Microwave holograms and microwave reconstruction

Microwave holographic reconstruction of the metal objects inside a purse was achieved by a nonscanning method. A microwave field was used for both constructing and reconstructing the holographic image. The visualisation of the microwave field was performed by both Polaroid and liquid-crystal techniques. The frequency used was 34.26GHz.     

Real-time optical reconstruction of microwave holograms

Real-time optical reconstruction from microwave hologram data is accomplished by means of a noncoherent-to-coherent image converter based on an erasable liquid-crystal photoconductor cell operating in a dynamic scattering mode. Reconstructed image quality is shown to be comparable to that which is obtained directly from photographic transparency records of the microwave hologram.     

影响数字全息再现像像质的因素分析

详细地讨论了CCD的感光像元、抽样间隔和感光面积这三个结构参量对数字全息再现像的影响,并分析了结构参量之间的相互关系.结果表明,CCD的三个结构参量不仅分别影响着数字全息图再现像的亮度、大小和分辨率,同时还与记录光路相关联并相互影响.在CCD的结构参量、物体大小及记录波长都确定的情况下,设计了最佳全息记录光路,使CCD的带宽得到了充分利用,获取的信息量也达到了极限.通过综合归纳,给出了充分利用CCD结构参量进行高质量数字全息记录的建议,并进行了实验验证.     

Tomographic microwave diversity image reconstruction employingunitary compression

Data compression through a unitary transform is utilized in tomographic microwave diversity image reconstruction in order to reduce the dimensionality and to extract the features in the data space. The unitary compression is derived by minimizing the mean-square error (MSE) and the unitary transform is made of eigenvectors of the data's covariance, regarded as a Karhunen-Loeve transform. Tomographic microwave frequency-swept imaging was developed using a unique target-derived reference technique to access the three dimensional Fourier space of the scatterer and an image reconstruction algorithm based on the projection slice theorem. It is shown that centimeter resolution of a complex object can be preserved even when half of the data set is compressed and that the reconstructed image remains identifiable by a human observer even when 2/3 of the data set is compressed     

电子全息的数字重现及其在微电磁场测量中的应用

阐述了用数」字方法重现电子全息图及实现噪声消除、位相差放大的基本原理,将它应用到微电磁场测量中,给出了实验结果,报道了光学重现未能观测到的新现象。     

Three-dimensional nonlinear image reconstruction for microwave biomedical imaging

Active microwave imaging has attracted significant interests in biomedical applications, in particular for breast imaging. However, the high electrical contrasts in breast tissue also increases the difficulty of forming an accurate image because of the increased multiple scattering. To model such strong three-dimensional (3-D) multiple scattering effects in biomedical imaging applications, we develop a full 3-D inverse scattering algorithm based on the combination of the contrast source inversion and the fast Fourier transform algorithm. Numerical results show that our algorithm can accurately invert for the high-contrast media in breast tissue.     

Singular value analysis of the Jacobian matrix in microwave image reconstruction

For non-linear inverse scattering problems utilizing Gauss-Newton methods, the Jacobian matrix encodes rich information concerning the system performance and algorithm efficiency. In this paper, we perform an analytical evaluation of a single-iteration Jacobian matrix based on a previously derived nodal adjoint representation. Concepts for studying linear ill-posed problems, such as the degree-of-ill-posedness, are used to assess the impact of important system parameters on the expected image quality. Analytical singular value decomposition (SVD) of the Jacobian matrix for a circular imaging domain is derived along with the numerical SVD for optimizing imaging system configurations. The results show significant reductions in the degree-of-ill-posedness when signal frequency, antenna array density and property parameter sampling are increased. Specifically, the decay rate in the singular spectrum of the Jacobian decreases monotonically with signal frequency being approximately 1/3 of its 0.1 GHz value at 3 GHz, is improved with antenna array density up to about 35 equally-spaced circumferentially positioned elements and drops significantly with increased property parameter sampling to more than twice the amount of measurement data. These results should serve as useful guidelines in the development of design specifications for an optimized hardware installation.     

Image reconstruction from acoustical holograms using microprocessors

An acoustic imaging system using holographic techniques is described. Insonification of the object is achieved using a phased array scanner. Image reconstruction uses Fourier Transformations to produce a 128 × 128 pixel TV display. The system has the ability to focus in the ‘near field’ of its transducer arrays and has a frame rate of approximately 0.3 Hz.     

