Complete elimination of the beam hardening effect in quantitative absorption tomography using polychromatic x-rays with single-component specimens

Absorption tomography has been a unique technique for 3D imaging in many scientific research and applications. However, the use of polychromatic x-ray sources for quantitative tomography has been limited due to the well-known beam hardening effect. In this article, we will describe the theoretical details of a new technique for quantitative tomography using polychromatic x-rays. The technique incorporates the full spectral information of the incident beam into the tomographic reconstruction process. Consequently, beam-hardening effects are eliminated and the result is quantitative. We will also demonstrate the technique experimentally using a single-component specimen. This technique promises great opportunities for laboratory-based x-ray sources to be used in quantitative applications.     

Tomographic examination of magnetic nanoparticles used as drug carriers

Tumors grown on animals and treated with magnetic drug targeting and magnetic hyperthermia have been analyzed by microcomputed X-ray tomography to study the three-dimensional nanoparticle distribution. The measurements have been performed in two laboratories, with a polychromatic X-ray cone beam as well as with monochromatic parallel beam. Due to the poor resolution in the first case, the distribution of the magnetic nanoparticles can be studied only qualitatively. With the polychromatic beam semi-quantitative results can be achieved. In this paper, the results from both methods are presented and compared.     

X-ray beam hardening correction for measuring density in linear accelerator industrial computed tomography

Due to X-ray attenuation being approximately proportional to material density, it is possible to measure the inner density through Industrial Computed Tomography (ICT) images accurately. In practice, however, a number of factors including the non-linear effects of beam hardening and diffuse scattered radia-tion complicate the quantitative measurement of density variations in materials. This paper is based on the linearization method of beam hardening correction, and uses polynomial fitting coefficient which is obtained by the curvature of iron polychromatic beam data to fit other materials. Through theoretical deduction, the paper proves that the density measure error is less than 2% if using pre-filters to make the spectrum of linear accelerator range mainly 0.3 MeV to 3 MeV. Experiment had been set up at an ICT system with a 9 MeV electron linear accelerator. The result is satisfactory. This technique makes the beam hardening correction easy and simple, and it is valuable for measuring the ICT density and making use of the CT images to recognize materials.     

Quantitative in-line phase-contrast imaging with multienergy X rays.

We present a new method for quantitative nondestructive characterization of objects by x-ray phase-contrast imaging. Spatial distributions of the projected values of the complex refractive index in the sample are reconstructed by processing near-field images collected at a fixed sample-to-detector distance using a polychromatic incident beam and an energy-sensitive area detector, such as a CCD used in the photon-counting spectroscopy mode. The method has the potential advantages of decreased radiation dose and increased accuracy compared to conventional techniques of x-ray imaging.     

KB scanning of X‐ray beam for Laue microdiffraction on accelero‐phobic samples: application to in situ mechanically loaded nanowires

A mapping technique has been developed where a sub‐micrometer focused polychromatic X‐ray beam is scanned across a stationary sample instead of scanning the sample in front of the X‐ray microbeam. This method is applied to a gold nanowire during its mechanical loading using the tip of an atomic force microscope. During the loading process, such a sample is `accelero‐phobic', i.e. the sample scanning stages must not to be moved to avoid parasitic additional load. Without beam scanning, only one single position within the sample can be probed during the test. The probed material point may even change because of drifts or movements induced by the test itself. The new scanning approach facilitates the in situ mapping of the entire wire giving access to the evolution of the wire shape as well as to the boundary conditions. This novel scanning technique opens promising perspectives for studies where sample motion is forbidden because of the sample environment.     

Beam hardening correction for a cone-beam CT system and its effect on spatial resolution

In this paper, we present a beam hardening correction (BHC) method in three-dimension space for a cone-beam computed tomography (CBCT) system in a mono-material case and investigate its effect on the spatial resolution. Due to the polychromatic character of the X-ray spectrum used, cupping and streak artifacts called beam hardening artifacts arise in the reconstructed CT images, causing reduced image quality. In addition, enhanced edges are introduced in the reconstructed CT images because of the beam hardening effect. The spatial resolution of the CBCT system is calculated from the edge response function (ERF) on different planes in space. Thus, in the CT images with beam hardening artifacts, enhanced ERFs will be extracted to calculate the modulation transfer function (MTF), obtaining a better spatial resolution that deviates from the real value. Reasonable spatial resolution can be obtained after reducing the artifacts. The 10% MTF value and the full width at half maximum (FWHM) of the point spread function with and without BHC are presented.     

