双能X射线光栅相衬成像的研究

X射线光栅相衬成像存在系统复杂、成像效率低、步进精度要求高、光栅加工难度大等问题.本文设计了一种双能阵列X射线源和双能分析光栅,并应用于X射线光栅相衬成像,提出了一种双能X射线光栅相衬成像系统,阐述了该成像系统的成像原理和相位信息提取方法.提出的成像系统不需要精密步进平台,精简了成像系统,避免了步进误差导致的成像质量降低问题;两次曝光就可以成像,提高了成像效率;双能阵列X射线源、双能分析光栅的应用避免了源光栅、分析光栅难以加工的问题.对提出的成像系统及其相位提取方法进行了仿真,仿真结果显示成像系统可以正常成像,提取到的检测样本的X射线相衬成像相位一阶导数分布与相关文献实验所得结果一致.     

Development of x-ray scintillator functioning also as an analyser grating used in grating-based x-ray differential phase contrast imaging

In order to push the grating-based phase contrast imaging system to be used in hospital and laboratories,this paper designs and develops a novel structure of x-ray scintillator functioning also as an analyser grating,which has been proposed for grating-based x-ray differential phase contrast imaging. According to this design,the scintillator should have a periodical structure in one dimension with the pitch equaling the period of self-image of the phase grating at the Talbot distance,where one half of the pitch is pixellated and is made of x-ray sensitive fluorescent material,such as CsI(Tl),and the remaining part of the pitch is made of x-ray insensitive material,such as silicon. To realize the design,a deep pore array with a high aspect ratio and specially designed grating pattern are successfully manufactured on 5 inch silicon wafer by the photo-assisted electrochemical etching method. The related other problems,such as oxidation-caused geometrical distortion,the filling of CsI(Tl) into deep pores and the removal of inside bubbles,have been overcome. Its pixel size,depth and grating pitch are 3 μm×7.5 μm,150 μm and 3 μm,respectively. The microstructure of the scintillator has been examined microscopically and macroscopically by scanning electron microscope and x-ray resolution chart testing,respectively. The preliminary measurements have shown that the proposed scintillator,also functioning as an analyser grating,has been successfully designed and developed.     

Investigation of noise properties in grating-based x-ray phase tomography with reverse projection method

The relationship between noise variance and spatial resolution in grating-based x-ray phase computed tomography(PCT) imaging is investigated with reverse projection extraction method, and the noise variances of the reconstructed absorption coefficient and refractive index decrement are compared. For the differential phase contrast method, the noise variance in the differential projection images follows the same inverse-square law with spatial resolution as in conventional absorption-based x-ray imaging projections. However, both theoretical analysis and simulations demonstrate that in PCT the noise variance of the reconstructed refractive index decrement scales with spatial resolution follows an inverse linear relationship at fixed slice thickness, while the noise variance of the reconstructed absorption coefficient conforms with the inverse cubic law. The results indicate that, for the same noise variance level, PCT imaging may enable higher spatial resolution than conventional absorption computed tomography(ACT), while ACT benefits more from degraded spatial resolution. This could be a useful guidance in imaging the inner structure of the sample in higher spatial resolution.     

X-ray phase-sensitive microscope imaging with a grating interferometer: Theory and simulation

A general theoretical framework is presented to explain the formation of the phase signal in an x-ray microscope integrated with a grating interferometer, which simultaneously enables the high spatial resolution imaging and the improved image contrast. By using this theory, several key parameters of phase contrast imaging can be predicted, for instance, the fringe visibility and period, and the conversion condition from the differential phase imaging (DPI) to the phase difference imaging (PDI). Additionally, numerical simulations are performed with certain x-ray optical components and imaging geometry. Comparison with the available experimental measurement [Appl. Phys. Lett. 113 063105 (2018)] demonstrates the accuracy of this developed quantitative analysis method of x-ray phase-sensitive microscope imaging.     

