基于穿越长度权重迭代重建算法的研究

目的:传统的CT迭代算法中为了简化运算,投影系数的计算选择用成像射线路径是否穿过像素内来确定投影系数矩阵中元素的数值,穿过为1,不穿过为0。有些射线只穿过像素的边缘,也被赋值为1,扩大了该射线对相应像素投影值的贡献。为减小图像重建的误差,提高图像重建质量,提出了基于穿越长度权重的迭代重建算法,通过精确建模,选择用成像射线路径在像素内的穿越长度来确定投影系数矩阵中元素的数值。方法:采用MATLAB7.0仿真工具,对Shepp-Logan模型进行计算机仿真扫描,分别以传统投影系数和穿越长度权重计算的投影系数进行ML-EM迭代重建图像。结果:仿真数据表明基于穿越长度权重投影系数的ML-EM迭代重建算法,相比基于传统投影系数的迭代重建算法,可以提高重建图像的质量。结论:基于穿越长度权重的ML-EM迭代重建算法,通过精确建模达到了控制噪声、减小误差、较为准确重建的目的。通过对成像的几何和物理因素进行精确的建模,能有效地控制其中的非随机部分的影响,减小图像重建误差,一方面对迭代重建算法提供一种新的投影系数的计算方法,另一方面进一步提高迭代算法在CT重建中的图像质量。     

一种基于反投影滤波的精确心脏重建算法

在分析心脏重建算法存在问题的基础上,结合反投影滤波重建算法及心脏重建的特性.提出一种基于反投影滤波的精确心脏重建算法。该算法通过M平面选取、心脏投影数据处理、M平面重建及重建图像重组等步骤,将滤波反投影重建算法应用于心脏重建中,避免原始非精确心脏重建算法中的锥角伪影、螺旋伪影,提高重建时间分辨率。以一患者的临床图像进行重建。结果对比表明,所提出的重建算法在冠脉血管CT均值上提高8%,最大值上提高4.4%,有效地减少心脏重建所需的投影数据范围,并为实现心脏低剂量超短扫描提供算法支持。     

运用兆伏锥形束计算机断层成像校正千伏计算机断层成像中假牙的金属伪影

本文提出一种运用兆伏锥形束CT(MVCBCT)校正千伏CT(kVCT)图像中假牙金属伪影的新方法。该方法分别用kVCT和MVCBCT扫描佩戴假牙的患者,得到两种CT图像。在kVCT中阈值分割得到金属图像,运用MVCBCT和kVCT融合得到先验图像,对先验图像前投影来替代原始金属区投影,最后通过滤波反投影(FBP)重建图像。将本文方法校正后效果与归一化金属伪影校正法(NMAR)、以MVCBCT为先验图像的归一化金属伪影校正法(NMAR-MV),以及线性插值法(LIMAR)这三种常用伪影校正方法进行比较,计算其归一化均方根偏差(NRMSD)和平均绝对偏差(MAD)。实验结果显示本文方法去除了严重的金属伪影且没有引入其他伪影,基于参考图像计算的NRMSD值和MAD值最小。NMAR、NMAR-MV、LIMAR以及本文方法的NRMSD值分别为21.0%、22.1%、41.9%、17.0%;MAD值分别为232、235、553、205 HU。本文提出的伪影校正方法能较好地去除假牙的金属伪影,大幅改善CT图像质量。     

计算机模拟CT滤波反投影重建的方法

目的:选择合适的滤波函数处理对CT反投影图像进行滤波,从而达到消除反投影所产生的星形伪影的目的。使得CT滤波反投影重建的方法直观地演示出来,也便于比较不同滤波函数的效果,寻找更好的滤波函数。方法:采用计算机软件编程实现计算机模拟CT反投影重建,以VC软件平台IDE集成开发环境,开发出一个具有可视图形界面的上位机软件,通过选择不同的滤波反投影参数,对重建的图像的效果进行定量的比较,探讨滤波函数中不同参数对结果的影响。结果:通过软件编程实现了标准体模图像重建的模拟,界面简单,可以直观地对CT图像进行反投影重建,同时所选择的基于频率域变换的滤波函数也部分消除了重建过程中造成的星形伪影,提高了图像均匀度,使标准体模的重建图像达到预期的理想效果。结论:开发的软件实现了CT反投影重建,能够将标准体模反投影重建的成像过程以更清晰的方式展现出来,有利于在医学物理学理论课和实验课的教学中,直观演示星形伪影及滤波效果,使学生更好地学习CT技术的原理。也有利于尝试新的投影算法,寻找新的滤波函数。     

