A novel "ghost"-free tomographic image reconstruction method applicable to rotary-shadowed replica specimens

Electron tomography by conventional filtered back-projection is often seriously impaired by anisotropic resolution due to unavoidable limitation in specimen tilt-angles. We propose a new approach to overcome the problem for thin film-like replica-type specimens in which internal density is supposed as homogenous and contiguously distributed, by imposing a reasonable constraint of density-existing region in the reconstruction procedure. The objects were approximated as a distribution of binary voxels and the intensity of the projected images being proportional to the thickness along the projection ray. The new reconstruction algorithm consists of initial determination of approximate constraint region by a topographic analysis by stereo-photogrammetry, followed by iterative computation to find the unique solution of simultaneous equations, so that all the intensity distribution in tilt-series images are included within pre-determined voxel arrangement. During a trial run with a new methodology, we realized its significantly advantageous feature that much less number of projection images than conventional back-projection is required to perform the reconstruction of almost equivalent quality. Here, we show the performance of this novel algorithm by 3-D reconstruction of quick-freeze deep-etch replica specimens without any trace of spurious ghosting caused by missing-wedge problems.     

Implementing a back-projection tomographic reconstruction algorithm using a fan-shaped beam

A back-projection tomographic reconstruction algorithm using a fan-shaped beam is considered. The freeware that implements the method of back projections is mainly applicable only in the case of parallel radiation. Distinctions in how this approach is applied when using parallel and fan-shaped beams are described in part that is related to the geometry of an experiment. The implementation of the algorithm with allowance for the peculiarities of reconstruction with a divergent beam is presented as a flow chart. The results provided in the paper demonstrate the working capacity of the algorithm.     

Fully three-dimensional defocus-gradient corrected backprojection in cryoelectron microscopy

Recognizing that the microscope depth of field is a significant resolution-limiting factor in 3D cryoelectron microscopy, Jensen and Kornberg proposed a concept they called defocus-gradient corrected backprojection (DGCBP) and illustrated by computer simulations that DGCBP can effectively eliminate the depth of field limitation. They did not provide a mathematical justification for their concept. Our paper provides this, by showing (in the idealized case of noiseless data being available for all projection directions) that the reconstructions obtained based on DGCBP from data produced with distance-dependent blurring are essentially the same as what is obtained by a classical method of reconstruction of a 3D object from its line integrals. The approach is general enough to be applicable for correcting for any distance-dependent blurring during projection data collection. We present a new implementation of the DGCBP concept, one that closely follows the mathematics of its justifications, and illustrate it using mathematically described phantoms and their reconstructions from finitely many distance-dependently blurred projections.     

铁谱磨粒图像多焦点融合算法研究

铁谱分析技术是一种常用的磨损监测技术。受限于高倍物镜下的景深限制,一张铁谱大磨粒图像往往只有局部聚焦清晰的特征。为了能够解决在高倍物镜下铁谱大磨粒图像的自动化清晰采集以及高质量图像融合问题,设计并构建一套自动化扫描显微系统,该系统可进行多焦点铁谱图像的自动扫描采集;同时,提出一种基于相位一致性的铁谱磨粒图像多焦点融合算法,对自动扫描的多焦点图像进行融合,得到清晰的磨粒图像。实验结果表明,设计的自动化扫描显微系统能快速完成多焦点铁谱图像的自动化采集流程,提出的图像融合算法相较于传统的小波图像融合算法具有更高的图像评价质量,并能获得更加清晰的图像边缘信息。     

Optical microscopy with flexible axial capabilities using a vari‐focus liquid lens

The axial imaging range of optical microscopy is restricted by its fixed working plane and limited depth of field. In this paper, the axial capabilities of an off‐the‐shelf microscope is improved by inserting a liquid lens, which can be controlled by a driving electrical voltage, into the optical path of the microscope. First, the numerical formulas of the working distance and the magnification with the variation of the focus of the liquid lens are inferred using a ray tracing method and conclusion is obtained that the best position for inserting a liquid lens with consistent magnification is the aperture plane and the rear focal plane of the objective lens. Second, with the liquid lens embedded in the microscope, the numerical relationship between the magnification and the working distance of the proposed flexible‐axial‐capability microscope and the liquid lens driving voltage is calibrated and fitted using the inferred numerical formulas. Third, techniques including autofocus, extending depth of field and three‐dimensional imaging are researched and applied, improving the designed microscope to not only flexibly control its working distance, but also to extend the depth of field near the variable working plane. Experiments show that the presented flexible‐axial‐capability microscope has a long working distance range of 8 mm, and by calibrating the magnification curve within the working distance range, samples can be observed and measured precisely. The depth of field can be extended to 400 μm from the variable working plane and is 20 times that of the off‐the‐shelf microscope.     

