Wavefront Arrival Times Research Articles

An aberration correction approach for single and dual aperture ultrasound imaging of the abdomen

Abdominal ultrasound image quality is hampered by phase aberration, that is mainly caused by the large speed-of-sound (SoS) differences between fat and muscle tissue in the abdominal wall. The mismatch between the assumed and actual SoS distribution introduces general blurring of the ultrasound images, and acoustic refraction can lead to geometric distortion of the imaged features. Large aperture imaging or dual-transducer imaging can improve abdominal imaging at deep locations by providing increased contrast and resolution. However, aberration effects for large aperture imaging can be even more severe, which limits its full potential. In this study, a model-based aberration correction method for arbitrary acquisition schemes is introduced for delay-and-sum (DAS) beamforming and its performance was analyzed for both single- and dual-transducer ultrasound imaging. The method employs aberration corrected wavefront arrival times, using manually assigned local SoS values. Two wavefront models were compared. The first model is based on a straight ray approximation, and the second model on the Eikonal equation, which is solved by a multi-stencils fast marching method. Their accuracy for abdominal imaging was evaluated in acoustic simulations and phantom experiments involving tissue-mimicking and porcine material with large SoS contrast (∼100 m/s). The lateral resolution was improved by up to 90% in simulations and up to 65% in experiments compared to standard DAS, in which the use of Eikonal beamforming generally outperformed straight ray beamforming. Moreover, geometric distortions were mitigated in multi-aperture imaging, leading to a reduction in position error of around 80%. A study on the sensitivity of the aberration correction to shape and SoS of aberrating layers was performed, showing that even with imperfect segmentations or SoS values, aberration correction still outperforms standard DAS.

Unfocused Field Analysis of a Density-Tapered Spiral Array for High-Volume-Rate 3-D Ultrasound Imaging.

Spiral array transducers with a sparse 2-D aperture have demonstrated their potential in realizing 3-D ultrasound imaging with reduced data rates. Nevertheless, their feasibility in high-volume-rate imaging based on unfocused transmissions has yet to be established. From a metrology standpoint, it is essential to characterize the acoustic field of unfocused transmissions from spiral arrays not only to assess their safety but also to identify the root cause of imaging irregularities due to the array's sparse aperture. Here, we present a field profile analysis of unfocused transmissions from a density-tapered spiral array transducer (256 hexagonal elements, 220- [Formula: see text] element diameter, and 1-cm aperture diameter) through both simulations and hydrophone measurements. We investigated plane- and diverging-wave transmissions (five-cycle, 7.5-MHz pulses) from 0° to 10° steering for their beam intensity characteristics and wavefront arrival time profiles. Unfocused firings were also tested for B-mode imaging performance (ten compounded angles, -5° to 5° span). The array was found to produce unfocused transmissions with a peak negative pressure of 93.9 kPa at 2 cm depth. All transmissions steered up to 5° were free of secondary lobes within 12 dB of the main beam peak intensity. All wavefront arrival time profiles were found to closely match the expected profiles with maximum root-mean-squared errors of [Formula: see text] for plane wave (PW) and [Formula: see text] for diverging wave. The B-mode images showed good spatial resolution with a penetration depth of 22 mm in PW imaging. Overall, these results demonstrate that the density-tapered spiral array can facilitate unfocused transmissions below regulatory limits (mechanical index: 0.034; spatial-peak, pulse-average intensity: 0.298 W/cm2) and with suppressed secondary lobes while maintaining smooth wavefronts.

