基于光时分复用技术的高速成像系统北大核心CSCD

提出了一种基于光时分复用技术的高速成像系统。飞秒激光器中心波长1557nm,脉冲宽度90fs,对USAF-1951分辨率板线性扫描成像,扫描频率为38.88 MHz。在连续时间序列编码放大显微成像技术的基础上,运用光时分复用技术,复制光脉冲信号并携带检测物体相同的空间信息。原光脉冲和复制光脉冲以相同的采样率分别采样,通过相应的数据处理将两次采样数据整合在一起还原图像。实验结果表明,与传统的超快成像方法相比,成像系统利用10GHz的数字采样设备可以达到20GHz的采样率,采样点数是传统超快成像方法的两倍。该方法有效克服了成像系统采样率不足的问题,提高了成像系统的空间分辨率。与此同时,该系统算法复杂程度不高,有利于进一步促进超高速成像技术的发展。     

基于成像差分吸收光谱技术和压缩感知理论的烟囱烟羽断层重建研究

烟羽断层重建质量受两方面条件限制：其中一个限制条件是遥感设备的时间分辨率。以往的研究多使用多轴差分吸收光谱仪（MAX-DOAS）进行CT重建,受采集数据速度的限制,重建图像的时间分辨率较低。另一个限制条件是,采集到的数据量有限,是典型的不完全角度重建。过去多使用代数迭代重建算法或统计迭代重建算法,重建图像受测量误差的影响比较大,分辨率较低且伪影较多。构造了基于成像差分吸收光谱技术（IDOAS）的光谱数据采集系统,与多轴差分吸收光谱仪构造的系统相比,数据采集的时间分辨率提高了160多倍,基本解决了时间分辨率的问题。提出了一种基于压缩感知理论和低三阶导数模型的烟羽断层重建算法--投影凸函数集低三阶导数法,简称为POCS-LTD。在投影的过程中,使用代数重建算法使重建图像符合投影方程;在全变分迭代的过程中使用了优化算法,将低三阶导数模型的全变分归一化值作为优化算法的迭代方向,前次迭代运算结果与本次投影运算的差值的模作为迭代步长。对重建算法进行了数值模拟,并以重建图像的接近度和一致性相关因子为指标,对重建结果进行了分析。数值模拟表明,算法具有良好的抗误差能力,与传统的低三阶导数法相比,本文提出的算法将重建接近度减小了80%以上。使用烟羽数据采集系统进行了外场实验,用POCS-LTD算法对外场实验的数据进行了烟羽重建,重建图像显示烟羽图像清晰,伪影得到了较好的抑制。介绍的烟羽断层数据采集系统和烟羽断层重建算法,提高了烟羽断层重建图像的时间分辨率,减少了重建图像的伪影,扩大了光谱测量技术的应用范围。     

压缩感知光谱成像技术的编码孔径与探测器匹配优化

编码孔径光谱成像仪在实际应用中存在着编码模板与探测器分辨率不匹配从而降低系统分辨率的问题。针对该问题进行了两种情况分析,并通过数学理论建模给出了相应的解决方案。对于编码模板分辨率高于探测器分辨率这一情况,提出引入邻域嵌入超分辨技术的方法,实现了基于压缩感知的超分辨光谱成像。对于编码模板分辨率低于探测器分辨率这一情况,提出区块阈值划分的编码孔径,将编码微元按照区块阈值重新划分并进行灰度分级,从而实现低分辨率编码模板的高分辨率编码孔径。利用梯度投影稀疏重构(GPSR)算法进行数据立方体重建,实验结果表明:运用基于超分辨理论的编码孔径快照光谱成像系统所测得的光谱图像更精准,内容更丰富;采用基于区块阈值划分的编码孔径的编码孔径快照光谱成像系统具有更高的空间分辨率和光谱分辨率。结果证实优化后的编码孔径快照光谱成像系统,其分辨率和成像质量大幅度提升,并实现了高分辨率元件的100%利用。     

