High-resolution Image Formation Research Articles

Studies on High-Resolution Airborne Synthetic Aperture Radar Image Formation with Pseudo-Random Agility of Interpulse Waveform Parameters

By means of alteration of the transmitted linear frequency modulation (LFM) signal waveform parameters, such as pulse width or chirp rate, initial phase, pulse repetition interval (PRI), and chirp rate polarity at every position of synthetic apertures, the pseudo-random agility technology of interpulse waveform parameters in airborne Synthetic Aperture Radar (SAR) actively increases the complexity and uncertainty of radar waveforms. This technology confuses jamming interception receivers, thus improving its anti-interference ability for active coherent jamming, which is one of the main research interests of airborne SAR technology. But the pseudo-random agility technology for interpulse waveform parameters faces certain challenges of large computation and complex system design, which need to be further studied and solved. To address these issues, a processing scheme of high-resolution SAR image formation which is appropriate for agile interpulse waveform parameters is proposed in this paper. This method can deal with multiple agile parameters, not only single ones as in most existing literature. Its computation load is nearly comparable to that of traditional SAR image formation with constant waveform parameters. The high-resolution SAR imaging results obtained by processing SAR raw data with agile interpulse waveform parameters demonstrate the effectiveness of the proposed method. In addition, real SAR images with resolutions of 0.5 m and 0.15 m, which are rarely found in the public literature, are shown under the circumstance of randomly changing the transmitted wideband LFM signal pulse parameters one by one.

Fourier Ptychography Part II: Phase Retrieval and High-Resolution Image Formation

Abstract: This article is the second within a three-part series on Fourier ptychography, which is a computational microscopy technique for high-resolution, large field-of-view imaging. While the first article laid out the basics of Fourier ptychography, this second part sheds light on its algorithmic ingredients. We present a non-technical discussion of phase retrieval, which allows for the synthesis of high-resolution images from a sequence of low-resolution raw data. Fourier ptychographic phase retrieval can be carried out on standard, widefield microscopy platforms with the simple addition of a low-cost LED array, thus offering a convenient alternative to other phase-sensitive techniques that require more elaborate hardware such as differential interference contrast and digital holography.

Air-Coupled Ultrasonic Spiral Phased Array for High-Precision Beamforming and Imaging

Sparse spiral phased arrays are advantageous for many emerging air-coupled ultrasonic applications, since grating lobes are prevented without being constrained to the half-wavelength element spacing requirement of well-known dense arrays. As a result, the limitation on the maximum transducer diameter is omitted and the aperture can be enlarged for improving the beamforming precision without requiring the number of transducers to be increased. We demonstrate that in-air imaging, in particular, benefits from these features, enabling large-volume, unambiguous and high-resolution image formation. Therefore, we created an air-coupled ultrasonic phased array based on the Fermat spiral, capable of transmit, receive and pulse-echo operation, as well as 3D imaging. The array consists of 64 piezoelectric 40-kHz transducers (Murata MA40S4S), spanning an aperture of 200mm. First, we provide an application-independent numerical and experimental characterization of the conventional beamforming performance of all operation modes for varying focal directions and distances. Second, we examine the resulting imaging capabilities using the single line transmission technique. Apart from the high maximum sound pressure level of 152 dB, we validate that unambiguous high-accuracy 3D imaging is possible in a wide field of view (±80°), long range (20cm to 5m+) and with a high angular resolution of up to 2.3°. Additionally, we demonstrate that object shapes and patterns of multiple reflectors are recognizable in the images generated using a simple threshold for separation. In total, the imaging capabilities achieved are promising to open up further possibilities, e.g. robust object classification in harsh environments based on ultrasonic images.

High resolution ply-by-ply ultrasound imaging of impact damage in thick CFRP laminates by high-frequency acoustic microscopy

Impact behavior of the carbon fiber reinforced polymers (CFRP) is still actual issue despite long history of laminate application and investigation of impact resistance. Expansion of knowledge on fracture mechanisms in reinforced composites could be related to development of methods for composite material investigations. In this work the high-frequency (50 MHz) impulse acoustic microscopy has been proposed for visualization and investigation of microstructure damages in the volume of CFRP laminates under low-velocity impact. Short probing pulses of high-frequency focused ultrasound provide both high lateral and axial resolution in layered structures. It has been shown possibilities of the acoustic microscopy to high-resolution imaging of delamination, matrix cracking and fiber fractures through the thickness of thick laminates. Mechanisms of high-resolution acoustical image formation have been described. The results of ply-by-ply ultrasound imaging of carbon fiber reinforced laminates with [+45°/0°/−45°/90°]NS lay-up have been supplemented by impact force histories and calculated values of the energies.