Feasibility of high-resolution optical images from microwave far-field holograms

It is demonstrated, with the aid of simple facsimile far-field (Fourier-transform) holograms, that optical images can be obtained from limited far-field information and that the quality of these images is also dependent on the spatial frequency of the `carrier fringes? which lace the hologram. The use of a synthetic reference wave in a practical case offers advantageously the possibility of high `carrier-fringe? frequencies and hence high-quality optical images.     

Real-time detection of metallic objects using liquid-crystal microwave holograms

A variation is suggested of the recently described microwave hologram procedure for detecting concealed weapons, whereby merely the hologram itself is employed (without reconstruction), with the hologram being displayed immediately by means of liquid crystals.     

Confocal microwave imaging for breast cancer detection: delay-multiply-and-sum image reconstruction algorithm

A new image reconstruction algorithm, termed as delay-multiply-and-sum (DMAS), for breast cancer detection using an ultra-wideband confocal microwave imaging technique is proposed. In DMAS algorithm, the backscattered signals received from numerical breast phantoms simulated using the finite-difference time-domain method are time shifted, multiplied in pair, and the products are summed to form a synthetic focal point. The effectiveness of the DMAS algorithm is shown by applying it to backscattered signals received from a variety of numerical breast phantoms. The reconstructed images illustrate improvement in identification of embedded malignant tumors over the delay-and-sum algorithm. Successful detection and localization of tumors as small as 2 mm in diameter are also demonstrated.     

Improving displayed resolution in convolution reconstruction of digital holograms

In digital holography,the hologramis recorded by aCCDor CMOSandthe holographici mageis numericallyreconstructed by a computer.Compared with conven-tional holography,digital holographycan be usedtosi m-ultaneously obtain the amplitude and phase information…     

The multidimensional phase unwrapping integral and applications to microwave tomographical image reconstruction.

Spatial unwrapping of the phase component of time varying electromagnetic fields has important implications in a range of disciplines including synthetic aperture radar (SAR) interferometry, MRI, optical confocal microscopy, and microwave tomography. This paper presents a fundamental framework based on the phase unwrapping integral, especially in the complex case where phase singularities are enclosed within the closed path integral. With respect to the phase unwrapping required when utilized in Gauss-Newton iterative microwave image reconstruction, the concept of dynamic phase unwrapping is introduced where the singularity location varies as a function of the iteratively modified property distributions. Strategies for dynamic phase unwrapping in the microwave problem were developed and successfully tested in simulations and clinical experiments utilizing large, high contrast targets to validate the approach.     

Digital decoding of in-line holograms for imaging fractal aggregates

The digital inverse filtering technique developed for improving the quality of in-line holograms by suppressing the twin image is shown to be effective for aerosol aggregate holograms. The aggregates are simulated as fractal patterns. Conventional reconstruction results are compared with the results of the digital decoding.     

Bayesian approach with the maximum entropy principle in image reconstruction from microwave scattered field data

Microwave imaging is of great interest in medical applications owing to its high sensitivity with respect to dielectric properties. It allows detection of very small inhomogeneities. The image reconstruction employing the microwave inverse scattering consists of reconstructing the image of an object from the scattered field measured behind the object. This reconstruction runs up against the nonuniqueness of the solution of the inverse scattering problem. The authors propose to solve the ill-posed inverse problem by a statistical regularization method based on the Bayesian maximum a posteriori estimation where the principle of maximum entropy is used for assigning the a priori laws. The results obtained demonstrate the power and potential of this method in image reconstruction.     

Digital microwave receiver technology

This paper reports the impact of digital signal processing on microwave receiver technology. The majority of modern receiver designs are based on digital technology. Wide-and narrow-band receivers are presented. The wide-band receivers cover approximately 1-GHz instantaneous bandwidth and are used to intercept radar pulses. Current narrow-band receivers cover up to 50-MHz instantaneous bandwidth and are primarily used for receiving communication signals. Two approaches for wide-band receiver design are discussed. One is the conventional digital receiver. The other one is called the monobit receiver, which has slightly inferior performance in some respects, but can be built on a single chip. Narrow-band receivers are best implemented in software because they can more adapt to changes. Two types of receivers are discussed. One is the software Global Positioning System receiver. The other one is called a transform-domain communication system. The object of this system is to avoid interference in a hostile communication environment     

Digital image processing

A review of the field of digital image processing is presented, with concentration upon image formation and recording processes, digital sampling and digital image display, and with in-depth coverage of image coding and image restoration. New results in image restoration are also presented, covering restoration by use of an eye-model constraint and nonlinear restoration by maximization of the posterior density function.