Time Reversal Algorithm Research Articles

A Unique Time-Reversal Algorithm-Enabled Flexible Ultrasound Transducer with a Controllable Acoustic Field.

Flexible ultrasonic devices represent a feasible technology for providing timely signal detection and even a non-invasive disease treatment for the human brain. However, the deformation of the devices is always accompanied by a change in the acoustic field, making it hard for accurate focusing. Herein, we report a stable and flexible transducer. This device can generate a high-intensity acoustic signal with a controllable acoustic field even when the device is bent. The key is to use a low-impedance piezoelectric material and an island-bridge device structure, as well as to design a unique time-reversal algorithm to correct the deviation of signals after transcranial propagation. To provide an in-depth study of the acoustic field of flexible devices, we also analyze the effects of mechanical deformation and structural parameters on the corresponding acoustic response.

Simulation of PD Localization Through Digital Twin Model and Time-Reversal Algorithm Under the Semi-Enclosed Condition

Simulation of PD Localization Through Digital Twin Model and Time-Reversal Algorithm Under the Semi-Enclosed Condition

Limit Values Acquisition in Field Control Problems Using Information-Based Time-Reversal Technique

Despite numerous theories proposed to describe the near-field focusing problem, a non-negligible error still exists between the calculation result and the target, and the process involved is quite complicated. While the time reversal algorithm has greatly reduced the problem’s complexity and improved accuracy under complex scenarios, it has limitations in problems involving large angles and complex area focusing. Both traditional and time reversal algorithms experience errors due to the loss of different types of information. In this paper, we propose an accurate focusing algorithm that combines time reversal with the vector fitting method. This algorithm effectively solves the limitations of traditional focusing algorithms in problems involving large angles, complex area focusing, and other scenarios. The proposed algorithm has the advantages of being fast, enabling global optimization, and not accumulating errors. To verify the accuracy of the algorithm, we design a tunable phase-modulated metasurface. By controlling different vias metallization, we obtain a 360° transmission phase shift with a transmission loss of less than 1 dB.

Chipless RFID identification based on time reversal algorithm

A chipless RFID identification based on time reversal algorithm is presented in this work. The key problems affecting the reliability of chipless RFID identification are analyzed by using the energy focusing feature of time reversal algorithm. Analysis carried out on the energy focusing model revealed that the time reversal array can realize the multi-antenna energy focusing at the tag, so as to improve the reading distance of chipless RFID tag. Furthermore, time reversal energy self-focusing algorithm was used to suppress the noise come from other scatters near the tag. Multi-tag identification can be realized by the application of selective focusing algorithm. MIMO time reveal antenna array and C shaped resonators chipless RFID tag simulation model is constructed to verify the correctness of the proposed method. The results show that two chipless RFID tags with a distance of 0.2 m in azimuth can be identified from 1 m away.

Wideband Multiple Bessel Beams With Desired Directions and Energy Distribution Proportion Based on Time Reversal and Genetic Algorithm in Microwave

In this paper, a simple synthesis technique for versatile and flexible generation of multiple bessel beams with desired directions and energy distribution proportion based on time reversal (TR) and genetic algorithm (GA) is provided. As example, three transmission metasurfaces (TMSs) are designed based on the proposed technique: (1) Single Bessel beam along z axis direction. (2) Dual bessel beams with equal energy distribution proportion for (θ = 30°,φ = 45°) and (θ = -30°,φ = 45°). (3) Dual Bessel Beam with desired energy distribution proportion (1:0.8) for (θ = 30°,φ = 45°) and (θ = -30°,φ = 45°). The TMSs with size 8.21λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ×8.21λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> are formed by units with three-metal layer, which can provide high transmission amplitude (>0.9) and 360° phase shift in a wide frequency band from 10GHz to 18GHz. For validating the design technique, TMS (3) is fabricated and measured, and the simulated and measured results agree well. There are three innovations in the paper: (1) Flexible design technique based on TR and GA. (2) High efficiency TMSs. The maximum Bessel beam conversion efficiency is 37%. (3) wideband 57.1%. The developed design technique can be used in designing high efficiency TMSs in the near field application scenarios such as dense channel communication, high-resolution microwave imaging, near-field detection, efficient wireless energy transmission, medical hyperthermia, etc.

