1. Physical principles of low-frequency ultrasound therapy

1.1 Acoustic cavitation

Although some studies point to the direct effects of ultrasound on cell walls, it is generally accepted that the effects of low-frequency ultrasound therapy are induced by the acoustic cavitation of gas microbubbles. These gas microbubbles can be either exogenous (e.g. the use of contrast agents for ultrasound) or endogenous (e.g. microbubbles formed from dissolved blood gases such as oxygen and carbon dioxide). In fact, under the influence of the ultrasound wave, which is a pressure/decompression wave propagating in liquids, the local pressure becomes for some time (depending on the pressure of the wave) lower than the partial vaporization pressure of the gaseous species in the blood, inducing the nucleation of an unstable gas microbubble, which, depending...