Performance evaluation of the backprojection filtered （BPF） algorithm in circular fan-beam and cone-beam CT

In this article we introduce an exact backprojection filtered （BPF） type reconstruction algorithm for cone-beam scans based on Zou and Pan＇s work. The algorithm can reconstruct images using only the projection data passing through the parallel PI-line segments in reduced scans. Computer simulations and practical experiments are carded out to evaluate this algorithm. The BPF algorithm has a higher computational efficiency than the famous FDK algorithm. The BPF algorithm is evaluated using the practical CT projection data on a 450 keV X-ray CT system with a flat-panel detector （FPD）. From the practical experiments, we get the spatial resolution of this CT system. The algorithm could achieve the spatial resolution of 2.4 lp/mm and satisfies the practical applications in industrial CT inspection.     

On neutron phase contrast imaging

Improving the spatial resolution conditions in a neutron imaging experiment enables the detection of phase-based contrast in addition to attenuation contrast. Addressing not only the amplitude but also the phase of radiation in an imaging experiment allows for obtaining additional information about the sample. The so-called neutron phase contrast method improves imaging results mainly by edge enhancement which increases the visibility of sub-resolution structures and of low attenuation contrast materials. These effects have been found at high intensity synchrotron X-ray sources before and have been applied to neutron imaging recently. However, the excellent coherence conditions and spatial resolution of imaging instruments at state-of-the-art synchrotron sources can hardly be compared to neutron imaging. Nevertheless, edge enhancement has been found for increased resolution (coherence) conditions in neutron experiments as well. As for X-ray instruments the effects have been explained by diffraction, although typical interference fringes have never been recorded. In contrast this article will explain the effects measured with neutron radiation by refraction and total reflection. Both of these do not require high spatial coherence. Therefore improved resolution and not increased coherence will be considered as precondition to obtain the reported signals and to understand the results. Considerations concerning relaxed collimation requirements for improved detector resolutions will be presented as a consequence.     

The micro-imaging station of the TopoTomo beamline at the ANKA synchrotron light source

The TopoTomo bending magnet beamline at the ANKA synchrotron facility in Karlsruhe (Germany) operates in the hard X-ray regime (above 6 keV). Recently, an X-ray micro-imaging station has been installed at TopoTomo. For typical imaging applications, a filtered white beam or from 2009 on a double-multilayer monochromator is used. In order to optimize the field of view and the resolution of the available indirect pixel detectors, different optical systems have been installed, adapted, respectively, to a large field of view (macroscope) and to high spatial resolution (microscope). They can be combined with different camera systems, ranging from 14-bit dynamic range CCDs to fast CMOS cameras. The spatial resolution can be brought substantially beyond the micrometer limit by using a Bragg magnifier. Due to the moderate flux of the beamline compared to insertion-device beamlines on third generation light sources, special emphasis has been put on the efficiency of the detectors via a dedicated scintillator concept. The layout of the beamline optics makes optimal use of the coherence properties. Thus, absorption contrast, phase-contrast and analyzer-based imaging can be applied. Additionally, white beam synchrotron topography is performed, using digital indirect X-ray pixel detectors as well as X-ray film.     

2D and 3D refraction-based visualization of breast cancer for early clinical check

Duetal carcinoma in-situ （DCIS） has been visualized by 2D XDFI （X-ray dark-field imaging） and further by a 3D X-ray CT, and the data was acquired by the X-ray optics DEI （diffraction-enhanced imaging）. A newly made al- gorithm was used for CT, Data of 900 projections with interval of 0.2 degrees were used. Ductus lactiferi, microcalci- fication in a 3D form have been clearly visible. The spatial resolution available was approximately 30 μm.     

评价衍射增强成像空间分辨率的方法

衍射增强成像是X射线成像领域的前沿科技。相比于传统的吸收成像，衍射增强成像能大幅提高图像的衬度，尤其是对于由C、H、O、N等低原子序数元素构成的物体，这一特性使得衍射增强成像在医学诊断方面具有突出的应用价值。目前还没有系统评价衍射增强成像空间分辨率的方法。本文通过构建成像系统的调制传递函数模型，推导出衍射增强成像的空间分辨率计算公式，从而对衍射增强实验平台的整体性能进行综合评价。调制传递函数综合考虑了摇摆曲线几何、CCD像素尺寸、闪烁体荧光弥散效应对系统空间分辨率的固有影响，并详细分析了系统调制传递函数各因素对空间分辨率的影响规律，为衍射增强实验平台的物理设计及设备选型提供理论依据。     

Two-dimensional “transXend” detector with band structure absorbers for third-generation energy-resolved computed tomography with improved spatial resolution

Energy-resolved computed tomography (CT) is a method that can generate multiple CT images reconstructed by the X-rays in specific energy ranges. We have previously discussed a lattice absorber type of X-ray detector for energy-resolved CT. We have now developed a band absorber type detector with the aim of improving spatial resolution. We compare spatial resolution and contrast-to-noise ratios for the two types of detectors.     

