (IEC) device. This compact pulsed neutron/x-ray line source can produce ∼2x1010 n/s 14.1-MeV D-T neutrons, ∼108 n/s 2.45-MeV D-D neutrons and 80 kV x-rays.Unlike prior neutron activation systems, this unit can provide a long line-like emission source to obtain broad coverage, providing very fast scan time for even large objects. The use of combined multi-energy neutron and x-ray sources, along with a 3-D detector array and fuzzy logic analysis system, are expected to provide high elemental identification accuracy, greatly decreasing false signals so commonly encountered in prior systems. Analysis techniques will employ both thermal neutron analysis (TNA)and pulsed fast neutron analysis (PFNA), accompanied by broad area x-ray imaging techniques.

Algorithms for three-dimensional chemical analysis via multi-energy synchrotron X-ray tomography

The conversion of X-ray tomography images into three-dimensional chemical composition requires accurate mass absorption values, high-quality images, and a robust fitting algorithm. The least-squares fits of the images to a three-dimensional chemical composition can proceed with several different options such as minimal vs. over-determined and/or constrained parameters. This project has investigated the impact of XAFS features and a limited CCD dynamic range. These simulated results are compared to a recent experimental project in which synchrotron X-ray tomography was used to image a polymer blend, and from those images, calculated three-dimensional chemical composition maps of the two-component flame retardant, a brominated phthalimide dimer, Saytex ™ BT-93 and a synergist, antimony(III) oxide (Sb 2O 3).

‘‘Hybrid’’ calibrations of a Dual Energy X-ray Scanner for material testing

Conventional x-ray tubes produce a fan-shaped x-ray beam covering a large spectrum of energies, which is why the fundamental law of x-ray attenuation is not readily applicable. As the mathematical formulation of the problem would be too cumbersome, calibrations using well-defined objects are carried out, which in turn allow the use of multienergy x-rays for measurements. Occasionally, such calibrations may not lead to the desired results. This could be for instance due to an insensitivity of x-rays towards low atomic number elements. Here we present such a case on hand the example of raw natural fibre. The DEXA parameters correlated with the fibre parameter wool base, but show distinct correlation for geographical regions of the origin of the wool. A calibration that is valid independently of geographical origin can be achieved by including independently measured parameters of the calibration body. We demonstrate a successful calibration that uses dual energy x-ray scanning technology as well as a size parameter of the fibre in the regression equation.

Development of a Novel Instrument for X-Ray Photoelectron Diffraction and Holography

For X-ray photoelectron diffraction (XPED) and holography measurements we developed a novel laboratory instrument with the multienergy high power X-ray source and the high energy and high angular resolution photoelectron spectrometer system. The photon intensities of Al–Kα, Cr–Lα and Cu–Kα were estimated at 4.6 × 1011 cps, 7.5 × 1010 cps and 7.2 × 1010 cps, respectively. Ag3d XPS also revealed that the energy resolutions of Al–Kα and Cr–Lα sources were 0.9 eV and 3.1 eV, respectively. XPS and XPED of h-BN/Ni(111) excited by Al–Kα and Cr–Lα elucidated the potential and the validity of the XPED and holography analysis by using this novel instrument. Cu–Kα excitation XPS and Ti1s XPED measurements of the SrTiO3 (001) surface have also been performed.

Analysis of Residual Stress Gradients Below the Surface of a Material Using a Multi-Energy Method

AbstractThe residual-stress-gradient distribution just below the surface of a material is an important factor to consider during the engineering and design of a component. With the availability of an intense energy-tunable synchrotron x-ray source, it becomes easier to analyze the stress gradient below the surface, using a multi-energy x-ray diffraction method. A program was developed to efficiently determine possible experimental parameters using a sample with a known stress gradient distribution. In addition, this program can also calculate the stress gradient distribution below the surface taking into account experimental results. It also includes a subroutine for calculating the x-ray absorption coefficients of all of the elements, generalizing it for use with any material. As an example, in the present study, the relationship between x-ray energy and the residual stress gradient is discussed according to the calculated result for a silicon nitride composition.

Recent Progress of Photoelectron Spectroscopy and Holography. Photoelectron Spectro-Holography.

Photoelectron spectro-holography, represented by differential photoelectron holography and multi-energy X-ray photoelectron diffraction, is a new technique to investigate the structures of surface and interface. Using photoelectron holograms effectively, differential photoelectron holography reconstructs more precious atomic images than other photoelectron techniques, while multi-energy X-ray photoelectron reveals the structures of surface and interface in wide depth range from adsorbates to buried interfaces. In this article, the principle, instrument and recent results of photoelectron spectro-holography method are overviewed briefly.

Principal components analysis of multienergy X-ray computed tomography of mineral samples

Principal components analysis (PCA) of X-ray transmission tomography images made at several different energies can produce images that allow individual materials to be distinguished more clearly than in single-energy images. Computer simulations and tests on a five-band mineral data set demonstrate the application of PCA.

Interference imaging and it's application to material and medical imaging

Recently several new imaging modalities in the domain of x-rays have been invented that show dramatically improved contrast over standard imaging techniques, where the contrast is based on attenuation only. Their working principle is either interference between scattered wave fronts from a sample with a reference wave field or the selective measure of refraction properties of the sample utilizing a crystal analyzer. The first category includes multi energy x-ray holography (MEXH) and phase contrast imaging (PHC) while the latter includes diffraction enhanced imaging (DEI). The basic theory will be discussed and some recent applications in material science as well as medical imaging will be reviewed.

Reconstruction of multi-energy X-ray computed tomography images of laboratory mice

A new X-ray computed tomography (CT) system is being developed at Oak Ridge National Laboratory to image laboratory mice for the purpose of rapid phenotype screening and identification. One implementation of this CT system allows simultaneous capture of several sets of sinogram data, each having a unique X-ray energy distribution. The goals of this paper are to (1) identify issues associated with the reconstruction of this energy-dependent data and (2) suggest preliminary approaches to address these issues. Due to varying numbers of photon counts within each set, both traditional (filtered backprojection, or FBP) and statistical (maximum likelihood, or ML) tomographic image reconstruction techniques have been applied to the energy-dependent sinogram data. Results of reconstructed images using both algorithms on sinogram data (high- and low-count) are presented. Also, tissue contrast within the energy-dependent images is compared to known X-ray attenuation coefficients of soft tissue (e.g. muscle, bone, and fat).