基于不同频率成份衰减矫正的光声成像方法

根据超声衰减理论,研究了光声信号不同频率成份随距离的衰减差异,及其对光声图像重建的影响;提出了对光声信号不同频率成份进行衰减矫正的成像方法,此方法增强了光声信号的高频成份,突出了吸收体的边界变化和细微的结构特征,提高了成像系统分辨率,实验结果显示系统分辨率由0.3 mm提高到0.2 mm.实验所用的光源为YAG激光器,波长为1064 nm,重复频率为20 Hz,脉宽为6 ns,探测器为针状的PVDF膜水听器,接收面积的直径为1 mm.     

Imaging resolution analysis in limited-view Laser Radar reflective tomography

In many applications of Laser radar reflective tomography, anisotropic resolution of images reconstructed from an incomplete set of reflective projections will degrade imaging quality. In this paper, we present the theoretical point spread function (PSF) of imaging system over full views for reflective tomography. The corresponding imaging resolution of the PSF was determined by Rayleigh's criterion, here the most common algorithm of filtered backprojection was used for image reconstruction. The theoretical imaging resolution was derived approximately from the range resolution resolved by the laser pulse. The simulated horizontal imaging resolutions of an ideal diffused single point target in different limited-views were compared with the theoretical value. Experimental reconstructed images from limit-view projections of letter “E” illustrated the effects of anisotropic resolution.     

Single‐cell resolution in high‐resolution synchrotron X‐ray CT imaging with gold nanoparticles

Gold nanoparticles are excellent intracellular markers in X‐ray imaging. Having shown previously the suitability of gold nanoparticles to detect small groups of cells with the synchrotron‐based computed tomography (CT) technique both ex vivo and in vivo, it is now demonstrated that even single‐cell resolution can be obtained in the brain at least ex vivo. Working in a small animal model of malignant brain tumour, the image quality obtained with different imaging modalities was compared. To generate the brain tumour, 1 × 10⁵ C6 glioma cells were loaded with gold nanoparticles and implanted in the right cerebral hemisphere of an adult rat. Raw data were acquired with absorption X‐ray CT followed by a local tomography technique based on synchrotron X‐ray absorption yielding single‐cell resolution. The reconstructed synchrotron X‐ray images were compared with images obtained by small animal magnetic resonance imaging. The presence of gold nanoparticles in the tumour tissue was verified in histological sections.     

A study on the evaluation of usefulness depending on the image reconstruction method in bone SPECT scan

The recent advances in image-processing techniques have led to the development of many methods to reduce the scan time without degrading the image quality. In particular, tomography has improved image reconstruction methods with the concomitant improvement of high-quality images. In this study, PRECEDENCE 16 was used to reconstruct images using the filtered back projection method, which is generally used, and the Astonish method and three-dimensional ordered-subsets expectation maximization method, which are based on repetition techniques. In qualitative and quantitative analysis of the reconstructed images, a comparison was made between images with different acquisition times and between images with the same acquisition time, which aimed at determining the optimal method for reconstructing high-quality images. A blind test for qualitative analysis confirmed almost no difference in image quality depending on the image acquisition time. Furthermore, in quantitative analysis, there was no significant difference in image quality depending on the image acquisition time. On the other hand, the results of the analysis in the image reconstruction method with the same acquisition time demonstrated a significant difference. The images reconstructed by the Astonish method, which uses a repetition technique, are believed to be excellent because they have high resolution and provide clinical diagnostic information. This study confirmed that the reconstruction method with a repetition technique could be used to improve image quality and reduce the scan time, despite not being in general use until recently due to the lengthy time needed for image reconstruction and lack of storage space.     

扩散张量成像的人脑模板构建

扩散张量脑模板包含丰富的大脑白质组织信息,在空间标准化或者脑图谱创建中具有重要价值,然而基于扩散张量模型构建的脑模板精度不高,特别是在脑部复杂的神经元微观结构区域中应用受到限制.针对这一问题,研究者们提出了基于高分辨率扩散成像构建大脑模板的方法.本文对使用扩散张量成像方法进行脑模板构建的研究进展进行了综述,首先介绍了扩散张量脑模板构建的发展进程,阐述了脑模板构建中解决的技术问题及同时存在的局限性;接着详细论述了基于扩散频谱成像及高角度分辨率扩散成像构建脑模板的不同方法间的差异,并总结了这些研究方法取得的重要进展;最后通过分析目前研究进展提出该研究问题中存在的不足以及未来的发展趋势.     