Glacial lake outburst flood hazard assessment by satellite Earth observation in the Himalayas (Chomolhari area, Bhutan)

Abstract. A case study of glacial lakes outburst flood (GLOF) hazard assessment by satellite Earth observation (EO) and numerical modelling is presented for the supraglacial and ice-contact lakes on Thangothang Chhu glacier, Chomolhari area (Bhutan). Detailed geomorphological mapping, including landslide and rock glacier inventories, as well as surface displacement determination using an interferometric SAR (InSAR) satellite, allowed a GLOF hazard assessment for lake Wa-007 to be performed. Outburst scenario modelling was achieved by combining both empirical and numerical modelling approaches, revealing that only a flood wave can have an impact on the two human settlements located downslope of Wa-007 lake. The worst-case scenario, modelled thanks to r.damflood, allowed the wave-front arrival time, the maximum water depth and the arrival time of maximum water height for the two human settlements to be quantified. A long-term monitoring strategy based entirely on EO data, with an update cycle of 5 years, is proposed to assess the future evolution of the area.

Temporal and spatial pressure distribution characteristics of hemispherical shell structure subjected to external explosion

It is necessary to master the propagation and distribution of the overpressure on a building under an explosive load before an anti-explosion design is achieved. However, there is very less available test data which can be applicable to multi-curved large-span space structure under a surface burst. In this work, a series of tests were performed to study the wave diffraction behaviour of a hemispherical structure by changing the nearest distance between the charge and structure and TNT equivalent. The purpose of the experiment was to develop an experimental dataset which could evaluate the accuracy and efficiency of the numerical model. A simplified method to study the blast load distribution on the hemispherical structure was proposed by numerical and geometric analysis. The numerical results showed that the peak overpressure, impulse, blast wave front arrival time, and positive phase duration were highly dependent on the span–distance ratio and span–charge ratio, which affect the significance of the reflecting and shielding effects. For a convenient engineering application, a series of pseudo-analytical formulas were suggested to estimate the pressure–time histories for an ideal hemisphere structure.

Wavelet-based analysis and detection of traveling waves due to DC faults in LCC HVDC systems

This paper presents qualitative and quantitative analysis of the traveling waves induced by faults on direct current (DC) transmission lines of line-commutated converter high-voltage direct current (LCC HVDC) systems for detecting the wavefront arrival times using the boundary wavelet coefficients from real-time stationary wavelet transform (RT-SWT). The qualitative analysis takes into account the steady-state operation and the detection of the inception times of both first and second wavefronts at the converter stations. The behavior of the boundary wavelet coefficients in DC transmission lines is examined considering the effects of the main parameters that influence the detection of the traveling waves, such as mother wavelets, sampling frequency, DC transmission line terminations, electrical noises, as well as fault resistance and distance. An algorithm designed to run in real-time and able to minimize the factors that hamper the performance of traveling wave-based protection (TWP) methods is proposed to detect the first and second surge arrival times. Quantitative results are achieved based on the accuracy of one- and two-terminal fault location estimation methods, and indicate the proper operation of the presented algorithm.

In vivo repeatability of the pulse wave inverse problem in human carotid arteries

Accurate arterial stiffness measurement would improve diagnosis and monitoring for many diseases. Atherosclerotic plaques and aneurysms are expected to involve focal changes in vessel wall properties; therefore, a method to image the stiffness variation would be a valuable clinical tool. The pulse wave inverse problem (PWIP) fits unknown parameters from a computational model of arterial pulse wave propagation to ultrasound-based measurements of vessel wall displacements by minimizing the difference between the model and measured displacements. The PWIP has been validated in phantoms, and this study presents the first in vivo demonstration. The common carotid arteries of five healthy volunteers were imaged five times in a single session with repositioning of the probe and subject between each scan. The 1D finite difference computational model used in the PWIP spanned from the start of the transducer to the carotid bifurcation, where a resistance outlet boundary condition was applied to approximately model the downstream reflection of the pulse wave. Unknown parameters that were estimated by the PWIP included a 10-segment linear piecewise compliance distribution and 16 discrete cosine transformation coefficients for each of the inlet boundary conditions. Input data was selected to include pulse waves resulting from the primary pulse and dicrotic notch. The recovered compliance maps indicate that the compliance increases close to the bifurcation, and the variability of the average pulse wave velocity estimated through the PWIP is on the order of 11%, which is similar to that of the conventional processing technique which tracks the wavefront arrival time (13%).

