When a bubble in a liquid is subjected to a periodic sound field, the resulting bubble oscillations can interact with the sound field, giving rise to the primary Bjerknes force. A simple undergraduate-level derivation, and a graphical illustration of the underlying processes, are given.

A Therasonic 1030 (Electro-Medical Supplies) therapeutic ultrasound generator operating at 1 Mkiz continuous wave was used to insonate aerated water at two temperatures, 22 "C and 37 "C. Using various acoustically reflecting materials, sound fields were set up with different standing wave components. Measurements of the acoustic pressure variations on the axis of the sound fields were made using a needle hydrophone and the results were compared with photographs of the spatial distributions of the image intensified sonoluminescent light output. The near field region was used, thereby simulating the clinical situation.Sustained sonoluminescence was observed for nominal intensities of 3 W cm-*, and acoustic reflections of greater than 40%. Under these conditions, if sonoluminescence did not appear spontaneously it could always be induced by rotating the transducer. Whenever bands of maximum light output formed they correlated closely with the pressure antinodes in the standing wave pattern. Very little light was produced by travelling wave fields.

Sonoluminescence, produced when a therapeutic ultrasound generator operating at 1 MHz was used to insonate a tank of water, was detected using a photomultiplier tube and analysed using pulse height analysis. Spectra of the number of counts per second were obtained for the complete range of observed pulse heights, under exposure conditions similar to those used in clinical practice. Water containing different concentrations of dissolved gases and an agar solution were investigated during the course of the experiments.Measurements were made to establish a threshold for sonoluminescence and the total sonoluminescent light output from tap water insonated with continuous wave ultrasound at 1 W cm-* was estimated. The density of free radicals produced under these conditions was also estimated. The effects of temperature and duty cycle were investigated. Sonoluminescence increased with temperature over the range 22-45 "C and pulsed regimens produced more sonoluminescence than continuous wave ultrasound over a significant part of the pulse height spectrum. ResumeEtude par sonoluminescence des effets de cavitation des ultrasons utilises en clinique: 2. Seuils de sonoluminescence pour u n faisceau d'ultrasons a usage therapeutique et effet de la temperature et des conditions d'utilisation.

In previous work the phenomenon of sonoluminescence (SL) has been used to find the conditions in which transient cavitation during exposure to ultrasound is likely to be maximum. This paper reports the effect of therapeutic ultrasound on growth of mouse tumour cells in monolayer culture when the cells are insonated either at a pressure antinode or at a pressure node in a standing wave ultrasound field that is known to produce strong bands of SL at the pressure antinodes. Reduced cell numbers 72 h after insonation were recorded when the cells were insonated at an antinode but not when they were at a node. The possibility that this effect might be an artefact of the experimental system, and further experiments that could elucidate the nature of the damage, are discussed.

Sonoluminescence can readily be seen when aerated water is insonated with continuous wave therapeutic ultrasound at room temperature but is not easily observed when short pulses of diagnostic ultrasound are used. In this work an ultrasound generator, operating in the region of 1 MHz and capable of producing pulses of different length and repetition rate, was used for insonation. The pulse repetition rate of the ultrasound was fixed at 1 kHz since this is characteristic of diagnostic machines, and a series of thresholds for sonoluminescence was obtained for two transducers, one therapeutic and one diagnostic, as the number of cycles in each pulse was varied. Sonoluminescence was observed for pulses of a few cycles, but the ultrasound intensity threshold for onset increased sharply with decreasing pulse iength. Under all conditions tested, sonoluminescence was more readily sustained than initiated. At about 20 cycles per pulse, peak negative pressures of about 400 kPa initiated sonoluminescence. These conditions are well within the range of some regimens for Doppler ultrasound and not far removed from the diagnostic situation.