融合ω-K和BP算法的圆柱扫描毫米波三维成像算法

结合主动式圆柱扫描毫米波3维成像安检仪的实际应用需求,该文提出一种用于3维场景重建的新方法。该方法采用ω-K算法实现天线阵列方向和距离方向的解耦合与聚焦,再采用后向投影(BP)算法进行距离方向和角度方向的合成孔径处理实现聚焦,进而实现3维场景的重建。3维人体模型仿真和实测数据处理结果表明,该方法具备理论可行性和工程适用性,除此之外,该文方法在CUDA平台下可以实现快速精确3维人体成像,并且能够适应非理想圆柱扫描轨迹3维成像应用。      

基于点探测器和柱面源双位置扫描的直接体积CT的研究

目前直接体积CT主要存在两大困难:一是如何获得三维图像精确重建所需的投影数据,包括扫描方式的可实现性;二是如何尽量避免康普顿散射对重建图像质量的影响.针对这些问题,本文提出一种新型的直接体积CT成像模式,即采用X线栅形扫描的柱面源、一组分布在大圆和垂直圆弧上的多个点状探测器采集数据,通过柱面源的一次旋转即可得到完全的投影数据.本文对该扫描装置的结构中X线柱面源的形状和尺寸、探测器的个数和位置分布进行了详细的说明,并对该装置的完全性条件进行论证.然后结合此扫描结构的特点,给出一种三维图像重建算法.计算机仿真实验的结果表明,该扫描结构在保证投影数据完全的条件下,实现了机械转动次数最少、有效抑制康普顿散射、提高三维图像重建速度和精度的目的,使直接体积CT的成像速度和重建精度比现有模式大大提高.     

圆弧轨迹Katsevich类型的锥束CT重建

基于Katsevich重建理论,提出了一种适合圆弧轨迹且具有移不变滤波反投影结构的锥束重建算法.分析了Radon平面绕投影射线连续转动过程中与轨迹平面相交的运动关系,并依据投影数据存储方式确定Radon平面的法线矢量方向.在此基础上,将检测器区域进行划分,进而探讨了与圆弧端点关联的结构因子在检测器阵列单元上的分布规律.同时,根据滤波线在检测器上的分布特点,推导了满足投影数据重采样精度并适合平板检测器的锥束重建公式.利用Forbild模型进行数值模拟,实验结果表明提出的算法有效地抑制了FDK类型重建算法所存在的锥束伪像并且适合长物体重建.     

高速旋转物体的CT图像重建方法

高速旋转物体的CT成像是难点问题,相关研究论文较少.由于CT检测系统的单幅投影的数据采集时间无法足够短,导致无法采集高速旋转物体准静止的CT扫描数据,从而使得基于逐条射线的经典CT成像模型不再适用于高速旋转物体CT成像.本文针对高速旋转物体CT,给出了数据采集准则,建立了射线混叠CT成像模型,并提出了相应迭代重建算法.仿真数据和实际数据的实验结果表明,本文提出的方法可以较好地对高速旋转物体进行CT成像.     

用反投影滤波算法实现CT图像的ROI重建

为了实现感兴趣区域(Region of Interest,ROI)的重建,使用了一种利用微分和有限希尔伯特变换的局部特性的反投影滤波(back projection filteration,BPF)算法。该算法是一种理论上精确的ROI重建算法,首先对仅仅覆盖ROI区域的投影数据微分,然后反投影到ROI区域,最后沿着覆盖ROI区域的PI线做有限希尔伯特滤波得到ROI图像。仿真实验表明BPF算法和经典的滤波反投影(Filtered Back Projection,FBP)算法的全局重建的精度基本相同,但该算法可以实现精确的ROI重建,而FBP算法因不具有局部特性,不能实现ROI重建。在实现有限希尔伯特变换时,采用了加权希尔伯特变换的方法,有效地避免了图像两边的亮条伪像。     

用反投影滤波(BPF)算法实现CT图像的ROI重建

为了实现感兴趣区域(Region of Interest,ROI)的重建,本文使用了一种利用微分和有限希尔伯特变换的局部特性的反投影滤波(back projection filteration,BPF)算法。这种算法是一种理论上精确的ROI重建算法,首先对仅仅覆盖ROI区域的投影数据微分,然后反投影到ROI区域,最后沿着覆盖ROI区域的PI线做有限希尔伯特滤波得到ROI图像。仿真实验表明BPF算法和经典的滤波反投影(filtered back projection, FBP)算法的全局重建的精度基本相同,但本文算法可以实现精确的ROI重建,而FBP算法因不具有局部特性,不能实现ROI重建。本文在实现有限希尔伯特变换时,采用了加权希尔伯特变换的方法,有效地避免了图像两边的亮条伪像。     