Two-Terminal Traveling-Wave-Based Transmission-Line Protection

This paper proposes a two-terminal traveling-wave-based transmission-line protection using only the first wavefront arrival time at each line terminal (polarity and magnitude of transients are not required), leading to a simple, accurate, and fast protection. In real time, the protection system detects internal and external faults and identifies the fault directionality. In addition, the protection zone and the protection operating time are also formulated. The effects of the sampling rate and the traveling-wave propagation velocity estimation are considered, and some paradigms, such as the need of a high-sampling frequency and an accurate traveling-wave propagation velocity estimation are properly addressed. Two traveling-wave-based protective relays were implemented and evaluated in realistic real-time hardware-in-the-loop simulations considering the communication equipment between the relays, in which reliability and protection operating time could be assessed accordingly.

Transient Source Localization Methodology and Laboratory Validation

Transient pressures occur in water distribution systems (WDS) and have the potential to cause structural damage to assets, such as bursts and leakage which can also have impacts on surrounding infrastructure. Transients also have the potential to adversely affect aspects of potable water quality, including inducing low or negative pressure events with the potential to cause contaminant intrusion or through shock loadings which can mobilise otherwise stable material layers from the pipe wall.This paper outlines a transient source location methodology. The basis of the method considers the varying transit times of a transient pressure primary wave front to different locations of a pipeline or network. The method relies on a comparison between determined wave front arrival times at multiple, time synchronised pressure data acquisition locations and estimated wave transit times calculated from system connectivity and known or estimated pipe parameters and characteristics, to estimate transient source locations. Validation of the method is provided by data from a laboratory based physical model which explores transient pressure propagation in a looped and branched pipe network.

Impulsive loading on reinforced concrete wall

This paper deals with the air blast and ground shock parameters for reinforced concrete wall panels. In this investigation peak pressure, pressure wave front arrival time, rising time, decreasing time and duration of the positive pressure phase of air blast time history parameters, and peak particle acceleration, arrival time, shock wave duration and time lag between ground shock and air blast pressure arrival at structures of ground shock owing to air blast were determined experimentally. Empirical relationships have been developed between different parameters. Typical values measured due to various charge weights have been compared with results of previous researchers and Conwep. The SAP2000 was used for stress analysis of wall panels using experimental results. The methodology outlined can be used to estimate the blast response of reinforced concrete walls. It is concluded that an accurate analysis of structure response and damage of structures caused by a nearby surface explosion requires simultaneous consideration of ground shock and air blast pressure.

Towards spatially distributed quantitative assessment of tsunami inundation models

This paper presents a framework and data for spatially distributed assessment of tsunami inundation models. Our associated validation test is based upon the 2004 Indian Ocean tsunami, which affords a uniquely large amount of observational data for events of this kind. Specifically, we use eyewitness accounts to assess onshore flow depths and speeds as well as a detailed inundation survey of Patong City, Thailand to compare modelled and observed inundation. Model predictions matched well the detailed inundation survey as well as altimetry data from the JASON satellite, eyewitness accounts of wave front arrival times and onshore flow speeds. Important buildings and other structures were incorporated into the underlying elevation model and are shown to have a large influence on inundation extent.

Prediction of airblast loads on structures behind a protective barrier

Experimental results and numerical simulations have demonstrated that a protective barrier can effectively reduce blast load and, therefore, protect structures from an external explosion. However, there are no formulae in the open literature that can be used to estimate the blast loads on a structure behind a barrier. In this paper, pseudo-analytical formulae based on numerical results are derived to estimate the reflected pressure-time history on a rigid wall behind a protective barrier. Numerical simulations of blast wave propagation are carried out to estimate the peak reflected pressure and the impulse on a rigid wall behind a blast barrier. The shock wave front arrival time and positive phase duration are extracted from the numerical results. Pseudo-analytical formulae, which are derived from the best-fitted curves of the numerical results, are suggested. These formulae can be used with those given in TM5-1300 or other methods for blast pressure estimation in the no-barrier case, to estimate pressure-time histories at various building locations behind a protective barrier.

