大型工业CT中的三维显示与交互检查技术

介绍了三维显示和交互检查技术在大型工业CT图像检查中的应用，总结了目前工业CT三维图像显示领域主要应用的面绘制和体绘制方法，并分别对这两类主要的绘制算法进行了评价，重点对工业CT多断层图像数据三维显示的各个技术细节进行了具体的分析，并总结了大型工业CT图像检查分系统方案设计中存在的技术难点。     

松弛系数-有序子集-最大似然期望算法在60Co集装箱CT检测中的应用

将松弛系数和迭代间像素值约束引入有序子集一最大似然期望算法中,构造出应用于发射CT中的ROSEM算法引入到透射式工业CT中。不仅加快了图像重建的收敛速度而且改善了重建图像的质量。针对ROSEM、代数迭代算法（ART）和卷积反投影算法（CBP）,分别做了模拟实验和实际汽车CT检测以及集装箱CT检测实验,并分析比较了它们在^60Co集装箱CT检测中的应用。ROSEM算法无论在重建速度还是重建质量上都优于ART算法,其重建图像质量和CBP算法相当。     

基于硬件实现的锥束CT图像重建系统的存储机制设计

为了实现三维锥束CT图像重建加速系统的小型化,建立了基于FPGA的三维图像重建系统。并对该系统中所采用的FDK重建算法所需的数据存储量和数据传输量以及由SDRAM、SRAM和FPGA内部缓存组成的存储系统的数据吞吐率进行了研究。首先,根据FDK算法的滤波与反投影两个步骤介绍了三维锥束CT图像重建系统的数据规模。接着,介绍了一种以SDRAM为外部主存,以SRAM为外部缓存和以FPGA内部SRAM资源作为内部高速缓存的存储机制。然后,介绍了该存储机制的实现方法以及测试方法。最后对该三维图像重建系统的数据吞吐能力进行了测试,并将之与FDK算法所需的数据传输量进行了对比分析。试验结果表明:该存储机制的数据连续存取速度为151.9MB/s,数据随机存取速度为100MB/s,基本满足小规模的三维图像重建的数据存储与传输带宽的要求。     

代数迭代算法进行CT图像重建的研究

对CT代数迭代重建算法(ART)进行了研究,讨论了射线数目对图像重建效果的影响,并采用平滑方法,改善了重建图像质量,加快了收敛速度,得出了效果满意的工业CT图像。     

基于FDK算法的锥束CT重建近似算法性能比较

相比通常的扇束和平行束CT重建,锥束CT有诸多优点(如空间分辨率高,扫描速度快等)。近似锥束重建算法理论结构简单,重建速度快,易于实际应用。对新出现的几种近似锥束重建算法作了简单介绍,并利用这些算法对3D Shepp-logan模型的仿真模拟对这些算法的特点作了比较和讨论。     

PET局部脑氧代谢率测定

应用PET和示踪动力学原理进行无创性局部脑氧代谢率（rCMRO2）测定为研究正常脑功能以及疾病状态下脑的病理生理状况提供了有力工具。本文对rCMRO2测定中的关键技术，包括局部氧摄取分数（rOEF）、局部脑血流量（rCBF）和局部脑血容量（rCBV）测定进行了详细介绍。     

静态CT行包安检系统关键技术研究

本文介绍了安检用静态CT技术的基本原理,对基于超顺排碳纳米管技术的冷阴极X射线源、基于光子计数技术的多能谱X射线成像技术和安检用静态CT系统图像重建算法等进行了研究。冷阴极X射线源由基于超顺排碳纳米管技术的冷阴极X射线管、两套高压系统和1个控制系统组成,在130 kV以下可稳定工作。研制并测试了一款具有三能谱成像功能的光子计数型X射线探测器系统。对安检用静态CT系统图像重建算法进行了讨论,提出了一种解决方法。安检用静态CT系统关键技术的研究为安检用静态CT系统的研制提供了技术基础。     

基于电子束扫描阵列微焦点射线源的倒置结构Micro-CT成像研究

倒置几何结构CT成像系统理论上具有扫描视场大、图像信噪比好、锥束伪影小等优点,但其多焦点、稀疏角的扫描模式亦会带来投影数据截断、稀疏和局部数据冗余等问题。本文利用电子束扫描阵列微焦点射线源和高分辨率小面积探测器等构建了一种新型高分辨率、大视场的倒置几何结构Micro CT实验系统(IG Micro CT)。针对该系统多焦点、稀疏角扫描模式下的投影数据截断和稀疏等特点,提出一种图像先验约束的全变分正则化SART迭代CT图像重建算法。首先通过仿真实验对图像重建算法进行了验证,并对阵列微焦点数量、旋转扫描分度数量等扫描参数进行优化,最后在IG Micro CT实验系统上获得了高分辨的Micro CT测试图像。实验结果表明,本文提出的算法适用于IG Micro CT系统,解决了投影数据截断、稀疏采样和局部数据冗余带来的条状等伪影问题,并验证了IG Micro CT这种新型成像方式的可行性。     