Vertical array signal recovery method based on normalized virtual time reversal mirror

Focusing on the vertical array source signal recovery method, aiming at the estimation deviation caused by environment mismatch in the engineering application of virtual time reversal mirror technology, this paper proposes the normalized virtual time reversal technology through theoretical derivation and analyzes the performance of the virtual time reversal technology under the change of environment and signal-to-noise ratio through simulation. Finally, the practicability of the normalized virtual time reversal method and the effectiveness of model correction are verified through experimental data. The data processing results show that the source level estimation error of vertical array based on normalized virtual time reversal algorithm is less than 5.2dB. In the experimental environment of this paper, the correlation of signal spectral structure recovered by this method is about 5% higher than that by the traditional method.

High-resolution ultrasonic imaging technology for the damage of concrete structures based on total focusing method

Concrete has diverse internal acoustic interface, and the scattering of ultrasonic wave is serious and complicated. Based on total focusing method (TFM) imaging, this paper investigates the concrete inspection resolution optimization method. Firstly, the reinforced concrete model was simulated, using time reversal (TR) algorithm to process the signal, the defects and rebars signal amplitudes increased by 7.6 dB and 4.7 dB, respectively. Then in order to reduce the structural noise, the phase coherence imaging (PCI) algorithm was applied, resulting in a signal-to-noise ratio (SNR) improvement of about 22.2 dB on average. Finally, the imaging positions were corrected. In the experiment, to eliminate the influence of the probe ringing on the detection signal, digital filtering was introduced to restore the signal wave packet,and the image SNR was improved by 3.2 dB after TR processing and by 9.4 dB after PCI processing.

Time-reversal imaging for concrete hole defect based on implantable sensing technology

Time-reversal imaging algorithm based on subspace theory has been verified as an efficient technique for localization of targets and damages in aerospace field. However, it is still a challenge to apply this technique for damage imaging of civil structures due to multi-phase concrete material and the interference of noises. To close the gap, this study proposes a novel subspace-based time-reversal imaging (STRI) algorithm enabled by implantable sensing technology. The STRI algorithm constructs a special three-dimensional transfer matrix and decomposes the time-reversal operator (TRO) into a certain frequency band. The eigenvectors corresponding to the noise subspace are used to reconstruct the imaging map. A novel transducer named cylindrical concrete implantable module (CCIM) is developed to install on the host structure for sending and receiving probe signals; its basic layout and fabrication are introduced in detail. Both the numerical and experimental studies validate the performance of the CCIM and STRI algorithm for hole damage imaging in concrete. Results show that the STRI algorithm overcomes the impacts of noises, offering a higher accuracy of damage areas compared to conventional algorithm. The proposed STRI algorithm enabled by implantable sensing technology has potential applications for real structural hole damage profiling and condition assessment.

Correcting for Target Motion in Time-Reversal Spectral Focusing

It is well-known that an iterative time-reversal (TR) process, applied via a distributed sensing system, can be used to produce a space-time waveform that maximizes the energy scattered from a stationary target back to the sensors. The TR process accomplishes this by automatically focusing energy, both spatially and spectrally, on the stationary target. When scatterers are moving, however, the TR focusing can break down. This article shows how to modify the TR process so that it automatically focuses energy on a moving target. This new TR process automatically generates a distributed beam that: 1) follows the target as it moves and 2) enhances the target resonances by concentrating the energy spectrally. This focusing occurs without a priori knowledge of the target’s location or spectral response. The new TR algorithm is derived through a careful analysis of the idealized case of a single isotropic moving point scatterer. This article includes simulations that compare the TR focus both with and without the new modification. Although the simulations are carried out for the electromagnetic case, the theory applies equally well to the acoustic case when the target speed is significantly less than the ambient sound speed. Appendices are included with details of the calculations and with analysis of the range of relative velocities for which the modified TR process should be used.

The feasibility study of XACT imaging for characterizing osteoporosis.

