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。     

X射线相位衬度CT

X射线相位衬度CT指的是在通过X射线光源来对物体进行成像过程中使用图像的位相衬度来反映物体的密度或者厚度分布,适用于弱吸收物体,还可以减少吸收剂量,放宽成像条件中光源强度的限制和减少对样品(尤其是生物样品)的损伤.介绍了目前用于X射线相位衬度CT的三种方法,实验及图像重建的过程,并分析了各自的优缺点.     

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.     

Reproducibility of global and local reconstruction of three-dimensional micro-computed tomography of iliac crest biopsies

Variation in computed tomography (CT) image grayscale and spatial geometry due to specimen orientation, magnification, voxel size, differences in X-ray photon energy and limited field-of-view during the scan, were evaluated in repeated micro-CT scans of iliac crest biopsies and test phantoms. Using the micro-CT scanner on beamline X2B at the Brookhaven National Laboratory's National Synchrotron Light Source, 3-D micro-CT images were generated. They consisted of up to 1024 X 2400², 4-mum cubic voxels, each with 16-bit gray-scale. We also reconstructed the images at 16-, 32-, and 48-mum voxel resolution. Scan data were reconstructed from the complete profiles using filtered back-projection and from truncated profiles using profile-extension and with a Local reconstruction algorithm. Three biopsies and one bonelike test phantom were each rescanned at three different times at annual intervals. For the full-data-set reconstructions, the reproducibility of the estimates of mineral content of bone at mean bone opacity value, was plusmn28.8 mg/cm³ , i.e., 2.56%, in a 4-mum cubic voxel at the 95% confidence level. The reproducibility decreased with increased voxel size. The interscan difference in imaged bone volume ranged from 0.86 plusmn 0.64% at 4-mum voxel resolution, and 2.64 plusmn 2.48% at 48 mum.     

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.     

Wavelet-based reconstruction for limited-angle X-ray tomography

The aim of X-ray tomography is to reconstruct an unknown physical body from a collection of projection images. When the projection images are only available from a limited angle of view, the reconstruction problem is a severely ill-posed inverse problem. Statistical inversion allows stable solution of the limited-angle tomography problem by complementing the measurement data by a priori information. In this work, the unknown attenuation distribution inside the body is represented as a wavelet expansion, and a Besov space prior distribution together with positivity constraint is used. The wavelet expansion is thresholded before reconstruction to reduce the dimension of the computational problem. Feasibility of the method is demonstrated by numerical examples using in vitro data from mammography and dental radiology.     

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     

Extraction of the hepatic vasculature in rats using 3-D micro-CT images

High-resolution micro-computed tomography (CT) scanners now exist for imaging small animals. In particular, such a scanner can generate very large three-dimensional (3-D) digital images of the rat's hepatic vasculature. These images provide data on the overall structure and function of such complex vascular trees. Unfortunately, human operators have extreme difficulty in extracting the extensive vasculature contained in the images. Also, no suitable tree representation exists that permits straight-forward structural analysis and information retrieval. This work proposes an automatic procedure for extracting and representing such a vascular tree. The procedure is both computation and memory efficient and runs on current PCs. As the results demonstrate, the procedure faithfully follows human-defined measurements and provides far more information than can be defined interactively.     

X射线计算机断层扫描测定沉积物中水合物微观分布

建立了利用X射线计算机断层成像(X-CT,X-ray c omputed tomography)研究天然气水合物在沉积物空隙中填充过程和赋存状况的测试系统。 针对数据图像 中水和水合物灰度相近不易区分的难题,采用提高密度差和添加标定物的方法对其进行 改善。通过重建的二维和三维影像,可清晰地观察到自由气、沉积物、水和水合物 的微观分布状态。在自行研制的可控温高压反应装置内合成CH₄水合物,并对其生 长过程进行了跟踪扫描。反应过程前后图像数据的对比表明,水合物最初主要在沉 积物颗粒表面与气液界面处生长,随后逐渐占据大部分液相空间,最终连同所有的 沉积物颗粒胶结在一起。实验结果证明,高精度X-CT应用 于含水合物的沉积物样品内部结构测试以及水合物在沉积物空隙中的动力学机理研 究是可行且有效的。     

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.     

预混燃烧火焰的光偏折层析三维动态可视化

使用光偏折层析(CT)方法研究了非稳态预混燃烧流 场的参数测量及火焰结构表征。建立了适用于瞬态流场层 析的叠栅光偏折投影条纹图有序阵列采样系统,实现了单光栅副、单CCD、多方向、同时和 同光路条件的动 态采样。构建适用于少数投影光偏折计算层析的混合正则化重建算法。实验测量中,对C 4H10/空气预混燃烧 流场进行6视角动态采样,使用混合正则化算法对燃烧流场进行截面二维温度分布重建,使 用Sobel梯度 算子进行火焰内外区域温度分布的阈值分割。对4个不同时刻共计150 条莫尔条纹进行信息提取并重建出 20个截面的温度分布,使用移动立方体面绘制算法和光线投射体绘制 算法对三维体数据进行 显示,实现了燃烧流场参量分布及火焰结构的三维动态可视化。     

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.     

