Simulation of internal stress waves generated by laser-induced damage in multilayer dielectric gratings.

Multilayer dielectric (MLD) gratings used in ultrahigh-intensity laser systems often exhibit a laser-induced damage performance below that of their constituent materials. Reduced performance may arise from fabrication- and/or design-related issues. Finite element models were developed to simulate stress waves in MLD grating structures generated by laser-induced damage events. These models specifically investigate the influence of geometric and material parameters on how stress waves can lead to degradation of material structural integrity that can have adverse effects on its optical performance under subsequent laser irradiation: closer impedance matching of the layer materials reduces maximum interface stresses by ~20% to 30%; increasing sole thickness from 50 nm to 500 nm reduces maximum interface stresses by ~50%.

Non-invasive quantitative reconstruction of tissue elasticity using an iterative forward approach.

A novel iterative approach is presented to estimate Young's modulus in homogeneous soft tissues using vibration sonoelastography. A low-frequency (below 100 Hz) external vibration is applied and three or more consecutive frames of B-scan image data are recorded. The internal vibrational motion of the soft tissue structures is calculated from 2D displacements between pairs of consecutive frames, which are estimated using a mesh-based speckle tracking method. An iterative forward finite element approach has been developed to reconstruct Young's modulus from the measured vibrational motion. This is accomplished by subdividing the 2D image domain into sample blocks in which Young's modulus is assumed to be constant. Because the finite element equations are internally consistent, boundary values other than displacement are not required. The sensitivity of the results to Poisson's ratio and the damping coefficient (viscosity) is investigated. The approach is verified using simulated displacement data and using data from tissue-mimicking phantoms.

Nonlinear propagation and the output indices.

By ignoring the effects of nonlinear propagation, current exposimetry protocols may yield significant underestimates of the acoustic pressure in situ. This problem can be avoided simply by (1) extrapolating pressures linearly from low amplitude measurements in water and (2) linearly derating these values to obtain estimates of fields in situ. The mechanical index was designed to provide an indication of temporal peak acoustic fields for use in prediction of nonthermal biological effects in tissues. At low outputs, the mechanical index, together with the frequency, gives the peak negative pressure near the focus of the field. As currently formulated, however, the pressure used in the mechanical index may be far from the focus at high output levels. Recommendations of the World Federation of Ultrasound in Medicine and Biology avoid the underestimate associated with nonlinear propagation as well as other problems with the mechanical index and may be preferable in dealing with non-thermal bioeffects. The thermal indices that are implemented currently in the Output Display Standard (American Institute of Ultrasound in Medicine/National Electrical Manufacturers' Association) are affected less seriously by nonlinear propagation.

Finite difference predictions of P-SV wave propagation inside submerged solids. I. Liquid-solid interface conditions.

A finite difference scheme has been developed to analyze internal strains in submerged elastic solids of irregular geometry subjected to ultrasonic wave sources that simulate a clinical lithotripter. In part I of this paper, the finite difference formulation that accounts for arbitrary liquid-solid interfaces is presented and sample numerical results are discussed. Two different methods for discretizing the liquid-solid interface conditions are developed. The first treats the interface conditions explicitly. The second integrates the heterogeneous wave equations across the interface using the divergence theorem. Both schemes account for varying grid sizes and give similar results for a test problem consisting of a radially diverging source incident on the rectangular solid. The time sequence obtained numerically for strain at the center of a rectangular solid matches well with the experimental results [S. M. Gracewski et al., J. Acoust. Soc. Am. 94, 652-661 (1993)] in terms of the arrival times and the relative amplitudes of the peaks. In addition, strain contours are plotted to visualize the propagation of P (longitudinal) and S (shear vertical) waves inside a circular solid. The reflection from the concave back surface of the circular solid has a focusing effect with the subsequent formation of focal zones, known as caustics, where peak strains occur. In part II of this paper, the finite difference scheme is used to study the effects of geometry changes on the internal stresses and caustics predicted in model stones subjected to lithotripter pulses.

Finite difference predictions of P-SV wave propagation inside submerged solids. II. Effect of geometry.

To understand better direct stress wave contributions to stone fragmentation during extracorporeal shock wave lithotripsy (ESWL), the numerical formulation developed in part I is applied to study the time evolution of stress wave fields produced inside submerged isotropic elastic solids having irregular geometries. Cut spheres are used to model stones that have already had an initial fracture. Ellipses are used to approximate other deviations from a spherical geometry. The propagation and focusing of the longitudinal (P) and shear (S) wave fronts are visualized by presenting internal strain contours. Internal strain measurements are obtained from strain gauges embedded inside plaster specimens to confirm the focusing effect obtained from the concave back surfaces of the stones. Fragmentation experiments indicate damage caused by spalling and direct stress wave focusing as well as a front surface pit presumably created by cavitation activity.

Response of constrained and unconstrained bubbles to lithotripter shock wave pulses.

The Gilmore formulation for spherical bubble dynamics [F. R. Gilmore, The Growth or Collapse of a Spherical Bubble in a Viscous Compressible Liquid (California Institute of Technology, Pasadena, CA, 1952), Rep. No. 26-4] is used to investigate the response of air bubbles to a variety of lithotripter shock waveforms. A modification of the Gilmore model is proposed to account for partial constraint of the bubble expansion that can be caused by bubble coatings (such as in echo contrast agents) and by tissues or vessels surrounding bubbles in vivo. In the modified formulation, a viscoelastic membrane is assumed to exist at the bubble interface to include the possible effects of the nonlinear elasticity and strain rate dependent viscosity on the bubble response. The stress induced in the membrane is assumed to be an exponential function of the bubble radius, which tends to restrict the bubble expansion. The viscosity is assumed to increase with the strain rate. In the absence of the membrane, the maximum bubble wall pressure induced by a negative (tensile) pulse is much larger than that induced by a positive (compressive) pulse of the same pressure waveform and amplitude. This difference increases with decreasing initial bubble radius. The addition of the viscoelastic membrane significantly decreases the predicted maximum bubble pressure and the difference in response between the positive and negative pulses. The effect of the time delay between double pulses (positive followed by negative or negative followed by positive) is also investigated for unconstrained bubbles.

