High‐intensity focused ultrasound: a revolution in non‐invasive ultrasound treatment?

The possibility of tissue destruction through the use of high-energy ultrasound waves that destroy cells by raising temperatures to above 60◦C, causing protein denaturation and cell death, was demonstrated as early as the 1940s1. However, it is only in the last decade that clinical applications of this technology have appeared. It is most often employed in the destruction of various types of tumors. The central principle behind high-intensity focused ultrasound (HIFUS) is its highly focused beam at a defined point, which makes it possible from a distance to heat and ‘destroy’ the target tissue without damaging other organs. The depth of the focal point is variable within the range of the machine. Various investigations have shown that HIFUS causes cell necrosis2. The treatment effect can be evident somewhat beyond the target area, thus methods to monitor temperature change are important. It has been postulated that injury to a single cell may generate apoptosis mediators that spread through gap junctions and extend tissue destruction3,4. Local ischemia resulting from interrupted vascular supply could widen the effect. This issue remains to be clarified. The target area must be exactly defined so that the beam is focused precisely to treat the targeted tissue and protect its surroundings. HIFUS has its advantages as a minimally invasive treatment modality, however, it also carries risk of side effects and other complications. There is danger of damage to the surrounding tissues, necessitating the strictest scrutiny of the rise in temperature surrounding the target area during the procedure. The coupling of HIFUS with magnetic resonance imaging (MRI) provides high-contrast images to define target areas and allow more precise treatment to target tissue. In addition, the capability for real-time monitoring of temperatures in the target and surrounding tissues (Figure 1), thus enhancing the safety of HIFUS, has made MRI-guided HIFUS a superior option for non-invasive treatment. In several trials this modality was successful in treating uterine myomas, breast fibroadenomas and renal, prostate, liver and even brain tumors. In each of these organs tumor necrosis occurred with occlusion of blood vessels supplying the tumors5–18. MRI guidance is superior in cases of obesity: it images the target organs much more clearly than does ultrasound. Additionally, it is more difficult to set ultrasound and maintain the image in position to a very specifically defined point, especially for the 3-hour procedure required in ablation of leiomyomas. It is likely that in the future, for shorter procedures, ultrasound guidance would be sufficient. The greatest advantage of MRI-HIFUS seems to be the non-invasive nature of HIFUS. That is, it is performed 154 mm 150 mm