基于声透镜成像系统的光声层析成像

光声成像是采用“光激发-超声波成像”的新型成像技术，它是检测强散射介质内部光吸收分布的一种有效医学影像技术.用短脉冲激光照射强散射介质(如生物组织)，强散射介质由于光声效应产生超声信号，使用具有成像能力的声透镜把声压分布成像于像面上，然后利用具有空间分辨能力的阵列光声传感器，把同一像面上的光声信号强度记录下来，最后根据光声信号强度的空间分布进行图像重组.根据成像系统物像共轭原理，同一物平面的光声信号到达像面的时延相等，而不同物面的光声信号到达同一个探测器平面的时延各不相同，因此，利用BOXCAR的门控积分 关键词： 光声层析成像 声透镜 光声信号     

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。     

S波段微波热致超声成像系统研究

微波热致超声成像技术通过向物体发射微波脉冲, 导致物体吸收电磁波温度迅速升高, 产生瞬时压力波, 从而激发产生超声波信号, 通过传感器对产生的超声波信号进行采集并成像, 最终还原了反映物体吸收电磁波能量特性的图像, 由于此方法兼具了微波成像的高对比性和超声成像的高分辨率特点, 理论上验证了热声成像技术对早期乳腺肿瘤检测的可行性. 本实验兼顾系统成像深度和分辨率, 采用S波段的微波脉冲信号源对物体进行辐射, 利用圆形扫描方式对待测物体进行检测, 同时为了更好的验证成像性能, 本实验同时使用了肿瘤仿体及实际生物组织进行成像实验. 通过实验分析, 验证了该系统对肿瘤仿体和生物组织检测的有效性, 以及系统的高分辨率和高对比度特性, 为早期乳房肿瘤检测提供了进一步的理论支撑.     

基于Hartley变换的磁共振成象

许多磁共振成象的应用场合需要利用正交双通道来采集一个具有时间和空间分辨力的对象序列。传统的基于Ｆｏｕｒｉｅｒ变换的成象方法，一方面，图象序列的重建时各帧图象是独立地进行重建的，因而图象序列的时间分辨力受到编码的限制；另一方面，来自两个通道之间的Ｆａｒｔｌｅｙ变换的磁共振成象技术。     

轴类超声检测的波束扩散成像*

大型轴类工件安装完成后通常只能从其端面进行检测,危害性的裂纹类缺陷不易检出,针对上述问题,本文提出了轴类对象基于波束扩散的端面网格化超声检测层析成像方法,搭建了轴类超声检测试验平台,在端面对其内部的平底孔缺陷进行检测,基于波束扩散设计叠加算法进行数据处理,然后根据结果重构出代表每个深度内部信息的伪彩色横截面图像,以方便快速识别内部缺陷位置和范围。与常规方法对比得出,基于波束扩散的叠加算法处理数据成像质量更佳,提高缺陷检测灵敏度达到12dB以上。     

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.     

纵向带状裂隙形貌的逆时偏移超声成像

传统的工业超声成像方法通常只能确定缺陷的位置与横向尺寸,无法获得缺陷的形貌信息.一些特殊的缺陷,如纵向裂纹,是典型的例子.基于多阵元技术,开展了固体介质缺陷逆时偏移超声成像方法的数值与实验研究.针对铝块平底纵裂纹及内部纵裂纹两种传统方法无法有效成像的缺陷,首先开展了单分量逆时偏移成像方法研究,给出了基于数值仿真的逆时偏移成像结果以及基于多阵元超声成像实验系统实验测试的逆时偏移成像结果.进一步开展了基于多分量位移检测与转换横波分离的逆时偏移成像方法研究,并提出了基于新型多分量激光干涉仪进行检测的思路.数值仿真结果证实了多分量逆时偏移图像重建结果可以克服单分量方式的缺点,得到明显优于单分量检测时的图像.     

基于滤波反投影的光声图像重建

 为了进一步提高光声重建图像的质量,采用滤波反投影算法进行图像重建。实验所用的光源为YAG激光器,波长为1 064 nm,重复频率为20 Hz,脉宽为7 ns,探测器为针状的PVDF膜水听器,接收面的直径为1mm,得到了4个圆形吸收体和5根头发的光声重建图像。仿真和实验结果表明：通过对信号进行滤波,能很好地抑制噪声信号,明显提高图像的对比度和分辨率。     

Lamb waves topological imaging combining with Green's function retrieval theory to detect near filed defects in isotropic plates

A method of combining Green's function retrieval theory and ultrasonic array imaging using Lamb waves is presented to solve near filed defects in thin aluminum plates.The defects are close to the ultrasonic phased array and satisfy the near field calculation formula.Near field acoustic information of defects is obscured by the nonlinear effects of initial wave signal in a directly acquired response using the full matrix capture mode.A reconstructed full matrix of inter-element responses is produced from cross-correlation of directly received ultrasonic signals between sensor pairs.This new matrix eliminates the nonlinear interference and restores the near-field defect information.The topological imaging method that was developed in recent ultrasonic inspection is used for displaying the scatterers.The experiments are conducted on both thin aluminum plates containing two and four defects, respectively.The results show that these defects are clearly identified when using a reconstructed full matrix.The spatial resolution is equal to about one wavelength of the selectively excited mode and the identifiable defect is about one fifth of the wavelength.However, in a conventional directly captured image,the images of defects overlap together and cannot be distinguished.The proposed method reduces the background noise and allows for effective topological imaging of near field defects.     

