代数重建技术中投影序列选择次序的研究

在代数重建技术中投影序列的选择次序对重建收敛速度及精度均具有重要影响。针对在360°方向上均匀获取投影数据的场合,提出加权距离正交投影排序方法。该方法将投影序列分成若干组,在每一组内前后选择的投影序列彼此正交,同时相邻组之间投影序列尽可能均匀分布。该方法能提高重建精度及收敛速度,同时利用前后正交投影角度下射线经过像素信息关系,可将重建速度提高1倍。     

Flame four-dimensional deflection tomography with compressed-sensing-revision reconstruction

Deflection tomography with limited angle projections was investigated to visualize a premixed flame. A projection sampling system for deflection tomography was used to obtain chronological deflectogram arrays at six view angles with only a pair of gratings. A new iterative reconstruction algorithm with deflection angle compressed-sensing revision was developed to improve reconstruction-distribution quality from incomplete projection data. Numerical simulation and error analysis provided a good indication of algorithm precision and convergence. In the experiment, 150 fringes were processed, and temperature distributions in 20 cross-sections were reconstructed from projection data in four instants. Four-dimensional flame structures and temperature distributions in the flame interior were visualized using the visualization toolkit. The experimental reconstruction was then compared with the result obtained from computational fluid dynamic analysis.     

Measurement of the laser beam intensity distribution using optical tomography

Optical tomography is proposed as a method for studying the intensity distribution in the laser beam cross section. The tomographic projection of the beam cross section is obtained with the help of an optical fiber possessing a high radiation resistance, which makes it possible to employ this technique for studying high-power laser radiation. The results of tomographic reconstruction of the bean cross section from experimental projections are compared with the results of reconstruction from model projections.     

Omega-space adaptive acquisition technique for magnetic resonance imaging from projections

An omega-space adaptive acquisition technique for MRI from projections is presented. It is based on the evaluation of the information content of a set composed of four initial projections, measured at angles 0 degrees, 45 degrees, 90 degrees, and 135 degrees, followed by the selection of new angles where the information content is maximum. An entropy function is defined on the power spectrum of the projections that is useful for evaluating the information content of each projection. The method makes it possible to reduce the total acquisition time with little degradation of the reconstructed image and it adapts to the arbitrary shape of the sample. For this reason, it can be particularly useful in those applications where acquisition from projections is strongly recommended to save acquisition time, such as functional MRI, imaging of species having very short T(2), or angiography. The method has been tested both on simulated data and on experimental data collected by a commercial MRI apparatus. The method has also been compared to the regular acquisition method, that is, the standard acquisition method in MRI from projections.     

Multiple-gate time domain diffuse fluorescence tomography allows more sparse tissue sampling without compromising image quality

Diffuse fluorescence tomography systems that employ highly sensitive photo-multiplier tubes for single-photon detection are pushing the sensitivity limits of the field. However, each of these detectors only offers a single data projection to be collected, implying these imaging systems either require many detectors or long scan times to collect full data sets for image reconstruction. This study presents a method of utilizing the time-resolved collection capabilities of time-correlated single-photon counting techniques to increase spatial resolution and to reduce the number of data projections to produce reliable fluorescence reconstructions. Experimental tissue phantom results demonstrate that using data at 10 time gates in the fluorescence reconstructions for only 40 data projections provided superior image accuracy when compared to reconstructions on 320 continuous-wave data projections.     

Algorithm Study on Reconstruction of Refractive Angles in Fan Beam Diffraction Enhanced Computed Tomography

Based on the 360° computing method of refractive angle for parallel beam diffraction enhanced imaging computed tomography （DE-CT） technique, a new algorithm used to calculate the refractive angle for fan-beam DE-CT technique is developed. The refractive index gradient can be obtained by using the new algorithm with projection data of an object in the range of 0 - 360°, and the new algorithm only needs to set the analyser at half slope position of the rocking curve.     

基于三投影方向的层析重建分析

针对传统的层析方法中,投影方向多、系统复杂等特点,提出了基于三个投影方向的层析重建方法.论文选取适用于少投影方向的代数迭代重建算法,采用参量修正方法,能较好的提高重建准确度和抑制椒盐噪音.同时对影响重建图像质量和速度的迭代次数、不同原图像结构对重建图像质量的影响进行了分析,给出了相应分析模拟结果.结果表明,迭代300次后,重建结果基本稳定.同时对于具有轴对称结构物体,三个方向投影数据能够较好地实现三维层析重建.分析结果为后期开展的基于单幅层析全息图的三维层析重建实验研究提供了理论依据.     

Dual-projection optical diffusion tomography

We propose a new approach to optical diffusion tomography that incorporates two orthogonal projections. All the data obtained in a double projection measurement are treated simultaneously. The second projection improves image quality due to the fact that the depth and transverse directions are interchanged. An image reconstruction algorithm is derived and illustrated with simulations. It is shown that the spatial resolution of images improves by a factor of 4-5 due to the second projection.     

