Resolution in high field echo planar microscopy

The application of echo planar imaging to NMR microscopy offers a temporal resolution unparalleled by other techniques. However, a major difficulty in imaging at the high field strengths used for microscopy is the effect of local field inhomogeneities caused by magnetic susceptibility effects. This can give rise to both image distortion and signal loss. In addition, the effect of diffusion in the presence of the large imaging gradients gives rise to a broadening of the point spread function and hence loss of true resolution. We compare the sensitivity of two techniques, MBEST and PEPI, to both of these effects. Analytic expressions for the signal in each echo of the two sequences are developed, and the point spread functions for the two techniques are calculated. Using PEPI, we have been able to produce images with an in-plane resolution of 50 micrometer from a single free induction decay. This technique has been extended to three dimensions allowing the generation of 64(3) images with an isotropic resolution of 80 micrometer.     

基于压缩感知超分辨鬼成像

分辨率是成像系统的一个重要参数, 获得高分辨率图像一直是鬼成像系统的一个目标. 本文提出了以成像系统点扩散函数作为先验知识, 基于稀疏测量的超分辨压缩感知鬼成像重建模型. 搭建了一套计算鬼成像实验装置, 用于验证该模型对于提高鬼成像系统分辨率的有效性, 并与传统的鬼成像计算模型进行了对比. 实验表明, 利用该模型可突破成像系统衍射极限分辨率的限制, 得到超分辨鬼成像. 关键词： 鬼成像 压缩感知 超分辨 稀疏测量     

MAP-regularized robust reconstruction for underwater imaging detection

In order to enhance the visual quality of underwater images, applications such as enhancement and restoration can be applied, but the resolution is still limited. Super-resolution reconstruction is a widely used technique for improving resolution beyond the limit of imaging system. With knowledge of the point spread function and techniques of regularization, the performance of reconstruction can be further enhanced. The presented effort proposed a robust image super-resolution reconstruction method under maximum a posteriori framework with regularization by the point spread function for underwater imaging detection. Objective image quality metrics are used to quantify the effectiveness of the reconstruction. Experimental results showed that the proposed method can effectively improve the resolution and quality of underwater imaging detection.     

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.     

球面参考光预放大数字全息成像系统分辨力

为了提高数字全息成像系统的成像分辨力,对以球面波作为参考光波的预放大数字全息成像系统的记录和再现过程进行了理论推导,得出了该系统的脉冲响应表达式,并由此分析了决定该系统成像分辨力的因素。在CCD成像分辨力高于显微物镜分辨力的情况下,通过对分辨力测试板进行实验,讨论了系统分辨力与记录距离的关系。结果表明:再现像的分辨力对记录距离不敏感;但在记录距离为零,即像面数字全息系统下,再现像具有更高的分辨力。实验结果与理论分析一致。     

Deconvolution of vibroacoustic images using a simulation model based on a three dimensional point spread function

Vibro-acoustography (VA) is a medical imaging method based on the difference-frequency generation produced by the mixture of two focused ultrasound beams. VA has been applied to different problems in medical imaging such as imaging bones, microcalcifications in the breast, mass lesions, and calcified arteries. The obtained images may have a resolution of 0.7–0.8 mm. Current VA systems based on confocal or linear array transducers generate C-scan images at the beam focal plane. Images on the axial plane are also possible, however the system resolution along depth worsens when compared to the lateral one. Typical axial resolution is about 1.0 cm. Furthermore, the elevation resolution of linear array systems is larger than that in lateral direction. This asymmetry degrades C-scan images obtained using linear arrays. The purpose of this article is to study VA image restoration based on a 3D point spread function (PSF) using classical deconvolution algorithms: Wiener, constrained least-squares (CLSs), and geometric mean filters. To assess the filters’ performance on the restored images, we use an image quality index that accounts for correlation loss, luminance and contrast distortion. Results for simulated VA images show that the quality index achieved with the Wiener filter is 0.9 (when the index is 1.0 this indicates perfect restoration). This filter yielded the best result in comparison with the other ones. Moreover, the deconvolution algorithms were applied to an experimental VA image of a phantom composed of three stretched 0.5 mm wires. Experiments were performed using transducer driven at two frequencies, 3075 kHz and 3125 kHz, which resulted in the difference-frequency of 50 kHz. Restorations with the theoretical line spread function (LSF) did not recover sufficient information to identify the wires in the images. However, using an estimated LSF the obtained results displayed enough information to spot the wires in the images. It is demonstrated that the phase of the theoretical and the experimental PSFs are dissimilar. This fact prevents VA image restoration with the current theoretical PSF. This study is a preliminary step towards understanding the restoration of VA images through the application of deconvolution filters.     

