PNAS：我国X射线相位衬度成像研究获重大突破 医疗CT技术有望实现新飞跃

中国科学技术大学研究员吴自玉领导的北京同步辐射装置和合肥国家同步辐射实验室联合成像科研小组,在X射线相位衬度成像研究领域取得重大突破,其研究成果克服了医学X射线CT技术应用X射线相位衬度成像方法的障碍,为形成更加快速、灵敏度更高、更安全的X射线相位CT技术奠定了基础。专家预测,这项新技术的诞生。将催生新型X射线相位CT产业。     

硬X射线相位衬度成像的实验研究

介绍了硬X射线(类同轴)相位衬度成像的工作原理及其实验研究结果。X射线波长为0．08860nm,样品为未经任何处理的飞蛾，记录介质为X射线胶片。胶片经处理以后，用光学显微镜读出，可以看出样品的许多细节,尤其在折射率突变处。而同样条件下基于吸收衬度机制的硬X射线吸收成像，由于是弱吸收样品．没有观察到任何图像。     

相位共轭反馈光波用于图像衬度控制

本文提出了一种利用双相位共轭反射镜的实时图像衬度控制方法。给出了理论分析,提出了一种实验方案,给出了模拟实验结果。     

硬X射线相位相衬成像研究进展

自从20世纪90年代中叶以来,硬X射线相位相衬成像技术因其对弱吸收材料的极高灵敏度引起了国内外广泛的关注,医学及生物成像是相位相衬成像的主要目标.详细论述了各种相位相衬成像技术的研究进展情况.     

数字X射线图像处理及其应用

概述了数字Ｘ射线图像处理，并叙述了它在医学和工业探伤方面的应用。     

光栅相衬CT中吸收信号环形伪影的去除方法

提出一种光栅X射线相衬断层扫描中吸收信号环形伪影的去除方法。该方法采用正弦图域和重建图域结合的处理算法。对于弱伪影,通过正弦图域的排序和滤波去除。而对于强伪影,首先根据环形伪影在极坐标系的表现,计算残差图像,转换到笛卡尔坐标系得到伪影像素和样品的边界;进而使用基于机器学习的图像分割方法获取每一类样品的分布,同时为了保护边界信息,通过形态学操作获得样本的内部区域;最后再利用残差图像的分布特征定位伪影像素,并使用临近非伪影像素均值替代。实验结果表明该方法可以在不破坏样品边界的前提下有效地去除图像中的环形伪影。     

衍射增强成像折射衬度的应用

折射衬度是衍射增强成像(diffraction enhanced imaging,DEI)中的一种重要衬度,在弱吸收物质的成像中,折射衬度远超过吸收衬度.折射衬度应用的关键是提取出样品的折射信息,折射信息由相应的折射图像表示.根据生物样品和材料样品的特点,研究和比较了两种折射信息提取方法.利用样品的折射图像,对聚苯乙烯样品进行了简单的定量分析,比较了正常肝组织和肝血管瘤组织.实验结果表明,折射衬度清楚地描述了肝组织的微细结构和血管瘤损伤,精确地显示了无法在传统X射线成像技术中获得的聚苯乙烯样品的清晰结构.     

相敏X射线成像

自从伦琴发现X射线100多年以来,X射线成像基本原理和射线照相术的解释基本上没有什么变化。普通方法依赖于X射线吸收,它是反差的唯一来源,也是唯一采用射线或几何光学来描述并解释像的形成。该法忽略另一个更有用的潜在反差来源──相位信息。相敏技术,能用波动光学而不是射线光学来理解它,结合相位信息提供扩大和补充标准的吸收反差法。     

Dynamic X-ray computed tomography

Dynamic computed tomography (CT) imaging aims at reconstructing image sequences where the dynamic nature of the living human body is of primary interest. The main applications concerned are image-guided interventional procedures, functional studies and cardiac imaging. The introduction of ultra-fast rotating gantries along with multi-row detectors and in near future area detectors allows huge progress toward the imaging of moving organs with low-contrast resolution. This paper gives an overview of the different concepts used in dynamic CT. A new reconstruction algorithm based on a voxel-specific dynamic evolution compensation is also presented. It provides four-dimensional image sequences with accurate spatio-temporal information, where each frame is reconstructed using a long-scan acquisition mode on several half-turns. In the same time, this technique permits to reduce the dose delivered per rotation while keeping the same signal to noise ratio for every frame using an adaptive motion-compensated temporal averaging. Results are illustrated on simulated data.     

