First X-ray fluorescence CT experimental results at the SSRF X-ray imaging beamline

X-ray fluorescence CT is a non-destructive technique for detecting elemental composition and distribution inside a specimen. In this paper, the first experimental results of X-ray fluorescence CT obtained at the SSRF X-ray imaging beamline (BL13W1) are described. The test samples were investigated and the 2D elemental image was reconstructed using a filtered back-projection algorithm. In the sample the element Cd was observed. Up to now, the X-ray fluorescence CT could be carried out at the SSRF X-ray imaging beamline.     

First X-ray fluorescence CT experimental results at the SSRF X-ray imaging beamline

X-ray fluorescence CT is a non-destructive technique for detecting elemental composition and distribution inside a specimen. In this paper, the first experimental results of X-ray fluorescence CT obtained at the SSRF X-ray imaging beamline (BL13W1) are described. The test samples were investigated and the 2D elemental image was reconstructed using a filtered back-projection algorithm. In the sample the element Cd was observed. Up to now, the X-ray fluorescence CT could be carried out at the SSRF X-ray imaging beamline.     

X射线衍射增强成像中吸收、折射以及散射衬度的计算层析

X射线相位衬度成像是一种新型的X射线成像技术,通过记录射线穿过物体后相位的改变对物体进行成像,可以提供比传统的X射线吸收成像更高的图像衬度以及空间分辨力。衍射增强成像方法(Diffraction enhancedimaging,DEI)是X射线相位衬度成像方法之一,利用一块放置在物体和探测器之间的分析晶体提取物体的吸收、折射以及散射信息并进行成像。将衍射增强成像方法与计算机断层成像技术(Computerized Tomography)进行结合,利用吸收、散射以及折射信息,分别采用滤波反投影以及雷登(Radon)变换,对昆虫样品——蜜蜂进行计算层析重建,获得了好于X射线吸收计算层析的重建结果,验证了衍射增强成像信息分离计算层析的优越性。     

样品内部非平行界面的反射式terahertz波层析研究

已有的反射式terahertz(THz)波层析研究都是针对平行界面进行，开展了样品内非平行界面的反射式THz波层析成像的数值模拟研究.根据物理模型给出了重构算法，并编制了相应的重构软件，数值计算结果表明非平行界面的重构是可行的.分析表明，重构误差主要来源于计算中的近似处理和累积误差.讨论了曲线轮廓界面的近似处理方法，提出当曲线轮廓各点的曲率半径都比较大时，可以把曲线分割成多段直线处理.最后分析了影响纵向分辨率的因素.结果表明，输入脉冲的脉宽越大则纵向分辨率越低，一般情况下纵向分辨率可达到亚毫米量级. 关键词： terahertz波 反射式层析成像     

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.     

螺旋锥束计算机断层成像倾斜扇束反投影滤波局部重建算法

螺旋锥束计算机断层成像(CT)作为常用的临床诊断工具,如何尽可能地减少其辐射剂量是热点研究领域之一.局部成像利用准直器减小射线直照区域,能够有效降低CT辐射剂量.然而,局部成像会造成投影数据横向截断,产生局部重建问题.现有螺旋反投影滤波(BPF)算法只能实现局部曲面重建,难以实现局部体区域重建.在圆轨迹扇束BPF算法的基础上,通过加权修正和坐标扩展,提出了螺旋锥束CT倾斜扇束反投影滤波(TFB-BPF)重建算法.该算法把重建区域按层划分,对各层构建倾斜的扇形束几何,并使用经过加权修正的TFB-BPF算法逐层进行重建.该算法最大的优势是滤波线(与原始螺旋锥束BPF算法中PI线等价)在二维平面内选择,算法更加简洁高效,并且能够应用于局部体区域的重建.实验结果表明,算法能够有效重建物体局部体区域,并且重建图像质量较好,没有明显的截断伪影.     

Frequency-swept ultrasound-modulated optical tomography in biological tissue by use of parallel detection

A frequency-swept ultrasonic beam was focused into a biological tissue sample to modulate the laser light passing through the ultrasonic beam inside the tissue. Parallel detection of the speckle field formed by the transmitted laser light was implemented with the source-synchronous-illumination lock-in technique to improve the signal-to-noise ratio. The ultrasound-modulated laser light reflects the local optical and mechanical properties in the ultrasonic beam and can be used for tomographic imaging of the tissue. Sweeping the ultrasonic frequency provides spatial resolution along the ultrasonic axis, which is scalable with the frequency span of the sweep. Two-dimensional images of biological tissue with buried objects were successfully obtained experimentally.     

