Basic Physical and Technical Principles

Abstract

Ultrasound is the term applied to mechanical pressure waves with frequencies above 20 000 Hz (beyond the audible range). A medium must be present for ultrasound propagation to occur. In biologic tissues, as in fluids, ultrasonic energy is propagated in the form of longitudinal waves. Ultrasonic waves in the megahertz (high-frequency) range can be emitted like a directional beam from transducers of practical size (e.g., transducer diameter D=1 cm) (Table 1.1). The ultrasound beam behaves similarly to a light beam in that it can be reflected, refracted, and diffracted. It can also be absorbed by the media it traverses. In this process the kinetic energy of the beam is converted to heat, resulting in a progressive decrease in the intensity of the ultrasound energy. In biologic tissue this “attenuation” is on the order of 1 dB per centimeter of tissue traversed and per megahertz of frequency.