基于深度神经网络的时空编码磁共振成像超分辨率重建方法

单扫描时空编码磁共振成像是一种新型超快速磁共振成像技术,它对磁场不均匀和化学位移伪影有较强的抵抗性,但是其固有的空间分辨率较低,因此通常需要进行超分辨率重建,以在不增加采样点数的情况下提高时空编码磁共振图像的空间分辨率.然而,现有的重建方法存在迭代求解时间长、重建结果有混叠伪影残留等问题.为此,本文提出了一种基于深度神经网络的单扫描时空编码磁共振成像超分辨率重建方法.该方法采用模拟样本训练深度神经网络,再利用训练好的网络模型对实际采样信号进行重建.数值模拟、水模和活体鼠脑的实验结果表明,该方法能快速重建出无残留混叠伪影、纹理信息清楚的超分辨率时空编码磁共振图像.适当增加训练样本数量以及在训练样本中加入适当的随机噪声水平,有助于改善重建效果.     

电厂烟囱烟羽截面断层重建研究

针对多轴差分吸收光谱仪重建图像的时间分辨率较低的问题,将成像差分吸收光谱仪技术与非负最小二乘法相结合,提出了一种新的重建二氧化硫(SO2)气体的空间二维分布的方法。采用Savitzky-Golay滤波器对采样得到的柱浓度数据进行预处理,使用顺序坐标法进行重建,并利用克里金插值平滑重建图像。数值仿真结果表明,重建图像的接近度最低可达0.11。实验结果表明,所提方法从采集数据到完成图像重建的时间为52.37s,适用于捕捉烟羽截面的瞬态浓度,可实时重建SO_2分布。     

基于Hadamard优化矩阵的多分辨显微关联成像研究

为满足显微成像领域的多样化需求,解决实际应用中成像质量与成像时间之间的矛盾,提出一种基于数字微镜器件的多分辨显微关联成像方法.该方法利用LED光源作为背景照射光源,对科研级荧光正置显微镜原光路改装设计为关联成像光路,采用多分辨Hadamard优化矩阵作为数字微镜器件的预置图样,实现了生物组织样品的连续多分辨成像.实验结果表明,多分辨显微关联成像系统的分辨率可达218 nm,单组测量后可同时输出8组不同分辨率图像,能够根据实际应用中不同图像质量需求选择不同的分辨率,减少成像时间和存储空间,极大地提高了显微成像的灵活性.这种新型多分辨显微关联成像方法可以扩展至细胞筛选、细胞实时成像等领域,对推动关联成像在细胞和生物组织显微成像领域的应用具有重要意义.     

遥感干涉超光谱图像压缩编码

基于卫星干涉超光谱成像光谱仪成像原理的分析,提出了一种新的遥感超光谱图像压缩方案，利用成像推扫平移特性提出一种低存储量，帧间小波域匹配的序列压缩，只需存储两帧图像，比起单帧处理提高图像PSNR 3-4dB.为了保护图像的光谱特征，系统采用了一种新的感兴趣区域（Region of interest, ROI） 编码技术，使系统的压缩比提高8倍以上.该感兴趣区域（ROI）编码采用率失真优化斜率提升，而不是比特平面移位，使图像在相同的光谱分辨率下拥有更好的空间分辨率.试验数据表明，算法大大保护了图像的光谱特性，在8倍压缩比情况时，满足卫星干涉超光谱遥感图像要求.     

基于合成孔径聚焦技术的混凝土断面二维成像方法研究

针对提高混凝土结构超声无损检测分辨率的需要,提出了两种处理方法.在对低频8扫描数据进行合成孔径聚焦处理时,由于低频探测信号时间宽度大,波包会被拉伸,发生波包变形现象,为此,引入波包分解技术来解决这个问题.波包分解技术实质是用一些模型参数去描述检测信号,可以避免对回波信号所自.采样点进行处理,不仅解决了波包变形问题,而且可以简化计算.对于实验中商用超声传感器引起的余振问题,提出一种数字滤波的补偿方法,通过水浸方式采集数据建立传感器系统补偿模型,用于实际检测信号中振铃的抑制,可提高成像的时空分辨率.数值仿真和实验结果证明,采用这两种方法后内嵌目标定位更准确,成像分辨率明显提高.     

