Acoustic Impedance Mismatch Research Articles

Transurethral ultrasound therapy of the prostate in the presence of calcifications: A simulation study.

Transurethral ultrasound therapy is an investigational treatment modality which could potentially be used for the localized treatment of prostate cancer. One of the limiting factors of this therapy is prostatic calcifications. These attenuate and reflect ultrasound and thus reduce the efficacy of the heating. The aim of this study is to investigate how prostatic calcifications affect therapeutic efficacy, and to identify the best sonication strategy when calcifications are present. Realistic computational models were used on clinical patient data in order to simulate different therapeutic situations with naturally occurring calcifications as well as artificial calcifications of different sizes (1-10mm) and distances (5-15mm). Furthermore, different sonication strategies were tested in order to deliver therapy to the untreated tissue regions behind the calcifications. The presence of calcifications in front of the ultrasound field was found to increase the peak pressure by 100% on average while the maximum temperature only rose by 9% during a 20-s sonication. Losses in ultrasound energy were due to the relatively large acoustic impedance mismatch between the prostate tissue and the calcifications (1.63 vs 3.20 MRayl) and high attenuation coefficient (0.78 vs 2.64dB/MHz1.1 /cm), which together left untreated tissue regions behind the calcifications. In addition, elevated temperatures were seen in the region between the transducer and the calcifications. Lower sonication frequencies (1-4MHz) were not able to penetrate through the calcifications effectively, but longer sonication durations (20-60s) with selective transducer elements were effective in treating the tissue regions behind the calcifications. Prostatic calcifications limit the reach of therapeutic ultrasound treatment due to reflections and attenuation. The tissue regions behind the calcifications can possibly be treated using longer sonication durations combined with proper transducer element selection. However, caution should be taken with calcifications located close to sensitive organs such as the urethra, bladder neck, or rectal wall.

Effect of acoustic impedance mismatch between skin and bone on transcranial ultrasound transmission

A major problem with transcranial Doppler (TCD) ultrasound is the poor transmission of ultrasound through the skull bone causing image quality degradation. The reasons for the poor image quality are (1) acoustic impedance mismatch along the wave propagation path and (2) bone frequency-dependent attenuation. Transmission loss due to acoustic impedance mismatch is typically ignored in the literature. Therefore, the objective of this paper is to study the effect of acoustic impedance mismatch on the transmitted energy as a function of frequency. To achieve this, the wave propagation was modelled analytically from the ultrasonic transducer into the brain. The model calculates frequency-dependent transmission coefficient for a given skin and bone thickness combination. The model incorporates both attenuation and acoustic impedance mismatch effects. The model was validated experimentally by comparing with measurements using a bone phantom plate mimicking the acoustic properties of the skull. The average error on the skin thickness between the model and the experiment was less than 6% for a constant bone thickness. Further analysis of the simulation suggests that an optimized excitation frequency can be chosen based on skin and bone thicknesses that would improve ultrasound transmission.

Fabrication of PMN-PT/Epoxy 2–2 Composite Ultrasonic Transducers and Analysis Based on Equivalent Circuit Model

Single-crystal (1 − x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 (PMN-PT) has great potential for use in ultrasonic applications; however, it is challenging to achieve both broad bandwidth and high sensitivity owing to the mismatch in acoustic impedance between the piezoelectric materials and loading medium. Fabrication of a matching layer for transducers is challenging owing to the increase in the center frequency of the transducer. In this study, a 2–2 PMN-PT-crystal composite was designed and fabricated, exhibiting desirable piezoelectric properties and lower acoustic impedance compared with single-crystal PMN-PT. Transducers based on the 2–2 PMN-PT composite were fabricated and analyzed in detail to investigate their sensitivity and bandwidth. Moreover, the effects of a backing layer on the transducer were investigated. The measured results showed that the bandwidth improved from 21.28% to 35.54% at the cost of reduced sensitivity of the transducer. Systematic analysis of this phenomenon using an equivalent circuit based on the Mason model is presented.

