Steven G. Kargl

Physical mechanisms of the therapeutic effect of ultrasound (a review)

Therapeutic ultrasound is an emerging field with many medical applications. High intensity focused ultrasound (HIFU) provides the ability to localize the deposition of acoustic energy within the body, which can cause tissue necrosis and hemostasis. Similarly, shock waves from a lithotripter penetrate the body to comminute kidney stones, and transcutaneous ultrasound enhances the transport of chemotherapy agents. New medical applications have required advances in transducer design and advances in numerical and experimental studies of the interaction of sound with biological tissues and fluids. The primary physical mechanism in HIFU is the conversion of acoustic energy into heat, which is often enhanced by nonlinear acoustic propagation and nonlinear scattering from bubbles. Other mechanical effects from ultrasound appear to stimulate an immune response, and bubble dynamics play an important role in lithotripsy and ultrasound-enhanced drug delivery. A dramatic shift to understand and exploit these nonlinear and mechanical mechanisms has occurred over the last few years. Specific challenges remain, such as treatment protocol planning and real-time treatment monitoring. An improved understanding of the physical mechanisms is essential to meet these challenges and to further advance therapeutic ultrasound.

Observations and modeling of the backscattering of short tone bursts from a spherical shell: Lamb wave echoes, glory, and axial reverberations

Tone bursts having durations of 3 or 4 cycles were incident on an air‐filled stainless steel shell in water. The resulting sequence of echoes included a specular reflection and echoes radiated by Lamb waves on the shell. Echo structure was studied for ka of 24 to 75, where a denotes the outer radius; b/a=0.838, where b denotes the inner radius. The amplitudes of Lamb wave echoes were modeled using an elastic generalization of the geometrical theory of diffraction (GTD) [P. L. Marston, J. Acoust. Soc. Am. 83, 25–37 (1988)]. The required Lamb wave parameters (the phase velocity cl and damping βl ) were found by the Sommerfeld–Watson method; an efficient numerical method for the computation of the required complex root νl is described. The echoes were identified by comparing arrival times with predictions; bursts reflected from a solid tungsten carbide sphere were used for a reference amplitude. Measurements with ka=24 of the largest Lamb wave echo (which was due to a flexural wave) were made at various back...

Acoustic scattering from a solid aluminum cylinder in contact with a sand sediment: Measurements, modeling, and interpretation

Understanding acoustic scattering from objects placed on the interface between two media requires incorporation of scattering off the interface. Here, this class of problems is studied in the particular context of a 61 cm long, 30.5 cm diameter solid aluminum cylinder placed on a flattened sand interface. Experimental results are presented for the monostatic scattering from this cylinder for azimuthal scattering angles from 0 degrees to 90 degrees and frequencies from 1 to 30 kHz. In addition, synthetic aperture sonar (SAS) processing is carried out. Next, details seen within these experimental results are explained using insight derived from physical acoustics. Subsequently, target strength results are compared to finite-element (FE) calculations. The simplest calculation assumes that the source and receiver are at infinity and uses the FE result for the cylinder in free space along with image cylinders for approximating the target/interface interaction. Then the effect of finite geometries and inclusion of a more complete Greens function for the target/interface interaction is examined. These first two calculations use the axial symmetry of the cylinder in carrying out the analysis. Finally, the results from a three dimensional FE analysis are presented and compared to both the experiment and the axially symmetric calculations.

Ray synthesis of Lamb wave contributions to the total scattering cross section for an elastic spherical shell

The optical theorem relates the extinction cross section σe(ka) to the forward‐scattering amplitude f (θ=0,ka). Here, θ denotes the scattering angle, k is the wavenumber of the incident sound, and a is the radius of the scatterer. If the absorption by the scatterer is negligible so that the scatterer is elastic, σe is equal to the total scattering cross section σt. By applying this theorem to the partial wave series for f (0,ka), an expression can be obtained for σt for an elastic spherical shell in water. However, the series representation of σt does not facilitate a direct understanding of the rich structure caused by the shell’s elastic response. In particular, the elastic response is attributable to leaky Lamb waves. A generalization of the geometrical theory of diffraction [P. L. Marston, J. Acoust. Soc. Am. 83, 25–37 (1988)] is employed to synthesize f (0,ka). This simple ray acoustic synthesis contains a component for ordinary diffraction by the shell and distinct contributions for the individual L...

High-frequency subcritical acoustic penetration into a sandy sediment

During the sediment acoustics experiment, SAX99, a hydrophone array was deployed in sandy sediment near Fort Walton Beach, Florida, in a water depth of 18 m. Acoustic methods were used to determine array element positions with an accuracy of about 0.5 cm, permitting coherent beamforming at frequencies in the range 11-50 kHz. Comparing data and simulations, it has been concluded that the primary cause of subcritical acoustic penetration was diffraction by sand ripples that were dominant at this site. These ripples had a wavelength of approximately 50 cm and RMS relief of about 1 cm. The level and angular dependence of the sound field in the sediment agree within experimental uncertainties with predictions made using small-roughness perturbation theory.

