Charles C. Church

The effects of an elastic solid surface layer on the radial pulsations of gas bubbles

Most previous theoretical investigations of gas bubble dynamics have assumed an uncontaminated gas–liquid interface. Recently, however, the potential importance of layers of surface active agents on bubble dynamics has been increasingly recognized. In this work it is assumed that a continuous layer of incompressible, solid elastic material separates the gas from the bulk Newtonian liquid. Elasticity is modeled to include viscous damping. A Rayleigh–Plesset‐like equation describing the dynamics of such surface‐contaminated gas bubbles is derived. The equation predicts that the surface layer supports a strain that counters the Laplace pressure and thereby stabilizes the bubble against dissolution. An analytical solution to this equation which includes both the fundamental and second‐harmonic response is presented. The dispersion relation describing the propagation of linear pressure waves in liquids containing suspensions of these bubbles also is presented. It is found that (1) the resonance frequencies of ...

Sonoluminescence and bubble dynamics for a single, stable, cavitation bubble

High‐amplitude radial pulsations of a single gas bubble in several glycerine and water mixtures have been observed in an acoustic stationary wave system at acoustic pressure amplitudes on the order of 150 kPa (1.5 atm) at 21–25 kHz. Sonoluminescence (SL), a phenomenon generally attributed to the high temperatures generated during the collapse of cavitation bubbles, was observed as short light pulses occurring once every acoustic period. These emissions can be seen to originate at the geometric center of the bubble when observed through a microscope. It was observed that the light emissions occurred simultaneously with the bubble collapse. Using a laser scattering technique, experimental radius‐time curves have been obtained which confirm the absence of surface waves, which are expected at pressure amplitudes above 100 kPa. [S. Horsburgh, Ph.D. dissertation, University of Mississippi (1990)]. Also from these radius‐time curves, measurements of the pulsation amplitude, the timing of the major bubble collaps...

A theoretical study of cavitation generated by an extracorporeal shock wave lithotripter

The intense acoustic wave generated at the focus of an extracorporeal shock wave lithotripter is modeled as the impulse response of a parallel RLC circuit. The shock wave consists of a zero rise time positive spike that falls to 0 at 1 microsecond followed by a negative pressure component 6 microseconds long with amplitudes scaled to +1000 and -160 bars, P+ and P-, respectively. This pressure wave drives the Gilmore-Akulichev formulation for bubble dynamics; the zero-order effect of gas diffusion on bubble response is included. The negative pressure component of a 1000-bar shock wave will cause a preexisting bubble in the 1- to 10-microns range to expand to over 100 times its initial size, R0, for 250 microseconds, with a peak radius of approximately 1400 microns, then collapse very violently, emitting far UV or soft x-ray photons (black body). Gas diffusion does not appreciably mitigate the amplitude of the pressure wave radiated at the primary collapse, but does significantly reduce the collapse temperature. Diffusion also increases the bubble radius from R0 up to 40 microns and extends the duration of ringing following the primary collapse, assuming that the bubble does not break up or shed microbubbles. Results are sensitive to P+/P- and to the duration of the negative pressure cycle but not to rise time.

Transient pulsations of small gas bubbles in water

Transient behavior of small gas bubbles in a liquid set into violent motion by ultrasonic pressure waves is of interest because of widespread use of microsecond pulses in diagnostic ultrasound. Such pulses contain only a few pressure cycles and the transient pulsations of bubbles set in motion by such pulses would determine the bubble–ultrasound interaction. A computer study has been made to obtain a global representation of the pulsation amplitudes R(t) of small gas bubbles (nuclei) in water during the first few cycles of a cw ultrasonic pressure. One objective was to obtain a better understanding of cavitation phenomena where many nuclei with initial radii Rn from 0.1–20 μm are set in motion at pressures ranging from 0.5–5 bars and at frequencies from 0.1–10 MHz. Results allowed construction of surfaces showing the relative bubble amplitude R/Rn as a function of Rn and of the time t/TA, where TA is the acoustic period. One finding is that, in the range of peak pressures found in diagnostic pulses, trans...

Lysis of erythrocytes by exposure to CW ultrasound

The threshold for lysis of erythrocytes suspended at concentrations of 0.5-1% in saline or plasma in rotating cylindrical exposure vessels is approximately spatial peak intensities of 2 W/cm2 at 1 MHz continuous wave (CW). Results of a series of experiments in which cell concentration, viscosity and gas composition of the suspending medium and rotation speed of the exposure vessel were varied combined with observations of sonoluminescence are all consistent with a hypothesis that cells are lysed by inertial (transient) acoustic cavitation. For the proposed mechanism to operate in cell suspensions, it is necessary that bubbles be brought into contact with the cells. Rotation of the chamber recycles bubbles that are driven by radiation forces to the far wall of the chamber in a matter of milliseconds. The physical and chemical properties of the wall of the chamber appear to be important as stabilizing sites for nuclei that serve as seeds for cavitation events.

