ACOUSTIC CAVITATION AND MEDICAL ULTRASOUND

The acoustic pressure amplitudes common to medical ultrasound devices are often significantly in excess of the cavitation threshold. For example, negative pressure amplitudes in excess of 15 MPa are used in a variety of therapeutic ultrasound devices such as those used in shock wave lithotripsy and high intensity focused ultrasound (HIFU) surgery; accordingly, cavitation is a common occurrence in the utilization of these devices. In some cases, this cavitation can be useful, if not essential, such as in the comminution of kidney stones by the intense shock waves used in lithotripsy. However, if the cavitation is not controlled, then these gas-filled cavities can act as a highly reflective barrier against further acoustic propagation. We have determined, for example, that there is a maximum pulse repetition rate for shock wave lithotripsy. When HIFU therapy systems are used for tumor necrosis, the acoustic intensities can achieve levels in excess of several kilowatts/cm 2 . With these intensities, cavitation can readily be achieved, even in the high-threshold, high-frequency realm of medical ultrasound. Because the ultimate goal of a HIFU therapy system is to necrose tissue, the appearance of cavitation was previously thought to be undesirable; however, in some cases, the initiation of cavitation can lead to enhanced heating, and also to protocols that prevent collateral eects of HIFU. In this presentation, a review of the occurrence of cavitation in medical ultrasound applications will be presented as well as some recent experimental results.