基于FDK算法的三维CT系统性能评估理论方法研究

随着医学应用和工业应用对X射线剂量、三维物体的成像速度的要求越来越严格,三维CT成像已经成为必然趋势,因此有必要对三维CT系统性能进行研究。本文首先建立三维CT图像噪声的理论估计方法,接着提出了基于FDK算法的三维CT系统性能评估的4种解析计算方法,使用高斯差模型作为通道函数。通过实验我们验证了本文提出的三维CT系统性...     

锥束CT技术在口腔临床中的应用

将锥束CT应用于口腔部位的三维成像,是近年来CT研究人员和口腔医生共同关注的一个热点.特别是随着牙齿种植技术的兴起,利用CBCT进行牙齿种植计划和手术导板的设计已成为一项新的研究方向.本文综述了口腔CBCT的成像原理和临床应用,比较了CBCT与传统CT在口腔成像中的特点,并总结了当前的关键技术和研究热点,供相关的技术及医学研究人员参考.     

基于特征学习的低剂量CT成像算法研究进展

随着CT（computed tomography）技术在临床中的大量应用,其辐射伤害问题也越来越受到人们的关注。与此同时,高性能低剂量的成像也已经成为近年来CT研究领域中的重要研究方向。随着学习型算法的提出及广泛应用,为低剂量CT成像算法的发展带来了新的方向。在影像大数据环境下,基于特征学习方法的低剂量CT成像有着更广阔的发展空间。本文将从稀疏表示和深度学习两个方面,介绍一些国内外应用于改善CT成像质量的相关技术,包括CT成像技术的发展趋势,特征学习相关算法的研究现状,提高低剂量CT扫描成像质量的相关方案等。本文对近年来在低剂量CT成像及特种学习算法等领域的研究成果进行了介绍,并进行相关总结和分析。      

X射线三维显微镜及其典型应用

X射线三维显微镜（显微CT）是目前发展最热门、成长最快的CT成像技术门类之一,在微纳制造技术、新材料以及电子科学等领域起着十分重要的作用并得到广泛的应用。X射线三维显微镜能够以亚微米的细节分辨能力对被检测对象内部结构进行无损的三维成像,是一种新型的检测设备。本文介绍了三英精密仪器有限公司研制的nano Voxel-2000系列X射线三维显微镜的结构和工作原理,展示了该仪器在系统设计、成像方法及应用分析方面的特色。该系列的X射线三维显微镜最大管电压为150 k V,采用5μm的微焦源实现低于500 nm的成像分辨率,能够进行不同尺寸样品、不同分辨率的成像。在此基础上,探讨了X射线显微镜的典型应用,主要对样品的内部结构、形貌进行表征。希望典型应用的实例能起到抛砖引玉的作用,吸引更多学者利用X射线三维显微成像技术开展科学研究,拓宽X射线三维显微镜的应用领域。     

感兴趣区域CT图像重建方法及模拟实验

虽然CT技术已经发展得相当成熟,但在大物体成像、高分辨率成像和减少辐射剂量等方面现有的CT成像方法仍然存在较大的困难。事实上,很多工程应用中并不要求对完整的物体进行全局CT成像,只需要获得某些感兴趣区域的物体图像即可,特别是医疗临床诊断中只要能够实现对可疑病灶部位的成像即可。因此,本文研究了针对感兴趣区域的CT图像重建方法,以及X射线束的视野只覆盖感兴趣区域的扫描方法设计,感兴趣区域CT成像研究的关键是如何在投影数据截断情况下实现断层图像的精确、稳定重建;本文介绍了三种投影数据在不同截断方式下的图像重建方法,并设计了相应的数值模拟实验,给出了数值模拟实验结果。最后,对感兴趣区域CT成像技术在工程中应用潜力做了展望。     

基于微米CT的磨介对产物三维形貌影响实验设计

本文设计研究球形和六棱柱形研磨介质对煤炭破碎产物三维形貌特征影响的实验,具体过程包括不同时间批次煤炭破碎和筛分,基于高分辨三维X射线显微成像系统的粗粒级煤炭无损检测以及基于Dragonfly三维数据分析软件的完整颗粒分割提取和三维形貌特征参数计算等环节。结果显示：在球磨机转速率较低的前提下,钢球和钢锻的总表面积或总质量相同时,钢球作用下物料的初始粒级破碎速率略高于钢锻,且-1 mm细粒级产率最高。研磨作用促使粗粒级物料的体积和表面积降低,3种研磨条件下表面积最大的钢锻对样品的“磨剥”作用最为明显,颗粒球形度最高。     