Ultrasound Biosafety Considerations for the Practicing Sonographer and Sonologist

The purpose of this article is to present the practicing sonographer and sonologist with an overview of the biohazards of ultrasound and guidelines for safe use.

A model for the dynamics of gas bubbles in soft tissue

Understanding the behavior of cavitation bubbles driven by ultrasonic fields is an important problem in biomedical acoustics. Keller-Miksis equation, which can account for the large amplitude oscillations of bubbles, is rederived in this paper and combined with a viscoelastic model to account for the strain-stress relation. The viscoelastic model used in this study is the Voigt model. It is shown that only the viscous damping term in the original equation needs to be modified to account for the effect of elasticity. With experiment determined viscoelastic properties, the effects of elasticity on bubble oscillations are studied. Specifically, the inertial cavitation thresholds are determined using R(max)/R(0), and subharmonic signals from the emission of an oscillating bubble are estimated. The results show that the presence of the elasticity increases the threshold pressure for a bubble to oscillate inertially, and subharmonic signals may only be detectable in certain ranges of radius and pressure amplitude. These results should be easy to verify experimentally, and they may also be useful in cavitation detection and bubble-enhanced imaging.

A mechanism for the generation of cavitation maxima by pulsed ultrasound

A train of 1-MHz pulses can generate maxima of cavitation activity [V. Ciaravino, H. G. Flynn, and M. W. Miller, Ultrasound Med. Biol. 7, 159-166 (1981)] at pulse lengths of 6 and 60 ms and at pressure amplitudes, PA, between 5.4 and 9.4 bars (or intensities between 10 and 30 W/cm2). Generation of maxima at PA between these limits on pressure amplitude implies that the increase in cavitation activity originates from gas nuclei with radii lying in a critical size range centered at about 0.08 micron. The mechanism proposed for this phenomenon suggests that nuclei in this critical range are unstabilized nuclei generated in one pulse and surviving to the next with an appreciable fraction of the survivors lying in the critical range. Transient cavities that grow from such small nuclei are shown to behave as isolated mechanical systems that on reaching maximum size collapse as imploding spheres. The maximum pressures reached in such imploding cavities would then approximate those calculated for the spherical collapse of cavities. The occurrence of the observed maxima is ascribed to the spherical collapse of transient cavities.

SPONTANEOUS HOMOGENEOUS NUCLEATION, INERTIAL CAVITATION AND THE SAFETY OF DIAGNOSTIC ULTRASOUND

Gas bubbles of sufficient size to serve as cavitation nuclei may form spontaneously in tissue in regions of very low interfacial tension. In the absence of an acoustic wave or other mechanical stress, such nuclei will quickly dissolve and disappear from the medium. Under the influence of an acoustic wave, however, these microbubbles may grow to many times their initial size and then collapse violently, a process known as inertial cavitation. In this work, the in vivo energetics and dynamics of the nucleation-cavitation process were modeled by treating tissue as a homogeneous fluid. The assumption of a viscosity of 10(-3) Pa s (i.e., that of water) resulted in the lowest acoustic rarefactional pressure threshold for nucleation-cavitation events, approximately 4.0 MPa, which was essentially frequency-independent over the range 1 to 15 MHz. The rarefactional pressure threshold for a viscosity of 5 x 10(-3) Pa s (that of blood) also was approximately 4.0 MPa at 1 MHz, but the threshold for this higher viscosity increased nearly linearly with frequency above approximately 5 MHz, never being more than approximately 0.2 MPa below the equivalent derated peak rarefactional pressure calculated assuming MI = 1.9, the current USFDA guideline.

American Institute of Ultrasound in Medicine consensus report on potential bioeffects of diagnostic ultrasound: Executive summary

The continued examination of potential biological effects of ultrasound and their relationship to clinical practice is a key element in evaluating the safety of diagnostic ultrasound. Periodically, the American Institute of Ultrasound in Medicine (AIUM) sponsors conferences bringing experts together to examine the literature on ultrasound bioeffects and to develop conclusions and recommendations related to diagnostic ultrasound. The most recent effort included the examination of effects whose origins were thermal or nonthermal, with separate evaluations for potential effects related to fetal ultrasound. In addition, potential effects due to the introduction of ultrasound contrast agents were summarized. This information can be used to assess risks in comparison to the benefits of diagnostic ultrasound. The conclusions and recommendations are organized into 5 broad categories, with a comprehensive background and evaluation of each topic provided in the corresponding articles in this issue. The following summary is not meant as a substitute for the detailed examination of issues presented in each of the articles but rather as a means to facilitate further study of this consensus report and implementation of its recommendations. The conclusions and recommendations are the result of several rounds of deliberations at the consensus conference, subsequent review by the Bioeffects Committee of the AIUM, and approval by the AIUM Board of Governors.