Inertial cavitation is a more violent phenomenon, in which the bubble grows during the rarefaction phase and then rapidly PD184352 datasheet collapses which leads to its destruction. The collapse is often accompanied by the loss of bubble sphericity and formation of high velocity liquid jets. If the bubble collapse occurs next to a cell, the jets may be powerful enough to cause disruption of the cell membrane Inhibitors,research,lifescience,medical (25),(26). In blood vessels, violently collapsing bubbles can damage the lining of the vessel
wall or even disrupt the vessel altogether. One may assume that the disruption occurs due to bubble growth and corresponding distension of the vessel wall. However, it was shown that most damage occurs Inhibitors,research,lifescience,medical as the bubble rapidly collapses and the vessel wall is bent inward or invaginated, causing high amplitude shear stress (27). Stable cavitation may lead to a phenomenon called “microstreaming” (rapid movement of fluid near the bubble due to its oscillating motion). Microstreaming can produce high shear forces close to the bubble that can disrupt cell membranes and may play a role in ultrasound-enhanced drug or gene delivery when damage to the cell membrane is transient (28). Cavitation activity is the major mechanism that is utilized when mechanical damage to tissue is a goal. At its extreme, when very high rarefactional pressures (> 20 MPa) are used, a cloud of cavitating bubbles can cause
Inhibitors,research,lifescience,medical complete tissue lysis at the focus (29). In such treatments the thermal effect is usually to be avoided, therefore, short bursts of very high amplitude ultrasound of low frequency (usually below 2 MHz) are used. The time-averaged intensity remains low, and the thermal dose delivered to the tissue is not sufficient to cause thermal damage. Cavitation can also promote heating if longer HIFU pulses or Inhibitors,research,lifescience,medical continuous ultrasound is used (30)-(32). The energy of the incident ultrasound wave is transferred very efficiently into stable oscillation of resonant-size bubbles. This oscillatory motion causes microstreaming around the bubbles and that, in turn, leads to additional Inhibitors,research,lifescience,medical tissue heating through
viscous friction, which can lead to coagulative necrosis. Cediranib (AZD2171) Nonlinear ultrasound propagation effects Nonlinear effects of ultrasound propagation are observed at high acoustic intensities and manifest themselves as distortion of the pressure waveform: a sinusoidal wave initially generated by an ultrasound transducer becomes sawtooth-shaped as it propagates through water or tissue (Figure 2D). This distortion represents the conversion of energy contained in the fundamental frequency to higher harmonics that are more rapidly absorbed in tissue since ultrasound absorption coefficient increases with frequency. As a result, tissue is heated much faster than it would if nonlinear effects did not occur. Therefore, it is critical to account for nonlinear effects when estimating a thermal dose that a certain HIFU exposure would deliver.