X-ray computed tomography for medical imaging

The article looks at reconstruction in 2-D and 3-D tomography. We have not dealt with some of the issues in reconstruction such as sampling and aliasing artifacts, finite detector aperture artifacts, beam hardening artifacts, etc., in greater detail since these are beyond the scope of an introductory tutorial. We examine the physical and mathematical concepts of the Radon (1917) transform, and the basic parallel beam reconstruction algorithms are discussed. We also develop the algorithms for fan-beam CT, and discuss the mathematical principles of cone-beam CT     

Developments with maximum likelihood X-ray computed tomography

An approach to the maximum-likelihood estimation of attenuation coefficients in transmission tomography is presented as an extension of earlier theoretical work by K. Lange and R. Carson (J. Comput. Assist. Tomography, vol.8, p.306-16, 1984). The reconstruction algorithm is based on the expectation-maximization (EM) algorithm. Several simplifying approximations are introduced which make the maximization step of the algorithm available. Computer simulations are presented using noise-free and Poisson randomized projections. The images obtained with the EM-type method are compared to those reconstructed with the EM method of Lange and Carson and with filtered backprojection. Preliminary results show that there are potential advantages in using the maximum likelihood approaches in situations where a high-contrast object, such as bone, is embedded in low-contrast soft tissue.     

Statistical image reconstruction for polyenergetic X-ray computed tomography

This paper describes a statistical image reconstruction method for X-ray computed tomography (CT) that is based on a physical model that accounts for the polyenergetic X-ray source spectrum and the measurement nonlinearities caused by energy-dependent attenuation. We assume that the object consists of a given number of nonoverlapping materials, such as soft tissue and bone. The attenuation coefficient of each voxel is the product of its unknown density and a known energy-dependent mass attenuation coefficient. We formulate a penalized-likelihood function for this polyenergetic model and develop an ordered-subsets iterative algorithm for estimating the unknown densities in each voxel. The algorithm monotonically decreases the cost function at each iteration when one subset is used. Applying this method to simulated X-ray CT measurements of objects containing both bone and soft tissue yields images with significantly reduced beam hardening artifacts.     

Monotonic penalized-likelihood image reconstruction for X-ray fluorescence computed tomography

In this paper, we derive a monotonic penalized-likelihood algorithm for image reconstruction in X-ray fluorescence computed tomography (XFCT) when the attenuation maps at the energies of the fluorescence X-rays are unknown. In XFCT, a sample is irradiated with pencil beams of monochromatic synchrotron radiation that stimulate the emission of fluorescence X-rays from atoms of elements whose K- or L-edges lie below the energy of the stimulating beam. Scanning and rotating the object through the beam allows for acquisition of a tomographic dataset that can be used to reconstruct images of the distribution of the elements in question. XFCT is a stimulated emission tomography modality, and it is thus necessary to correct for attenuation of the incident and fluorescence photons. The attenuation map is, however, generally known only at the stimulating beam energy and not at the energies of the various fluorescence X-rays of interest. We have developed a penalized-likelihood image reconstruction strategy for this problem. The approach alternates between updating the distribution of a given element and updating the attenuation map for that element's fluorescence X-rays. The approach is guaranteed to increase the penalized likelihood at each iteration. Because the joint objective function is not necessarily concave, the approach may drive the solution to a local maximum. To encourage the algorithm to seek out a reasonable local maximum, we include in the objective function a prior that encourages a relationship, based on physical considerations, between the fluorescence attenuation map and the distribution of the element being reconstructed.     

