**5. Clinical devices in use**

There are two main types of transducers used for HIFU. In general HIFU needs a high power ultrasound transducer. Such kind of transducers is made of low loss piezoceramic materials or piezocomposites working in resonant mode. The two major types are the self- focusing spherical shaped piezoceramic transducers with fixed aperture and fixed focal length which is the simplest, and the phased array transducers made up of a large number of separate transducer elements. Thereby each element is driven by a separate electric radio frequency signal with an individual phase, frequency and amplitude. The advantages of phased arrays are the possibility for an adaptive selection of elements, the fast electronic beam steering and the beam forming capability. Such transducers are much more expensive and the number of elements in such transducers can reach thousands in some special cases [22].

On the other hands we can divide the transducers into extracorporeal used for the treatment of cutaneous and abdominal tumors, transrectal as those used for the prostate, and interstitial and endoscopic ones used for the treatment of biliary or esophageal lesions [24].