圆轨迹SAR快速成像处理

由于圆轨迹SAR特殊的运动轨迹,直线SAR的成像算法不能直接应用于圆轨迹SAR数据处理。然而,对于圆轨迹SAR系统而言,其响应函数具有"沿角度维平移不变"的特性。因此可以利用这一特性,借鉴条带SAR成像算法的思想,在频域研究圆轨迹SAR数据的快速成像方法。该文推导了完全精确的柱面坐标系下圆轨迹SAR回波频谱解析表达式。利用条带SAR成像算法的思想,通过对频谱的合理近似,给出了圆轨迹SAR的快速成像算法。仿真实验验证了算法的有效性。     

可结合CSAR时域成像处理的改进PGA方法

以后向投影算法（Back Projection, BP）为代表的时域类成像算法能够适用于复杂轨迹下的雷达图像重建,且具有精确成像能力。然而,这类算法的成像过程往往不满足结合传统自聚焦算法的条件,从而限制了其对实测数据的成像性能。针对这个问题,本文提出了一种可结合圆周合成孔径雷达（Circular Synthetic Aperture Radar, CSAR）时域成像处理的改进相位梯度自聚焦算法（Phase Gradient Autofocus, PGA）。首先,基于CSAR的成像几何,推导了运动误差形式;在此基础上建立了运动误差条件下的BP成像处理信号模型;接下来,提出了可结合CSAR时域成像处理的改进PGA算法,并给出了详细的处理流程;最后,利用实测数据成像处理结果证明了理论分析的正确性和所提算法的有效性。      

基于局部直角坐标和子区域处理的弹载SAR频域成像算法

针对弹载SAR平台非直线轨迹运动和大斜视成像的特点,该文提出一种基于局部直角坐标和子区域处理的频域成像算法。对大斜视成像模式,采用局部直角坐标系建模的方法,使距离走动校正后的方位信号具有更高的近似精度;为精确补偿包络和相位的方位空变,采用子区域成像的方式,可显著提高场景边缘处的聚焦质量;将各子区域图像校正至统一的地面坐标系下,即可得到最终的SAR图像。点目标仿真和实测数据验证了所提算法的有效性。     

直线模式下基于圆弧线投影层析成象的算法

本文探讨了感测器沿直线移动时由反射信号数据重建物体表平面或截平面图象的方法。文中推导了直线模式下基于圆弧线投影层析成象处理的基本关系式和快速实现算法,给出了一些计算模拟结果。本文的方法可应用于雷达对地面局部区域成象中,也可应用于反射式超声CT竖断面成象中。     

Region of interest reconstruction from truncated data in circular cone-beam CT

The circular scanning trajectory is one of the most widely adopted data-acquisition configurations in computed tomography (CT). The Feldkamp, Davis, Kress (FDK) algorithm and its various modifications have been developed for reconstructing approximately three-dimensional images from circular cone-beam data. When data contain transverse truncations, however, these algorithms may reconstruct images with significant truncation artifacts. It is of practical significance to develop algorithms that can reconstruct region-of-interest (ROI) images from truncated circular cone-beam data that are free of truncation artifacts and that have an accuracy comparable to that obtained from nontruncated cone-beam data. In this work, we have investigated and developed a backprojection-filtration (BPF)-based algorithm for ROI-image reconstruction from circular cone-beam data containing transverse truncations. Furthermore, we have developed a weighted BPF algorithm to exploit "redundant" information in data for improving image quality. In an effort to validate and evaluate the proposed BPF algorithms for circular cone-beam CT, we have performed numerical studies by using both computer-simulation data and experimental data acquired with a radiotherapy cone-beam CT system. Quantitative results in these studies demonstrate that the proposed BPF algorithms for circular cone-beam CT can reconstruct ROI images free of truncation artifacts.     

