Transient Acoustic Radiation Force Research Articles

Numerical Simulation of the Radiation Force from Transient Acoustic Fields: Application to Laser-Guided Acoustic Tweezers

Using pulsed acoustic waves could provide a superior selectivity for microscale acoustic tweezers. However, the theory for the radiation force of pulsed acoustic waves has only been recently derived and no numerical implementations are available. In this paper, we present a finite-element implementation of this model to simulate the transient acoustic radiation force on small spheres. We use the model to simulate laser-guided acoustic tweezers and optimize their performance. By enabling numerical simulations of the transient radiation force, this work may accelerate the rational design of pulse-based high-selectivity acoustic tweezer devices.

Optical coherence viscometry.

We report a technique, named optical coherence viscometry (OCV), to measure the viscosity of Newtonian fluids in a noncontact manner. According to linear wave theory with small amplitudes, capillary waves are associated with fluid mechanical properties. To perform this measurement and avoid the overdamped effects of capillary waves in viscous fluids, transient acoustic radiation force was applied to generate capillary waves. Within a very limited field-of-view using optical coherence tomography, wave motion acquired in the time domain was analyzed using Fourier methods to study the wave velocity dispersion and attenuation relationships for capillary waves, which can reduce the fluid quantity drastically into tissue culture scale. We measure the viscosities of water, water–glycerol solutions with three concentrations, and biological plasma using the proposed OCV and compare the experimental results to theoretical calculations. OCV is sensitive to wave perturbations and can be a promising technique for measuring the viscosity of biological fluids and could be applied in future applications for measurements for lipid membranes in cell biology and tissue engineering investigation.

Magnetic resonance shear wave elastography using transient acoustic radiation force excitations and sinusoidal displacement encoding

A magnetic resonance (MR) shear wave elastography technique that uses transient acoustic radiation force impulses from a focused ultrasound (FUS) transducer and a sinusoidal-shaped MR displacement encoding strategy is presented. Using this encoding strategy, an analytic expression for calculating the shear wave speed in a heterogeneous medium was derived. Green’s function-based simulations were used to evaluate the feasibility of calculating shear wave speed maps using the analytic expression. Accuracy of simulation technique was confirmed experimentally in a homogeneous gelatin phantom. The elastography measurement was compared to harmonic MR elastography in a homogeneous phantom experiment and the measured shear wave speed values differed by less than 14%. This new transient elastography approach was able to map the position and shape of inclusions sized from 8.5 to 14 mm in an inclusion phantom experiment. These preliminary results demonstrate the feasibility of using a straightforward analytic expression to generate shear wave speed maps from MR images where sinusoidal-shaped motion encoding gradients are used to encode the displacement-time history of a transiently propagating wave-packet. This new measurement technique may be particularly well suited for performing elastography before, during, and after MR-guided FUS therapies since the same device used for therapy is also used as an excitation source for elastography.

Ultrasound-modulated optical tomography with intense acoustic bursts

Ultrasound-modulated optical tomography (UOT) detects ultrasonically modulated light to spatially localize multiply scattered photons in turbid media with the ultimate goal of imaging the optical properties in living subjects. A principal challenge of the technique is weak modulated signal strength. We discuss ways to push the limits of signal enhancement with intense acoustic bursts while conforming to optical and ultrasonic safety standards. A CCD-based speckle-contrast detection scheme is used to detect acoustically modulated light by measuring changes in speckle statistics between ultrasound-on and ultrasound-off states. The CCD image capture is synchronized with the ultrasound burst pulse sequence. Transient acoustic radiation force, a consequence of bursts, is seen to produce slight signal enhancement over pure ultrasonic-modulation mechanisms for bursts and CCD exposure times of the order of milliseconds. However, acoustic radiation-force-induced shear waves are launched away from the acoustic sample volume, which degrade UOT spatial resolution. By time gating the CCD camera to capture modulated light before radiation force has an opportunity to accumulate significant tissue displacement, we reduce the effects of shear-wave image degradation, while enabling very high signal-to-noise ratios. Additionally, we maintain high-resolution images representative of optical and not mechanical contrast. Signal-to-noise levels are sufficiently high so as to enable acquisition of 2D images of phantoms with one acoustic burst per pixel.

Transient displacement induced in shear wave elastography: Comparison between analytical results and ultrasound measurements

It is now accepted that an effective way to investigate the elastic properties of soft tissues is to generate a localized transient acoustic radiation force and to follow the associated displacements in the time/space domain. Shear waves induced by this stress field are particularly interesting in this kind of medium because they are governed by the shear elastic modulus μ, which is directly linked to the Young modulus, and spatial distribution and temporal evolution of the transient motion induced must therefore be obtained in detail. We report here a model based on the elastodynamic Green’s function formalism to describe these displacements. 3D simulation of radiation force in homogenous elastic media was performed and the displacement curves computed at different radial distances for different temporal force profiles. Amplitude and duration of displacement were found to be reliable parameters to characterize the elastic properties of the medium. Experimental measurements were performed in a homogeneous agar-gelatin tissue-mimicking phantom, and two transducers were used to generate the radiation force and follow the induced displacements. Displacements obtained from different lateral locations around the applied force axis were then used to reconstruct the shear-wave propagation in a scan plane as a function of time. The experimental displacements/curves agreed with the theoretical profiles obtained by the elastodynamic Green’s function formalism.

2260: Transient ultrasound radiation force elastography: A preliminary comparison with surface palpation elastography

Objectives: Ultrasound elasticity imaging is being developed for evaluating the mechanical properties of soft tissues in vivo, but each research group tends to specialize in a specific approach. Our objective was to assess the advantages and disadvantages of generating tissue strain by deep loading with transient acoustic radiation force, versus the use of nearly static surface loading.

Analysis of contrast in images generated with transient acoustic radiation force

Several mechanical imaging methods are under investigation that use focused ultrasound (US) as a source of mechanical excitation. Images are then generated of the tissue response to this localized excitation. One such method, acoustic radiation force impulse (ARFI) imaging, utilizes a single US transducer on a commercial US system to transmit brief, high-energy, focused acoustic pulses to generate radiation force in tissue and correlation-based US methods to detect the resulting tissue displacements. Local displacements reflect relative mechanical properties of tissue. The resolution of these images is comparable with that of conventional B-mode imaging. The response of tissue to focused radiation force excitation is complex and depends upon tissue geometry, forcing function geometry ( i.e., region of excitation, or ROE) and tissue mechanical and acoustic properties. Finite element method (FEM) simulations using an experimentally validated model and phantom experiments have been performed using varying systems, system configurations and tissue-mimicking phantoms to determine their impact on image quality. Image quality is assessed by lesion contrast. Due to the dynamic nature of ARFI excitation, lesion contrast is temporally-dependent. Contrast of spherical inclusions is highest immediately after force cessation, decreases with time postforce and then reverses, due to shear wave interaction with internal boundaries, differences in shear modulus between lesions and background and inertial effects. In images generated immediately after force cessation, contrast does not vary with applied force, increases with lesion stiffness and increases as the ROE size decreases relative to the size of the structure being imaged. These studies indicate that improved contrast in radiation force-generated images will be achieved as ROE size decreases; however, frame rate and thermal considerations present trade-offs with small ROE size. (E-mail: kathy.nightingale@duke.edu)