Wave-based Imaging Research Articles

Learning a stable approximation of an existing but unknown inverse mapping: application to the half-time circular Radon transform

Supervised deep learning-based methods have inspired a new wave of image reconstruction methods that implicitly learn effective regularization strategies from a set of training data. While they hold potential for improving image quality, they have also raised concerns regarding their robustness. Instabilities can manifest when learned methods are applied to find approximate solutions to ill-posed image reconstruction problems for which a unique and stable inverse mapping does not exist, which is a typical use case. In this study, we investigate the performance of supervised deep learning-based image reconstruction in an alternate use case in which a stable inverse mapping is known to exist but is not yet analytically available in closed form. For such problems, a deep learning-based method can learn a stable approximation of the unknown inverse mapping that generalizes well to data that differ significantly from the training set. The learned approximation of the inverse mapping eliminates the need to employ an implicit (optimization-based) reconstruction method and can potentially yield insights into the unknown analytic inverse formula. The specific problem addressed is image reconstruction from a particular case of radially truncated circular Radon transform (CRT) data, referred to as ‘half-time’ measurement data. For the half-time image reconstruction problem, we develop and investigate a learned filtered backprojection method that employs a convolutional neural network to approximate the unknown filtering operation. We demonstrate that this method behaves stably and readily generalizes to data that differ significantly from training data. The developed method may find application to wave-based imaging modalities that include photoacoustic computed tomography.

In Vivo Microwave-Induced Thermoacoustic Endoscopy for Colorectal Tumor Detection in Deep Tissue.

Optical endoscopy, as one of the common clinical diagnostic modalities, provides irreplaceable advantages in the diagnosis and treatment of internal organs. However, the approach is limited to the characterization of superficial tissues due to the strong optical scattering properties of tissue. In this work, a microwave-induced thermoacoustic (TA) endoscope (MTAE) was developed and evaluated. The MTAE system integrated a homemade monopole sleeve antenna (diameter = 7 mm) for providing homogenized pulsed microwave irradiation to induce a TA signal in the colorectal cavity and a side-viewing focus ultrasonic transducer (diameter = 3 mm) for detecting the TA signal in the ultrasonic spectrum to construct the image. Our MTAE, system combined microwave excitation and acoustic detection; produced images with dielectric contrast and high spatial resolution at several centimeters deep in soft tissues, overcome the current limitations of the imaging depth of optical endoscopy and mechanical wave-based imaging contrast of ultrasound endoscopy, and had the ability to extract complete features for deep location tumors that could be infiltrating and invading adjacent structures. The practical feasibility of the MTAE system was evaluated i n vivo with rabbits having colorectal tumors. The results demonstrated that colorectal tumor progression could be visualized from the changes in electromagnetic parameters of the tissue via MTAE, showing its potential clinical application.

Cohesive framework for non-line-of-sight imaging based on Dirac notation.

The non-line-of-sight (NLOS) imaging field encompasses both experimental and computational frameworks that focus on imaging elements that are out of the direct line-of-sight, for example, imaging elements that are around a corner. Current NLOS imaging methods offer a compromise between accuracy and reconstruction time as experimental setups have become more reliable, faster, and more accurate. However, all these imaging methods implement different assumptions and light transport models that are only valid under particular circumstances. This paper lays down the foundation for a cohesive theoretical framework which provides insights about the limitations and virtues of existing approaches in a rigorous mathematical manner. In particular, we adopt Dirac notation and concepts borrowed from quantum mechanics to define a set of simple equations that enable: i) the derivation of other NLOS imaging methods from such single equation (we provide examples of the three most used frameworks in NLOS imaging: back-propagation, phasor fields, and f-k migration); ii) the demonstration that the Rayleigh-Sommerfeld diffraction operator is the propagation operator for wave-based imaging methods; and iii) the demonstration that back-propagation and wave-based imaging formulations are equivalent since, as we show, propagation operators are unitary. We expect that our proposed framework will deepen our understanding of the NLOS field and expand its utility in practical cases by providing a cohesive intuition on how to image complex NLOS scenes independently of the underlying reconstruction method.