显微热成像系统自适应位置标定方法

利用光学微扫描技术,可在不改变探测器结构的情况下,提高显微热成像系统空间分辨率。但为了获得高质量过采样重构图像,微扫描位置需要标定。基于零点定标,提出一种各点自适应定标的微扫描位置标定方法。模拟实际系统在定标前后的欠采样图像,采用不同重构方法进行仿真对比验证;完成了实际采集的欠采样显微热图像序列的重构对比实验。实验结果表明该方法明显改善了显微热成像系统的过采样重构图像质量,提高了系统空间分辨率。此方法还可以应用在其他光电成像系统中。     

毫米波步进频连续波SAR成像系统研究

研究了一种基于毫米波的步进频连续波合成孔径雷达成像系统.基于算法研究,搭建了成像硬件及软件系统,实现信号扫描及回波信号的采集与处理.处理后的信号通过成像算法计算,获得目标物体的三维图像数据.实验测试了系统x、y、z方向的分辨率,测试结果表明系统x方向分辨率在8～9 mm,y方向分辨率为12 mm,z方向分辨率为40 m...     

基于多元相控技术的一体化光声快速成像系统

利用多元相控技术发展了一体化光声快速成像系统,该成像系统集成了激光传输、光声激发和光声信号探测.多元线性阵列探测器以反射模式接收光声信号,并通过相控聚焦技术快速成像,得到了不同深度的猪油模拟样品的光声像和活体动物不同直径血管的光声像,系统的横向分辨率为0.5 mm,光声采集成像时间为5 s.该一体化光声快速成像系统与现有的光声成像系统相比,具有快速方便等特点.     

Optimization of sparse synthetic transmit aperture imaging with coded excitation and frequency division

An effective aperture approach is used for optimization of a sparse synthetic transmit aperture (STA) imaging system with coded excitation and frequency division. A new two-stage algorithm is proposed for optimization of both the positions of the transmit elements and the weights of the receive elements. In order to increase the signal-to-noise ratio in a synthetic aperture system, temporal encoding of the excitation signals is employed. When comparing the excitation by linear frequency modulation (LFM) signals and phase shift key modulation (PSKM) signals, the analysis shows that chirps are better for excitation, since at the output of a compression filter the sidelobes generated are much smaller than those produced by the binary PSKM signals. Here, an implementation of a fast STA imaging is studied by spatial encoding with frequency division of the LFM signals. The proposed system employs a 64-element array with only four active elements used during transmit. The two-dimensional point spread function (PSF) produced by such a sparse STA system is compared to the PSF produced by an equivalent phased array system, using the Field II simulation program. The analysis demonstrates the superiority of the new sparse STA imaging system while using coded excitation and frequency division. Compared to a conventional phased array imaging system, this system acquires images of equivalent quality 60 times faster, when the transmit elements are fired in pairs consecutively and the power level used during transmit is very low. The fastest acquisition time is achieved when all transmit elements are fired simultaneously, which improves detectability, but at the cost of a slight degradation of the axial resolution. In real-time implementation, however, it must be borne in mind that the frame rate of a STA imaging system depends not only on the acquisition time of the data but also on the processing time needed for image reconstruction. Comparing to phased array imaging, a significant increase in the frame rate of a STA imaging system is possible if and only if an equivalent time efficient algorithm is used for image reconstruction.     

Photonic time-stretch analog-to-digital converter employing envelope removing technique

The previously proposed differential time-stretch preprocessor can eliminate the distortions induced by the non-uniformity of optical pulse and the pulse-to-pulse variations of the optical source. However, the time-delay element in this method reduces the processing speed of the system. In this paper, we propose an envelope removing technique to mitigate these distortions and improve the time efficiency of time-stretch system simultaneously. To verify the feasibility of this technique, a comprehensive mathematical model for this technique is developed and numerical simulation is also presented. Simulation results show that this method can effectively eliminate non-uniformity and pulse-to-pulse variations in time-stretch system. This method has two times faster processing speed than the differential time-stretch preprocessor.     