Osteoporosis is a progressive bone disease that is characterized by a decrease in bone mass and the deterioration in bone microarchitecture, which might be related to age and space travel. An unmet need exists for the development of novel imaging technologies to characterize osteoporosis. The purpose of our study is to investigate the feasibility of X-ray-induced acoustic computed tomography (XACT) imaging for osteoporosis detection. An in-house simulation workflow was developed to assess the ability of XACT for osteoporosis detection. To evaluate this simulation workflow, a three-dimensional digital bone phantom for XACT imaging was created by a series of two-dimensional micro-computed tomography (micro-CT) slices of normal and osteoporotic bones in mice. In XACT imaging, the initial acoustic pressure rise caused by the X-ray induce acoustic (XA) effect is proportional to bone density. First, region growing was deployed for image segmentation of different materials inside the bone. Then k-wave simulations were deployed to model XA wave propagation, attenuation, and detection. Finally, the time-varying pressure signals detected at each transducer location were used to reconstruct the XACT image with a time-reversal reconstruction algorithm. Through the simulated XACT images, cortical porosity has been calculated, and XA signal spectra slopes have been analyzed for the detection of osteoporosis. The results have demonstrated that osteoporotic bones have lower bone mineral density and higher spectra slopes. These findings from XACT images were in good agreement with porosity calculation from micro-CT images. This work explores the feasibility of using XACT imaging as a new imaging tool for Osteoporosis detection. Considering that acoustic signals are generated by X-ray absorption, XACT imaging can be combined with traditional X-ray imaging that holds potential for clinical management of osteoporosis and other bone diseases.

An efficient Lamb wave-based virtual refined time-reversal method for damage localization in plates using broadband measurements

This study proposes a modified virtual time-reversal (VTR) algorithm for baseline signal-free damage detection in plate-like structures. The physical actuation and sensing of Lamb waves are performed using a broadband Gaussian excitation instead of the conventional narrowband modulated tone burst excitations. The forward response and the reconstructed signal due to the time-reversal process for a narrowband input signal are then constructed virtually using the broadband transfer function. The method eliminates the possibility of numerical errors encountered in the conventional VTR method based on narrowband excitations. It is also more efficient than the traditional VTR algorithm because it can probe at multiple excitation frequencies using a single measurement for each sensing path. This modified VTR algorithm is employed in the recently developed refined time-reversal method (RTRM), which uses an extended signal length of the reconstructed signal for computing damage index (DI) and probes the structure at the best reconstruction frequency. The new method is termed the virtual refined time-reversal method (VRTRM). The DIs based on the VRTRM are used in the reconstruction algorithm for probabilistic inspection of defects to achieve baseline signal-free localization of damages. The efficacy of the proposed VRTRM for damage localization is experimentally verified with the established technique RTRM. Experiments are performed in an aluminium plate equipped with a network of surface-bonded piezoelectric patch transducers to illustrate the conventional VTR’s pitfalls and the modified VTR’s accuracy for a single mass damage scenario. The results show that the proposed VRTRM is as accurate as the established technique RTRM in estimating the reconstructed signals and localizing a block mass damage. Finally, the VRTRM is shown to localize in a dual damage scenario with excellent accuracy. In contrast, the conventional main mode-based VTR method fails to localize the damages with or without single-mode tuning.

An Electromagnetic Time-Reversal Imaging Algorithm for Moisture Detection in Polymer Foam in an Industrial Microwave Drying System.

Microwave tomography (MWT) based control is a novel idea in industrial heating systems for drying polymer foam. In this work, an X-band MWT module is designed and developed using a fixed antenna array configuration and integrated with the HEPHAISTOS industrial heating system. A decomposition of the time-reversal operator (DORT) algorithm with a proper Green’s function of multilayered media is utilized to localize the moisture location. The derived Green’s function can be applied to the media with low or high contrast layers. It is shown that the time-reversal imaging (TRI) with the proposed Green’s function can be applied to the multilayered media with a moderately rough surface. Moreover, a single frequency TRI is proposed to decrease the measurement time. Numerical results for different moisture scenarios are presented to demonstrate the efficacy of the proposed method. The developed method is then tested on the experimental data for different moisture scenarios from our developed MWT experimental prototype. Image reconstruction results show promising capabilities of the TRI algorithm in estimating the moisture location in the polymer foam.

Supporting Visualization Analysis in Industrial Process Tomography by Using Augmented Reality-A Case Study of an Industrial Microwave Drying System.