Simultaneous in-situ measurements of gas temperature and pyrolysis of biomass smoldering via X-ray computed tomography

In-situ X-ray computed tomography (XCT) imaging is employed to investigate the smoldering dynamics of biomass at the sub-millimeter scale. This technique provides simultaneous and spatially-resolved information about the gas temperature and the biomass density, thereby enabling tracking of the pyrolysis and char oxidation fronts. To achieve well-controlled heating and flow conditioning, oak biomass samples are instrumented above a diffusion flame inside a tube, with total oxygen concentrations of 6% and 11% per volume. Experiments are performed on a laboratory XCT system. The flow is diluted with Kr to increase X-ray attenuation in the gas phase thus allowing for simultaneous 3D measurements of sample density and surrounding temperature. XCT scans are acquired every 90 s at a spatial resolution of 135 µm. The high spatial resolution enables the volumetric visualization of the smoldering process that is associated with pyrolysis and char oxidation. These measurements show how the grain structure affects flame stabilization and induces fingering of the pyrolysis front, while crack formation accelerates the char oxidation locally. Evaluations of the sample mass via XCT are compared with load cell measurements, showing good agreement. A low-order model is developed to evaluate the propagation speeds of pyrolysis and oxidation fronts from the X-ray data over time, and comparisons are made with the surface recess speed.     

Remote photoacoustic imaging on solid material using a two-wave mixing interferometer

We report on remote and contactless photoacoustic imaging (PAI) for the inspection of solid materials using a two-wave mixing interferometer. In this Letter, a semitransparent sample was excited with picosecond laser pulses. The local absorption of the electromagnetic radiation led to generation of broadband ultrasonic waves inside the sample. Ultrasonic waves arriving at the sample surface were detected utilizing a two-wave mixing interferometer. After data acquisition, the initial pressure distribution was reconstructed using a Fourier space synthetic aperture technique algorithm. We show the potential of PAI for the inspection of semitransparent solid materials.     

Characterization of porosity in a 19th century painting ground by synchrotron radiation X-ray tomography

The study of the early oeuvre of the Swiss painter Cuno Amiet (1868–1961) has revealed that, up to 1907, many of his grounds were hand applied and are mainly composed of chalk, bound in protein. These grounds are not only lean and absorbent, but also, as Synchrotron radiation X-ray microtomography has shown, porous. Our approach to the characterization of pore structure and quantity, their connectivity, and homogeneity is based on image segmentation and application of a clustering algorithm to high-resolution X-ray tomographic data. The issues associated with the segmentation of the different components of a ground sample based on X-ray imaging data are discussed. The approach applied to a sample taken from “Portrait of Max Leu” (1899) by Amiet revealed the presence of three sublayers within the ground with distinct porosity features, which had not been observed optically in cross-section. The upper and lower layers are highly porous with important connectivity and thus prone to water uptake/storage. The middle layer however shows low and nonconnected porosity at the resolution level of the X-ray tomography images, so that few direct water absorption paths through the entire sample exist. The potential of the method to characterize porosity and to understand moisture-related issues in paint layer degradation are discussed.     

影响同步辐射X射线螺旋显微CT的若干因素研究

同步辐射X射线是扁平光, 传统显微CT难以对长样品实现快速、高分辨、低剂量三维成像, 螺旋扫描的引入可以很好地解决这一难题. 本文采用数字模拟和实验的方法较为系统地研究了螺距、光斑竖直高度和180° 投影数对成像质量、速度的影响. 模拟结果表明, 在螺距不大于2的前提下, 180° 投影数增大, 误差减小, CT成像速度变慢; 光斑竖直高度增大, 速度加快; 螺距增大, 速度加快, 且误差在允许范围内; 选取合适的参数值使旋转平移比为整数, 误差最小. 同时, 优化参数并在上海光源实现螺旋显微CT, 从实验上验证了模拟结果. 研究表明, 通过优化各参数, 同步辐射螺旋显微CT可以实现快速高质量三维成像, 为以后的实际应用提供了依据.     

Beyond the Rayleigh criterion: grating assisted far-field optical diffraction tomography

We propose an optical imaging system, in which both illumination and collection are done in far field, that presents a power of resolution better than one-tenth of the wavelength. This is achieved by depositing the sample on a periodically nanostructured substrate illuminated under various angles of incidence. The superresolution is due to the high spatial frequencies of the field illuminating the sample and to the use of an inversion algorithm for reconstructing the map of relative permittivity from the diffracted far field. Thus, we are able to obtain wide-field images with near-field resolution without scanning a probe in the vicinity of the sample.     

