The importance of nonlinear acoustic wave propagation and ultrasound-induced cavitation in the acceleration of thermal lesion production by high intensity focused ultrasound was investigated experimentally and theoretically in a transparent protein-containing gel. A numerical model that accounted for nonlinear acoustic propagation was used to simulate experimental conditions. Various exposure regimes with equal total ultrasound energy but variable peak acoustic pressure were studied for single lesions and lesion stripes obtained by moving the transducer. Static overpressure was applied to suppress cavitation. Strong enhancement of lesion production was observed for high amplitude waves and was supported by modeling. Through overpressure experiments it was shown that both nonlinear propagation and cavitation mechanisms participate in accelerating lesion inception and growth. Using B-mode ultrasound, cavitation was observed at normal ambient pressure as weakly enhanced echogenicity in the focal region, but was not detected with overpressure. Formation of tadpole-shaped lesions, shifted toward the transducer, was always observed to be due to boiling. Boiling bubbles were visible in the gel and were evident as strongly echogenic regions in B-mode images. These experiments indicate that nonlinear propagation and cavitation accelerate heating, but no lesion displacement or distortion was observed in the absence of boiling.

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The potential role of therapeutic ultrasound in medicine is promising. Currently, medical devices are being developed that utilize high-intensity focused ultrasound as a noninvasive method to treat tumors and to stop bleeding (hemostasis). The primary advantage of ultrasound that lends the technique so readily to use in noninvasive therapy is its ability to penetrate deep into the body and deliver to a specific site thermal or mechanical energy with submillimeter accuracy. Realizing the full potential of acoustic therapy, however, requires precise targeting and monitoring. Fortunately, several imaging modalities can be utilized for this purpose, thus leading to the concept of image-guided acoustic therapy. This article presents a review of high-intensity focused ultrasound therapy, including its mechanisms of action, the imaging modalities used for guidance and monitoring, some current applications, and the requirements and technology associated with this exciting and promising field.

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