Fast noise reduction in computed tomography for improved 3-D visualization

Computed tomography (CT) has a trend towards higher resolution and higher noise. This development has increased the interest in anisotropic smoothing techniques for CT, which aim to reduce noise while preserving structures of interest. However, existing smoothing techniques are slow, which makes clinical application difficult. Furthermore, the published methods have limitations with respect to preserving small details in CT data. This paper presents a widely applicable speed optimized framework for anisotropic smoothing techniques. A second contribution of this paper is an extension to an existing smoothing technique aimed at better preserving small structures of interest in CT data. Based on second-order image structure, the method first determines an importance map, which indicates potentially relevant structures that should be preserved. Subsequently an anisotropic diffusion process is started. The diffused data is used in most parts of the images, while structures with significant second-order information are preserved. The method is qualitatively evaluated against an anisotropic diffusion method without structure preservation in an observer study to assess the improvement of 3-D visualizations of CT series and quantitatively by determining the reduction of the difference between low and high dose CT scans of in vitro carotid plaques.      

Maximum entropy image reconstruction in X-ray and diffraction tomography

The authors propose a Bayesian approach with maximum-entropy (ME) priors to reconstruct an object from either the Fourier domain data (the Fourier transform of diffracted field measurements) in the case of diffraction tomography, or directly from the original projection data in the case of X-ray tomography. The objective function obtained is composed of a quadratic term resulting from chi(2) statistics and an entropy term that is minimized using variational techniques and a conjugate-gradient iterative method. The computational cost and practical implementation of the algorithm are discussed. Some simulated results in X-ray and diffraction tomography are given to compare this method to the classical ones.     

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.     

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.     

光学相干层析技术微流场三维可视化测速方法

为实现微流场3D 可视化速度测量,建立了基于光学相干层析技术的微粒子跟踪速度测量系统。对系统组成原理、微粒子图像提取、匹配和速度计算方法等进行研究。介绍了频域光学相干层析技术、微流场速度测量系统组成及对渗入微粒子的微流场扫描及三维成像方法。利用中值滤波、最大类间方差二值化和体积滤波等方法搜索流场中各个微粒子,实现全流场流动特性3D 可视化;利用微粒子之间距离、灰度二阶矩建立代价函数,对不同时刻扫描得到的微粒子进行匹配,根据微粒子三维坐标求其运动速度。对对流流场进行了测量,实现了微米级空间分辨的微粒子图像与速度矢量显示。适合于复杂微流场的三维速度检测,对微流动器件流动特性研究具有重要意义。     

Computerized tomography with X-ray, emission, and ultrasound sources

This paper reviews the major developments that have taken place during the last three years in imaging with computed tomography (CT) using X-ray, emission, and ultrasound sources. Space limitations have resulted in some selection of topics by the author. There are four major sections dealing with algorithms, X-ray CT, emission CT, and ultrosound CT. Since most of the currently used algorithms are of filtered-backprojection type, we have concentrated on these in the section on algorithms (with emphasis on their implementation aspects). In X-ray CT an important question raised during the last few years has concerned the parameter measured by a CT scanner, given the fact that the X-rays used in CT scanners are polychromatic and the fact that tissue attenuation coefficients are energy dependent. Answers to this question are reviewed in the section on X-ray CT where we have also discussed the artifacts caused by the polychromaticity of the X-ray photons. Methods for the removal of these artifacts have also been reviewed. In emission CT the biggest development of the last three years is the great interest in positron tomography, although space constraints have dictated an essentially introductory treatment and not all aspects of the single photon and positron tomography have been surveyed. Finally, we have reviewed recent developments in ultrasound CT. We have pointed out that because of the sensitivity of this technique to refraction, it is currently limited to soft tissue structures, with ultrasonic detection of tumors in the female breast a significant application.     

Iterative reconstruction in X-ray fluorescence tomography based on radon inversion

We describe a new approach for the inversion of the generalized attenuated radon transform in X-ray fluorescence computed tomography (XFCT). The approach consists of using the radon inverse as an approximation for the actual one, followed by an iterative refinement. Also, we analyze the problem of retrieving the attenuation map directly from the emission data, giving rise to a novel alternating method for the solution. We applied our approach to real and simulated XFCT data and compared its performance to previous inversion algorithms for the problem, showing its main advantages: better images than those obtained by other analytic methods and much faster than iterative methods in the discrete setting.     

Penalized weighted least-squares image reconstruction for dual energy X-ray transmission tomography

We present a dual-energy (DE) transmission computed tomography (CT) reconstruction method. It is statistically motivated and features nonnegativity constraints in the density domain. A penalized weighted least squares (PWLS) objective function has been chosen to handle the non-Poisson noise added by amorphous silicon (aSi:H) detectors. A Gauss-Seidel algorithm has been used to minimize the objective function. The behavior of the method in terms of bias/standard deviation tradeoff has been compared to that of a DE method that is based on filtered back projection (FBP). The advantages of the DE PWLS method are largest for high noise and/or low flux cases. Qualitative results suggest this as well. Also, the reconstructed images of an object with opaque regions are presented. Possible applications of the method are: attenuation correction for positron emission tomography (PET) images, various quantitative computed tomography (QCT) methods such as bone mineral densitometry (BMD), and the removal of metal streak artifacts.