Study on pyroshock propagation through plates with joints and washers

Pyrotechnic release devices are widely used in various missions in order to reliably separate the structural parts of the system. However, the pyroshock induced by an explosive pyrotechnic event can lead to fatal malfunctions of the adjacent electrical components. In order to mitigate these pyroshock issues, an accurate understanding of the pyroshock propagation in structures is essential. In this study, an experimental setup for the pyroshock propagation experiments is developed with the pyroshock excitation using pyrotechnic initiators. The pyroshock propagation analysis environment with a commercial hydrocode is established and verified through comparison between the analysis and the experimental results. The pyroshock propagates through the thin plates in the form of flexural waves (or anti-symmetric Lamb waves). Using the established numerical and experimental techniques, the effects of the pyroshock attenuation by the joints and washers are investigated. The plates connected by joints with different materials and the plates connected by joints with inserted washers made of different materials and in different thickness are considered. The experimental and numerical results are in good agreement: the pyroshock attenuation is highly effective when the joints are made of higher density and stiffness materials and when the thickness of the washers is increased. The primary reason for the pyroshock attenuation due to the joints and washers is the flexural wave reflection at the discontinuities caused by acoustic impedance mismatching.

Optimizing piezoelectric receivers for acoustic power transfer applications

In this paper, we aim to optimize piezoelectric plate receivers for acoustic power transfer applications by analyzing the influence of the losses and of the acoustic boundary conditions. We derive the analytic expressions of the efficiency of the receiver with the optimal electric loads attached, and analyze the maximum efficiency value and its frequency with different loss and acoustic boundary conditions. To validate the analytical expressions that we have derived, we perform experiments in water with composite transducers of different filling fractions, and see that a lower acoustic impedance mismatch can compensate the influence of large dielectric and acoustic losses to achieve a good performance. Finally, we briefly compare the advantages and drawbacks of composite transducers and pure PZT (lead zirconate titanate) plates as acoustic power receivers, and conclude that 1–3 composites can achieve similar efficiency values in low power applications due to their adjustable acoustic impedance.

Cross-plane coherent acoustic phonons in two-dimensional organic-inorganic hybrid perovskites

Two-dimensional Ruddlesden–Popper organic–inorganic hybrid layered perovskites (2D RPs) are solution-grown semiconductors with prospective applications in next-generation optoelectronics. The heat-carrying, low-energy acoustic phonons, which are important for heat management of 2D RP-based devices, have remained unexplored. Here we report on the generation and propagation of coherent longitudinal acoustic phonons along the cross-plane direction of 2D RPs, following separate characterizations of below-bandgap refractive indices. Through experiments on single crystals of systematically varied perovskite layer thickness, we demonstrate significant reduction in both group velocity and propagation length of acoustic phonons in 2D RPs as compared to the three-dimensional methylammonium lead iodide counterpart. As borne out by a minimal coarse-grained model, these vibrational properties arise from a large acoustic impedance mismatch between the alternating layers of perovskite sheets and bulky organic cations. Our results inform on thermal transport in highly impedance-mismatched crystal sub-lattices and provide insights towards design of materials that exhibit highly anisotropic thermal dissipation properties.

Simple and inexpensive micromachined aluminum microfluidic devices for acoustic focusing of particles and cells

We introduce a new method to construct microfluidic devices especially useful for bulk acoustic wave (BAW)-based manipulation of cells and microparticles. To obtain efficient acoustic focusing, BAW devices require materials that have high acoustic impedance mismatch relative to the medium in which the cells/microparticles are suspended and materials with a high-quality factor. To date, silicon and glass have been the materials of choice for BAW-based acoustofluidic channel fabrication. Silicon- and glass-based fabrication is typically performed in clean room facilities, generates hazardous waste, and can take several hours to complete the microfabrication. To address some of the drawbacks in fabricating conventional BAW devices, we explored a new approach by micromachining microfluidic channels in aluminum substrates. Additionally, we demonstrate plasma bonding of poly(dimethylsiloxane) (PDMS) onto micromachined aluminum substrates. Our goal was to achieve an approach that is both low cost and effective in BAW applications. To this end, we micromachined aluminum 6061 plates and enclosed the systems with a thin PDMS cover layer. These aluminum/PDMS hybrid microfluidic devices use inexpensive materials and are simply constructed outside a clean room environment. Moreover, these devices demonstrate effectiveness in BAW applications as demonstrated by efficient acoustic focusing of polystyrene microspheres, bovine red blood cells, and Jurkat cells and the generation of multiple focused streams in flow-through systems. Graphical abstract The aluminum acoustofluidic device and the generation of multinode focusing of particles.

