The various gas microbubble contrast media are generally safe with low toxicity in humans. The tolerance of these agents is much higher than that of most x-ray agents, a reflection perhaps of the higher expectation of safety and convenience for ultrasound.
Extensive preclinical and clinical trials have demonstrated an excellent ultrasound contrast agent safety profile, the main side effect being a mild and transient local discomfort at the injection site which results from the high osmolality of these agents. Each contrast agent has its own profile of adverse effects, but all have been trivial thus far.

See also Ultrasonic Contrast Agents.

(UCA / USCA) Ultrasonography is the most commonly performed diagnostic imaging procedure. The introduction of sonographic contrast media into routine practice modifies the use of ultrasound in a variety of clinical applications. USCAs consist of microbubbles filled with air or gases and can be classified according to their pharmacokinetics. Among the blood pool agents, transpulmonary ultrasound contrast agents offer higher diagnostic potential compared to agents that cannot pass the pulmonary capillary bed after a peripheral intravenous injection. In addition to their vascular phase, some USCAs can exhibit a tissue- or organ-specific phase.
The sonogram image quality is improved either by decreasing the reflectivity of the undesired interfaces or by increasing the backscattered echoes from the desired regions.

Different types of ultrasound contrast agents:
Ultrasound contrast agents act as echo-enhancers, because of the high different acoustic impedance at the interface between gas and blood. The enhanced echo intensity is proportional to the change in acoustical impedance as the sound beam crosses from the blood to the gas in the bubbles.

The ideal qualities of an ultrasound contrast agent:
A typical ultrasound contrast agent consists of a thin flexible or rigid shell composed of albumin, lipid, or polymer confining a gas such as nitrogen, or a perfluorocarbon. The choice of the microbubble shell and gas has an important influence on the properties of the agent.
Current generations of microbubbles have a diameter from 1 μm to 5 μm. The success of these agents is mostly dependent on the small size and on the stability of their shell, which allows passage of the microbubbles through the pulmonary circulation. Microbubbles must be made smaller than the diameter of capillaries or they would embolize and be ineffective and perhaps even dangerous.
The reflectivity of these microbubbles is proportional to the fourth power of a particle diameter but also directly proportional to the concentration of the contrast agent particles themselves.
Ultrasound contrast agents produce unique acoustic signatures that allow to separate their signal from tissue echoes and to depict whether they are moving or stationary. This enables the detection of capillary flow and of targeted microbubbles that are retained in tissues such as normal liver.
The new generation of contrast media is characterized by prolonged persistence in the vascular bed which provides consistent enhancement of the arterial Doppler signal. Contrast agents make it also possible to perform dynamic and perfusion studies. Targeted contrast imaging agents are for example taken up by the phagocytic cell systems and thus have liver/spleen specific effects.

See also Ultrasound Contrast Agent Safety, Adverse Reaction, Tissue-Specific Ultrasound Contrast Agent, and Bubble Specific Imaging.

An ultrasound couplant is a material that propagates acoustical waves. The couplant eliminates air from the interface and adapts the contours of the transducer to the skin. The used material is an ultrasound gel that mimics the acoustic characteristics of biological tissue.

An echo is defined as the repetition of a sound by reflection of sound waves from a surface.
Echo types used in ultrasound imaging:
Specular echoes are created from relatively large, regularly shaped objects with smooth surfaces. Specular echoes are relatively intense and angle dependent.
Scattered echoes are created from relatively small, weakly reflective, irregularly shaped objects. Scattered echoes are less angle dependant and less intense.

See also Specular Echo, and Scattered Echo.

The ultrasound equipment includes the ultrasound machine, the coaxial cable, the transducer assembly, different modalities to print out and store the ultrasound pictures, ultrasound gel, and a couch for the patients.
Often, the ultrasound system is connected with the internal radiology information system which allows the takeover of patient data, and a picture archiving and communication system to store images.

See also Ultrasound System Performance.