Shape parameter estimation of wavefronts with known waveform

Active sensor arrays are useful to parametrically estimate the wavefronts in most applications where the excitation signal (waveform) is known or can be consistently estimated (e.g., in remote sensing, radar and communication systems). For the estimation of the shape parameters (SPs) of the wavefronts several algorithms have been summarized from the literature and compared. The algorithm proposed here is based on singular value decomposition (SVD) and has been proven to be effective in reducing coherent noise arising from interfering wavefronts. The problem of SP estimation is treated as a whole for plane and curved wavefronts, for different geometries (1-D and 2-D array of sensors), and for wide and narrowband waveforms. Wavefield distortion due to propagating medium inhomogeneities is analyzed for increasing distortion. Compared with the methods adapted for SP estimation of narrowband wavefront, the maximum likelihood estimator (ML) and the method based on SVD show better resolution of the wavefront arrival time since they correlate with the known waveform. The Cramér–Rao bound is derived and some useful properties are also established. Finally the paper contains a numerical study of the SP estimators for incoherent and coherent noise.

Laser-generated elastic waves in carbon-epoxy composite

New contactless techniques using laser-generated ultrasound have been applied to the inspection of composite materials. Transient elastic waves were launched thermoelastically in half cylindrical composite samples using a long pulse dye laser or a Q-switched Nd-YAG laser. The waves were detected with piezoelectric transducers or with an optical heterodyne interferometer. The measurements have been carried out on two different lay up design composites: carbon/epoxy: undirectional and cross-ply 0 degrees /45 degrees /90 degrees /-45 degrees . Quasi-longitudinal, quasi-shear, and shear bulk waves and head waves are clearly discerned in the stacking of a large number of waveforms. Velocities of the different types of waves simultaneously generated are compared to the phase and group velocities computed using Christoffel equations and an hexagonal model. It is shown with this point-source measurement technique that the wavefront arrival times agree with the energy velocities rather than with the phase velocities. A pronounced anisotropy is observed in the amplitudes of the wave arrivals. Angular directivity patterns of quasi-longitudinal, quasi-shear, and transverse bulk waves are plotted.

Autofocusing ultrasonic propagation in composite media based on laser generated ultrasound

Quantitative results on laser generated acoustic waves in carbon/ epoxy composites was presented. With this point-source technique, it is shown that the wavefront arrival times agree with the energy velocities rather than with the phase velocities. A pronounced anisotropy has been observed in the amplitudes of the wave fronts. The comparison between experimental directivity patterns of quasi-longitudinal, quasi-shear, transverse bulk waves and simulations demonstrates that this phenomenon can be ascribed to the self-focusing effect. It was shown that in anisotropic media, the energy flow is much more concentrated in some directions than others.

Orthogonal Electrode Catheter Array for Mapping of Endocardial Focal Site of Ventricular Activation

Precise location of the endocardial site of origin of ventricular tachycardia may facilitate surgical and catheter ablation of this arrhythmia. The endocardial catheter mapping technique can locate the site of ventricular tachycardia within 4-8 cm2 of the earliest site recorded by the catheter. This report describes an orthogonal electrode catheter array (OECA) for mapping and radiofrequency ablation (RFA) of endocardial focal site of origin of a plunge electrode paced model of ventricular activation in dogs. The OECA is an 8 F five pole catheter with four peripheral electrodes and one central electrode (total surface area 0.8 cm2. In eight mongrel dogs, mapping was performed by arbitrarily dividing the left ventricle (LV) into four segments. Each segment was mapped with OECA to find the earliest segment. Bipolar and unipolar electrograms were obtained. The plunge electrode (not visible on fluoroscopy) site was identified by the earliest wave front arrival times of -30 msec or earlier at two or more electrodes (unipolar electrograms) with reference to the earliest recorded surface ECG (I, AVF, and V1). Validation of the proximity of the five electrodes of the OECA to the plunge electrode was performed by digital radiography and RFA. Pathological examination was performed to document the proximity of the OECA to the plunge electrode and also for the width, depth, and microscopic changes of the ablation. To find the segment with the earliest LV activation a total of 10 +/- 3 (mean +/- SD) positions were mapped. Mean arrival times at the two earlier electrodes were -39 +/- 4 msec and -35 +/- 3 msec. Digital radiography showed the plunge electrode to be within the area covered by all five electrodes in all eight dogs. The plunge electrode was within 1 cm2 area of the region of RFA in all eight dogs. The width and depth of ablation were 5 +/- 3.5 and 7 +/- 3.5 mm, respectively. Microscopic changes revealed coagulative necrosis, hemorrhage, and inflammatory changes in all RFAs. In conclusion, the OECA can map the endocardial focal site of origin of paced ventricular activation within 1 cm2 area in a canine model. RFA from the OECA can cause discrete ablations representing all five electrodes or cross-shaped ablation connecting central electrode to all four peripheral electrodes. This catheter holds promise for extending surgical and clinical catheter ablation procedures.

