一种广角扇束扫描γ射线工业CT实验仪

一种用于大学物理教学的广角扇束扫描γ射线工业CT实验仪已研制完成.通过实验可使学生了解和掌握CT基本理论、CT扫描数据采集过程、图像重建等知识.该仪器系统包括一个活度为7.4×108Bq的137Cs同位素放射源、五通道塑料闪烁体+光电倍增管"探测系统、一台计算机和一套图像重建处理软件.机械运动控制、数据采集软件用Visual C++开发,CT图像重建可使用系统提供的软件也可以由学生使用MATLAB 编程.     

工业CT教学实验仪图像重建的设计

工业CT教学实验仪被应用于物理和放射医学专业的教学,图像重建是教学实验仪软件的重要组成部分.论文阐述了CT的物理原理和第一代CT重建算法,描述了工业CT教学实验仪的扫描过程;分析了旋转中心偏移的伪影,实现了中心偏移校正方法,设计了图像重建软件,实际应用表明此软件重建图像清晰质量较高.     

同步辐射X射线光斑信息对扫描相干衍射成像的影晌

本文采用拓展重叠关联迭代引擎算法,系统地模拟研究了x射线光斑尺寸、扫描点数及波前形貌对扫描相干衍射成像重建图像质量的影响。结果表明,在实际入射光斑尺寸不易确定时,算法中初始猜测光斑尺寸可以由模拟入射光斑最大光强值的1％对应边界确定。对物体进行扫描时,入射光斑个数大于7×7且重叠度优于70％可以重建出较好的图像。入射光斑的振幅和位相分布对重建图像质量也有着显著影响。     

X射线非完整扫描情况下的缺陷检测研究

CT技术是一种先进的无损检测手段,被广泛应用于医学和工业领域.针对大型工业构件的无损检测问题,提出了一种x射线非完整扫描情况下的缺陷检测方法.在构件使用之前预先采集一套原始的非完整扫描投影数据,采用代数重建算法进行图像重建,将重建图像作为基准图像.再采集构件使用之后不同时刻的非完整扫描投影数据,将其重建图像与基准图像进...     

探测器分块排列的CT系统及其图像重建

为解决CT系统中扫描视野受限及低剂量成像问题,提出一种探测器分块排列的CT系统及其稀疏角度CT图像迭代重建方法。探测器分块排列可扩大CT图像物体扫描视野,稀疏角度采集投影数据能有效降低辐射剂量,同时采用ART-TV迭代算法重建CT图像。仿真实验表明:该方法可有效扩大CT系统扫描视野,在稀疏角度获取投影数据采集方式下,利用ART-TV迭代重建算法可重建优质CT图像,实现降低辐射剂量的目的。     

基于小波阈值去噪的CT图像重建

实际扫描过程中投影数据不可避免地受到各种噪声的干扰,进而影响了图像重建的精度.提出了一种基于小波阈值去噪的图像重建方法,首先对不同角度下的投影数据进行小波分解,并对分解后的小波系数进行阀值化去噪处理,然后经过逆小波变换得到去噪后的投影数据,再由这些去噪后的投影数据进行图像重建.实验结果表明本文的方法非常有效,与传统的重建方法相比,由去噪处理后的投影数据所重建的图像质量明显改善.     

中间函数法CT图像重建算法研究

CT自从被发明以来其技术已发生了很大的变化.这些变化不仅体现在计算机技术、探测器技术和X线技术方面,同时CT的图像重建算法也在不断地发展.本文提出的中间函数重建算法,是一种可以应用于扇束和锥束扫描的图像重建算法.研究它的目的是为将来的锥束扫描提供直接重建算法.     

教学用工业CT实验仪器的设计与开发研究

计算机层析成像(CT)技术采用射线从多个角度穿透物体来重建断层图像或三维图像,该技术已经广泛地应用于医学诊断和治疗、工业无损检测和无损评价.因此,对于大学学生来讲,了解和掌握CT技术的基本原理具有重要意义.本文详细介绍了用于大学教学的CD-50BG型CT实验仪.该仪器采用"平移 旋转"的扫描方式,具有活度为0.74 GBq的137Cs γ射线源、单个塑料闪烁体 光电二极管的探测器和50 mm的视场直径,其中射线源安装在钨合金的屏蔽容器中.同时,开发了图像处理软件对采集数据进行处理,重建断层图像和重构三维图像.重建断层图像可达到11 p/mm的空间分辨率和1%的密度分辨率.目前,已经有包括清华大学、北京大学在内的十余所高校将该仪器用于基础教学.     