基于CdZnTe像素阵列探测技术的伽玛源成像

根据高能射线针孔成像理论,采用CdZnTe像素阵列探测器建立了直接成像探测模式的伽玛源针孔探测系统。测试分析了CdZnTe像素阵列探测器的能量分辨力及峰值效率,讨论研究了针孔成像探测系统的调制传递函数和附加噪声特性,测试获得直径5mm137Cs源的探测图像,采用Lucy-Richardson迭代算法得到了137Cs源的复原图像。实验结果表明:CdZnTe探测器对662keV137Cs源的能量分辨力为6.25%～7.50%,峰值效率65.0%～72.5%;成像系统探测图像存在一定扩散现象,所采用的Lucy-Richardson迭代复原算法能较好地修正图像扩散,提高探测图像中心区域细节分辨力;估算所得137Cs源尺寸误差约0.5mm,所建立的CdZnTe针孔成像探测系统能有效得到小尺寸伽玛源的辐照强度分布及尺寸信息。     

被动式太赫兹图像目标检测研究

研究提出一种被动式太赫兹扫描成像的目标检测方法。用0.22 THz被动式扫描成像系统采集原始图像,在滤波去噪后用针孔像分析法获得太赫兹扫描成像系统的点扩展函数,进而用Lucy-Richardson算法重构图像,通过灰度变换和边缘检测增强图像对比度和目标分辨力。实验结果表明,算法能有效改善被动式太赫兹图像质量,提高成像系统探测隐藏可疑物的能力。     

Lucy-Richardson算法用于针孔图像的恢复

计算了X射线针孔成像系统的点扩散函数矩阵，利用Lucy-Richardson算法对模拟和真实的针孔图像进行了较好的恢复，分析了恢复后得到的结果，给出了合理的迭代次数、恢复方法细节和恢复图像的分辨率，并用模拟实验验证了Lucy-Richardson算法对针孔图像进行恢复的可靠性. 关键词： 针孔成像 点扩散函数 Lucy-Richardson算法     

Focused array transducer for two-dimensional optoacoustic tomography

Optoacoustic (OA) imaging utilizes short laser pulses to create acoustic sources in tissue and time resolved detection of generated pressure profiles for image reconstruction. The ultrasonic transients provide information on the distribution of optical absorption coefficient that can be useful for early cancer diagnostics. In this work a new design of wide-band array transducer is developed and tested. The array consists of 32 focused piezo-elements made of PVDF slabs imposed on a cylindrical surface. A single array element response to an OA signal coming from arbitrarily located point source is investigated theoretically and experimentally. The measured signals correspond well to numerically calculated ones. Focal zone maps of the elements with aperture angles 30 degrees and 60 degrees are presented and discussed; the resolution in direction perpendicular to the imaging plane is determined. Point spread function of the whole array is calculated using experimentally obtained signals from the sources located at different distances from the array. Backprojection algorithm is employed for reconstruction of the optoacoustic images. It is shown that the spatial resolution of the images yielded by the proposed array increases significantly compared to previous transducer designs.     

利用小尺寸电荷耦合器件实现数字全息高分辨成像

为了在降低数字全息显微成像系统成本的同时实现高分辨成像,对像面数字全息显微术的记录与再现过程进行了理论分析,结合系统的点扩散函数,对该系统的横向分辨率进行了分析. 得到了如下结论：像面数字全息显微系统的横向分辨率对电荷耦合器件（CCD）光敏面尺寸变化不敏感;对于常见的CCD 器件,其像元尺寸的变化对该系统的横向分辨率影响甚微. 此外,对像面数字全息显微系统的成像特点进行了分析,结果表明：利用像面数字全息系统可以实现物体信息的完整记录与再现,其成像分辨率及像质优于预放大数字全息系统. 利用搭建的数字全息实验记录系统,从强度及位相两方面对理论分析结果进行了验证,实验结果表明了理论分析的正确性. 关键词： 像面数字全息术 横向分辨率 点扩散函数 小尺寸电荷耦合器件     

基于亚像素扫描的超分辨技术在高分辨X射线显微镜中的应用

在不改变现有硬件条件的情况下,开展超分辨扫描重建方法,可以在不增加系统成本的基础上提高高分辨X射线显微镜的成像性能.设计了基于亚像素扫描的超分辨扫描模式,按照设计的调制方式进行亚像素位移的移动,采集多幅具有互补信息的低分辨率图像;然后基于系统的点扩散函数,对高分辨率图像进行复原;最后结合POCS超分辨重建算法重建出高分辨图像.实验结果表明,10倍光耦探测器下的衬度噪声比提高了20%左右,空间分辨力提高了0.2μm(约15%),细节分辨能力超过探测器像素尺寸1.35μm的限制,可以看到在低分辨率图像中看不到的细节.实验说明用超分辨技术提高高分辨X射线显微镜的分辨率是有意义的.     

共聚焦成像系统中探测器对分辨率的影响

基于点扩展函数,推导出共焦系统的成像公式,模拟了探测器尺寸对分辨率的影响,并提出最佳针孔半径.为验证理论分析,设计并建立了一套共焦系统,测量了不同针孔时的轴向响应,并得到了鉴别率板的共焦和离焦像.     

