Laser Ultrasound Imaging Research Articles

Image quality improvement in multi-mode laser ultrasound imaging using reconstructed synthetic aperture focusing technique

The synthetic aperture focusing technique (SAFT) is used as a laser ultrasonic imaging algorithm for non-destructive internal defect testing, and multi-mode time domain SAFT (MMT-SAFT) imaging is developed to enhance the imaging capability of defect detection. However MMT-SAFT algorithm has the problem of artifacts caused by the laser ultrasound (LU) modes waves. Thus to solve this problem, an automatic imaging technique based on MMT-SAFT called Reconstructed synthetic aperture focusing technique (Re-SAFT) is proposed. This technique consists of two steps: a preliminary MMT-SAFT is firstly performed to locate the defect position tentatively and futher separate LU modes wave signals using the defect-reflected signal; then a secondary reconstructed MMT-SAFT based on the priori defect location information is achieved for artifacts removal. In addition, directivity patterns of LU modes wave have been used in Re-SAFT to achieve more accurate defect imaging in the second step. Results indicate that by using Re-SAFT, the typical artifact caused by laser ultrasonic waves can be removed and the imaging quality can be significantly improved from 19.67 to 33.85 dB. Further, the consumed time of Re-SAFT is economized by optimizing the imaging pixel size without compromising the image quality. Therefore, Re-SAFT has potential applications in industrial laser ultrasonic testing.

Laser ultrasound imaging of mechanical stresses near holes and indented areas: Experimental results and theoretical model

Experimental results on the behavior of laser ultrasonic signals near holes in stressed ceramics and metals are presented. The data obtained are analyzed within the framework of the standard theory of thermoelasticity. It is shown that this approach is not able to explain the behavior of laser ultrasonic signals in stressed materials. A new model of thermoelasticity taking into account thermal perturbation of non-stationary defect states with relaxation is presented and analyzed. It is demonstrated that strong stress dependence of the laser ultrasonic signals provides an opportunity to estimate mechanical stresses in different materials. An example of mechanical stress effect on the laser ultrasonic image near Rockwell indentation zones in metals is presented.

Features of the behavior of laser ultrasonic signals near a hole in duralumin when exposed to two misaligned mechanical stresses

The effect of misaligned mechanical stresses on the excitation of ultrasonic vibrations by focused laser radiation near a small-diameter hole in aluminum alloy is investigated. The analysis of the behavior of the signal at various mutual angles and values of two uniaxial stresses is carried out. It is shown that at certain angles under the action of mechanical stresses, not only the contrast of laser ultrasound images changes, but also their rotation as a whole relative to the center of the hole can occur. The possibility of disappearance of the linear component of laser ultrasonic signals at certain characteristics of stress and appearance of a nonlinear component is also demonstrated. Keywords: diagnostics, nondestructive evaluation, mechanical stress, laser ultrasound.

High-resolution air-coupled laser ultrasound imaging of microstructure and defects in braided CFRP

Defect inspection of braided carbon fiber reinforced polymer (CFRP) is very difficult due to its nonhomogeneity, anisotropy, and sensitivity to coupling agents. This paper presents a hybrid system for detecting microstructure and defects in braided CFRP that combines the advantages of laser-ultrasound and air-coupled ultrasonic testing. Through the finite element method, the ultrasonic field propagating on the braided CFRP was simulated during the laser-induced ultrasound, and the influences of laser parameters and surface braided structure were analyzed on the laser ultrasonic signal. This detection method based on air-coupled laser ultrasound can provide the near-surface microstructure characterization of the resin pockets and braided fiber bundles, and also can achieve both shallow and deep defect detection of CFRP sheets which is comparable to that of the contact-type high-frequency phased array with higher contrast and less distortion. These results indicate that it has the potential to develop a non-contact, high-resolution, and low-cost method for the detection and repairment of advanced composite materials.

Full noncontact laser ultrasound: first human data

Full noncontact laser ultrasound (LUS) imaging has several distinct advantages over current medical ultrasound (US) technologies: elimination of the coupling mediums (gel/water), operator-independent image quality, improved repeatability, and volumetric imaging. Current light-based ultrasound utilizing tissue-penetrating photoacoustics (PA) generally uses traditional piezoelectric transducers in contact with the imaged tissue or carries an optical fiber detector close to the imaging site. Unlike PA, the LUS design presented here minimizes the optical penetration and specifically restricts optical-to-acoustic energy transduction at the tissue surface, maximizing the generated acoustic source amplitude. With an appropriate optical design and interferometry, any exposed tissue surfaces can become viable acoustic sources and detectors. LUS operates analogously to conventional ultrasound but uses light instead of piezoelectric elements. Here, we present full noncontact LUS results, imaging targets at ~5 cm depths and at a meter-scale standoff from the target surface. Experimental results demonstrating volumetric imaging and the first LUS images on humans are presented, all at eye- and skin-safe optical exposure levels. The progression of LUS imaging from tissue-mimicking phantoms, to excised animal tissue, to humans in vivo is shown, with validation from conventional ultrasound images. The LUS system design insights and results presented here inspire further LUS development and are a significant step toward the clinical implementation of LUS.