通用CT算法研究实验平台的软件设计

使用高级语言编程研究CT重建算法有着运行效率高、速度快的优点,但开发工作繁重、效率低.提出了一个半开放式通用CT算法研究实验平台的设计,提供了投影数据获取和数据可视化及分析处理功能,并可以方便地使用C 语言编写重建程序加入实验平台,同时使用面向对象的程序设计方法大大提高了重建程序的可重用性,解决了使用高级语言编程进行算法研究工作时开发效率低的问题.还给出了一个按此设计实现的实验平台软件.     

基于自适应NLM修正的CT重建算法

针对稀疏角度CT图像重建问题,J.Huang等人提出了ART-NLM算法。该算法利用NLM对每次ART重建后的图像进行滤波去噪。虽然噪声抑制得到明显改善,但图像结构边缘模糊。论文在ART-NLM基础上改进NLM,通过自适应选取滤波参数及相似块的旋转变换,对待重建图像进行自适应NLM滤波,以达到同时去除噪声和保护边缘的目的。Shepp-Logan体模仿真实验表明,与传统ART-NLM算法相比,新算法提高了重建图像质量,有效平衡了图像去噪和边缘保护。     

Apparent Linear Attenuation Coefficients in Phase Contrast X-Ray Tomography

In the inline phase contrast X-ray tomography the reconstructed apparent linear attenuation coefficient values may be greatly larger than sample’s linear attenuation coefficients or even be negative. In this work we present a general formula to quantitatively relate the apparent linear attenuation coefficient values in cone-beam phase contrast tomography to sample’s linear attenuation coefficients and refractive indices. This formula overcomes the gross inaccuracy of the existing formula in the literature in analyzing high-resolution phase contrast tomography, and it will be useful for correctly interpreting and quantifying the apparent linear attenuation coefficients in cone-beam X-ray phase contrast tomography.     

工业CT中探测器失效伪影的校正

在工业CT的扫描重建过程中，如果出现探测器失效的情况，重建图像中会出现严重的伪影，必须进行校正。如果用插值方法得到失效探测器数据，会引进不真实数据，而只用滤波方法处理图像，则不能完全消除伪影。本文提出了一种用结合中值滤波的Ordered Subsets Convex方法对探测器失效伪影进行校正，在校正速度和质量上都得到了较好的校正效果。     

Generalized flow pattern image reconstruction algorithm for electrical capacitance tomography

Successful applications of electrical capacitance tomography (ECT) depend on the speed and precision of the image reconstruction algorithms. In this paper, based on the semiparametric model, a generalized objective functional that considers the outliers in the measured capacitance data and the model error is proposed. A regularized combination minimax estimation is developed. An efficient algorithm, which integrates the advantages of the homotopy method where the homotopy equation is designed by the fixed-point homotopy and solved using the fixed-point iteration algorithm based on the alternate iteration scheme, the quantum particle swarm optimization algorithm that is coupled with the crossover and mutation operators, and the simulated annealing algorithm, is proposed. This algorithm is tested by the noise-free capacitance data and the noise-contaminated capacitance data, and encouraging results are observed. Numerical simulation results reveal the effectiveness and superiority of the proposed algorithm. In the cases of the reconstructed objects considered in this paper, the reconstructed results by the proposed algorithm show great improvement in the spatial resolution and accuracy. The spatial resolution of the reconstructed images is enhanced, and the artifacts in the reconstructed images can be removed effectively. Furthermore, the reconstructed results by the proposed algorithm under the noise-contaminated capacitance data reveal that the proposed algorithm is very competent to deal with the inaccurate nature in the capacitance data. Consequently, a promising algorithm is introduced for ECT image reconstruction.     

工业CT散射校正方法的研究

在工业CT的扫描重建过程中，散射效应使得重建图像出现模糊，各点灰度值同实际物体的吸收系数之间出现偏差，需要进行校正。如果采用一般的卷积方法，则需要迭代多次才能收敛，所以在校正时间上不能达到要求。采用一种结合散射模型的Ordered Subsets Convex方法对散射效果进行校正，在校正速度和质量上都得到了很好的结果。     

