The ultrasonic power is the total amount of sound energy emitted by the transducer per unit of time.

See also Ultrasonic Heating.

In physics, the absorbed dose is the ultrasonic power absorbed per unit of mass of an object, and is measured in watts per kilogram (W/kg). The absorption increases with ultrasound intensity and frequency.
The thermal index describes the potential for heating of the patient's tissue due to the application of energy.

See also Thermal Effect, Ultrasound Safety, Ultrasound Regulations.

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:

(dB) A customary logarithmic measure most commonly used (in various ways) for measuring sound. Decibel is a way to express the ratio of two sound intensities: dB=10log10I1/I2 being I1 the reference. If one sound is 1 bel (10 decibel) 'louder' than another, this means the louder sound is 10 times louder than the fainter one. A difference of 20 decibel corresponds to an increase of 10 x 10 or 100 times in intensity.
The intensity of ultrasound decreases during the propagation and is measured in db/cm.
For sound pressure (the pressure exerted by the sound waves) 0 decibel equals 20 microPascal (μPa), and for ultrasonic power 0 decibel sometimes equals 1 picoWatt.

See also dB/dt, Phon, and Logarithms.

The earliest introduction of vascular ultrasound contrast agents (USCA) was by Gramiak and Shah in 1968, when they injected agitated saline into the ascending aorta and cardiac chambers during echocardiographic to opacify the left heart chamber. Strong echoes were produced within the heart, due to the acoustic mismatch between free air microbubbles in the saline and the surrounding blood.