An iterative maximum-likelihood polychromatic algorithm for CT

A new iterative maximum-likelihood reconstruction algorithm for X-ray computed tomography is presented. The algorithm prevents beam hardening artifacts by incorporating a polychromatic acquisition model. The continuous spectrum of the X-ray tube is modeled as a number of discrete energies. The energy dependence of the attenuation is taken into account by decomposing the linear attenuation coefficient into a photoelectric component and a Compton scatter component. The relative weight of these components is constrained based on prior material assumptions. Excellent results are obtained for simulations and for phantom measurements. Beam-hardening artifacts are effectively eliminated. The relation with existing algorithms is discussed. The results confirm that improving the acquisition model assumed by the reconstruction algorithm results in reduced artifacts. Preliminary results indicate that metal artifact reduction is a very promising application for this new algorithm.     

A comparative study of two postreconstruction beam hardening correction methods

The general nature of postreconstruction beam hardening correction methods is discussed. A methodology for choosing the energy of reconstruction is presented based on a technique of evaluating the "nearness" of two projection data sets. Two previously published postreconstruction beam hardening correction methods are described within a common framework. These methods differ at a number of independent places and so one can produce hybrid methods by interchanging some but not all of the choices. A basic difference between the methods is that one needs only the initial reconstruction during the postreconstruction correcting phase, while the other needs the original projection data as well. Both methods have been implemented and are compared (using a mathematical head phantom) from the points of view of the nearness of the corrected polychromatic projection data to the desired monochromatic projection data and the visual quality of the reconstructions. Variants and hybrids of the two methods are also investigated and recommendations based on the results are presented.     

基于变螺距螺旋FDK重建算法的研究

目前广泛应用的锥束螺旋CT主要采用螺旋FDK算法进行图像重建,但为了使算法能更应用于实际,许多学者对标准的螺旋轨迹进行了拓展.本文主要研究了变螺距螺旋扫描轨迹.变螺距螺旋扫描可以提高扫描速度,有更高的时间、空间分辨率.本文详细介绍了变螺距螺旋扫描轨迹,给出了具体的变螺距螺旋扫描轨迹方程.针对图像重建,本文主要从FDK重...     

Atmospheric correction against algorithm for NOAA-AVHRR products:theory and application

Investigation of the effect of atmospheric constituents on NOAA Advanced Very High Resolution Radiometer (AVHRR) visible and near-infrared data is presented. The general remote sensing equation, including scattering, absorption, and bidirectional reflectance effects for the AVHRR solar bands, is described. The magnitude of the atmospheric effects for AVHRR solar bands with respect to their impact on the normalized difference vegetation index (NDVI) and the surface bidirection reflectance is examined. Possible approaches for acquiring atmospheric information are discussed, and examples of atmospheric correction of surface reflectance and NDVI are given. Invariant effects (ozone absorption and molecular scattering) and variant effects (water vapor absorption and aerosol scattering) are shown to dominate the atmospheric effects in the AVHRR solar bands     

An estimation/correction algorithm for detecting bone edges in CT images

The normal direction of the bone contour in computed tomography (CT) images provides important anatomical information and can guide segmentation algorithms. Since various bones in CT images have different sizes, and the intensity values of bone pixels are generally nonuniform and noisy, estimation of the normal direction using a single scale is not reliable. We propose a multiscale approach to estimate the normal direction of bone edges. The reliability of the estimation is calculated from the estimated results and, after re-scaling, the reliability is used to further correct the normal direction. The optimal scale at each point is obtained while estimating the normal direction; this scale is then used in a simple edge detector. Our experimental results have shown that use of this estimated/corrected normal direction improves the segmentation quality by decreasing the number of unexpected edges and discontinuities (gaps) of real contours. The corrected normal direction could also be used in postprocessing to delete false edges. Our segmentation algorithm is automatic, and its performance is evaluated on CT images of the human pelvis, leg, and wrist.     

Image denoising based on multiscale singularity detection for cone beam CT breast imaging

It was recently reported that the real-time flat panel detector-based cone-beam computed tomography (CBCT) breast imaging can help improve the detectability of small breast tumors with an X-ray dose comparable to that of the conventional mammography. In this paper, an efficient denoising algorithm is proposed to further reduce the X-ray exposure level required by a CBCT scan to acquire acceptable image quality. The proposed wavelet-based denoising algorithm possesses three significant characteristics: 1) wavelet coefficients at each scale are classified into two categories: irregular coefficients, and edge-related and regular coefficients; 2) noise in irregular coefficients is reduced as much as possible without producing artifacts to the denoised images; and 3) for the edge-related and regular coefficients, if they are at the first decomposition level, they are further denoised, otherwise, no modifications are made to them so as to obtain good visual quality for diagnosis. By applying the proposed denoising algorithm to the filtered projection images, the X-ray exposure level necessary for the CBCT scan can be reduced by up to 60% while obtaining clinically acceptable image quality. This denoising result indicates that in the clinical application of CBCT breast imaging, the patient radiation dose can be significantly reduced.     