Nonlinear air-coupled thermosonics for fatigue micro-damage detection and localisation

Over the years, traditional active infrared thermography has played a pivotal role in ensuring that a component is free of any damage. However, whilst optical thermography is still not sufficiently sensitive to the presence of material micro-flaws, current vibro-thermography requires inspection solutions that are not always feasible, such as the use of coupling materials between the sensing probe and the monitored structure. This paper presents a valid alternative to current thermographic systems by developing and experimentally validating nonlinear air-coupled thermosonics for a contactless, rapid and accurate detection of fatigue micro-cracks in an aero-engine turbine blade. The proposed thermographic method combines the high sensitivity to micro-damage of nonlinear ultrasonic techniques with non-contact air-coupled ultrasonic transducers and thermographic equipment. Narrowband frequency sweeps were performed to identify local damage resonance frequencies in order to generate large vibrational amplitudes at the damage location and compensate for signal losses caused by the high acoustic impedance mismatch between the air and the sample. An infrared camera was then used to acquire the thermal response generated by frictional heat at the crack interfaces. Moreover, an image processing method based on a combination of morphological opening and a Savitzky-Golay smoothing filter was employed to enhance the quality of thermal images affected by anisotropic heating and thermal noise effects. Nonlinear air-coupled thermosonics experiments were validated with laser Doppler vibrometry scan measurements and compared with both flash and pulsed phase thermography. Thermal imaging results showed that the proposed nonlinear air-coupled thermosonics was the only thermographic technique able to detect fatigue micro-cracks, thus demonstrating its potential as an efficient and sensitive inspection tool for micro-damage detection in geometrically complex components.

Primary blast-induced traumatic brain injury: lessons from lithotripsy

Traumatic injury caused by explosive or blast events is traditionally divided into four mechanisms: primary, secondary, tertiary, and quaternary blast injury. The mechanisms of blast-induced traumatic brain injury (bTBI) are biomechanically distinct and can be modeled in both in vivo and in vitro systems. The primary bTBI injury mechanism is associated with the response of brain tissue to the initial blast wave. Among the four mechanisms of bTBI, there is a remarkable lack of information regarding the mechanism of primary bTBI. On the other hand, 30 years of research on the medical application of shock waves (SWs) has given us insight into the mechanisms of tissue and cellular damage in bTBI, including both air-mediated and underwater SW sources. From a basic physics perspective, the typical blast wave consists of a lead SW followed by shock-accelerated flow. The resultant tissue injury includes several features observed in primary bTBI, such as hemorrhage, edema, pseudo-aneurysm formation, vasoconstriction, and induction of apoptosis. These are well-described pathological findings within the SW literature. Acoustic impedance mismatch, penetration of tissue by shock/bubble interaction, geometry of the skull, shear stress, tensile stress, and subsequent cavitation formation are all important factors in determining the extent of SW-induced tissue and cellular injury. In addition, neuropsychiatric aspects of blast events need to be taken into account, as evidenced by reports of comorbidity and of some similar symptoms between physical injury resulting in bTBI and the psychiatric sequelae of post-traumatic stress. Research into blast injury biophysics is important to elucidate specific pathophysiologic mechanisms of blast injury, which enable accurate differential diagnosis, as well as development of effective treatments. Herein we describe the requirements for an adequate experimental setup when investigating blast-induced tissue and cellular injury; review SW physics, research, and the importance of engineering validation (visualization/pressure measurement/numerical simulation); and, based upon our findings of SW-induced injury, discuss the potential underlying mechanisms of primary bTBI.

