Thyroid Ultrasound Physics

Some animal species such as dolphins, whales, and bats are capable of creating a “visual” image based on receiving reflected sound waves. Man’s unassisted vision is limited to electromagnetic waves in the spectrum of visible light. Humans require technology and an understanding of physics to use sound to create a picture. This chapter will explore how man has developed a technique for creating a visual image from sound waves (1).

Sound is transmitted as mechanical energy, in contrast to light, which is transmitted as electromagnetic energy. Unlike electromagnetic waves, sound waves require a propagating medium. Light is capable of traveling through a vacuum, but sound will not transmit through a vacuum. The qualities of the transmitting medium directly affect how sound is propagated. Materials have different speeds of sound transmission. Speed of sound is constant for a specific material and does not vary with sound frequency (Fig. 2.1). Acoustic impedance is the inverse of the capacity of a material to transmit sound. Acoustic impedance of a material depends on its density, stiffness and speed of sound. When sound travels through a material and encounters a change in acoustic impedance a portion of the sound energy will be reflected, and the remainder will be transmitted. The amount reflected is proportionate to the degree of mismatch of acoustic impedance.