An efficient ultrasonic wavenumber-domain plane wave imaging method towards the inspection of curved structures.

Ultrasonic phased array testing is commonly employed for inspecting curved structures. Conventional plane wave imaging techniques, based on delay-and-sum in the time-domain, offer high image quality and inspection accuracy but suffer from low frame rates due to their high computational complexity. In this work, an efficient wavenumber-domain imaging method that combines non-stationary wavefield extrapolation and f-k migration is proposed for curved structure inspection. Special emission focal laws are designed to generate a sequence of steered plane waves through the curved interface. The raw data is then extrapolated to the top boundary of the region of interest, followed by f-k migration to reconstruct images with high time efficiency. Simulation and experimental evaluations demonstrate a time reduction by a factor of up to 32.24 compared to conventional time-domain plane wave image reconstruction with equivalent image quality, highlighting its potential for monitoring flaws in real-time.

Ultrasonic imaging through reverberation media.

Thin layered media like thermal barrier and corrosion resistant coating layers, are important components to protect and strengthen the base materials. Nevertheless, the non-destructive testing (NDT) of base materials under thin layer media are still strongly demanded. However, surface and reverberation waves propagating in the top thin layer deteriorate the ultrasonic imaging results of base materials. Particularly, they overwhelm the reflection waves of defects, making ultrasonic NDT of base materials a challenge. These waves are determined by the structural of testing objects and called structural noises. Here, a pre-process of ultrasonic total focusing method is proposed to remove the structural noises. The pre-process utilizes the different similarity characteristics of structural noises and defect signals in diagonal matrices to suppress the noises by principal component analysis. The experimental results show that it can effectively improve the SNR about 5-8 dB and reduce array performance indicators about 30-40%.

3-D ultrasonic image reconstruction in frequency domain using a virtual transducer model.

In ultrasonic non-destructive testing, image reconstruction is essential to restore the diffracted ultrasound signals to improve the lateral resolution of images. Some reconstruction methods, like DAS-based synthetic aperture imaging, are inefficient, especially for reconstructing three-dimensional (3-D) images. Other methods do not provide high-resolution results, because they neglect the distortion effect introduced by transducer geometry. To overcome these disadvantages, we propose a 3-D ultrasonic image reconstruction method based on synthetic aperture wavenumber algorithm. It considers wave diffraction and transducer geometry effects, and can refocus the reflectors even in non-focal zone, which suits for large depth range imaging. This method builds a virtual transducer model in frequency domain by treating the focused transducer as a virtual planar transducer on its focal plane. In addition, the method uses non-uniform fast Fourier transform and deconvolution operation to achieve the 3-D image reconstruction, which has remarkably improved the efficiency and accuracy. According to the experimental results, the lateral resolution of an image reconstructed by the proposed method can reach 290.2 µm, exceeding the lateral resolution limitation of the 15 MHz focused transducer (523.24 µm). Furthermore, the proposed method only takes 0.744 s to reconstruct a 3-D image with 1000×100×100 pixels, while the time domain SAFT takes about 1163.8 s. It shows the potential for real-time 3-D imaging under advanced hardware.

Temperature field regulation of a droplet using an acoustothermal heater.

Heating a droplet without contamination is desired for the emerging applications of microfluidic devices in life science and materials science, especially in the form of controllable temperature distribution. Microfluidic heaters using surface acoustic waves have been recently demonstrated, which highlights an urgent need for an insight into the detailed heating mechanism to guide the development of temperature regulation methodologies. Here, we show that the temperature field of a droplet on the path of a travelling wave can be regulated by modulating the heat source distribution and thermal conduction inside the target. We model the acoustothermal process of the droplet including the effects of electric dissipation, acoustic dissipation, and acoustic-induced steady flow. The electric-mechanical-acoustic coupling contributes to the dominant heat source, and we call it acoustic heat source. The nonlinear effects of incident waves generate acoustic vortexes with a velocity of up to 20 mm s-1, inducing forced convection inside the droplet to enhance heat transfer. The equilibrium temperature field of a droplet is determined by a synergy of dissipative acoustic attenuation and acoustic streaming. We demonstrate that the distribution of the acoustic heat source and the patterns of acoustic streaming can be modulated by fluid viscosity and droplet size. Various spatial combinations of the acoustic heat source and steady streaming make different temperature fields in the droplet. We also propose a phase diagram of the temperature distribution in the droplet. This methodology enables opportunities for temperature-related processing inside a droplet bioparticle carrier or microreactor.

