The relevance of bubbles in clinical high-intensity focused ultrasound treatments

Abstract

Until recently there has been a dichotomy of opinion as to the utility of bubbles in clinical HIFU treatments. On the one hand uncontrolled acoustic cavitation can lead to lesions of unpredictable shape, size, and position, while on the other, lesions containing bubbles are visible on conventional B-mode ultrasound images, and clinically are generally regarded as being indicative of tissue destruction. In some HIFU treatments of cancer, therefore, the ultrasound exposure levels are increased until bright ups are seen on the real time ultrasound scan of the target volume. It is now known that, in the presence of acoustic cavitation, the rate of heating is considerably enhanced, and investigations are under way to establish whether controlled cavitation induction can be used to speed up clinical HIFU treatments. When temperatures in excess of 100<th>°C are achieved in tissue, tissue water boiling will occur. This too will result in enhanced scatter in the ultrasound image. Boiling will also result in tissue destruction. The clinical implications of bubble activity will be reviewed in this paper.