Passive synthetic aperture imaging using an achromatic grating interferometer

Synthetic aperture imaging with active systems is relatively well developed. The implementation of passive systems, on the other hand, is being hindered by a low SNR and the difficulty in achieving 2-D aperture fill. The achromaticity of the grating interferometer permits the use of a wider instantaneous spectral bandwidth to produce better SNR, and it provides a simple means to synthesize a 2-D aperture. A passive synthetic aperture imaging approach implemented with an achromatic grating interferometer is described, and laboratory experimental results are presented to demonstrate the proposed concept.

Hidden object imaging is challenging problem in the fields of computer vision and image processing, and it’s a key step in many application fields, include intelligent video surveillance, visual tracking and scene understanding. Recently, the camera array synthetic aperture imaging has been proved to be a powerful technology for hidden object detection, and the state-of-art synthetic imaging method can focus on multiple parallel planes so as to achieve seeing hidden through severe occlusion. However, due to the depth variation of hidden object’s surface, it’s difficult for existing method to get a complete clear image. This paper presents a novel synthetic aperture imaging and fusion approach, the main characteristics of proposed method include three parts: (1) A novel imaging framework is presented which integrate synthetic aperture imaging and fusion seamlessly. (2) Several multiple scale image fusion methods have been adopted in our framework to create high resolution and highly detailed images of occluded object. (3)A camera array synthetic aperture imaging and fusion system has been developed based on the proposed approach, and the Stanford light field public dataset and our camera array occlusion object dataset have been used to evaluate the performance of this algorithm. Extensive experiment results demonstrate that compared to the existing state-of-art camera array synthetic aperture imaging approach, our approach can enhance the image detail and quality of hidden objects under occlusion.

GPU-Based Minimum Variance Beamformer for Synthetic Aperture Imaging of the Eye

The occlusion problem is a major challenge in the field of computer vision. Synthetic aperture imaging (SAI) is often used for surface reconstruction of occluded objects. However, SAI usually relies on high-speed information transmission devices. In addition, a large amount of information in the scene is lost, when handling low-resolution input images. This limitation results in unclear reconstructed regions in the synthetic aperture image and thus hinders the application of SAI in downstream tasks. We propose a multi-view super-resolution SAI method. It aims to generate high-resolution synthetic aperture images using images acquired by few of low-resolution acquisition devices. The main contributions of this paper are: (1) a multi-view super-resolution algorithm is proposed. It can generate clear synthetic aperture images in an array with a limited number of cameras. (2) By exploiting the correlation between views, the proposed algorithm can generate super-resolution synthetic aperture images with more accurate image structure and sharper image edges. (3) A feature extraction module is proposed. It can effectively extract the complementary relationship between pictures from different perspectives. The experimental results show that the proposed method can generate a reconstructed image of the occluded object surface with clear edges and accurate structure. Compared to conventional SAI, our method improves 5.7 % / 21.1 % on peak signal-to-noise ratio (PSNR)/structure similarity index measure (SSIM) and 4.4 % / 9.2 % on PSNR/SSIM respectively on two datasets compared to other state-of-the-art super-resolution methods.

Synthetic magnetic resonance imaging (SyMRI) allows to obtain different weighted-images using the multiple-dynamic multiple-echo sequence lasting 6min. The aim is to compare quantitatively and qualitatively synthetic- and conventional MRI in patients with multiple sclerosis (MS) and controls assessing the contrast (C), the signal to noise ratio (SNR), and the contrast to noise ratio (CNR). We evaluated the lesion count and lesion-to-white matter contrast ([Formula: see text] in the MS patients. 51 patients underwent synthetic- and conventional MRI. Qualitative analysis was evaluated by assigning scores to all synthetic- and conventional MRI sequences by two neuroradiologists. Lesions were counted in MS patients both in the conventional- and synthetic T2-FLAIR. Regions of interest were placed in the cerebrospinal fluid, in the white- and grey matter. For the sequences were evaluated: C, CNR, and SNR. Synthetic T2-FLAIR images are qualitatively inferior. C and CNR were significantly higher in synthetic T1W and T2W images compared to conventional images, but not for T2-FLAIR. The SNR value was always lower in synthetic images than in conventional ones. SyMRI can be used in clinical practice because it has a similar diagnostic accuracy which reduces the scanning time compared to the conventional one. However, synthetic T2-FLAIR images need to be improved.

