Modeling the axial extension of a transmission line source within iterative reconstruction via multiple transmission sources

Reconstruction algorithms for transmission tomography have generally assumed that the photons reaching a particular detector bin at a particular angle originate from a single point source. In this paper, we highlight several cases of extended transmission sources, in which it may be useful to approach the estimation of attenuation coefficients as a problem involving multiple transmission point sources. Examined in detail is the case of a fixed transmission line source with a fan-beam collimator. This geometry can result in attenuation images that have significant axial blur. Herein it is also shown, empirically, that extended transmission sources can result in biased estimates of the average attenuation, and an explanation is proposed. The finite axial resolution of the transmission line source configuration is modeled within iterative reconstruction using an expectation-maximization algorithm that was previously derived for estimating attenuation coefficients from single photon emission computed tomography (SPECT) emission data. The same algorithm is applicable to both problems because both can be thought of as involving multiple transmission sources. It is shown that modeling axial blur within reconstruction removes the bias in the average estimated attenuation and substantially improves the axial resolution of attenuation images.     

An analytical scatter correction for singles-mode transmission data in PET

We present an analytical scatter correction, based upon the Klein-Nishina formula, for singles-mode transmission data in positron emission tomography (PET) and its implementation as part of an iterative image reconstruction algorithm. We compared our analytically-calculated scatter sinogram data with previously validated simulation data for a small animal PET scanner with 68 Ge (a positron emitter) and 57 Co (approximately 122-keV photon emitter) transmission sources using four different phantom configurations (three uniform water cylinders with radii of 25, 30, and 45 mm and a nonuniform phantom consisting of water, Teflon, and air). Our scatter calculation correctly predicts the contribution from single-scattered (one incoherent scatter interaction) photons to the simulated sinogram data and provides good agreement for the percent scatter fraction (SF) per sinogram for all phantoms and both transmission sources. We then applied our scatter correction as part of an iterative reconstruction algorithm for PET transmission data for simulated and experimental data using uniform and nonuniform phantoms. For both simulated and experimental data, the reconstructed linear attenuation coefficients (mu-values-values) agreed with expected values to within 4% when scatter corrections were applied, for both the 68 Ge and 57 Co transmission sources. We also tested our reconstruction and scatter correction procedure for two experimental rodent studies (a mouse and rat). For the rodent studies, we found that the average mu-values for soft-tissue regions of interest agreed with expected values to within 4%. Using a 2.2-GHz processor, each scatter correction iteration required between 6-27 min of CPU time (without any code optimization) depending on the phantom size and source used. This extra calculation time does not seem unreasonable considering that, without scatter corrections, errors in the reconstructed mu-values were between 18%-45% depending on the phantom size and transmission source used.     

康普顿散射对X射线透射谱的影响

采用一定组分的材料对X射线进行辐射屏蔽,可以吸收大部分的低能光子,减少X射线对电子系统的辐射损伤。但通过研究发现,由于康普顿散射的影响,在一定条件下,透射谱中的低能光子数不但没有减少反而大于入射谱,这对于电子元器件的辐射防护是不利的。通过改变入射源光子能谱、探测器位置、屏蔽材料类型以及屏蔽材料厚度等参量,采用蒙特卡罗方法计算并分析了散射光子对X射线透射谱的影响,得到了康普顿散射光子对X射线透射谱的影响规律,并提出了降低散射光子影响的几种措施。     

Incorporation of system resolution compensation (RC) in the ordered-subset transmission (OSTR) algorithm for transmission imaging in SPECT

