Acoustic Inhomogeneities Research Articles

ON THE PROPERTIES OF A FAST ALGORITHM FOR SOLVING A THREE-DIMENSIONAL INVERSE PROBLEM OF SCALAR ACOUSTICS

t Theinverse single-frequency problem of scalar acoustics in three-dimensional space is considered. It consists in determining the characteristics of acoustic inhomogeneities lying in a flat layer from the distribution of the complex amplitude of the acoustic field in a flat recording layer. To effectively solve it, a special numerical algorithm is used, previously proposed and justified by the authors. As its component part, the algorithm uses solutions to special one-dimensional Fredholm equations of the 1st kind, and in the case of ambiguity in the solution of the latter, their normal solutions (solutions with a minimum norm) are sought. To regularize this ill-posed inverse problem, Tikhonov regularization and the TSVD method are used. A systematic numerical study is carried out of the influence of various parameters in the data recording scheme on the accuracy of the approximate solutions obtained using the algorithm. In particular, we study the dependence of this accuracy on the position of point sources and composite sources that cause sound vibrations, its dependence on the position of the data recording layer, and on the number of planes in which the recording sensors lie. It is shown that acceptable accuracy of approximate solutions can be obtained even with two such layers. In addition, an approach to the numerical study of the ambiguity of the solution to the inverse problem under consideration is proposed. It is based on special theoretical statements given in the article, and on the same numerical algorithm for solving the inverse problem. The approach is focused on comparing several normal solutions of one-dimensional integral equations from the problem under consideration. These normal solutions are calculated with respect to various elements. An example of using this theory to numerically estimate the non-uniqueness of a solution is given.

Deep-Learning-Enabled Microwave-Induced Thermoacoustic Tomography Based on ResAttU-Net for Transcranial Brain Hemorrhage Detection.

Hemorrhagic stroke is a leading threat to human's health. The fast-developing microwave-induced thermoacoustic tomography (MITAT) technique holds potential to do brain imaging. However, transcranial brain imaging based on MITAT is still challenging due to the involved huge heterogeneity in speed of sound and acoustic attenuation of human skull. This work aims to address the adverse effect of the acoustic heterogeneity using a deep-learning-based MITAT (DL-MITAT) approach for transcranial brain hemorrhage detection. We establish a new network structure, a residual attention U-Net (ResAttU-Net), for the proposed DL-MITAT technique, which exhibits improved performance as compared to some traditionally used networks. We use simulation method to build training sets and take images obtained by traditional imaging algorithms as the input of the network. We present ex-vivo transcranial brain hemorrhage detection as a proof-of-concept validation. By using an 8.1-mm thick bovine skull and porcine brain tissues to perform ex-vivo experiments, we demonstrate that the trained ResAttU-Net is capable of efficiently eliminating image artifacts and accurately restoring the hemorrhage spot. It is proved that the DL-MITAT method can reliably suppress false positive rate and detect a hemorrhage spot as small as 3 mm. We also study effects of several factors of the DL-MITAT technique to further reveal its robustness and limitations. The proposed ResAttU-Net-based DL-MITAT method is promising for mitigating the acoustic inhomogeneity issue and performing transcranial brain hemorrhage detection. This work provides a novel ResAttU-Net-based DL-MITAT paradigm and paves a compelling route for transcranial brain hemorrhage detection as well as other transcranial brain imaging applications.

Phenomenology of acoustic standing waves as applied to the VVER-1200 reactor plant

The insufficiently studied issues of acoustic standing waves (ASW) in the main circulation circuits of the VVER reactor plants are considered. For a long time no proper attention has been given to this phenomenon both by the researchers and NPP experts. In general, generation of ASWs requires the acoustic inhomogeneities of the medium in the planes perpendicular to the direction of propagation of the longitudinal wave, in which a jump in acoustic resistance occurs, this is shown by the authors based on an example of the wave equation solution (D’Alembert equation) for a certain function of two variables. The ASW classification has been developed based on the obtained experimental material, 6 ASW types have been described, and their key parameters have been specified. The amplitude distributions have been plotted for all major ASW types proceeding from the phase relations of signals from the pressure pulsation detectors and accelerometers installed on the MCC pipelines. The nature of these distributions is general and they are valid for all VVER types. For the first time the globality of all lowest ASW types is identified. Four attribute properties of the ASWs have been formulated. The first attribute is the regular ASW temperature dependences, which is the source of the diagnostic information in the process of heating/cooling of the VVER unit. The linear experimental dependences of the ASW frequencies on coolant temperature have been obtained. The frequencies, at which the MCC resonant excitation due to coincidence of the ASW frequencies with the RCP rotational frequency harmonics, have been found experimentally. The ASW energy, which origin has resulted from the RCP operation, is estimated. The RCP operation can be presented as continuous generation of pressure pulsations, which fall onto the acoustic path inhomogeneities in the form of a traveling wave and generate a standing wave after reflection from them.

