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 constructed 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 the 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 connected with this feature. A layer of gelatine foam was also used to introduce gas into the gel and in this case the temperature rise was about 12 +/- 5 degrees C under similar conditions. Without gaseous inclusions, the mean temperature rise in gel in 1 min was 2.3 +/- 0.2 degree 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 agreement with the report by Hynynen (1991) of an excess temperature elevation of 60 degrees C in dogs' muscle in vivo during a 1 s pulse at 250 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 temperature rise greater than 0.3 degrees C was observed.