三维锥形束CT成像FDK重建算法发展综述

在各种三维锥束重建算法中,近似算法由于数学形式上简单,实现起来容易,而且在锥角比较小的情况下,能够取得较好的重建效果,所以在实际中有着广泛的应用。在各种基于滤波反投影的近似算法中,FDK类型的算法一直是实际应用中的主流。鉴于FDK方法巨大的实用价值,本文对它的历史和目前的发展,包括标准FDK、推广到任意轨道的G-FDK、以及着眼于提高重建速度或重建精度的其它FDK衍生算法等作了一个回顾和总结,并对实际应用中FDK存在的问题作了进一步的讨论分析,最后对FDK算法理论的发展进行了展望。     

新的超分辨CT扫描模式和图像重建方法

针对通常工业CT转台特点,提出了一种新的基于扇束等距采样的超分辨CT扫描模式和相应的重建算法。分别向左和右偏移CT旋转中心1/4探测器宽度,得到的两组相叠采样的投影数据。通过求解一组具有双对角矩阵的线性方程组,得到高分辨的CT虚拟投影数据,进而由扇束滤波反投影算法重建出高分辨的CT图像。数值模拟结果验证了本文方法的有效性。     

半像素错位技术在三维CT重建中的应用

本文介绍了一种基于半像素错位的多幅锥束投影数据重建高分辨率投影数据的技术,利用在水平和垂直方向上进行半像素错位的方法提高了锥束投影数据的分辨率,并给出了具体的实现方法,同时对重建后的投影数据用FDK方法进行重建,实验结果表明,重建出来的三维物体的质量在水平和垂直方向都获得了的提高,而且还有抑制噪声的效果。     

满三维图像重建的卷积反投影算法

本文用构造δ序列的方法,给出了一种n维Radon变换反演的卷积反投影算法,证明了该算法在图像的连续点收敛于原图像,当n=2时,就是广泛用于图像重建的卷积反投影算法,由此也给出了卷积反投影方法收敛性的另一种推导。当n=3时,是新的满三维重建的卷积反投影算法,由于三维反演公式具有局部性,该算法对研究三维局部重建有启示。并基于三维情形,利用模拟数据进行了数值仿真,说明该算法是一种有效的三维图像重建算法。     

转台一次偏置扫描的ICT重建算法

目的工业CT的检测对象大小不一,在探测器尺寸一定的情况下,工程上往往通过设计不同的扫描方式以及所对应的重建算法来获得更大的成像范围,比如转台偏置、探测器偏置等扫描模式。本文提出一种用于转台一次偏置扫描模式下的重建算法。该算法首先用Parker类型函数对重复采样的投影数据进行加权,然后用扇束重建算法得到断层图像。实验结果证明了这种方法的有效性和优越性。     

基于光子计数器能谱响应优化的能谱CT造影剂K边缘分解方法

受到光子计数探测器(PCD)电荷共享、光子逃逸以及散射等效应的影响，在K边缘附近的入射能谱在探测器下会产生失真，降低了能谱CT的重建和材料分解性能。为此，提出了基于能谱恢复优化的投影域K-edge材料分解算法。该方法由两个步骤组成：首先，对探测器的能量响应函数(Energy Response Function, ERF)进行建模，通过MLEM方法恢复入射能谱；然后，基于恢复的投影数据进行能谱CT的K边缘重建和材料分解，得到造影像和组织像的正确分布。碘造影剂的仿真实验表明，能谱恢复较好地实现了造影剂与组织的图像分离，可以有效地提高K边缘重建图像中碘造影剂信号，提升能谱CT的造影剂分辨能力。     

CT图像中金属伪影的快速校正

CT系统由于检测对象含有金属夹杂而使得重建图像中出现金属伪影,影响图像的质量。针对金属伪影,提出一种快速而稳定的校正方法。首先判断金属投影区域,然后对金属投影进行插值,然后重建图像。最后,在重建的图像中插入金属部分,得到完整的重建图像。数值模拟和实际CT系统的实验表明,该方法能有效的消除金属伪影,并能保留金属信息,恢复图像的完整。该算法计算复杂度很小,计算快速,具有较高的实用价值。     

基于不可分离MRA的小波图像重建

本文提出了一种基于不可分离MRA的小波重建算法。算法将投影数据进行二通道的小波分解，在分解抽样中，使用不同的抽样方法。算法结果，相比通常所使用的基于可分离MRA的小波重建算法基本相同的局部重建特性，重建质量和运算速度都优于后者。     

序列图像缩放系数的一种实时估计算法

照相机在靠近或远离目标景物的运动过程中，如保持聚焦中心点不变，则前后两帧图像间存在一致的缩放关系。此时，可根据尺度变化下图像灰度累积投影曲线的特点，分别计算参考帧和当前帧的投影曲线上一对极大一极小值点的坐标差值，以它们的比值作为缩放系数，实现实时求取。最后以实例验证了本算法。     

CT旋转中心的精确确定方法

CT旋转中心（COR）的精确确定是图像准确重建的一个前提条件,COR的偏差会导致重建图像出现伪影,降低成像质量,本文对现有的COR确定方法进行了分析研究,在此基础上,利用中心投影具有π分割数据一致性的特点,提出了一种利用投影原始数据精确确定CT投影中心的方法。本方法无需专用校正模体,不需知道CT测量的几何参数,不需要全周扫描数据,对于射线源与旋转中心的连线不垂直于探测器的情况同样适用。实验结果证明,本方法适用范围广,抗随机噪声能力强,具有很好的应用前景。     

