Multiple helical scans and the reconstruction of over FOV-sized objects in cone-beam CT

In cone-beam computed tomography （CBCT）, there are often cases where the size of the specimen is larger than the field of view （FOV） （referred to as over FOV-sized （OFS））. To acquire the complete projection data for OFS objects, some scan modes have been developed for long objects and short but over-wide objects. However, these modes still cannot meet the requirements for both longitudinally long and transversely wide objects. In this paper, we propose a multiple helical scan mode and a corresponding reconstruction algorithm for both longitudinally long and transversely wide objects. The simulation results show that our model can deal with the problem and that the results are acceptable, while the OFS object is twice as long compared with the FOV in the same latitude.

HYBRID REGULARIZED CONE-BEAM RECONSTRUCTION FOR AXIALLY SYMMETRIC OBJECT TOMOGRAPHY

In this paper,we consider 3 D tomographic reconstruction for axially symmetric objects from a single radiograph formed by cone-beam X-rays.All contemporary density reconstruction methods in high-energy X-ray radiography are based on the assumption that the cone beam can be treated as fan beams located at parallel planes perpendicular to the symmetric axis,so that the density of the whole object can be recovered layer by layer.Considering the relationship between different layers,we undertake the cone-beam global reconstruction to solve the ambiguity effect at the material interfaces of the reconstruction results.In view of the anisotropy of classical discrete total variations,a new discretization of total variation which yields sharp edges and has better isotropy is introduced in our reconstruction model.Furthermore,considering that the object density consists of continually changing parts and jumps,a high-order regularization term is introduced.The final hybrid regularization model is solved using the alternating proximal gradient method,which was recently applied in image processing.Density reconstruction results are presented for simulated radiographs,which shows that the proposed method has led to an improvement in terms of the preservation of edge location.

Advanced 4-dimensional cone-beam computed tomography reconstruction by combining motion estimation, motioncompensated reconstruction, biomechanical modeling and deep learning

4-Dimensional cone-beam computed tomography(4D-CBCT)offers several key advantages over conventional 3DCBCT in moving target localization/delineation,structure de-blurring,target motion tracking,treatment dose accumulation and adaptive radiation therapy.However,the use of the 4D-CBCT in current radiation therapy practices has been limited,mostly due to its sub-optimal image quality from limited angular sampling of conebeam projections.In this study,we summarized the recent developments of 4D-CBCT reconstruction techniques for image quality improvement,and introduced our developments of a new 4D-CBCT reconstruction technique which features simultaneous motion estimation and image reconstruction(SMEIR).Based on the original SMEIR scheme,biomechanical modeling-guided SMEIR(SMEIR-Bio)was introduced to further improve the reconstruction accuracy of fine details in lung 4D-CBCTs.To improve the efficiency of reconstruction,we recently developed a U-net-based deformation-vector-field(DVF)optimization technique to leverage a population-based deep learning scheme to improve the accuracy of intra-lung DVFs(SMEIR-Unet),without explicit biomechanical modeling.Details of each of the SMEIR,SMEIR-Bio and SMEIR-Unet techniques were included in this study,along with the corresponding results comparing the reconstruction accuracy in terms of CBCT images and the DVFs.We also discussed the application prospects of the SMEIR-type techniques in image-guided radiation therapy and adaptive radiation therapy,and presented potential schemes on future developments to achieve faster and more accurate 4D-CBCT imaging.

拟平移不变信号的自适应采样与重构

在采样问题的研究过程中,时间编码器作为易实现且高效的采样装置被提出。为达到高效重构信号的目的,基于时间编码器对拟平移不变空间中信号的自适应采样与重构展开研究,重点研究拟平移不变信号的重构算法。首先,基于“Crossing”和“Integrate-and-Fire”时间编码器,分别考虑了2种自适应采样方式,并建立了相应的精确重构算法。其次,讨论了一种可以产生有限自适应平均样本的IF采样器,并给出了基于样本的近似重构算法。结果表明,当时间编码器的稠密度满足一定衰减性时,建立的精确重构算法具有指数收敛性;当生成元满足一定衰减性时,近似重构算法的收敛速度线性依赖于IF采样器的阈值参数。

Accuracy Assessment of Three-dimensional Surface Reconstructions of In vivo Teeth from Cone-beam Computed Tomography