A human ear-inspired ultrasonic transducer (HEUT) for 3D localization of sub-wavelength scatterers

The proposed technology aims to enable 3D localization of scatterers using single element ultrasonic transducers, which are traditionally limited to 1D measurements. This is achieved by designing a bespoke acoustic lens with a spiral-shaped pattern similar to the human outer ear, a shape that has evolved for sound source localization. This lens breaks the surface symmetry of the transducer, allowing ultrasonic waves arriving from different directions to be encoded in a certain way that can later be decoded to extract directional information. By employing the mechanism of spatial-encoding of the received signals and decoding via signal processing, the location of sub-wavelength scatterers can be detected in 3D with a single measurement for sparsely distributed scatterers. The proposed technology is first verified through a simulation study, and then 3D printed acoustic lenses are used to demonstrate the 3D encoding functionality of the Human Ear-inspired Ultrasonic Transducer (HEUT) experimentally. A framework is created to localize scatterers in 3D by processing received signals acquired by a HEUT prototype. With this technology, a single transducer can obtain multi-dimensional information with a single pulse-echo measurement, reducing the number of elements required for performing 3D ultrasound localization. The proposed spatial-encoding and -decoding technology can be applied to other wave-based imaging methods to develop affordable, practical, and compact sensing devices.

Imaging the delamination in multilayer structure

To achieve the interlaminar delamination imaging of multilayer structures, we proposed a systematic methodology named Integrated Reflected–Transmitted wave-based Imaging (IRTI) by using the interface wave. Based on the interaction mechanism between the interface wave and interlaminar delamination, the first step of IRTI is to extract the interlaminar delamination information. Then, the IRTI designs an appropriate probe layout according to the excitation characteristics of the interface wave. Next, two algorithms are adopted respectively in the IRTI for reflected wave-based imaging and transmitted wave-based imaging. Lastly, the two imaging results are fused to obtain the final result. Furthermore, the effectiveness of the proposed IRTI is verified by experiments on the double-layer plates with delamination. For a rectangle-shaped and the XJTU-shaped delamination with different shape characteristics, the imaging results are legible, and the identified delamination shape is with accurate contour and location.

Far-field acoustic subwavelength imaging with blind structured illumination

The blind structured illumination microscopy (blind-SIM) is an optical method to surpass the diffraction limit, a fundamental limitation to all wave-based imaging techniques. By illuminating the object with multiple structured illumination patterns, the spatial frequency mixing between the object and the illuminations converts evanescent waves to propagating waves that can be detected by sensors in the far field. In this work, we develop blind-SIM in the acoustic domain. The blind acoustic structured illuminations (ASI) are generated by randomly located scatterers with subwavelength spatial distributions. The image of the object can be reconstructed with a subwavelength resolution from multiple far-field measurements (each produced by an ASI pattern) utilizing a compressed sensing algorithm. We demonstrate subwavelength acoustic imaging resolution in both kilohertz airborne and megahertz underwater acoustic imaging systems. The developed blind-SIM method has great potential in medical imaging, non-destructive testing, and underwater acoustic imaging for improving the imaging resolution of objects far away from the sensors.

Study on an artificial phenomenon observed in terahertz biological imaging.

Terahertz (THz) wave-based imaging of biological samples is an emerging but promising field. In the present work, we report an artificial phenomenon observed in imaging melanoma slices, which can lead to mistakenly interpretation of the experimental results. It was observed that a structure similar to but smaller than the sample contour appeared inside the melanoma slice image. The underlying mechanism of this phenomenon was then investigated both experimentally and theoretically. By imaging a regular standard sample (vinyl coverslip) with a THz time domain spectroscopy (THz-TDS) system and reconstructing its images at 0.8 and 1.2 THz, we can clearly observe the afore-mentioned artifacts. The experimental results are highly consistent with the simulations based on the Fresnel-Kirchhoff diffraction theory in which possible optical aberrations were incorporated. It can be concluded that this artifact was caused by the frequency-dependent diffraction of the sample edge. The work demonstrated here is essential for correct interpretation of the images obtained by the THz-TDS technique.