Sonic-Boom Propagation in a Random Atmosphere

A discussion is given of the propagation of a nominally planar N-shaped pressure pulse of time duration T through a random atmosphere characterized by a scale size l and standard deviation Δv of velocity fluctuations. {This extends the author's previous discussion [J. Acoust. Soc. Amer. 44, 1052–1061 (1968)] of a refraction-diffraction mechanism that tentatively explains the anomalous spikes on sonic-boom waveforms.} Effects are classified as (a) gross effects representing distortions in the waveform's central portion and (b) onset effects that produce distortions near the leading and trailing edges. Gross effects are interpreted on the basis of harmonic waves of period 2T. Those caused by inhomogeneities at distances greater than l2/(cT) are generally negligible. To a limited extent, onset effects may be interpreted within the framework of the Keller-Friedlander geometrical acoustics theory of singular wavefronts. Over large distances, ray paths become tangled such that many singular surfaces may pass a given observer point. A discussion is given of the effect of caustics and of the distribution of wavefront arrival times. The estimated expected effective shock rise time is of the order of (r/l)12(Δv)l/c2, where r is propagation distance. Although this is partially offset by nonlinear steepening, it could explain the finite rise times of sonic-boom onsets.

Waveforms and relative phase stability of transients radiated from a helicopter-supported antenna wire

A nearly vertical antenna wire varying in length from 3000 to 8000 ft was shock-excited by a high-voltage source. The resulting radiation fields were received at distances of about 600 km by two stations separated laterally so as to subtend an angle of about 15\deg at the source. The difference in wavefront arrival times at the two stations, observed for 101 transmissions, showed an average absolute deviation from the mean of about only \frac{1}{2}\mu S. This figure includes the effects of possible source asymmetries, sferics noise, instrumental errors, and wave propagation. Typical oscillograms of antenna currents and radiation fields are given. It is shown that the amplitudes and durations of the pulse envelopes, the amplitudes and durations of the first half-cycles, and the quasi-frequencies of the pulses are in good agreement with a theory that takes energy losses into account.

Transient Compressional Waves in an Infinite Elastic Plate With a Circular Cylindrical Cavity

The problem treated is that of an infinite free plate with a circular cylindrical cavity subjected to a step normal displacement. The linear equations of elasticity are employed and the formal solution is obtained using a multi-integral transform technique, necessitating the introduction of extended Hankel transforms, and residue theory. Some properties of the Rayleigh-Lamb frequency equation, pertinent to the inversion process, are derived. Numerical information for the far field, showing the effect of the hole radius on the displacements, is obtained using the stationary phase method and, in the case of the radial displacement, the solution is compared with the corresponding slab solution. The results show that at a given station the plate-cavity solution approaches that for the slab, as the hole radius decreases. The head of the pulse and stationary-phase approximations to the corresponding horizontal slab displacement are also compared, and some discrepancies between the two are found in the vicinity of the wave-front arrival time.