基于变螺距螺旋FDK重建算法的研究

目前广泛应用的锥束螺旋CT主要采用螺旋FDK算法进行图像重建,但为了使算法能更应用于实际,许多学者对标准的螺旋轨迹进行了拓展.本文主要研究了变螺距螺旋扫描轨迹.变螺距螺旋扫描可以提高扫描速度,有更高的时间、空间分辨率.本文详细介绍了变螺距螺旋扫描轨迹,给出了具体的变螺距螺旋扫描轨迹方程.针对图像重建,本文主要从FDK重...     

A cone-beam reconstruction algorithm for circle-plus-arc data-acquisition geometry

In cone-beam computerized tomography (CT), projections acquired with the focal spot constrained on a planar orbit cannot provide a complete set of data to reconstruct the object function exactly. There are severe distortions in the reconstructed noncentral transverse planes when the cone angle is large. In this work, a new method is proposed which can obtain a complete set of data by acquiring cone-beam projections along a circle-plus-arc orbit. A reconstruction algorithm using this circle-plus-arc orbit is developed, based on the Radon transform and Grangeat's formula. This algorithm first transforms the cone-beam projection data of an object to the first derivative of the three-dimensional (3-D) Radon transform, using Grangeat's formula, and then reconstructs the object using the inverse Radon transform. In order to reduce interpolation errors, new rebinning equations have been derived accurately, which allows one-dimensional (1-D) interpolation to be used in the rebinning process instead of 3-D interpolation. A noise-free Defrise phantom and a Poisson noise-added Shepp-Logan phantom were simulated and reconstructed for algorithm validation. The results from the computer simulation indicate that the new cone-beam data-acquisition scheme can provide a complete set of projection data and the image reconstruction algorithm can achieve exact reconstruction. Potentially, the algorithm can be applied in practice for both a standard CT gantry-based volume tomographic imaging system and a C-arm-based cone-beam tomographic imaging system, with little mechanical modification required.     

Vessel tree reconstruction in thoracic CT scans with application to nodule detection

Vessel tree reconstruction in volumetric data is a necessary prerequisite in various medical imaging applications. Specifically, when considering the application of automated lung nodule detection in thoracic computed tomography (CT) scans, vessel trees can be used to resolve local ambiguities based on global considerations and so improve the performance of nodule detection algorithms. In this study, a novel approach to vessel tree reconstruction and its application to nodule detection in thoracic CT scans was developed by using correlation-based enhancement filters and a fuzzy shape representation of the data. The proposed correlation-based enhancement filters depend on first-order partial derivatives and so are less sensitive to noise compared with Hessian-based filters. Additionally, multiple sets of eigenvalues are used so that a distinction between nodules and vessel junctions becomes possible. The proposed fuzzy shape representation is based on regulated morphological operations that are less sensitive to noise. Consequently, the vessel tree reconstruction algorithm can accommodate vessel bifurcation and discontinuities. A quantitative performance evaluation of the enhancement filters and of the vessel tree reconstruction algorithm was performed. Moreover, the proposed vessel tree reconstruction algorithm reduced the number of false positives generated by an existing nodule detection algorithm by 38%.     

一种适用于大尺寸工件的定点检测快速重建方法

为提高大尺寸工件的缺陷检测效率,本文在滤波反投影(FBP)算法的基础上,提出了一种快速获得指定点CT数的定点重建方法,为实现大尺寸工件缺陷区域的快速局部重建提供了有效手段.模拟的断层重建检测结果表明,该定点重建方法局部重建的图像清晰,缺陷分辨率高,重建速度快,具有很好的应用价值.     

红外运动目标轨迹重构动态仿真平台

提出了一种红外热成像视频中运动目标识别追踪以及轨迹重构的动态仿真方法。通过仿真环境中虚拟红外图像的生成方式与成像的基本模型,对得到的图像进行一系列预处理。以空对空场景搭建了基于Gazebo与OpenCV的动态仿真平台,利用平滑约束算法对追踪目标进行实时动态轨迹重构,提出了误差分析模型,并分析出轨迹重构算法的性能以及仿真平台的效能。实验结果表明,该方法针对空对空场景下的红外运动目标轨迹重构具有较好的精度与鲁棒性,对目标的运动模型基本没有约束,同时仿真平台具有较高的运行效能与实时性,普通家用电脑即可实现高于60 fps的实时动态仿真,满足轨迹重构算法性能测试与训练的需求,其核心算法亦可迁移至机载计算平台实现真实场景下的实时轨迹重构。所提出的单路热成像视频中运动目标轨迹重构动态仿真方法对空间目标三维轨迹重建与动态测距定位的研究具有重要意义。     