Suppression of acoustic reflection artifact in endoscopic photoacoustic tomographic images based on approximation of ideal signals.

BACKGROUND: In endoscopic photoacoustic tomography (EPAT), the photoacoustically induced ultrasonic wave reflects at tissue boundaries due to the acoustic inhomogeneity of the imaged tissue, resulting in reflection artifacts (RAs) in the reconstructed images.OBJECTIVE: To suppress RAs in EPAT image reconstruction for improving the image quality.METHODS: A method was presented to render the cross-sectional images of the optical absorption with reduced RAs from acoustic measurements. The ideal photoacoustic signal was recovered from acoustic signals collected by the detector through solving a least square problem. Then, high-quality images of the optical absorption distribution were reconstructed from the ideal signal.RESULTS: The results demonstrated the improvement in the quality of the images rendered by this method in comparison with the conventional back-projection (BP) reconstructions. Compared with the short lag spatial coherence (SLSC) method, the peak signal-to-noise ratio (PSNR), normalized mean square absolute distance (NMSAD), and structural similarity (SSIM) were improved by up to 8%, 20%, and 5%, respectively.CONCLUSIONS: This method was capable of rendering images displaying the complex tissue types with reduced RAs and lower computational burden in comparison with previously developed methods.

Fast Solution Algorithm for a Three-Dimensional Inverse Multifrequency Problem of Scalar Acoustics with Data in a Cylindrical Domain

A new algorithm for the stable solution of a three-dimensional scalar inverse problem of acoustic sounding of an inhomogeneous medium in a cylindrical region is proposed. The data of the problem is the complex amplitude of the wave field, measured outside the region of acoustic inhomogeneities in a cylindrical layer. Using the Fourier transform and Fourier series, the inverse problem is reduced to solving a set of one-dimensional Fredholm integral equations of the first kind, to the subsequent calculation of the complex amplitude of the wave field in the region of inhomogeneity, and then to finding the required sound velocity field in this region. The algorithm allows solving the inverse problem on a personal computer of average performance for sufficiently fine three-dimensional grids in tens of seconds. A numerical study of the accuracy of the proposed algorithm for solving model inverse problems at various frequencies is carried out, and the issues of stability of the algorithm with respect to data perturbations are investigated.

On deep-sea acoustic inhomogeneities in the bottom layers in the Sea of Okhotsk and in the Sea of Japan

On deep-sea acoustic inhomogeneities in the bottom layers in the Sea of Okhotsk and in the Sea of Japan

An autofocus method to reduce acoustic inhomogeneity in microwave-induced thermo-acoustic tomography based on basis pursuit

In microwave-induced thermo-acoustic tomography, the quality of reconstructed images is seriously influenced by the speed of sound distribution, which is usually set according to the speed of sound (SoS) of the background coupling medium or an empirical value. To reduce defocusing and distortions induced by acoustic inhomogeneity in microwave-induced thermo-acoustic tomography, a method is proposed. We classify the initial reconstructed pressure by the difference in dielectrics and update the acoustic property accordingly. A spare spectrum is obtained by the proposed method and partitions the initial pressure into several clusters, and then SoS will be updated based on clinical statistics with which the pressure is reconstructed again. The proposed method is first validated in the anatomically realistic breast phantom; an ex vivo experiment based on a prototype system is carried out for further validation. The results prove that the proposed method has a reliable effect on reducing distortions than the state-of-art methods under the condition of a low signal to noise ratio.