Industrial process tomography (IPT) based process control is an advisable approach in industrial heating processes for improving system efficiency and quality. When using it, appropriate dataflow pipelines and visualizations are key for domain users to implement precise data acquisition and analysis. In this article, we propose a complete data processing and visualizing workflow regarding a specific case—microwave tomography (MWT) controlled industrial microwave drying system. Furthermore, we present the up-to-date augmented reality (AR) technique to support the corresponding data visualization and on-site analysis. As a pioneering study of using AR to benefit IPT systems, the proposed AR module provides straightforward and comprehensible visualizations pertaining to the process data to the related users. Inside the dataflow of the case, a time reversal imaging algorithm, a post-imaging segmentation, and a volumetric visualization module are included. For the time reversal algorithm, we exhaustively introduce each step for MWT image reconstruction and then present the simulated results. For the post-imaging segmentation, an automatic tomographic segmentation algorithm is utilized to reveal the significant information contained in the reconstructed images. For volumetric visualization, the 3D generated information is displayed. Finally, the proposed AR system is integrated with the on-going process data, including reconstructed, segmented, and volumetric images, which are used for facilitating interactive on-site data analysis for domain users. The central part of the AR system is implemented by a mobile app that is currently supported on iOS/Android platforms.

Breast Acoustic Parameter Reconstruction Method Based on Capacitive Micromachined Ultrasonic Transducer Array.

Ultrasound computed tomography (USCT) systems based on capacitive micromachined ultrasonic transducer (CMUT) arrays have a wide range of application prospects. For this paper, a high-precision image reconstruction method based on the propagation path of ultrasound in breast tissue are designed for the CMUT ring array; that is, time-reversal algorithms and FBP algorithms are respectively used to reconstruct sound speed distribution and acoustic attenuation distribution. The feasibility of this reconstruction method is verified by numerical simulation and breast model experiments. According to reconstruction results, sound speed distribution reconstruction deviation can be reduced by 53.15% through a time-reversal algorithm based on wave propagation theory. The attenuation coefficient distribution reconstruction deviation can be reduced by 61.53% through FBP based on ray propagation theory. The research results in this paper will provide key technological support for a new generation of ultrasound computed tomography systems.

An improved technique based on microwave thermoacoustic method for breast cancer screening

Thermoacoustic tomography is an emerging medical imaging technique combining the benefits of microwave imaging and ultrasound. This imaging method has a great potential to be used for breast cancer screening due to detecting early-stage breast cancer with high contrast and high resolution. It has been proven that applying a special regime of microwave pulses can evoke the resonance phenomenon in thermoacoustic. Here, the parameters affecting the intensity of the acoustic signal and the resonance phenomenon are examined. Then, a breast tumor detection technique is proposed based on utilizing the resonance phenomenon and the relationship between sample size and resonance bandwidth. The location of the tumor is also estimated by an explicit method. This technique eliminates the need for image reconstruction. In this method, the size and location of the tumor can be obtained using only three acoustic transducers. Finally, the precision of this method is compared with the time-reversal image reconstruction algorithm. The results demonstrate the feasibility of a safe, fast, and low-cost thermoacoustic detection method for skin and breast cancers.

A Hybrid Synthetic Aperture and Time-Reversal MUSIC Algorithm for Subwavelength Radar Imaging

A promising chipless RFID approach uses millimeter-wave synthetic aperture radar (SAR) to image metal ink-printed ID tags from a meter or more away. Due to printing cost, it is desirable to minimize the size and spacing of metal patches within a tag, preferably into the subwavelength regime. Although circular SAR (CSAR) has a sharply peaked point response in 2D, its side lobes of closely-spaced targets interfere strongly with each other to distort the image. An alternative 2D subwavelength imaging approach with minimal side lobes is Time-Reversal MUSIC (TR-MUSIC). Traditional TR-MUSIC, however, requires a large number of transmitters and receivers. We propose a hybrid synthetic aperture TR-MUSIC algorithm (SATR-MUSIC) that combines the benefits of both approaches. Using relatively few transceivers, SATR-MUSIC is able to resolve objects separated by approximately in 2D with minimal background artifacts. It does so by averaging TR-MUSIC’s imaging kernel incoherently over the synthetic aperture.