АЛГОРИТМЫ ПЕРВИЧНОЙ И ВТОРИЧНОЙ ОБРАБОТКИ СИГНАЛОВ В СИСТЕМАХ АКТИВНОГО АПЕРТУРНОГО СИНТЕЗА

A method for processing spatio-temporal radio signals, which allows the formation of high spatial resolution images from aerospace carriers, has been synthesized. In this case, a swath is ±15° from the nadir relative to the flight line is considered. Traditionally this swath is not visible from aerospace carriers. A block diagram of an active-passive type radar is proposed. Initially the image obtained at the output of such a radar has a low quality of visual perception. Such image is called primary, since it was obtained at the stage of primary signal processing. The physical sense of such image is investigated. It approximately represents the operation of convolution of the ambiguity function with a spatial function proportional to the ideal image. The physical sense of the ideal image depends on the characteristics of the imaging system. In the problem under consideration, this is the specific radar cross section. In order to improve the quality of the image it is proposed additional processing (filtering). The paper substantiates the method of filtering (secondary processing) of images based on the solution of the inverse integral equation. It is shown that the exact solution to this equation is difficult to obtain because the form of the ambiguity function is not known exactly and can be estimated only approximately and, in addition, not for each ambiguity function it is possible to obtain a non-singular solution of the inversion equation. Therefore, an approximate solution to the problem is given. It solution is sufficient to solve practical problems of improving image quality. Using numerical simulation methods, the influence of the error in determining the shape of the uncertainty function on the task of improving image quality is studied. Examples of primary and secondary images obtained at the output of wideband active aperture synthesis systems with various geometries of spatially distributed antenna systems are given.

Spaceborne Azimuth Multichannel BiSAR With Geosynchronous Transmitter and LEO Receiver: High-Resolution Wide-Swath Image Formation

Spaceborne Azimuth Multichannel BiSAR With Geosynchronous Transmitter and LEO Receiver: High-Resolution Wide-Swath Image Formation

Target-Oriented High-Resolution SAR Image Formation via Semantic Information Guided Regularizations

Sparsity-regularized synthetic aperture radar (SAR) imaging framework has shown its remarkable performance to generate a feature-enhanced high-resolution image, in which a sparsity-inducing regularizer is involved by exploiting the sparsity priors of some visual features in the underlying image. However, since the simple prior of low-level features is insufficient to describe different semantic contents in the image, this type of regularizer will be incapable of distinguishing between the target of interest and unconcerned background clutters. As a consequence, the features belonging to the target and clutters are simultaneously affected in the generated image without concerning their underlying semantic labels. To address this problem, we propose a novel semantic information guided generative framework for target-oriented SAR image formation, which aims at enhancing the interested target scatters while suppressing the background clutters. First, we develop a new semantics-specific regularizer for image formation by exploiting the statistical properties of different semantic categories in a target scene SAR image. In order to infer the semantic label for each pixel in an unsupervised way, we moreover induce a novel high-level prior-driven regularizer and some semantic causal rules from the prior knowledge. Finally, our regularized framework for image formation is further derived as a simple iteratively reweighted $\ell _{1}$ minimization problem that can be conveniently solved by many off-the-shelf solvers. Experimental results demonstrate the effectiveness and superiority of our framework for SAR image formation in terms of target enhancement and clutters suppression, compared with the state of the arts. Additionally, the proposed framework opens a new direction of devoting some machine learning strategies to image formation, which can benefit the subsequent decision-making tasks.

Calibration of High-Resolution Synthetic Aperture Radar (SAR) Image Features by Ground-Based Experiments

ABSTRACTResults of recent experiments reported in the literature with frequency-modulated continuous wave (FMCW) mode of synthetic aperture radar (SAR) imaging are enabling high-resolution, day and night remote sensing of ground terrain. Also, FMCW-based SAR techniques are accepted for ground-based SAR (GB-SAR) instrumentation for early warning of fragile slopes such as in coal mines, avalanche detection. Features of images generated by FMCW SAR sensors need to be calibrated for interpretation purposes as like other forms of microwave imaging. In this paper, we demonstrate a ground-based method to calibrate high-resolution FMCW SAR image features by experimenting with a long railSAR testbed. Signal processing algorithms are derived to support high-resolution SAR image formation according to the requirement of the experiment. Trihedral corner reflectors (CRs) of submeter dimensions are placed on terrains with different normalized radar cross-section (RCS) coefficient (σ0) to measure backscattering properties from targets and surrounding clutter from different heights and varied look angles. Calibration of the features of CRs is performed with respect to their physical dimensions by measuring the impulse response functions and spatial resolution in the SAR images. Target RCS and speckle properties derived from single-look and multilook processing of experimental data closely follows target and scene parameters such as radiometric and geometric measures of RCS of the CRs and σ0 of surroundings such as tar road and concrete road. By this experiment the features of high-resolution FMCW SAR images are measured and calibrated without flying the radar sensor on airborne platform.