Nondestructive measurement of the coating thickness for simulated TRISO-coated fuel particles by using phase contrast X-ray radiography

The TRISO-coated fuel particle for a HTGR (high temperature gas-cooled reactor) is composed of a nuclear fuel kernel and outer coating layers. The coating layers consist of a buffer PyC (pyrolytic carbon) layer, an inner PyC (I-PyC) layer, a SiC layer, and an outer PyC (O-PyC) layer. X-ray radiography is one of the nondestructive alternatives to measure a coating thickness without generating a radioactive waste. Phase contrast X-ray radiography technology is more powerful for acquiring a radiograph with clear boundaries, when compared with a conventional X-ray radiography. The contrast can be enhanced for weakly absorbing materials in a phase contrast X-ray radiograph by detecting an intensity variation due to the variation of a phase of the X-rays in the boundary between two objects. Phase contrast X-ray radiograph was acquired from a simulated TRISO-coated fuel particle with a micro-focus X-ray imaging system. The coating thickness was nondestructively measured from the phase contrast X-ray image for the fuel particle.     

Cross-Sectional Imaging of Large and Dense Materials by High Energy X-Ray CT Using Linear Accelerator

A prototype high energy X-ray CT (computed tomography) system has been developed which employs a linear accelerator as the X-ray source (max. photon energy: 12MeV).

One problem encountered in development of this CT system was to reduce the scattered photons from adjacent detectors, i.e. crosstalk, due to high energy X-rays. This crosstalk was reduced to 2% by means of detector shields using tungsten spacers. Spatial resolution was not affected by such small crosstalk as confirmed by numerical simulations. A second problem was to reduce the scattered photons from the test object. This was done using collimators. A third concern was to realize a wide dynamic range data processing which would allow applications to large and dense objects. This problem was solved by using a sample and hold data acquisition method to reduce the dark current of the photo detectors. The dynamic range of this system was experimentally confirmed over 60 dB. It was demonstrated that slits (width : 2 mm) in an iron object (diameter : 25 cm) could be imaged by this prototype CT system.     

3D strain imaging in sub-micrometer crystals using cross-reciprocal space measurements: Numerical feasibility and experimental methodology

Direct inversion of coherent X-ray diffraction patterns is a powerful method to image strains in individual crystals with a high spatial resolution, less than 10 nm. The possibility to invert the diffraction pattern is in principle ensured by the oversampling of the measurement. In addition, the resolution of the reconstructed object requires the measurement of the intensity distribution as far as possible in reciprocal space. Thus, fulfilling the oversampling and resolution constraints, in 3D, implies very long acquisition times, hardly compatible with the stability of the X-ray synchrotron source or sample. To overcome this problem, we present a method based on partial crossed intensity measurements: it consists in two irregularly spaced intensity measurements taken in two orthogonal directions. This paper demonstrates that partial crossed intensity measurements are in principle sufficient to reconstruct 2D objects using a phase retrieval algorithm. We also describe how such measurements can be achieved in practice for 3D imaging. 3D intensity measurements taken in two orthogonal directions for a 111 Bragg peak of a sub-micrometer Au grain are shown.     