基于投影矩阵广义逆的CT重建迭代算法

目的在CT检查时,有限角度投影和稀疏矩阵投影能够减少X射线的剂量,然而这会导致投影数据不足,给图像重建带来一定的困难。为了克服这一难题得到较好的重建图像,本文提出一种基于计算投影矩阵广义逆的CT迭代重建算法。方法该算法在计算过程中,将重建图像表示为投影矩阵以及其广义逆的乘积。首先使用一阶迭代计算广义逆矩阵,但是由于投影矩阵和其广义逆矩阵都比较大,在迭代过程中以投影和滤波反投影代替。然后通过不同的算法分别对平行束投影、有限角度投影、稀疏矩阵投影的数据进行重建,并对重建结果的均方差、通用图像质量指标以及图像互信息进行比较。结果本文提出的方法重建出图像的均方差、通用图像质量指标和图像互信息更优,而且重建时间较短。结论该方法能够在没有未知图像先验结构信息和伪影猜想的情况下有效地提高重建图像的质量,而且该算法不需要计算投影过程,重建过程简单易行。     

基于全局方差的感兴趣区域断层CT图像重建算法

感兴趣区域断层CT图像重建在医疗诊断、工业无损检测领域广泛应用,重建精度直接影响医疗诊断结果与工业检测结果,为此提出基于全局方差的感兴趣区域断层CT图像重建算法。利用全局方差的断层CT图像增强算法步骤和流程,增强全局方差与全局均值的图像效果,同时获取区域内像素灰度级方差。在此基础上,导入ART模拟与投影,得到Radon逆变换ART迭代函数,重建增强效果后的感兴趣区域断层CT图像。研究结果表明,基于全局方差的重建算法可有效重建感兴趣区域图像,重建图像的归一化均方误差判距与归一化平均绝对距离判距均低于0.2,是一种可靠的感兴趣区域断层CT图像重建算法。     

CT图像重建的快速卷积反投影算法的优化

卷积反投影是最重要的一种CT图像重建算法，此算法简单，重建精度高，重建速度较快，应用最广泛。但是CT投影数据量巨大，重建时间比较长。根据卷积反投影算法的特点，提出了此算法的快速实现方法，在PC机上实现时，重建速度约提高20倍。     

一种快速校正CT环形伪影的方法

背景：环形伪影严重影响了CT图像质量,对图像后处理造成困难以及容易造成误诊断。目前去除环形伪影必不可少。 目的：去除CT重建图像中的环形伪影,提高CT图像质量以及后续处理和量化分析的精度,便于诊断。 方法：首先把含环形伪影的CT图像进行线性变换,将灰度图像转换成浮点类型的图像。接着由直角坐标变换到极坐标,这样原来的环形伪影就被变换成线形伪影。设计多维滤波器,计算每个象素滤波后均值以及方差,通过与阈值比较确定伪影范围。最后通过对伪影范围进行修正以及对图像进行坐标变换,变为灰度图像。 结果与结论：通过Matlab 7.0软件设计程序,处理含环形伪影的CT图像。实验表明,此方法能有效快速地校正CT环形伪影,是一种属于图像后处理的校正方法。     

结合各向异性扩散滤波Thin Plate先验正电子发射断层图像重建的算法

背景：在正电子发射断层成像中,MAP重建方法通过引入先验分布约束,可以明显提高重建图像的质量,但不合适的先验分布项可能会造成重建图像过度平滑或出现阶梯状边缘伪影。 目的：针对基于传统局部先验信息的MAP方法易于导致重建图像过平滑或产生阶梯状边缘伪影的问题,提出了一种结合各向异性扩散滤波的、基于Thin Plate先验的改进MAP重建算法。 方法：重建算法由两步组成：基于双向扩散系数的PDE各向异性扩散滤波和基于Thin Plate先验的MAP估计。重建图像通过这两步交替迭代得到。文中采用归一化均方根误差和信噪比定量评价重建图像质量。 结果与结论：结合了基于双向扩散系数的PDE各向异性扩散滤波,并将Thin Plate二次二阶先验模型引入到MAP重建算法中,所获得的重建结果图像在抑制噪声、边缘保持方面取得了良好的效果,SNR、RMSE以及视觉评价等方面均有较大程度的改善。     

CT图像锥形伪影的实验研究

目的：研究螺旋CT插值算法引起的图像伪影的规律。方法：采用自制的有机玻璃锥形体模在Siemens Plus4和GE Lightspeed16扫描仪上设置不同参数进行扫描。基于MATLAB7．0编程对图像进行分析，计算伪影的面积占原像面积的比重。结果：伪影的面积比随着螺距的增大而增大，随着体模半锥角的正切值和重建位置的半径的增大分别呈现增大和减小的趋势。临床照片中，锥形伪影突出表现在边界明显的部位。结论：锥形伪影与螺距和被扫描物属性存在确定关系。     