In order to reconstruct attenuation maps with improved spatial resolution and quantitative accuracy, we developed an approximate method of incorporating system resolution compensation (RC) in the ordered-subset transmission (OSTR) algorithm for transmission reconstruction. Our method approximately models the blur caused by the finite intrinsic detector resolution, the nonideal source collimation and detector collimation. We derived the formulation using the optimization transfer principle as in the derivation of the OSTR algorithm. The formulation includes one forward-blur step and one back-blur step, which do not severely slow down reconstruction. The formulation could be applicable to various transmission geometries, such as point-source, line-source, and sheet-source systems. Through computer simulations of the MCAT phantom and transmission measurements of the air-filled Data Spectrum Deluxe single photo emission computed tomography (SPECT) Phantom on a system which employed a cone-beam geometry and a system which employed a scanning-line-source geometry, we showed that incorporation of RC increased spatial resolution and improved the quantitative accuracy of reconstruction. In simulation studies, attenuation maps reconstructed with RC correction improved the quantitative accuracy of emission reconstruction.     

Penalized-likelihood estimators and noise analysis forrandoms-precorrected PET transmission scans

This paper analyzes and compares image reconstruction methods based on practical approximations to the exact log likelihood of randoms-precorrected positron emission tomography (PET) measurements. The methods apply to both emission and transmission tomography, however, in this paper we focus on transmission tomography. The results of experimental PET transmission scans and variance approximations demonstrate that the shifted Poisson (SP) method avoids the systematic bias of the conventional data-weighted least squares (WLS) method and leads to significantly lower variance than conventional statistical methods based on the log likelihood of the ordinary Poisson (OP) model. We develop covariance approximations to analyze the propagation of noise from attenuation maps into emission images via the attenuation correction factors (ACF's). Empirical pixel and region variances from real transmission data agree closely with the analytical predictions. Both the approximations and the empirical results show that the performance differences between the OP model and SP model are even larger, when considering noise propagation from the transmission images into the final emission images, than the differences in the attenuation maps themselves.     

W-band Imaging Detector with Incoherent Illumination

A single-pixel imaging detector operating in W-band is described. It is based on a wideband receiving module using direct detecting circuitry and a transmitting unit with a pulse-modulated wideband noise source. The detector consists of a single transmit / receive channel mounted on a programmable scanner in an X-Y plane configuration. A LabView interface integrated with MatLab is employed allowing image continues reconstruction line-by-line. The application of incoherent illumination generated by a frequency-limited noise source improves image quality compared with conventional coherent sources used in active imagers. An image processing algorithm using a binary threshold level is considered with some examples of image reconstruction employing the detector.     

Orthogonal switched beams for downlink diversity transmission

A downlink transmission scheme in a switched-beam antenna system for providing diversity and beamforming simultaneously based on the space-time block coding technique is proposed. The scheme involves selecting multiple beams based on the uplink reception of the switched-beam antenna system, and determining multiple orthogonal beams from the selected beams. With the orthogonal transmit beams, the proposed switched-beam antenna system outperforms conventional switched-beam systems and approaches the optimal performance achievable by a much complex adaptive antenna system.     

基于多测量矢量压缩感知的超分辨荧光显微成像研究

在超分辨荧光显微成像技术中,单分子定位显微方法是被广泛应用的技术之一。根据荧光显微成像原理构造多测量矢量压缩感知模型（Multiple Measurement Vector-Compressed Sensing, MMV-CS）,并采用多重稀疏贝叶斯学习算法进行求解,来实现超分辨荧光图像重建。分析了有效像元大小、荧光分子生成的光子数和背景信号泊松化噪声对重建结果的影响,以及在图像进行分块处理时算法运行时间的分析。模拟和实验计算分析表明,当点扩展函数的标准差在160 nm时,有效像元大小在120、160、200 nm能取得较好的重构效果,而在60 nm时效果较差。探测器收集的光子数越多,重构效果越好,随着背景信号光子数增加时,离得越近的样品结构越不能分辨。在同样的分块处理情况下,MMV-CS比同伦算法(L1-Homotopy, L1-H）和凸优化算法(CVX)分别快一个数量级和三个数量级,因此,在研究三维超分辨荧光显微成像时,MMV-CS算法在运行时间上具有更大的优势。     

A maximum NEC criterion for Compton collimation to accurately identify true coincidences in PET