用于HL-2A装置的新型硬X射线探测器

介绍了一种应用于托卡马克等离子诊断领域的新型硬X射线探测器,该探测器主要由硅酸钇镥(Lutetium-yttrium Oxyorthosilicate,LYSO)闪烁体和硅光电倍增管(Silicon Photomultiplier,Si PM)组成。基于项目应用需求,对探测器的选型和基于粒子输运蒙特卡罗仿真软件GEANT4的探测器设计加以阐述,并搭建了前端的探测系统,在HL-2A托卡马克装置上进行了现场诊断实验。实现了硬X射线空间能谱和辐射强度分布的诊断测量。实验表明该探测器阵列具有10 ms的时间分辨率,同时具有结构紧凑、使用方便等特点,能够满足等离子体诊断中硬X射线动态能谱和强度分布测量的诊断需求。     

基于电子束扫描阵列微焦点射线源的倒置结构Micro-CT成像研究

倒置几何结构CT成像系统理论上具有扫描视场大、图像信噪比好、锥束伪影小等优点，但其多焦点、稀疏角的扫描模式亦会带来投影数据截断、稀疏和局部数据冗余等问题。本文利用电子束扫描阵列微焦点射线源和高分辨率小面积探测器等构建了一种新型高分辨率、大视场的倒置几何结构Micro CT实验系统(IG Micro CT)。针对该系统多焦点、稀疏角扫描模式下的投影数据截断和稀疏等特点，提出一种图像先验约束的全变分正则化SART迭代CT图像重建算法。首先通过仿真实验对图像重建算法进行了验证，并对阵列微焦点数量、旋转扫描分度数量等扫描参数进行优化，最后在IG Micro CT实验系统上获得了高分辨的Micro CT测试图像。实验结果表明，本文提出的算法适用于IG Micro CT系统，解决了投影数据截断、稀疏采样和局部数据冗余带来的条状等伪影问题，并验证了IG Micro CT这种新型成像方式的可行性。     

静态CT行包安检系统关键技术研究

本文介绍了安检用静态CT技术的基本原理,对基于超顺排碳纳米管技术的冷阴极X射线源、基于光子计数技术的多能谱X射线成像技术和安检用静态CT系统图像重建算法等进行了研究。冷阴极X射线源由基于超顺排碳纳米管技术的冷阴极X射线管、两套高压系统和1个控制系统组成,在130 kV以下可稳定工作。研制并测试了一款具有三能谱成像功能的光子计数型X射线探测器系统。对安检用静态CT系统图像重建算法进行了讨论,提出了一种解决方法。安检用静态CT系统关键技术的研究为安检用静态CT系统的研制提供了技术基础。     

Impedance imaging of two-phase flow field with mesh grouping method

There have been many attempts to measure the bubble distribution in two-phase flow fields and various techniques have been devised. However, the existing techniques require much improvement for imaging two-phase flow fields. In this study, the EIT (Electrical Impedance Tomography) technique is introduced for two-phase flow visualization. In the EIT, a static image reconstruction algorithm providing a higher spatial resolution is required. Using the conventional iterative static image reconstruction algorithms, however, the processing time increases rapidly with poor convergence characteristics as we try to obtain a higher spatial resolution. In order to overcome this problem, we propose an adaptive mesh grouping method utilizing the genetic algorithm and the fuzzy set theory. Computer simulations using the improved Newton–Raphson method with the proposed method show promising results indicating that we can significantly reduce the image reconstruction time without sacrificing spatial resolution.     

Display Processing of Computed Tomography Images

In this paper, we review some of the possibilities for digital enhancement of Computed Tomography (CT) images. The basic hypothesis is that the array of computed x-ray absorbtion values obtained in CT contains more information than the unaided eye can generally grasp. Examples of novel contrast enhancement and noise smoothing algorithms are included to support this hypothesis. Particular attention is paid to properties of the human visual system which we consider an element of the overall information processing loop.     

上海光源X射线成像实验站相位衬度CT初步结果

X射线相位衬度CT能获得样品内部结构的边缘增强图像,可观察到传统X射线吸收CT无法观察到的生物软组织内部微细结构,具有巨大发展潜力.经初步调试,上海光源X射线成像实验站的8-72.5 keV单色X射线输出已能用于实验研究.本文在X射线成像实验站上开展生物样品(蝗虫)同轴X射线相位衬度CT研究,获得蝗虫样品的切片重构图像和三维重建图像.结果图像中,蝗虫样品的翅膀、表面纹理和内部组织分布情况清晰可见.     

Low-dose exposure energy-resolved X-ray computed tomography using a contrast agent with a high-energy K-edge

X-ray computed tomography (CT) with iodine contrast agent is widely employed to locate cancers. However, this method has shortcomings such as high-radiation dose exposure, iodine side effects, and a beam hardening effect. We have been working on the energy-resolved CT measurement method using a novel X-ray detection system, the “transXend” detector, which measures X-rays as electric currents and gives the energy distribution of incident X-rays after analysis. In the present study, we propose a method for low-dose exposure CT that involves the combination of the energy-resolved CT method, which is free from the beam hardening effect, and a harmless contrast agent with high-energy K-edge absorption, such as gold nanoparticles expected as a future contrast agent. Comparisons of radiation dose exposures as functions of aluminum filter thickness at the exit aperture of an X-ray tube and the K-edge energies of contrast agents are described.     

