【作者】 陈毅；

【导师】 赵俊；

【作者基本信息】 上海交通大学 ， 生物医学工程， 2017， 博士

【摘要】 计算机断层成像(CT)是医学成像中常用的影像诊断技术。近年来,小型化CT射线源技术的不断发展让静止式CT的可行性和经济性逐渐明朗。所谓的静止式CT,是由多个分布式X射线源组成的CT结构,无需旋转就能采集到不同角度的投影数据。静止式CT有一些潜在的优点:首先,它能消除源或探测器在扫描过程中高速旋转带来的副作用,包括机械结构不稳定带来的图像质量下降以及采集投影过程中射线源移动引起的投影数据不匹配。其次,分布式源的结构使得不需要机械旋转就能采集多个角度的投影,因此能提高成像速度和时间分辨率。再次,由于角度上的投影角度不仅限于按顺序采集,这允许我们通过设计不同的采样序列来进一步提高图像质量。本研究主要围绕静止式CT成像的一些关键问题,包括重建算法、动态成像、去散射等。本文分为以下三个部分来总结这些研究成果。(1)针对多源螺旋静止式CT,本文提出了它的Katsevich类型直接重建算法,该算法能减少重建过程中的中间步骤,节省计算量和提高重建精度。此外,针对螺旋静止式CT中数据缺失问题,我们基于精确重建理论,提出一种移动扫描法,使得图像重建所需要的最小检测窗能避开数据缺失区域,从而保证数据的完备性。结果表明所提出的移动扫描法能解决数据缺失问题。(2)动态成像中,已有的算法多数是通过缩短时间窗的方式来提高时间分辨率,而缩短时间窗意味着可用的投影数据变少,可能会降低图像质量。为了解决这个问题,本文提出了时间加权动态重建算法(TWIR),该算法没有将投影数据截断,而是根据待重建时刻和投影采集时刻之间相距的远近来对投影分配以不同的权重,从而让可用的数据更多。实验结果表明,TWIR算法能提高动态成像的图像质量。此外,针对多源静止式CT本身,本文提出了投影的间隔采集方式。多源静止式CT中,由于射线源是分布式的,因此,我们可以让各个射线源以间隔的方式先后开启,从而能在有限的时间窗内让投影尽可能均匀地分布于各个角度。与顺序扫描方式相比,虽然单位时间内采集到的投影数不变,但间隔扫描方式采集到的数据之间冗余度更低,因此,可以潜在地提高每个时间窗内重建图像的质量。实验结果表明,静止式CT的这种间隔采集方式能大幅提高动态成像中每个时刻的图像质量。(3)对于去散射问题:由于传统的防散滤线栅无法在静止式CT中使用。为解决这个问题,我们提出了一种基于射束调制器的散射估计法。该方法在不阻挡直射射线的前提下,能估算出散射大小,从而能够在重建之前将其减去,以消除散射的影响。通过模拟实验和实际实验,我们验证了该算法,并取得了优于现有算法的结果。更多还原

【Abstract】 Computed tomography is one of the most commonly used imaging modality for diagnosis.Thanks to the development of small-sized X-ray tube during the past few years,the feasibility of stationary CT is increased in terms of technical aspects and cost.Stationary CT is composed of multiple X-ray sources which are distributed at different positions to obtain projection data from different angles without the rotation of gantry.Stationary CT possesses several potential benefits.Firstly,it avoids the image degradation caused by mechanical instability and enlarged X-ray focal spots along the motion direction.Secondly,the temporal resolution can be improved because the projection data from different angles can be obtained in short time without rotation of the gantry.Thirdly,the sources can be individually controlled,which allows to design various acquisition sequential for further improve the image quality.This research focuses on the imaging methods of stationary CT,including reconstruction algorithm,dynamic imaging and scatter correction.The main contributions are introduced as follows.(1)A Katsevich-type reconstruction algorithm was proposed for the helical stationary CT.The proposed algorithm is advantageous in terms of reconstruction accuracy and efficiency because data rebinning for ring detector is not required.To solve the data truncation problem in stationary CT,a moving scan method was proposed.It is a method that moves the table or gantry during scanning,thus reshaping the effective source trajectory so that a sufficient dataset can be obtained.(2)Existing reconstruction algorithms for time-resolved CT are mainly accomplished by shorten the time interval of scanning,while shorter time interval sometimes means less data for reconstruction and reduces image quality.We have proposed a temporal-weighting iterative reconstruction(TWIR)algorithm to solve this problem.The algorithm doesn’t truncate the projection data,but assigns different weight to each projection data according to the reliability of projection data in temporal dimension.TWIR algorithm enables more data for reconstruction.Results suggest that this algorithm improves the image quality as well as temporal resolution in time-resolved imaging.For the stationary CT,a interlaced acquisition sequence was proposed.As the sources in stationary CT can be individually controlled,the acquisition sequence can be interlaced instead of sequential to make the projections data less redundant in short temporal window.Thus the image quality and temporal resolution can be improved.Results show that this interlaced acquisition sequential improves the image quality for each time frame.(3)Scatter reduces the image quality in computed tomography(CT).Anti-scatter-grid cannot be used in stationary CT.In this study,an optimization-based scatter estimation algorithm(OSE)is proposed to estimate and correct scatter.In the concept of primary modulation,the primary is modulated,but the scatter remains smooth by inserting a modulator between the X-ray source and the object.In the proposed algorithm,an objective function is designed for separating the scatter and primary.Prior knowledge is incorporated in the optimization-based framework to improve the accuracy of the estimation:(1)the primary is always positive;(2)the primary is locally smooth and the scatter is smooth;(3)location of penumbra can be determined;and(4)scatter-contaminated data provide knowledge about which part is smooth.Result shows that the proposed OSE algorithm improves the robustness and accuracy in scatter estimation and correction.This method is promising for scatter correction of various kinds of X-ray imaging modalities,such as X-ray radiography,cone beam CT,and the fourth-generation CT.更多还原