The dimension of the ultrasound beam and the transducer array are the origin of the slice thickness artifact. This artifact is also called partial volume artifact or volume averaging artifact.

See also Validation.

As the sound travels through a relatively homogeneous medium it propagates in essentially a straight line. When the sound reaches an interface a part of the incident beam is reflected, and a part is refracted (transmitted). Snells law governs the direction of the transmitted beam when refraction occurs:
sin qt = (c2/c1) x sin qi (qt is the transmit and qi is the incident angle)
The amount of sound that is reflected depends on the degree of difference between the two media; the greater the acoustic mismatch, the greater the amount of sound reflected. In addition, the amount of ultrasound reflected or refracted depends on the angle at which the sound beam hits the interface between the different media. As the angle of incidence approaches 90°, a higher percentage of the ultrasound is reflected.

See also Sonographic Features.

(TIS) The bone thermal index is an exposure model for the case that the ultrasound beam heats primarily soft tissue.

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 dimension of a spectral reflector is greater than the wavelength of the ultrasound beam. With specular echo reflection, the sound strikes the target at perpendicular incidence and the sound is reflected back towards the receiver. Specular reflections produce bright echoes.