免分析光栅一次曝光相位衬度成像方法

X射线光栅微分相位衬度成像技术可以观察到常规吸收衬度成像难以分辨的弱吸收物质的精细结构信息,因而在医学、材料学等研究领域具有巨大的应用前景.但传统的X射线光栅微分相位衬度成像技术由于采用分析光栅作为空间滤波器,需要采用相位步进法扫描分析光栅来获得样品的多张投影图像才能够分离出样品的吸收、折射和散射信息,因此存在样品曝光时间长、辐射剂量高以及X射线光通量利用率低等问题,限制了其在各个学科领域的应用研究.为克服上述问题,本文提出一种基于免分析光栅相位衬度成像系统的一次曝光样品信息提取算法.该算法只需要利用一块相位光栅,进而采用高分辨探测器进行样品投影数据的一次采集即可提取样品的吸收、折射和散射信息.理论和模拟研究结果表明:与传统相位步进法相比,该算法具有样品信息提取精度高,且不受光栅的自成像周期需为探测器像素尺寸的整数倍条件的限制.此外,该算法还能够有效地减少对生物样品的辐射损伤,因此在生物医学成像等研究领域中具有广泛的应用前景.     

A new method for information retrieval in two-dimensional grating-based X-ray phase contrast imaging

Grating-based X-ray phase contrast imaging has been demonstrated to be an extremely powerful phase-sensitive imaging technique.By using two-dimensional(2D) gratings,the observable contrast is extended to two refraction directions.Recently,we have developed a novel reverse-projection(RP) method,which is capable of retrieving the object information efficiently with one-dimensional(1D) grating-based phase contrast imaging.In this contribution,we present its extension to the 2D grating-based X-ray phase contrast imaging,named the two-dimensional reverseprojection(2D-RP) method,for information retrieval.The method takes into account the nonlinear contributions of two refraction directions and allows the retrieval of the absorption,the horizontal and the vertical refraction images.The obtained information can be used for the reconstruction of the three-dimensional phase gradient field,and for an improved phase map retrieval and reconstruction.Numerical experiments are carried out,and the results confirm the validity of the 2D-RP method.     

Shifting curves based on the detector integration effect for x-ray phase contrast imaging

In theory, we find that the actual function of the analyzer grating in the Talbot–Lau interferometer is segmenting the self-images of the phase grating and choosing integral areas, which make sure that each period of self-images in one detector pixel contributes the same signal to the detector. Furthermore, in the case of the lack of an analyzer grating, the shifting curves are still existent in theory as long as the number of fringes is non-integral in a detector pixel, which is a sufficient condition for creating shifting curve. The sufficient condition is available for not only the Talbot–Lau interferometer and the inverse geometry of Talbot–Lau interferometer, but also the x-ray phase contrast imaging system based on geometrical optics. In practical applications, we propose a method to improve the performances of the existing systems by employing the sufficient condition. This method can shorten the system length, is applicable to large period gratings, and can use the detectors with large pixels and large field of view. In addition, the experimental arrangement can be simplified due to the lack of an analyzer grating. In order to improve detection sensitivity and resolution, we also give an optimal fringe period.We believe that the theory and method proposed here is a step forward for x-ray phase contrast imaging.     

硬X射线相位光栅的设计与研制

针对在普通实验室和医院实现40—100keVX射线相衬成像的需求,考虑到成像系统参数、X射线源空间相干特性及光栅衍射效率,设计出硅基相位光栅结构参数.利用我们已发展的光助电化学刻蚀技术研制出直径为5英寸的相位光栅,其空间周期为5.6μm,线宽为2.8μm,深度为40—70μm.在理论分析的基础上,通过提高硅片两端有效工作电压和修正Lehmann电流密度公式,解决了实际刻蚀过程中出现的钻蚀问题.由实验结果可知,本方案对制作大面积高精度相位光栅十分有效。     