基于CUDA架构与B样条的实时锥束CT重建算法

随着X线探测板数据采集速度的快速发展,研究者开始利用C臂机采集投影数据并重建断层图像,用于手术导航或者放射治疗.但是普通PC的重建速度慢,很难匹配硬件数据采集速度,限制了其在实时临床环境中的应用.本文提出一种基于CUDA(Compute Unified Device Architecture)架构的改进FDK算法,利用GPU(Graphic Porcessing Unit)显卡的并行计算能力实现了实时CT重建,并通过B样条插值提高重建图像的质量,在实时临床环境中具有很好的应用价值.     

X-ray CT metal artifact reduction using wavelets: an application for imaging total hip prostheses

Traditional computed tomography (CT) reconstructions of total joint prostheses are limited by metal artifacts from corrupted projection data. Published metal artifact reduction methods are based on the assumption that severe attenuation of X-rays by prostheses renders corresponding portions of projection data unavailable, hence the "missing" data are either avoided (in iterative reconstruction) or interpolated (in filtered backprojection with data completion; typically, with filling data "gaps" via linear functions). In this paper, we propose a wavelet-based multiresolution analysis method for metal artifact reduction, in which information is extracted from corrupted projection data. The wavelet method improves image quality by a successive interpolation in the wavelet domain. Theoretical analysis and experimental results demonstrate that the metal artifacts due to both photon starving and beam hardening can be effectively suppressed using our method. As compared to the filtered backprojection after linear interpolation, the wavelet-based reconstruction is significantly more accurate for depiction of anatomical structures, especially in the immediate neighborhood of the prostheses. This superior imaging precision is highly advantageous in geometric modeling for fitting hip prostheses.     

Real-time error correction algorithm for noise-corrupted JPEGbit-streams

An effective post-processing algorithm for error recovery in noise-corrupted JPEG bit streams is presented. The good performances provided in terms of perceptual quality and real-time working capability makes the algorithm highly suitable for applications such as outdoor video-surveillance, characterised by severe constraints on image quality and processing time     

基于OFDM的WLAN相位校正算法研究

相位校正对基于OFDM的高速数据传输系统至关重要.文章针对802.11a协议提出了一种使用导频进行相位校正的算法.该算法对接收导频样本进行最小二乘估计以获得相偏参数,算法综合考虑了抽样时钟偏移(SCO)和载波频偏(CFO)所引起的相位偏移.文章通过仿真比较了该算法与另一种相位校正算法在不同参数下的性能,结果证明在相偏主要由SCO和CFO引起的情况下该算法性能较好.     

Time-resolved interventional cardiac C-arm cone-beam CT: an application of the PICCS algorithm

Time-resolved cardiac imaging is particularly interesting in the interventional setting since it would provide both image guidance for accurate procedural planning and cardiac functional evaluations directly in the operating room. Imaging the heart in vivo using a slowly rotating C-arm system is extremely challenging due to the limitations of the data acquisition system and the high temporal resolution required to avoid motion artifacts. In this paper, a data acquisition scheme and an image reconstruction method are proposed to achieve time-resolved cardiac cone-beam computed tomography imaging with isotropic spatial resolution and high temporal resolution using a slowly rotating C-arm system. The data are acquired within 14 s using a single gantry rotation with a short scan angular range. The enabling image reconstruction method is the prior image constrained compressed sensing (PICCS) algorithm. The prior image is reconstructed from data acquired over all cardiac phases. Each cardiac phase is then reconstructed from the retrospectively gated cardiac data using the PICCS algorithm. To validate the method, several studies were performed. Both numerical simulations using a hybrid motion phantom with static background anatomy as well as physical phantom studies have been used to demonstrate that the proposed method enables accurate reconstruction of image objects with a high isotropic spatial resolution. A canine animal model scanned in vivo was used to further validate the method.     

基于BP算法的焊缝X射线成像校正算法研究

为解决长输管道焊缝X射线成像时畸变问题,文中根据长输管道的特点基于BP神经网络方法进行图像校正,采用聚类与调节权值、阈值的方式应用于图像校正,在分析算法的特点的基础上,优化并改进了该算法,从而对长输管道成像畸变完成了校正。能够明显地抑制计算量、提高校正的精确度。首次在工业上完成了对X射线的图像校正。     

滞空式炮射雷达干扰弹干扰效能评估研究

为了解决滞空式炮射雷达干扰弹干扰效能评估难题,针对滞空式炮射雷达干扰弹工作原理及配试雷达特性,提出了静态悬挂多部干扰机、实弹射击填砂弹作为合作目标的半动态试验方法。并进行了由多部干扰机至单部干扰机、由配试雷达到真实作战对象的评估方法研究,通过外场试验,评估了滞空式炮射雷达干扰弹干扰效能。     

A four-dimensional registration algorithm with application to joint correction of motion and slice timing in fMRI

Existing groupwise image registration algorithms for longitudinal data generally ignore continuous movements and signal changes that occur throughout image acquisition. We emphasize the case of functional magnetic resonance images, which present spatio-temporal distortion due to the combination of head motion during scanning and staggered slice acquisition. While there exist techniques to correct for motion and slice timing separately, a common dilemma is to determine which correction should be applied first. This paper proposes a four-dimensional realignment algorithm to perform both tasks simultaneously. Experiments conducted on simulated datasets with known movements suggest that the proposed algorithm provides more accurate image reconstruction than the classical two-step realignment procedure (temporal then spatial) as implemented, for instance, in the statistical parametric mapping software.     