Model-based measurement of internal geometry of solid parts with sub-PSF accuracy using laser-ultrasonic imaging

The problem of workpiece tolerance verification is of high interest in the field of nondestructive testing and evaluation. In this paper, reflection mode immersion 2D laser-ultrasound tomography is proposed for control of dimensions of solid objects with piecewise linear surface profiles. Pulsed laser radiation is used for generation of short ultrasonic probe pulse providing high spatial resolution. Wideband array of piezofilm sensors is used for detection of the waves reflected by the external and internal surfaces of the object. The image reconstruction algorithm accounts for refraction of acoustic waves at the liquid-solid interface. The contrast-enhanced reconstructed image is processed using the virtual model of the inspected part and the least-squares method to measure its actual dimensions. The accuracy obtained in the experiments with PMMA and duralumin samples is better than 100 μm in axial direction and is better than the size of the point-spread function in the material of the inspected part.

Studying internal structure of quartz by broadband ultrasonic tomography under cyclic compression

The article considers the possibility of studying and identifying defects in the internal structure of synthetic quartz crystals using the parameters of acoustic emission and laser-ultrasound tomography. In the course of cyclic compressive loading, the nature of the propagation of discontinuities in a sample was investigated. It is revealed that the properties of the material, the inhomogeneity of which is associated with the presence of microdefects and anisotropy, change in the initial stages of mechanical load. With repeated application of the load on the sample, previously formed defects cause significant changes in its internal structure. With the help of numerical modeling, the obtained results were verified.

Characterization of laser ultrasound source signals in biological tissues for imaging applications.

Short optical pulses emitted from a tunable Q-switched laser (800 to 2000nm) generate laser ultrasound (LUS) signals at the surface of biological tissue. The LUS signal's acoustic frequency content, dependence on sample type, and optical wavelength are observed in the far field. The experiments yield a reference dataset for the design of noncontact LUS imaging systems. Measurements show that the majority of LUS signal energy in biological tissues is within the 0.5 and 3MHz frequency bands and the total acoustic energy generated increases with the optical absorption coefficient of water, which governs tissue optical absorption in the infrared range. The experimental results also link tissue surface roughness and acoustic attenuation with limited LUS signal bandwidth in biological tissue. Images constructed using 810-, 1064-, 1550-, and 2000-nm generation laser wavelengths and a contact piezoelectric receiver demonstrates the impact of the generation laser wavelength on image quality. A noncontact LUS-based medical imaging system has the potential to be an effective medical imaging device. Such a system may mitigate interoperator variability associated with current medical ultrasound imaging techniques and expand the scope of imaging applications for ultrasound.

A refraction-corrected tomographic algorithm for immersion laser-ultrasonic imaging of solids with piecewise linear surface profile

In this paper, a technique for reflection mode immersion 2D laser-ultrasound tomography of solid objects with piecewise linear 2D surface profiles is presented. Pulsed laser radiation was used for generation of short ultrasonic probe pulses, providing high spatial resolution. A piezofilm sensor array was used for detection of the waves reflected by the surface and internal inhomogeneities of the object. The original ultrasonic image reconstruction algorithm accounting for refraction of acoustic waves at the liquid-solid interface provided longitudinal resolution better than 100 μm in the polymethyl methacrylate sample object.

Real-Time Laser Ultrasound Tomography for Profilometry of Solids

We studied the possibility of applying laser ultrasound tomography for profilometry of solids. The proposed approach provides high spatial resolution and efficiency, as well as profilometry of contaminated objects or objects submerged in liquids. The algorithms for the construction of tomograms and recognition of the profiles of studied objects using the parallel programming technology NDIVIA CUDA are proposed. A prototype of the real-time laser ultrasound profilometer was used to obtain the profiles of solid surfaces of revolution. The proposed method allows the real-time determination of the surface position for cylindrical objects with an approximation accuracy of up to 16 μm.

A method of laser ultrasound tomography for solid surfaces mapping

Surface control is an important problem for production inspection in industry. Traditionally it is solved using mechanical and optical profilometry, providing high spatial resolution. However, such methods are inappropriate for bodies immersed in opaque liquids and require preliminary preparation of samples. Current work focuses on an application of a method of laser ultrasound tomography to mapping of solid surfaces. The suggested approach combines high acoustical resolution, high performance and allows profilometry of contaminated and immersed samples. An automated prototype for real-time surface profile measurements is constructed and tested on several bodies of revolution. Linear approximation of a cylinder profile obtained by the prototype shows maximum approximation inconsistency of 15μm. The results are verified by those obtained by the conventional x-ray method.

Nonconfocal all-optical laser-ultrasound and photoacoustic imaging system for angle-dependent deep tissue imaging.