2004年中国体视学学会-CT和三维成像学术会议征文通知

中国体视学学会CT理论与应用分会(CSSCT)和清华同方威视技术股份有限公司主办、《CT理论与应用研究》编辑部协办,并欢迎有关单位参加主办、协办,将于2004年10月10日前后在北京市开会 :研讨“21世纪科技进步与CT及三维成像理论与应用,交流国外内CT和三维成像的理论、应用技术的成就和展望”。会议分5项议题: Symp1: CT理论、计算方法和软件:显微CT、锥束CT成像理论和算法; Symp2:工业CT和数字扫描成像、新型CT机和微焦点CT的设计和性能,信息反馈工程和其他应用;CT与NDT在线检测;Symp3: 医学CT扫描成像诊断: 高质量CT/MRI/PE…     

利用混合法构建三维数字岩心(英文)

在弹性波有三维数字岩心描述了岩石的微观孔隙结构。X射线CT扫描是获取三维数字岩心最准确和直接的方法,但实验成本高。本文结合沉积过程模拟和模拟退火算法,提出了重建三维数字岩心的混合法,基于岩石二维图像重建三维数字岩心。利用岩石颗粒沉积算法构建初始数字岩心,作为模拟退火算法的初始状态。运用模拟退火算法调整岩石颗粒和孔隙的相对位置,使三维数字岩心与岩心二维图像具有相似的自相关函数,从而建立三维数字岩心。与传统模拟退火算法相比,该方法计算时间明显减小。运用局部孔隙度理论定量比较了重建数字岩心和岩心X射线微CT图像,两种数字岩心具有相似的均质性和孔隙连通性。利用有限元方法和格子玻尔兹曼方法分别模拟了重建三维数字岩心的地层因素和渗透率,数值模拟结果与实验结果相符。相比传统模拟退火算法,混合法重建数字岩心的传导特性更接近真实岩心的传导特性。     

X射线三维CT技术在元器件失效分析中的应用

通过比较X射线DR,普通断层CT技术,扫描声学显微镜的功能特点,分析了X射线三维CT技术的优势。通过对焊接空洞、微动开关失效案例以及不同封装形式器件的分析,研究了X射线三维CT技术在电子元器件失效分析领域的应用。同时根据X射线三维CT成像的特点,分析了该技术发展的前景。     

原位岩土CT及其关键技术探讨

当前应用工业X射线CT技术观测岩土内部微观结构和力学特性成为研究工程地质的热点课题。然而,现有CT技术的“笨、大、重、险、贵”限制了现场应用能力,且常规岩土工程勘察方法存在丢失或破坏原有地质信息的问题。本文提出原位CT技术的概念,并指出其在岩土工程勘察领域应用中的关键技术。基于超小型冷阴极脉冲式X射线管及成像设备搭建随钻原位CT实验平台,实验结果表明该方法能够实现原位岩心数字采样与CT快速重构。在此基础上明确后续研究重点和关键技术,主要包括2D/3D CT快速重构新理论和新方法,解析CT重构图像与岩土颗粒特性之间的关系以及局部精细结构重建算法,构建精确的原位岩心3D微观结构模型;结合3D打印技术制备原位置岩土准样本,研究等效准样本的力学特性;建立时-空四维岩土工程勘察数据库,引入人工智能算法形成具有学习和预测的反馈评价体系,对丰富当前岩土工程勘察方法和完善原位CT技术具有重要的科学意义。      

Porosity of bimodal sediment mixture with particle filling

The porosity of a bimodal sediment mixture is affected by the filling of fine particles in the voids of coarse particles when the particle size range is wide. The classical ideal packing models tend to overestimate the filling, and, thus, underestimate the porosity of the mixture. In this study, an existing random filling model is improved by considering a three-dimensional packing configuration, and a new model is developed by considering how many fine particles are required and how many are available to cover the surface of coarse particles in the sediment mixture. The developed models are validated using measured data and compared with existing models. The new model can reproduce well the variation trend of the mixture porosity as the fraction of fine particles varies.     