Prior image constrained scatter correction in cone-beam computed tomography image-guided radiation therapy

X-ray scatter is a significant problem in cone-beam computed tomography when thicker objects and larger cone angles are used, as scattered radiation can lead to reduced contrast and CT number inaccuracy. Advances have been made in x-ray computed tomography (CT) by incorporating a high quality prior image into the image reconstruction process. In this paper, we extend this idea to correct scatter-induced shading artifacts in cone-beam CT image-guided radiation therapy. Specifically, this paper presents a new scatter correction algorithm which uses a prior image with low scatter artifacts to reduce shading artifacts in cone-beam CT images acquired under conditions of high scatter. The proposed correction algorithm begins with an empirical hypothesis that the target image can be written as a weighted summation of a series of basis images that are generated by raising the raw cone-beam projection data to different powers, and then, reconstructing using the standard filtered backprojection algorithm. The weight for each basis image is calculated by minimizing the difference between the target image and the prior image. The performance of the scatter correction algorithm is qualitatively and quantitatively evaluated through phantom studies using a Varian 2100 EX System with an on-board imager. Results show that the proposed scatter correction algorithm using a prior image with low scatter artifacts can substantially mitigate scatter-induced shading artifacts in both full-fan and half-fan modes.     

Removal of ring artifacts in computed tomographic imaging using iterative center weighted median filter

A new iterative center weighted median filter (ICWMF) for ring artifact reduction from the micro-computed tomographic (μ-CT) image is proposed in this paper. The center weight of the median filter is computed based on the characteristic of the ring artifact in the mean curve of the projection data. The filter operates on the deviation of the mean curve to smooth the ring generating peaks and troughs iteratively while preserving the details due to image. A convergence criterion for the iterative algorithm is determined from the distribution of the local deviation computed from the mean curve deviation. The estimate of the mean curve obtained using the ICWMF is used to correct the ring corrupted projection data from which reconstruction gives the ring artifact suppressed μ-CT image. Test results on both the synthetic and real images demonstrate that the ring artifacts can be more effectively suppressed using our method as compared to other ring removal techniques reported in the literature.     

A reconstruction algorithm for coherent scatter computed tomography based on filtered back-projection

Coherent scatter computed tomography (CSCT) is a reconstructive x-ray imaging technique that yields the spatially resolved coherent-scatter form factor of the investigated object. Reconstruction from coherently scattered x-rays is commonly done using algebraic reconstruction techniques (ART). In this paper, we propose an alternative approach based on filtered back-projection. For the first time, a three-dimensional (3D) filtered back-projection technique using curved 3D back-projection lines is applied to two-dimensional coherent scatter projection data. The proposed algorithm is tested with simulated projection data as well as with projection data acquired with a demonstrator setup similar to a multi-line CT scanner geometry. While yielding comparable image quality as ART reconstruction, the modified 3D filtered back-projection algorithm is about two orders of magnitude faster. In contrast to iterative reconstruction schemes, it has the advantage that subfield-of-view reconstruction becomes feasible. This allows a selective reconstruction of the coherent-scatter form factor for a region of interest. The proposed modified 3D filtered back-projection algorithm is a powerful reconstruction technique to be implemented in a CSCT scanning system. This method gives coherent scatter CT the potential of becoming a competitive modality for medical imaging or nondestructive testing.     

Application of TACT to the evaluation of total joint arthroplasty

Three-dimensional imaging may be valuable in the evaluation of total joint arthroplasty. The partial-volume effect and streak artifacts from the metallic components of these devices limit the applicability of computed tomography (CT) to this application. Tuned-aperture computed tomography (TACT) is a tomographic approach that has been successfully used in other medical applications. In TACT, the acquisition geometry is inferred from the localization of fiducial markers in a series of projection images. The flexibility and robustness of TACT, as well as the fact that through an appropriate choice of reconstruction algorithms it can suppress streak artifacts, potentially makes it an appropriate approach for evaluating total joint arthroplasty. A simple computer-simulated model of a total knee replacement (TKR) was generated and used to evaluate the accuracy and artifacts associated with three-dimensional (3-D) renderings produced using TACT. A knee specimen from a cadaver that had received a TKR was used to investigate further the potential of TACT for this application. In both tests, TACT provided high-quality 3-D representations of the object. Both simple back-projection and minimum pixel back-projection were used to reconstruct the data. Minimum pixel back-projection provided high-contrast images that appeared to be relatively free of tomosynthetic artifacts. In summary, the potential of TACT in the evaluation of total joint arthroplasty was demonstrated. Future investigations will study TACT's ability to quantify the spatial relationship between the metallic components of these devices as well as TACT's ability to identify bony changes of diagnostic consequence.     

