Search

The visibility samples generated by the synthetic aperture interferometric radiometer (SAIR) under near-field observation conditions contain information about the distance from the target to the instrument. This requires a precise understanding of the target–instrument distance to guarantee imaging quality in near-field SAIR applications. In this paper, we introduce a novel distance estimate approach for near-field SAIR systems, which achieves satisfactory imaging performance in the absence of prior information on target–instrument distance. First, we reformulate the signal model of near-field SAIR from the fractional Fourier transform (FRFT) perspective. This formulation ties the distance that is variable to the visibility function in a straightforward manner, offering an efficient solution for image reconstruction in near-field SAIR. Subsequently, we present an iterative strategy for target–instrument distance estimation based on simulated annealing (SA). In each iteration, the modified average gradient (MAG) of images reconstructed within the FRFT framework is evaluated, and based on the Metropolis criterion, the estimated target–instrument distance is optimally updated iteratively. Finally, the validity and effectiveness of the proposed distance estimation method for near-field SAIR imaging systems are demonstrated through numerical simulation and real experiments. [ABSTRACT FROM AUTHOR]

High-frequency endoscopic ultrasound (HFEUS) imaging is an important tool commonly used in clinical practice for imaging hollow organs. The virtual source synthetic aperture (VSSA) method is effective in improving the imaging quality of HFEUS. However, interference from the motor control unit severely affects the accuracy of the conventional delay and sum (DAS) method, thus compromising the effectiveness of VSSA. In this article, a new computational method based on phase correction was proposed to overcome these shortcomings, which is named phase-corrected-and-sum (PCAS). Meanwhile, the parameters of coherence factor weighting (CFW) can be obtained from the correlation coefficient of the superimposed signals to further increase the imaging quality. Three kinds of imaging experiments were designed to evaluate the proposed method. Compared with the conventional method, the results show that the PCAS-CFW method improves the lateral resolution by about 10% and the contrast-to-noise ratio (CNR) by about 44%. Therefore, this proposed method is capable of significantly improving HFEUS image quality, and this method can be easily integrated into current HFEUS imaging systems, showing great potential for clinical applications. [ABSTRACT FROM AUTHOR]

Highlights • DB-SAT yields better, and depth independent, lateral resolution compared to CFB. • The frame –rate achievable using DB-SAT is several times more compared to CFB. • DB-SAT with only 8-TX/64-RX elements achieved the above characteristics. Abstract In this paper, a technique combining Diverging Beam with Synthetic Aperture Technique (DB-SAT) is demonstrated that utilizes only limited number of active elements yet provides better quality images at higher frame rates than possible with Conventional Focused Beamforming (CFB). The DB-SAT has been investigated in simulations and experiments on wire and tissue-mimicking phantoms, and the results are compared with routinely used CFB with Linear Array (CFB-LA). The estimated lateral resolution at the focal point was 0.41 mm and 0.34 mm for CFB-LA and DB-SAT, respectively, in simulations. These were estimated to be 0.78 mm and 0.71 mm, respectively, in experiments. Experimentally computed contrast resolution (contrast-to-noise ratio) for the cyst located at 60 mm depth were 0.50 (1.31 dB) and 0.58 (2.33 dB) for CFB-LA and DB-SAT, respectively. The frame-rate achieved by DB-SAT was 8 times and 2 times higher than that achieved by CFB-LA when transmit sub-apertures had an overlap 0% and 75%, respectively. Therefore, it can be concluded from the results that DB-SAT using 8 active transmit elements and 64 active receive elements yields better quality images at higher frame-rates than those obtained using CFB-LA with 64 active elements in transmit and receive. Since there is a reduction in the number of active transmit elements in the case of DB-SAT, it leads to a reduction in the overall system complexity. [ABSTRACT FROM AUTHOR]

This paper describes a way to synthesize a larger coherent aperture from smaller apertures combined with motion, when only intensities are measured. It relies on collecting intensity patterns in two planes for each aperture, for example, the aperture plane and an image plane, and using a phase-retrieval algorithm to reconstruct the optical field in the aperture plane. As the sensor moves forward, a larger two-dimensional aperture is synthesized, allowing a much finer resolution image to be reconstructed. An algorithm to correct for the relative pointing (tip and tilt phases) and piston errors between different apertures and at different times is needed to phase up the synthetic aperture. Results of simulations, including the effects of speckle, are shown, and practical considerations are evaluated. [ABSTRACT FROM AUTHOR]

