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.

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.

The dimension of the ultrasound beam and the transducer array are the origin of the beam width artifact or volume averaging artifact. When the ultrasound beam is wider than the diameter of the lesion being scanned, normal tissues which lie immediately adjacent to the lesion arc included within the beam width, and their echotexture is averaged in with that of the lesion.
Thus, what appears to be the echogenicity of the lesion is really that of the lesion plus the averaged normal tissues. Because of volume averaging, cystic lesions may falsely appear to be solid, and some subtle solid lesions may become impossible to distinguish from surrounding normal tissue and, therefore, not identified at all.

See also Ultrasound Picture and Vector Array Transducer.

Echogenic is a tissue that structures are capable of producing echoes when they are in the path of a sound beam.

See also Echogenicity, and Isoechogenic.

Gray scale [also grayscale, grey scale = brit.] produces basically black and white images with series of shades of gray. Solid areas appear white and fluid areas appear black, varying from black at the weakest intensity to white at the strongest. Gray scale resolves artifacts as small as 1 mm. The display is made by transmitting bursts of energy and analyzing the returning signal. Gray scale pictures are limited to the gray scale tones; color pictures display more information because the color is added to the gray scale.
Most ultrasound contrast agents also improve gray scale visualization of the flowing blood to such a degree that the tissue echogenicity increases. Gray scale enhancement of flow in an organ promises to improve lesion detection, along with the ability to differentiate between normal and abnormal areas, using many of the criteria already routinely used in CT and MRI.

See also Compress, Densitometry, Triplex Exam and QB-Mode.