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.     

Advances and dilemmas in computed tomography

The technology of computed tomography has advanced markedly in recent years, and promises further improvements in medical imaging. The questions for physicians and health care evaluators are the extent to which technically improved images contribute to better patient care and to improved health outcome, and at what cost. Assessing the clinical efficacy and cost of new technology is difficult, but necessary for rational decisions in the care of patients and use of health resources.     

Distinguishability of conductivities by electric current computed tomography

We give criteria for the distinguishability of two different conductivity distributions inside a body by electric current computed tomography (ECCT) systems with a specified precision. It is shown in a special case how these criteria can be used to determine the measurement precision needed to distinguish between two different conductivity distributions. It is also shown how to select the patterns of current to apply to the body in order to best distinguish given conductivity distributions with an ECCT system of finite precision.     

Spatial-resolution enhancement in computed tomography

We propose an approach that combines an asymmetric fan-beam configuration and a new reconstruction algorithm to enhancing spatial resolution in computed tomography (CT). The asymmetric configuration can be achieved by changing the center of rotation (COR) from the conventional symmetric configuration. It does not, however, require new detectors and X-ray source nor alter the relative geometry between the detector and the X-ray source. By effectively reducing the distance of the COR to the X-ray source, the asymmetric configuration can increase the effective sampling density in projection data without reducing the size of the field of view (FOV). The proposed algorithm, on the other hand, can reconstruct images directly from data acquired with this asymmetric configuration. We performed numerical studies to demonstrate and validate the proposed acquisition/reconstruction approach. Results in these studies confirm that the proposed approach can lead to enhanced spatial resolution in reconstructed images. The proposed acquisition/reconstruction approach may find applications in micro-CT and industrial CT in which the CORs may be changed.     

High-resolution three-dimensional scanning transmission electron microscopy characterization of oxide-nitride-oxide layer interfaces in Si-based semiconductors using computed tomography

Oxide-nitride-oxide (ONO) layer structures are widely used for charge storage in flash memory devices. The ONO layer interfaces should be as flat as possible, so measurement of the nanoscale roughness of those interfaces is needed. In this study, quantification of an ONO film from a commercially available flash memory device was carried out with a pillar-shaped specimen using scanning transmission electron microscopy (STEM) and computed tomography. The ONO area contained only low Z- and low STEM-contrast materials, which makes high-quality reconstruction difficult. The optimum three-dimensional reconstruction was achieved with an STEM annular dark-field detector inner collection angle of 32?mrad, a sample tilt range from -78° to +78° and 25 iterations for the simultaneous iterative reconstruction technique.     

Polygonal and polyhedral contour reconstruction in computed tomography

This paper is about three-dimensional (3-D) reconstruction of a binary image from its X-ray tomographic data. We study the special case of a compact uniform polyhedron totally included in a uniform background and directly perform the polyhedral surface estimation. We formulate this problem as a nonlinear inverse problem using the Bayesian framework. Vertice estimation is done without using a voxel approximation of the 3-D image. It is based on the construction and optimization of a regularized criterion that accounts for surface smoothness. We investigate original deterministic local algorithms, based on the exact computation of the line projections, their update, and their derivatives with respect to the vertice coordinates. Results are first derived in the two-dimensional (2-D) case, which consists of reconstructing a 2-D object of deformable polygonal contour from its tomographic data. Then, we investigate the 3-D extension that requires technical adaptations. Simulation results illustrate the performance of polygonal and polyhedral reconstruction algorithms in terms of quality and computation time.     

