Time Reversal Acoustic Focusing System Research Articles

Acoustic diver deterrent in a shallow harbor using time reversal acoustics

Protection of domestic harbors against surface and underwater threats is an important task of port security. A significant security risk is associated with scuba divers ability to carry explosives. The diver detection sonars have been developed and there is a need to compliment it with low cost acoustic swimmer deterrent. Previous research demonstrated that time reversal acoustic (TRA) system can focus intensive sound using acoustic noise from a diver. This paper discusses the feasibility of applying TRA principles for the focusing of sonar ping-pulse reflected from a diver. The advantages of the TRA focusing system and relevant operating parameters are demonstrated using the model of shallow sea, where propagation of sonar pulse is affected by numerous reflections from surface and bottom. The developed model was used to estimate and compare an effective zone of the diver deterrence taking into consideration the sonar pulse reflections from diver, bottom and surface. The main objective was to estimate if NAVY ship sonar is capable of producing sufficient and spatially localized sound pressure to disorient and divert the diver away. A secondary objective was to ensure that sound pressure level outside the focal zone in the diver proximity remains not harmful to marine life.

Time-reversal techniques in ultrasound-assisted deep vein thrombosis treatment: Technology development and in vitro evaluation.

We describe a thrombolysis method that combines time-reversal acoustic (TRA) focusing with an AngioJet thrombectomy catheter to improve the dissolution of thrombus in the treatment of deep venous thrombosis (DVT). Commercially available ultrasound-assisted DVT systems have been shown to accelerate thrombolysis by up to 40% and reduce the amount of thrombolytic drug required for treatment by 50%–70%. Current ultrasound-assisted DVT treatments utilize a specially designed intravascular catheter with therapeutic ultrasound transducer on the tip and side. Additionally, MRI guided high intensity focused ultrasound systems have shown to rapidly lyse thrombus. We describe a novel TRA focusing system that is used to infuse fluids into the thrombus while simultaneously exposing the clot to safe levels of 1-MHz ultrasound energy. The system includes a combined infusion catheter-hydrophone, a ten-channel ultralow-output impedance amplifier, a broad-band ultrasound resonator, and MATLAB-based, TRA control and user-interface. TRA allows easy focusing of ultrasound therapy to the catheter tip from an external source, without complex phase-correction and array design. The system has been tested in vitro in a DVT phantom model, and results show that it provides 1-mm spatial resolution focusing. We present results in using TRA focusing to lyse bovine blood clots in the physiological size DVT phantom.

Time-reversal acoustic focusing system as a virtual random phased array

This paper compares the performance of two different systems for dynamic focusing of ultrasonic waves: conventional 2-D phased arrays (PA) and a focusing system based on the principles of time-reversed acoustics (TRA). Focused ultrasound fields obtained in the experiments with the TRA focusing system (TRA FS), which employs a liquid-filled reverberator with 4 piezotransducers attached to its wall, are compared with the focused fields obtained by mathematical simulation of PAs comprised from several tens to several hundreds of elements distributed randomly on the array surface. The experimental and simulated focusing systems had the same aperture and operated at a frequency centered about 600 kHz. Experimental results demonstrated that the TRA FS with a small number of channels can produce complex focused patterns and can steer them with efficiency comparable to that of a PA with hundreds of elements. It is shown that the TRA FS can be realized using an extremely simple means, such as a reverberator made of a water-filled plastic bottle with just a few piezotransducers attached to its walls.

Time Reversal Acoustic focusing with random reverberator

Time Reversal Acoustic (TRA) focusing system based on an external reverberator can provide good focusing with minimum side lobes even with a few transducers. The focusing ability of such system can be increased using reverberator with rough boundaries or by adding internal random scatterers. The experiments were conducted with a reverberator made of a polyethylene bottle filled with water and a single piezoceramic disk attached externally to its wall. Experiments demonstrated that inserting in the bottle random hard scatterers or altering of the bottle surface significantly decreases the size of the focused spot and the level of side lobes. The new pseudo-impulse method of TRA focusing allowed suppressing piezotransducer resonances and provided formation of short wide band signals. The ability of such TRA focusing system based on random reverberator to form simultaneously multiple focuses and produce focal spots of the complex shape has been demonstrated. The application of binary radiation regime led to several fold increasing of the amplitude of the focal spot while the focal structure practically was not affected. Applications of such random focusing system for medical imaging and HIFU treatment is discussed.