Flexible Thin-Film InGaAs Photodiode Focal Plane Array

Natural imaging systems such as the retina and the compound eye employ a conformal architecture that provides an aberration-free image with wide field of view (FOV) and very low f/number. However, most artificial imagers such as conventional cameras are limited to a planar architecture demanded by the use of brittle semiconductor focal plane arrays (FPAs). High-resolution image formation on this flat field requires multiple bulky optical elements. Here we demonstrate a general approach to fabricating complex circuits and in particular FPAs on flexible and/or conformable substrates that can be shaped to overcome these fundamental limitations. An 8 × 100, lightweight, thin-film In0.53Ga0.47As p-i-n photodiode FPA with sensitivity to wavelengths as long as λ = 1650 nm is fabricated on a thin flexible plastic foil following transfer by adhesive-free bonding of the epitaxial layers that are subsequently lifted off from the parent InP substrate. The array is shaped into either a convex cylindrically curved imag...

Evaluation of microbubble contrast agents for dynamic imaging with x-ray phase contrast.

X-rays are commonly used as a means to image the inside of objects opaque to visible light, as their short wavelength allows penetration through matter and the formation of high spatial resolution images. This physical effect has found particular importance in medicine where x-ray based imaging is routinely used as a diagnostic tool. Increasingly, however, imaging modalities that provide functional as well as morphological information are required. In this study the potential to use x-ray phase based imaging as a functional modality through the use of microbubbles that can be targeted to specific biological processes is explored. We show that the concentration of a microbubble suspension can be monitored quantitatively whilst in flow using x-ray phase contrast imaging. This could provide the basis for a dynamic imaging technique that combines the tissue penetration, spatial resolution, and high contrast of x-ray phase based imaging with the functional information offered by targeted imaging modalities.

Compressive SAR imaging with joint sparsity and local similarity exploitation.

Compressive sensing-based synthetic aperture radar (SAR) imaging has shown its superior capability in high-resolution image formation. However, most of those works focus on the scenes that can be sparsely represented in fixed spaces. When dealing with complicated scenes, these fixed spaces lack adaptivity in characterizing varied image contents. To solve this problem, a new compressive sensing-based radar imaging approach with adaptive sparse representation is proposed. Specifically, an autoregressive model is introduced to adaptively exploit the structural sparsity of an image. In addition, similarity among pixels is integrated into the autoregressive model to further promote the capability and thus an adaptive sparse representation facilitated by a weighted autoregressive model is derived. Since the weighted autoregressive model is inherently determined by the unknown image, we propose a joint optimization scheme by iterative SAR imaging and updating of the weighted autoregressive model to solve this problem. Eventually, experimental results demonstrated the validity and generality of the proposed approach.

B1 transmit phase gradient coil for single-axis TRASE RF encoding

PurposeTRASE (Transmit Array Spatial Encoding) MRI uses RF transmit phase gradients instead of B0 field gradients for k-space traversal and high-resolution MR image formation. Transmit coil performance is a key determinant of TRASE image quality. The purpose of this work is to design an optimized RF transmit phase gradient array for spatial encoding in a transverse direction (x- or y- axis) for a 0.2T vertical B0 field MRI system, using a single transmitter channel. This requires the generation of two transmit B1 RF fields with uniform amplitude and positive and negative linear phase gradients respectively over the imaging volume. Materials and MethodsA two-element array consisting of a double Maxwell-type coil and a Helmholtz-type coil was designed using 3D field simulations. The phase gradient polarity is set by the relative phase of the RF signals driving the simultaneously energized elements. ResultsField mapping and 1D TRASE imaging experiments confirmed that the constructed coil produced the fields and operated as designed. A substantially larger imaging volume relative to that obtainable from a non-optimized Maxwell-Helmholtz design was achieved. ConclusionThe Maxwell (sine)–Helmholtz (cosine) approach has proven successful for a horizontal phase gradient coil. A similar approach may be useful for other phase-gradient coil designs.