Correction for resolution nonuniformities caused by anode angulation in computed tomography

Most X-ray tubes comprise a rotating anode that is bombarded with electrons to produce X-rays. A substantial amount of heat is generated, and to increase the area of the anode exposed to the electrons, without increasing the apparent size of the focal spot, the focal track of the anode is generally beveled with a very shallow angle (typically 5 degrees-7 degrees in a computed tomography (CT) tube). Due to the line focus principle, this allows a fairly large area of the focal track to be exposed to electrons while retaining a fairly small effective projected focal spot. One side effect of anode angulation is that the focal spot appears different from different positions in the detector array; the effective focal spot size at a constant distance from the tube will be larger for a peripheral detector channel than for a central one. These differences in the effective size of the focal spot across the field-of-view lead to worse resolution in the periphery than in the center of reconstructed images. In this work we describe a method for achieving more uniform resolution in fanbeam CT images by correcting for these focal spot angulation effects. We do so by modeling the effects as a series of local blurrings in the space of transmitted CT intensities and determining the effective coefficients of the corresponding discrete convolutions. The effect of these blurrings can then be compensated for in the sinogram domain through the use of a penalized-likelihood sinogram restoration model we have recently developed.     

Image restoration in computed tomography: restoration of experimental CT images

It is pointed out that to restore experimental computed tomography (CT) images which have been blurred by a spatially variant point spread function (PSF), a quadrant symmetry method which simultaneously optimizes storage requirements for the estimated PSFs and computational speed is used. The quadrant symmetry approach required less than 0.579 Megawords of storage for 9x9 pixel spatially variant PSFs in a 256x256 pixel image, allowing image restoration in Cartesian coordinates. A locally adaptive restoration method formulated using a noise prewhitening filter derived from the measured noise power spectrum, the conjugate gradient method was used to obtain a numerical solution. Restored images for a resolution phantom and for a cadaver femur are presented.     

Longitudinal aliasing in multislice helical computed tomography: sampling and cone-beam effects

In this study, we examine longitudinal aliasing properties in multislice helical computed tomography (CT) volumes reconstructed under the multiple parallel fanbeam approximation by use of a 180LI-type algorithm. We focus on the differences between the multislice case and the single-slice case, which has been studied previously. Specifically, we examine longitudinal aliasing properties in four-slice scanners for helical pitches 3 and 6, which are sometimes called "preferred" in four-slice helical CT, because it is believed that the effective longitudinal sampling intervals at these pitches are equivalent to those in single-slice helical CT operating at pitches 1 and 2, respectively. While these equivalences have been supported by comparative studies of slice-sensitivity profiles in single- and multislice helical CT, artifacts have been observed in pitch-3 and pitch-6 multislice images that were not evident in their purported single-slice counterparts. We attribute these differences to aliasing arising in the multislice reconstructions that is not present in the single-slice counterparts. We find that the aliasing has two principal origins: sampling effects similar to those in the single-slice case and cone-beam effects. The difference between the multislice, pitch-3 and single-slice, pitch-1 results is attributed to the small cone angle in multislice helical CT, which introduces inconsistencies among the measurements of different detector rows. The difference between multislice, pitch-6 and single-slice, pitch-2 results is attributed to a combination of the cone angle and genuine differences in sampling patterns. It is argued, however, that the lack of strict equivalence with single-slice counterparts does not necessarily undermine the claim that pitches 3 and 6 are "preferred" relative to other pitches in multislice helical CT.     

On intermediate view estimation in computed tomography

In many applications in computed tomography, practical limitations in data acquisition restrict the number of projections (views). The use of the standard convolution backprojection algorithm for reconstruction from an inadequate number of projections results in view aliasing artifacts. One approach to alleviating the effects of such artifacts consists of artificially increasing the number of views, by estimating a set of intermediate views. Two possible methods of estimating the intermediate views are interpolation and reprojection. In this paper, a study of the two is considered. Based on the merits and demerits of the two methods, a combination of the two methods is investigated. Specifically, a reconstruction from the available sinogram augmented by intermediate view reprojections, and the projections interpolated from the original views and the reprojections, provide an additional improvement with respect to view aliasing artifacts. The advantage of computing reprojections over smaller regions of interest is discussed. When the number of available projections is reasonably high but not adequate to produce an artifact-free reconstruction, estimating the intermediate views by interpolation provides an improvement without much additional degradation, at minimal computational cost.This work was supported by Siemens Medical Systems and by the Medical Research Council of Canada (through grant no. MT-13356). The work of Dr. Holdsworth was supported in part by a Research Scholarship from the Heart and Stroke Foundation of Canada.     