X射线相衬成像应用于惯性约束核聚变多层球壳靶丸检测

随着惯性约束核聚变(ICF)研究的逐步深入,尤其是氘氚(DT)燃料层球壳梯度掺杂的广泛应用,对轻物质界面的诊断需求日益增长.在北京同步辐射形貌成像站的微米CT机上利用类同轴相衬成像方法获取了三层球壳靶丸的相衬图像,最佳空间分辨率达到了2μupm,衬度达到12%;通过分析边缘增强函数和衬度传递函数之间的关系,总结出一套完整的类同轴相衬成像方法,同时将相衬成像结果与吸收成像结果对比.实验结果表明,X射线相衬成像在轻物质界面的分辨中具有明显的优势,能够广泛应用于ICF研究、医学断层扫描CT装置和生物结构等前沿科学领域.     

第三代同步辐射光源X射线相干性测量研究

随着高性能第三代同步辐射光源的建成开放,基于X射线相干特性的实验方法得到了快速发展和广泛应用.作为一个典型的例子,X射线相位衬度成像已经成为常规的X射线实验方法并向用户开放.相干散射、相干衍射成像、光子关联谱等X射线实验方法正日益受到重视,在高空间分辨、时间分辨等研究领域已显示出其独特的优越性.因此,研究和测量第三代同步辐射的空间相干特性对进一步发展这些新的实验方法具有重要意义.基于Talbot自成像原理成功测量了上海光源X射线成像线站发射的X射线的空间相干长度,并进而测得了相应光源的空间尺度.光子能量为33.2 keV时,测得的X射线光束垂直方向空间相干长度为8.84μm,对应的光源尺寸为23μm,测量结果与理论分析相符.     

Hard x-ray phase tomography with low-brilliance sources

We report on a method for tomographic phase contrast imaging of centimeter sized objects. As opposed to existing techniques, our approach can be used with low-brilliance, lab based x-ray sources and thus is of interest for a wide range of applications in medicine, biology, and nondestructive testing. The work is based on the recent development of a hard x-ray grating interferometer, which has been demonstrated to yield differential phase contrast projection images. Here we particularly focus on how this method can be used for tomographic reconstructions using filtered back projection algorithms to yield quantitative volumetric information of both the real and imaginary part of the samples's refractive index.     

Quantitative x-ray phase-contrast imaging using a single grating of comparable pitch to sample feature size

The ability to quantitatively retrieve transverse phase maps during imaging by using coherent x rays often requires a precise grating or analyzer-crystal-based setup. Imaging of live animals presents further challenges when these methods require multiple exposures for image reconstruction. We present a simple method of single-exposure, single-grating quantitative phase contrast for a regime in which the grating period is much greater than the effective pixel size. A grating is used to create a high-visibility reference pattern incident on the sample, which is distorted according to the complex refractive index and thickness of the sample. The resolution, along a line parallel to the grating, is not restricted by the grating spacing, and the detector resolution becomes the primary determinant of the spatial resolution. We present a method of analysis that maps the displacement of interrogation windows in order to retrieve a quantitative phase map. Application of this analysis to the imaging of known phantoms shows excellent correspondence.     

Imaging the ultrasmall-angle x-ray scattering distribution with grating interferometry

X-ray imaging with grating interferometry has previously been regarded as a technique providing information only in direct space. It delivers absorption, phase, and dark-field contrast, which can be viewed as parameters of the underlying but unresolved scattering distribution. Here, we present a method that provides the ultrasmall-angle x-ray scattering distribution and, thus, allows simultaneous access to direct and reciprocal space information.     

