THE EFFECT OF GAS-BUBBLES ON THE PRODUCTION OF ULTRASOUND HYPERTHERMIA AT 0.75 MHZ - A PHANTOM STUDY

Transparent phantoms, made of bovine hide gelatine, have been construc ted in order to study the consequences of the occurrence of cavitation in tissues. Gas pockets of about resonant size, physically introduced into the gel, lead to a mean temperature rise of 41 +/- 15-degrees-C in 1 min, when the gel of concentration 11.4% (w/v) is sonicated in th e continuous-wave (cw) mode at 1 W cm-2 (spatial average) and 0.75 MHz . Nyborg (1965) has shown that gas bubbles in a sound field can act as acoustic amplifiers and the observations reported here may be connect ed with this feature. A layer of gelatine foam was also used to introd uce gas into the gel and in this case the temperature rise was about 1 2 +/- 5-degrees-C under similar conditions. Without gaseous inclusions , the mean temperature rise in gel in 1 min was 2.3 +/- 0.2-degrees-C. At a gel/air interface, the rise per unit intensity per minute was 4. 4-degrees-C. It is concluded that in clinical situations, cavitation ( or degassing due to supersaturation), when it does occur, is likely to be an undesirable consequence of ultrasound treatment. This finding, of large temperature rises in proximity to gas bubbles, is in broad ag reement with the report by Hynynen (1991) of an excess temperature ele vation of 60-degrees-C in dogs' muscle in vivo during a 1 s pulse at 2 50 W cm-2 and 0.56 MHz. Other studies, by ter Haar and Daniels (1981) and Daniels and ter Haar (1986), of sonicated animal tissues in vivo, have found thresholds for bubble inception but no consequent temperatu re rise greater than 0.3-degrees-C was observed.