An analytical reconstruction algorithm for multifocal converging-beam SPECT

Multifocal converging-beam collimation has been suggested for cardiac SPECT imaging to increase sensitivity over the heart without truncation of the activity distribution in the chest. In this study, an analytical reconstruction algorithm is derived for multifocal fan-beam and multifocal cone-beam tomography. In the algorithm, the projection data are differently weighted and filtered, depending on the distance from the detector. For a given image point, the set of filtered data corresponding to the distance between this point and detector is backprojected to determine the pixel value of the point. Thus, the backprojection is done only once at each projection view. To evaluate the algorithm, simulation studies are performed using a 3D Defrise slab phantom without considering photon attenuation and scatter, detector response, and statistical noise. Reconstructed images demonstrate that reasonable quality can be achieved with a modest focal-length variation rate and with a small radius of rotation. A collimator with a focal length increasing quickly near its centre provides better quality in the image region distant from the central plane of the cone geometry, but produces more severe artifacts at the centre of the reconstructed image, compared to a collimator with an initially slowly varying focal length.     

Weighted expectation maximization reconstruction algorithms with application to gated megavoltage tomography

We propose and investigate weighted expectation maximization (EM) algorithms for image reconstruction in x-ray tomography. The development of the algorithms is motivated by the respiratory-gated megavoltage tomography problem, in which the acquired asymmetric cone-beam projections are limited in number and unevenly sampled over view angle. In these cases, images reconstructed by use of the conventional EM algorithm can contain ring- and streak-like artefacts that are attributable to a combination of data inconsistencies and truncation of the projection data. By use of computer-simulated and clinical gated fan-beam megavoltage projection data, we demonstrate that the proposed weighted EM algorithms effectively mitigate such image artefacts.     

Differential attenuation method for simultaneous estimation of activity and attenuation in multiemission single photon emission computed tomography

A penalized weighted least squares reconstruction algorithm is described that simultaneously estimates activity and attenuation distributions from emission sinogram data alone. This estimation technique is based on differential attenuation information and is applicable to any single photon emission computed tomography imaging isotope with emissions at two or more distinct energies, after accurate compensation for Compton scatter. A rotation-based forward projector is used to efficiently model photon attenuation at multiple emission energies, as well as distance-dependent spatial resolution. The algorithm was tested using simulated scatter-free 201T1 projection data from a single-slice numerical cardiac phantom with and without cold myocardial defects. Poisson noise was added to the projection data to mimic clinically realistic count densities. The activity estimates resulting from the proposed method had fewer artifacts and were substantially more accurate than images reconstructed with filtered backprojection without compensation for attenuation. Several techniques were employed to reduce the time required for the iterative routine to converge and to reduce the sensitivity of the solution to noise in the projection data. These included: (1) a preconditioning image variable transformation; (2) a coarse-to-fine grid initialization schedule; and (3) a convex hull image mask determined directly from the data. The combined effect of these techniques substantially reduced the compute time required for the reconstruction.     

Sparse angular CT reconstruction using non-local means based iterative-correction POCS

In divergent-beam computed tomography (CT), sparse angular sampling frequently leads to conspicuous streak artifacts. In this paper, we propose a novel non-local means (NL-means) based iterative-correction projection onto convex sets (POCS) algorithm, named as NLMIC-POCS, for effective and robust sparse angular CT reconstruction. The motivation for using NLMIC-POCS is that NL-means filtered image can produce an acceptable priori solution for sequential POCS iterative reconstruction. The NLMIC-POCS algorithm has been tested on simulated and real phantom data. The experimental results show that the presented NLMIC-POCS algorithm can significantly improve the image quality of the sparse angular CT reconstruction in suppressing streak artifacts and preserving the edges of the image.     

Optimal noise control in and fast reconstruction of fan-beam computed tomography image.

We proposed a linear approach that exploits statistically complementary information inherent in the projection data of fan-beam computed tomography (CT) for achieving a bias-free image-variance reduction in fan-beam CT. This linear approach leads to the development of infinite classes of hybrid algorithms for image reconstruction in fan-beam CT. These hybrid algorithms are computationally more efficient and numerically less susceptible to data noise and to the effect of finite sampling than the conventional fan-beam filtered back-projection (FFBP) algorithm. We also developed infinite classes of generalized fan-beam filtered back-projection (GFFBP) algorithms, which include the conventional FFBP algorithm as a special member. We demonstrated theoretically and quantitatively that the hybrid and GFFBP algorithms are identical (or different) in the absence (or presence) of data noise and of the effect of finite sampling. More importantly, we identified the statistically optimal hybrid algorithm that may have potentially significant implication to image reconstruction in fan-beam CT. Extensive numerical results of computer-simulation studies validated our theoretical results.