Centric scan SPRITE magnetic resonance imaging: optimization of SNR, resolution, and relaxation time mapping

Two strategies for the optimization of centric scan SPRITE (single point ramped imaging with T1 enhancement) magnetic resonance imaging techniques are presented. Point spread functions (PSF) for the centric scan SPRITE methodologies are numerically simulated, and the blurring manifested in a centric scan SPRITE image through PSF convolution is characterized. Optimal choices of imaging parameters and k-space sampling scheme are predicted to obtain maximum signal-to-noise ratio (SNR) while maintaining acceptable image resolution. The point spread function simulation predictions are verified experimentally. The acquisition of multiple FID points following each RF excitation is described and the use of the Chirp z-Transform algorithm for the scaling of field of view (FOV) of the reconstructed images is illustrated. Effective recombination of the rescaled images for SNR improvement and T*2 mapping is demonstrated.     

利用维纳滤波改善声透镜光声成像系统的分辨率

为了克服衍射效应对光声成像系统分辨率的限制,需要采用逆卷积方法进行图像反演.从理论上分析了声透镜成像原理,模拟仿真了声透镜的点扩展函数对声透镜成像系统分辨率的影响和维纳滤波解卷积方法复原光声成像的过程,并利用自搭建的声透镜光声成像系统进行了深入的实验研究,得到了物平面上相距4 mm和3 mm的两个黑胶带点的直接成像光声...     

An exact, linear solution to the problem of imaging through turbulence

We show how, in principle, to solve the ‘blind deconvolution' problem. This is in the context of the problem of imaging through atmospheric turbulence. The approach is digital but not iterative, and requires as input data but two short-exposure intensity images, without the need for reference point sources. By taking the Fourier transform of each image and dividing, a set of linear equations is generated whose unknowns are sampled values of the two random point spread functions that degraded the images. An oversampling by 50% in Fourier space equalizes the number of unknowns and independent equations. With some prior knowledge of spread function support, and in the absence of added noise of image detection, the inverted equations give exact solutions. The two observed images are then inverse filtered to reconstruct the object.     

Application of multi-frame approach in single-frame blind deconvolution

A new single-frame blind deconvolution algorithm for the linear shift-invariant imaging system is presented. The algorithm processes the partial images segmented from one single degraded image by multi-frame approach to recover the point spread function (PSF). Then a deconvolution method is employed to restore the whole image with the recovered PSF. In addition, in order to improve the fidelity and resolution of the recovered PSF, the coprimeness of the partial images is utilized. Results of simulated and real atmospheric turbulence degraded images using the algorithm are reported.     

Fourier optics approach to imaging with sub-wavelength resolution through metal-dielectric multilayers

Metal-dielectric layered stacks for imaging with sub-wavelength resolution are regarded as linear isoplanatic systems — a concept popular in Fourier optics and in scalar diffraction theory. In this context, a layered flat lens is a one-dimensional spatial filter characterised by the point spread function. However, depending on the model of the source, the definition of the point spread function for multilayers with sub-wavelength resolution may be formulated in several ways. Here, a distinction is made between a soft source and hard electric or magnetic sources. Each of these definitions leads to a different meaning of perfect imaging. It is shown that some simple interpretations of the PSF, such as the relation of its width to the resolution of the imaging system are ambiguous for the multilayers with sub-wavelenth resolution. These differences must be observed in point spread function engineering of layered systems with sub-wavelength sized PSF.     

Surface imaging using diffracted electrons

Refection electron microscopy has been applied for studying single crystal surfaces of noble metals in a conventional electron microscope using diffracted electrons for the imaging. This is equivalent to dark field imaging in transmission electron microscopy. The image contains far more contrast due to deviation from the ideal surface structure than any other surface imaging technique. A resolution of the order of 20 Å has been achieved for directions perpendicular to the incident beam. In directions parallel to the incident beam the resolution is much worse and the magnification is reduced with a factor 15 to 100 because the surface is viewed at the angle of the diffracted beam used for imaging (foreshortening). Nevertheless the achieved resolution is one to two orders of magnitude better than for other surface imaging methods. Since a correspondence can be established between image and diffraction pattern by means of selected area diffraction, the present technique should be very useful in structure analysis of surfaces.     

基于灰度投影法运动估计的成像CCD平移补偿法

为了提高成像质量,得到高分辨率的图像,通过对灰度投影法进行分析,提出一种基于灰度投影法运动估计的成像CCD平移补偿法,给出其系统结构图。该方法采取只平移成像CCD,探测CCD固定,即在成像CCD补偿位移量的过程中,探测CCD不补偿位移,探测CCD在成像开始的第一帧为全局参考帧,其他帧与之比较来获得位移量。验证了分辨率标板在各种运动情况下系统的补偿效果,对拍摄的实物图像进行恢复,结果证明：该补偿法对慢速运动图像和随机振动图像具有较高的恢复能力。