Imaging in Random Media by Two-Point Coherent Interferometry

This paper considers wave-based imaging through a heterogeneous (random) scattering medium. The goal is to estimate the support of the reflectivity function of a remote scene from measurements of t...

Superresolution Imaging via Subwavelength Hole Resonances

This work presents a new super-resolution imaging approach by using subwavelength hole resonances. We employ a subwavelength structure in which an array of tiny holes are etched in a metallic slab with the neighboring distance $\ell$ that is smaller than half of the wavelength. By tuning the incident wave at resonant frequencies, the subwavelength structure generates strong illumination patterns that are able to probe both low and high spatial frequency components of the imaging sample sitting above the structure. The image of the sample is obtained by performing stable numerical reconstruction from the far-field measurement of the diffracted wave. It is demonstrated that a resolution of $\ell/2$ can be obtained for reconstructed images, thus one can achieve super-resolution by arranging multiple holes within one wavelength. The proposed approach may find applications in wave-based imaging such as electromagnetic and ultrasound imaging. It attains two advantages that are important for practical realization. It avoids the difficulty to control the distance the between the probe and the sample surface with high precision. In addition, the numerical reconstructed images are very stable against noise by only using the low frequency band of the far-field data in the numerical reconstruction.

Imaging techniques for defect detection of fiber reinforced polymer‐bonded civil infrastructures

The present article provides a state-of-the-art review of imaging techniques used for defect detection of fiber reinforced polymer (FRP)-bonded civil engineering structures. Compared to the conventional techniques by equipping a structure with stress wave sensors, the contactless imaging techniques feature efficient instrumentation, convenient data acquisition, and rapid evaluation in the procedures of nondestructive testing (NDT). Recently, a substantial progress in utilizing electromagnetic waves for development of imaging NDT techniques (e.g., synthetic aperture radar, infrared thermography, laser shearography, and laser reflection technique) with the purpose to identify debonding or delamination of FRP-bonded structural systems has been made. As another electromagnetic wave-based imaging technique, X-ray computed tomography is promising for exploration of the damage evolution in this structural system, despite little application in structural health monitoring of real infrastructures. Apart from these imaging techniques, more recently, there have been computer-aided motion magnification techniques for defect identification. The motion magnification technique only requires a digital camera and a computer with vision algorithm, which can amplify the motion of defect region and render it visible in a video. This advanced imaging technique achieves high-resolution measurement, simultaneous full-field inspection, and straightforward defect identification in the scene. In this review, both Eulerian motion magnification and phase-based motion magnification techniques for structural visual inspection are presented and discussed. Furthermore, the present article recommends the combination of imaging NDT techniques with artificial intelligence approaches (e.g., deep learning algorithms) to realize the automated and efficient defect detection towards FRP-bonded civil infrastructures.

Digital Sequence and Virtual Path Construction-Based Impact Imaging Method for Onboard Monitoring of Aerospace Composite Structures

Abstract Because of the rapidly growing use and poor impact resistance of composite materials, impact monitoring of composite structures has become more and more important, especially for aerospace engineering applications. Among the existing impact monitoring methods, piezoelectric transducer (PZT) network and guided wave-based imaging method has proved to be an effective structural health monitoring (SHM) technology. However, the pursuit of accurate localization of ordinary impact imaging methods comes at a cost, the high requirement for the SHM hardware system making these methods inapplicable for onboard impact monitoring. In order to realize accurate impact monitoring of aerospace composite structures onboard, this paper proposes a digital sequence and virtual path construction-based impact imaging method, which is simple enough to work in a digital impact monitor with a greatly simplified hardware system. The monitor is used to convert the impact response signals of PZTs into digital sequences, based on which the method first recognizes the impact occurring sub-region. Then, the virtual pitch–catch paths of the recognized sub-region are innovatively defined and constructed to estimate the impact-induced influence on these paths so that the path-synthesis imaging algorithm-based impact imaging can be performed with little computation cost. To verify the feasibility and effectiveness of the proposed method, a total of 90 impacts are applied on a composite unmanned aerial vehicle (UAV) wing, and experimental results show the good performance of accurate impact imaging and localization.