Visual geometry Group-UNet: Deep learning ultrasonic image reconstruction for curved parts.

Detecting small defects in curved parts through classical monostatic pulse-echo ultrasonic imaging is known to be a challenge. Hence, a robot-assisted ultrasonic testing system with the track-scan imaging method is studied to improve the detecting coverage and contrast of ultrasonic images. To further improve the image resolution, we propose a visual geometry group-UNet (VGG-UNet) deep learning network to optimize the ultrasonic images reconstructed by the track-scan imaging method. The VGG-UNet uses VGG to extract advanced information from ultrasonic images and takes advantage of UNet for small dataset segmentation. A comparison of the reconstructed images on the simulation dataset with ground truth reveals that the peak signal-to-noise ratio (PSNR) and structural similarity index measure (SSIM) can reach 39 dB and 0.99, respectively. Meanwhile, the trained network is also robust against the noise and environmental factors according to experimental results. The experiments indicate that the PSNR and SSIM can reach 32 dB and 0.99, respectively. The resolution of ultrasonic images reconstructed by track-scan imaging method is increased approximately 10 times. All the results verify that the proposed method can improve the resolution of reconstructed ultrasonic images with high computation efficiency.

Design of acoustofluidic device for localized trapping.

State of the art acoustofluidics typically treat micro-particles in a multi-wavelength range due to the scale limitations of the established ultrasound field. Here, we report a spatial selective acoustofluidic device that allows trapping micro-particles and cells in a wavelength scale. A pair of interdigital transducers with a concentric-arc shape is used to compress the beam width, while pulsed actuation is adopted to localize the acoustic radiation force in the wave propagating direction. Unlike the traditional usage of geometrical focus, the proposed device is designed by properly superposing the convergent section of two focused surface acoustic waves. We successfully demonstrate a single-column alignment of 15-µm polystyrene particles and double-column alignment of 8-µm T cells in a wavelength scale. Through proof-of-concept experiments, the proposed acoustofluidic device shows potential applications in on-chip biological and chemical analyses, where localized handing is required.

Enhancing ultrasonic time-of-flight diffraction measurement through an adaptive deconvolution method.

Deconvolution is generally applied to improve the temporal resolution of ultrasonic signals. However, using this process in the time-of-flight diffraction (TOFD) measurement of small and shallow defects is challenging because TOFD signals are dispersive in space-frequency distribution. Particularly, determining the reference signal for deconvolution remains a critical barrier. To this end, an adaptive deconvolution method is proposed in this study. Using wavelet transform, we firstly decompose the TOFD signals into sub-band signals to standardise the space-frequency distribution. Then, sub-band signals with strong coherences are adaptively selected on the basis of coherence coefficient metric. Upon the opted sub-band signals, a lateral wave can be readily used as the reference signal, and TOFD signals can be reconstructed with established Wiener filtering and spectral extrapolation methods. The feasibility of the proposed method is validated with the TOFD measurement of a small side-drilled hole near the surface. Results show that the proposed method effectively separates overlapping TOFD signals and improves the axial resolution of a TOFD image.

Frequency domain synthetic aperture focusing technique for variable-diameter cylindrical components.

Ultrasonic non-destructive testing (UNDT) plays an important role in ensuring the quality of cylindrical components of equipment such as pipes and axles. As the acoustic beam width widens along propagation depths, the diffraction of acoustic wave becomes serious and the images of defects will be interfered with. To precisely evaluate the dimensions of defects and flaws concealed in components, the synthetic aperture focusing technique (SAFT) is introduced to enhance the image resolutions. Conventional SAFTs have been successfully implemented for the ultrasonic imaging of normal cylinders, while solutions for complex ones, such as variable-diameter cylinders, are still lacking. To overcome this problem, a frequency-domain SAFT for variable-diameter cylindrical components is proposed. This algorithm is mainly based on acoustic field extrapolation, which is modified from cylindrical phase shift migration with the aid of split-step Fourier. After a series of extrapolations, a high-resolution ultrasound image can be reconstructed using a particular imaging condition. According to the experimental results, the proposed method yields low side lobes and high resolutions for flat transducers. Its attainable angular resolution relies on the transducer diameter D and scanning radius R and approximates D/(2R).