Background： The accuracy of three-dimensional （3 D） reconstructions from cone-beam computed tomography （CBCT） has been particularly important in dentistry, which will affect the effectiveness of diagnosis, treatment plan, and outcome in clinical practice. The aims of this study were to assess the linear, volumetric, and geometric accuracy of 3 D reconstructions from CBCT and to investigate the influence of voxel size and CBCT system on the reconstructions results. Methods： Fifty teeth from 18 orthodontic patients were assigned to three groups as NewTom VG 0.15 mm group （NewTom VG; voxel size： 0.15 ram; n = 17）, NewTom VG 0.30 mm group （NewTom VG; voxel size： 0.30 ram; n - 16）, and VATECH DCTPRO 0.30 mm group （VATECH DCTPRO; voxel size： 0.30 ram; n = 17）. The 3D reconstruction models of the teeth were segmented from CBCT data manually using Mimics 18.0 （Materialise Dental, Leuven, Belgium）, and the extracted teeth were scanned by 3Shape optical scanner （3Shape A/S, Denmark）. Linear and volumetric deviations were separately assessed by comparing the length and volume of the 3D reconstruction model with physical measurement by paired t-test Geometric deviations were assessed by the root mean square value of the imposed 3D reconstruction and optical models by one-sample t-test. To assess the influence of voxel size and CBCT system on 3D reconstruction, analysis of variance （ANOVA） was used （α = 0.05）. Results： The linear, volumetric, and geometric deviations were -0.03±0.48 mm, -5.4 ±2.8%, and 0.117 ± 0.018 mm for NewTom VG 0.15 mm group; -0.45 ± 0.42 mm, -4.5±3.4%, and 0.116 ＋ 0.014 mm for NewTom VG 0.30 mm group; and 0.93 ± 0.40 ram, 4.8 ± 5.1%, and 0.194 ± 0.117 mm for VATECH DCTPRO 0.30 mm group, respectively. There were statistically significant differences between groups in terms of linear measurement （P 〈 0.001）, but no significant difference in terms of volumetric measurement （P 0.774）. No statistically significant difference were found on geometric measurement between NewTom VG 0.15 mm and NewTom VG 0.30 mm groups （P - 0.999） while a significant difference was found between VATECH DCTPRO 0.30 mm and NewTom VG 0.30 mm groups （P = 0.006）. Conclusions： The 3D reconstruction from CBCT data can achieve a high linear, volumetric, and geometric accuracy. Increasing voxel resolution from 0.30 to 0.15 mm does not result in increased accuracy of 3D tooth reconstruction while different systems can affect the accuracy.

A robust compact least-squares reconstruction method for compressible turbulent flow simulations of complex configurations

For the second-order finite volume method,implicit schemes and reconstruction methods are two main algorithms which influence the robustness and efficiency of the numerical simulations of compressible turbulent flows.In this paper,a compact least-squares reconstruction method is proposed to calculate the gradients for the distribution of flow field variables approximation.The compactness of the new reconstruction method is reflected in the gradient calculation process.The geometries of the face-neighboring elements are no longer utilized,and the weighted average values at the centroid of the interfaces are used to calculate the gradients instead of the values at the centroid of the face-neighboring elements.Meanwhile,an exact Jacobian solving strategy is developed for implicit temporal discretization.The accurate processing of Jacobian matrix can extensively improve the invertibility of the Jacobian matrix and avoid introducing extra numerical errors.In addition,a modified Venkatakrishnan limiter is applied to deal with the shock which may appear in transonic flows and the applicability of the mentioned methods is enhanced further.The combination of the proposed methods makes the numerical simulations of turbulent flow converge rapidly and steadily with an adaptive increasing CFL number.The numerical results of several benchmarks indicate that the proposed methods perform well in terms of robustness,efficiency and accuracy,and have good application potential in turbulent flow simulations of complex configurations.