Design and fabrication of distributed Bragg reflector multilayers for dynamic pressure sensing.

A novel 2D-surface shock pressure sensor is designed and tested based on 1D-Photonic Crystal, i.e., Distributed Bragg Reflector Multilayer (DBR/ML) structures. The fast opto-mechanical response of these structures to changes in layer thicknesses and refractive indices are ideally suited for dynamic pressure sensing. They offer the potential to minimize acoustic impedance mismatch between the material layers, and most importantly, the potential to monitor both temporal and spatial (lateral) variations during shock compression. In this feasibility study, different materials and device designs are investigated to identify material/device design combinations with optimum response to dynamic loading. Structural and material effects are studied in terms of spectral and mechanical properties, structure stability, and the ease of fabrication process. Structures comprising of different numbers of SiO1.5/SiO1.7 bilayer stacks are modeled, and fabricated. A 10-bilayer structure placed under a dynamic compressive load of ~7.2 GPa, exhibits a blueshift of 29 nm with a response time of ~5 ns which is well within the shock pressure rise time measured with PDV velocimetry. This promising result successfully demonstrates the feasibility of the specifically designed DBR/ML structure as a dynamic pressure sensor.

Controllable direction of liquid jets generated by thermocavitation within a droplet.

A high-velocity fluid stream ejected from an orifice or nozzle is a common mechanism to produce liquid jets in inkjet printers or to produce sprays among other applications. In the present research, we show the generation of liquid jets of controllable direction produced within a sessile water droplet by thermocavitation. The jets are driven by an acoustic shock wave emitted by the collapse of a hemispherical vapor bubble at the liquid-solid/substrate interface. The generated shock wave is reflected at the liquid-air interface due to acoustic impedance mismatch generating multiple reflections inside the droplet. During each reflection, a force is exerted on the interface driving the jets. Depending on the position of the generation of the bubble within the droplet, the mechanical energy of the shock wave is focused on different regions at the liquid-air interface, ejecting cylindrical liquid jets at different angles. The ejected jet angle dependence is explained by a simple ray tracing model of the propagation of the acoustic shock wave inside the droplet.

Predictions of defect detection performance of air-coupled ultrasonic rail inspection system

A rail inspection system based on ultrasonic guided waves and non-contact (air-coupled) ultrasound transduction is under development at the University of California at San Diego. The system targets defects in the rail head that are major causes of train accidents. Because of the high acoustic impedance mismatch between air and steel, the non-contact system poses severe challenges and questions on the defect detection performance. This article presents an extensive numerical study, conducted with a local interaction simulation approach, to model the ultrasound propagation and interaction with defects in the proposed system. This model was used to predict the expected detection performance of the system in the presence of various defects of different sizes and positions, and at varying levels of signal-to-noise ratios. When possible, operating variables for the model were chosen consistently with the field test of an experimental prototype that was conducted in 2014. The defect detection performance was evaluated through the computation of receiver operating characteristic curves in terms of probability of detection versus probability of false alarms. The study indicates that despite the challenges of non-contact probing of the rail, quite satisfactory inspection performance can be expected for a variety of defect types, sizes, and positions. Beyond the specific cases examined in this article, this numerical framework can also be used in the future to examine a larger variety of field test conditions.

Novel Contrast‐Enhanced Ultrasound Evaluation in Neonatal Hypoxic Ischemic Injury: Clinical Application and Future Directions

Sensitive, specific, and safe bedside evaluation of brain perfusion is key to the early diagnosis, treatment, and improved survival of neonates with hypoxic ischemic injury. Contrast-enhanced ultrasound (US) imaging is a novel imaging technique in which intravenously injected gas-filled microbubbles generate enhanced US echoes from an acoustic impedance mismatch. This article describes contrast-enhanced US imaging in 2 neonates with hypoxic ischemic injury and future directions on developing quantitative contrast-enhanced US techniques for improved characterization of perfusion abnormalities. The importance of studying the temporal evolution of brain perfusion in neonatal hypoxic ischemic injury is also highlighted.