Porosity of bimodal and trimodal sediment mixtures

This study focuses on the prediction of the porosity of nonuniform sediment mixtures,considering the effects of particle packing.A random particle packing model has been developed for the porosity of bimodal mixtures by extending the existing random particle filling theory.Coefficients in the developed model are calibrated by fitting the model to measured data for a variety of bimodal mixtures,including spherical glass particles,rounded quarry grains,and natural sediments.The model coefficients ...     

A study of pore geometry effects on anisotropy in hydraulic permeability using the lattice-Boltzmann method

We hypothesize that anisotropy in soil properties arises from pore-scale heterogeneity caused by the alignment of aspherical soil particles. We developed a method to predict the permeability tensor from particle shape and packing structure. Digital geometry maps were created for the pore space in regular cubic and random packs of particles with various aspect ratios using a numerical packing algorithm. The lattice-Boltzmann method was used to simulate saturated flow through these packs, and the effect of particle shape and degree of alignment on the permeability tensor was characterized. Results show that the degree of anisotropy in permeability depends not only upon particle shape and alignment, but also on the three-dimensional structure of the pack. In random packs, more oblate particles and higher degrees of particle alignment lead to reduced permeability perpendicular to the direction of particle alignment compared to the direction parallel to particle alignment.     

On the predictivity of pore-scale simulations: Estimating uncertainties with multilevel Monte Carlo

A fast method with tunable accuracy is proposed to estimate errors and uncertainties in pore-scale and Digital Rock Physics (DRP) problems. The overall predictivity of these studies can be, in fact, hindered by many factors including sample heterogeneity, computational and imaging limitations, model inadequacy and not perfectly known physical parameters. The typical objective of pore-scale studies is the estimation of macroscopic effective parameters such as permeability, effective diffusivity and hydrodynamic dispersion. However, these are often non-deterministic quantities (i.e., results obtained for specific pore-scale sample and setup are not totally reproducible by another “equivalent” sample and setup). The stochastic nature can arise due to the multi-scale heterogeneity, the computational and experimental limitations in considering large samples, and the complexity of the physical models. These approximations, in fact, introduce an error that, being dependent on a large number of complex factors, can be modeled as random. We propose a general simulation tool, based on multilevel Monte Carlo, that can reduce drastically the computational cost needed for computing accurate statistics of effective parameters and other quantities of interest, under any of these random errors. This is, to our knowledge, the first attempt to include Uncertainty Quantification (UQ) in pore-scale physics and simulation. The method can also provide estimates of the discretization error and it is tested on three-dimensional transport problems in heterogeneous materials, where the sampling procedure is done by generation algorithms able to reproduce realistic consolidated and unconsolidated random sphere and ellipsoid packings and arrangements. A totally automatic workflow is developed in an open-source code [1], that include rigid body physics and random packing algorithms, unstructured mesh discretization, finite volume solvers, extrapolation and post-processing techniques. The proposed method can be efficiently used in many porous media applications for problems such as stochastic homogenization/upscaling, propagation of uncertainty from microscopic fluid and rock properties to macro-scale parameters, robust estimation of Representative Elementary Volume size for arbitrary physics.     

A general Lagrangian tracking methodology for riverine flow and transport

This article presents a new public domain tool for generalized Lagrangian particle tracking in rivers. The approach can be applied with a variety of two- and three-dimensional flow solvers. Particle advection by the flow is incorporated using flow fields from the chosen solver assuming particles follow the Reynolds-averaged flow, although some other simple passive and active particle behaviors are also treated. Turbulence effects are treated using a random walk algorithm with spatial step lengths randomly chosen from Gaussian distributions characterized by the diffusivity from the flow solver. Our work extends this concept to a general framework that is solver and coordinate system independent to allow easy comparisons between differing flow treatments. To better treat problems where detailed information is required in specific regions, the approach includes novel cloning and colligation algorithms which enhance local resolution at modest computational expense. We also provide tools for computing local concentrations and total exposure over a user-specified time interval. Several examples of predictions are provided to illustrate applications of the technique, including examination of the role of curvature-driven secondary flows, storage in lateral separation eddies, treatment of larval drift, treatment of fuel spill dispersion, river-floodplain connections, and sedimentation in floodplain ponds by tie channel connections. We also demonstrate that the model can reproduce analytically derived concentration profiles for simple diffusivities. These examples show that the Lagrangian particle tracking approach and the extensions proposed here are broadly applicable and viable for treating difficult river problems with multiple temporal and spatial scales. The examples also illustrate the utility of the cloning/colligation extensions and show how these can decrease the computational effort required on problems where high local resolution is required. Enhancement of the tools and even broader applicability can be achieved through the inclusion of multiple particle populations and particle–particle interactions.     

