G: High intensity ultrasound

High-intensity focused ultrasound (HIFU) is a technique that was first investigated in the 1940s as a method of destroying selective regions within the brain in neuro-surgical research. An ultrasound beam can be brought to a tight focus at a distance from its source, and if sufficient energy is concentrated within the focus, the cells lying within this focal volume are killed, whereas those lying elsewhere are spared. This is a noninvasive method of producing selective and trackless tissue de-struction in deep seated targets in the body, without damage to overlying tissues. This field, known both as HIFU and focused ultrasound surgery (FUS), is re-viewed in this article. Copyright © 2001 by W.B. Saunders Company. Key words: Therapeutic ultrasound, urology, oncology, hyperthermia, thermal ablation. /t ~ edical ultrasound is best known for its use- / 1 as an imaging modality. It is often the diagnostic technique of choice in obstetrics, for example, because it is believed to be safe and not able to induce deleterious effects in the fetus or mother. The fact that it is used by physiothera-pists for the treatment of soft tissue injuries shows that it is capable of inducing biological change. The main difference between diagnostic ultrasound and physiotherapy ultrasound lies in the way the sound energy is delivered, and in the acoustic power used. Ultrasound images are pro-duced by using very short pulses (1 to 10 As) and acoustic power levels around 200 mW. Physio-therapy uses longer pulses (10 to 100 ms) and power levels of about 1 W. The biological changes sought from physiotherapy ultrasound are usu-ally reversible, with the aim being to facilitate function and stimulate tissue repair. If the ultra-sonic power used is increased yet further (above 100 W) and pulses lasting 1 to 3 seconds are used, cell death may result. It is these high-power lev-els that are used in focused ultrasound surgery (FUS), a noninvasive method of inducing thermal ablation. This technique is also known as high-intensity focused ultrasound (HIFU)