**2. Basic physics of waves and ultrasound**

To understand the basic principle of high intensity focused ultrasound as an ablative treatment modality for prostate cancer, the main properties of sound waves should be known and are summarized in figure 1 and 2.

**Figure 1.** Physical parameters of sound physics

Sound is defined as a mechanical disturbance from a state of equilibrium that propagates through an elastic material medium in the form of vibrating waves. These vibrating waves transport energy from its source to another area as long as a medium is present. The energy transported by sound is known as acoustic energy or sonic energy. Sonic intensity (SI), defined as a time-average rate of sonic energy-flow through a unit area (SI unit: W/cm2), is proportional to sonic pressure square and has a positive correlation with the power and energy of sound. This implies that the amount of energy accumulating at a target area is larger when the sonic pressure or intensity increases. The latter parameter varies with space and time, and it is usually expressed as peak or average intensity, and both quantities can refer to either a spatial or temporal dimension (e.g. ISP = spatial peak intensity, ISATA = spatial average, temporal average intensity). The frequency of a wave is derived from the amount of cycles per unit of time. Ultrasound (US) is a form of sound that has a frequency higher than the frequency detected by human ear (> 20.000 Hz vs. 20-20.000 Hz). HIFU therapy utilizes high intensity US waves that are propagated through human tissues. In contrast to other ablative therapies, HIFU therapy operates without the need of an electrode or antenna to deliver its waves. However, the main challenge of such a technique is focusing energy-accumulation at the target area to

**Figure 2.** HIFU uses high energy US waves generated from numerous sources and focuses them to a small spot. Acoustic pressure is rapidly elevated near focus where tissue temperatures are also raised to level that is sufficient for coagulation necrosis

induce significant biological reactions without causing harm to the intervening and surround‐ ing tissues. Recently, a phased array US transducer technique was adopted for focal HIFU treatment. It sends temporally different sets of electronic signals to each specific transducer component, thus enabling beam-steering and focusing, which can move a focal spot in virtually any direction within physically allowed ranges. This system is not only more versatile than other systems but also highly efficient without any sonic attenuation [20].