Biomedical imaging systems incorporating both photoacoustic (PA) and ultrasound capabilities are of interest for obtaining optical and acoustic properties deep in tissue. While most dual-modality systems utilize piezoelectric transducers, all-optical systems can obtain broadband high-resolution data with hands-free operation. Previously described reflection-mode all-optical laser-ultrasound (LUS) systems use a confocal source and detector; however, angle-dependent raypaths are lost in this configuration. As a result, the overall imaging aperture is reduced, which becomes increasingly problematic with depth. We present a reflection-mode nonconfocal LUS and PA imaging system that uses signals recorded on all-optical hardware to create angle-dependent images. We use reverse-time migration and time reversal to reconstruct the LUS and PA images. We demonstrate this methodology with both a numerical model and tissue phantom experiment to image a steep-curvature vessel with a limited aperture 2-cm beneath the surface. Nonconfocal imaging demonstrates improved focusing by 30% and 15% compared to images acquired with a single LUS source in the numerical and experimental LUS images, respectively. The appearance of artifacts is also reduced. Complementary PA images are straightforward to acquire with the nonconfocal system by tuning the source wavelength and can be further developed for quantitative multiview PA imaging.

Application of laser ultrasound imaging technology in the frequency domain based on Wigner–Ville algorithm for detecting defect

Abstract According to the thermoelastic and heat conduction equations of laser ultrasonic, ultrasonic propagation images are obtained by solving finite element solution equations. Based on thermoelastic mechanism, a Wigner–Ville transform ultrasonic propagation imaging method of stronger frequency selectivity was proposed. An aluminum plate with defects was scanned by a pulsed laser, the ultrasonic waves produced by a pulsed laser were received by an ultrasonic sensor, and then ultrasonic propagation imaging on the aluminum plate was analyzed in detail. In order to isolate a damage-related ultrasonic wave, a Wigner–Ville algorithm as a frequency selectivity method was proposed to convert a complex time domain multiple wavefield into a simple frequency domain single wavefield. At last, we compared the conventional ultrasonic propagation imaging method with the Wigner–Ville transform ultrasonic propagation imaging method, which demonstrated that the Wigner–Ville transform ultrasonic propagation imaging method can effectively and intuitively evaluate the sizes of damages or flaws without any reference data, which should promote the application of the technique in the industry.

激光超声波成像技术在奥氏体不锈钢焊缝检测中的研究

激光超声波成像技术在奥氏体不锈钢焊缝检测中的研究

基于Hilbert变换的激光超声波成像技术在缺陷检测中的应用

基于Hilbert变换的激光超声波成像技术在缺陷检测中的应用

Simultaneous three-dimensional photoacoustic and laser-ultrasound tomography.

A tomographic setup that provides the co-registration of photoacoustic (PA) and ultrasound (US) images is presented. For pulse-echo US-tomography laser-induced broadband plane ultrasonic waves are produced by illuminating an optically absorbing target with a short near-infrared laser pulse. Part of the same pulse is frequency doubled and used for the generation of PA waves within the object of interest. The laser-generated plane waves are scattered at the imaging object and measured with the same interferometric detector that also acquires the photoacoustic signals. After collection and separation of the data image reconstruction is done using back-projection resulting in three-dimensional, co-registered PA and US images. The setup is characterized and the resolution in PA and US mode is estimated to be about 85 µm and 40 µm, respectively. Besides measurements on phantoms the performance is also tested on a biological sample.

Multichannel, time-resolved picosecond laser ultrasound imaging and spectroscopy with custom complementary metal-oxide-semiconductor detector

This paper presents a multichannel, time-resolved picosecond laser ultrasound system that uses a custom complementary metal-oxide-semiconductor linear array detector. This novel sensor allows parallel phase-sensitive detection of very low contrast modulated signals with performance in each channel comparable to that of a discrete photodiode and a lock-in amplifier. Application of the instrument is demonstrated by parallelizing spatial measurements to produce two-dimensional thickness maps on a layered sample, and spectroscopic parallelization is demonstrated by presenting the measured Brillouin oscillations from a gallium arsenide wafer. This paper demonstrates the significant advantages of our approach to pump probe systems, especially picosecond ultrasonics.

Laser–ultrasound image restoration using independent components analysis

A novel image enhancement method for laser ultrasound images is proposed. The method is based on Independent Component Analysis (ICA). We assume that the speckle noise comes from a signal source, which accompanies but is independent of the 'true signal source' and that their statistical characteristics are not the same. Thus the image restoration problem can also be described as 'signal source separation and enhancement problem. Then we classify the basis images that span into two different subspaces, namely 'true signal subspace' and 'speckle subspace'. Based on the classification result, we build our spatially adaptive filter and obtain the recovered image after a mixing transform. Our experimental results show that the proposed method achieves an improvement as compared to other methods for laser ultrasound images of structured objects blurred by speckle noises.