Simultaneous sum-frequency and vibro-acoustography imaging for nondestructive evaluation and testing applications

High-resolution ultrasound imaging systems for inspection of defects and flaws in materials are of great demand in many industries. Among these systems, Vibro-acoustography (VA) has shown excellent capabilities as a noncontact method for nondestructive high-resolution imaging applications. This method consists of mixing two confocal ultrasound beams, slightly shifted in frequency, to produce a dynamic (oscillatory) radiation force in the region of their intersection. This force vibrates the object placed at the focus of the confocal transducer. As a result of the applied force, an acoustic emission field at the difference frequency of the primary incident ultrasound beams is produced. In addition to the difference frequency acoustic emission signal, there exists another signal at the sum frequency, formed in the intersection region of the two primary beams. The goal of this study is to investigate the formation of high-resolution images using the sum frequency of ultrasound waves in VA while concurrently forming the conventional difference-frequency VA image, thereby increasing the amount of information acquired during a single scan. A theoretical model describing the sum-frequency wave propagation, including beam forming and image formation in the confocal configuration, is developed and verified experimentally. Moreover, sample experiments are performed on a flawed fiber-reinforced ceramic composite plate. Images at both the difference and sum frequencies are compared and discussed. Results show that the sum-frequency image produces a high-resolution C scan of the plate by which the flaws and structural details of the plate can be detected.

Rewritable Imaging on the Surface of Frozen Ionic Liquids

The write way: A technique for high-resolution rewritable image formation has been derived by using frozen ionic liquids as a temperature-controllable canvas that allows the simple erasure of data. This technique is very flexible as the physicochemical properties of ionic liquids can be fine-tuned through manipulation of the chemical/structural composition of the individual ions.

Synthetic‐aperture technique for high‐resolution composite imaging of the inside walls of tubular specimens

AbstractHigh‐resolution survey of the inside walls of tubular specimens is a unique application of synthetic‐aperture composite imaging. This article describes the data acquisition process, 3D motion estimation and compensation, image registration, and superposition for the formation of high‐resolution composite images from conventional video sequences. Experiment results from the survey of an oil well are used to demonstrate the capability of the technique. © 2004 Wiley Periodicals, Inc. Int J Imaging Syst Technol 14, 167–169, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ima.20020

High resolution image formation from low resolution frames using delaunay triangulation

An algorithm based on spatial tessellation and approximation of each triangle patch in the Delaunay (1934) triangulation (with smoothness constraints) by a bivariate polynomial is advanced to construct a high resolution (HR) high quality image from a set of low resolution (LR) frames. The high resolution algorithm is accompanied by a site-insertion algorithm for update of the initial HR image with the availability of more LR frames till the desired image quality is attained. This algorithm, followed by post filtering, is suitable for real-time image sequence processing because of the fast expected (average) time construction of Delaunay triangulation and the local update feature.

Zeitvarianzabbildung, ein Verfahren zur Detektion von Ultraschallkontrastmitteln

Ultrasound contrast agents which are injected into the blood stream mostly use encapsulated microbubbles as acoustical scatterers. The acoustic properties of microbubbles differ significantly from those of scatterers in tissue. When insonified, bubbles oscillate in a nonlinear mode, which is determined by a mechanical resonance frequency. The insonification can cause changes of the bubble diameters, complete or partial destruction of the shells as well as splitting or fusion of bubbles. Received echoes resulting from sequences of short pulses reflect the alterations of the acoustic properties of the bubbles and, therefore, allow the formation of high resolution images which show the distribution of the contrast agent within the tissue. Consequently, this new imaging technique has the potential to improve the visualization of small vessels and the assessment of perfusion conditions.

Broad-source image plane holography as a confocal imaging process

The process of image plane holography with incoherent illumination has many significant properties. The process can produce extremely high-quality, low-noise images, section slicing, image formation through inhomogeneities, and high-resolution image formation through small apertures. The process of confocal imaging has similar properties. We describe the similarities and differences between the two processes.

Parallel implementation of synthetic aperture radar algorithms

Two sequences of operations necessary for implementation of high resolution image formation in strip and spotlight modes of the synthetic aperture radar (SAR) are presented. The sequences are mapped onto a mesh-connected SIMD architecture. The mapping includes not only parallel implementation of all the basic computation steps, but also all the necessary data transformation and communication operations. Detailed estimates of the processing times are provided for Hughes Research Laboratories Systolic/Cellular architecture.

Instrumental Limitations and Current Prospects for Materials Research with the Latest Generation of High-Resolution Electron Microscopes

The macroscopic properties of most materials depend directly on their microstructure and its local variability at the atomic level. Recent trends in high resolution electron microscopes (HREMs) have led to resolving powers on this scale, which in turn has made these instruments invaluable to many materials science investigations. The purposes of this short review are firstly to outline some of the fundamentals of high resolution image formation and interpretation and then to summarise some of the latest instrumental developments. Some recent applications are briefly described to provide some appreciation of the wide range of materials currently being investigated with the HREM. The impact of this work should be apparent from reference to other papers in this volume as well as several recent reviews [1–3] and special conference proceedings [4–5]. The likelihood of further developments in instrumentation and the necessity for complementary information from other techniques are also briefly considered.