The acoustic fields of the Wolf electrohydraulic lithotripter.

Electric sparks are used as the sources for both intra- and extracorporeal shock wave lithotripters. Upon ignition, a pressure pulse, headed by a shock, is generated that propagates as a spherically diverging wave. Simultaneously, a bubble is created that, in the case of the Wolf Model 2137.50 Electrohydraulic Lithotripter, expands to a radius of approximately 5 mm and collapses spontaneously after approximately 1 msec. Upon rebound, the bubble generates a second pressure pulse that is almost equal in amplitude and acoustic energy to the first shock wave. Measured pressures are almost entirely positive and decrease in amplitude with the reciprocal of the distance from the source. For the Wolf lithotripter at its maximum output setting, the pressure amplitude at a distance of 3 cm from the spark is typically 3 MPa.

Erratum: transient pulsations of small gas bubbles in water [J. Acoust. Soc. Am. 84, 985-998 (1988)] [corrected and republished].

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 microns are set in motion at pressures ranging from 0.5-5 bars and at frequencies from 0.5-10 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, transient cavities would be generated during the first pressure cycle from nuclei with initial radii as small as a few microns (micron). Nuclei that grow into transient cavities in the first pressure cycle are here called "prompt" nuclei. At a specified pressure, the size range of radii Rn in which they occur decreases with increasing frequency. At 5 bars, the range of Rn for prompt nuclei is 0.166-11.35 microns at 0.5 MHz and vanishes at 10 MHz.

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.

The exposure vessel as a factor in ultrasonically-induced mammalian cell lysis--II. An explanation of the need to rotate exposure tubes.

This paper is an attempt to explain the need to rotate a polystyrene tube containing a cell suspension in order to obtain cell lysis. Calculations, based on known physical laws, were performed in order to determine the important forces on cells and bubbles and the movements and interactions which these forces are likely to cause. These calculations support the following conclusions: (1) in the absence of rotation, cells and bubbles larger than resonance size are trapped at pressure minima while bubbles smaller than resonance size are trapped at pressure maxima, (2) at 1 W/cm2 with rotation, lysis is caused by cells sweeping through arrays of trapped small bubbles, (3) at higher intensities lysis is caused by both trapped and non-trapped small bubbles.