锥束CT 精确重建算法研究最新进展(英文)

第八届三维图像重建及核医疗学国际会议于2005年7月在美国盐湖城召开。该会议是在CT、PET 及SPECT 图像重建领域最负盛名的会议之一。本文主要介绍在本次会议上提出的几种最新锥束CT 精确重建算法,包括MD-FBP 算法、R-line 算法等;还讨论了这两种精确锥束重建算法的各自优点,并对CT图像重建领域下一步的研究方向做了展望。

现代结构系统可靠性评估理论研究进展

简要介绍大型结构系统可靠性评估理论的发展概况，对各个时期有代表性的估算方法进行了比较系统的分析，重点阐述了复杂系统可靠性精确评估理论的整体框架，对近年来兴起的随机有限元法，直接ＭｏｎｔｅＣａｒｌｏ模拟法，响应面法和广义失效面及广义母体分布重构方法的应用前景作了粗略的预测。

单圆轨道扫描的Grangeat改进算法

为减少ICT(Industrial Computed Tomography)重建图像的伪影,研究了Grangeat精确重建算法的原理.基于平板探测器依次通过对投影数据沿行、列一阶偏微分,求Radon导以及两步反投影等步骤仿真实现了Grangeat算法.针对单圆轨道锥束扫描投影数据的不完备性,分析了Radon空间的数据缺失情况,提出了通过近邻插值、线性插值、二次三点插值和一元线性回归插值的方法对Radon空间缺失数据进行插值改进该算法的几种途径.计算机仿真结果证明了改进算法的正确性和可行性.

一般直线-圆弧扫描轨迹CT的三维精确重建

近年,C-arm CT扫描系统的推出,给实际的扫描轨迹提供了很大的灵活性,比如圆弧-圆弧及直线-圆弧等扫描轨迹,针对于不同的成像及放疗部位,它们将具有很好的应用前景.最近提出的BPF算法不但可以处理投影数据的纵向截断问题,而且也可以处理投影数据的横向截断问题,这使得精确的局部重建成为可能.为了更好的进行局部重建,本文提出了一种一般形式的直线-圆弧扫描轨迹,并且针对这种扫描轨迹,基于上述的BPF算法,提出了一个通用的三维精确重建算法,它可以适用于直线和圆弧所在平面成任意角度的成像系统.这将更进一步的接近真正意义上的局部重建.文章最后的实验结果也证明了我们算法的有效性.

基于 Radon 变换导数的精确三维 CT 图像重建理论与改进方法介绍

本文主要介绍了一种基于Ｒａｄｏｎ变换导数的精确３Ｄ－ＣＴ图像重建的改进方法，它的计算复杂性和成像质量与传统方法相比有很大改善。

三维锥束精确重建的完全条件和源点轨迹

本文从理论上分析推导了３Ｄ锥束精确重建的完全条件，并结合一些具体的源点轨迹进行了讨论。

一种指数型动态螺距螺旋轨道的三维精确重建

寻求新型轨道,以适应形态各异的感兴趣区域的三维精确重建。基于动态螺距螺旋轨道精确重建的充分条件,提出一种指数型动态螺距的螺旋轨道。理论分析表明,该新型轨道完全满足这一充分条件,可实现锥形束CT的精确重建,并具有在物理上易于实现的优点及易于变形的特性。仿真1采用多个参数下的指数型螺旋轨道与标准螺旋轨道,对钟表模型实现重建;仿真2采用线性组合的指数型螺旋轨道与标准螺旋轨道,对简化的腹部模型感兴趣区域进行实验重建。对钟表模型的重建,各参数下的指数型螺旋轨道与标准螺旋轨道具有相似的重建效果;对简化的腹部模型中感兴趣区域重建,指数型螺旋轨道可同时降低整体与局部剂量,与标准螺旋轨道的重建结果相似。指数型动态螺距螺旋轨道可依据感兴趣区域的形态而灵活变换形态,并实现精确重建,可较好地逼近多种扫描轨道,且重建的效果相似。

基于改进核范数的2DPCA人脸识别算法研究

传统的二维主成分分析法广泛应用于图像特征提取,为了使此算法更加有效,提出了一种结构化二维算法,即核范数2DPCA算法(N-2-DPCA)。该算法基于核范数重构误差准则,将核范数最优化问题转化为基于F范数的最优化问题,然后通过采用迭代方法寻找到最佳投影矩阵,最后运用最小欧氏距离规则识别出待识别人脸的身份。在此基础之上,将N-2-DPCA扩展到基于双边投影的算法(N-B2-DPCA),采用曲线搜索算法寻找到双边投影矩阵,继而进行识别。最后将提出的算法在FERET和Yale B人脸数据库中进行人脸识别评估,实验结果表明所提出的算法与L1-2DPCA相比,重建误差降低了2.19%,识别率提高了2.03%,性能更好。

一种适用于小视野的快速精确重建算法

分析了反投影滤波(BPF)精确重建算法在重建小视野图像重建中存在的速度缓慢问题,提出一种适用于小视野的快速精确重建算法.该算法通过多尺度采样概念重新定义PI平面数据,利用两次不同尺度PI平面数据的插值获得最终结果,避免了原始算法在PI平面的无用计算.实验结果表明,该算法能够在保证重建图像质量的前提下,大大降低PI平面的计算量,从而提高BPF精确重建算法的重建速度,有效地解决了精确重建算法在小视野重建中的无用计算问题,为精确重建算法应用于实际提供了条件.