Anomalous diffusion of acoustic waves in 2D periodic media: Radiative transport and renormalization analysis.

This work investigates the occurrence of anomalous transport of acoustic waves propagating in two-dimensional (2D) perfectly periodic media and presents dedicated analysis tools to explore and understand the properties of the medium controlling the transitions between different transport regimes. By leveraging a two-fold approach that combines both radiative transport and renormalization theory, the propagation properties of the inhomogeneous medium can be characterized both near and at the transition from normal to anomalous diffusion. The proposed approach builds upon the classical radiative transfer theory of bulk materials, and it is specifically designed to study 2D systems. The ability to simulate and interpret the field quantities that describe such transport mechanisms can play a significant role in the development of wave-based imaging technology for highly inhomogeneous and scattering media.

Exact Multistatic Interferometric Imaging via Generalized Wirtinger Flow

In this paper, we present a novel approach that can exactly recover extended targets in wave-based multistatic interferometric imaging, based on Generalized Wirtinger Flow (GWF) theory [1]. Interferometric imaging is a generalization of phase retrieval, which arises from cross-correlation of measurements from pairs of receivers in multistatic configuration. Unlike standard Wirtinger Flow, GWF theory guarantees exact recovery for arbitrary lifted forward models that satisfy the restricted isometry property over rank-1, positive semi-definite (PSD) matrices with a sufficiently small restricted isometry constant (RIC). To this end, we design a deterministic, lifted forward model for interferometric multistatic radar satisfying the exact recovery conditions of the GWF theory. Our results quantify a lower limit on the pixel spacing and the minimal sample complexity for exact multistatic radar imaging via GWF. We provide a numerical study of our RIC and pixel spacing bounds, which shows that GWF can achieve exact recovery with super-resolution. While our primary interest lies in radar imaging, our method is also applicable to other multistatic wave-based imaging problems such as those arising in acoustics and geophysics.

Optimal experimental design for joint reflection-transmission ultrasound breast imaging: From ray- to wave-based methods.

Ultrasound computed tomography (USCT) is an emerging modality to image the acoustic properties of the breast tissue for cancer diagnosis. With the need of improving the diagnostic accuracy of USCT, while maintaining the cost low, recent research is mainly focused on improving (1) the reconstruction methods and (2) the acquisition systems. D-optimal sequential experimental design (D-SOED) offers a method to integrate these aspects into a common systematic framework. The transducer configuration is optimized to minimize the uncertainties in the estimated model parameters, and to reduce the time to solution by identifying redundancies in the data. This work presents a formulation to jointly optimize the experiment for transmission and reflection data and, in particular, to estimate the speed of sound and reflectivity of the tissue using either ray-based or wave-based imaging methods. Uncertainties in the parameters can be quantified by extracting properties of the posterior covariance operator, which is analytically computed by linearizing the forward problem with respect to the prior knowledge about parameters. D-SOED is first introduced by an illustrative toy example, and then applied to real data. This shows that the time to solution can be substantially reduced, without altering the final image, by selecting the most informative measurements.

Steady Delivery: Wireless Local Positioning Systems for Tracking and Autonomous Navigation of Transport Vehicles and Mobile Robots

For many players in the service and manufacturing sectors, the cost associated with transportation processes is a key factor in competitiveness and market success. Innovations such as wireless connectivity, RF identification (RFID), and the Internet of Things have already transformed these industries, and this trend-stimulated by modern wireless technologies, wave-based imaging systems, and autonomous driving-is set to continue.