Progressive EPR imaging with adaptive projection acquisition

Continuous wave electron paramagnetic resonance imaging (EPRI) of living biological systems requires rapid acquisition and visualization of free radical images. In the commonly used multiple-stage back-projection image reconstruction algorithm, the EPR image cannot be reconstructed until a complete set of projections is collected. If the data acquisition is incomplete, the previously acquired incomplete data set is no longer useful. In this work, a 3-dimensional progressive EPRI technique was implemented based on inverse Radon transform in which a 3-dimensional EPR image is acquired and reconstructed gradually from low resolution to high resolution. An adaptive data acquisition strategy is proposed to determine the significance of projections and acquire them in an order from the most significant to the least significant. The image acquisition can be terminated at any time if further collection of projections does not improve the image resolution distinctly, providing flexibility to trade image quality with imaging time. The progressive imaging technique was validated using computer simulations as well as imaging experiments. The adaptive acquisition uses 50-70% less projections as compared to the regular acquisition. In conclusion, adaptive data acquisition with progressive image reconstruction should be very useful for the accelerated acquisition and visualization of free radical distribution.     

利用匹配区域的纹理特征改善重构三维图像的视觉质量

研究了计算机重构三维图像时分辨率低的问题,提出一种改善3D计算机全景重构图像的视觉质量的方法,该方法利用3D空间的物体部分在每个元素图像中形成的匹配区域的纹理特征,从两个相邻的元素图像中的匹配区域提取出多个像素,经过加权计算重构出相应的图像区域.该方法与传统的计算机重构方法相比,提高了图像分辨率,同时也消除了从每个元素图像中提取多个像素直接重构图像时存在的"像素块"效应,改善了重构图像的视觉质量.     

超声虚源成像中自适应双向空间逐点聚焦方法

针对传统超声成像中图像分辨率和对比度随深度下降的问题,提出了一种基于虚源的自适应双向空间逐点聚焦超声成像方法。首先,使用超声换能器线列阵分子孔径分别定焦点发射和接收超声波,采集扫描线数据;然后将焦点视为虚拟点声源,计算虚源到空间成像点的延时,利用合成孔径原理再次进行空间逐点聚焦;在合成过程中采用相干系数进行自适应加权。采用空间脉冲响应法对不同深度的点目标和囊目标仿真成像,从而量化分辨率和对比度。在F数为1.5、焦距为10 mm时(对应子孔径阵元数为17)可以获得与64通道定焦点发射、动态聚焦接收相当的图像质量且在所有深度上保持一致。实际硬件平台的体模成像实验进一步验证了方法的有效性。该方法可在整个成像深度范围内保持和常规成像一致的分辨率和对比度,从而获得更优的整体成像效果。      

基于代数重建算法的有限角度扫描的光声成像

由于滤波反投影重建算法要求对成像区域进行全方位扫描以获取完全投影数据,它需要较长时间采集大量数据,使其在医学上的应用受到限制。研究了在有限角度下采用代数重建算法进行光声成像的方法,实验用的光源为YAG激光器,波长为1064nm,重复频率20Hz,脉宽为6ns,探测器为针状的磺化聚二氟乙烯(PVDF)膜水听器,接收面积的直径为1mm,从仿真和实验结果表明该方法适用于“非完备投影数据”的光声层析成像。从图像重建效果上与滤波反投影算法相比较,该成像算法提高了重建图像的分辨率和对比度。采用代数重建算法的有限角度的光声成像方法,对临床医学的无损伤检测,具有重要的意义。     

Feasibility of photoacoustic tomography for ophthalmology

For the eyeball composed of membrane and liquid, the contrast of ultrasound imaging is not high due to its small variance in acoustic impedance. As a new imaging modality, photoacoustic tomography combines the advantages of pure optical and ultrasonic imaging together and can provide high resolution, high contrast images. In this paper, the feasibility of photoacoustic tomography for ophthalmology is studied experimentally. A Q-switched Nd:YAG pulsed laser with 7-ns pulse width is used to generate photoacoustic signal of a porcine eyeball in vitro. The two-dimensional (2D) optical absorption image of the entire eyeball is reconstructed by time-domain spherical back projection algorithm. The imaging results agree well with the histological structure of the eyeball and show a high imaging contrast.     

Sensor resolution enhancement for remote imaging by synthesizing mask-based camera array images

In this paper, we propose an architecture based on camera array with masks to enhance the sensor resolution of remote imaging system. The sensor resolution is enhanced by multiplexing the sensor images of each camera, in which the angular resolution bandwidth is converted using masks to expand the spatial resolution bandwidth. The improvement of the sensor resolution depends on the number of cameras in the array. The theory of improving the sensor resolution is discussed both in Fourier domain and spatial domain. We verify resolution enhancement of the architecture by ray-tracing imaging simulation. A simulation model is built to verify the resolution-enhancing ability of the camera array architecture. From each camera in the array, we get a resolution-limited image. The reconstructed image is synthesized from all the images of the cameras. The post-synthesized image has finer information details compared with the images of each camera. The resolution improvement varies with the object distance. The optimal resolution improvement of the reconstructed image is equal to the total sensor pixels of the camera array.