Microwave Imaging of Breast Skin Utilizing Elliptical UWB Antenna and Reverse Problems Algorithm.

Skin cancer is one of the most widespread and fast growing of all kinds of cancer since it affects the human body easily due to exposure to the Sun’s rays. Microwave imaging has shown better outcomes with higher resolution, faster processing time, mobility, and less cutter and artifact effects. A miniaturized elliptical ultra-wideband (UWB) antenna and its semi-spherical array arrangement were used for signal transmission and reception from the defected locations in the breast skin. Several conditions such as various arrays of three, six, and nine antenna elements, smaller tumor, multi-tumors, and skin on a larger breast sample of 30 cm were considered. To assess the ability of the system, a breast shape container with a diameter of 130 mm and height of 60 mm was 3D printed and then filled with fabricated skin and breast fat to perform the experimental investigation. An improved modified time-reversal algorithm (IMTR) was used to recreate 2D images of tumors with the smallest radius of 1.75 mm in any location within the breast skin. The reconstructed images using both simulated and experimental data verified that the system can be a reliable imaging system for skin cancer diagnosis having a high structural similarity index and resolution.

A Passive Location Method Based on Virtual Time Reversal of Cross Antenna Sensor Array and Tikhonov Regularized TLS

The multi-station direction finding and cross positioning method is often used in three-dimensional passive detection system when the measurement information only has DOA estimation value. However, the essence of this algorithm is to linearize the nonlinear problem, which is very vulnerable to the influence of direction finding error. In order to solve the above problem, the anti-interference virtual time reversal (VTR) algorithm combined with Tikhonov regularization total least squares (TLS) are for passive location. A multi-station passive location model is built and the cross antenna sensor array is used to receive signals. According to the established array model, the coordinate transformation formula of the observation station is derived and the coordinate formula of each array element is obtained by the transformation matrix, then the signal receiving model is established. The coordinates of virtual points are transformed for DOA estimation which accomplished by VTR, then the direction lines obtained. The results of the VTR direction finding are transformed into new space angles and the observation equations of the space angles are constructed. In order to overcome the influence of noise on the positioning solution, Tikhonov regularized TLS method is used to solve the equations for obtaining target position. The simulation results show that the algorithm can achieve high-resolution DOA estimation of the target at low SNR and the positioning errors under different number of observation stations are analyzed. The positioning accuracy has better performance compared with the unimproved least square method.

Physical-virtual time reversing of nonlinear Lamb waves for fatigue crack detection and quantification

This article presents the investigation of a nonlinear Lamb wave time reversing technique for fatigue crack detection and quantification. A 2D analytical framework is initially presented, modeling Lamb wave generation, propagation, wave crack linear and nonlinear interaction, and reception. This study extends the Time Reversal (TR) techniques into the realm of nonlinear Lamb waves. Due to the structural transfer function variation between the forward and backward transmission process, the Virtual Time Reversal (VTR) algorithm reveals obvious deviation for predicting nonlinear Lamb waves, given that it replaces the backward TR procedure with the forward transfer function. However, this study demonstrates that the difference between the physical nonlinear TR method and the conventional VTR algorithm proves to be sensitive to detect and quantify fatigue cracks. Fatigue tests on a thin aluminum plate with a rivet hole are conducted to induce a fatigue crack. The experimental results further illuminate that the proposed physical-virtual nonlinear Lamb wave TR technique possesses remarkable sensitivity to the nucleation and growth of fatigue cracks. The paper finishes with discussion, concluding remarks, and suggestions for future work.

Parametrix for the inverse source problem of thermoacoustic tomography with reduced data

Our goal is to solve the inverse source problem of thermo- and photoacoustic tomography, with data registered on an open surface partially surrounding the source of acoustic waves. The proposed modified time reversal algorithm recovers the source term up to an infinitely smooth error term. Similarly to (Eller M et al 2020 Inverse Problems 36 085012), numerical simulations show that the error term is quite small in practical terms. Unlike the latter method, the present technique is applicable in the presence of a known variable speed of sound. It is also significantly more efficient from a computational standpoint. It can be implemented using either standard finite difference techniques or through methods based on separation of variables, that for special geometries yield extremely fast image reconstruction. We illustrate our results with numerical simulations in 2 and 3 spatial dimensions.