Ghost imaging in the random paraxial regime

In this paper we analyze a wave-based imaging modality called ghost imaging that can produce an image of an object illuminated by a partially coherent source. The image of the object is obtained by correlating the intensities measured by two detectors, one that does not view the object and another one that does view the object. More exactly, a high-resolution detector measures the intensity of a wave field emitted by a partially coherent source which has not interacted with the object to be imaged. A bucket (or single-pixel) detector collects the total (spatially-integrated) intensity of the wave field emitted by the same source that has interacted with the object. The correlation of the intensity measured at the high-resolution detector with the intensity measured by the bucket detector gives an image of the object. In this paper we analyze this imaging modality when the medium through which the waves propagate is random. We discuss the relation with time reversal focusing and with correlation-based imaging using ambient noise sources. We clarify the role of the partial coherence of the source and we study how scattering affects the resolution properties of the ghost imaging function in the paraxial regime: the image resolution is all the better as the source is less coherent, and all the worse as the medium is more scattering.

Using the synthetic aperture focusing technique with velocity responses to detect the defects inside reinforced concrete structures

In civil engineering, imaging can be a useful non-destructive testing (NDT) tool for revealing defects in a concrete structure. To date, there is no effective method for imaging the defects inside in situ concrete structures. In this study, a new elastic wave-based NDT method has been developed and then applied to reveal the defects inside concrete specimens using an imaging method. This method integrates the point-source/point-receiver method with the synthetic aperture focusing technique (SAFT) to achieve a scanning effect when using a phase array system. This method operates with a large functioning depth because the transient elastic wave method possesses a high-energy feature, whereas the traditional ultrasonic method does not. As an alternative to using displacement responses, this method uses velocity responses to construct the SAFT image. Based on the numerical simulation and the experimental results, using the proposed method allowed a high image quality to be produced and successfully revealed defects underneath a layer of rebars. The results of this study indicate the great potential for using the elastic wave-based imaging method to inspect defects in situ concrete structures.

Superoscillations without sidebands: power-efficient sub-diffraction imaging with propagating waves.

A superoscillation wave is a special superposition of propagating electromagnetic (EM) waves which varies with sub-diffraction resolution inside a fixed region. This special property allows superoscillation waves to carry sub-diffraction details of an object into the far-field, and makes it an attractive candidate technology for super-resolution devices. However, the Shannon limit seemingly requires that superoscillations must exist alongside high-energy sidebands, which can impede its widespread application. In this work we show that, contrary to prior understanding, one can selectively synthesize a portion of a superoscillation wave and thereby remove its high-energy region. Moreover, we show that by removing the high-energy region of a superoscillation wave-based imaging device, one can increase its power efficiency by two orders of magnitude. We describe the concept behind this development, elucidate conditions under which this phenomenon occurs, then report fullwave simulations which demonstrate the successful, power-efficient generation of sub-wavelength focal spots from propagating waves.

Lamb wave-based damage imaging method for damage detection of rectangular composite plates

Lamb wave-based damage imaging method for damage detection of rectangular composite plates

Baseline-free guided wave imaging via adaptive source removal

Structural health monitoring systems employing in situ guided wave transducers are being considered for civil, mechanical, and aerospace applications. In particular, guided wave-based imaging methods have been applied to graphically depict the area of interest and localize the damage. Most existing imaging techniques have relied on scattered waves extracted by baseline subtraction to quantify the effects of damage on the guided waves. In many cases, however, baseline data may not have been recorded or may be ineffective for baseline subtraction because of uncompensated variations between baseline and in-service signals. Proposed here is an alternative approach whereby source signals are first adaptively estimated from the in-service signals, direct arrivals are then calculated by forward propagation, and scattered signals are obtained by subtraction of the direct arrivals. Images using these scattered signals can then be created by, for example, delay-and-sum methods. This approach is demonstrated using synthetic signals generated by ray tracing and conventional delay-and-sum imaging. Its feasibility is further demonstrated with experimental data recorded on an aluminum plate using a single in situ transducer as a source and a laser vibrometer to record guided signals around the periphery of the region of interest.