microbubbles form behind propeller, courtesy of U.S. Navy

Rounding out our recent trifecta of biosafety posts is a description of cavitation. Cavitation is the formation of microbubbles in liquid which has been subjected to rapid pressure changes. This can happen from a variety of causes from beating Dolphin tails, propellers, cracking your knuckles, and with ultrasound. The Mechanical Index is used to represent the risk of cavitation in tissue during ultrasound evaluation, though most authorities do not think cavitation occurs in the normal operating parameters of diagnostic ultrasound.

During rarefaction (the low pressure portion of the ultrasound pressure wave) air-filled structures expand. They then quickly contract again during the remaining phases of the sound wave. Cavitation is deliberately employed in lithotrypsy, as well as non-medical applications such as metal cleaning.

According to Wikipedia:

The physical process of cavitation inception is similar to boiling. The major difference between the two is the thermodynamic paths that precede the formation of the vapor. Boiling occurs when the local vapor pressure of the liquid rises above its local ambient pressure and sufficient energy is present to cause the phase change to a gas. Cavitation inception occurs when the local pressure falls sufficiently far below the saturated vapor pressure, a value given by the tensile strength of the liquid at a certain temperature.

So there are two major bioeffects of ultrsound: Heat and cavitation. The risks of either are vanishingly small with normal diagnostic ultrasound use. No studies have demonstrated any ill effects of diagnostic ultrasound in humans or even fetuses. But understanding these processes at least helps us recognize the issues behind bioeffect concerns.