Design optimization of embedded ultrasonic transducers for concrete structures assessment

In the last decades, the field of structural health monitoring and damage detection has been intensively explored. Active vibration techniques allow to excite structures at high frequency vibrations which are sensitive to small damage. Piezoelectric PZT transducers are perfect candidates for such testing due to their small size, low cost and large bandwidth. Current ultrasonic systems are based on external piezoelectric transducers which need to be placed on two faces of the concrete specimen. The limited accessibility of in-service structures makes such an arrangement often impractical. An alternative is to embed permanently low-cost transducers inside the structure. Such types of transducers have been applied successfully for the in-situ estimation of the P-wave velocity in fresh concrete, and for crack monitoring. Up to now, the design of such transducers was essentially based on trial and error, or in a few cases, on the limitation of the acoustic impedance mismatch between the PZT and concrete. In the present study, we explore the working principles of embedded piezoelectric transducers which are found to be significantly different from external transducers. One of the major challenges concerning embedded transducers is to produce very low cost transducers. We show that a practical way to achieve this imperative is to consider the radial mode of actuation of bulk PZT elements. This is done by developing a simple finite element model of a piezoelectric transducer embedded in an infinite medium. The model is coupled with a multi-objective genetic algorithm which is used to design specific ultrasonic embedded transducers both for hard and fresh concrete monitoring. The results show the efficiency of the approach and a few designs are proposed which are optimal for hard concrete, fresh concrete, or both, in a given frequency band of interest.

The influence of acoustic impedance on gaseous layered detonations bounded by an inert gas

Gaseous detonations propagating through a layer of reactants that is bounded by an inert gas were simulated by solving the two-dimensional reactive Euler equations. A single-step chemical reaction model was used with thermochemical properties that are representative of a highly reactive fuel–oxidizer mixture, such as stoichiometric ethylene and oxygen. A series of cases with varying acoustic impedance ratios between the inert and reactant gases, Z, were studied to explore the influence of acoustic impedance mismatch on the propagation of a detonation through the reactant layer. The detonation failed when Z ∼ 1 due to a loss of triple points at the interface. The detonation propagated stably when Z is high and the impedance of the inert gas is much higher than the reactants. Reflected shocks are produced from the interaction of the Mach stem of a detonation cell with the interface between the reactant and inert gases. These reflected shocks, in turn, detach and generate new triple points that are necessary to propagate the detonation. The detonation was also stable when the acoustic impedance of the inert gas is much lower than the reactants. A gas dynamic structure forms that involves a detached shock in the inert gas and a series of oblique shocks and slip lines in the reactants. A small local explosion is triggered when the Mach stem of a detonation cell interacts with the compressed reactants behind one of these oblique shocks. The resulting retonation wave produces a new Mach stem and a new triple point that leads to a stable detonation. These results suggest that the acoustic impedance and shock structure in the inert gas can have a significant influence on the stable propagation of a layered gaseous detonation.

Fully-disposable multilayered phononic crystal liquid sensor with symmetry reduction and a resonant cavity

Phononic crystals are artificial structures with unique capabilities to control the transmission of acoustic waves. These novel periodic composite structures bring new possibilities for developing a fundamentally new sensor principle that combines features of both ultrasonic and resonant sensors. This paper reports the design, fabrication and evaluation of a phononic crystal sensor for biomedical applications, especially for its implementation in point of care testing technologies. The key feature of the sensor system is a fully-disposable multi-layered phononic crystal liquid sensor element with symmetry reduction and a resonant cavity. The phononic crystal structure consists of eleven layers with high acoustic impedance mismatch. A defect mode was utilized in order to generate a well-defined transmission peak inside the bandgap that can be used as a measure. The design of the structures has been optimized with simulations using a transmission line model. Experimental realizations were performed to evaluate the frequency response of the designed sensor using different liquid analytes. The frequency of the characteristic transmission peaks showed to be dependent on the properties of the analytes used in the experiments. Multi-layered phononic crystal sensors can be used in applications, like point of care testing, where the on-line measurement of small liquid samples is required.