In this work, we propose a new method to increase the accuracy of identifying true coincidence events for positron emission tomography (PET). This approach requires 3-D detectors with the ability to position each photon interaction in multi-interaction photon events. When multiple interactions occur in the detector, the incident direction of the photon can be estimated using the Compton scatter kinematics (Compton Collimation). If the difference between the estimated incident direction of the photon relative to a second, coincident photon lies within a certain angular range around colinearity, the line of response between the two photons is identified as a true coincidence and used for image reconstruction. We present an algorithm for choosing the incident photon direction window threshold that maximizes the noise equivalent counts of the PET system. For simulated data, the direction window removed 56%-67% of random coincidences while retaining > 94% of true coincidences from image reconstruction as well as accurately extracted 70% of true coincidences from multiple coincidences.     

激光光束在海水中的空间传输特性分析

由于激光的初始发散角以及海水对光的散射作用,激光在水下传输时光斑大小发生改变,从而影响接收机的接收范围。采用波长为514 nm的蓝绿激光,基于水下无线光通信信道模型,模拟光子在海水中的传输过程,通过统计不同传输距离时的光子数和光子位置,建立了海水信道中的光斑扩展模型,分析了海水水质对高斯光束的光斑空间特性的影响。结果表明:高斯光束在海水信道中传输时,根据光强比值定义光斑大小的位置,1/e光斑半径与传输距离呈线性关系,线性增加系数为光源的发散半角;当接收机灵敏度为确定值时,传输较短距离后光斑逐渐减小;光斑大小由接收机灵敏度决定,随着接收机灵敏度的增加,在相同距离下,接收到的光斑大小基本呈线性增加。     

An optimization approach to the frequency-domain inverse problemfor a nonuniform LCRG transmission line

The inverse problem for a nonuniform LCRG transmission line is considered in the frequency domain. Imbedding equations for the reflection and transmission coefficients are derived through the concept of wave-splitting. An optimization approach is applied to reconstruct the line parameters as functions of the position using band-limited reflection and/or transmission data. Exact and explicit expressions for the gradients are derived, and the reconstruction algorithm (based upon a conjugate gradient method) is tested with both clean and noisy data, The problem of the nonuniqueness is also discussed     

An algorithm for three-dimensional reconstruction incorporating cross-plane rays

Conventional multislice positron cameras reconstruct a three-dimensional distribution of a positron-emitting radioscope as a set of two-dimensional transverse sections. Consequently, annihilation photons which cross two or more planes are eliminated from the data. Such an approach makes efficient use of the emitted photon flux. A method is proposed which makes more efficient use of the available photons by including both oblique and transverse section in the reconstruction. The implementation of the method consists of centering a scaled convolution filter on each detected coincidence event line and backprojecting the filter values through the three-dimensional reconstruction volume. The final image is normalized to allow for the different number of oblique and transverse sections that contribute to each point in the imaging volume. The method has been evaluated using both simulated data and measured data obtained with a routing area detector positron camera.     

Stationary positron emission tomography and its image reconstruction

Feasibility of stationary positron emission tomography (PET) using discrete detectors has been investigated by simulation studies. To enable full utilization of detector resolution, a "bank array" of detectors is proposed and an EM algorithm is adopted for image reconstruction. The bank array consists of an odd number of detector banks arranged on a circular ring with a gap equal to one half the detector width. The EM algorithm [11] is used with some modifications for reducing the quantity of computation, improving the convergence speed, and suppressing statistical noise, so as to meet the present purpose. Simulation studies involving several phantoms show that the stationary PET with the new detector array provides image quality which is good enough for clinical applications. For fast dynamic studies with low spatial resolution, the convolution-backprojection method is efficient, but for high-resolution static imaging, resolution enhancement by an iterative method is required. Problems arising in the corrections for attenuation of photons and detector sensitivity, etc., are also discussed. A totally stationary PET avoids the mechanical problems associated with accurate movement of heavy assemblies and is particularly advantageous in gated cardiac imaging or in fast dynamic studies. Elimination of a scan along the detector plane allows a quick scan in the axial direction to achieve three-dimensional imaging with a small number of detector rings.     

