**5.2 Cavitation**

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 rigid wall using a high-speed camera9.

Due to the depression caused by the negative part of the ultrasound wave, intracellular water may enter the gaseous phase.

That would lead to the development of microbubbles.

When they reach the size of resonance, these bubbles suddenly collapse and produce highpressure shock waves, destroying adjacent tissue.

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 investigated by a high-speed photography method.

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 cloud structure.

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 structure.

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

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 cloud structure.

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 showed the dynamics of the crown-shape bubble cloud structure.

The remaining 4% exhibited the interchanging of these two structures.