Application of acoustic reflection tomography to sonar imaging

Computer-aided tomography is a technique for providing a two-dimensional cross-sectional view of a three-dimensional object through the digital processing of many one-dimensional views (or projections) taken at different look directions. In acoustic reflection tomography, insonifying the object and then recording the backscattered signal provides the projection information for a given look direction (or aspect angle). Processing the projection information for all possible aspect angles enables an image to be reconstructed that represents the two-dimensional spatial distribution of the object's acoustic reflectivity function when projected on the imaging plane. The shape of an idealized object, which is an elliptical cylinder, is reconstructed by applying standard backprojection, Radon transform inversion (using both convolution and filtered backprojections), and direct Fourier inversion to simulated projection data. The relative merits of the various reconstruction algorithms are assessed and the resulting shape estimates compared. For bandpass sonar data, however, the wave number components of the acoustic reflectivity function that are outside the passband are absent. This leads to the consideration of image reconstruction for bandpass data. Tomographic image reconstruction is applied to real data collected with an ultra-wideband sonar transducer to form high-resolution acoustic images of various underwater objects when the sonar and object are widely separated.     

衍射增强计算机断层技术研究

对衍射增强计算机断层技术进行了分析研究，提出了一种新方法.应用该方法只需在摇摆曲线一点处对物体进行360°范围内旋转投影成像，即可获得物体的折射率梯度分布以及吸收系数与散射系数和分布.该方法较以前的方法，大大简化了实验步骤. 关键词： 同步辐射 衍射增强 计算机断层     

Quantitative damage imaging using Lamb wave diffraction tomography

In this paper, we investigate the diffraction tomography for quantitative imaging damages of partly through-thickness holes with various shapes in isotropic plates by using converted and non-converted scattered Lamb waves generated numerically. Finite element simulations are carried out to provide the scattered wave data. The validity of the finite element model is confirmed by the comparison of scattering directivity pattern(SDP) of circle blind hole damage between the finite element simulations and the analytical results. The imaging method is based on a theoretical relation between the one-dimensional(1D) Fourier transform of the scattered projection and two-dimensional(2D) spatial Fourier transform of the scattering object. A quantitative image of the damage is obtained by carrying out the 2D inverse Fourier transform of the scattering object. The proposed approach employs a circle transducer network containing forward and backward projections, which lead to so-called transmission mode(TMDT) and reflection mode diffraction tomography(RMDT),respectively. The reconstructed results of the two projections for a non-converted S0 scattered mode are investigated to illuminate the influence of the scattering field data. The results show that Lamb wave diffraction tomography using the combination of TMDT and RMDT improves the imaging effect compared with by using only the TMDT or RMDT. The scattered data of the converted A0 mode are also used to assess the performance of the diffraction tomography method. It is found that the circle and elliptical shaped damages can still be reasonably identified from the reconstructed images while the reconstructed results of other complex shaped damages like crisscross rectangles and racecourse are relatively poor.     

Mojette变换层析技术中的投影角度布局方法

Mojette变换是一种最小冗余采样的离散Radon变换,能够用较少角度的投影数据进行精确的计算层析(computed tomography,CT)重建,为少量投影角度CT技术的实现提供了一种新思路.投影角度的空间布局决定了层析重建最少所需投影的数量.为了获得Mojette变换层析技术中的最优投影空间角度布局方案,本文对三维Mojette变换数学模型及其精确重建条件进行了研究.以此为基础,在考虑实际探测器像素数目受限的条件下,提出了确定最优投影角度的方法.研究结果表明:所有探测器围绕被测物体在同一水平面内进行平行投影采集是最优的投影角度布局方案,此时投影模型为二维Mojette变换,所需的投影角度和探测器像素数最少,投影角度范围最小;若在实际的测量中该投影条件无法满足,则投影矢量中|pi|和|qi|的值越小越好.该研究可为实际层析系统的建立提供理论基础.     

多金属物体CT图像的金属伪影校正

针对多金属伪影的校正问题,本文通过仿真实验分析了多金属伪影的成因,并提出了一种基于投影校正的多金属伪影校正方法.该方法首先直接从投影域分割出金属区域,然后建立对金属区域投影值的校正模型,最后通过调整模型参数达到校正目的.模型以重建图像的灰度熵为目标函数,采用单纯形法迭代求解使熵最小时的校正参数.仿真和实际数据的实验结果表明,本文算法对多金属伪影的校正起到了良好的效果,且校正后的图像质量优于插值校正法.     

Feature tracking for projection registration in laboratory-scale reflective tomography laser radar imaging

In range-resolved reflective tomography laser radar imaging system, for accurate image reconstruction, all projections must be aligned to the center of target rotation. Due to the unknown location of the axis of rotation and random target translation, the center of target rotation in each projection cannot be measured accurately and images reconstructed based on range-resolved projections without registration would be dislocated. Here, we apply a feature tracking method to align projections data for tomographic reconstruction in laboratory-scale laser radar imaging. The target was placed on a spin table with an unknown and fixed axis, and the oscillatory motion of the target translation was simulated by a second order inertia filter to accord with the real satellites vibrations. The experimental simulation results demonstrate the effectiveness of this method to improve images reconstruction equality. The application of this method to actual radar imaging in remote distance was discussed and the differences were compared with the lab setups.     