Fluorescence-lifetime-based tomography for turbid media

We derive a novel algorithm to recover the in vivo distributions of fluorophores based on an asymptotic life-time analysis of time-domain fluorescence measurements with turbid tissue. We experimentally demonstrate the advantage offered by this method in localizing fluorophores with distinct lifetimes. This algorithm has wide applicability for diagnostic fluorescence imaging in the presence of several-centimeter-thick biological tissue, since fluorescence lifetime is a sensitive indicator of local tissue environment and interactions at the molecular level.     

Fluorescence tomography with simulated data based on the equation of radiative transfer

The quantification of a nonuniform quantum yield or fluorophore absorption distribution is of major interest in molecular imaging of biological tissue. We introduce what is believed to be the first fluorescence image reconstruction algorithm based on the equation of radiative transfer that recovers the spatial distribution of light-emitting fluorophores inside a highly scattering medium from measurements made on the surface of the medium. We obtain images of either the quantum yield or the fluorophore absorption.     

Imaging of articular cartilage--data matching using X-ray tomography, SEM, FIB slicing and conventional histology

The study was aimed at demonstrating a true cellular resolution for articular cartilage using synchrotron radiation-based X-ray microcomputed tomography (SR-μCT) with a sample-specific optimization of the phase contrast. The generated tomographic data were later used to prepare a matching histological sample from the full volume specimen. We used highly coherent and monochromatic X-rays from a synchrotron source to image a tissue sample of bovine articular cartilage after deparaffinization. Phase contrast enhancement was achieved by using five different sample to detector distances for the same X-ray energy. After tomography, the sample was re-embedded into resin while retaining a dedicated sample orientation for subsequent sectioning and polishing, which was conducted until a previously defined spatial position was achieved. The protocol for resin embedding was developed to inhibit morphological changes during embedding. Giemsa staining was applied for better structural and morphological discrimination. Data from tomography and lightmicroscopy were exactly matched and finally compared to results from FIB/SEM imaging. Image detail was achieved at a single cell resolution. Image detail was achieved at a single cell resolution, which has been estimated to be 0.833μm/voxel in the tomographic data. SR-μCT with optimized phase contrast properties represents a method to investigate biological tissues in certain areas of interest, where true cellular resolution or enhanced volumetric imaging is needed. In this study, we demonstrate that this method can compete with conventional histology using light microscopy but even surpasses it due to the possibility of retrieving volumetric data.     

An ART iterative reconstruction algorithm for computed tomography of diffraction enhanced imaging

X-ray diffraction enhanced imaging (DEI) has extremely high sensitivity for weakly absorbing low-Z samples in medical and biological fields. In this paper, we propose an Algebra Reconstruction Technique (ART) iterative reconstruction algorithm for computed tomography of diffraction enhanced imaging (DEI-CT). An Ordered Subsets (OS) technique is used to accelerate the ART reconstruction. Few-view reconstruction is also studied, and a partial differential equation (PDE) type filter which has the ability of edge-preserving and denoising is used to improve the image quality and eliminate the artifacts. The proposed algorithm is validated with both the numerical simulations and the experiment at the Beijing synchrotron radiation facility (BSRF).     

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.     

Image formation in holographic tomography

Tomography has been applied to holographic imaging systems recently to improve the 3D imaging performance. However, there are two distinct ways to achieve this: either by rotation of the object or by rotation of the illumination beam. We provide a transfer function analysis to distinguish between these two techniques and to predict the 3D imaging performance in holographic tomography when diffraction effects are considered. The results show that the configuration of rotating the illumination beam in one direction while fixing the sample leads to different 3D imaging performance as compared to the configuration of rotating the sample. The spatial frequency cutoff is nonisotropic in the case of rotating the illumination, and a curved line of singularity is observed. Rotating of the sample, on the contrary, has more symmetry in spatial frequency coverage but has a single point of singularity. The 3D transfer function derived can be used for 3D image reconstruction.     

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     

Cosine fitting radiography and computed tomography

A new method in diffraction-enhanced imaging computed tomography(DEI-CT) that follows the idea developed by Chapman et al.[Chapman D,Thomlinson W,Johnston R E,Washburn D,Pisano E,Gmur N,Zhong Z,Menk R,Arfelli F and Sayers D 1997 Phys.Med.Biol.42 2015] in 1997 is proposed in this paper.Merged with a "reverse projections"algorithm,only two sets of projection datasets at two defined orientations of the analyzer crystal are needed to reconstruct the linear absorption coefficient,the decrement of the real part of the refractive index and the linear scattering coefficient of the sample.Not only does this method reduce the delivered dose to the sample without degrading the image quality,but,compared with the existing DEI-CT approaches,it simplifies data-acquisition procedures.Experimental results confirm the reliability of this new method for DEI-CT applications.