一种新的天线阵列位置误差校正算法

天线阵元的位置误差会影响天线阵元所接收到信号的相位.基于特征值分解的波达方向算法对信号的相位误差非常敏感,因此有必要对阵元的位置误差所带来的接收信号相位误差进行校正.提出了一种基于噪声子空间的最优化算法能有效的校正直线阵列的位置误差.与原方法相比,该算法的优化参数仅为原来的一半,提高了梯度优化算法的收敛速度.数值计算结果表明该算法是有效的.     

Genetic algorithm procedure for linear array failure correction

A genetic algorithm procedure is proposed for array failure correction. This approach enables the minimum number of array elements to be found for which the excitation must be adjusted in order to recover the original pattern. The method has been applied to restore the pattern of an array with three failures by perturbing the amplitude of less than a third of the unfailed array elements     

A new iterative algorithm for extrapolation of data available in multiple restricted regions with application to radar imaging

A new iterative method for extrapolation of incomplete segmented data available in multiple separated bands is proposed and tested. The method uses the Burg algorithm to find the linear prediction parameters and an iterative procedure to improve the estimation of the linear prediction parameters and the extrapolation of the data. This method is especially effective when the spectra (Fourier transform of the observed data) are in discrete forms, in the context of radar imaging represented here, this means the objects consist of distinctly spaced scattering centers. The advantages of this algorithm are demonstrated using both numerically generated and realistic experimental data pertaining to high resolution radar imaging.     

Surface normal overlap: a computer-aided detection algorithm with application to colonic polyps and lung nodules in helical CT

We developed a novel computer-aided detection (CAD) algorithm called the surface normal overlap method that we applied to colonic polyp detection and lung nodule detection in helical computed tomography (CT) images. We demonstrate some of the theoretical aspects of this algorithm using a statistical shape model. The algorithm was then optimized on simulated CT data and evaluated using a per-lesion cross-validation on 8 CT colonography datasets and on 8 chest CT datasets. It is able to achieve 100% sensitivity for colonic polyps 10 mm and larger at 7.0 false positives (FPs)/dataset and 90% sensitivity for solid lung nodules 6 mm and larger at 5.6 FP/dataset.     

Signal-to-noise ratios for attenuation correction in PET imaging

Attenuation correction is an important part of accurate image reconstruction in positron tomography. The usual correction method involves direct measurement of attenuation correction factors (ACFs). A reconstruct-reproject method, which has been suggested as providing superior noise properties, is sometimes employed; an attenuation image is first reconstructed from the measurement and then ACFs are obtained by reprojection through this image. Here the authors present a model which follows the signal-to-noise ratio (SNR) through the attenuation correction by both the direct and reconstruct-reproject methods. This model is applicable to both 2D and 3D imaging geometry, but applies to the central elements of emission and transmission objects with circular symmetry and constant amplitude. For this simplified geometry, the model predicts that the SNR of the emission image following attenuation correction is the same for both direct and reconstruct-reproject methods, although the SNRs of the ACFs are themselves substantially different. The authors also present the measured SNR at the various steps of attenuation correction for both the direct and reconstruct-reproject methods using simulated transmission and emission data. The measured SNRs agree with the model; no significant difference between the direct and reconstruct-reproject SNRs was observed.     

Window function and interpolation algorithm for OFDM frequency-offset correction

In orthogonal frequency-division multiplexing (OFDM) digital communication systems, an uncorrected frequency offset can lead to a severe degradation of the system performance. The paper describes a technique to mitigate the influence of frequency offset on system performance by applying a weighted discrete Fourier transform (DFT) to a novel OFDM frequency-assignment scheme in which carriers are regularly inserted among modulated data carriers. This frequency-assignment scheme makes possible the application of a window function at the demodulator to broaden the signal spectrum and renders the demodulated signal more immune to the influence of frequency offset. In addition, a variant of this frequency-assignment scheme also leads to a DFT-based measurement technique for easy and accurate determination of the frequency offset. Interpolation algorithms and frequency-assignment schemes suitable for both initial frequency acquisition and subsequent frequency tracking are described in detail. The Rife and Vincent (Class-I) windows (see Rife, D.C. and Vincent, G.A., Bell Syst. Tech. J., vol.49, p.197-228, 1976) are examined and their estimation results are obtained for an additive white Gaussian noise (AWGN) and a multipath fading channel.