During the data processing and interpretation of mine transient electromagnetic method (MTEM), in order to improve the identification of the geo‐electrical interface between low resistivity abnormal body and surrounding rock in advanced detection, transient electromagnetic field was transformed to pseudo‐seismic wave‐field based on the function relationship between spreading electromagnetic field and seismic wave‐field in whole‐space. Meanwhile, the signal of pseudo‐seismic after being transformed was processed with correlative stack to strengthen the amplitude and improve signal‐noise‐ratio (SNR) using the synthetic aperture imaging (SAI). The data of different detection directions on one surveying point in MTEM were imaged as aperture data, which would highlight weak abnormality and improve SNR. In addition, the application effect of SAI was testified with the in‐site advanced detection in roadway using MTEM. The results showed that the SAI of the MTEM could improve SNR of the electrical interface information and highlight the geometrical resolution to increase exploration accuracy. The technique is significant theoretically and practically in accurate advanced detection of aquifer structure by MTEM.

High‐resolution millimetre wave images of contiguous targets often suffer from the influence of sidelobe artefacts, partial correlation between targets and so on. Owing to the characteristics of near‐field synthetic aperture imaging radiometers [such as the changing point spread function (PSF) and slender sideline], the existing CLEAN algorithms are unsuitable for near‐field synthetic aperture imaging. This study is devoted to establishing a novel CLEAN algorithm (named adaptive CLEAN) to clean the reconstructed millimetre wave images accurately. First, the characteristics of near‐field synthetic aperture imaging are studied. Then, the adaptive CLEAN algorithm is established based on these characteristics. In this study, the authors amend the amplitude of the targets and select the matching PSF for them according to their azimuths. Unlike other CLEAN algorithms, the parameters of this adaptive CLEAN algorithm are calculated by a formula, which is concluded from a lot of simulation experiments. Finally, the effectiveness of the proposed adaptive CLEAN algorithm is tested by several simulation experiments, and the superiority is also demonstrated by comparing it with the existing CLEAN algorithm. The results demonstrate that the proposed method is an efficient, feasible algorithm for cleaning the reconstructed images, irrespective of the point or contiguous targets.

The purpose of this paper is to extend the technology of synthetic aperture (SA) imaging used in radar (SAR) and sonar (SAS) to the THz range (SAT). This novel approach in THz will be applied in the domain of non-destructive testing (NDT). The paper presents simulated SAT images which are demonstrating the performance of a synthetic aperture time domain reconstruction processing. The generated SAT target responses of broadband radiation show high resolution and high signal-to-noise ratio (SNR) which lead to an improvement of non-destructive defect identification.

A novel multi-object detection method in complex scene using synthetic aperture imaging

Synthetic aperture imaging (SAI) is able to achieve the see through effect by blurring out the off-focus foreground occlusions and reconstructing the in-focus occluded targets from multi-view images. However, very dense occlusions and extreme lighting conditions may bring significant disturbances to the SAI based on conventional frame-based cameras, leading to performance degeneration. To address these problems, we propose a novel SAI system based on the event camera which can produce asynchronous events with extremely low latency and high dynamic range. Thus, it can eliminate the interference of dense occlusions by measuring with almost continuous views, and simultaneously tackle the over/under exposure problems. To reconstruct the occluded targets, we propose a hybrid encoder-decoder network composed of spiking neural networks (SNNs) and convolutional neural networks (CNNs). In the hybrid network, the spatio-temporal information of the collected events is first encoded by SNN layers, and then transformed to the visual image of the occluded targets by a style-transfer CNN decoder. Through experiments, the proposed method shows remarkable performance in dealing with very dense occlusions and extreme lighting conditions, and high quality visual images can be reconstructed using pure event data.

The synthetic aperture imaging (SAI) and backpropagation imaging (BPI) algorithms are compared for reconstructing a representation of scattering objects from time-domain ground-penetrating radar data. Synthetic data produced by a FDTD forward solver is used as a basis for the comparison. We assume initially a homogeneous, lossless host medium. The BPI algorithm, which includes waveform diffractions, is shown to result in a better focused image than SAI, with fewer artifacts. The superiority of the BPI algorithm is particularly evident at lower frequencies, where the scattering objects are on the order of the wavelength of the incident field.

