Studies of the cavitational effects of clinical ultrasound by sonoluminescence: 2. Thresholds for sonoluminescence from a therapeutic ultrasound beam and the effect of temperature and duty cycle

Abstract

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.

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“…It is still controversial how temperature affects cavitational effects. Chendke and Fogler showed that sonoluminescence, a cavitational effect, was decreased as temperature increased in the range 11-89 • C using 20 KHz ultrasound sonicating to water [28], while Pickworth et al reported that, by also detecting sonoluminescence, the cavitation threshold decreased as temperature rose in the range 22-45 • C, using a 1 MHz continuous or pulsed ultrasound at intensities up to 3 W/cm 2 in water [29]. Recently, Entezari and Kruus showed that the effect of cavitation is dependent on temperature and ultrasound intensity.…”

Effects and Mechanisms

“…Iernetti found a maximum at 12° C for aerated water. Pickworth et al [261] found that the sonoluminescence from areated water in a 1-MHz standing-wave field increased at both 1 and 3 W cm" 2 (/SATA) in the range 22-45° C. An experimental study of cavitation, rather than sonoluminescence, by Blake [262] showed that cavitation thresholds decrease with temperature in the range -10-50° C.…”

“…In 1988, Pickworth et al [261] observed an interesting form of pulse enhancement when studying the distribution of energies of the sonoluminescent pulses as detected by a photomultiplication system connected to a pulse height analyser. The apparatus is shown in Figure 5.39.…”

“…One might expect the cross-over seen in Figure 5.41 to disappear, and the count rates to be increased in the order of duty cycle 1:7,1:4 and 1:2, with the most luminescence from continuous-wave insonation, the count rate now being simply a function of the total time of insonation. Pickworth et al [261,296] obtained the pulse-energy distribution spectrum in fluid 1.875 g/1 agar at 22° C. This substance had a viscosity of 170 ± 10 cp, and so was still fluid; since viscosity is a second-order effect when describing the actual bubble oscillations (equation (4.81)), then the actual cavitational behaviour will be similar to that in water. What will be different is the scale of the migrations, which will be greatly reduced in agar, because of the increased viscosity [319].…”

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“…Although it is not a surprise to expect light emission due to the high temperature releasing by transiently cavitating bubbles collapse, the precise mechanism of light production is still unclear. Sonoluminescence may be occurred due to the blackbody radiation, bremsstrahlung radiation, recombination radiation, or combinations thereof [19,[22][23][24][25].…”