Reconstruction of Endoscopic Photoacoustic Tomographic Images for Attenuating Acoustic Tissues: A Preliminary Study

Biological endoscopic photoacoustic tomography (EPAT) Has been rapidly developed in the past decades as a hybrid functional imaging modality with multi-physics coupling. The acoustic inhomogeneity of biological tissues results in acoustic reflection, scattering, diffusion and absorption, thus the attenuated photoacoustic signals measured by the detector. The idealized assumptions on the acoustic homogeneity usually lead to reconstructed images with reduced quality and imaging depth. This paper focuses on a preliminary study of EPAT image reconstruction accounting for acoustic attenuation. The correlation equation related to ideal and measured photoacoustic signals is deduced. The equation is then solved by using singular value decomposition (SVD) to recover the approximation of the ideal signals from the measured signals. Finally, the cross-sectional images of optical distribution are reconstructed from the ideal signals. Numerical results indicate that this method can effectively reduce image artifacts caused by acoustic attenuation and improve image focusing. Comparison results suggest that it outperforms the method of correcting the frequency-dependent attenuation which requires the prior knowledge about attenuation coefficients of tissues.

Scattering of Retarded Wave Fields by a Point Acoustic Inhomogeneity

The scattering of a monochromatic wave by an inhomogeneity with the small wave size is considered. The scattering coefficients are introduced, and sets of their allowable values are obtained with and without absorption inside the inhomogeneity. The solution to the inverse scattering problem is analyzed for different values of the scattering coefficients, from the viewpoint of the adequacy of the reconstruction result and its stability.

Measurement of acoustic backscattering from rocky outcrops in Monterey Bay

Acoustic scattering is an interaction between acoustic waves and inhomogeneities (interface or volume) which results in acoustic energy being dispersed in all directions. Scattering strength, a measure of the incoherent scattered energy, depends on grazing angle, frequency, bottom roughness, and/or sediment type and can be a source of false alarms for target detection systems. Understanding acoustic backscatter relationships of the sea floor allows for remote sensing applications, as well as target detection performance, in these areas. Acoustic backscatter from sand and mud bottoms has been studied extensively, but only six rocky bottom studies have been performed to date, motivating our study of high frequency acoustic scattering from rocky environments. For this project, acoustic data were collected and analyzed to measure the acoustic backscattering strength of rocky seafloors as a function of grazing angle, bottom roughness, and frequency in Monterey Bay, CA. High-frequency acoustic surveys with two narrowband split-beam echosounders, 200 kHz and 420 kHz, were conducted. Backscatter measurements were compared to existing bathymetric and geologic surveys from California State University, Monterey Bay and the California Geologic Survey to qualitatively compare backscattering strength to roughness features and geology.Acoustic scattering is an interaction between acoustic waves and inhomogeneities (interface or volume) which results in acoustic energy being dispersed in all directions. Scattering strength, a measure of the incoherent scattered energy, depends on grazing angle, frequency, bottom roughness, and/or sediment type and can be a source of false alarms for target detection systems. Understanding acoustic backscatter relationships of the sea floor allows for remote sensing applications, as well as target detection performance, in these areas. Acoustic backscatter from sand and mud bottoms has been studied extensively, but only six rocky bottom studies have been performed to date, motivating our study of high frequency acoustic scattering from rocky environments. For this project, acoustic data were collected and analyzed to measure the acoustic backscattering strength of rocky seafloors as a function of grazing angle, bottom roughness, and frequency in Monterey Bay, CA. High-frequency acoustic surveys with two ...

Reducing Acoustic Inhomogeneity Based on Speed of Sound Autofocus in Microwave Induced Thermoacoustic Tomography.

Microwave induced thermoacoustic tomography is a newly developing non-invasive and non-ionizing modality. In practical applications, such as breast tumor detection and brain imaging, the acoustic properties in the tissue to be detected are usually unknown and spatially non-uniform, which results in distortion and blurring of the buried targets. In this paper, a reconstruction method based on speed of sound (SoS) autofocus is proposed to reduce the effect of acoustic inhomogeneity in different soft tissues. According to this method, the number of tissue types, which are referred to as clusters in this work, can be automatically determined by a decision graph. To distinguish the boundaries of different tissues, a Gaussian Mixture Model (GMM) is fitted to the obtained image data for soft clustering instead of traditional hard clustering. Through fixing the tissue centers which are characterized by corresponding data density peaks as the means of Gaussian parameters rather than choosing them randomly, adaptive and robust reconstruction performance can be guaranteed. After performing an iterative GMM optimization, the SoS autofocus is achieved. Image reconstructed by using the updated SoS distribution is with higher accuracy than that with homogeneous assumption. Compared with the existing similar methods, the proposed method strategy obviates the need of extra experiment costs, and possesses good robustness with respect to hard assignment model errors when the medium is relatively complex. Realistic breast model and brain model simulations combined with experiments of agar phantom and pig's brain are provided to demonstrate the effectiveness of the proposed method.