超声层析成像检测系统的研究与实现

为获得更清晰的图像检测效果，在超声波波动传播原理的基础上，采用超声阵列检测方法设计超声层析成像（CT）系统。将射线追踪算法引入到层析成像中，基于SIRT重建算法，实现速度重建。通过数值仿真表明：层析成像效果得到明显改善。     

Sensitivity of photon-counting based K-edge imaging in X-ray computed tomography

The feasibility of K-edge imaging using energy-resolved, photon-counting transmission measurements in X-ray computed tomography (CT) has been demonstrated by simulations and experiments. The method is based on probing the discontinuities of the attenuation coefficient of heavy elements above and below the K-edge energy by using energy-sensitive, photon counting X-ray detectors. In this paper, we investigate the dependence of the sensitivity of K-edge imaging on the atomic number Z of the contrast material, on the object diameter D , on the spectral response of the X-ray detector and on the X-ray tube voltage. We assume a photon-counting detector equipped with six adjustable energy thresholds. Physical effects leading to a degradation of the energy resolution of the detector are taken into account using the concept of a spectral response function R(E,U) for which we assume four different models. As a validation of our analytical considerations and in order to investigate the influence of elliptically shaped phantoms, we provide CT simulations of an anthropomorphic Forbild-Abdomen phantom containing a gold-contrast agent. The dependence on the values of the energy thresholds is taken into account by optimizing the achievable signal-to-noise ratios (SNR) with respect to the threshold values. We find that for a given X-ray spectrum and object size the SNR in the heavy element's basis material image peaks for a certain atomic number Z. The dependence of the SNR in the high- Z basis-material image on the object diameter is the natural, exponential decrease with particularly deteriorating effects in the case where the attenuation from the object itself causes a total signal loss below the K-edge. The influence of the energy-response of the detector is very important. We observed that the optimal SNR values obtained with an ideal detector and with a CdTe pixel detector whose response, showing significant tailing, has been determined at a synchrotron differ by factors of about two to three. The potentially very important impact of scattered X-ray radiation and pulse pile-up occurring at high photon rates on the sensitivity of the technique is qualitatively discussed.     

High-speed three-dimensional X-ray computed tomography: The dynamic spatial reconstructor

Most X-ray CT scanners require a few seconds to produce a single two-dimensional (2-D) image of a cross section of the body. The accuracy of full three-dimensional (3-D) images of the body synthesized from a contiguous set of 2-D images produced by sequential CT scanning of adjacent body slices is limited by 1) slice-to-slice registration (positioning of patient); 2) slice thickness; and 3) motion, both voluntary and involuntary, which occurs during the total time required to scan all slices. Therefore, this method is inadequate for true dynamic 3-D imaging of moving organs like the heart, lungs, and circulation. To circumvent these problems, the Dynamic Spatial Reconstructor (DSR) was designed by the Biodynamics Research Unit at the Mayo Clinic to provide synchronous volume imaging, that is stop-action (1/100 s), high-repetition rate (up to 60/s), simultaneous scanning of many parallel thin cross sections (up to 240, each 0.45 mm thick, 0.9 mm apart) spanning the entire anatomic extent of the bodily organ(s)of interest. These capabilities are achieved by using multiple X-ray sources and multiple 2-D fluoroscopic video camera assemblies on a continually rotating gantry. Desired tradeoffs between temporal, spatial, and density resolution can be achieved by retrospective selection and processing of appropriate subsets of the total data recorded during a continuous DSR scan sequence.     

超声散射CT信号的特征提取

超声检测技术中,缺陷的定性分类这一技术难题至今尚未得以彻底解决。研究了希尔伯特-黄变换(HHT)和小波包变换(WPT)在超声散射CT特征提取中的应用。以荷兰试块作为测试对象,针对3个不同的检测面,采用HHT和WPT提取能量特征,采用基于距离的类别可分性判据对特征提取结果进行评价,起了定量衡量尺度的作用。实验结果表明:对于本实验中信号频率比较单一的超声散射CT信号,WPT提取的能量特征比HHT提取的能量特征更为有效。     

基于相衬CT的肝纤维化血管三维可视化研究

利用相衬CT分别获得了轻度和重度肝纤维化样品的血管三维结构,并对它们的血管内壁纹理参数进行了测量和分析,同时对重度肝纤维化样品中的血栓进行了测量和分析。实验结果表明,相衬CT清楚地展现了不同病变程度的肝纤维化血管三维微观结构,精确地显示了血管内壁和血栓的结构,在肝纤维化疾病辅助诊断中具有应用前景。