Anatomic structures respond with characteristic features on ultrasound scanning.
There are some ultrasound terms, referring to the echo appearance, that describes tissue appearance in a uniform manner:

Tendons characteristically are hyperechoic on ultrasound because of the fibrillar pattern. Ligaments appear hyperechoic when the beam is perpendicular to the tissue. Peripheral nerves are hyperechoic relative to muscle.
Muscle appears relatively hypoechoic to tendon fibers. Close observation reveals hypoechoic muscle fibers separated by hyperechoic septae that converge on a hyperechoic aponeurosis. Articular hyaline cartilage appears hypoechoic. The presence of fluid within the joint outlining the cartilage produces a thin bright echo at this interface.
Sound beams do not penetrate the bone cortex. The very bright echo produced at the interface allows both recognition of the bone cortex but also can demonstrate fracture, spurring and bone callus bridging. Abnormal soft tissue calcification and ossification also produces bright reflective echoes.
Cysts or fluid filled areas are without internal echoes and are called echo free or anechoic and may demonstrate enhanced soft tissue echoes posterior to the fluid collection. Inflamed metatarsal bursae and calcaneal bursae clearly depict fluid swelling.

See also Beam Pattern and Zero Offset.

The echo is the part of the transmitted ultrasound that is reflected back to the receiving transducer crystal.

See also Sonographic Features.

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.

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 near field (also called Fresnel zone) is the proximal part of an ultrasound beam. The Fresnel zone is adjacent to the transducer surface and has a converging sound beam profile. A narrow beam shape is maintained in the near field owing to constructive and destructive interference patterns of sound wavelets emitted from the transducer crystal.
The length of the near field is equal to
r²/l = d²/4l
where r is the radius, l is the ultrasound wavelength in the medium of propagation and d the diameter of the piezoelectric crystal.

See also Beam Pattern, and Sonographic Features.