Investigating the influence of gamma ray energies and steel fiber on attenuation properties of reactive powder concrete

A smoothing algorithm for energy spectrum based on differential nonlinearity (DNL) error elimination with total counts conservation for high-energy particle detector systems is presented.It is physics based and is only determined by the DNL error of analog-to-digital converter device itself.From the experimental results,this algorithm slightly improves both noise performance and energy resolution,while greatly reduces the testing errors by almost a half compared to their original values.In addition,the reduced-x2 statistic for evaluating the Gaussian fitting goodness is significantly reduced by almost two orders after smoothing.As a typical verification example,this algorithm is successfully applied in the ground calibration of the Low Energy X-ray Instrument onboard the Hard X-ray Modulation Telescope (HXMT-LE) satellite,lending it a powerful,nondestructive and low-cost tool for both calibration and data processing for high-energy particle detector systems.     

Ion beam studies of InAs/GaAs self assembled quantum dots

Self assembled InAs/GaAs quantum dots (QD’s) emit in the wavelength range (1.3-1.55 μm) revealing an enormous potential to become the active elements of low threshold lasers and light emitting diodes for communication systems. However, the luminescence is dramatically quenched at room temperature (and even below) due to the defects in the GaAs matrix which open non-radiative recombination paths.In this study we combine Rutherford backscattering/channelling (RBS-C) and high resolution X-ray diffraction (HRXRD) to study the structural properties of the InAs/GaAs structures. The InAs/GaAs QD heterostructures were grown by atmospheric pressure metal organic vapour phase epitaxy. Channelling measurements reveal a good crystalline quality along the main axial directions with minimum yields in the range of 4-6% through the entire capping layer. An increase on the dechannelling rate was observed in the region where the InAs quantum dots were buried. The channelling results also give evidence for the presence of defects preferentially oriented. Detailed angular scans in a structure with a 28 nm cap allowed the study of the In orientation with respect to the GaAs matrix and a perfect alignment was found along the growth direction. The strain in the dots shifts the angular curves along the tilt directions.     

Radiation effects on hydroxypropyl methylcellulose phthalate in aqueous system

A water-insoluble cellulose derivative, hydroxypropyl methylcellulose phthalate (HPMCP) hydrogels, was converted to Na type to form hydrogel in paste-like status by radiation crosslinking. Mechanism for radiation crosslinking of cellulose-derivatives in paste-like status was discussed. Crosslinkers, i.e. methyl N,N-bis-acrylamide (MBA) or ethyleneglycol dimethacrylate (EGDMA) has been used to decrease gelation dose (Dg) of synthesis HPMCP hydrogels and improve its mechanical properties. HPMCP-MBA hydrogels were found to be more rigid and HPMCP-EGDMA hydrogels were more flexible. Swelling degree of HPMCP hydrogel in many kinds of salt solutions followed Hofmeister series, which is ubiquitous in polyelectrolyte hydrogel. Specific reswelling was observed in concentrated KF solution, implying a very strong F⁻ binding ability of benzyl group. The comprehensive results obtained in this study will be utilized on the design of HPMCP-based controlled release system.     

X-ray micro-tomography studies on carbon based composite materials for porosity network characterization

Quantitative characterization of the porosity structure of the carbon reinforced carbon fibre (CFC) materials is carried out by high resolution cone-beam X-ray micro-tomography (CBμCT). It is shown that CBμCT provides useful information pertaining to the in-depth fuel migration into carbon tiles function of the CFC material structure. The investigated materials comprised two types of non-irradiated CFC samples (former ITER reference CFC NB31 and JET CFC DMS780) and a series of CFC N11 samples in the frame of the Deuterium Inventory in Tore Supra (DITS) post-mortem analysis. A procedure for the quantitative evaluation of the CFC porosity factor has been introduced and tested. Useful information about the pores connectivity as well as the metal impregnation inside the CFC macroscopic pores, in case of heat sink region of the TS CFC, can be retrieved. The method can be used for the quality control monitoring of the new CFC ITER reference materials.