Simulation and experimental study of the sensor emitting frequency for ultrasonic tomography system in a conducting pipe

Ultrasonic tomography techniques provide flow visualization capability, non-invasively and non-intrusively, to enhance the understanding of complex flow processes. There is limited ultrasonic research in tomography imaging systems in the tomogram analysis of fluid flow in a conducting pipe because of a high acoustic impedance mismatch, which means that very little ultrasonic energy can be transmitted through the interface. The majority of industrial pipelines are constructed from metallic composites. Therefore, the development and improvement of ultrasonic measurement methods to accommodate a stainless steel pipe are proposed in this paper. Experimental and simulation distribution studies of the ultrasonic emitting frequency in acrylic versus stainless steel pipes were studied, measured and analyzed. During the simulation, ultrasonic transducers were placed on the surface of the investigated pipe to inspect the ultrasonic sensing field. The distribution of the sound wave acoustic pressure was simulated based on the physical dimensions and parameters of the actual experimental hardware set-up. We developed ultrasonic acoustic models using the finite element method with COMSOL software, and experiments were carried out to validate the simulation results. Finally, by performing the static phantoms tests, a feasibility study of ultrasonic tomography system was presented to investigate the void fraction of liquid column inside a stainless steel pipe.

Highly sensitive piezoelectric micromachined ultrasonic transducer operated in air

Piezoelectric micromachined ultrasonic transducer (pMUT) is gaining increasing research interest. It overcomes the inherent shortcomings of conventional bulk ultrasonic transducers such as acoustic impedance mismatching and poor sensitivity. In addition, pMUT does not require the extremely large input voltage as capacitive micromachined ultrasonic transducer. Therefore the pMUT is potential to be integrated into portable electronics. A pMUT array operated in air using the lead zirconate titanate (PZT) thin film polled by high voltage pulses is realised and fully investigated. The high voltage pulses are found to be more effective for PZT poling, where the piezoelectric constant d 31 is elevated to 105 pm/V. Benefited from such high performance PZT thin film and optimised structure, the fabricated pMUT (500 × 300 μm) achieves a displacement sensitivity of 807 nm/V at its resonant frequency (482 kHz) without DC offset. Compared with previously reported PZT pMUTs, the sensitivity is superior to them even with a smaller membrane. The in-air transmitting performance is also evaluated. A single pMUT element is able to generate 63.7 dB sound pressure level at 10 mm in air with only 2 V input. The proposed high performance pMUT shows its promise for practical applications in portable electronics.

Enhancement of backward second harmonic generation of acoustic waves in a resonant cavity by using a superlattice

In this paper, the enhancement effect of backward second harmonic generation of acoustic waves in the system, consisting of a linear solid superlattice and a nonlinear liquid resonant cavity, is investigated analytically and numerically. Results show the nonlinear effect in the system can be enhanced drastically due to the existence of defective modes and the large acoustic impedance mismatching between solids and liquids. Because of the latter, the dual-resonance phenomenon can always take place approximately and automatically. These results are different from previous conclusions in optical fields.

Cavity Resonance Sensor With Disposable Analyte Container for Point of Care Testing

The use of phononic crystals and resonant structures as sensing platforms paves the way to the development of new biomedical technologies. An acoustic sensor with a resonant cavity and a disposable element was investigated in this paper. The sensor consists of seven layers with high acoustic impedance mismatch. The disposable element used was a glass spectrophotometry cuvette and, during the experimentation, it was filled with different liquid analytes showing characteristic transmission features that could be used as measures to differentiate and identify them. Experimental transmission curves were obtained using an electronic characterization system that uses a double sideband modulation technique to acquire valuable information about the structure being analyzed. Simulations using the 1-D transmission line method were performed to support the experimental realizations. The frequency of maximum transmission has been found to be strongly dependent on the speed of sound of the analyte under test.