An adaptive approach to metal artifact reduction in helical computed tomography for radiation therapy treatment planning: experimental and clinical studies

PURPOSE: In this article, an approach to metal artifact reduction is proposed that is practical for clinical use in radiation therapy. It is based on a new interpolation scheme of the projections associated with metal implants in helical computed tomography (CT) scanners. METHODS AND MATERIALS: A three-step approach was developed consisting of an automatic algorithm for metal implant detection, a correction algorithm for helical projections, and a new, efficient algorithm for projection interpolation. The modified raw projection data are transferred back to the CT scanner device where CT slices are regenerated using the built-in reconstruction operator. The algorithm was tested on a CT calibration phantom in which the density of inserted objects are known and on clinical prostate cases with two hip prostheses. The results are evaluated using the CT number and shape of the objects. RESULTS: The validations on a CT calibration phantom with various inserts of known densities show that the algorithm improved the overall image quality by restoring the shape and the representative CT number of the objects in the image. For the clinical hip replacement cases, a large fraction of the bladder, rectum, and prostate that were not visible on the original CT slices were recovered using the algorithm. Precise contouring of the target volume was thus feasible. Without this enhancement, physicians would have drawn bigger margins to be sure to include the target and, at the same time, could have prescribed a lower dose to keep the same level of normal tissue toxicity. CONCLUSIONS: In both phantom experiment and patient studies, the algorithm resulted in significant artifact reduction with increases in the reliability of planning procedure for the case of metallic hip prostheses. This algorithm is now clinically used as a preprocessing before treatment planning for metal artifact reduction.     

Incorporation of local dependent reliability information into the Prior Image Constrained Compressed Sensing (PICCS) reconstruction algorithm

PurposeThe reduction of dose in cone beam computer tomography (CBCT) arises from the decrease of the tube current for each projection as well as from the reduction of the number of projections. In order to maintain good image quality, sophisticated image reconstruction techniques are required. The Prior Image Constrained Compressed Sensing (PICCS) incorporates prior images into the reconstruction algorithm and outperforms the widespread used Feldkamp-Davis-Kress-algorithm (FDK) when the number of projections is reduced. However, prior images that contain major variations are not appropriately considered so far in PICCS. We therefore propose the partial-PICCS (pPICCS) algorithm. This framework is a problem-specific extension of PICCS and enables the incorporation of the reliability of the prior images additionally.Material and MethodsWe assumed that the prior images are composed of areas with large and small deviations. Accordingly, a weighting matrix considered the assigned areas in the objective function. We applied our algorithm to the problem of image reconstruction from few views by simulations with a computer phantom as well as on clinical CBCT projections from a head-and-neck case. All prior images contained large local variations. The reconstructed images were compared to the reconstruction results by the FDK-algorithm, by Compressed Sensing (CS) and by PICCS. To show the gain of image quality we compared image details with the reference image and used quantitative metrics (root-mean-square error (RMSE), contrast-to-noise-ratio (CNR)).ResultsThe pPICCS reconstruction framework yield images with substantially improved quality even when the number of projections was very small. The images contained less streaking, blurring and inaccurately reconstructed structures compared to the images reconstructed by FDK, CS and conventional PICCS. The increased image quality is also reflected in large RMSE differences.ConclusionsWe proposed a modification of the original PICCS algorithm. The pPICCS algorithm incorporates prior images as well as information about location dependent uncertainties of the prior images into the algorithm. The computer phantom and experimental data studies indicate the potential to lowering the radiation dose to the patient due to imaging while maintaining good image quality.     

一种改进的光线投射方法

光线投射算法是体绘制中的经典算法,但其绘制速度较慢。本文对传统的光线投射算法中等间距重采样进行了改进,引入包围盒方法,采用变步长的采样方法,减少冗余数据量和投射光线数量,优化重采样过程,提高采样效率。医学图像可视化实验表明,改进方法能够在保证图像质量的同时,提高绘制速度。     

三维形体任意剖面轮廓线的提取方法

提出了一种快速提取由多个断层轮廓线组成的三维形体任意方向剖面轮廓线的方法。该方法首先对这些断层轮廓线进行插值、排序和旋转处理,然后用虚拟平面从任意给定方向和位置剖切由这些轮廓线构成的三维形体,根据切面与各个断层轮廓线的交点,直接提取出切面轮廓线,避免了三维重建运算和大量的求交运算,加快了处理速度,实现了三维形体中任意剖面轮廓线的快速投影显示。实验证明,该方法快速、有效。     

基于光线投射的锥束CT正投影算法研究

计算机断层成像（computed tomography,CT）作为一种现代成像技术已经被广泛应用于医学诊断、工业无损检测等领域。在研究CT重建算法的过程中,正投影模拟数据是必不可少的一部分。本文提出一种基于光线投射算法的圆轨迹锥束扫描模式下的正投影算法,并结合CUDA（compute unified device architecture,统一计算设备架构）技术,实现了GPU（graphics processing unit,图形处理器）加速正投影计算。通过与传统的正投影算法相比,本文算法在投影图像质量上有一定的提高,并具有更高的计算效率。