Planck 2015 Constraints on the Non-flat XCDM Inflation Model

Abstract We examine the Planck 2015 cosmic microwave background (CMB) anisotropy data by using a physically consistent energy density inhomogeneity power spectrum generated by quantum-mechanical fluctuations during an early epoch of inflation in the non-flat XCDM model. Here dark energy is parameterized using a fluid with a negative equation of state parameter but with the speed of fluid acoustic inhomogeneities set to the speed of light. We find that the Planck 2015 data in conjunction with baryon acoustic oscillation distance measurements are reasonably well fit by a closed-XCDM model in which spatial curvature contributes a percent of the current cosmological energy density budget. In this model, the measured non-relativistic matter density parameter and Hubble constant are in good agreement with values determined using most other data. Depending on cosmological parameter values, the closed-XCDM model has reduced power, relative to the tilted, spatially flat ΛCDM case, and can partially alleviate the low multipole CMB temperature anisotropy deficit and can help partially reconcile the CMB anisotropy and weak lensing σ 8 constraints, at the expense of somewhat worsening the fit to higher multipole CMB temperature anisotropy data. However, the closed-XCDM inflation model does not seem to improve the agreement much, if at all, compared to the closed ΛCDM inflation case, even though it has one additional free parameter.

Effects of breast structure on high-intensity focused ultrasound focal error

BackgroundThe development of imaging technologies and breast cancer screening allowed early detection of breast cancers. High-intensity focused ultrasound (HIFU) is a non-invasive cancer treatment, but the success of HIFU ablation was depending on the system type, imaging technique, ablation protocol, and patient selection. Therefore, we aimed to determine the relationship between breast tissue structure and focal error during breast cancer HIFU treatment.MethodsNumerical simulations of the breast cancer HIFU ablation were performed using digital breast phantoms constructed using the magnetic resonance imaging data obtained from 12 patients.ResultsThe focal shapes were distorted despite breast tissue representing soft tissue. Focal errors are caused by the complex distribution of fibroglandular tissue, and they depend on the target position and the arrangement of the transducer. We demonstrated that the focusing ratio increases with the decrease in the local acoustic inhomogeneity, implying that it may be used as an indicator to reduce the HIFU focal error depending on the breast structure.ConclusionsThe obtained results demonstrated that the focal error observed during the breast cancer HIFU treatment is highly dependent on the structure of fibroglandular tissue. The optimal arrangement of the transducer to the target can be obtained by minimizing the local acoustic inhomogeneity before the breast cancer HIFU treatment.

Study on the Influence of Acoustically Inhomogeneous Characteristics on Magneto-acoustic-electrical Tomography

Magneto-acoustic-electrical tomography (MAET) is a method of non-invasive conductivity imaging, with three couplings of magnetic field, acoustic field and electric field. It is suitable for the conductivity imaging of low-conductivity fluid or approximate fluid medium. MAET will have a broad application prospect in the field of bio-impedance imaging. Acoustic and electrical characteristics of biological tissue are important factors in magneto-acoustic-electrical tomography, it is essential to evaluate the effect of acoustic inhomogeneity on the sound pressure distribution and the electrical signal. In this paper, a numerical analysis of the positive problem of magneto-acoustic -electrical tomography of eight groups of human tissues with different density and sound velocity, such as blood and cerebrospinal fluid, is carried out. Firstly, establishing a two-dimensional plane model of pure fluid and applying ultrasonic excitation. Secondly, combining with the current continuity theorem and getting the target body potential distribution. Finally, the voltage signal can be calculated. The simulation results show that compared to the acoustic parameter value usually used in the approximate calculation, the voltage signal error of the muscle tissue is the largest of the seven groups of parameters under the same condition. The results of the study laid the foundation for the study of the influence of the acoustically inhomogeneous model of human tissue on the magneto–acoustic -electrical tomography.

Pattern-Noise in Ultrasonic Inspection of Articles Made of Complexly Structured Materials

Pattern-noise (PN) that occurs due to combined reflection of an ultrasonic probing signal from acoustic inhomogeneities in the material structure is an interference that conceals echo-signals from flaws. The most efficient method for extracting echo-signals from PN is multichannel spacetime signal processing (STSP). Different inspection purposes dictate different configurations of STSP schemes with the parameters that are determined by PN correlation characteristics. A theoretical model has been used to determine mutual PN correlation characteristics for the main types of transducers. The theoretical results have been confirmed experimentally.

