(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.

Contrast microbubbles can be destroyed by intense ultrasound and the scattered signal level can increase abruptly for a short time during microbubble destruction, resulting in an acoustical flash (sudden increase in echogenicity).
Intermittent imaging with high acoustic output utilizes the properties of contrast microbubbles to improve blood-to-tissue image contrast by imaging intermittently at very low frame rates.
The frame rate is usually reduced to about one frame per second, or it is synchronized with cardiac cycles so that enough contrast microbubbles can flow into the imaging site where most microbubbles have been destroyed by the previous acoustic pulse. Because bubbles are destroyed by ultrasound, controlling the delay time between frames produces images whose contrast emphasizes regions with rapid blood flow rate or regions with high or low blood volume.

Reflection of the sound beam occurs when it hits a boundary between materials having different acoustic impedance. The reflection (echo) is the portion of a sound that is returned from the boundary. The reflection time (the time taken for the wave to return to the probe) can be used to determine the depth of the object.
The reflection within the body produces the ultrasound image, but should be minimized at an ultrasound couplant to skin boundary where the couplant acts as an acoustic window through which the image is seen. The amount of sound waves, which are reflected back at the interface between two tissues is depend on the angle of incidence and the difference between the acoustic impedance values of the two tissues.
If the difference is great, a large part of the sound waves will be reflected back. If too much sound is reflected back and not enough waves are remaining to be able to penetrate the tissue, the imaging will be poor.
If the difference is small, a small amount will be reflected back. Enough sound signal remains to continue with ultrasound imaging.
If the ultrasound beam meets a rough surface or small object, the beam is scattered in all directions and only a small amount will be received by the probe.

See also False Distance Artifact, Target Strength, and Snells Law.

Ultrasound waves are reflected when there is a change in acoustic impedance. The larger the change, the more ultrasound is reflected. Microbubbles have an enormous difference in acoustic impedance as compared to surrounding fluid due to the large differences in density, elasticity and compressibility.
At low acoustic power (mechanical index less than 0.1), the mechanism of ultrasound reflection is that of Rayleigh scattering and the microbubbles may be regarded as point scatterers. The scattering strength of a point scatterer is proportional to the sixth power of the particle radius and to the fourth power of the ultrasound frequency;; the echogenicity of such contrast agent is therefore highly dependent upon particle size and transmit frequency. The backscattered intensity of a group of point scatterers is furthermore directly proportional to the total number of scatterers in the insonified volume. The concentration of the contrast medium is of importance.

See also Backscatter Energy, Cross-section Scattering.

A point scatterer is a reflector with a diameter much smaller than the ultrasound wavelength. The reflection from blood is a typical example of point scattering. Red blood cells are with 7μm versus 0.44 mm wavelength at 3.5 MHz, smaller than any US wavelength. The individual cells are not only the point scatterers, ultrasound is scattered whenever there is a change in acoustic impedance, and in blood such changes are caused by variable cell concentration. These local fluctuations in cell concentration have a spatial extent that is also much smaller than the ultrasound wavelength, and they therefore act as point scatterers.
A point scatterer gives rise to spherical wavelets spreading out in all directions with the scatterer itself at the center of the sphere. The spherical wavelets from one single point scatterer are much too weak to be detected by the transducer, but constructive interference between numerous wavelets will produce backscattering of higher amplitude echoes with parallel wavefronts, also in the direction of the ultrasound transducer.

See also Rayleigh Scattering.