Preliminary research on dual-energy X-ray phase-contrast imaging

Dual-energy X-ray absorptiometry(DEXA) has been widely applied to measure the bone mineral density(BMD) and soft-tissue composition of the human body. However, the use of DEXA is greatly limited for lowZ materials such as soft tissues due to their weak absorption, while X-ray phase-contrast imaging(XPCI) shows significantly improved contrast in comparison with the conventional standard absorption-based X-ray imaging for soft tissues. In this paper, we propose a novel X-ray phase-contrast method to measure the area density of low-Z materials, including a single-energy method and a dual-energy method. The single-energy method is for the area density calculation of one low-Z material, while the dual-energy method aims to calculate the area densities of two low-Z materials simultaneously. Comparing the experimental and simulation results with the theoretical ones, the new method proves to have the potential to replace DEXA in area density measurement. The new method sets the prerequisites for a future precise and low-dose area density calculation method for low-Z materials.     

无吸收光栅的X射线相位衬度成像实验研究

通过对基于空间相干源和具有分析光栅功能的X射线转换屏的微分干涉X射线相位衬度成像 系统的理论分析, 利用线发射体阵列结构阳极X射线管和光助电化学刻蚀技术 研制的相位光栅和具有分析光栅功能的X射线转换屏, 组建了一种无吸收光栅的X射线微分干涉相衬成像系统. 在此系统上开展了生物样品的实验研究, 获得了较传统吸收成像更为清晰和更多样品结构信息的相位衬度图像. 从而试验验证了该系统方案设计的可行性, 为X射线相衬成像技术从实验室走向临床应用提供了有效途径.     

A refractive index interferometry method based on the X-ray crystallography

The absorption contrast image is very low for those weakly absorbing materials or weak phase object, such as the soft biological tissues, however, the phase item value of refractive index is a thousand larger than the absorption item, therefore the refractive index interferometry method is expected to be a new imaging tool for them. An interferometry method based on the X-ray crystallography is explored to the measurement of refractive index in this paper. The theoretical foundation, the optical design, the crystal transistor performance are the key parts to this interferometry method based on the X-ray crystallography. We give out a special refractive index detection scheme, using a laboratory hard X-ray source, and four single Si crystals. With this instrument, refractive index profile of those weak phase object can be unfolded accurately using this method. This refractive index interferometry method based on the X-ray crystallography will provide a new research tool for those special material properties or biological tissues study.     

同轴X射线相位衬度计算机X射线断层摄影术研究

基于北京同步辐射装置(BSRF)开展了同轴X射线相位衬度计算机X射线断层摄影术(CT)研究.利用北京同步辐射的14 keV单色X射线作为光源,以高分辨能力的X射线胶片作为探测器,分别开展吸收衬度和同轴相位衬度成像的比较研究以及相位衬度计算机X射线断层摄影术研究.相位衬度计算机X射线断层摄影术重建采用Bronnikov提出的算法.结果显示,与传统的吸收衬度图像相比,相位衬度图像具有更好的衬度和更高的空间分辨力;实验获得人工样品和蝗虫的相位衬度计算机X射线断层摄影术重建图像.重建图像中可见样品的一些结构细节.实验结果表明,相位衬度X射线成像更适合于研究弱吸收或吸收差异很小的材料;利用北京同步辐射开展同轴X射线相位衬度计算机X射线断层摄影术研究是可行的.     

X-ray phase-attenuation duality and phase retrieval

Phase retrieval is the key to quantitative x-ray phase-contrast imaging. To retrieve the phase image of an x-ray wave field, in general one needs multiple phase-contrast images. We have made a new observation of phase-attenuation duality for soft tissues, and we show how only a single phase-contrast image is needed for successful phase retrieval based on this duality. The phase-retrieval formula based on a single phase-contrast image of inhomogeneous soft tissue is derived and presented. We show the striking enhancement of the tissue contrast in simulated phase images that this new approach produces.     