三维重建技术在工业CT中的应用

用图像的三维重建技术,实现连续断层CT图像的三维可视化是近年来国内工业CT领域内的-个重要研究方向.用三角面片连接轮廓线方法,实现了连续等间隔工业CT图像的三维重建,获得了扫描物体的三维立体显示图像.通过计算机交互操作,实现多角度、多方位观察物体,无损测量物体三维参数等功能.利用重建出的三维图像,检测人员可以获得物体更多的空间结构信息,从而提高无损检测的方便性和准确性.     

超分辨重建方法在STXM图像处理中的应用

介绍了数字图像超分辨重建技术在上海光源扫描透射软X射线谱学显微(STXM)图像处理中的应用.对基于亚像素位移的STXM低分辨率图像序列,通过超分辨重建方法重构出一幅高分辨率图像.在STXM线站现有的实验装置情况下,初步探讨了获取亚像素位移低分辨率图像序列的实验方法,并研究了重构所需低分辨图像的帧数问题.实验和数据分析表...     

Tomographic Algorithms for General Line Integrals

This paper extends the constrained optimization image reconstruction techniques to curved-ray projections for a number of scanning systems and for 2-D and 3-D reconstructions. Teh treatment presented is general in that it includes the previous results on parallel-ray and divergent-ray geometries as special casss. Employing the projection data as constraints, the reconstruction problem is expressed as a generalized constrained optimization problem whose solution leads to a relationship (model) between the reconstruction and the associate Lagrange multiplier functions, thus introducting the models for a number of prescribed cost criteria and scanning systems involing curved-ray projections. Using these new models a category of subspaces of a Hirbert space, the elements of which are the functions (objects) for reconstruction, is presented. The projections and the minimum energy reconstruction are shown to be projections of the object into these new subspaces. The projection slice theorem is extended by introducing a new transform, a slice of which is equal to the Fourier series of the corresponding projection, irrespective of the projection geometry. The ART methods are extended to curved-ray scanning systems. Also a convergent iterative algorithms which evaluates the Lagrange multipliers from general line integrals and then calculates the reconstruction, is introduced. The iterations of this algorithm are identical to those of the additive ART, thus resulting in the same solution.     

Compressed sensing and Otsu’s method based binary CT image reconstruction technique in non-destructive detection

This paper tries to address the problem of binary CT image reconstruction in non-destructive detection with an algorithm based on compressed sensing(CS) and Otsu's method, which could reconstruct binary CT image of test object from incomplete detection data. According to binary CT image characteristics, we employ Splitbregman method based on L1/2regularization to solve piecewise constant region reconstruction. To improve the reconstructed image quality from incomplete detection data, we utilize a priori knowledge and Otsu's method as the optimization constraint. In our study, we make numerical simulation to investigate our proposed method,and compare reconstructed results from different reconstruction methods. Finally, the experimental results demonstrate that the proposed method could effectively reduce noise and suppress artifacts, and reconstruct high-quality binary image from incomplete detection data.     

基于基图像TV模型的CT射束硬化校正方法

X射线CT中,X射束硬化导致重建图像中出现伪影,严重影响了图像质量。论文深入分析了射束硬化对原始投影数据的影响,并提出了一种基于基图像TV模型的射束硬化校正方法。该方法首先依据射束硬化的物理特征建立了带有可调参数的初步校正模型;其次,在不同的可调参数条件下,原始投影数据经该模型预处理变换得到多组预处理投影序列;再次,分别对预处理投影序列进行重建得到一系列校正基图像;最后,将基图像的线性组合作为最终重建图像。其中,加权系数的最优解是以最终重建图像的全变分(Total Variation,TV)函数作为目标函数,通过迭代法求得。为验证该算法,对真实的铝柱体模和工业检测件进行了实验,结果表明,该算法对射束硬化引起的杯状和条状伪影均有显著的抑制效果。     

Derivative-Hilbert-Backprojection based image reconstruction from truncated projections in helical cone-beam CT

In helical cone-beam computed tomography(CT), Feldkamp-Davis-Kress(FDK) based image reconstruction algorithms are by far the most popular. However, artifacts are commonly met in the presence of lateral projection truncation. The reason is that the ramp filter is global. To restrain the truncation artifacts, an approximate reconstruction formula is proposed based on the Derivative-Hilbert-Backprojection(DHB) framework. In the method, the first order derivative filter is followed by the Hilbert transform. Since the filtered projection values are almost zero by the first order derivative filter, the following Hilbert transform has little influence on the projection values, even though the projections are laterally truncated. The proposed method has two main advantages. First, it has comparable computational efficiency and image quality as well as the conventional helical FDK algorithm for non-truncated projections. The second advantage is that images can be reconstructed with acceptable quality and much lower computational cost in comparison to the Laplace operator based algorithm in cases with truncated projections. To point out the advantages of our method, simulations on the computer and real data experiments on our laboratory industrial cone-beam CT are conducted. The simulated and experimental results demonstrate that the method is feasible for image reconstruction in the case of projection truncation.     

