**4. Mechanisms of action of high intensity focused ultrasound**

The technique is based on the generation of focused ultrasound field. The focused ultrasound field is generated by a piezoelectric transducer with a center frequency of 1 to 7 MHz. The field is coupled into the body and aimed into a target region. Ultrasound waves are absorbed by tissues throughout their propagation and the acoustic energy will be converted to heat with a high energy density inside the focal zone [22]. The result is a temperature rise in the target zone within seconds to a level exceeding the threshold level of protein denaturation which is around 60 degrees. The result will be protein coagulation and coagulative necrosis [22-24]. In contrast there is a low acoustic energy density in the surrounding structures which keep them spared. This will create elliptical small volume lesions of 50-300 mm3 . Larger volume can be ablated without gaps by combining single lesions.

Between sonication a pause is necessary to prevent boiling and bubbles formations which can reflects and distort the ultrasound field and the consequence is unpredictable lesion growth and lesion formation in unwanted areas. So to ablate a large solid tumor it can take several hours. Thus HIFU is a very time consuming procedure [22]. HIFU has the advantage over other thermal ablative techniques of being a mini-invasive and non-ionizing with no long-term cumulative effects which means that it could be safely repeated. Further more it is interesting non invasive surgical tool because it can cause cell death in a tissue that is distant from the source of US [23].