Internal stress wave measurements in solids subjected to lithotripter pulses.

Semiconductor strain gauges were used to measure the internal strain along the axes of spherical and disk plaster specimens when subjected to lithotripter shock pulses. The pulses were produced by one of two lithotripters. The first source generates spherically diverging shock waves of peak pressure approximately 1 MPa at the surface of the specimen. For this source, the incident and first reflected pressure (P) waves in both sphere and disk specimens were identified. In addition, waves reflected by the disk circumference were found to contribute significantly to the strain fields along the disk axis. Experimental results compared favorably to a ray theory analysis of a spherically diverging shock wave striking either concretion. For the sphere, pressure contours for the incident P wave and caustic lines were determined theoretically for an incident spherical shock wave. These caustic lines indicate the location of the highest stresses within the sphere and therefore the areas where damage may occur. Results were also presented for a second source that uses an ellipsoidal reflector to generate a 30-MPa focused shock wave, more closely approximating the wave fields of a clinical extracorporeal lithotripter.

Gallstone movement during lithotripsy: mechanisms and effects on fragmentation.

We sought to examine the mechanisms of gallstone movement and its effect on gallstone fragmentation in vitro. Two experiments were performed in four specially constructed phantoms that allowed decreasing degrees of movement during the application of shock waves. Shock waves caused displacement of the stone from the focus, but the stone and its fragments were returned to the focus by streaming movements in the coupling liquid when the volume of surrounding fluid was small. Streaming movements were ineffective in large volumes. Restraining movements of the gallstone did not improve the results of fragmentation. We conclude that radiation force and the streaming motion of the surrounding liquid account for movements of the stone and fragments during lithotripsy. Lithotripsy is more effective when smaller volumes are used because streaming brings fragments back to the focus of the lithotripter. Total immobilization of the stone in the focus of the lithotripter, however, offers no benefit, probably because it inhibits rotational movement of the stone.

Ability of high-intensity ultrasound to ablate human atherosclerotic plaques and minimize debris size.

To investigate whether high-intensity ultrasound can destroy atherosclerotic plaques while sparing the normal arterial wall, 279 normal human aortic sites and 119 fibrous and 193 calcified plaques, obtained from 24 necropsies, were insonified in a water tank, at 20 kHz and at 5 different power intensities, ranging from 68 W/cm2 (P1) to 150 W/cm2 (P5). These intensities were associated with a total excursion of the ultrasound irradiation apparatus tip from 90 to 268 microns, respectively. Time to perforate normal aortic sites and fibrous and calcified plaques was recorded at each intensity. There was no difference in perforation time between normal aortic sites and fibrous and calcified plaques when high-power levels (P2 to P5) were used. However, at the lowest power (P1), perforation time for the normal aortic wall was significantly longer than for fibrous and calcified plaques: 30 +/- 18 seconds (166 observations), 14 +/- 7 seconds (p less than 0.001) (78 observations) and 12 +/- 8 seconds (p less than 0.001) (115 observations), respectively. When perforation times for normal vessel wall versus fibrous plaque and normal vessel wall versus calcified plaque from the same necropsy specimen were compared in a pairwise manner, the results were: 29 +/- 13 vs 16 +/- 7 (p less than 0.001) (48 paired observations) and 26 +/- 9 vs 10 +/- 5 seconds (p less than 0.001) (55 paired observations), respectively. Regardless of whether paired or unpaired comparison was applied, no significant difference was found in perforation time between fibrous and calcified plaques. The debris did not differ in size as measured separately for normal sites and fibrous and calcified plaques by a computer-interfaced Channelizer and Coulter Counter system.(ABSTRACT TRUNCATED AT 250 WORDS)

High-frequency attenuation measurements using an acoustic microscope.

An acoustic microscope was used to measure excess attenuation of aqueous solutions of sugars and proteins at 1.0 GHz. Interference pattern spacing and peak amplitude reduction of V(z) curves, obtained with these solutions as the acoustic microscope coupling liquid, were related to the solution wavespeed and attenuation, respectively. Consistent with published results for lower frequencies, solutions with molecular weight greater than 10,000 had a higher specific absorption than those with a molecular weight less than 1000 and within these two molecular weight ranges specific absorption was independent of concentration.

Optical generation of coherent surface acoustics: an optically based probe of surface structure and dynamics.

Above-band-gap picosecond pulses are used to generate holographically high-frequency, coherent, surface acoustic modes on semiconductor surfaces. Optical diffraction from the surface acoustics is in superposition to a free-carrier phase grating that acts as an amplifying cross term in the diffraction process. The detection limits are of the order of 10(-4) nm for the surface displacement. Frequencies up to 2 GHz have been realized, with frequencies >20 GHz possible. A quantitative theory for the photothermal coupling has been developed. In addition, propagation of the optically generated surface modes has revealed a solid-liquid phase transition of the water layer at TiO(2)-H(2)O interfaces.