小型静态CT用多焦点X射线源电子束偏转系统研究

第5代全静态电子束CT（EBCT）能在被检测物体、射线源和探测器均处于静止状态下完成CT扫描成像。这种EBCT成像方式既可应用于粉体材料、生物活体样品等易形变物体的CT检测，又可应用于物流包裹、在役管线等的快速CT检测。高精度的电子束偏转系统是静态CT成像多焦点射线源的关键技术之一。本文基于麦克斯韦电磁场相互作用理论，探讨了电子束在均匀磁场、小偏转角、近轴区域内的散焦、畸变及灵敏度特性，研究了影响电子束扫描系统焦点位置重复精度的物理参数及电子束在靶平面上的扫描偏转量与偏转线圈电流的线性关系。并设计、加工制作了一种小型高精度多焦点X射线的电子束偏转扫描系统，实验采用直径0.1 mm标准针规电子束扫描DR投影图像序列，验证了电子束偏转系统有良好的偏转线性，偏转量正比于O点的磁场强度，而与电子束的位置等初始条件无关。实验通过电子束偏转系统控制电子束偏转扫描，获得了熔断式保险管（Ø100 μm）的250个 X 射线视角投影，完成了第5代全静态三维CT重建图像。     

基于半扇束扫描的CT图像迭代重建算法

为解决探测器尺寸受限导致的CT图像重建区域面积受限以及X射线辐射剂量较大问题,提出一种半扇束扫描模式下的CT图像迭代重建算法。该算法将探测器水平偏置,稀疏角度下采集物体投影数据,利用ART-TV迭代算法对投影数据进行重建,达到预设迭代次数后,输出重建图像。实验结果表明:半扫描可扩大重建面积,降低探测器成本;利用迭代重建算法,降低剂量的同时重建优质CT图像。     

PHASE CORRECTION AND BAND-PASS FILTERS IN CT IMAGING

This paper reports the use of the fast Fourier transform (FFT) in the direct Fourier transform method (DFM) in Computerized Tomography (CT) reconstruction. Phase corrections are needed in the CT reconstruction. In order to eliminate the image distortion from the basic DFM, Padding and band-pass filters are used in the improved DFM. Finally, some reconstructions from simulated projections and several experimental results are given.     

高能CT检测中图像重建算法的积分因子仿真计算

针对核装置的具体情况,设计一种提高其图像指标的算法。利用核装置的大量先验知识并结合蒙特卡罗程序来修正CT检测中图像重建算法的积分因子,通过图像重建仿真实验验证了该算法可有效抑制散射与串扰对图像重建的影响,大幅提高了检测结果的精度。     

Non-Linear Techniques in Multi-Spectral X-Ray Imaging

The sensitivity of medical radiographs for the contrast agent iodine can be improved by multispectral imaging techniques. Three different shadow projections of the object to be imaged are taken by using three distinct primary X-ray spectra. The corresponding material-selective projections of iodine, bone and tissue masses are generated by combining appropriately the shadow projections. A nonlinear reconstruction algorithm is presented thereby improving the reconstruction accuracy compared with known linear techniques. The algorithm is tested both in computer simulation and in experiment.     

Investigation of optimal scanning protocol for X-ray computed tomography polymer gel dosimetry

X-ray computed tomography is one of the potential tool used to evaluate the polymer gel dosimeters in three dimensions. The purpose of this study is to investigate the factors which affect the image noise for X-ray CT polymer gel dosimetry. A cylindrical water filled phantom was imaged with single slice Siemens Somatom Emotion CT scanner. The imaging parameters like tube voltage, tube current, slice scan time, slice thickness and reconstruction algorithm were varied independently to study the dependence of noise on each other. Reductions of noise with number of images to be averaged and spatial uniformity of the image were also investigated. Normoxic polymer gel PAGAT was manufactured and irradiated using Siemens Primus linear accelerator. The radiation induced change in CT number was evaluated using X-ray CT scanner. From this study it is clear that image noise is reduced with increase in tube voltage, tube current, slice scan time, slice thickness and also reduced with increasing the number of images averaged. However to reduce the tube load and total scan time, it was concluded that tube voltage of 130 kV, tube current of 200 mA, scan time of 1.5 s, slice thickness of 3 mm for high dose gradient and 5 mm for low dose gradient were optimal scanning protocols for this scanner. Optimum number of images to be averaged was concluded to be 25 for X-ray CT polymer gel dosimetry. Choice of reconstruction algorithm was also critical. From the study it is also clear that CT number increase with imaging tube voltage and shows the energy dependency of polymer gel dosimeter. Hence for evaluation of polymer gel dosimeters with X-ray CT scanner needs the optimization of scanning protocols to reduce the image noise.