Nonuniform noise propagation by using the ramp filter in fan-beam computed tomography

It is observed that when the homogeneity property of the ramp filter is used to derive a filtered backprojection algorithm in fan-beam tomography, the reconstructed images have nonstationary frequency components and nonstationary noise. When a short focal-length is used, higher frequency components are amplified more at the edge of the image than at the center of the image, resulting in higher noise at the edge of the image.     

多排CT评价冠状动脉斑块变化与血脂的关系

目的以无创多排螺旋CT观察冠状动脉斑块的变化及其与血脂的关系。方法冠脉CT患者46例,平均年龄(56.1±1.7)岁,平均随访时间(251.8±13.1)d,同时检查空腹血脂(总胆固醇,甘油三酯,高密度脂蛋白胆固醇,低密度脂蛋白胆固醇)。取前后两次CT检查同一部位血管断面图测非钙化斑块面积,血管狭窄程度和CT值,CTHU值(-10)~158定义为斑块,>159为管腔。按血管狭窄程度加重和减轻分为恶化组和改善/稳定组,比较两组血脂控制情况,并进行相关分析。结果总体血脂控制情况一般,TC和LDL-C有轻度增加,狭窄程度也有增加趋势,斑块CT值下降。稳定组TC控制情况明显好于恶化组(3.86±0.83vs4.53±1.32,P=0.0240),LDL-C控制情况也明显好于恶化组(2.11±0.56vs2.57±0.99,P=0.0301)。回归分析表明狭窄减轻和LDL-C呈显著负相关(r=0.3099,P=0.0361),病变无进展的LDL-C界值为1.94mmol/L。结论控脂可以减缓冠脉斑块进展,冠脉斑块和血管狭窄的变化可用无创多排CT监测。     

Collimator effects in high resolution x-ray computed tomography

This paper complements other studies in the field of image restoration in X-ray computed tomography by including the source collimator among the sources of image degradation. The overall blur is describable in terms of a space variant 2-D impulse response or point spread function, which is initially calculated for an arbitrary scanning eccentricity, a uniform source intensity distribution, a uniform detector sensitivity, and arbitrary collimator aperture and position, and subsequently derived for more complicated sources and detectors. Numerous 3-D hidden-line displays are used throughout to illustrate the complicated 2-D functions emerging from the analysis.     

太赫兹计算机辅助层析成像发展近况

由于太赫兹波可穿透塑料、纸张、衣服等非金属、非极性物质,较X辐射具有较低的光子能量,且计算机辅助层析成像(CT)可获得物体内部结构信息,并可重构出物体的三维图像,因此太赫兹CT受到国际广泛关注。重点介绍了近年来太赫兹CT研究重点及具体状况,并分析了存在的问题和发展趋势。可为我国太赫兹CT技术的发展提供技术借鉴。     

Single-slice rebinning reconstruction in spiral cone-beam computed tomography

At the advent of multislice computed tomography ICT) a variety of approximate cone-beam algorithms have been proposed suited for reconstruction of small cone-angle CT data in a spiral mode of operation. The goal of this study is to identify a practical and efficient approximate cone-beam method, extend its potential for medical use, and demonstrate its performance at medium cone-angles required for area detector CT. We will investigate two different approximate single-slice rebinning algorithms for cone-beam CT: the multirow Fourier reconstruction (MFR) and an extension of the advanced single-slice rebinning method (ASSR), which combines the idea of ASSR with a z-filtering approach. Thus, both algorithms, MFR and ASSR, are formulated in the framework of z-filtering using optimized spiral interpolation algorithms. In each view, X-ray samples to be used for reconstruction are identified, which describe an approximation to a virtual reconstruction plane. The performance of approximate reconstruction should improve as the virtual reconstruction plane better fits the spiral focus path. The image quality of the respective reconstruction will be assessed with respect to image artifacts, spatial resolution, contrast resolution, and image noise. It turns out that the ASSR method using tilted reconstruction planes is a practical and efficient algorithm, providing image quality comparable to that of a single-row scanning system even with a 46-row detector at a table feed of 64 mm. Both algorithms tolerate any table feed below the maximum value associated to the detector height. Due to the z-filter approach, all detector data sampled can be used for image reconstruction.     