微焦点源X射线相衬成像技术

 相衬成像方法利用硬X射线对低密度弱吸收物质成像，可获得高衬度图像。用菲涅尔衍射理论分析了X射线图像的形成机理。在频域中根据光学传递函数，对物像距离、样品空间频率等对图像相位衬度的影响进行了分析。分辨率和衬度是决定图像可见度的两个依据，分辨率主要依赖于光源的空间相干性，空间相干性又决定于源点尺寸，而时间相干性（单色性）是一个不重要的影响因子。利用多色微焦点源实现了X射线相衬成像技术，获得了有价值的相衬图像，如低原子序数低密度泡沫材料的硬X射线相衬图像，与吸收衬度成像相比，其图像质量得到了很大提高，能观察到泡沫材料的细微结构，分辨率可达μm量级。     

A variable-wavelength-based approach of phase retrieval for contrast transfer function based methods

X-ray phase-contrast imaging has emerged as an important method for improving contrast and sensitivity in the field of X-ray imaging. This increase in the sensitivity is attributed to the fact that, in the hard X-ray regime, the phase shift is more prominent as compared with the attenuation for materials having a low X-ray absorption coefficient. Among all the methods using the X-ray phase-contrast technique, in-line phase-contrast imaging scores over the other methods in terms of ease of implementation and efficient use of available X-ray flux. In order to retrieve the projected phase map of the object from the recorded intensity pattern, a large number of algorithms have been proposed. These algorithms generally use either the transport of intensity or contrast transfer function based approach for phase retrieval. In this paper it is proposed to use multiple wavelengths for phase retrieval using the contrast transfer function based formalism.     

Optimization of the in-line X-ray phase-contrast imaging setup considering edge-contrast enhancement and spatial resolution

Employing the approximation theory based on refraction and the definition of the total point-spread-function of the imaging system, the variation in the edge contrast of simple model samples is discussed with different source-to-sample and sample-to-detector distances, which actually means different spatial resolutions of the imaging system. The experiments were carried out with the Beamline 4W1A imaging setup at the Beijing Synchrotron Radiation Facility for simple model and insect samples. The results show that to obtain clear phase-contrast images of biologic tissues for the X-ray in-line imaging setup, with determined parameters such as the size of the X-ray source, the pixel size of the detector and the fixed source-to-sample distance, there is a range of optimized sample-to-detector distances. The analysis method discussed in this article can be helpful in optimizing the setup of X-ray in-line phase-contrast imaging.     

基于激光驱动等离子体X光源的X射线相衬成像

为了诊断惯性约束聚变(ICF)内爆靶丸球壳的多层信息,在神光Ⅱ激光器上对激光驱动等离子体X光源的相衬成像进行了研究。利用神光Ⅱ第9路激光驱动平面Ti靶获得X光源,在10μm的针孔约束下作为次级点光源对样品成像,用X光胶片记录。成功地将相衬成像技术应用于ICF实验,综合考虑成像放大倍数、分辨力、成像衬度和抑制烧蚀碎片等因素,选择合适的实验条件,成功获得了清晰的双层内爆靶丸球壳结构,空间分辨力优于10μm。     

X-ray spatial harmonic imaging of phase objects

Refractive index gradients in materials or at material interfaces lead to x-ray diffraction. Interference of this radiation with adjacent x-ray waves causes phase contrast that can be used for imaging purposes if an x-ray source with sufficient spatial coherence is used. The imaging modality presented here uses hard x radiation diffracted at interfaces, but requires only little spatial coherence. We report experiments showing, first, that image contrast is not diminished by motional blurring, and second, that contrast can be increased by orders of magnitude relative to in-line x-ray phase-contrast imaging. These properties substantially broaden the applicability of phase-sensitive imaging to moving samples and very weak density gradients.     

采用吸收修正Bronnikov算法的有机复合样品的X射线显微计算机层析研究

有机复合样品如大多数由低原子序数物质构成的多聚物组合材料、生物软组织等具有混合相位和振幅特性,采用传统吸收计算机层析(CT)或纯相位重构算法难以达到较高密度分辨的目的。采用基于吸收修正的Bronnikov相位恢复算法的X射线同轴相衬CT技术对高分子混合组份材料和药物医学样品进行了实验研究。结果表明该相位恢复算法可以实现多聚物组合材料的无损分辨和精确测量,能够清晰辨别各组份材料的电子密度分布;验证了药物样品多孔性结构三维测量的可行性,能够有效提供孔隙测量的阈值选择。该方法在材料科学尤其是聚合物组合材料、泡沫多孔材料都会发挥很好的作用;单距的低辐射优势,尤其适用于生物医学样品三维无损成像。     

相移吸收二元性算法用于X射线混合衬度定量显微CT的可行性研究

本文采用相移吸收二元性(PAD)相位恢复算法来实现混合 衬度样品内部不同密度组分的定量成像, 采用数字模拟和实验研究验证该方法的可行性. 模拟结果表明, 对于三种不同材料其重构误差均小于1%, 且误差值随材料折射率的增大而减小. 利用上海光源X射线成像线站开展了实验研究, 结果表明用单距PAD相位恢复算法可获取样品的定量信息. 与模拟结果相比, 实验中的重构精度相对较低, 环状伪影可能是影响精度的主要原因. 模拟和实验研究过程中, 均只采用了一组单距投影数据. 可以认为, 相移吸收二元性算法可用于混合衬度样品的定量信息分析研究. 由于剂量相对较低, 应可适合于软组织和骨骼同时存在时生物医学样品的定量相衬CT研究.     