• The current Fourier ptychographic imaging need high aperture overlapping ratio to ensure the convergence. This limitation dictates that the images can only be acquired by dense aperture scanning. We proposed a new framework to solve the Fourier ptychographic imaging with sparse aperture. Our method breaks the limitation of dense aperture scanning and allows multiple separate imaging systems to capture images simultaneously, which could simplify the imaging configuration and improve the acquisition efficiency. • In our work, the aperture overlapping ratio is reduced to less than 25% for the first time. The imaging quality of proposed method also outperforms than the state-of-the-art methods under the same imaging parameters. Our work shows the potential for single-shot imaging by using a camera array instead of aperture scanning mentioned in previously published works, so that the observation of rapidly changing scenes can be realized. Synthetic aperture technique could effectively expand the passband and beyond the diffraction limit of aperture. Macroscopic Fourier ptychography (FP) makes synthetic aperture feasible in visible light by incorporating phase retrieval process into synthetic aperture. In FP, a dense aperture sampling is necessary to overcome the ill-posedness of phase retrieval, which leads to long acquisition time and heavy computation. Here, a total variation regularization-based Fourier ptychographic (TVFP) reconstruction method is proposed for long-range imaging. The TVFP method imposes total variation constraints to improve the ill-posedness of the imaging model, and recovers high-quality image from sparse aperture sampling. For the imaging system with aberration, a joint optimization of pupil function and target image is also proposed to achieve the aberration free image and the estimated pupil function simultaneously. The results show the aperture overlapping ratio could be reduced to 25% or less. The proposed method simplifies the imaging configuration and improves the robustness for long-range imaging. [Display omitted] [ABSTRACT FROM AUTHOR]

The main objective of this study is to improve the ultrasound image by employing a new algorithm based on transducer array element beam pattern correction implemented in the synthetic transmit aperture (STA) method combined with emission of mutually orthogonal complementary Golay sequences. Orthogonal Golay sequences can be transmitted and received by different transducer elements simultaneously, thereby decreasing the time of image reconstruction, which plays an important role in medical diagnostic imaging. The paper presents the preliminary results of computer simulation of the synthetic aperture method combined with the orthogonal Golay sequences in a linear transducer array. The transmission of long waveforms characterized by a particular autocorrelation function allows to increase the total energy of the transmitted signal without increasing the peak pressure. It can also improve the signal-to-noise ratio and increase the visualization depth maintaining the ultrasound image resolution. In the work, the 128-element linear transducer array with a 0.3 mm pitch excited by 8-bits Golay coded sequences as well as one cycle at nominal frequencies of 4 MHz were used. The comparison of 2D ultrasound images of the phantoms is presented to demonstrate the benefits of a coded transmission. The image reconstruction was performed using the synthetic STA algorithm with transmit and receive signals correction based on a single element directivity function. [ABSTRACT FROM AUTHOR]

In the paper the concept of synthetic aperture used for high resolution/high frame rate ultrasonic imaging is reviewed. The synthetic aperture technique allows building extended 'virtual' apertures, synthesized from smaller real aperture resulting in improved lateral resolution along full penetration depth without sacrificing the frame rate. Especially, four methods, synthetic aperture focusing (SAF), multi-element synthetic aperture focusing (M-SAF), synthetic receive aperture (SRA) and synthetic transmit aperture (STA) are addressed. The effective aperture function, describing two-way, far field radiation is a useful tool in beam pattern analysis. Some basic notations, which are used to calculate the effective aperture are introduced in Appendix. [ABSTRACT FROM AUTHOR]

A synthetic aperture is an effective approach to enhancement of the resolution of telescopes. In this paper, a new method to improve the resolution by using a pair of gratings is proposed. It allows collection of parts of the light diffracted from the object which could not previously reach the imaging device. This method improves the resolution and light energy utilization ratio of telescopes without introducing new chromatic aberration. An experiment for resolution testing was carried out and the feasibility and availability of the method were verified. [ABSTRACT FROM PUBLISHER]

Drew lesson from the synthetic aperture radar, and made the further processing to the fictitious wave-field data, that was correlation and synthesizing processing. Specifically, developed the traditional mode of processing that mainly based on profiles into a new method,in which, several apertures synthesis with the measure point as the center, lapse point-by-point, finally, multiple coverage was adopted. In hopes to improves the signal-to-noise ratio, highlight the abnormal, improves the resolution and increases the exploration depth. Through the processing of theoretical model data, verified the effectiveness of this method. The different theoretical model processing suggests that related to enhanced synthesis of useful signal, to improve the signal to noise ratio, resolution enhancement effect. The feasibility and validity of synthetic aperture of transient electromagnetic imaging numerical calculation was demonstrated. [ABSTRACT FROM AUTHOR]

