**5.3 Heat damage**

The tissue ablation induced by high-intensity ultrasound results primarily from bulk heating, with possible contributions from boiling and acoustic cavitation. Bubbles, when present, may enhance local absorption (Fig. 6).

The position and shape of the heated region are determined by the intensity of the field near the focus, the attenuation and the effects of diffusion.

The rate of change of temperature at any point is proportional to the absorbed power density and hence to the incident beam power and attenuation11.

The irreversible changes in proteins associated with tissue denaturation and coagulation start at low temperature.

When temperatures of approximately 60°C are approached, the rate of denaturation becomes so great that irreversible changes can occur in seconds.

Temperature reached during HIFU treatment is clearly higher, as reported above: When lesioning occurs, the attenuation within the treated volume increases12, thus altering the absorbed power distribution, depending on the biologic feature of the prostate.

Consequently, the region in which heat is deposited may be expected to change during the heating process.

By a macroscopic point of view, we can say that heat growth is maximal in the middle of the treated volume and minimal in the external area of the treated volume.

Cavitation is due to the gas microbubble (bubble clouds) vibration dissolved in prostate tissue. Lindau and Lauterborn investigated the collapse and rebound of a cavitation bubble near a flat

Due to the depression caused by the negative part of the ultrasound wave, intracellular water

When they reach the size of resonance, these bubbles suddenly collapse and produce high-

In a study carried out by Chen H and colleagues10, the dynamics of cavitation bubble clouds generated at the tissue boundary in continuous HIFU fields has been experimentally

The experimental results revealed that the cavitation bubble clouds organize into two shapes, which were named ''cone-shape" bubble cloud structure and ''crown-shape" bubble

The cavitation bubble cloud is visible at the tissue surface at 200 µs; then a tiny tip becomes obvious at 600 µs. The elongated tip leads to the formation of a cone-shape bubble cloud

The bubble cluster grows larger and developes a crown-like shape. Meanwhile, it moves forward and finally hits the tissue boundary forming the crown-shape cavitation bubble

Among the 171 image series recorded in the study carried out by Chen H et al, 85% showed the evolution of the cone-shape bubble cloud structure. Another 11% of the image series

The tissue ablation induced by high-intensity ultrasound results primarily from bulk heating, with possible contributions from boiling and acoustic cavitation. Bubbles, when

The position and shape of the heated region are determined by the intensity of the field near

The rate of change of temperature at any point is proportional to the absorbed power

The irreversible changes in proteins associated with tissue denaturation and coagulation

When temperatures of approximately 60°C are approached, the rate of denaturation

Temperature reached during HIFU treatment is clearly higher, as reported above: When lesioning occurs, the attenuation within the treated volume increases12, thus altering the

Consequently, the region in which heat is deposited may be expected to change during the

By a macroscopic point of view, we can say that heat growth is maximal in the middle of the

absorbed power distribution, depending on the biologic feature of the prostate.

treated volume and minimal in the external area of the treated volume.

After 1.8 ms, the cone-shape bubble cloud attaines a dynamically stable state.

showed the dynamics of the crown-shape bubble cloud structure. The remaining 4% exhibited the interchanging of these two structures.

density and hence to the incident beam power and attenuation11.

becomes so great that irreversible changes can occur in seconds.

present, may enhance local absorption (Fig. 6).

the focus, the attenuation and the effects of diffusion.

**5.2 Cavitation** 

cloud structure.

cloud structure.

**5.3 Heat damage** 

start at low temperature.

heating process.

structure.

rigid wall using a high-speed camera9.

That would lead to the development of microbubbles.

pressure shock waves, destroying adjacent tissue.

investigated by a high-speed photography method.

may enter the gaseous phase.

This difference allows to surely set the treatment outlines and save the prostate apex, and the striated sphincter and vasculo-nervous bundles.

Fig. 6. Elementary lesion formation and its effects on prostate parenchyma, due to heat and mechanical damage.