Interventional 4-D C-arm CT perfusion imaging using interleaved scanning and partial reconstruction interpolation

Tissue perfusion measurement during catheter-guided stroke treatment in the interventional suite is currently not possible. In this work, we present a novel approach that uses a C-arm angiography system capable of computed tomography (CT)-like imaging (C-arm CT) for this purpose. With C-arm CT one reconstructed volume can be obtained every 4-6 s which makes it challenging to measure the flow of an injected contrast bolus. We have developed an interleaved scanning (IS) protocol that uses several scan sequences to increase temporal sampling. Using a dedicated 4-D reconstruction approach based on partial reconstruction interpolation (PRI) we can optimally process our data. We evaluated our combined approach (IS-PRI) with simulations and a study in five healthy pigs. In our simulations, the cerebral blood flow values (unit: ml/100 g/min) were 60 (healthy tissue) and 20 (pathological tissue). For one scan sequence the values were estimated with standard deviations of 14.3 and 2.9, respectively. For two interleaved sequences the standard deviations decreased to 3.6 and 1.5, respectively. We used perfusion CT to validate the in vivo results. With two interleaved sequences we achieved promising correlations ranging from r=0.63 to r=0.94. The results suggest that C-arm CT tissue perfusion imaging is feasible with two interleaved scan sequences.     

Radial imaging with multipolar magnetic encoding fields

We present reconstruction methods for radial magnetic resonance imaging (MRI) data which were spatially encoded using a pair of orthogonal multipolar magnetic fields for in-plane encoding and parallel imaging. It is shown that a direct method exists in addition to iterative reconstruction. Standard direct projection reconstruction algorithms can be combined with a previously developed direct reconstruction for multipolar encoding fields acquired with Cartesian trajectories. The algorithm is simplified by recasting the reconstruction problem into polar coordinates. In this formulation distortion and aliasing become separate effects. Distortion occurs only along the radial direction and aliasing along the azimuthal direction. Moreover, aliased points are equidistantly distributed in this representation, and, consequently, Cartesian SENSE is directly applicable with highly effective unfolding properties for radio-frequency coils arranged with a radial symmetry. The direct and iterative methods are applied to simulated data to analyze basic properties of the algorithms and for the first time also measured in vivo data are presented. The results are compared to linear spatial encoding using a radial trajectory and quadrupolar encoding using a Cartesian trajectory. The direct reconstruction gives good results for fully sampled datasets. Undersampled datasets, however, show star-shaped artifacts, which are significantly reduced with the iterative reconstruction.     

CT重建中滤波器的设计与实现

CT重建算法主要包括解析法和迭代法,因为解析法重建速度快,所需数据存储空间较小,所以在实际中的应用更为广泛。在商业CT中,几乎毫无例外地采用卷积反投影重建算法,滤波算子是这一算法中一个非常重要的部分。目前CT重建算法的研究热点一方面是改进算法提高图像重建速度,另一研究内容主要集中在预处理滤波器的设计与实现,以便得到边缘清晰、平滑及噪声较少的图像。本文从滤波器的设计定义出发,分析讨论了它的性质,并在此基础上提出来一种新的滤波器。经过实验验证,新的滤波器取得比较理想的结果。     

Dictionary Learning-Based Image Reconstruction for Terahertz Computed Tomography

Terahertz computed tomography (THz CT) demonstrates its advantages in aspects of nonmetallic and nonpolar materials penetration, 3D internal structure visualization, etc. To perform satisfied reconstruction results, it is necessary to obtain complete measurements from many different views. However, this process is time-consuming and we usually obtain incomplete projections for THz CT in practice, which generates artifacts in the final reconstructed images. To address this issue, dictionary learning-based THz CT reconstruction (DLTR) model is proposed in this study. Especially, the image patches are extracted from other state-of-the-art reconstructed images to train the initial dictionary by using the K-SVD algorithm. Then, the dictionary can be adaptively updated during THz CT reconstruction. Finally, the updated dictionary is used for further updating reconstructed images. In order to verify the accuracy and quality of DLTR method, the filtered back-projection (FBP), simultaneous algebraic reconstruction technique (SART), and total variation (TV) reconstruction are chosen as comparisons. The experiment results show that the DLTR method has a good capability for noise suppression and structures preservation.