Ultrasonic focused waveform generation using time reversal acoustic focusing system

Abstract The concept of Time Reversal Acoustics (TRA) provides an elegant possibility of both temporal and spatial concentrating of acoustic energy in highly inhomogeneous media. We explored the possibility of generating acoustical signals with arbitrary waveforms using the TRA Focusing System (TRA FS). A method has been developed to predict TRA-focused ultrasound waveforms and spatial distribution by using the measurements of transfer function of transfer function relating the signal at the TRA transmitter to that at the focusing point. The developed approach for TRA-focused signal waveform prediction from the results of direct signal measurements was tested on ten-channel TRA FS based on aluminum resonator with glued piezotransducers. The TRA FS operated in the frequency band of 100–1000 kHz. The formation of ultrasonic signals with various envelopes was demonstrated experimentally. The calculated and experimentally measured waveforms and spatial distributions were practically identical. We formed triangular, rectangular, and amplitude modulated tone burst signals with different modulation frequencies in the focal region. The level of side lobes in the generated signals was much lower than that for standard TRA focusing.

Advantages of time reversal acoustic focusing system in biomedical applications

The development and biomedical applications of time reversal acoustics (TRA) systems for focusing and manipulating ultrasound beams are reviewed. The TRA focusing system (TRA FS) is capable to deliver ultrasound energy to the chosen region in highly inhomogeneous medium (including soft tissues and bones) with focusing efficacy hardly achievable using conventional phased array transmitters. TRA FS is able to focus and stir ultrasound beams in a 3‐D volume using just a few piezoceramic transducers glued to the facets an aluminum block. Another advantage of TRA FS is its ability to produce pulses with arbitrary waveforms in a wide frequency band. A custom‐designed compact multichannel TRA system operating in a wide frequency range from 0.01 to 10 MHz has been developed. Measurements of TRA field structure were conducted in a large variety of inhomogeneous tissue phantoms and ex vivo bones and soft tissues. Principles of TRA focusing optimization based on acoustical properties of the resonator material, parameters of the sonicated medium, and the coupling of the TRA resonator with the medium were developed and applied in the tested TRA systems. [Work was supported by NIH.]

Optimization of time-reversal acoustic focusing

Time-reversal acoustic (TRA) focusing systems are typically based on an acoustical resonator (a cavity) in which the multiple reflections of a bouncing acoustic pulse act as a virtual phased array. The efficiency of the TRA focusing system depends on the spectral and temporal parameters of the focused pulse and parameters of the resonator, such as its shape, size, and the relative position of its transducers. The acoustical properties of the resonator material, parameters of the sonicated medium, and the coupling of the TRA resonator and sonicated medium may also affect efficiency. Optimization of such a multiparametric system is a complex problem which is nearly impossible to achieve empirically without an adequate theoretical model. A theoretical model for predicting the efficacy of TRA focusing has been developed and experiments testing theoretical predictions were conducted. The experiments were conducted using a 10-channel TRA system developed in Artann with several broadband resonators in the frequency band of 0.1–2 MHz. The measured dependences of the focused signal on the acoustic pulse spectrum and duration, coupling of the resonator with the sonication medium, and the modes of TRA signal formation (analog or binary) confirmed theoretical predictions. [Work supported by NIH.]

Radiation force produced by time reversal acoustic focusing system

An ultrasonic induced radiation force is an efficient tool for remote probing of internal anatomical structures and evaluating tissue viscoelastic properties, which are closely related to tissue functional state and abnormalities. Time Reversal Acoustic Focusing System (TRA FS) can provide efficient ultrasound focusing in highly inhomogeneous media. Furthermore, numerous reflections from boundaries, which distort focusing in conventional ultrasound focusing systems and are viewed as a significant technical hurdle, lead to an improvement of the focusing ability of the TRA system. In this work the TRA FS field structure and radiation force in a transcranial phantom were investigated. A simple TRA FS comprising a plane piezoceramic transducer attached to an external resonator such as an aluminum block was acoustically coupled to the tested transcranial phantom. A custom-designed compact electronic unit for TRA FS provided receiving, digitizing, storing, time reversing and transmitting of acoustic signals in a wide frequency range from 0.01 to 10 MHz. The radiation force produced by ultrasonic pulses was investigated as a function of the transmitted ultrasound temporal parameters. The simplest TRA FS provided focusing of 500 kHz ultrasound pulses and the generation of a radiation force with an efficacy hardly achievable using conventional sophisticated phased array transmitters. [Work supported by NIH.]