A cone-beam compensated back-projection algorithm for X-ray particle tracking velocimetry

Characterizing multiphase or granular flows is difficult due to the opaque nature of the system. While invasive measurement techniques provide detailed information about a single point, assessing the entire system is a laborious task due to the large number of samples required. Therefore, significant work has gone into developing noninvasive methods of measuring these flow systems. In this study, identical pairs of X-ray source/detector systems are used to provide two simultaneous but independent X-ray radiographic projections, which are then coupled together to perform X-ray stereographic imaging of a granular flow. A cone-beam compensated back-projection algorithm is developed for X-ray particle tracking velocimetry (XPTV). This method accurately corrects for the X-ray׳s cone-beam geometry, which is ignored in parallel-beam back-projection methods. To demonstrate the need for the cone-beam compensation, a direct comparison between the cone-beam and parallel-beam back-projection algorithms is used, and significant differences are presented. These methods are then used to perform XPTV in a double screw mixer, allowing the position and velocity of individual tracer particles to be characterized.     

一种改进的未知环境无人机三维地图实时创建方法

提出一种针对RGB-D传感器获取未知环境三维地图的方法。该方法将RGB-D传感器获得的数据信息实时地转化为三维点云地图。在扫描环境之前,引入径向畸变和切向畸变模型,完成对RGB-D传感器彩色摄像头和深度摄像头的标定,获得摄像机参数; 同时,对RGB-D传感器红外模式和深度模式下的图进行偏差矫正,提高三维点云计算的精度。实时地图构建时,利用投影和反投影将获得的彩色图片和深度图片转换成局部三维地图; 在RGB-D传感器移动过程中,实时提取RGB图片上的特征进行位姿估计和关键帧处理,从而获得整个地图。实验表明,     

Bright-field scanning confocal electron microscopy using a double aberration-corrected transmission electron microscope

Scanning confocal electron microscopy (SCEM) offers a mechanism for three-dimensional imaging of materials, which makes use of the reduced depth of field in an aberration-corrected transmission electron microscope. The simplest configuration of SCEM is the bright-field mode. In this paper we present experimental data and simulations showing the form of bright-field SCEM images. We show that the depth dependence of the three-dimensional image can be explained in terms of two-dimensional images formed in the detector plane. For a crystalline sample, this so-called probe image is shown to be similar to a conventional diffraction pattern. Experimental results and simulations show how the diffracted probes in this image are elongated in thicker crystals and the use of this elongation to estimate sample thickness is explored.     

Concentration measurements of bubbles in a water column using an optical tomography system

Optical tomography provides a means for the determination of the spatial distribution of materials with different optical density in a volume by non-intrusive means. This paper presents results of concentration measurements of gas bubbles in a water column using an optical tomography system. A hydraulic flow rig is used to generate vertical air-water two-phase flows with controllable bubble flow rate. Two approaches are investigated. The first aims to obtain an average gas concentration at the measurement section, the second aims to obtain a gas distribution profile by using tomographic imaging. A hybrid back-projection algorithm is used to calculate concentration profiles from measured sensor values to provide a tomographic image of the measurement cross-section. The algorithm combines the characteristic of an optical sensor as a hard field sensor and the linear back projection algorithm.     

Multifocal displays,diffractive depth of focus,and visual comfort

Volumetric displays with numerous fronto-parallel planes-images stimulating eye accommodation are considered. The depth of the focal field is determined as a function of the axial intensity distribution. Results of interpolation of images between the planes are presented. It is shown that the best resolution and visual comfort are reached if the distance between the planes does not exceed the diffractive depth of focus.     

Measurement and restoration of the point spread function of fluorescence confocal microscopy

The point spread function of an objective lens of a fluorescence confocal microscope was directly measured by imaging fluorescent beads. We analysed how the measurement of the point spread function was influenced by the diameter of the fluorescent beads and how the restoration technique with a deconvolution algorithm improved the measuring performance. Numerical and experimental results are presented for a typical point spread function and a zero‐centred point spread function.     