Elemental analysis by high‐energy electron excitation

The possibility of using high‐energy β‐particles (10²–10³ keV) to induce the emission of characteristic x‐rays from pure chemical elements, with important improvements with respect to conventional excitation methods, has been recently reviewed. An excitation procedure named BIXE (β‐induced x‐ray emission) is used for implementing a spectrometric technique along the lines developed for EPMA (electron probe microanalysis). We have found that by using BIXE it is possible to determine binary sample compositions of elements present at concentrations higher than 1%, by comparisons with reference samples to obtain calibration curves. Experience with EPMA shows that when ternary and higher order samples are analyzed, the use of reference samples is not enough and it is necessary to perform theoretical corrections to the relationship between the line intensity and the corresponding concentrations, as a suitable complement to the analytical procedure. Semi‐quantitative results are thus obtained with corrections applied through the ZAF method usually used in EPMA. In this work we concentrated in finding whether calibration curves as used in EPMA (where the electron beam is monochromatic) can be used in BIXE, where the electron beam is polychromatic (the β spectrum). We expect that BIXE can be developed as a spectrometric technique whose main advantages are that it is a low‐cost technique suitable for in situ studies and that the experimental arrangement and data acquisition and its evaluation are comparatively simple. Copyright © 2004 John Wiley & Sons, Ltd.     

Generalized eikonal of partially coherent beams and its use in quantitative imaging

The generalized eikonal of a partially coherent paraxial wave is introduced via a differential equation describing the evolution of the time-averaged intensity. The theoretical formalism provides an analytical tool for the study of partially coherent imaging systems. It also makes possible quantitative phase retrieval and compositional mapping of weakly absorbing samples using phase-contrast imaging with broadband polychromatic radiation of known spectral distribution. An experimental demonstration is presented of the quantitative reconstruction of the projected thickness of a sample, given a phase-contrast image obtained using a polychromatic microfocus x-ray source.     

Lens axicon illuminated by polychromatic Gaussian beams for generating uniform focal segments

In this paper, we investigate a lens axicon, which actually is a lens with spherical aberration, illuminated by a polychromatic Gaussian beam for producing an extended axial line image of a desired length and nearly uniform intensity. A numerical calculation is performed to investigate the dependence of the axial intensity distribution of the focal segment on the parameters of the incident polychromatic Gaussian beam. It is shown that, compared with monochromatic Gaussian beam illumination, the illumination of the polychromatic Gaussian beam may improve the uniformity of the distribution of the axial intensity, and this improvement in the uniformity of the axis intensity is strongly dependent on the spectral width of the incident Gaussian beam. Moreover, apodization with annular super-Gaussian amplitude distribution is employed to reduce the undesired oscillation of the axial intensity.     

On the formation of X-ray microbeams utilizing a short-focus refractive lens and a laboratory radiation source

The possibility of focusing an X-ray beam from a laboratory radiation source using a short-focus refractive composite lens is shown. The lens consists of 161 spherical biconcave epoxy lenses, each with a curvature radius of 50 μm. A Metal Jet (Excillium^TM) microfocus X-ray tube, with a focal-spot size of 20 μm and containing a liquid helium anode, is used as a radiation source. The size of the focal spot in the image plane is 2.4 μm, which corresponds to the theoretical estimate. The possibility of using the composite refractive lens to form a parallel polychromatic X-ray beam is demonstrated. The results obtained allow discussion of the possibility of applying short-focus refractive X-ray lenses for X-ray microanalysis using laboratory sources; such microanalysis is currently a prerogative of synchrotron radiation sources only.     

Immobilization of Scapharca HbI crystals improves data quality in time-resolved crystallographic experiments

Time-resolved crystallography is a powerful technique that allows structural transitions to be followed in real time during the course of a chemical reaction. The extension of the time resolution of this technique to nanosecond and picosecond time scales require a short laser pulse to initiate the transition and a rapid polychromatic X-ray pulse to probe the structural perturbations. Unfortunately, polychromatic diffraction patterns are quite sensitive to subtle crystal movements that can occur from laser pulses used to trigger the structural transition. The immobilization of crystals within capillary tubes dramatically improves data quality and allows the utilization of more intense laser pulses for the initiation step. This leads to an increase in the signal to noise present in the electron density maps.     

Advances in X-ray excitation of Kossel patterns by a focusing polycapillary lens

In this paper, the polychromatic X-ray excitation of Kossel patterns by an X-ray tube and a focusing polycapillary lens will be presented. Additionally, it will be shown that the lateral resolution of the Kossel technique under X-ray tube excitation can be improved and the exposure times can be strongly reduced by using a polycapillary lens. The advantageous combination of X-ray fluorescence and crystal structure analysis by means of Kossel microdiffraction will be demonstrated.     

Polychromatic microdiffraction characterization of defect gradients in severely deformed materials

This paper analyzes local lattice rotations introduced in severely deformed polycrystalline titanium by friction stir welding. Nondestructive three-dimensional (3D) spatially resolved polychromatic X-ray microdiffraction, is used to resolve the local crystal structure of the restructured surface from neighboring local structures in the sample material. The measurements reveal strong gradients of strain and geometrically necessary dislocations near the surface and illustrate the potential of polychromatic microdiffraction for the study of deformation in complex materials systems.     