固态激光雷达输电线路实时建模及压缩技术北大核心CSCD

输电线路的三维重建是电网巡检的重要任务之一。为实现电路巡检自动化,提出了一种基于固态激光雷达的输电线路实时三维重建及数据压缩技术。首先,针对固态激光雷达的特点对经典激光SLAM框架进行改进,优化其特征提取过程并加入了运动补偿,使用改进的算法对输电线路进行实时建模;然后,使用加入权重因子改进的模糊C均值聚类方法对点云模型进行降噪滤波,去除离群点及噪点;最后,为了降低大型点云数据的储存及传输开销,设计基于时空编码的方法对输电线路模型进行压缩。实验结果表明,该方法可以实现输电线路场景的高精度实时建模,压缩后的模型可以满足储存及传输要求。     

The Use of Filtering Methods to Compensate for Constant Attenuation in Single-Photon Emission Computed Tomography

A back projection of filtered projection (BKFIL) reconstruction algorithm is presented that is applicable to single-photon emission computed tomography (ECT) in the presence of a constant attenuating medium such as the brain. The filters used in transmission computed tomography (TCT)-comprised of a ramp multiplied by window functions?are modified so that the single-photon ECT filter is a function of the constant attenuation coefficient. The filters give good reconstruction results with sufficient angular and lateral sampling. With continuous samples the BKFIL algorithm has a point spread function that is the Hankel transform of the window function. The resolution and statistical properties of the filters are demonstrated by various simulations which assume an ideal detector response. Statistical formulas for the reconstructed image show that the square of the percent-root-mean-square (percent-rms) uncertainty of the reconstruction is inversely proportional to the total measured counts. The results indicate that constant attenuation can be compensated for by using an attenuation-dependent filter that reconstructs the transverse section reliably. Computer time requirements are two times that of conventional TCT or positron ECT and there is no increase in memory requirements.     

Optimal use of Markov models for DPCM picture transmission over noisy channels

Joint source/channel decoders that use the residual redundancy in the source are investigated for differential pulse code modulation (DPCM) picture transmission over a binary symmetric channel. Markov sequence decoders employing the Viterbi algorithm that use first-order source statistics are reviewed, and generalized for decoders that use second-order source statistics. To make optimal use of the source correlation in both horizontal and vertical directions, it is necessary to generalize the conventional Viterbi decoding algorithm for a one higher-dimensional trellis. The paths through the trellis become two-dimensional "sheets", thus, the technique is coined "sheet decoding". By objective [reconstruction signal-to-noise ratio (SNR)] and subjective measure, it is found that the sheet decoders outperform the Markov sequence decoders that use a first-order Markov model, and outperform two other known decoders (modified maximum a posteriori probability and maximal SNR) that use a second-order Markov model. Moreover, it is found that the use of a simple rate-2/3 block code in conjunction with Markov model-aided decoding (MMAD) offers significant performance improvement for a 2-bit DPCM system. For the example Lenna image, it is observed that the rate-2/3 block code is superior to a rate-2/3 convolutional code for channel-error rates higher than 0.035. The block code is easily incorporated into any of the MMAD DPCM systems and results in a 2-bit MMAD DPCM system that significantly outperforms the noncoded 3-bit MMAD DPCM systems for channel-error rates higher than 0.04.     

