Cavitation

Cavitation is the formation of vapour cavities in a liquid – i.e. small liquid-free zones ("bubbles" or "voids") – that are the consequence of forces acting upon the liquid. It usually occurs when a liquid is subjected to rapid changes of pressure that cause the formation of cavities where the pressure is relatively low. When subjected to higher pressure, the voids implode and can generate an intense shockwave.

There are two broad categories of cavitation:

• stable

• transient

Stable cavitation (also called non-inertial) is production and oscillation of these bubbles.

Transient cavitation (also called unstable, inertial, or collapse cavitation) is when the bubbles violently burst.  The peak rarefaction (lowest) pressure is the key factor in determining when transient cavitation will occur.

When transient cavitation does occur there is a transient dramatic increase in the temperature and mechanical forces surrounding the collapsed bubble. These forces have the potential to induce biological effects in tissue.

Thankfully the relatively weak pressures generated by diagnostic ultrasound have not been found to cause cavitation in human tissues that do not already contain gas bubbles.  This means that cavitation is not thought to occur using diagnostic ultrasound to perform cardiac, abdominal, gynecologic, or obstetrical ultrasound.  However, cavitation is a potential mechanism for bioeffects and may be of concern in applications where bubbles already exist such as during lung ultrasound or with the use of contrast agents.  

Interesting aside:

As a therapeutic procedure, transient cavitation is now being exploited in a new non-invasive surgical technique known as histotripsy.  The adjacent video explains this new therapeutic technique well.  Histotripsy is similar to HIFU (high intensity focused ultrasound) which is currently used as an ablative treatment.  However, histotripsy uses cavitation rather than heating to destroy the diseased tissue.