A novel compression algorithm for infrared thermal image sequence based on K-means method

High resolution in space and time is becoming the new trend of thermographic inspection of equipments, therefore, the development of a fast and precise processing and data store technique of high resolution thermal image should be well studied. This article will propose a novel global compression algorithm, which will provide an effective way to improve the precision and processing speed of thermal image data. This new algorithm is based on the decay of the temperature of thermograph and the feature of thermal image morphology. Firstly, it will sort the data in space according to K-means method. Then it will employ classic fitting calculation to fit all the typical temperature decay curves. At last, it will use the fitting parameters of the curves as the parameters for compression and reconstruction of thermal image sequence to achieve the method for which the thermal image sequence can be compressed in space and time simultaneously. To validate the proposed new algorithm, the authors used two embedded defective specimens made of different materials to do the experiment. The results show that the proposed infrared thermal image sequence compression processing algorithm is an effective solution with high speed and high precision. Compared to the conventional method, the global compression algorithm is not only noise resistant but also can improve the computing speed in hundreds of times.     

A measurement-domain adaptive beamforming approach for ultrasound instrument based on distributed compressed sensing: Initial development

High efficient acquisition of the sensor array signals and accurate reconstruction of the backscattering medium are important issues in ultrasound imaging instrument. This paper presents a novel measurement-domain adaptive beamforming approach (MABF) based on distributed compressed sensing (DCS) which seeks to simultaneously measure signals that are each individually sparse in some domain(s) and also mutually correlated with much few measurements under the Nyquist rate. Instead of sampling conventional backscattering signals at the Nyquist rate, few linear projections of the returned signal with random vectors are taken as measurements, which can reduce the amount of samples per channel greatly and makes the real-time transmission of sensor array data possible. Then high resolution ultrasound image is reconstructed from the few measurements of DCS directly by the proposed MABF algorithm without recovering the raw sensor signals with complex convex optimization algorithm. The simulated results show the effectiveness of the proposed method.     

超宽带微波检测早期乳腺肿瘤三维仿真

超宽带微波成像技术可作为有效的方法用于乳房早期乳腺肿瘤的检测. 该方法基于乳房组织和肿瘤之间较大的电学特性差异的特点进行成像, 能提供足够的分辨率以及足够的穿透深度. 本文采用时域有限差分方法建立超宽带微波信号在三维乳房组织中传播的模型, 并采用单极德拜模型完成了生物组织色散特性的模拟. 利用8发9收的天线阵列实现微波肿瘤探测, 利用共焦成像算法对乳房组织进行图像重构并进行肿瘤定位. 仿真结果显示共焦成像算法能够实现最小直径3 mm的肿瘤的检测, 同时证实了超宽带微波成像技术应用于早期乳腺肿瘤检测的有效性. 关键词： 微波成像 肿瘤检测 时域有限差分 共焦成像算法     

Super-resolution imaging via sparsity constraint and sparse speckle illumination

We present an imaging approach via sparsity constraint and sparse speckle illumination which can dramatically enhance the optical system's imaging resolution. When the object is illuminated by some sparse speckles and the sparse reconstruction algorithm is utilized to restore the blur image, numerical simulated results demonstrate that the image,whose resolution exceeds the Rayleigh limit, can be stably reconstructed even if the detection signal-to-noise ratio(SNR)is less than 10 d B. Factors affecting the quality of the reconstructed image, such as the coded pattern's sparsity and the detection SNR, are also studied.     

Fourier Telescopy extrapolation based on the sampling theorem

Fourier Telecopy (FT) technique plays a promising role in deep space target detection. To improve the imaging quality of circular transmitting telescope array FT, a new processing method which adopts an extrapolation method based on the sampling theorem is proposed. Simulation results show that the extrapolation method can obviously improve system imaging resolution as well as reconstructed image quality. Even at a relative low SNR level the method improves system imaging resolution to a certain extent.     

互相关自适应加权的医学超声成像算法研究

根据超声成像系统的超声回波信号互相关性,提出互相关自适应加权超声成像算法.该算法根据散射点回波信号之间的空间相关性设置加权系数,对不同位置的散射点进行自适应加权成像,从而降低了成像系统的旁瓣,抑制了相关性较差的噪声.通过Field II仿真的点目标和吸声斑目标处理结果表明该方法成像的横向和纵向分辨率高,成像速度快.相对于延时叠加(DAS)算法,该算法对散射点成像可提高对比度16 dB,对于吸声斑成像可提高对比度0.85 dB.最后采用完备数据集进行实验,结果表明该算法成像分辨率优于DAS算法,对比度提高了17 dB.