Echogenicity is the ability of a medium to create an echo, for example to return a signal when tissue is in the path of the sound beam. The ultrasound echogenicity is dependent on characteristics of tissues or contrast agents and is measured by calculating the backscattering and transmission coefficients as a function of frequency.
The fundamental parameters that determine echogenicity are density and compressibility. Blood is two to three orders of magnitude less echogenic than tissue due to the relatively small impedance differences between red blood cells and plasma. The tissue echogenicity can be increased by ultrasound contrast agents. Encapsulated microbubbles are highly echogenic due to differences in their compressibility and density, compared to tissue or plasma.
Microbubbles are 10,000 times more compressible than red blood cells. The compressibility of air is 7.65 x 10−6 m2/N, in comparison with 4.5 x 10-11 m2/N for water (on the same order of magnitude as tissue and plasma). This impedance mismatch results in a very high echogenicity. An echo from an individual contrast agent can be detected by a clinical ultrasound system sensitive to a volume on the order of 0.004 pl.

See also Isoechogenic, Retrolenticular Afterglow, and Sonographic Features.

From Fukuda Denshi Co., Ltd.;
'Operation-friendly Switches and Controls backlit to permit the operator to identify them in a darkroom
Image Optimization through Ultrasonic Frequency Changeover without replacement of the probe
High frame rate enhancement mode for Cardiac echo
Cine memory for selection of best data through SCROLL, LOOP, or MANUAL review
Programmable function keys allow to get predefined for measurement parameters.
Special measurement software usable in a wide range.
Measurement Report page on the display can be printed as required.'

The far field (also called Fraunhofer zone) is the distal part of an ultrasound beam characterized by a diverging shape and continuous loss of ultrasound intensity with distance from the transducer. The angle of divergence increases with lower transducer frequency and with smaller transducer diameter.

See also Sonographic Features.

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.
The disadvantage of this microbubbles produced by agitation, was that the air quickly leak from the thin bubble shell into the blood, where it dissolved. In addition, the small bubbles that were capable of traversing the capillary bed did not survive long enough for imaging because the air quickly dissipated into the blood. Aside from agitated saline, also hydrogen peroxide, indocyanine green dye, and iodinated contrast has been tested. The commercial development of contrast agents began in the 1980s with greatest effort to the stabilization of small microbubbles.

The development generations by now:
To pass through the lung capillaries and enter into the systemic circulation, microspheres should be less than 10 μm in diameter. Air bubbles in that size range persist in solution for only a short time; too short for systemic vascular use.
The first developed agent was Echovist (1982), which enabled the enhancement of the right heart. The second generation of echogenic agents, sonicated 5% human albumin-containing air bubbles (Albunex), were capable of transpulmonary passage but often failed to produce adequate imaging of the left heart. Both Albunex and Levovist utilize air as the gas component of the microbubble.
In the 1990s newer developed agents with fluorocarbon gases and albumin, surfactant, lipid, or polymer shells have an increased persistence of the microspheres. This smaller, more stable microbubble agents, and improvements in ultrasound technology, have resulted in a wider range of application including myocardial perfusion.

See also First Generation USCA, Second Generation USCA, and Third Generation USCA.

A hydrophone is a special transducer for underwater measurement of acoustic fields. The diameter of a hydrophone should be smaller than the wavelength of the measured ultrasound, in combination with a large bandwidth.