层析成像技术在多相流中的研究动态

在多相流检测中,层析成像技术可获取多相流体二维/三维的时空局部的、微观的分布信息,是目前多相流参数检测技术研究发展的前沿和趋势之一.本文较系统阐述了国内外应用电阻层析、电容层析、射线层析、超声层析技术进行多相流检测的原理、研究现状以及层析成像技术在多相流应用方面的发展.     

伴随成像及其在地球深部结构研究中的应用

基于高精度的三维波场模拟和伴随方法, 伴随成像成功实现了天然地震全波形成像在区域和全球尺度下的应用。伴随成像技术基于谱元法, 使用全三维、 多参数的初始模型对地震波场进行数值模拟, 通过正演波场和伴随波场的相互作用快速求取目标函数的梯度, 结合高性能计算技术, 实现大尺度全波场成像。相比于传统的地震层析成像方法, 伴随成像对地球内部结构异常体的描述更加精细和全面, 更适用于构造活跃地区的深部动力学研究。本文首先介绍伴随成像方法的基本原理, 随后阐述其具体实现流程, 并回顾该方法在地球深部结构研究中的应用, 最后对其未来发展做出展望。     

CL系统扫描长方体构件的投影计算机仿真

x射线投影计算机仿真可以排除实际x射线断层扫描过程中的随机因素,更好地验证扫描方式、图像重建方法等的正确性,为实际系统的实现、完善以及重建算法的改进等提供理论依据。本文基于一种典型的计算机分层成像（CL）系统,针对电子线路板大多为长方体薄板的事实,研究了长方体构件的投影仿真计算方法;设计了仿真芯片模体,对其进行投影计算机仿真;并与系统实验结果对比,得到了较为满意的结果。该方法为后续改进图像重建算法、分析误差来源、校正伪影等研究提供了良好的基础。     

基于三维X射线显微镜的孔隙性砂岩中变形带微观结构解析

孔隙性砂岩中变形带对揭示其相关地质作用过程和工程应用具有重要的意义,而其微观结构是其相关科学与工程研究的关键点。常规的岩石微观结构分析手段对于全面解析岩石内部的微观结构具有一定的局限性,而近年发展起来的三维X射线显微成像技术可以无损地获得岩石内部微米尺度的三维精细数字化图像,是建立岩石微观结构并对其进行解析的有效手段。通过三维X射线显微镜对孔隙性砂岩中变形带扫面,建立了高分辨率三维微观结构图像,利用高密度切片图像,对其微观结构特征进行解析。根据变形带微观产出状带和颗粒破碎过程的解析,建立颗粒破碎-破碎颗粒与残余破碎颗粒重新排列-局部颗粒破碎带-弱颗粒破碎变形带-强颗粒破碎变形带-强弱颗粒破碎变形带相间排列-簇状变形带的完整时间发育序列。      

Effect of a Starting Model on the Solution of a Travel Time Seismic Tomography Problem

In the problems of three-dimensional (3D) travel time seismic tomography where the data are travel times of diving waves and the starting model is a system of plane layers where the velocity is a function of depth alone, the solution turns out to strongly depend on the selection of the starting model. This is due to the fact that in the different starting models, the rays between the same points can intersect different layers, which makes the tomography problem fundamentally nonlinear. This effect is demonstrated by the model example. Based on the same example, it is shown how the starting model should be selected to ensure a solution close to the true velocity distribution. The starting model (the average dependence of the seismic velocity on depth) should be determined by the method of successive iterations at each step of which the horizontal velocity variations in the layers are determined by solving the two-dimensional tomography problem. An example illustrating the application of this technique to the P-wave travel time data in the region of the Black Sea basin is presented.