关于X射线同轴位相衬度成像的研究

利用模糊函数对部分相干源相X射线衬成像进行了详细的理论分析.列举了不同条件下的实验结果.通过与吸收成像相比较，相衬成像无疑对生物样品的内部结构有更高的对比度和可见度.     

衍射增强成像方法在计算机断层成像中的应用

衍射增强成像是X射线相位衬度成像方法之一，这种方法具有较高的衬度和空间分辨率. 利用它对由轻元素组成的生物、医学样品成像可以观察到常规吸收成像无法观察到的内部微细结构. 这种方法在生物、医学、材料科学等无损检测领域中的应用研究，已成为当前国际上X射线成像领域中的研究热点. 讨论衍射增强成像方法和该方法在计算机断层成像中的应用. 实验结果表明，使用衍射增强成像方法获得的数据源能够重建出微米级的生物组织结构，这些组织结构信息在常规X射线断层成像中是难以得到的. 关键词： 衍射增强成像 CT重建 同步辐射 微细结构     

Phase-contrast imaging of nanostructures by soft x rays from a femtosecond-laser plasma

The possibility of phase-contrast imaging of nanostructures has been analyzed with the use of a femtosecond-laser plasma as a spatially coherent soft x-ray source and a LiF crystal as an x-ray detector having both the submicron spatial resolution in a wide field of view and a high contrast. It is demonstrated that the spatial coherence length of radiation in the wavelength range 1–13 nm at a distance of 30 cm from the femtosecond-laser plasma source is ?1.5 μm. The achieved spatial coherence of the source is sufficient to obtain high-quality phase-contrast x-ray images of foils with various chemical compositions and a thickness of ?100 nm.     

Soft X-ray imaging of thick carbon-based materials using the normal incidence multilayer optics

The high transparency of carbon-containing materials in the spectral region of “carbon window” (λ ～ 4.5–5 nm) introduces new opportunities for various soft X-ray microscopy applications. The development of efficient multilayer coated X-ray optics operating at the wavelengths of about 4.5 nm has stimulated a series of our imaging experiments to study thick biological and synthetic objects. Our experimental set-up consisted of a laser plasma X-ray source generated with the 2nd harmonics of Nd–glass laser, scandium-based thin-film filters, Co/C multilayer mirror and X-ray film UF-4. All soft X-ray images were produced with a single nanosecond exposure and demonstrated appropriate absorption contrast and detector-limited spatial resolution. A special attention was paid to the 3D imaging of thick low-density foam materials to be used in design of laser fusion targets.     

Noise analysis of grating-based x-ray differential phase-contrast imaging with angular signal radiography

X-ray phase-contrast imaging is one of the novel techniques,and has potential to enhance image quality and provide the details of inner structures nondestructively.In this work,we investigate quantitatively signal-to-noise ratio(SNR) of grating-based x-ray phase contrast imaging(GBPCI) system by employing angular signal radiography(ASR).Moreover,photon statistics and mechanical error that is a major source of noise are investigated in detail.Results show the dependence of SNR on the system parameters and the effects on the extracted absorption,refraction and scattering images.Our conclusions can be used to optimize the system design for upcoming practical applications in the areas such as material science and biomedical imaging.     

基于同步辐射X射线相衬显微CT技术的竹木复合材料胶合界面特征研究

第三代同步辐射光源X射线相位衬度显微CT能获得样品内部结构的边缘增强图像,实现对低Z材料成像。利用上海同步辐射光源X射线相衬显微成像技术,实现了竹木复合材料中EPI和MUF胶合界面和胶黏剂渗透特征的无损探测,并基于这些特征分析了几种不同加工工艺对胶黏剂在木材和竹材中渗透的影响。该技术为人造板工艺解剖学研究提供了一种重要无损检测手段。     

The use of high energy laser-plasma sources in soft X-ray contact microscopy of living biological samples

In this paper the results of an experiment on soft X-ray contact microscopy using a laser-plasma source are presented. A resolution of 50 nm has been achieved imaging pig sperm cells, while other specimens, such as algae and yeast cells, showed internal details, proving the technique to be a powerful tool for biological investigations. Original biological information has been obtained and the conditions for optimal image formation have been studied. Received 5 June 2002 Published online 24 September 2002 RID="a" ID="a"e-mail: batani@mib.infn.it