Quantitative quasi-local tomography using absorption and phase contrast

Possibilities are investigated for quantitative computed tomography (CT) reconstruction of the spatial distribution of refractive index in a region of interest (ROI) inside an object which is larger than the field of view. The analysis is presented for the case of conventional (absorption) CT, as well as diffraction (phase-contrast) CT. It is shown that in both cases an accurate reconstruction can be achieved using the projection data corresponding to rays passing through a sufficiently wide vicinity of a ROI. Our analysis also indicates that X-ray phase-contrast CT can typically be localized to smaller regions compared to absorption CT. In particular, quasi-local hard X-ray micro-CT of regions of the order of 100 μm or even smaller in size, appears feasible with the use of propagation-based phase contrast. Numerical and experimental tests confirm the possibility of accurate quantitative CT reconstruction from truncated projections.     

An iterative image reconstruction algorithm combined with forward and backward diffusion filtering for in‐line X‐ray phase‐contrast computed tomography

In‐line X‐ray phase‐contrast computed tomography (IL‐PCCT) can reveal fine inner structures for low‐Z materials (e.g. biological soft tissues), and shows high potential to become clinically applicable. Typically, IL‐PCCT utilizes filtered back‐projection (FBP) as the standard reconstruction algorithm. However, the FBP algorithm requires a large amount of projection data, and subsequently a large radiation dose is needed to reconstruct a high‐quality image, which hampers its clinical application in IL‐PCCT. In this study, an iterative reconstruction algorithm for IL‐PCCT was proposed by combining the simultaneous algebraic reconstruction technique (SART) with eight‐neighbour forward and backward (FAB8) diffusion filtering, and the reconstruction was performed using the Shepp–Logan phantom simulation and a real synchrotron IL‐PCCT experiment. The results showed that the proposed algorithm was able to produce high‐quality computed tomography images from few‐view projections while improving the convergence rate of the computed tomography reconstruction, indicating that the proposed algorithm is an effective method of dose reduction for IL‐PCCT.     

Radiography registration for mosaic tomography

A hybrid method of stitching X‐ray computed tomography (CT) datasets is proposed and the feasibility to apply the scheme in a synchrotron tomography beamline with micrometre resolution is shown. The proposed method enables the field of view of the system to be extended while spatial resolution and experimental setup remain unchanged. The approach relies on taking full tomographic datasets at different positions in a mosaic array and registering the frames using Fourier phase correlation and a residue‐based correlation. To ensure correlation correctness, the limits for the shifts are determined from the experimental motor position readouts. The masked correlation image is then minimized to obtain the correct shift. The partial datasets are blended in the sinogram space to be compatible with common CT reconstructors. The feasibility to use the algorithm to blend the partial datasets in projection space is also shown, creating a new single dataset, and standard reconstruction algorithms are used to restore high‐resolution slices even with a small number of projections.     

A modified discrete algebraic reconstruction technique for multiple grey image reconstruction for limited angle range tomography

The `missing wedge', which is due to a restricted rotation range, is a major challenge for quantitative analysis of an object using tomography. With prior knowledge of the grey levels, the discrete algebraic reconstruction technique (DART) is able to reconstruct objects accurately with projections in a limited angle range. However, the quality of the reconstructions declines as the number of grey levels increases. In this paper, a modified DART (MDART) was proposed, in which each independent region of homogeneous material was chosen as a research object, instead of the grey values. The grey values of each discrete region were estimated according to the solution of the linear projection equations. The iterative process of boundary pixels updating and correcting the grey values of each region was executed alternately. Simulation experiments of binary phantoms as well as multiple grey phantoms show that MDART is capable of achieving high‐quality reconstructions with projections in a limited angle range. The interesting advancement of MDART is that neither prior knowledge of the grey values nor the number of grey levels is necessary.     

乳腺超声断层成像系统的有限元仿真分析

本文利用有限元仿真软件COMSOL进行乳腺超声断层成像仿真,仿真模型置于水中,重建区域包含乳腺软组织和一个圆形铁块障碍物。256个换能器等角度环形分布于乳腺外部,采用一个换能器发射、其余所有换能器接收的方式进行,依次进行256次仿真来实现环形扫描。仿真得到随时间变化的二维声压分布云图,提取接收电压获得信号矩阵。利用等角扇束滤波反投影重建算法进行图像重建,仿真实验结果证明了换能器呈360?进行环形扫描的方式可行。     

Investigation of Bubbly Flow by Ultrasonic Tomography

Computerized tomography (CT) means reconstruction of pictures from projections taken at different angles. This method is well known in fields such as medical diagnostics and nondestructive testing. The application of CT to gas/liquid flow measurement results in a non-intrusive method for measuring the spatial distribution of the gas phase in a cross-section of the flow. In our application the projection data are obtained from measurement of the extinction along a set of ultrasonic rays in this cross-section, using piezo-ceramic ultrasonic transducers. The transducers are arranged in a circular sensor array which is directly integrated in the pipe wall of the flow; instead of mechanical rotation, the sensors are sequentially switched.