Geometric deconvolution: a meta-algorithm for limited view computed tomography

Images reconstructed using a limited number of projections spanning a narrow angular range suffer from a systematic geometric distortion due to the two-dimensional point spread function of the reconstruction process. Applying the projection theorem, we show that the problem of removing this distortion reduces to that of estimating the one-dimensional spread function and deconvolving projections computed for a complementary set of new angles from the initial reconstruction. A second reconstruction is performed using the deconvolved projections along with the original set of projections, thus incorporating wider angular coverage. We present here initial results of such geometric deconvolution performed via inverse filtering using fast Fourier transform techniques. While the results are noisy due to well-known problems associated with inverse filtering, they illustrate the plausibility of the underlying ideas.     

用全息干涉C—T技术重建三维折射率场

本文结合折射率场的先验知识，研究了全自干涉Ｃ－Ｔ技术重建三维折射率场的四种方法。通过计算机模拟，考查了数据误差和总观察角大小对重建精度的影响。作为一个应用实例，。计算了火焰温度场的一截面的温度分析，并与热电偶测量的值进行了比较．     

锥形束CT和全颌曲面断层片在根尖区牙根外吸收诊断中的对比研究

目的:比较锥形束CT(cone-beam computed tom CT)和全颌曲面断层片(全景片)在根尖区牙根外吸收(根尖吸收)诊断中的差异.方法:选取80颗单根前磨牙,用以模拟4种不同程度的根尖吸收:无(完整的牙根)、轻度(根尖区表面1.0mm直径和深度的洞形)、中度(1.0mm牙根缩短)、重度(3.0mm牙根缩短).模拟完成后,对所有牙齿拍摄CBCT和全景片获取两组X线图像.两名正畸医师通过对图像资料的分析独立地评估所有根尖吸收的严重程度.结果:采用CBCT和全景片评估轻度根尖吸收的正确率分别为100％和27.5％,差异有统计学意义(P＜ 0.001);评估全部根尖吸收的正确率分别为81.9％和64.4％,差异有统计学意义(P＜0.001).结论:相比于全景片,CBCT对根尖吸收具有更高的敏感度.在正畸临床中,CBCT可用于对根尖吸收进行早期诊断,从而为继续或修正当前的正畸治疗提供必要的参考意见.     

Image restoration in computed tomography: the spatially invariant point spread function

Image restoration to deblur smoothing caused by the finite-size X-ray beam profile for a simulated computed tomography (CT) system is presented. Three simple image restoration methods are compared when the point-spread-function (PSF) is spatially invariant. In the first restoration method, an iterative least squares solution, regularized with the image norm and constrained by the boundary of the object, is obtained from the projection data. In the second method, a Wiener filter, designed using the power spectrum of CT noise, is applied to the reconstructed CT image. The third method obtains a weighted least-squares solution, by iteration, from the reconstructed CT image; the solution is regularized with the weighted image norm. Restored images were compared with the image obtained using filtered backprojection method. Differences between these images were evaluated qualitatively.     

应用锥形束CT分析调强放疗中头颈部肿瘤的摆位误差

背景与目的：采用锥形束CT（Cone-beam computed tomography,CBCT）影像技术研究本院头颈部肿瘤调强放射治疗中的摆位误差,并计算出CTV的外扩边界PTV的范围。方法：应用Varian图像引导放疗系统（Image-guided radiation therapy,IGRT）治疗头颈部肿瘤31例,每周均获取锥形束断层扫描（CBCT）各一次,将CBCT图像和计划CT图像及其靶中心匹配,获得靶中心x、y、z方向的误差,分析误差及其分布规律。CTV到PTV外扩边界的大小由公式：2.5∑＋0.7σ计算得出,其中∑为系统误差,σ为随机误差。结果：得出本院头颈部肿瘤PTV的理论边界x、y、z（左右、头脚、前后）方向上分别为：1.9mm,1.6mm,1.3mm。结论：通过CBCT测量摆位误差并进行调整可提高患者摆位精度,为外扩CTV边界提供了理论依据,使治疗计划的实施更精确。