工业CT在工件检测中X射线硬化校正

X射线工业CT中,由于X射线能谱具有多色性,X射线在透射物质时,能量较低的射线优先被吸收,X射线能量越高,衰减系数越低。也即较高能量的X射线的衰减系数比较低能量的X射线的衰减系数小。射线随透射厚度增大,变得更易穿透,也就是发生了能谱硬化现象。由于射线硬化现象使图像重建时出现伪影,因此必须修正。文中对X射线硬化现象进行了分析,探讨了在均匀物质中,X射线射束和与透射厚度的关系。并根据Beer定律和X射线与物质作用的特点,通过获取X射线射束和数据,拟合出射束和与透射厚度的关系式。然后得出在同一透射厚度时,X射线射束和校正为单色等效射束和的关系及其等效方法。最终得出X射线等效单色射线的衰减系数的拟合值。再对此衰减系数拟合值进行卷积反投影重构,即可有效消除X射线射束硬化的影响。     

Polychromatic intensity point spread function of a birefringent lens

We study the intensity point spread function (IPSF) of a uniaxial birefringent lens sandwiched between two linear polarizers under broadband illumination having flat-top spectral profile. The optic axis of the birefringent crystal is perpendicular to the lens axis. For monochromatic input illumination, a low-power birefringent lens offers enhanced resolution under parallel-polarizers configuration and apodization under crossed-polarizers configuration with a single focus as well. At a relatively high value of birefringence parameter, the proposed system behaves as a double-focus lens. The present communication uses a novel technique to study the effect of polychromatic illumination on the imaging characteristics of birefringent lenses. The studies reveal that the imaging characteristics of the low-power birefrigent lens does not deteriorate considerably under polychromatic illumination. However, for a polychromatic beam with large bandwidth, the imaging behavior of the system having high birefringence parameter deviates appreciably from that under strictly monochromatic illumination. The said system even loses its inherent bi-focal nature due to depolarization of polarized polychromatic light upon passage through the birefringent crystal.     

Effect of chromaticity on quarter-wave retarder performance

When a polarized polychromatic beam passes through a birefringent medium, the constituent spectral components suffer different change of state of polarization. As a result when the beam passes through an analyzer, the intensity of the resultant beam changes depending on the orientation of the analyzer, state of polarization of the input beam, spectral and spatial intensity distribution of the source and the polarizing properties of the birefringent medium. In the present study the intensity variation of the resulting beam passing through a quarter-wave retarder plate is observed theoretically with the variation of the azimuthal angle of the analyzer for monochromatic and polychromatic source of light. A simple experiment is conducted to observe the intensity variation for an incandescent lamp and HeNe laser as polychromatic and monochromatic source of light passing through a birefringent polymer-based quarter-wave retarder plate. The experimentally obtained values are compared with theoretical values and a good agreement is observed.     

大气湍流对多色高斯-谢尔模型光束扩展的影响

基于广义惠更斯-菲涅耳原理,采用Rytov相位结构函数二次近似和硬边窗口函数的复高斯展开法,推导了受光阑限制的多色高斯-谢尔模型(GSM)光束在大气湍流中的二阶矩束宽公式。研究表明:二阶矩束宽随着大气湍流折射率结构常数、源光谱带宽和光束传输距离的增加而增大,随着光束截断参数和光束相干参数的增加而减小;并且,当源光谱带宽越大、光束截断参数和光束相干参数越小,则湍流对受光阑限制的多色GSM光束扩展的影响就越小。关键词:      

Generalization of dual-energy method for material identification with X-ray introscopy

X-ray nondestructive testing was used for flaw location, for examination of the content of a package, and for fault detection. However, a monoenergetic beam can determine only the density variations of the analyzed object, not its structure. Study of the structure of an object was carried out by dual-energy X-ray beams. It is established that materials have different dependences of the X-ray extinction coefficient on its energy. The possibility of determination the composition of the material using two polychromatic X-ray beams with different energy spectra is shown. A variant of the algorithm for determination of the atomic number Z in the case of a weakly absorbing material is proposed.     

A simple polarimeter for quantifying synchrotron polarization

There are currently no simple and easy to apply techniques for determining the degree of linear polarization of a soft X-ray beam delivered by a synchrotron beamline despite the fact that this parameter is important for a wide range of synchrotron-based soft X-ray experiments. This work presents a new method for the quantitative determination of the linear polarization state of synchrotron radiation using an HOPG crystal as the standard sample. The method is straightforward to perform without the need for experimental apparatus beyond that commonly available at beamlines in this energy range. In addition, the high-degree of order that can easily be achieved in a HOPG sample means that it is potentially more accurate than currently employed comparative methods. We show how the method can be applied to experimentally determine the linear polarization state of various soft X-ray synchrotron beamlines and we discuss the wide variations in the degree of linear polarization that are measured.     

大气湍流对多色高斯-谢尔模型光束扩展的影响

基于广义惠更斯-菲涅耳原理,采用Rytov相位结构函数二次近似和硬边窗口函数的复高斯展开法,推导了受光阑限制的多色高斯-谢尔模型(GSM)光束在大气湍流中的二阶矩束宽公式。研究表明：二阶矩束宽随着大气湍流折射率结构常数、源光谱带宽和光束传输距离的增加而增大,随着光束截断参数和光束相干参数的增加而减小;并且,当源光谱带宽越大、光束截断参数和光束相干参数越小,则湍流对受光阑限制的多色GSM光束扩展的影响就越小。关键词： 