Floquet wave and element-by-element summation representations are implemented to investigate the radiation from a periodic, rectangular array of linearly phased/sequentially pulsed dipoles in the frequency- and time-domains. A synthetic aperture type formulation for Floquet wave representation and an asymptotic formulation for the element-by-element summation representation are developed. Numerical results are presented to demonstrate the efficiency of the Floquet wave representation in terms of radiating, non-radiating and diffracted wave constituents and then validated via comparison with direct summation over the contributions of individual dipole radiations. [ABSTRACT FROM AUTHOR]

High-frequency ultrasound (HFUS) imaging has emerged as an essential tool for pre-clinical studies and clinical applications such as ophthalmic and dermatologic imaging. HFUS imaging systems based on array transducers capable of dynamic receive focusing have considerably improved the image quality in terms of spatial resolution and signal-to-noise ratio (SNR) compared to those by the single-element transducer-based one. However, the array system still suffers from low spatial resolution and SNR in out-of-focus regions, resulting in a blurred image and a limited penetration depth. In this paper, we present synthetic aperture imaging with a virtual source (SA-VS) for an ophthalmic application using a high-frequency convex array transducer. The performances of the SA-VS were evaluated with phantom and ex vivo experiments in comparison with the conventional dynamic receive focusing method. Pre-beamformed radio-frequency (RF) data from phantoms and excised bovine eye were acquired using a custom-built 64-channel imaging system. In the phantom experiments, the SA-VS method showed improved lateral resolution (>10%) and sidelobe level (>4.4 dB) compared to those by the conventional method. The SNR was also improved, resulting in an increased penetration depth: 16 mm and 23 mm for the conventional and SA-VS methods, respectively. Ex vivo images with the SA-VS showed improved image quality at the entire depth and visualized structures that were obscured by noise in conventional imaging. [ABSTRACT FROM AUTHOR]

Synthetic aperture imaging radiometers (SAIRs) are powerful passive microwave systems for high-resolution imaging by use of synthetic aperture technique. However, the ill-posed inverse problem for SAIRs makes it difficult to reconstruct the high-precision brightness temperature map. The traditional regularization methods add a unique penalty to all the frequency bands of the solution, which may cause the reconstructed result to be too smooth to retain certain features of the original brightness temperature map such as the edge information. In this paper, a multi-parameter regularization method is proposed to reconstruct SAIR brightness temperature distribution. Different from classical single-parameter regularization, the multi-parameter regularization adds multiple different penalties which can exhibit multi-scale characteristics of the original distribution. Multiple regularization parameters are selected by use of the simplified multi-dimensional generalized cross-validation method. The experimental results show that, compared with the conventional total variation, Tikhonov, and band-limited regularization methods, the multi-parameter regularization method can retain more detailed information and better improve the accuracy of the reconstructed brightness temperature distribution, and exhibit superior noise suppression, demonstrating the effectiveness and the robustness of the proposed method. [ABSTRACT FROM AUTHOR]

In the buried-object search technique using a pulsed radar, the estimation method for the buried-pipe location and the underground propagation velocity of the electromagnetic wave by means of a synthetic aperture method has been studied. In general, the electromagnetic wave attenuates as it propagates m the underground and the amplitude of the received signal often becomes extremely small. Therefore, it has been difficult in the signal processing using the amplitude value to recognize several buried objects at different depths by an identical discrimination standard. To resolve this difficulty, a new zero-crossed synthetic aperture method is developed. In the underground cross-sectional image, the waves reflected from a buried pipe form a hyperbolic pattern dependent on the underground propagation velocity of the electromagnetic wave. By evaluating only the edge of this hyperbolic pattern, the underground propagation velocity of the electromagnetic wave and the location of the buried object are estimated. First, to extract the edge, the amplitude value at the time when the observed signal has a ‘0’ amplitude is replaced with ‘1’ if the phase of the original waveform changes from minus to plus whereas with ‘-1’ if it changes from plus to minus. The amplitude at other time points is replaced with ‘0.’ Subsequently, a synthetic aperture process (addition process) is carried out. Since the absolute value of the amplitude is 1 or 0, it is not necessary to consider the effect of the attenuation of the amplitude due to propagation of the electromagnetic wave as a discrimination reference in the estimation of the location of the buried pipe after the addition process. The location of the buried pipe can be detected accurately. [ABSTRACT FROM AUTHOR]

High-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 [ABSTRACT FROM AUTHOR]