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.

A thyroid ultrasound evaluates the size and shape of the thyroid gland and parathyroid glands. A thyroid ultrasound can show nodules, cysts, tumors, and an enlargement, but a sonogram cannot determine the function of the thyroid. Ultrasound guides the placement of the needle during a thyroid fine needle aspiration biopsy.

See also Sonographic Features, Ultrasound Imaging Modes, Anechoic, Beam Width Artifact and Enhancement Artifact.

Ultrasound physics is based on the fact that periodic motion emitted of a vibrating object causes pressure waves. Ultrasonic waves are made of high pressure and low pressure (rarefactional pressure) pulses traveling through a medium.

Properties of sound waves:

The speed of ultrasound depends on the mass and spacing of the tissue molecules and the attracting force between the particles of the medium. Ultrasonic waves travels faster in dense materials and slower in compressible materials. Ultrasound is reflected at interfaces between tissues of different acoustic impedance e.g., soft tissue - air, bone - air, or soft tissue - bone.
The sound waves are produced and received by the piezoelectric crystal of the transducer. The fast Fourier transformation converts the signal into a gray scale ultrasound picture.

The ultrasonic transmission and absorption is dependend on:
See also Sonographic Features, Doppler Effect and Thermal Effect.