基于感知组织的输电线路结构识别方法

为了提高输电线路缺陷诊断正确率，有效降低各种复杂背景纹理及光线对识别输电线路结构的影响，从Gestalt感知理论着手，研究一种多感知识别输电线路结构的方法。在图像识别的底层，提取不同方向、不同宽度的线段，研究了一种融合计算Gestalt定律的近似性、连续性、共线性的多级搜索算法，获得显著的、完整的输电线路人造对象轮廓；在图像识别的中层，研究一种基于分块与合并的计算方法能视觉感知近平行线、近对称交叉的结构，设计了一个三级分类器感知聚类平行线组；在图像识别的高层，研究输电线路的知识模型，建立识别输电线路组成结构的约束机制，进而从语义上唯一地识别输电线路的结构。通过无人机巡检采集的输电线路图像，验证这种方法能有效识别输电线路组成的杆塔、导线、地线及绝缘子所在区域。     

Near–Far Resistant ICA based Detector for DS-CDMA Systems in the Downlink

The performance of a conventional single user DS-CDMA receiver is severely limited by multiple access interference (MAI) and near–far effects. This severity has motivated research into adaptive filtering, near–far resistant detectors and power control strategies for DS-CDMA systems. In this paper, we propose a near–far resistant detector based on independent component analysis (ICA) of the received signal. Since ICA is a blind technique, the proposed ICA based detector has the potential to combat the near–far problem. The ICA is a higher order statistical technique based on the assumption of independence of source signals. The assumptions in ICA algorithm are the realistic conditions in a DS-CDMA system and therefore ICA algorithm can be applied successfully to detect the signal of the desired user. The focus of this paper is to illustrate the near–far resistance capability of the ICA based detector. Simulation studies performed on the proposed detector show that it is resistant to the near-far problem and has low bit error rate.     

Achieving energy‐neutral data transmission by adjusting transmission power for energy‐harvesting wireless sensor networks

Recently, benefiting from rapid development of energy harvesting technologies, the research trend of wireless sensor networks has shifted from the battery‐powered network to the one that can harvest energy from ambient environments. In such networks, a proper use of harvested energy poses plenty of challenges caused by numerous influence factors and complex application environments. Although numerous works have been based on the energy status of sensor nodes, no work refers to the issue of minimizing the overall data transmission cost by adjusting transmission power of nodes in energy‐harvesting wireless sensor networks. In this paper, we consider the optimization problem of deriving the energy‐neutral minimum cost paths between the source nodes and the sink node. By introducing the concept of energy‐neutral operation, we first propose a polynomial‐time optimal algorithm for finding the optimal path from a single source to the sink by adjusting the transmission powers. Based on the work earlier, another polynomial‐time algorithm is further proposed for finding the approximated optimal paths from multiple sources to the sink node. Also, we analyze the network capacity and present a near‐optimal algorithm based on the Ford–Fulkerson algorithm for approaching the maximum flow in the given network. We have validated our algorithms by various numerical results in terms of path capacity, least energy of nodes, energy ratio, and path cost. Simulation results show that the proposed algorithms achieve significant performance enhancements over existing schemes. Copyright © 2016 John Wiley & Sons, Ltd.     

一种红外目标模拟器的大气传输校准方法研究

红外辐射在大气中传输会在大气分子、气溶胶粒子的吸收和散射以及大气自身辐射的影响下发生变化,导致红外辐射测量精度的降低。为消除大气在红外目标模拟器校准中的影响,在基于恒定标准源的宽动态红外辐射测量方法的基础上,提出了一种红外目标模拟器的大气传输校准方法。在水平均匀大气近距离的红外目标模拟器校准中,利用卷积神经网络的数据分析能力建立了不同波段、不同温度、不同距离下的大气透过率和大气程辐射的动态模型,将探测器输出电压作为基于编码器-解码器结构的卷积神经网络的输入,按照训练流程对网络进行训练,在实验环境下预测了大气传输对红外辐射的影响。所建模型能够反映大气透过率和大气程辐射的动态变化规律,并通过红外辐射反演对提出的方法进行了验证。实验结果表明：基于编码器-解码器结构的卷积神经网络算法能够较好地预测大气透过率和大气程辐射,在三个波段下的平均误差为3.078 3%、3.818 6%、5.345 2%,低于传统方法,降低了大气透过率和大气程辐射的影响,从而减小了红外辐射的测量误差,提高了校准精度。