Phase-difference amplification using a Sagnac interferometer

In this paper, we describe a new method for achieving phase-difference amplification, which is quick and convenient, operates in real time, and requires no photographic steps. Magnification factors of 2, 4 or 6 are achieved easily in one step. Because the system operates in real time, phase stepping may be applied to extract the amplified phase distributions. Our method is a variation on longitudinally reversed shearing interferometry, using first- or higher-order diffraction from a grating (hologram) which is in fact the interferogram of the wavefront under test. The grating is derived from a standard two-beam interferometer which is phase-stepped, and displayed in real time on a spatial light modulator in the phase-difference amplification setup. It is illuminated by the two output beams from a Sagnac interferometer, similar to the set up used by (Barnes et al. Barnes TH, Eiju T, Matsuda K. Appl Opt 1986; 25:1864). for spectral resolution enhancement, and a phase-amplified fringe pattern is obtained by spatial filtering using a Fourier transform lens. We demonstrate operation of the phase amplifier and show amplified phase maps retrieved by phase-stepping. We believe this is the first time that real-time phase amplification without photographic steps and with phase stepping has been demonstrated.     

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.     

Two-dimensional x-ray beam phase sensing

We present a new method to analyze quantitatively the wave front of a partially coherent x-ray beam. The technique is based on the use of two-dimensional speckle patterns combined with digital image correlation algorithms and offers a pixel size resolution, a high accuracy, and a reduced sensitivity to mechanical vibrations thanks to a very simple setup. The requirements on transverse and longitudinal coherence are also low. Finally, we show how the method can be used for phase contrast imaging applications by a single sample exposure process.     

光栅成像的锥束相位CT重建算法

X射线相位衬度成像对弱吸收物质有着独有的高密度分辨率,在医学、生物学、材料学等领域显示出良好的应用前景.但是其中的干涉成像法、衍射增强成像法和同轴成像法具有一定局限性,难以被广泛应用.光栅成像克服了以往相位衬度成像的缺陷,也使相位衬度成像向锥束成像发展成为可能.本文致力于锥束相位X射线计算机断层摄影术(CT)重建算法的研究,根据基于光栅的锥束相位成像的特点,利用吸收CT中FDK (Feldkamp-Davis-Kress)重建算法的思想,发展出适用于锥束相位成像的CT重建方法.该方法为滤波反投影类型,以相位一阶导数像为投影值,直接重建物体的相位项.通过仿真验证了算法的正确性.     

Quantitative coherence analysis of dual phase grating x-ray interferometry with source grating

Dual phase grating x-ray interferometry is compatible with common imaging detectors,and abandons the use of an absorption analyzer grating to reduce the radiation dose.When using x-ray tubes,an absorbing source grating must be introduced into the dual phase grating interferometer.In order to attain a high fringe visibility,in this work we conduct a quantitative coherence analysis of dual phase grating interferometry to find how the source grating affects the fringe visibility.Theoretical analysis shows that with the generalized Lau condition satisfied,the fringe visibility is influenced by the duty cycle of the source grating and the transmission through the grating bar.And the influence of the source grating profile on the fringe visibility is independent of the phase grating type.Numerical results illustrate that the maximum achievable fringe visibility decreases significantly with increasing transmission in the grating bar.Under a given transmission,one can always find an optimal duty cycle to maximize the fringe visibility.These results can be used as general guidelines for designing and optimizing dual phase grating x-ray interferometers for potential applications.     

五维自由度衍射光栅精密测量系统（英文）

为了在保证结构简单的前提下,实现衍射光栅精密测量系统的大量程、高精度、多维度测量,设计了能够同时测量位移和角度的五维自由度衍射光栅精密测量系统。基于利特罗对称式光路结构,采用高刻线密度的一维衍射光栅以及外差干涉原理实现了沿光栅矢量方向和光栅法线方向的二维位移测量;通过引入高精度的位置灵敏探测器,结合±1级衍射光与光栅之间的角度变化关系实现了对光栅俯仰、偏摆和滚转三个维度的角度误差测量。实验结果表明:该衍射光栅精密测量系统能够实现分辨力优于4 nm的二维位移测量以及分辨力优于1″的三维角度测量,其位移测量范围只受限于光栅的尺寸,量程大大增加。该衍射光栅精密测量系统在精密测量领域有重要意义。     

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.