Cavitation is any activity of highly compressible transient or stable microbubbles of gas and/or vapour, generated by ultrasonic power in the propagation medium. Cavitation can be described as inertial or non-inertial. Inertial cavitation has the most potential to damage tissue and occurs when a gas-filled cavity grows, during pressure rarefaction of the ultrasound pulse, and contracts, during the compression phase. Collapses of bubbles can generate local high temperatures and pressures. Transient cavitation can cause tissue damage.
The threshold for cavitation is high and does not occur at current levels of diagnostic ultrasound. The introduction of contrast agents leads to the formation of microbubbles that potentially provide gas nuclei for cavitation. The use of contrast agents can lower the threshold at which cavitation occurs.

Types of cavitation:

The main advantage of ultrasound is that certain structures can be observed without using radiation. However, ultrasound is energy and there are ultrasound safety regulations, because two bioeffects of ultrasound are heat and cavitation. Ultrasound is a mechanical energy in which a pressure wave travels through tissue. Reflection and scattering back to the transducer are used to form the image. As sound energy is transmitted through the tissue, some energy is reflected and some power is absorbed.
Possible physical effects with ultrasound:
The ultrasound safety is based on two indices, the mechanical index (MI) and the thermal index (TI). The WFUMB guidelines state that ultrasound that produces temperature rises of less than 1.5°C may be used without reservation. They also state that ultrasonic exposure causing temperature rises of greater than 4°C for over 5 min should be considered potentially hazardous. This leaves a wide range of temperature increases which are within the capability of diagnostic ultrasound equipment to produce and for which no time limits are recommended. However, it has not been determined that medical ultrasound causes any adverse reaction or deleterious effect.
The American Institute of Ultrasound in Medicine states that as of 1982, no independently confirmed significant biologic effects had been observed in mammalian tissue below (medical usage) 100mW/cm2.

See also Ultrasound Regulations and Ultrasound Radiation Force.

Increasing the frequency of the transmitted power improves the image quality of ultrasound, but the improvement in resolution results in a decreased signal to noise ratio (SNR). Higher acoustic power levels can prevent the loss in SNR, but among other reasons, ultrasound regulations limit this to avoid heating or cavitation.
Coded excitation increase the signal to noise ratio without the loss of resolution by using coded waveforms. Coded excitation allows transmitting a long wide-band pulse with more acoustic power and high penetration of the sound beam.

(MI) The mechanical index is an estimate of the maximum amplitude of the pressure pulse in tissue. It is an indicator of the likelihood of mechanical bioeffects (streaming and cavitation). The mechanical index of the ultrasound beam is the amount of negative acoustic pressure within a ultrasonic field and is used to modulate the output signature of US contrast agents and to incite different microbubble responses.
The mechanical index is defined as the peak rarefactional pressure (negative pressure) divided by the square root of the ultrasound frequency.
The FDA ultrasound regulations allow a mechanical index of up to 1.9 to be used for all applications except ophthalmic (maximum 0.23). The used range varies from 0.05 to 1.9.
At low acoustic power, the acoustic response is considered as linear. At a low MI (less than 0.2), the microbubbles undergo oscillation with compression and rarefaction that are equal in amplitude and no special contrast enhanced signal is created. Microbubbles act as strong scattering objects due to the difference in impedance between air and liquid, and the acoustic response is optimized at the resonant frequency of a microbubble.
At higher acoustic power (MI between 0.2-0.5), nonlinear oscillation occurs preferentially with the bubbles undergoing rarefaction that is greater than compression. Ultrasound waves are created at harmonics of the delivered frequency. The harmonic response frequencies are different from that of the incident wave (fundamental frequency) with subharmonics (half of the fundamental frequency), harmonics (including the second harmonic response at twice the fundamental frequency), and ultra-harmonics obtained at 1.5 or 2.5 times the fundamental frequency. These contrast enhanced ultrasound signals are microbubble-specific.
At high acoustic power (MI greater than 0.5), microbubble destruction begins with emission of high intensity transient signals very rich in nonlinear components. Intermittent imaging becomes needed to allow the capillaries to be refilled with fresh microbubbles. Microbubble destruction occurs to some degree at all mechanical indices. A mechanical index from 0.8 to 1.9 creates high microbubble destruction. The output signal is unique to the contrast agent.

From GE Healthcare;;
Optison is the first 'second generation USCA' marketed in the US.
Ultrasound contrast agents used during an ultrasound imaging procedure, enable more accurate diagnosis of the patient's heart condition. The application of Optison allows to image the endocardial borders of the heart, to see cardiac wall motion abnormalities and to guide the selection and monitoring of treatment.
Optison represents a class of microbubbles with a shell formed by sonicating a solution of capsules filled with a perfluoropropane gas. The high molecular weight slows microbubble dissolution and prolongs the enhancement for several minutes. The human albumin-stabilized cavitation bubbles have a surface tension of 0.9 N/m and a surface dilatational viscosity 0.08 msP.

'August 06, 2001 Molecular Biosystems Inc., a subsidiary of Alliance Pharmaceutical Corp, announced the amendment of the Optison Product Rights Agreement (OPRA) dated May 9, 2000 with Mallinckrodt Inc, a unit of Tyco Healthcare. Optison, an intravenous ultrasound contrast agent, was developed by MBI and is being marketed by Mallinckrodt in the U.S. and Europe. Under the amended agreement, MBI will receive an immediate cash payment plus additional unspecified royalties for a two-year period. The amendment of OPRA coincides with an announcement by Nycomed Amersham Imaging that Nycomed and Mallinckrodt will terminate their joint commercialization and development agreement for ultrasound contrast agents, including Optison, effective Dec. 31, 2001. Effective Jan. 1, 2002, all selling and marketing activities will be resumed solely by Nycomed Amersham.'