Monitoring bubble growth in supersaturated blood and tissue ex vivo and the relevance to marine mammal bioeffects

There have been several recent reports that active sonar systems can lead to serious bioeffects in marine mammals, particularly beaked whales, resulting in strandings, and in some cases, to their deaths. We have devised a series of experiments to determine the potential role of low-frequency acous- tic sources as a means to induce bubble nucleation and growth in supersatu- rated ex vivo bovine liver and kidney tissues, and blood. Bubble detection was achieved with a diagnostic ultrasound scanner. Under the conditions of this experiment, supersaturated tissues and blood led to extensive bubble produc- tion when exposed to short pulses of low frequency sound. right whales. 5 Although these cetaceans have not been associated with mass stranding events related to navy sonar systems, it is likely that other cetaceans will also undergo significant changes in behavior when subjected to high-intensity acoustic pulses. Rapid surfacing from a deep dive may lead to decompression sickness. In addition, it is known that exercising after diving can lead to decompression sickness in humans. 6 Analogously, abnormal extended activity resulting from sonar may induce decompression sickness in cetaceans. To address the role of direct bubble nucleation in tissue by a sound pulse, it is worthwhile to discuss the bioeffects induced by diagnostic ultrasound systems, used routinely worldwide to image the progress of healthy as well as pathological conditions in the human patient. It is no surprise, then, to recognize that ultrasound-induced bioeffects in human tissue have been studied extensively. To this date, no repeatable effects of diagnostic ultrasound exams have been reported in the general literature. This paucity of observable bioeffects was at first surprising because the acoustic pressure amplitudes used in imaging devices are in excess of the threshold for bubble nucleation and growth, i.e., cavitation—the most likely ultrasound-induced

A transition‐matrix formulation of scattering in homogeneous, saturated, porous media

Scattering by an obstacle embedded within a homogeneous host is analyzed in the context of Biot’s theory. Both host and scatterer are assumed to be fluid‐saturated poroelastic media. A transition matrix that accounts for the additional slow longitudinal wave is constructed from a Betti’s identity generalized to poroelastic media. Extinction, scattering, and absorption cross sections are defined for objects embedded in nonattenuating hosts. When attenuation is significant in the host (as is typical in saturated porous media), appropriately scaled, far‐field scattering amplitudes are used to investigate scattering dynamics. General results are presented and specialized to spherical objects where the elements of the transition matrix may be obtained in closed form. Using effective medium estimates of Biot parameters where needed, numerical calculations of cross sections and scattering amplitudes are presented for incident plane‐wave fields. Several limiting cases are discussed.

Ray synthesis of the form function for backscattering from an elastic spherical shell: Leaky Lamb waves and longitudinal resonances

An acoustic ray analysis is employed in synthesizing the form function for backscattering, f(θ=π,ka), from a fluid-loaded evacuated elastic spherical shell where k is the wave number of the incident plane wave and a is the outer radius of the shell. The synthesis contains a component associated with a specular reflection, fsp, and contributions from leaky Lamb waves. The contribution fl of the lth leaky Lamb wave is expressible in a Fabry–Perot resonator form [P. L. Marston, J. Acoust. Soc. Am. 83, 25–37 (1988)]. A comparison of the ray synthesis for f(ka) with the exact partial-wave series representation for a 440c stainless-steel shell verifies the usefulness of the ray synthesis for the present case of a shell. The present synthesis is also new in that it includes the effects of longitudinal resonances on fsp. A novel ray synthesis of fsp indicates a significant resonance effect near the condition kLh=nπ (n=1,2,…). The thickness of the shell is h, and kL=ω/cL is the longitudinal wave number where cL is...

Double monopole resonance of a gas-filled, spherical cavity in a sediment

The monopole response of a gas-filled, spherical cavity in a sediment is investigated. The sediment is either a fluid, elastic solid, or saturated poroelastic medium. The present method entails the scattering of an incident displacement field that preferentially excites the monopole resonance of the cavity. The main result demonstrates that a gas-filled, spherical cavity in a saturated poroelastic medium can exhibit two distinct monopole resonances. These resonances arise from the two distinct longitudinal modes of propagation in saturated poroelastic medium as described by Biot’s theory.

LOW- TO MID-FREQUENCY SCATTERING FROM ELASTIC OBJECTS ON A SAND SEA FLOOR: SIMULATION OF FREQUENCY AND ASPECT DEPENDENT STRUCTURAL ECHOES

The scattering from roughly meter-sized targets, such as pipes, cylinders and unexploded ordnance shells in the 130 kHz frequency band is studied by numerical simulations and compared to experimental results. The numerical tool used to compute the frequency and aspect-dependent target strength is a hybrid model, consisting of a local finite-element model for the vicinity of the target, based on the decomposition of the three-dimensional scattering problem for axially symmetric objects into a series of independent two-dimensional problems, and a propagation model based on the wavenumber spectral integral representation of the Greens functions for layered media.