Limited angle STIM and PIXE tomography of single cells

STIM (scanning transmission ion microscopy) tomography has been shown to be a valuable method for the three-dimensional characterization of microsamples. It has, however, rarely been employed for the study of single cells, since a free-standing sample is needed for an ordinary tomography experiment. This requirement places high demands on sample preparation techniques.In this study cells fixated on a substrate rather than free-standing were used for tomography. Since the substrate prevented a full rotation of the sample an algorithm for limited-angle tomography was devised. STIM projections covering only a limited angular range of ca. 120° were supplemented with simulated projections generated from a back and forth iteration between real space and Radon space. The energy loss caused by the substrate was subtracted from each projection. The cells were reconstructed using filtered backprojection. The surface of the cells as well as some interior structures could be reconstructed.Following the STIM projections a lesser number of PIXE (particle induced X-ray emission) projections were taken in order to obtain information about the elemental distribution of the sample. From the PIXE projections the three-dimensional phosphorus distribution within the cell was reconstructed using limited-angle tomography. Superimposition of the STIM and PIXE tomograms revealed the location of intracellular structures.Whereas STIM tomography is sensitive to density contrast, which are greatest at the surface, PIXE tomography is sensitive to changes in elemental concentration. Hence, the combination of the two methods can be very fruitful, while the limited angle approach can compensate some of the difficulties associated with tomography of single cells, namely preparation difficulties and excessive sample damage.     

基于平板探测器投影数据预处理的三维CT重构算法研究

大面积非晶硅平板探测器(flat panel detector-FPD)在工业透射射线成像领域得到越来越多的应用。基于FPD的圆轨道FDK型三维计算机层析成像技术(Computed Tomography,3D-CT)检测速度快,但成像质量不及2D-CT。为发挥3D-CT速度优势,抑制噪声同时提高图像重建的质量。以由平板探测器得到的二维投影数据为研究对象,提出了一种对投影数据分别进行高频和低频两次滤波的投影预处理方法,然后将分别重建的图像叠加最终得到高质量的重建图像。由算法的仿真实验结果表明,重建图像质量得到明显改善,系统噪声得到抑制。     

Tomography of emissivity for Doppler coherence imaging spectroscopy diagnostic in HL-2A

A newly developed Doppler coherence imaging spectroscopy(CIS) technique has been implemented in the HL-2 A tokamak for carbon impurity emissivity and flow measurement. In CIS diagnostics, the emissivity and flow profiles inside the plasma are measured by a camera from the line-integrated emissivity and line-averaged flow, respectively. A standard inference method, called tomographic inversion, is necessary. Such an inversion is relatively challenging due to the ill-conditioned nature. In this article, we report the recent application and comparison of two different tomography algorithms, Gaussian process tomography and Tikhonov tomography, on light intensity measured by CIS, including feasibility and benchmark studies.Finally, the tomographic results for real measurement data in HL-2A are presented.     

利用混合态分解算法提高重叠关联成像的质量

X射线重叠关联成像技术所采用的常规相位恢复迭代算法均把入射光视为完全相干的单色光(即单态入射光),把样品视为稳定平衡的(即单态样品),把实验过程中成像系统内所有光学元器件的响应视为固定不变的。然而在实际实验中,上述三个条件很难得到同时保证。在非理想的实验条件下,任何单态条件发生变化或者出现多重态混合,其造成的退相干效应均会使衍射图样变得模糊,严重降低利用单态相位恢复迭代算法获得的重建图像质量。为了消除各种来源的混合态效应的负面影响,本文利用多模式拓展重叠关联迭代引擎算法(Mm-e PIE)和子像素上采样拓展重叠关联迭代引擎算法(Us-e PIE)这两种不同的混合态分解算法对实验数据进行处理。重建结果表明,不同来源的混合态效应(退相干效应)的分解结果在重建探针函数和重建样品函数中均得到了体现,两种混合态分解算法均能够显著改善重建图像的质量。其中多模式算法Mm-e PIE在消除入射探针的非相干效应方面更胜一筹。     

Transverse phase space reconstruction study in Shanghai soft X-ray FEL facility

Phase space is one of the most important parameters used to describe beam properties. Computer tomography, as a method for reconstructing phase space and measuring beam emittance, has been used in many accelerators over the past few decades. In this paper, we demonstrate a transverse phase space reconstruction study in the Shanghai soft X-ray free electron laser facility. First,we discuss the basic principles of phase space reconstruction and the advantage of reconstructing beam distribution in normalized phase space. Then, the phase space reconstruction results by different computer tomography methods based on the maximum entropy(MENT) algorithm and the filtered back projection algorithm in normalized phase space are presented. The simulation results indicate that,with proper configuration of the phase advance between adjacent screens, the MENT algorithm is feasible and has good efficiency. The beam emittance and Twiss parameters are also calculated using the reconstructed phase space.     

Verification and validation of the flux reconstruction method for CANDU® applications

In this paper, we propose a new method for the cell local flux reconstruction for CANDU® applications. The method is based on the modulation method where the core local mesh flux is reconstructed by modulating the form function with an interpolated cell mesh flux along with an appropriate normalization technique. The reconstruction method was adapted to the diffusion finite difference method. In this paper we present: (a) the flux reconstruction method, (b) the verification of this method using two benchmarking problems and (c) its validation using the ZED-2 data generated from the defueled channel experiments.