Lateral motion estimation has been a challenge in ultrasound elastography mainly due to the low resolution, low sampling frequency, and lack of phase information in the lateral direction. Synthetic transmit aperture (STA) can achieve high resolution due to two-way focusing and can beamform high-density image lines for improved lateral motion estimation with the disadvantages of low signal-to-noise ratio (SNR) and limited penetration depth. In this study, Hadamard-encoded STA (Hadamard-STA) is proposed for the improvement of lateral motion estimation in elastography, and it is compared with STA and conventional focused wave (CFW) imaging. Simulations, phantom, and in vivo experiments were conducted to make the comparison. The normalized root mean square error (NRMSE) and the contrast-to-noise ratio (CNR) were calculated as the evaluation criteria in the simulations. The results show that, at a noise level of -10 dB and an applied strain of -1% (compression), Hadamard-STA decreases the NRMSEs of lateral displacements by 46.92% and 35.35%, decreases the NRMSEs of lateral strains by 52.34% and 39.75%, and increases the CNRs by 9.70 and 9.75 dB compared with STA and CFW, respectively. In the phantom experiments performed on a heterogeneous tissue-mimicking phantom, the sum of squared differences (SSD) between the reference and the motion-compensated RF data, and the CNR were calculated as the evaluation criteria. At an applied strain of -1.80%, Hadamard-STA is found to decrease the SSDs by 20.91% and 30.99% and increase the CNRs by 14.15 and 24.66 dB compared with STA and CFW, respectively. In the experiments performed on a breast phantom, Hadamard-STA achieves better visualization of the breast inclusion with a clearer boundary between the inclusion and the background than STA and CFW. The in vivo experiments were performed on a patient with a breast tumor, and the tumor could also be better visualized with a more homogeneous background in the strain image obtained by Hadamard-STA than by STA and CFW. These results demonstrate that Hadamard-STA achieves a substantial improvement in lateral motion estimation and maybe a competitive method for quasi-static elastography.

Currently available medical ultrasonic imaging systems are limited in some applications by their resolution. In particular: better resolution is needed to detect small lesions in the female breast. The apertures required to improve resolution can be obtained using large transducers at the expense of depth of field. Alternatively, synthetic aperture techniques offer the potential for achieving the necessary resolution without sacrificing depth of field. Advantages of synthetic aperture imaging are high focus, independent of depth, coupled with the absence of near field aberrations. The goal of this study is to develop a synthetic aperture ultrasonic imaging system specifically designed for breast scanning.

High‐resolution imaging of distant, dynamic targets is fundamentally limited by finite‐aperture diffraction. Unlike radar, optical detectors record only intensity and lack the temporal resolution for direct phase measurement, hindering straightforward extension of synthetic aperture techniques to the optical domain. While Macroscopic Fourier ptychography (FP) offers a promising computational alternative, its reliance on mechanical scanning or engineered illumination restricts its use to cooperative, static scenes. In this paper, we present Inverse Synthetic Aperture Macroscopic Fourier ptychography (ISAMFP), which, to the best of our knowledge, is the first practical realization of inverse synthetic aperture imaging for diffusely reflecting targets within the Fourier ptychography framework. By transforming the natural target motion from a detrimental factor into an effective source of angular diversity, ISAMFP eliminates the need for mechanical scanning or illumination modulation and achieves resolution enhancement far beyond the aperture limit using intensity‐only measurements. Experimental validations on USAF resolution targets, unmanned aerial vehicles, and complex multi‐target scenes highlight both the robustness and versatility of ISAMFP, establishing it as a powerful tool for non‐interferometric synthetic aperture imaging of moving, non‐cooperative targets, with broad implications for surveillance, defense, and remote sensing.

Photonics‐based radar enables a large operation bandwidth that is particularly favourable for high‐resolution synthetic aperture imaging. However, how to implement fast and high‐accuracy imaging with photonics‐based broadband radar is still an open question. In this Letter, the performance of photonics‐based inverse synthetic aperture radar imaging with frequency‐domain and time‐domain algorithms are experimentally investigated and compared. The results show that, frequency‐domain range–Doppler algorithm has fast imaging capability, but the image suffers from defocusing and distortions. Although migration compensation techniques can be applied to improve the imaging accuracy, the imaging speed is significantly reduced. On the other hand, using time‐domain back‐projection (BP) algorithm can achieve well‐focused images, and the imaging speed can be enhanced with fast BP methods. Therefore, the time‐domain imaging method is found to be the best choice for fast and high‐accuracy synthetic aperture imaging with photonics‐based broadband radars.