改进烟花算法计算热态轴类锻件内圆柱体空洞缺陷深度

针对高温状态下热态轴类锻件内部产生的圆柱体空洞缺陷测量困难的问题，提出基于改进的烟花算法检测空洞深度方法。根据锻件的内部微元模型和瞬态温度场传热微分方程推导圆柱体空洞温度场与空洞半径关系的微分方程，利用分离变量法求解；对烟花算法进行改进，以提高圆柱体空洞深度检测精度；构造缺陷深度的目标函数进行测试，得到最优解的相对误差为2.5%。最后，运用所提方法、PSO算法和传统的烟花算法于同一深度的空洞分别进行检测，验证所提方法的可行性。     

SPARK,a kernel of software programs for spatial reconstruction in electron microscopy

SPARK, an acronym for ‘SPAtial Reconstruction Kernel’, is the nucleus of a software library being developed for the three-dimensional (3-D) reconstruction of objects observed by the electron microscope. A unifying concept is used: the Fourier transform, known in several central sections, is resampled to obtain a 3-D Cartesian grid, which is inverted by a fast Fourier transform. This technique is used for both single-axis tilting (of 2-D periodic layers or of isolated objects) and for the random conical-tilt technique. The principles that make it possible to recover Cartesian grids in the two different geometries are illustrated and some preliminary results are reported. SPARK resamples the Cartesian grids with the use of a fast and efficient algorithm of Shannon interpolation developed by the authors. Compared to back-projection techniques the method shows a considerable improvement in execution time with no sacrifice in accuracy; it therefore allows the effects of a variety of parameters in a given reconstruction to be scrutinized in a reasonable time. Some new possibilities and future extensions of the library are briefly outlined.     

X-ray chest image reconstruction by Radon transform simulation with fan-beam geometry

A Radon transform algorithm which incorporates the correlated X-ray images (some stock chest radiographs) into the processing of back-projection with the fan-beam geometry reconstruction with the aim of improving image quality was developed and explored. These reconstruction images were evaluated and compared with the original image. Meanwhile, we also made the comparisons with the filtered back-projection method and the Radon transform method without fan-beam geometry; the results also reveal that the proposed method has the best image reconstruction capability than other two methods. The results show that the proposed method produces reconstructed images with quiet high peak-signal-to-noise ratio.     

Detection, classification and 3D reconstruction of biological macromolecules on hypercube computers.

In this work we present results of the mapping on hypercube computers of some of the key steps involved in the procedure for 3D structural determination from transmission electron microscopy images. The goal is the introduction of parallel processing tools in the field of electron microscopy image processing. We show how the rich topology of the hypercube, combined with an efficient programming strategy, allows for order-of-magnitude increase in computational capacity for such time-consuming tasks as calculation of multidimensional FFT's, cross-correlation coefficients, fuzzy partitioning functionals and the filtered back-projection 3D reconstruction method.     

微操作系统中微型物体上点的空间坐标的计算方法研究

为操作者提供微型物体的完整的、放大的三维信息,就可以使操作者像在他日常生活中那样精密地控制所使用的工具和被操作对象,解决了在显微镜下操作者的视野和观察方向受到限制以及缺少第三维信息的问题,而为了实时、无遮挡地生成物体的三维虚拟放大影像,需要获得被操作对象不同角度的图像信息并采用简洁、快速的算法以满足实时性要求.介绍在一个人类级微型物体虚拟遥操作系统中根据不同角度拍摄的图像计算物体上点的空间坐标时所采用的一些算法.     

A model based reconstruction technique for depth sectioning with scanning transmission electron microscopy

Depth sectioning in high angular annular dark field scanning transmission electron microscopy is considered a candidate for three-dimensional characterization on the atomic scale. However at present the depth resolution is still far from the atomic level, due to strong limitations in the opening angle of the beam. In this paper we introduce a new, parameter based tomographic reconstruction algorithm that allows to make maximal use of the prior knowledge about the constituent atom types and the microscope settings, so as to retrieve the atomic positions and push the resolution to the atomic level in all three dimensions.     

一种改进型灰度差分聚焦算法研究与实现

为解决显微图像采集中聚焦的准确性和快速性,设计并实现了一种包括图像采集端、图像处理算法实现模块、OSD菜单实现模块、系统控制模块和HDMI输出模块等硬件采集系统。结合实际应用和现有的图像处理算法,提出了适合数字显微图像的图像预处理自动聚焦算法,在灰度差分绝对值之和函数的基础上增加了对周围点的判断,提高了算法的精确度。实验结果表明,通过对切片图像在显微镜物镜10倍、40倍、100倍下采集到的离、聚焦图像以及聚焦曲线呈现单峰性,无偏性等特点,能实现良好的聚焦能力。     

Accurate structure determination from image reconstruction in ADF STEM

Annular dark-field (ADF) imaging in a scanning transmission electron microscope results in direct structure images of the atomic configuration of the specimen. Since such images are almost perfectly incoherent they can be treated as a convolution between a point-spread function, which is simply the intensity of the illuminating electron probe, and a sharply peaked object function that represents the projected structure of the specimen. Knowledge of the object function for an image region of perfect crystal allows the point-spread function to be directly determined for that image. We examine how the object function for an image can then be reconstructed using a Wiener filter, the CLEAN algorithm and a maximum entropy reconstruction. Prior information is required to perform a reconstruction, and we discuss what nature of prior information is suitable for ADF imaging.