融合通用形变模型信息的面部三维重建

基于图像对的立体重建是用于获取人脸三维信息的通用方法,但根据图像数据和重建算法所得到的三维重建结果存在各种误差,本文对通用形变模型进行改进并与三维立体重建融合以得到更精确的重建结果。首先使用Max-Margin对象检测算法来获取面部边界框,其中回归树集合法能直接从像素强度的稀疏子集识别面部特征点。然后通过PCA颜色模型生成形状和颜色的三维面部统计模型,利用ISOMAP算法将三维网格转换为二维表面并提取纹理信息,得到面部模型。最后在源网格上进行两步非刚性表面配准的变形过程:先通过对源网格进行二次采样来选择少量网格点来表示源的全局变化,并选取径向基函数(RBF)进行非刚性全局变形;再对源顶点进行Procrustes分析获得非刚性变换,再通过加权方案来进行k-近邻变换,得到平滑的局部变形。将单图像重建的面部模型,立体重建的面部模型和本文的面部变形模型与高质量扫描云图进行对齐比较,得到面部变形模型的3个RMS值分别为2.795 2,2.102 8和2.153 4,相比于其他模型,面部变形模型更接近高质量扫描云图,即与原图像一致性更高,误差更小。面部变形模型的定性和定量分析表明,立体重建与人脸一般形状信息的组合在几何信息的表达上优于基于通用模型的单个图像重建以及未考虑通用模型的立体重建。

螺旋锥束精确重建算法优化

精确重建算法是当今研究的热点,其反投影计算量很大。因此以Katsevich算法为例,将PI线二分法求端点转变为一元函数的迭代求根进行优化;利用锥束扫描模式存在的对称性和正、余弦的特性,减少计算投影位置的次数。实验表明,优化效果较好。

全局方向模板对非结构有限体积梯度与高阶导数重构的影响

模板选择方式对非结构有限体积方法的计算准确性会产生显著影响.在之前的工作中,基于局部方向模板存在的问题,我们探索了一种更加简单有效的全局方向模板选择方法,并将其应用于二阶精度非结构有限体积求解器.基于该方法找到的模板单元均沿着壁面法向与流向,可有效捕捉流场变化,反映流动的各向异性,并且模板选择过程脱离了对网格拓扑的依赖,避免了局部方向模板选择方法中复杂的阵面推进与方向判断过程,克服了在大压缩比三角形网格上模板单元偏离壁面法向的现象,同时在二阶精度求解器上得到了较高的计算精度与计算准确性.为了进一步验证全局方向模板在高阶精度非结构有限体积方法中应用的可行性,本文初步测试了该模板对变量梯度及高阶导数重构的影响.经检验,在不同类型的网格上,采用全局方向模板得到的变量梯度与高阶导数误差明显低于局部方向模板,同时也低于共点模板的计算误差.此外,在高斯积分点处由全局方向模板得到的变量点值与导数误差同样在三种模板中最低.因此该模板选择方法在非结构有限体积梯度与高阶导数重构方面具有较好的数值表现,具备在高阶精度非结构有限体积求解器中应用并推广的可行性.

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。

三源螺旋CT精确重建算法在动态重建中的应用

目的将三源CT精确重建算法应用到动态重建的场合,用于抑制心脏或呼吸运动在CT成像中产生的伪迹。方法采用单源与三源CT精确重建算法对两种动态数字模型进行仿真重建。通过比较三源CT与单源CT的重建结果,评价三源精确重建算法在动态重建方面的性能。结果三源CT的扫描方式能够有效抑制动态重建中的伪迹,改善重建质量。结论本文的结果证实了多源CT精确重建算法在动态重建中的有效性,对于进一步改善心脏成像及小动物成像质量具有积极的实际意义。