Theory of acoustooptical filtration of light beams in the inhomogeneous acoustic field

The theory of acoustooptical (AO) interaction between light beams in the longitudinally inhomogeneous acoustic field is constructed with allowance for conditions typical of AO filtration. In the case of smooth acoustic inhomogeneities, analytical expressions for the spatially spectral characteristic relating the spatial spectra of incident and diffracted light beams are derived. The situation where a Gaussian light beam undergoes filtration in the acoustic field excited by a rectangular transducer is discussed in detail. The narrowband quasi-collinear AO filter based on a paratellurite crystal is simulated. It is demonstrated that the maximum diffraction efficiency and transmission-function shape distortion (with respect to the plane-wave model) of the given device depend to the greatest extent on the misalignment between the directions of the group velocities of interacting light beams.

Photoacoustic reflection artifact reduction using photoacoustic-guided focused ultrasound: comparison between plane-wave and element-by-element synthetic backpropagation approach.

Reflection artifacts caused by acoustic inhomogeneities constitute a major problem in epi-mode biomedical photoacoustic imaging. Photoacoustic transients from the skin and superficial optical absorbers traverse into the tissue and reflect off echogenic structures to generate reflection artifacts. These artifacts cause difficulties in the interpretation of images and reduce contrast and imaging depth. We recently developed a method called PAFUSion (photoacoustic-guided focused ultrasound) to circumvent the problem of reflection artifacts in photoacoustic imaging. We already demonstrated that the photoacoustic signals can be backpropagated using synthetic aperture pulse-echo data for identifying and reducing reflection artifacts in vivo. In this work, we propose an alternative variant of PAFUSion in which synthetic backpropagation of photoacoustic signals is based on multi-angled plane-wave ultrasound measurements. We implemented plane-wave and synthetic aperture PAFUSion in a handheld ultrasound/photoacoustic imaging system and demonstrate reduction of reflection artifacts in phantoms and in vivo measurements on a human finger using both approaches. Our results suggest that, while both approaches are equivalent in terms of artifact reduction efficiency, plane-wave PAFUSion requires less pulse echo acquisitions when the skin absorption is the main cause of reflection artifacts.

Taking advantage of acoustic inhomogeneities in photoacoustic measurements.

This paper proposes a method for improving the localization and the quantification of the optical parameters in photoacoustic (PA) tomography of biological tissues that are intrinsically heterogeneous in both optical and acoustic properties. It is based on the exploitation of both the PA signal, generated by the heterogeneous optical structures, and the secondary acoustic echoes due to the interaction between a primary PA wave generated near the tissue surface and the heterogeneous acoustic structures. These secondary echoes can also be collected through proper measurements of the PA signals. The experimental procedure is presented along with the method to filter the signal and the reconstruction algorithm that includes the account of the acoustic information.

Vector field reconstruction of flows using the Novikov–Agaltsov functional algorithm and the additive correlation method

Results from numerical modeling of the Novikov–Agaltsov functional algorithm which allows us to reconstruct scalar-vector acoustic inhomogeneities are presented. On the other hand, a vector map of flow velocities is recovered using experimental data obtained via ultrasonic tomograph and processed by means of spatial correlation method.

Photoacoustic-guided focused ultrasound for accurate visualization of brachytherapy seeds with the photoacoustic needle.

An important problem in minimally invasive photoacoustic (PA) imaging of brachytherapy seeds is reflection artifacts caused by the high signal from the optical fiber/needle tip reflecting off the seed. The presence of these artifacts confounds interpretation of images. In this letter, we demonstrate a recently developed concept called photoacoustic-guided focused ultrasound (PAFUSion) for the first time in the context of interstitial illumination PA imaging to identify and remove reflection artifacts. In this method, ultrasound (US) from the transducer is focused on the region of the optical fiber/needle tip identified in a first step using PA imaging. The image developed from the US diverging from the focus zone at the tip region visualizes only the reflections from seeds and other acoustic inhomogeneities, allowing identification of the reflection artifacts of the first step. These artifacts can then be removed from the PA image. Using PAFUSion, we demonstrate reduction of reflection artifacts and thereby improved interstitial PA visualization of brachytherapy seeds in phantom and <italic<ex vivo</italic< measurements on porcine tissue.